Spring Framework Reference Documentation

Spring Framework Reference Documentation
4.2.7.RELEASE
Rod Johnson , Juergen Hoeller , Keith Donald , Colin Sampaleanu , Rob Harrop , Thomas Risberg , Alef
Arendsen , Darren Davison , Dmitriy Kopylenko , Mark Pollack , Thierry Templier , Erwin Vervaet , Portia
Tung , Ben Hale , Adrian Colyer , John Lewis , Costin Leau , Mark Fisher , Sam Brannen , Ramnivas
Laddad , Arjen Poutsma , Chris Beams , Tareq Abedrabbo , Andy Clement , Dave Syer , Oliver Gierke ,
Rossen Stoyanchev , Phillip Webb , Rob Winch , Brian Clozel , Stephane Nicoll , Sebastien Deleuze
Copyright © 2004-2015
Copies of this document may be made for your own use and for distribution to others, provided that you do not charge any fee
for such copies and further provided that each copy contains this Copyright Notice, whether distributed in print or electronically.
Spring Framework Reference Documentation
Table of Contents
I. Overview of Spring Framework ................................................................................................ 1
1. Getting Started with Spring ............................................................................................. 2
2. Introduction to the Spring Framework .............................................................................. 3
2.1. Dependency Injection and Inversion of Control ...................................................... 3
2.2. Modules .............................................................................................................. 3
Core Container .................................................................................................. 4
AOP and Instrumentation ................................................................................... 5
Messaging ......................................................................................................... 5
Data Access/Integration ...................................................................................... 5
Web .................................................................................................................. 5
Test ................................................................................................................... 6
2.3. Usage scenarios ................................................................................................. 6
Dependency Management and Naming Conventions ............................................ 9
Spring Dependencies and Depending on Spring ......................................... 11
Maven Dependency Management ............................................................. 11
Maven "Bill Of Materials" Dependency ....................................................... 12
Gradle Dependency Management ............................................................. 12
Ivy Dependency Management ................................................................... 13
Distribution Zip Files ................................................................................. 13
Logging ............................................................................................................ 13
Not Using Commons Logging ................................................................... 14
Using SLF4J ............................................................................................ 14
Using Log4J ............................................................................................. 15
II. What’s New in Spring Framework 4.x .................................................................................... 17
3. New Features and Enhancements in Spring Framework 4.0 ............................................ 18
3.1. Improved Getting Started Experience .................................................................. 18
3.2. Removed Deprecated Packages and Methods .................................................... 18
3.3. Java 8 (as well as 6 and 7) ............................................................................... 18
3.4. Java EE 6 and 7 ............................................................................................... 19
3.5. Groovy Bean Definition DSL .............................................................................. 19
3.6. Core Container Improvements ............................................................................ 19
3.7. General Web Improvements ............................................................................... 20
3.8. WebSocket, SockJS, and STOMP Messaging ..................................................... 20
3.9. Testing Improvements ........................................................................................ 21
4. New Features and Enhancements in Spring Framework 4.1 ............................................ 22
4.1. JMS Improvements ............................................................................................ 22
4.2. Caching Improvements ...................................................................................... 22
4.3. Web Improvements ............................................................................................ 23
4.4. WebSocket Messaging Improvements ................................................................. 24
4.5. Testing Improvements ........................................................................................ 24
5. New Features and Enhancements in Spring Framework 4.2 ............................................ 26
5.1. Core Container Improvements ............................................................................ 26
5.2. Data Access Improvements ................................................................................ 27
5.3. JMS Improvements ............................................................................................ 28
5.4. Web Improvements ............................................................................................ 28
5.5. WebSocket Messaging Improvements ................................................................. 29
5.6. Testing Improvements ........................................................................................ 29
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III. Core Technologies ..............................................................................................................
6. The IoC container ........................................................................................................
6.1. Introduction to the Spring IoC container and beans ..............................................
6.2. Container overview ............................................................................................
Configuration metadata .....................................................................................
Instantiating a container ....................................................................................
Composing XML-based configuration metadata ..........................................
Using the container ..........................................................................................
6.3. Bean overview ...................................................................................................
Naming beans ..................................................................................................
Aliasing a bean outside the bean definition ................................................
Instantiating beans ...........................................................................................
Instantiation with a constructor ..................................................................
Instantiation with a static factory method ....................................................
Instantiation using an instance factory method ...........................................
6.4. Dependencies ...................................................................................................
Dependency Injection .......................................................................................
Constructor-based dependency injection ....................................................
Setter-based dependency injection ............................................................
Dependency resolution process .................................................................
Examples of dependency injection .............................................................
Dependencies and configuration in detail ...........................................................
Straight values (primitives, Strings, and so on) ...........................................
References to other beans (collaborators) ..................................................
Inner beans ..............................................................................................
Collections ...............................................................................................
Null and empty string values .....................................................................
XML shortcut with the p-namespace ..........................................................
XML shortcut with the c-namespace ..........................................................
Compound property names .......................................................................
Using depends-on ............................................................................................
Lazy-initialized beans .......................................................................................
Autowiring collaborators ....................................................................................
Limitations and disadvantages of autowiring ...............................................
Excluding a bean from autowiring ..............................................................
Method injection ...............................................................................................
Lookup method injection ...........................................................................
Arbitrary method replacement ...................................................................
6.5. Bean scopes .....................................................................................................
The singleton scope .........................................................................................
The prototype scope .........................................................................................
Singleton beans with prototype-bean dependencies ............................................
Request, session, global session, application, and WebSocket scopes .................
Initial web configuration ............................................................................
Request scope .........................................................................................
Session scope ..........................................................................................
Global session scope ...............................................................................
Application scope .....................................................................................
Scoped beans as dependencies ................................................................
Custom scopes ................................................................................................
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Creating a custom scope .......................................................................... 71
Using a custom scope .............................................................................. 72
6.6. Customizing the nature of a bean ....................................................................... 73
Lifecycle callbacks ............................................................................................ 73
Initialization callbacks ............................................................................... 74
Destruction callbacks ................................................................................ 74
Default initialization and destroy methods .................................................. 75
Combining lifecycle mechanisms ............................................................... 76
Startup and shutdown callbacks ................................................................ 77
Shutting down the Spring IoC container gracefully in non-web applications
................................................................................................................. 79
ApplicationContextAware and BeanNameAware ................................................. 79
Other Aware interfaces ..................................................................................... 80
6.7. Bean definition inheritance ................................................................................. 81
6.8. Container Extension Points ................................................................................ 83
Customizing beans using a BeanPostProcessor ................................................. 83
Example: Hello World, BeanPostProcessor-style ........................................ 84
Example: The RequiredAnnotationBeanPostProcessor ............................... 86
Customizing configuration metadata with a BeanFactoryPostProcessor ................ 86
Example: the Class name substitution PropertyPlaceholderConfigurer .......... 87
Example: the PropertyOverrideConfigurer .................................................. 88
Customizing instantiation logic with a FactoryBean ............................................. 89
6.9. Annotation-based container configuration ............................................................ 89
@Required ....................................................................................................... 91
@Autowired ..................................................................................................... 91
Fine-tuning annotation-based autowiring with @Primary ..................................... 94
Fine-tuning annotation-based autowiring with qualifiers ....................................... 95
Using generics as autowiring qualifiers ............................................................ 100
CustomAutowireConfigurer .............................................................................. 100
@Resource .................................................................................................... 101
@PostConstruct and @PreDestroy .................................................................. 102
6.10. Classpath scanning and managed components ................................................ 102
@Component and further stereotype annotations ............................................. 103
Meta-annotations ............................................................................................ 103
Automatically detecting classes and registering bean definitions ........................ 104
Using filters to customize scanning .................................................................. 105
Defining bean metadata within components ..................................................... 106
Naming autodetected components ................................................................... 109
Providing a scope for autodetected components ............................................... 110
Providing qualifier metadata with annotations ................................................... 111
6.11. Using JSR 330 Standard Annotations ............................................................. 111
Dependency Injection with @Inject and @Named ............................................. 112
@Named: a standard equivalent to the @Component annotation ....................... 112
Limitations of JSR-330 standard annotations .................................................... 113
6.12. Java-based container configuration ................................................................. 115
Basic concepts: @Bean and @Configuration ................................................... 115
Instantiating the Spring container using AnnotationConfigApplicationContext ....... 115
Simple construction ................................................................................ 116
Building the container programmatically using register(Class<?>…) ........... 116
Enabling component scanning with scan(String…) .................................... 116
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Support for web applications with AnnotationConfigWebApplicationContext
............................................................................................................... 117
Using the @Bean annotation .......................................................................... 118
Declaring a bean .................................................................................... 118
Bean dependencies ................................................................................ 119
Receiving lifecycle callbacks ................................................................... 119
Specifying bean scope ............................................................................ 121
Customizing bean naming ....................................................................... 122
Bean aliasing ......................................................................................... 122
Bean description ..................................................................................... 122
Using the @Configuration annotation ............................................................... 122
Injecting inter-bean dependencies ............................................................ 122
Lookup method injection ......................................................................... 123
Further information about how Java-based configuration works internally.... 124
Composing Java-based configurations ............................................................. 125
Using the @Import annotation ................................................................. 125
Conditionally include @Configuration classes or @Bean methods .............. 129
Combining Java and XML configuration ................................................... 129
6.13. Environment abstraction ................................................................................. 131
Bean definition profiles ................................................................................... 132
@Profile ................................................................................................. 132
XML bean definition profiles ............................................................................ 134
Activating a profile .................................................................................. 135
Default profile ......................................................................................... 135
PropertySource abstraction ............................................................................. 136
@PropertySource ........................................................................................... 137
Placeholder resolution in statements ................................................................ 138
6.14. Registering a LoadTimeWeaver ...................................................................... 138
6.15. Additional Capabilities of the ApplicationContext .............................................. 138
Internationalization using MessageSource ........................................................ 139
Standard and Custom Events .......................................................................... 142
Annotation-based Event Listeners ............................................................ 145
Generic Events ....................................................................................... 147
Convenient access to low-level resources ........................................................ 147
Convenient ApplicationContext instantiation for web applications ....................... 148
Deploying a Spring ApplicationContext as a Java EE RAR file ........................... 148
6.16. The BeanFactory ........................................................................................... 149
BeanFactory or ApplicationContext? ................................................................ 149
Glue code and the evil singleton ..................................................................... 150
7. Resources .................................................................................................................. 152
7.1. Introduction ..................................................................................................... 152
7.2. The Resource interface .................................................................................... 152
7.3. Built-in Resource implementations .................................................................... 153
UrlResource ................................................................................................... 153
ClassPathResource ........................................................................................ 153
FileSystemResource ....................................................................................... 154
ServletContextResource .................................................................................. 154
InputStreamResource ..................................................................................... 154
ByteArrayResource ......................................................................................... 154
7.4. The ResourceLoader ....................................................................................... 154
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7.5. The ResourceLoaderAware interface ................................................................
7.6. Resources as dependencies .............................................................................
7.7. Application contexts and Resource paths ..........................................................
Constructing application contexts .....................................................................
Constructing ClassPathXmlApplicationContext instances - shortcuts ..........
Wildcards in application context constructor resource paths ...............................
Ant-style Patterns ...................................................................................
The Classpath*: portability classpath*: prefix ............................................
Other notes relating to wildcards .............................................................
FileSystemResource caveats ..........................................................................
8. Validation, Data Binding, and Type Conversion ............................................................
8.1. Introduction .....................................................................................................
8.2. Validation using Spring’s Validator interface ......................................................
8.3. Resolving codes to error messages ..................................................................
8.4. Bean manipulation and the BeanWrapper .........................................................
Setting and getting basic and nested properties ...............................................
Built-in PropertyEditor implementations ............................................................
Registering additional custom PropertyEditors ..........................................
8.5. Spring Type Conversion ...................................................................................
Converter SPI ................................................................................................
ConverterFactory ............................................................................................
GenericConverter ...........................................................................................
ConditionalGenericConverter ...................................................................
ConversionService API ...................................................................................
Configuring a ConversionService .....................................................................
Using a ConversionService programmatically ...................................................
8.6. Spring Field Formatting ....................................................................................
Formatter SPI .................................................................................................
Annotation-driven Formatting ...........................................................................
Format Annotation API ............................................................................
FormatterRegistry SPI .....................................................................................
FormatterRegistrar SPI ...................................................................................
Configuring Formatting in Spring MVC .............................................................
8.7. Configuring a global date & time format ............................................................
8.8. Spring Validation .............................................................................................
Overview of the JSR-303 Bean Validation API .................................................
Configuring a Bean Validation Provider ............................................................
Injecting a Validator ................................................................................
Configuring Custom Constraints ..............................................................
Spring-driven Method Validation ..............................................................
Additional Configuration Options ..............................................................
Configuring a DataBinder ................................................................................
Spring MVC 3 Validation .................................................................................
9. Spring Expression Language (SpEL) ...........................................................................
9.1. Introduction .....................................................................................................
9.2. Feature Overview ............................................................................................
9.3. Expression Evaluation using Spring’s Expression Interface .................................
The EvaluationContext interface ......................................................................
Type Conversion ....................................................................................
Parser configuration ........................................................................................
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SpEL compilation ............................................................................................
Compiler configuration ............................................................................
Compiler limitations ................................................................................
9.4. Expression support for defining bean definitions ................................................
XML based configuration ................................................................................
Annotation-based configuration ........................................................................
9.5. Language Reference ........................................................................................
Literal expressions ..........................................................................................
Properties, Arrays, Lists, Maps, Indexers .........................................................
Inline lists .......................................................................................................
Inline Maps ....................................................................................................
Array construction ...........................................................................................
Methods .........................................................................................................
Operators .......................................................................................................
Relational operators ................................................................................
Logical operators ....................................................................................
Mathematical operators ...........................................................................
Assignment ....................................................................................................
Types .............................................................................................................
Constructors ...................................................................................................
Variables ........................................................................................................
The #this and #root variables ..................................................................
Functions .......................................................................................................
Bean references .............................................................................................
Ternary Operator (If-Then-Else) .......................................................................
The Elvis Operator .........................................................................................
Safe Navigation operator ................................................................................
Collection Selection ........................................................................................
Collection Projection .......................................................................................
Expression templating .....................................................................................
9.6. Classes used in the examples ..........................................................................
10. Aspect Oriented Programming with Spring .................................................................
10.1. Introduction ....................................................................................................
AOP concepts ................................................................................................
Spring AOP capabilities and goals ...................................................................
AOP Proxies ..................................................................................................
10.2. @AspectJ support ..........................................................................................
Enabling @AspectJ Support ............................................................................
Enabling @AspectJ Support with Java configuration .................................
Enabling @AspectJ Support with XML configuration .................................
Declaring an aspect ........................................................................................
Declaring a pointcut ........................................................................................
Supported Pointcut Designators ..............................................................
Combining pointcut expressions ..............................................................
Sharing common pointcut definitions ........................................................
Examples ...............................................................................................
Writing good pointcuts ............................................................................
Declaring advice .............................................................................................
Before advice .........................................................................................
After returning advice ..............................................................................
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After throwing advice ..............................................................................
After (finally) advice ................................................................................
Around advice ........................................................................................
Advice parameters ..................................................................................
Advice ordering ......................................................................................
Introductions ...................................................................................................
Aspect instantiation models .............................................................................
Example .........................................................................................................
10.3. Schema-based AOP support ..........................................................................
Declaring an aspect ........................................................................................
Declaring a pointcut ........................................................................................
Declaring advice .............................................................................................
Before advice .........................................................................................
After returning advice ..............................................................................
After throwing advice ..............................................................................
After (finally) advice ................................................................................
Around advice ........................................................................................
Advice parameters ..................................................................................
Advice ordering ......................................................................................
Introductions ...................................................................................................
Aspect instantiation models .............................................................................
Advisors .........................................................................................................
Example .........................................................................................................
10.4. Choosing which AOP declaration style to use ..................................................
Spring AOP or full AspectJ? ...........................................................................
@AspectJ or XML for Spring AOP? .................................................................
10.5. Mixing aspect types .......................................................................................
10.6. Proxying mechanisms ....................................................................................
Understanding AOP proxies ............................................................................
10.7. Programmatic creation of @AspectJ Proxies ...................................................
10.8. Using AspectJ with Spring applications ...........................................................
Using AspectJ to dependency inject domain objects with Spring ........................
Unit testing @Configurable objects ..........................................................
Working with multiple application contexts ................................................
Other Spring aspects for AspectJ ....................................................................
Configuring AspectJ aspects using Spring IoC .................................................
Load-time weaving with AspectJ in the Spring Framework .................................
A first example .......................................................................................
Aspects ..................................................................................................
'META-INF/aop.xml' ................................................................................
Required libraries (JARS) ........................................................................
Spring configuration ................................................................................
Environment-specific configuration ...........................................................
10.9. Further Resources .........................................................................................
11. Spring AOP APIs ......................................................................................................
11.1. Introduction ....................................................................................................
11.2. Pointcut API in Spring ....................................................................................
Concepts ........................................................................................................
Operations on pointcuts ..................................................................................
AspectJ expression pointcuts ..........................................................................
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Convenience pointcut implementations ............................................................
Static pointcuts .......................................................................................
Dynamic pointcuts ..................................................................................
Pointcut superclasses .....................................................................................
Custom pointcuts ............................................................................................
11.3. Advice API in Spring ......................................................................................
Advice lifecycles .............................................................................................
Advice types in Spring ....................................................................................
Interception around advice ......................................................................
Before advice .........................................................................................
Throws advice ........................................................................................
After Returning advice ............................................................................
Introduction advice ..................................................................................
11.4. Advisor API in Spring .....................................................................................
11.5. Using the ProxyFactoryBean to create AOP proxies .........................................
Basics ............................................................................................................
JavaBean properties .......................................................................................
JDK- and CGLIB-based proxies ......................................................................
Proxying interfaces .........................................................................................
Proxying classes ............................................................................................
Using 'global' advisors ....................................................................................
11.6. Concise proxy definitions ................................................................................
11.7. Creating AOP proxies programmatically with the ProxyFactory ..........................
11.8. Manipulating advised objects ..........................................................................
11.9. Using the "auto-proxy" facility .........................................................................
Autoproxy bean definitions ..............................................................................
BeanNameAutoProxyCreator ...................................................................
DefaultAdvisorAutoProxyCreator ..............................................................
AbstractAdvisorAutoProxyCreator ............................................................
Using metadata-driven auto-proxying ...............................................................
11.10. Using TargetSources ....................................................................................
Hot swappable target sources .........................................................................
Pooling target sources ....................................................................................
Prototype target sources .................................................................................
ThreadLocal target sources .............................................................................
11.11. Defining new Advice types ............................................................................
11.12. Further resources .........................................................................................
IV. Testing .............................................................................................................................
12. Introduction to Spring Testing ....................................................................................
13. Unit Testing ..............................................................................................................
13.1. Mock Objects .................................................................................................
Environment ...................................................................................................
JNDI ..............................................................................................................
Servlet API .....................................................................................................
Portlet API .....................................................................................................
13.2. Unit Testing support Classes ..........................................................................
General testing utilities ...................................................................................
Spring MVC ...................................................................................................
14. Integration Testing ....................................................................................................
14.1. Overview .......................................................................................................
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14.2. Goals of Integration Testing ............................................................................
Context management and caching ..................................................................
Dependency Injection of test fixtures ...............................................................
Transaction management ................................................................................
Support classes for integration testing .............................................................
14.3. JDBC Testing Support ...................................................................................
14.4. Annotations ...................................................................................................
Spring Testing Annotations .............................................................................
Standard Annotation Support ..........................................................................
Spring JUnit Testing Annotations .....................................................................
Meta-Annotation Support for Testing ................................................................
14.5. Spring TestContext Framework .......................................................................
Key abstractions .............................................................................................
TestExecutionListener configuration .................................................................
Registering custom TestExecutionListeners ..............................................
Automatic discovery of default TestExecutionListeners ..............................
Ordering TestExecutionListeners .............................................................
Merging TestExecutionListeners ..............................................................
Context management ......................................................................................
Context configuration with XML resources ................................................
Context configuration with Groovy scripts .................................................
Context configuration with annotated classes ...........................................
Mixing XML, Groovy scripts, and annotated classes ..................................
Context configuration with context initializers ............................................
Context configuration inheritance .............................................................
Context configuration with environment profiles ........................................
Context configuration with test property sources .......................................
Loading a WebApplicationContext ...........................................................
Context caching ......................................................................................
Context hierarchies .................................................................................
Dependency injection of test fixtures ................................................................
Testing request and session scoped beans ......................................................
Transaction management ................................................................................
Test-managed transactions .....................................................................
Enabling and disabling transactions .........................................................
Transaction rollback and commit behavior ................................................
Programmatic transaction management ...................................................
Executing code outside of a transaction ...................................................
Configuring a transaction manager ..........................................................
Demonstration of all transaction-related annotations .................................
Executing SQL scripts ....................................................................................
Executing SQL scripts programmatically ..................................................
Executing SQL scripts declaratively with @Sql .........................................
TestContext Framework support classes ..........................................................
Spring JUnit Runner ...............................................................................
Spring JUnit Rules ..................................................................................
JUnit support classes ..............................................................................
TestNG support classes ..........................................................................
14.6. Spring MVC Test Framework ..........................................................................
Server-Side Tests ...........................................................................................
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Static Imports ......................................................................................... 336
Setup Options ........................................................................................ 336
Performing Requests .............................................................................. 338
Defining Expectations ............................................................................. 339
Filter Registrations .................................................................................. 340
Differences between Out-of-Container and End-to-End Integration Tests.... 340
Further Server-Side Test Examples ......................................................... 341
HtmlUnit Integration ........................................................................................ 341
Why HtmlUnit Integration? ...................................................................... 341
MockMvc and HtmlUnit ........................................................................... 344
MockMvc and WebDriver ........................................................................ 346
MockMvc and Geb ................................................................................. 351
Client-Side REST Tests .................................................................................. 352
Static Imports ......................................................................................... 353
Further Examples of Client-side REST Tests ............................................ 353
14.7. PetClinic Example .......................................................................................... 353
15. Further Resources .................................................................................................... 355
V. Data Access ...................................................................................................................... 356
16. Transaction Management .......................................................................................... 357
16.1. Introduction to Spring Framework transaction management .............................. 357
16.2. Advantages of the Spring Framework’s transaction support model ..................... 357
Global transactions ......................................................................................... 357
Local transactions ........................................................................................... 358
Spring Framework’s consistent programming model ......................................... 358
16.3. Understanding the Spring Framework transaction abstraction ............................ 359
16.4. Synchronizing resources with transactions ....................................................... 362
High-level synchronization approach ................................................................ 362
Low-level synchronization approach ................................................................. 363
TransactionAwareDataSourceProxy ................................................................. 363
16.5. Declarative transaction management ............................................................... 363
Understanding the Spring Framework’s declarative transaction implementation... 365
Example of declarative transaction implementation ........................................... 365
Rolling back a declarative transaction .............................................................. 369
Configuring different transactional semantics for different beans ........................ 370
<tx:advice/> settings ....................................................................................... 372
Using @Transactional ..................................................................................... 373
@Transactional settings .......................................................................... 378
Multiple Transaction Managers with @Transactional ................................. 379
Custom shortcut annotations ................................................................... 380
Transaction propagation .................................................................................. 380
Required ................................................................................................ 380
RequiresNew .......................................................................................... 381
Nested ................................................................................................... 381
Advising transactional operations ..................................................................... 381
Using @Transactional with AspectJ ................................................................. 384
16.6. Programmatic transaction management ........................................................... 385
Using the TransactionTemplate ....................................................................... 385
Specifying transaction settings ................................................................ 387
Using the PlatformTransactionManager ............................................................ 387
16.7. Choosing between programmatic and declarative transaction management ........ 388
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16.8. Transaction bound event ................................................................................
16.9. Application server-specific integration ..............................................................
IBM WebSphere .............................................................................................
Oracle WebLogic Server .................................................................................
16.10. Solutions to common problems .....................................................................
Use of the wrong transaction manager for a specific DataSource .......................
16.11. Further Resources .......................................................................................
17. DAO support ............................................................................................................
17.1. Introduction ....................................................................................................
17.2. Consistent exception hierarchy .......................................................................
17.3. Annotations used for configuring DAO or Repository classes ............................
18. Data access with JDBC ............................................................................................
18.1. Introduction to Spring Framework JDBC ..........................................................
Choosing an approach for JDBC database access ...........................................
Package hierarchy ..........................................................................................
18.2. Using the JDBC core classes to control basic JDBC processing and error
handling .................................................................................................................
JdbcTemplate .................................................................................................
Examples of JdbcTemplate class usage ...................................................
JdbcTemplate best practices ...................................................................
NamedParameterJdbcTemplate .......................................................................
SQLExceptionTranslator ..................................................................................
Executing statements ......................................................................................
Running queries .............................................................................................
Updating the database ....................................................................................
Retrieving auto-generated keys .......................................................................
18.3. Controlling database connections ....................................................................
DataSource ....................................................................................................
DataSourceUtils ..............................................................................................
SmartDataSource ...........................................................................................
AbstractDataSource ........................................................................................
SingleConnectionDataSource ..........................................................................
DriverManagerDataSource ..............................................................................
TransactionAwareDataSourceProxy .................................................................
DataSourceTransactionManager ......................................................................
NativeJdbcExtractor ........................................................................................
18.4. JDBC batch operations ..................................................................................
Basic batch operations with the JdbcTemplate .................................................
Batch operations with a List of objects .............................................................
Batch operations with multiple batches ............................................................
18.5. Simplifying JDBC operations with the SimpleJdbc classes ................................
Inserting data using SimpleJdbcInsert ..............................................................
Retrieving auto-generated keys using SimpleJdbcInsert ....................................
Specifying columns for a SimpleJdbcInsert ......................................................
Using SqlParameterSource to provide parameter values ...................................
Calling a stored procedure with SimpleJdbcCall ...............................................
Explicitly declaring parameters to use for a SimpleJdbcCall ...............................
How to define SqlParameters ..........................................................................
Calling a stored function using SimpleJdbcCall .................................................
Returning ResultSet/REF Cursor from a SimpleJdbcCall ...................................
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18.6. Modeling JDBC operations as Java objects .....................................................
SqlQuery ........................................................................................................
MappingSqlQuery ...........................................................................................
SqlUpdate ......................................................................................................
StoredProcedure .............................................................................................
18.7. Common problems with parameter and data value handling ..............................
Providing SQL type information for parameters .................................................
Handling BLOB and CLOB objects ..................................................................
Passing in lists of values for IN clause ............................................................
Handling complex types for stored procedure calls ...........................................
18.8. Embedded database support ..........................................................................
Why use an embedded database? ..................................................................
Creating an embedded database using Spring XML ..........................................
Creating an embedded database programmatically ...........................................
Selecting the embedded database type ...........................................................
Using HSQL ...........................................................................................
Using H2 ................................................................................................
Using Derby ...........................................................................................
Testing data access logic with an embedded database .....................................
Generating unique names for embedded databases .........................................
Extending the embedded database support ......................................................
18.9. Initializing a DataSource .................................................................................
Initializing a database using Spring XML ..........................................................
Initialization of other components that depend on the database ..................
19. Object Relational Mapping (ORM) Data Access ..........................................................
19.1. Introduction to ORM with Spring .....................................................................
19.2. General ORM integration considerations .........................................................
Resource and transaction management ...........................................................
Exception translation .......................................................................................
19.3. Hibernate .......................................................................................................
SessionFactory setup in a Spring container ......................................................
Implementing DAOs based on plain Hibernate API ...........................................
Declarative transaction demarcation ................................................................
Programmatic transaction demarcation ............................................................
Transaction management strategies ................................................................
Comparing container-managed and locally defined resources ............................
Spurious application server warnings with Hibernate .........................................
19.4. JDO ..............................................................................................................
PersistenceManagerFactory setup ...................................................................
Implementing DAOs based on the plain JDO API .............................................
Transaction management ................................................................................
JdoDialect ......................................................................................................
19.5. JPA ...............................................................................................................
Three options for JPA setup in a Spring environment ........................................
LocalEntityManagerFactoryBean ..............................................................
Obtaining an EntityManagerFactory from JNDI .........................................
LocalContainerEntityManagerFactoryBean ...............................................
Dealing with multiple persistence units .....................................................
Implementing DAOs based on plain JPA ..........................................................
Transaction Management ................................................................................
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JpaDialect ...................................................................................................... 454
20. Marshalling XML using O/X Mappers ......................................................................... 455
20.1. Introduction .................................................................................................... 455
Ease of configuration ...................................................................................... 455
Consistent Interfaces ...................................................................................... 455
Consistent Exception Hierarchy ....................................................................... 455
20.2. Marshaller and Unmarshaller .......................................................................... 455
Marshaller ...................................................................................................... 455
Unmarshaller .................................................................................................. 456
XmlMappingException ..................................................................................... 457
20.3. Using Marshaller and Unmarshaller ................................................................. 457
20.4. XML Schema-based Configuration .................................................................. 459
20.5. JAXB ............................................................................................................. 459
Jaxb2Marshaller ............................................................................................. 459
XML Schema-based Configuration ........................................................... 460
20.6. Castor ........................................................................................................... 460
CastorMarshaller ............................................................................................ 460
Mapping ......................................................................................................... 461
XML Schema-based Configuration ........................................................... 461
20.7. XMLBeans ..................................................................................................... 462
XmlBeansMarshaller ....................................................................................... 462
XML Schema-based Configuration ........................................................... 462
20.8. JiBX .............................................................................................................. 462
JibxMarshaller ................................................................................................ 463
XML Schema-based Configuration ........................................................... 463
20.9. XStream ........................................................................................................ 463
XStreamMarshaller ......................................................................................... 463
VI. The Web .......................................................................................................................... 465
21. Web MVC framework ................................................................................................ 466
21.1. Introduction to Spring Web MVC framework .................................................... 466
Features of Spring Web MVC ......................................................................... 466
Pluggability of other MVC implementations ...................................................... 468
21.2. The DispatcherServlet .................................................................................... 468
Special Bean Types In the WebApplicationContext ........................................... 472
Default DispatcherServlet Configuration ........................................................... 473
DispatcherServlet Processing Sequence .......................................................... 473
21.3. Implementing Controllers ................................................................................ 474
Defining a controller with @Controller .............................................................. 475
Mapping Requests With @RequestMapping ..................................................... 475
@Controller and AOP Proxying ............................................................... 477
New Support Classes for @RequestMapping methods in Spring MVC 3.1.. 477
URI Template Patterns ........................................................................... 478
URI Template Patterns with Regular Expressions ..................................... 479
Path Patterns ......................................................................................... 479
Path Pattern Comparison ........................................................................ 479
Path Patterns with Placeholders .............................................................. 480
Suffix Pattern Matching ........................................................................... 480
Suffix Pattern Matching and RFD ............................................................ 480
Matrix Variables ...................................................................................... 481
Consumable Media Types ....................................................................... 483
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Producible Media Types .......................................................................... 483
Request Parameters and Header Values ................................................. 484
Defining @RequestMapping handler methods .................................................. 484
Supported method argument types .......................................................... 485
Supported method return types ............................................................... 487
Binding request parameters to method parameters with @RequestParam... 488
Mapping the request body with the @RequestBody annotation .................. 488
Mapping the response body with the @ResponseBody annotation ............. 490
Creating REST Controllers with the @RestController annotation ................ 490
Using HttpEntity ...................................................................................... 490
Using @ModelAttribute on a method ....................................................... 491
Using @ModelAttribute on a method argument ......................................... 492
Using @SessionAttributes to store model attributes in the HTTP session
between requests ................................................................................... 493
Working with "application/x-www-form-urlencoded" data ............................ 494
Mapping cookie values with the @CookieValue annotation ........................ 494
Mapping request header attributes with the @RequestHeader annotation... 495
Method Parameters And Type Conversion ............................................... 495
Customizing WebDataBinder initialization ................................................. 495
Advising controllers with @ControllerAdvice ............................................. 497
Jackson Serialization View Support ......................................................... 497
Jackson JSONP Support ........................................................................ 498
Asynchronous Request Processing .................................................................. 499
Exception Handling for Async Requests ................................................... 500
Intercepting Async Requests ................................................................... 501
HTTP Streaming ..................................................................................... 501
HTTP Streaming With Server-Sent Events ............................................... 501
HTTP Streaming Directly To The OutputStream ....................................... 502
Configuring Asynchronous Request Processing ........................................ 502
Testing Controllers ......................................................................................... 503
21.4. Handler mappings .......................................................................................... 503
Intercepting requests with a HandlerInterceptor ................................................ 504
21.5. Resolving views ............................................................................................. 506
Resolving views with the ViewResolver interface .............................................. 506
Chaining ViewResolvers ................................................................................. 508
Redirecting to Views ....................................................................................... 508
RedirectView .......................................................................................... 509
The redirect: prefix ................................................................................. 510
The forward: prefix ................................................................................. 510
ContentNegotiatingViewResolver ..................................................................... 510
21.6. Using flash attributes ..................................................................................... 512
21.7. Building URIs ................................................................................................. 513
Building URIs to Controllers and methods ........................................................ 514
Building URIs to Controllers and methods from views ....................................... 515
21.8. Using locales ................................................................................................. 516
Obtaining Time Zone Information .................................................................... 516
AcceptHeaderLocaleResolver .......................................................................... 516
CookieLocaleResolver ..................................................................................... 516
SessionLocaleResolver ................................................................................... 517
LocaleChangeInterceptor ................................................................................ 517
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21.9. Using themes ................................................................................................
Overview of themes ........................................................................................
Defining themes .............................................................................................
Theme resolvers .............................................................................................
21.10. Spring’s multipart (file upload) support ...........................................................
Introduction ....................................................................................................
Using a MultipartResolver with Commons FileUpload ........................................
Using a MultipartResolver with Servlet 3.0 .......................................................
Handling a file upload in a form ......................................................................
Handling a file upload request from programmatic clients ..................................
21.11. Handling exceptions .....................................................................................
HandlerExceptionResolver ..............................................................................
@ExceptionHandler ........................................................................................
Handling Standard Spring MVC Exceptions ......................................................
Annotating Business Exceptions With @ResponseStatus ..................................
Customizing the Default Servlet Container Error Page ......................................
21.12. Web Security ...............................................................................................
21.13. Convention over configuration support ...........................................................
The Controller ControllerClassNameHandlerMapping ........................................
The Model ModelMap (ModelAndView) ............................................................
The View - RequestToViewNameTranslator .....................................................
21.14. HTTP caching support ..................................................................................
Cache-Control HTTP header ...........................................................................
HTTP caching support for static resources .......................................................
Support for the Cache-Control, ETag and Last-Modified response headers in
Controllers ......................................................................................................
Shallow ETag support .....................................................................................
21.15. Code-based Servlet container initialization .....................................................
21.16. Configuring Spring MVC ...............................................................................
Enabling the MVC Java Config or the MVC XML Namespace ............................
Customizing the Provided Configuration ...........................................................
Conversion and Formatting .............................................................................
Validation .......................................................................................................
Interceptors ....................................................................................................
Content Negotiation ........................................................................................
View Controllers .............................................................................................
View Resolvers ..............................................................................................
Serving of Resources .....................................................................................
Falling Back On the "Default" Servlet To Serve Resources ................................
Path Matching ................................................................................................
Message Converters .......................................................................................
Advanced Customizations with MVC Java Config .............................................
Advanced Customizations with the MVC Namespace ........................................
22. View technologies .....................................................................................................
22.1. Introduction ....................................................................................................
22.2. Thymeleaf .....................................................................................................
22.3. Groovy Markup Templates ..............................................................................
Configuration ..................................................................................................
Example .........................................................................................................
22.4. Velocity & FreeMarker ....................................................................................
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Dependencies ................................................................................................
Context configuration ......................................................................................
Creating templates .........................................................................................
Advanced configuration ...................................................................................
velocity.properties ...................................................................................
FreeMarker .............................................................................................
Bind support and form handling .......................................................................
The bind macros ....................................................................................
Simple binding ........................................................................................
Form input generation macros .................................................................
HTML escaping and XHTML compliance .................................................
22.5. JSP & JSTL ..................................................................................................
View resolvers ................................................................................................
'Plain-old' JSPs versus JSTL ...........................................................................
Additional tags facilitating development ............................................................
Using Spring’s form tag library ........................................................................
Configuration ..........................................................................................
The form tag ..........................................................................................
The input tag ..........................................................................................
The checkbox tag ...................................................................................
The checkboxes tag ...............................................................................
The radiobutton tag ................................................................................
The radiobuttons tag ...............................................................................
The password tag ...................................................................................
The select tag ........................................................................................
The option tag ........................................................................................
The options tag ......................................................................................
The textarea tag .....................................................................................
The hidden tag .......................................................................................
The errors tag ........................................................................................
HTTP Method Conversion .......................................................................
HTML5 Tags ..........................................................................................
22.6. Script templates .............................................................................................
Dependencies ................................................................................................
How to integrate script based templating ..........................................................
22.7. XML Marshalling View ....................................................................................
22.8. Tiles ..............................................................................................................
Dependencies ................................................................................................
How to integrate Tiles .....................................................................................
UrlBasedViewResolver ............................................................................
ResourceBundleViewResolver .................................................................
SimpleSpringPreparerFactory and SpringBeanPreparerFactory .................
22.9. XSLT .............................................................................................................
My First Words ...............................................................................................
Bean definitions ......................................................................................
Standard MVC controller code .................................................................
Document transformation ........................................................................
22.10. Document views (PDF/Excel) ........................................................................
Introduction ....................................................................................................
Configuration and setup ..................................................................................
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Document view definitions .......................................................................
Controller code .......................................................................................
Subclassing for Excel views ....................................................................
Subclassing for PDF views .....................................................................
22.11. JasperReports ..............................................................................................
Dependencies ................................................................................................
Configuration ..................................................................................................
Configuring the ViewResolver ..................................................................
Configuring the Views .............................................................................
About Report Files ..................................................................................
Using JasperReportsMultiFormatView ......................................................
Populating the ModelAndView .........................................................................
Working with Sub-Reports ...............................................................................
Configuring Sub-Report Files ..................................................................
Configuring Sub-Report Data Sources .....................................................
Configuring Exporter Parameters .....................................................................
22.12. Feed Views .................................................................................................
22.13. JSON Mapping View ....................................................................................
22.14. XML Mapping View ......................................................................................
23. Integrating with other web frameworks .......................................................................
23.1. Introduction ....................................................................................................
23.2. Common configuration ...................................................................................
23.3. JavaServer Faces 1.2 ....................................................................................
SpringBeanFacesELResolver (JSF 1.2+) .........................................................
FacesContextUtils ...........................................................................................
23.4. Apache Struts 2.x ..........................................................................................
23.5. Tapestry 5.x ..................................................................................................
23.6. Further Resources .........................................................................................
24. Portlet MVC Framework ............................................................................................
24.1. Introduction ....................................................................................................
Controllers - The C in MVC ............................................................................
Views - The V in MVC ....................................................................................
Web-scoped beans .........................................................................................
24.2. The DispatcherPortlet .....................................................................................
24.3. The ViewRendererServlet ...............................................................................
24.4. Controllers .....................................................................................................
AbstractController and PortletContentGenerator ...............................................
Other simple controllers ..................................................................................
Command Controllers .....................................................................................
PortletWrappingController ................................................................................
24.5. Handler mappings ..........................................................................................
PortletModeHandlerMapping ............................................................................
ParameterHandlerMapping ..............................................................................
PortletModeParameterHandlerMapping ............................................................
Adding HandlerInterceptors .............................................................................
HandlerInterceptorAdapter ...............................................................................
ParameterMappingInterceptor ..........................................................................
24.6. Views and resolving them ..............................................................................
24.7. Multipart (file upload) support .........................................................................
Using the PortletMultipartResolver ...................................................................
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Handling a file upload in a form ......................................................................
24.8. Handling exceptions .......................................................................................
24.9. Annotation-based controller configuration ........................................................
Setting up the dispatcher for annotation support ...............................................
Defining a controller with @Controller ..............................................................
Mapping requests with @RequestMapping .......................................................
Supported handler method arguments .............................................................
Binding request parameters to method parameters with @RequestParam ..........
Providing a link to data from the model with @ModelAttribute ............................
Specifying attributes to store in a Session with @SessionAttributes ....................
Customizing WebDataBinder initialization .........................................................
Customizing data binding with @InitBinder ...............................................
Configuring a custom WebBindingInitializer ..............................................
24.10. Portlet application deployment ......................................................................
25. WebSocket Support ..................................................................................................
25.1. Introduction ....................................................................................................
WebSocket Fallback Options ...........................................................................
A Messaging Architecture ...............................................................................
Sub-Protocol Support in WebSocket ................................................................
Should I Use WebSocket? ..............................................................................
25.2. WebSocket API .............................................................................................
Create and Configure a WebSocketHandler .....................................................
Customizing the WebSocket Handshake ..........................................................
WebSocketHandler Decoration ........................................................................
Deployment Considerations .............................................................................
Configuring the WebSocket Engine .................................................................
Configuring allowed origins .............................................................................
25.3. SockJS Fallback Options ................................................................................
Overview of SockJS .......................................................................................
Enable SockJS ...............................................................................................
HTTP Streaming in IE 8, 9: Ajax/XHR vs IFrame ..............................................
Heartbeat Messages .......................................................................................
Servlet 3 Async Requests ...............................................................................
CORS Headers for SockJS .............................................................................
SockJS Client .................................................................................................
25.4. STOMP Over WebSocket Messaging Architecture ...........................................
Overview of STOMP .......................................................................................
Enable STOMP over WebSocket .....................................................................
Flow of Messages ..........................................................................................
Annotation Message Handling .........................................................................
Sending Messages .........................................................................................
Simple Broker ................................................................................................
Full-Featured Broker .......................................................................................
Connections To Full-Featured Broker ..............................................................
Using Dot as Separator in @MessageMapping Destinations ..............................
Authentication .................................................................................................
User Destinations ...........................................................................................
Listening To ApplicationContext Events and Intercepting Messages ...................
STOMP Client ................................................................................................
WebSocket Scope ..........................................................................................
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Configuration and Performance .......................................................................
Runtime Monitoring .........................................................................................
Testing Annotated Controller Methods .............................................................
26. CORS Support .........................................................................................................
26.1. Introduction ....................................................................................................
26.2. Controller method CORS configuration ............................................................
26.3. Global CORS configuration .............................................................................
JavaConfig .....................................................................................................
XML namespace ............................................................................................
26.4. Advanced Customization ................................................................................
VII. Integration ........................................................................................................................
27. Remoting and web services using Spring ...................................................................
27.1. Introduction ....................................................................................................
27.2. Exposing services using RMI ..........................................................................
Exporting the service using the RmiServiceExporter .........................................
Linking in the service at the client ...................................................................
27.3. Using Hessian or Burlap to remotely call services via HTTP ..............................
Wiring up the DispatcherServlet for Hessian and co. .........................................
Exposing your beans by using the HessianServiceExporter ...............................
Linking in the service on the client ..................................................................
Using Burlap ..................................................................................................
Applying HTTP basic authentication to a service exposed through Hessian or
Burlap ............................................................................................................
27.4. Exposing services using HTTP invokers ..........................................................
Exposing the service object ............................................................................
Linking in the service at the client ...................................................................
27.5. Web services .................................................................................................
Exposing servlet-based web services using JAX-WS ........................................
Exporting standalone web services using JAX-WS ...........................................
Exporting web services using the JAX-WS RI’s Spring support ..........................
Accessing web services using JAX-WS ...........................................................
27.6. JMS ..............................................................................................................
Server-side configuration .................................................................................
Client-side configuration ..................................................................................
27.7. AMQP ...........................................................................................................
27.8. Auto-detection is not implemented for remote interfaces ...................................
27.9. Considerations when choosing a technology ....................................................
27.10. Accessing RESTful services on the Client .....................................................
RestTemplate .................................................................................................
Working with the URI ..............................................................................
Dealing with request and response headers .............................................
Jackson JSON Views support .................................................................
HTTP Message Conversion ............................................................................
StringHttpMessageConverter ...................................................................
FormHttpMessageConverter ....................................................................
ByteArrayHttpMessageConverter .............................................................
MarshallingHttpMessageConverter ...........................................................
MappingJackson2HttpMessageConverter .................................................
MappingJackson2XmlHttpMessageConverter ...........................................
SourceHttpMessageConverter .................................................................
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BufferedImageHttpMessageConverter ......................................................
Async RestTemplate .......................................................................................
28. Enterprise JavaBeans (EJB) integration .....................................................................
28.1. Introduction ....................................................................................................
28.2. Accessing EJBs .............................................................................................
Concepts ........................................................................................................
Accessing local SLSBs ...................................................................................
Accessing remote SLSBs ................................................................................
Accessing EJB 2.x SLSBs versus EJB 3 SLSBs ...............................................
28.3. Using Spring’s EJB implementation support classes .........................................
EJB 3 injection interceptor ..............................................................................
29. JMS (Java Message Service) ....................................................................................
29.1. Introduction ....................................................................................................
29.2. Using Spring JMS ..........................................................................................
JmsTemplate ..................................................................................................
Connections ...................................................................................................
Caching Messaging Resources ...............................................................
SingleConnectionFactory .........................................................................
CachingConnectionFactory ......................................................................
Destination Management .................................................................................
Message Listener Containers ..........................................................................
SimpleMessageListenerContainer ............................................................
DefaultMessageListenerContainer ............................................................
Transaction management ................................................................................
29.3. Sending a Message .......................................................................................
Using Message Converters .............................................................................
SessionCallback and ProducerCallback ...........................................................
29.4. Receiving a message .....................................................................................
Synchronous Reception ..................................................................................
Asynchronous Reception - Message-Driven POJOs ..........................................
the SessionAwareMessageListener interface ....................................................
the MessageListenerAdapter ...........................................................................
Processing messages within transactions ........................................................
29.5. Support for JCA Message Endpoints ...............................................................
29.6. Annotation-driven listener endpoints ................................................................
Enable listener endpoint annotations ...............................................................
Programmatic endpoints registration ................................................................
Annotated endpoint method signature ..............................................................
Response management ..................................................................................
29.7. JMS Namespace Support ...............................................................................
30. JMX .........................................................................................................................
30.1. Introduction ....................................................................................................
30.2. Exporting your beans to JMX ..........................................................................
Creating an MBeanServer ...............................................................................
Reusing an existing MBeanServer ...................................................................
Lazy-initialized MBeans ...................................................................................
Automatic registration of MBeans ....................................................................
Controlling the registration behavior .................................................................
30.3. Controlling the management interface of your beans ........................................
the MBeanInfoAssembler Interface ..................................................................
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Using Source-Level Metadata (Java annotations) .............................................
Source-Level Metadata Types .........................................................................
the AutodetectCapableMBeanInfoAssembler interface ......................................
Defining management interfaces using Java interfaces .....................................
Using MethodNameBasedMBeanInfoAssembler ...............................................
30.4. Controlling the ObjectNames for your beans ....................................................
Reading ObjectNames from Properties ............................................................
Using the MetadataNamingStrategy .................................................................
Configuring annotation based MBean export ....................................................
30.5. JSR-160 Connectors ......................................................................................
Server-side Connectors ...................................................................................
Client-side Connectors ....................................................................................
JMX over Burlap/Hessian/SOAP ......................................................................
30.6. Accessing MBeans via Proxies .......................................................................
30.7. Notifications ...................................................................................................
Registering Listeners for Notifications ..............................................................
Publishing Notifications ...................................................................................
30.8. Further Resources .........................................................................................
31. JCA CCI ..................................................................................................................
31.1. Introduction ....................................................................................................
31.2. Configuring CCI .............................................................................................
Connector configuration ..................................................................................
ConnectionFactory configuration in Spring .......................................................
Configuring CCI connections ...........................................................................
Using a single CCI connection ........................................................................
31.3. Using Spring’s CCI access support .................................................................
Record conversion ..........................................................................................
the CciTemplate .............................................................................................
DAO support ..................................................................................................
Automatic output record generation .................................................................
Summary .......................................................................................................
Using a CCI Connection and Interaction directly ...............................................
Example for CciTemplate usage ......................................................................
31.4. Modeling CCI access as operation objects ......................................................
MappingRecordOperation ................................................................................
MappingCommAreaOperation ..........................................................................
Automatic output record generation .................................................................
Summary .......................................................................................................
Example for MappingRecordOperation usage ...................................................
Example for MappingCommAreaOperation usage .............................................
31.5. Transactions ..................................................................................................
32. Email .......................................................................................................................
32.1. Introduction ....................................................................................................
32.2. Usage ...........................................................................................................
Basic MailSender and SimpleMailMessage usage ............................................
Using the JavaMailSender and the MimeMessagePreparator .............................
32.3. Using the JavaMail MimeMessageHelper ........................................................
Sending attachments and inline resources .......................................................
Attachments ...........................................................................................
Inline resources ......................................................................................
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Creating email content using a templating library ..............................................
A Velocity-based example .......................................................................
33. Task Execution and Scheduling .................................................................................
33.1. Introduction ....................................................................................................
33.2. The Spring TaskExecutor abstraction ..............................................................
TaskExecutor types ........................................................................................
Using a TaskExecutor .....................................................................................
33.3. The Spring TaskScheduler abstraction ............................................................
the Trigger interface .......................................................................................
Trigger implementations ..................................................................................
TaskScheduler implementations ......................................................................
33.4. Annotation Support for Scheduling and Asynchronous Execution .......................
Enable scheduling annotations ........................................................................
The @Scheduled Annotation ...........................................................................
The @Async Annotation .................................................................................
Executor qualification with @Async .................................................................
Exception management with @Async ..............................................................
33.5. The Task Namespace ....................................................................................
The 'scheduler' element ..................................................................................
The 'executor' element ....................................................................................
The 'scheduled-tasks' element .........................................................................
33.6. Using the Quartz Scheduler ............................................................................
Using the JobDetailFactoryBean ......................................................................
Using the MethodInvokingJobDetailFactoryBean ..............................................
Wiring up jobs using triggers and the SchedulerFactoryBean .............................
34. Dynamic language support ........................................................................................
34.1. Introduction ....................................................................................................
34.2. A first example ..............................................................................................
34.3. Defining beans that are backed by dynamic languages .....................................
Common concepts ..........................................................................................
The <lang:language/> element ................................................................
Refreshable beans ..................................................................................
Inline dynamic language source files .......................................................
Understanding Constructor Injection in the context of dynamic-languagebacked beans .........................................................................................
JRuby beans ..................................................................................................
Groovy beans .................................................................................................
Customizing Groovy objects via a callback ...............................................
BeanShell beans ............................................................................................
34.4. Scenarios ......................................................................................................
Scripted Spring MVC Controllers .....................................................................
Scripted Validators .........................................................................................
34.5. Bits and bobs ................................................................................................
AOP - advising scripted beans ........................................................................
Scoping ..........................................................................................................
34.6. Further Resources .........................................................................................
35. Cache Abstraction ....................................................................................................
35.1. Introduction ....................................................................................................
35.2. Understanding the cache abstraction ...............................................................
35.3. Declarative annotation-based caching .............................................................
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@Cacheable annotation ..................................................................................
Default Key Generation ...........................................................................
Custom Key Generation Declaration ........................................................
Default Cache Resolution ........................................................................
Custom cache resolution .........................................................................
Conditional caching ................................................................................
Available caching SpEL evaluation context ...............................................
@CachePut annotation ...................................................................................
@CacheEvict annotation .................................................................................
@Caching annotation .....................................................................................
@CacheConfig annotation ..............................................................................
Enable caching annotations ............................................................................
Using custom annotations ...............................................................................
35.4. JCache (JSR-107) annotations .......................................................................
Features summary ..........................................................................................
Enabling JSR-107 support ..............................................................................
35.5. Declarative XML-based caching ......................................................................
35.6. Configuring the cache storage ........................................................................
JDK ConcurrentMap-based Cache ...................................................................
EhCache-based Cache ...................................................................................
Guava Cache .................................................................................................
GemFire-based Cache ....................................................................................
JSR-107 Cache ..............................................................................................
Dealing with caches without a backing store ....................................................
35.7. Plugging-in different back-end caches .............................................................
35.8. How can I set the TTL/TTI/Eviction policy/XXX feature? ....................................
VIII. Appendices .....................................................................................................................
36. Migrating to Spring Framework 4.x ............................................................................
37. Spring Annotation Programming Model ......................................................................
38. Classic Spring Usage ...............................................................................................
38.1. Classic ORM usage .......................................................................................
Hibernate .......................................................................................................
The HibernateTemplate ...........................................................................
Implementing Spring-based DAOs without callbacks .................................
38.2. JMS Usage ...................................................................................................
JmsTemplate ..................................................................................................
Asynchronous Message Reception ..................................................................
Connections ...................................................................................................
Transaction Management ................................................................................
39. Classic Spring AOP Usage .......................................................................................
39.1. Pointcut API in Spring ....................................................................................
Concepts ........................................................................................................
Operations on pointcuts ..................................................................................
AspectJ expression pointcuts ..........................................................................
Convenience pointcut implementations ............................................................
Static pointcuts .......................................................................................
Dynamic pointcuts ..................................................................................
Pointcut superclasses .....................................................................................
Custom pointcuts ............................................................................................
39.2. Advice API in Spring ......................................................................................
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Advice lifecycles .............................................................................................
Advice types in Spring ....................................................................................
Interception around advice ......................................................................
Before advice .........................................................................................
Throws advice ........................................................................................
After Returning advice ............................................................................
Introduction advice ..................................................................................
39.3. Advisor API in Spring .....................................................................................
39.4. Using the ProxyFactoryBean to create AOP proxies .........................................
Basics ............................................................................................................
JavaBean properties .......................................................................................
JDK- and CGLIB-based proxies ......................................................................
Proxying interfaces .........................................................................................
Proxying classes ............................................................................................
Using 'global' advisors ....................................................................................
39.5. Concise proxy definitions ................................................................................
39.6. Creating AOP proxies programmatically with the ProxyFactory ..........................
39.7. Manipulating advised objects ..........................................................................
39.8. Using the "autoproxy" facility ..........................................................................
Autoproxy bean definitions ..............................................................................
BeanNameAutoProxyCreator ...................................................................
DefaultAdvisorAutoProxyCreator ..............................................................
AbstractAdvisorAutoProxyCreator ............................................................
Using metadata-driven auto-proxying ...............................................................
39.9. Using TargetSources ......................................................................................
Hot swappable target sources .........................................................................
Pooling target sources ....................................................................................
Prototype target sources .................................................................................
ThreadLocal target sources .............................................................................
39.10. Defining new Advice types ............................................................................
39.11. Further resources .........................................................................................
40. XML Schema-based configuration .............................................................................
40.1. Introduction ....................................................................................................
40.2. XML Schema-based configuration ...................................................................
Referencing the schemas ...............................................................................
the util schema ...............................................................................................
<util:constant/> .......................................................................................
<util:property-path/> ................................................................................
<util:properties/> .....................................................................................
<util:list/> ................................................................................................
<util:map/> .............................................................................................
<util:set/> ...............................................................................................
the jee schema ..............................................................................................
<jee:jndi-lookup/> (simple) .......................................................................
<jee:jndi-lookup/> (with single JNDI environment setting) ..........................
<jee:jndi-lookup/> (with multiple JNDI environment settings) ......................
<jee:jndi-lookup/> (complex) ....................................................................
<jee:local-slsb/> (simple) .........................................................................
<jee:local-slsb/> (complex) ......................................................................
<jee:remote-slsb/> ..................................................................................
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lang schema .............................................................................................
jms schema ..............................................................................................
tx (transaction) schema .............................................................................
aop schema .............................................................................................
context schema ........................................................................................
<property-placeholder/> ...........................................................................
<annotation-config/> ...............................................................................
<component-scan/> ................................................................................
<load-time-weaver/> ...............................................................................
<spring-configured/> ...............................................................................
<mbean-export/> ....................................................................................
the tool schema ..............................................................................................
the jdbc schema .............................................................................................
the cache schema ..........................................................................................
the beans schema ..........................................................................................
41. Extensible XML authoring .........................................................................................
41.1. Introduction ....................................................................................................
41.2. Authoring the schema ....................................................................................
41.3. Coding a NamespaceHandler .........................................................................
41.4. BeanDefinitionParser ......................................................................................
41.5. Registering the handler and the schema .........................................................
'META-INF/spring.handlers' .............................................................................
'META-INF/spring.schemas' .............................................................................
41.6. Using a custom extension in your Spring XML configuration ..............................
41.7. Meatier examples ...........................................................................................
Nesting custom tags within custom tags ..........................................................
Custom attributes on 'normal' elements ............................................................
41.8. Further Resources .........................................................................................
42. spring JSP Tag Library .............................................................................................
42.1. Introduction ....................................................................................................
42.2. The argument tag ..........................................................................................
42.3. The bind tag ..................................................................................................
42.4. The escapeBody tag ......................................................................................
42.5. The eval tag ..................................................................................................
42.6. The hasBindErrors tag ...................................................................................
42.7. The htmlEscape tag .......................................................................................
42.8. The message tag ...........................................................................................
42.9. The nestedPath tag .......................................................................................
42.10. The param tag .............................................................................................
42.11. The theme tag .............................................................................................
42.12. The transform tag ........................................................................................
42.13. The url tag ...................................................................................................
43. spring-form JSP Tag Library ......................................................................................
43.1. Introduction ....................................................................................................
43.2. The button tag ...............................................................................................
43.3. The checkbox tag ..........................................................................................
43.4. The checkboxes tag .......................................................................................
43.5. The errors tag ...............................................................................................
43.6. The form tag .................................................................................................
43.7. The hidden tag ..............................................................................................
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43.8. The input tag .................................................................................................
43.9. The label tag .................................................................................................
43.10. The option tag .............................................................................................
43.11. The options tag ............................................................................................
43.12. The password tag ........................................................................................
43.13. The radiobutton tag ......................................................................................
43.14. The radiobuttons tag ....................................................................................
43.15. The select tag ..............................................................................................
43.16. The textarea tag ..........................................................................................
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Part I. Overview of Spring Framework
The Spring Framework is a lightweight solution and a potential one-stop-shop for building your
enterprise-ready applications. However, Spring is modular, allowing you to use only those parts that you
need, without having to bring in the rest. You can use the IoC container, with any web framework on
top, but you can also use only the Hibernate integration code or the JDBC abstraction layer. The Spring
Framework supports declarative transaction management, remote access to your logic through RMI or
web services, and various options for persisting your data. It offers a full-featured MVC framework, and
enables you to integrate AOP transparently into your software.
Spring is designed to be non-intrusive, meaning that your domain logic code generally has no
dependencies on the framework itself. In your integration layer (such as the data access layer), some
dependencies on the data access technology and the Spring libraries will exist. However, it should be
easy to isolate these dependencies from the rest of your code base.
This document is a reference guide to Spring Framework features. If you have any requests, comments,
or questions on this document, please post them on the user mailing list. Questions on the Framework
itself should be asked on StackOverflow (see https://spring.io/questions).
Spring Framework Reference Documentation
1. Getting Started with Spring
This reference guide provides detailed information about the Spring Framework. It provides
comprehensive documentation for all features, as well as some background about the underlying
concepts (such as "Dependency Injection") that Spring has embraced.
If you are just getting started with Spring, you may want to begin using the Spring Framework by
creating a Spring Boot based application. Spring Boot provides a quick (and opinionated) way to create
a production-ready Spring based application. It is based on the Spring Framework, favors convention
over configuration, and is designed to get you up and running as quickly as possible.
You can use start.spring.io to generate a basic project or follow one of the "Getting Started" guides like
the Getting Started Building a RESTful Web Service one. As well as being easier to digest, these guides
are very task focused, and most of them are based on Spring Boot. They also cover other projects from
the Spring portfolio that you might want to consider when solving a particular problem.
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2. Introduction to the Spring Framework
The Spring Framework is a Java platform that provides comprehensive infrastructure support for
developing Java applications. Spring handles the infrastructure so you can focus on your application.
Spring enables you to build applications from "plain old Java objects" (POJOs) and to apply enterprise
services non-invasively to POJOs. This capability applies to the Java SE programming model and to
full and partial Java EE.
Examples of how you, as an application developer, can benefit from the Spring platform:
• Make a Java method execute in a database transaction without having to deal with transaction APIs.
• Make a local Java method a remote procedure without having to deal with remote APIs.
• Make a local Java method a management operation without having to deal with JMX APIs.
• Make a local Java method a message handler without having to deal with JMS APIs.
2.1 Dependency Injection and Inversion of Control
A Java application — a loose term that runs the gamut from constrained, embedded applications to n-tier,
server-side enterprise applications — typically consists of objects that collaborate to form the application
proper. Thus the objects in an application have dependencies on each other.
Although the Java platform provides a wealth of application development functionality, it lacks the
means to organize the basic building blocks into a coherent whole, leaving that task to architects and
developers. Although you can use design patterns such as Factory, Abstract Factory, Builder, Decorator,
and Service Locator to compose the various classes and object instances that make up an application,
these patterns are simply that: best practices given a name, with a description of what the pattern does,
where to apply it, the problems it addresses, and so forth. Patterns are formalized best practices that
you must implement yourself in your application.
The Spring Framework Inversion of Control (IoC) component addresses this concern by providing a
formalized means of composing disparate components into a fully working application ready for use.
The Spring Framework codifies formalized design patterns as first-class objects that you can integrate
into your own application(s). Numerous organizations and institutions use the Spring Framework in this
manner to engineer robust, maintainable applications.
Background
"The question is, what aspect of control are [they] inverting?" Martin Fowler posed this question
about Inversion of Control (IoC) on his site in 2004. Fowler suggested renaming the principle to
make it more self-explanatory and came up with Dependency Injection.
2.2 Modules
The Spring Framework consists of features organized into about 20 modules. These modules are
grouped into Core Container, Data Access/Integration, Web, AOP (Aspect Oriented Programming),
Instrumentation, Messaging, and Test, as shown in the following diagram.
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Figure 2.1. Overview of the Spring Framework
The following sections list the available modules for each feature along with their artifact names and the
topics they cover. Artifact names correlate to artifact IDs used in Dependency Management tools.
Core Container
The Core Container consists of the spring-core, spring-beans, spring-context, springcontext-support, and spring-expression (Spring Expression Language) modules.
The spring-core and spring-beans modules provide the fundamental parts of the framework,
including the IoC and Dependency Injection features. The BeanFactory is a sophisticated
implementation of the factory pattern. It removes the need for programmatic singletons and allows you
to decouple the configuration and specification of dependencies from your actual program logic.
The Context (spring-context) module builds on the solid base provided by the Core and Beans
modules: it is a means to access objects in a framework-style manner that is similar to a JNDI
registry. The Context module inherits its features from the Beans module and adds support for
internationalization (using, for example, resource bundles), event propagation, resource loading, and the
transparent creation of contexts by, for example, a Servlet container. The Context module also supports
Java EE features such as EJB, JMX, and basic remoting. The ApplicationContext interface is
the focal point of the Context module. spring-context-support provides support for integrating
common third-party libraries into a Spring application context for caching (EhCache, Guava, JCache),
mailing (JavaMail), scheduling (CommonJ, Quartz) and template engines (FreeMarker, JasperReports,
Velocity).
The spring-expression module provides a powerful Expression Language for querying and
manipulating an object graph at runtime. It is an extension of the unified expression language (unified
EL) as specified in the JSP 2.1 specification. The language supports setting and getting property values,
property assignment, method invocation, accessing the content of arrays, collections and indexers,
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logical and arithmetic operators, named variables, and retrieval of objects by name from Spring’s IoC
container. It also supports list projection and selection as well as common list aggregations.
AOP and Instrumentation
The spring-aop module provides an AOP Alliance-compliant aspect-oriented programming
implementation allowing you to define, for example, method interceptors and pointcuts to cleanly
decouple code that implements functionality that should be separated. Using source-level metadata
functionality, you can also incorporate behavioral information into your code, in a manner similar to that
of .NET attributes.
The separate spring-aspects module provides integration with AspectJ.
The spring-instrument module provides class instrumentation support and classloader
implementations to be used in certain application servers. The spring-instrument-tomcat module
contains Spring’s instrumentation agent for Tomcat.
Messaging
Spring Framework 4 includes a spring-messaging module with key abstractions from the Spring
Integration project such as Message, MessageChannel, MessageHandler, and others to serve as a
foundation for messaging-based applications. The module also includes a set of annotations for mapping
messages to methods, similar to the Spring MVC annotation based programming model.
Data Access/Integration
The Data Access/Integration layer consists of the JDBC, ORM, OXM, JMS, and Transaction modules.
The spring-jdbc module provides a JDBC-abstraction layer that removes the need to do tedious
JDBC coding and parsing of database-vendor specific error codes.
The spring-tx module supports programmatic and declarative transaction management for classes
that implement special interfaces and for all your POJOs (Plain Old Java Objects).
The spring-orm module provides integration layers for popular object-relational mapping APIs,
including JPA, JDO, and Hibernate. Using the spring-orm module you can use all of these O/Rmapping frameworks in combination with all of the other features Spring offers, such as the simple
declarative transaction management feature mentioned previously.
The spring-oxm module provides an abstraction layer that supports Object/XML mapping
implementations such as JAXB, Castor, XMLBeans, JiBX and XStream.
The spring-jms module (Java Messaging Service) contains features for producing and consuming
messages. Since Spring Framework 4.1, it provides integration with the spring-messaging module.
Web
The Web layer consists of the spring-web, spring-webmvc, spring-websocket, and springwebmvc-portlet modules.
The spring-web module provides basic web-oriented integration features such as multipart file upload
functionality and the initialization of the IoC container using Servlet listeners and a web-oriented
application context. It also contains an HTTP client and the web-related parts of Spring’s remoting
support.
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The spring-webmvc module (also known as the Web-Servlet module) contains Spring’s modelview-controller (MVC) and REST Web Services implementation for web applications. Spring’s MVC
framework provides a clean separation between domain model code and web forms and integrates with
all of the other features of the Spring Framework.
The spring-webmvc-portlet module (also known as the Web-Portlet module) provides the MVC
implementation to be used in a Portlet environment and mirrors the functionality of the spring-webmvc
module.
Test
The spring-test module supports the unit testing and integration testing of Spring components with
JUnit or TestNG. It provides consistent loading of Spring ApplicationContexts and caching of those
contexts. It also provides mock objects that you can use to test your code in isolation.
2.3 Usage scenarios
The building blocks described previously make Spring a logical choice in many scenarios, from
embedded applications that run on resource-constrained devices to full-fledged enterprise applications
that use Spring’s transaction management functionality and web framework integration.
Figure 2.2. Typical full-fledged Spring web application
Spring’s declarative transaction management features make the web application fully transactional, just
as it would be if you used EJB container-managed transactions. All your custom business logic can be
implemented with simple POJOs and managed by Spring’s IoC container. Additional services include
support for sending email and validation that is independent of the web layer, which lets you choose
where to execute validation rules. Spring’s ORM support is integrated with JPA, Hibernate and and
JDO; for example, when using Hibernate, you can continue to use your existing mapping files and
standard Hibernate SessionFactory configuration. Form controllers seamlessly integrate the web-
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layer with the domain model, removing the need for ActionForms or other classes that transform HTTP
parameters to values for your domain model.
Figure 2.3. Spring middle-tier using a third-party web framework
Sometimes circumstances do not allow you to completely switch to a different framework. The Spring
Framework does not force you to use everything within it; it is not an all-or-nothing solution. Existing
front-ends built with Struts, Tapestry, JSF or other UI frameworks can be integrated with a Springbased middle-tier, which allows you to use Spring transaction features. You simply need to wire up your
business logic using an ApplicationContext and use a WebApplicationContext to integrate
your web layer.
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Figure 2.4. Remoting usage scenario
When you need to access existing code through web services, you can use Spring’s Hessian-,
Burlap-, Rmi- or JaxRpcProxyFactory classes. Enabling remote access to existing applications
is not difficult.
Figure 2.5. EJBs - Wrapping existing POJOs
The Spring Framework also provides an access and abstraction layer for Enterprise JavaBeans,
enabling you to reuse your existing POJOs and wrap them in stateless session beans for use in scalable,
fail-safe web applications that might need declarative security.
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Dependency Management and Naming Conventions
Dependency management and dependency injection are different things. To get those nice features of
Spring into your application (like dependency injection) you need to assemble all the libraries needed (jar
files) and get them onto your classpath at runtime, and possibly at compile time. These dependencies
are not virtual components that are injected, but physical resources in a file system (typically). The
process of dependency management involves locating those resources, storing them and adding them
to classpaths. Dependencies can be direct (e.g. my application depends on Spring at runtime), or indirect
(e.g. my application depends on commons-dbcp which depends on commons-pool). The indirect
dependencies are also known as "transitive" and it is those dependencies that are hardest to identify
and manage.
If you are going to use Spring you need to get a copy of the jar libraries that comprise the pieces of
Spring that you need. To make this easier Spring is packaged as a set of modules that separate the
dependencies as much as possible, so for example if you don’t want to write a web application you
don’t need the spring-web modules. To refer to Spring library modules in this guide we use a shorthand
naming convention spring-* or spring-*.jar, where * represents the short name for the module
(e.g. spring-core, spring-webmvc, spring-jms, etc.). The actual jar file name that you use is
normally the module name concatenated with the version number (e.g. spring-core-4.2.7.RELEASE.jar).
Each release of the Spring Framework will publish artifacts to the following places:
• Maven Central, which is the default repository that Maven queries, and does not require any special
configuration to use. Many of the common libraries that Spring depends on also are available
from Maven Central and a large section of the Spring community uses Maven for dependency
management, so this is convenient for them. The names of the jars here are in the form spring-*<version>.jar and the Maven groupId is org.springframework.
• In a public Maven repository hosted specifically for Spring. In addition to the final GA releases, this
repository also hosts development snapshots and milestones. The jar file names are in the same form
as Maven Central, so this is a useful place to get development versions of Spring to use with other
libraries deployed in Maven Central. This repository also contains a bundle distribution zip file that
contains all Spring jars bundled together for easy download.
So the first thing you need to decide is how to manage your dependencies: we generally recommend the
use of an automated system like Maven, Gradle or Ivy, but you can also do it manually by downloading
all the jars yourself.
You will find bellow the list of Spring artifacts. For a more complete description of each modules, see
Section 2.2, “Modules”.
Table 2.1. Spring Framework Artifacts
GroupId
ArtifactId
Description
org.springframework
spring-aop
Proxy-based AOP support
org.springframework
spring-aspects
AspectJ based aspects
org.springframework
spring-beans
Beans support, including
Groovy
org.springframework
spring-context
Application context runtime,
including scheduling and
remoting abstractions
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GroupId
ArtifactId
Description
org.springframework
spring-context-support
Support classes for integrating
common third-party libraries
into a Spring application context
org.springframework
spring-core
Core utilities, used by many
other Spring modules
org.springframework
spring-expression
Spring Expression Language
(SpEL)
org.springframework
spring-instrument
Instrumentation agent for JVM
bootstrapping
org.springframework
spring-instrument-tomcat
Instrumentation agent for
Tomcat
org.springframework
spring-jdbc
JDBC support package,
including DataSource setup and
JDBC access support
org.springframework
spring-jms
JMS support package, including
helper classes to send and
receive JMS messages
org.springframework
spring-messaging
Support for messaging
architectures and protocols
org.springframework
spring-orm
Object/Relational Mapping,
including JPA and Hibernate
support
org.springframework
spring-oxm
Object/XML Mapping
org.springframework
spring-test
Support for unit testing and
integration testing Spring
components
org.springframework
spring-tx
Transaction infrastructure,
including DAO support and JCA
integration
org.springframework
spring-web
Web support packages,
including client and web
remoting
org.springframework
spring-webmvc
REST Web Services and
model-view-controller
implementation for web
applications
org.springframework
spring-webmvc-portlet
MVC implementation to be used
in a Portlet environment
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GroupId
ArtifactId
Description
org.springframework
spring-websocket
WebSocket and SockJS
implementations, including
STOMP support
Spring Dependencies and Depending on Spring
Although Spring provides integration and support for a huge range of enterprise and other external tools,
it intentionally keeps its mandatory dependencies to an absolute minimum: you shouldn’t have to locate
and download (even automatically) a large number of jar libraries in order to use Spring for simple use
cases. For basic dependency injection there is only one mandatory external dependency, and that is
for logging (see below for a more detailed description of logging options).
Next we outline the basic steps needed to configure an application that depends on Spring, first with
Maven and then with Gradle and finally using Ivy. In all cases, if anything is unclear, refer to the
documentation of your dependency management system, or look at some sample code - Spring itself
uses Gradle to manage dependencies when it is building, and our samples mostly use Gradle or Maven.
Maven Dependency Management
If you are using Maven for dependency management you don’t even need to supply the logging
dependency explicitly. For example, to create an application context and use dependency injection to
configure an application, your Maven dependencies will look like this:
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>4.2.7.RELEASE</version>
<scope>runtime</scope>
</dependency>
</dependencies>
That’s it. Note the scope can be declared as runtime if you don’t need to compile against Spring APIs,
which is typically the case for basic dependency injection use cases.
The example above works with the Maven Central repository. To use the Spring Maven repository
(e.g. for milestones or developer snapshots), you need to specify the repository location in your Maven
configuration. For full releases:
<repositories>
<repository>
<id>io.spring.repo.maven.release</id>
<url>http://repo.spring.io/release/</url>
<snapshots><enabled>false</enabled></snapshots>
</repository>
</repositories>
For milestones:
<repositories>
<repository>
<id>io.spring.repo.maven.milestone</id>
<url>http://repo.spring.io/milestone/</url>
<snapshots><enabled>false</enabled></snapshots>
</repository>
</repositories>
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And for snapshots:
<repositories>
<repository>
<id>io.spring.repo.maven.snapshot</id>
<url>http://repo.spring.io/snapshot/</url>
<snapshots><enabled>true</enabled></snapshots>
</repository>
</repositories>
Maven "Bill Of Materials" Dependency
It is possible to accidentally mix different versions of Spring JARs when using Maven. For example,
you may find that a third-party library, or another Spring project, pulls in a transitive dependency to an
older release. If you forget to explicitly declare a direct dependency yourself, all sorts of unexpected
issues can arise.
To overcome such problems Maven supports the concept of a "bill of materials" (BOM) dependency.
You can import the spring-framework-bom in your dependencyManagement section to ensure
that all spring dependencies (both direct and transitive) are at the same version.
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-framework-bom</artifactId>
<version>4.2.7.RELEASE</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
An added benefit of using the BOM is that you no longer need to specify the <version> attribute when
depending on Spring Framework artifacts:
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-web</artifactId>
</dependency>
<dependencies>
Gradle Dependency Management
To use the Spring repository with the Gradle build system, include the appropriate URL in the
repositories section:
repositories {
mavenCentral()
// and optionally...
maven { url "http://repo.spring.io/release" }
}
You can change the repositories URL from /release to /milestone or /snapshot as
appropriate. Once a repository has been configured, you can declare dependencies in the usual Gradle
way:
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dependencies {
compile("org.springframework:spring-context:4.2.7.RELEASE")
testCompile("org.springframework:spring-test:4.2.7.RELEASE")
}
Ivy Dependency Management
If you prefer to use Ivy to manage dependencies then there are similar configuration options.
To configure Ivy to point to the Spring repository add the following resolver to your ivysettings.xml:
<resolvers>
<ibiblio name="io.spring.repo.maven.release"
m2compatible="true"
root="http://repo.spring.io/release/"/>
</resolvers>
You can change the root URL from /release/ to /milestone/ or /snapshot/ as appropriate.
Once configured, you can add dependencies in the usual way. For example (in ivy.xml):
<dependency org="org.springframework"
name="spring-core" rev="4.2.7.RELEASE" conf="compile->runtime"/>
Distribution Zip Files
Although using a build system that supports dependency management is the recommended way to
obtain the Spring Framework, it is still possible to download a distribution zip file.
Distribution zips are published to the Spring Maven Repository (this is just for our convenience, you
don’t need Maven or any other build system in order to download them).
To download a distribution zip open a web browser to http://repo.spring.io/release/org/springframework/
spring and select the appropriate subfolder for the version that you want. Distribution files end dist.zip, for example spring-framework-{spring-version}-RELEASE-dist.zip. Distributions are also
published for milestones and snapshots.
Logging
Logging is a very important dependency for Spring because a) it is the only mandatory external
dependency, b) everyone likes to see some output from the tools they are using, and c) Spring integrates
with lots of other tools all of which have also made a choice of logging dependency. One of the goals
of an application developer is often to have unified logging configured in a central place for the whole
application, including all external components. This is more difficult than it might have been since there
are so many choices of logging framework.
The mandatory logging dependency in Spring is the Jakarta Commons Logging API (JCL). We compile
against JCL and we also make JCL Log objects visible for classes that extend the Spring Framework.
It’s important to users that all versions of Spring use the same logging library: migration is easy because
backwards compatibility is preserved even with applications that extend Spring. The way we do this
is to make one of the modules in Spring depend explicitly on commons-logging (the canonical
implementation of JCL), and then make all the other modules depend on that at compile time. If you are
using Maven for example, and wondering where you picked up the dependency on commons-logging,
then it is from Spring and specifically from the central module called spring-core.
The nice thing about commons-logging is that you don’t need anything else to make your application
work. It has a runtime discovery algorithm that looks for other logging frameworks in well known places
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on the classpath and uses one that it thinks is appropriate (or you can tell it which one if you need to).
If nothing else is available you get pretty nice looking logs just from the JDK (java.util.logging or JUL
for short). You should find that your Spring application works and logs happily to the console out of the
box in most situations, and that’s important.
Not Using Commons Logging
Unfortunately, the runtime discovery algorithm in commons-logging, while convenient for the enduser, is problematic. If we could turn back the clock and start Spring now as a new project it would use
a different logging dependency. The first choice would probably be the Simple Logging Facade for Java
( SLF4J), which is also used by a lot of other tools that people use with Spring inside their applications.
There are basically two ways to switch off commons-logging:
1. Exclude the dependency from the spring-core module (as it is the only module that explicitly
depends on commons-logging)
2. Depend on a special commons-logging dependency that replaces the library with an empty jar
(more details can be found in the SLF4J FAQ)
To exclude commons-logging, add the following to your dependencyManagement section:
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<version>4.2.7.RELEASE</version>
<exclusions>
<exclusion>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
</exclusion>
</exclusions>
</dependency>
</dependencies>
Now this application is probably broken because there is no implementation of the JCL API on the
classpath, so to fix it a new one has to be provided. In the next section we show you how to provide an
alternative implementation of JCL using SLF4J as an example.
Using SLF4J
SLF4J is a cleaner dependency and more efficient at runtime than commons-logging because it uses
compile-time bindings instead of runtime discovery of the other logging frameworks it integrates. This
also means that you have to be more explicit about what you want to happen at runtime, and declare it
or configure it accordingly. SLF4J provides bindings to many common logging frameworks, so you can
usually choose one that you already use, and bind to that for configuration and management.
SLF4J provides bindings to many common logging frameworks, including JCL, and it also does the
reverse: bridges between other logging frameworks and itself. So to use SLF4J with Spring you need
to replace the commons-logging dependency with the SLF4J-JCL bridge. Once you have done that
then logging calls from within Spring will be translated into logging calls to the SLF4J API, so if other
libraries in your application use that API, then you have a single place to configure and manage logging.
A common choice might be to bridge Spring to SLF4J, and then provide explicit binding from SLF4J to
Log4J. You need to supply 4 dependencies (and exclude the existing commons-logging): the bridge,
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the SLF4J API, the binding to Log4J, and the Log4J implementation itself. In Maven you would do that
like this
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<version>4.2.7.RELEASE</version>
<exclusions>
<exclusion>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>jcl-over-slf4j</artifactId>
<version>1.5.8</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.5.8</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
<version>1.5.8</version>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.14</version>
</dependency>
</dependencies>
That might seem like a lot of dependencies just to get some logging. Well it is, but it is optional, and it
should behave better than the vanilla commons-logging with respect to classloader issues, notably if
you are in a strict container like an OSGi platform. Allegedly there is also a performance benefit because
the bindings are at compile-time not runtime.
A more common choice amongst SLF4J users, which uses fewer steps and generates fewer
dependencies, is to bind directly to Logback. This removes the extra binding step because Logback
implements SLF4J directly, so you only need to depend on two libraries not four ( jcl-over-slf4j and
logback). If you do that you might also need to exclude the slf4j-api dependency from other external
dependencies (not Spring), because you only want one version of that API on the classpath.
Using Log4J
Many people use Log4j as a logging framework for configuration and management purposes. It’s efficient
and well-established, and in fact it’s what we use at runtime when we build and test Spring. Spring
also provides some utilities for configuring and initializing Log4j, so it has an optional compile-time
dependency on Log4j in some modules.
To make Log4j work with the default JCL dependency ( commons-logging) all you need to do is put
Log4j on the classpath, and provide it with a configuration file ( log4j.properties or log4j.xml in
the root of the classpath). So for Maven users this is your dependency declaration:
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<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<version>4.2.7.RELEASE</version>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.14</version>
</dependency>
</dependencies>
And here’s a sample log4j.properties for logging to the console:
log4j.rootCategory=INFO, stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %t %c{2}:%L - %m%n
log4j.category.org.springframework.beans.factory=DEBUG
Runtime Containers with Native JCL
Many people run their Spring applications in a container that itself provides an implementation
of JCL. IBM Websphere Application Server (WAS) is the archetype. This often causes problems,
and unfortunately there is no silver bullet solution; simply excluding commons-logging from your
application is not enough in most situations.
To be clear about this: the problems reported are usually not with JCL per se, or even with commonslogging: rather they are to do with binding commons-logging to another framework (often Log4J).
This can fail because commons-logging changed the way they do the runtime discovery in between
the older versions (1.0) found in some containers and the modern versions that most people use now
(1.1). Spring does not use any unusual parts of the JCL API, so nothing breaks there, but as soon as
Spring or your application tries to do any logging you can find that the bindings to Log4J are not working.
In such cases with WAS the easiest thing to do is to invert the class loader hierarchy (IBM calls it "parent
last") so that the application controls the JCL dependency, not the container. That option isn’t always
open, but there are plenty of other suggestions in the public domain for alternative approaches, and
your mileage may vary depending on the exact version and feature set of the container.
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3. New Features and Enhancements in Spring
Framework 4.0
The Spring Framework was first released in 2004; since then there have been significant major revisions:
Spring 2.0 provided XML namespaces and AspectJ support; Spring 2.5 embraced annotation-driven
configuration; Spring 3.0 introduced a strong Java 5+ foundation across the framework codebase, and
features such as the Java-based @Configuration model.
Version 4.0 is the latest major release of the Spring Framework and the first to fully support Java 8
features. You can still use Spring with older versions of Java, however, the minimum requirement has
now been raised to Java SE 6. We have also taken the opportunity of a major release to remove many
deprecated classes and methods.
A migration guide for upgrading to Spring 4.0 is available on the Spring Framework GitHub Wiki.
3.1 Improved Getting Started Experience
The new spring.io website provides a whole series of "Getting Started" guides to help you learn Spring.
You can read more about the guides in the Chapter 1, Getting Started with Spring section in this
document. The new website also provides a comprehensive overview of the many additional projects
that are released under the Spring umbrella.
If you are a Maven user you may also be interested in the helpful bill of materials POM file that is now
published with each Spring Framework release.
3.2 Removed Deprecated Packages and Methods
All deprecated packages, and many deprecated classes and methods have been removed with version
4.0. If you are upgrading from a previous release of Spring, you should ensure that you have fixed any
deprecated calls that you were making to outdated APIs.
For a complete set of changes, check out the API Differences Report.
Note that optional third-party dependencies have been raised to a 2010/2011 minimum (i.e. Spring 4
generally only supports versions released in late 2010 or later now): notably, Hibernate 3.6+, EhCache
2.1+, Quartz 1.8+, Groovy 1.8+, and Joda-Time 2.0+. As an exception to the rule, Spring 4 requires the
recent Hibernate Validator 4.3+, and support for Jackson has been focused on 2.0+ now (with Jackson
1.8/1.9 support retained for the time being where Spring 3.2 had it; now just in deprecated form).
3.3 Java 8 (as well as 6 and 7)
Spring Framework 4.0 provides support for several Java 8 features. You can make use of lambda
expressions and method references with Spring’s callback interfaces. There is first-class support for
java.time (JSR-310), and several existing annotations have been retrofitted as @Repeatable. You
can also use Java 8’s parameter name discovery (based on the -parameters compiler flag) as an
alternative to compiling your code with debug information enabled.
Spring remains compatible with older versions of Java and the JDK: concretely, Java SE 6 (specifically,
a minimum level equivalent to JDK 6 update 18, as released in January 2010) and above are still fully
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supported. However, for newly started development projects based on Spring 4, we recommend the
use of Java 7 or 8.
3.4 Java EE 6 and 7
Java EE version 6 or above is now considered the baseline for Spring Framework 4, with the JPA 2.0
and Servlet 3.0 specifications being of particular relevance. In order to remain compatible with Google
App Engine and older application servers, it is possible to deploy a Spring 4 application into a Servlet
2.5 environment. However, Servlet 3.0+ is strongly recommended and a prerequisite in Spring’s test
and mock packages for test setups in development environments.
Note
If you are a WebSphere 7 user, be sure to install the JPA 2.0 feature pack. On WebLogic 10.3.4
or higher, install the JPA 2.0 patch that comes with it. This turns both of those server generations
into Spring 4 compatible deployment environments.
On a more forward-looking note, Spring Framework 4.0 supports the Java EE 7 level of applicable
specifications now: in particular, JMS 2.0, JTA 1.2, JPA 2.1, Bean Validation 1.1, and JSR-236
Concurrency Utilities. As usual, this support focuses on individual use of those specifications, e.g. on
Tomcat or in standalone environments. However, it works equally well when a Spring application is
deployed to a Java EE 7 server.
Note that Hibernate 4.3 is a JPA 2.1 provider and therefore only supported as of Spring Framework 4.0.
The same applies to Hibernate Validator 5.0 as a Bean Validation 1.1 provider. Neither of the two are
officially supported with Spring Framework 3.2.
3.5 Groovy Bean Definition DSL
Beginning with Spring Framework 4.0, it is possible to define external bean configuration using a Groovy
DSL. This is similar in concept to using XML bean definitions but allows for a more concise syntax. Using
Groovy also allows you to easily embed bean definitions directly in your bootstrap code. For example:
def reader = new GroovyBeanDefinitionReader(myApplicationContext)
reader.beans {
dataSource(BasicDataSource) {
driverClassName = "org.hsqldb.jdbcDriver"
url = "jdbc:hsqldb:mem:grailsDB"
username = "sa"
password = ""
settings = [mynew:"setting"]
}
sessionFactory(SessionFactory) {
dataSource = dataSource
}
myService(MyService) {
nestedBean = { AnotherBean bean ->
dataSource = dataSource
}
}
}
For more information consult the GroovyBeanDefinitionReader javadocs.
3.6 Core Container Improvements
There have been several general improvements to the core container:
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• Spring now treats generic types as a form of qualifier when injecting Beans. For example, if you are
using a Spring Data Repository you can now easily inject a specific implementation: @Autowired
Repository<Customer> customerRepository.
• If you use Spring’s meta-annotation support, you can now develop custom annotations that expose
specific attributes from the source annotation.
• Beans can now be ordered when they are autowired into lists and arrays. Both the @Order annotation
and Ordered interface are supported.
• The @Lazy annotation can now be used on injection points, as well as on @Bean definitions.
• The @Description annotation has been introduced for developers using Java-based configuration.
• A generalized model for conditionally filtering beans has been added via the @Conditional
annotation. This is similar to @Profile support but allows for user-defined strategies to be developed
programmatically.
• CGLIB-based proxy classes no longer require a default constructor. Support is provided via the
objenesis library which is repackaged inline and distributed as part of the Spring Framework. With
this strategy, no constructor at all is being invoked for proxy instances anymore.
• There is managed time zone support across the framework now, e.g. on LocaleContext.
3.7 General Web Improvements
Deployment to Servlet 2.5 servers remains an option, but Spring Framework 4.0 is now focused primarily
on Servlet 3.0+ environments. If you are using the Spring MVC Test Framework you will need to ensure
that a Servlet 3.0 compatible JAR is in your test classpath.
In addition to the WebSocket support mentioned later, the following general improvements have been
made to Spring’s Web modules:
• You can use the new @RestController annotation with Spring MVC applications, removing the
need to add @ResponseBody to each of your @RequestMapping methods.
• The AsyncRestTemplate class has been added, allowing non-blocking asynchronous support
when developing REST clients.
• Spring now offers comprehensive timezone support when developing Spring MVC applications.
3.8 WebSocket, SockJS, and STOMP Messaging
A new spring-websocket module provides comprehensive support for WebSocket-based, two-way
communication between client and server in web applications. It is compatible with JSR-356, the Java
WebSocket API, and in addition provides SockJS-based fallback options (i.e. WebSocket emulation)
for use in browsers that don’t yet support the WebSocket protocol (e.g. Internet Explorer < 10).
A new spring-messaging module adds support for STOMP as the WebSocket sub-protocol to use in
applications along with an annotation programming model for routing and processing STOMP messages
from WebSocket clients. As a result an @Controller can now contain both @RequestMapping and
@MessageMapping methods for handling HTTP requests and messages from WebSocket-connected
clients. The new spring-messaging module also contains key abstractions formerly from the Spring
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Integration project such as Message, MessageChannel, MessageHandler, and others to serve as
a foundation for messaging-based applications.
For further details, including a more thorough introduction, see the Chapter 25, WebSocket Support
section.
3.9 Testing Improvements
In addition to pruning of deprecated code within the spring-test module, Spring Framework 4.0
introduces several new features for use in unit and integration testing.
• Almost all annotations in the spring-test module (e.g., @ContextConfiguration,
@WebAppConfiguration, @ContextHierarchy, @ActiveProfiles, etc.) can now be used
as meta-annotations to create custom composed annotations and reduce configuration duplication
across a test suite.
• Active bean definition profiles can now be resolved programmatically, simply by implementing
a custom ActiveProfilesResolver and registering it via the resolver attribute of
@ActiveProfiles.
• A new SocketUtils class has been introduced in the spring-core module which enables you to
scan for free TCP and UDP server ports on localhost. This functionality is not specific to testing but
can prove very useful when writing integration tests that require the use of sockets, for example tests
that start an in-memory SMTP server, FTP server, Servlet container, etc.
• As of Spring 4.0, the set of mocks in the org.springframework.mock.web package is
now based on the Servlet 3.0 API. Furthermore, several of the Servlet API mocks (e.g.,
MockHttpServletRequest, MockServletContext, etc.) have been updated with minor
enhancements and improved configurability.
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4. New Features and Enhancements in Spring
Framework 4.1
4.1 JMS Improvements
Spring 4.1 introduces a much simpler infrastructure to register JMS listener endpoints by annotating
bean methods with @JmsListener. The XML namespace has been enhanced to support this new
style (jms:annotation-driven), and it is also possible to fully configure the infrastructure using
Java config (@EnableJms, JmsListenerContainerFactory). It is also possible to register listener
endpoints programmatically using JmsListenerConfigurer.
Spring 4.1 also aligns its JMS support to allow you to benefit from the spring-messaging abstraction
introduced in 4.0, that is:
• Message listener endpoints can have a more flexible signature and benefit from standard messaging
annotations such as @Payload, @Header, @Headers, and @SendTo. It is also possible to use a
standard Message in lieu of javax.jms.Message as method argument.
• A new JmsMessageOperations interface is available and permits JmsTemplate like operations
using the Message abstraction.
Finally, Spring 4.1 provides additional miscellaneous improvements:
• Synchronous request-reply operations support in JmsTemplate
• Listener priority can be specified per <jms:listener/> element
• Recovery options for the message listener container are configurable using a BackOff
implementation
• JMS 2.0 shared consumers are supported
4.2 Caching Improvements
Spring 4.1 supports JCache (JSR-107) annotations using Spring’s existing cache configuration and
infrastructure abstraction; no changes are required to use the standard annotations.
Spring 4.1 also improves its own caching abstraction significantly:
• Caches can be resolved at runtime using a CacheResolver. As a result the value argument
defining the cache name(s) to use is no longer mandatory.
• More operation-level customizations: cache resolver, cache manager, key generator
• A new @CacheConfig class-level annotation allows common settings to be shared at the class level
without enabling any cache operation.
• Better exception handling of cached methods using CacheErrorHandler
Spring 4.1 also has a breaking change in the CacheInterface as a new putIfAbsent method has
been added.
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4.3 Web Improvements
• The existing support for resource handling based on the ResourceHttpRequestHandler
has been expanded with new abstractions ResourceResolver, ResourceTransformer, and
ResourceUrlProvider. A number of built-in implementations provide support for versioned
resource URLs (for effective HTTP caching), locating gzipped resources, generating an HTML 5
AppCache manifests, and more. See the section called “Serving of Resources”.
• JDK 1.8’s java.util.Optional is now supported for @RequestParam, @RequestHeader, and
@MatrixVariable controller method arguments.
• ListenableFuture is supported as a return value alternative to DeferredResult
where an underlying service (or perhaps a call to AsyncRestTemplate) already returns
ListenableFuture.
• @ModelAttribute methods are now invoked in an order that respects inter-dependencies. See
SPR-6299.
• Jackson’s @JsonView is supported directly on @ResponseBody and ResponseEntity controller
methods for serializing different amounts of detail for the same POJO (e.g. summary vs. detail page).
This is also supported with View-based rendering by adding the serialization view type as a model
attribute under a special key. See the section called “Jackson Serialization View Support” for details.
• JSONP is now supported with Jackson. See the section called “Jackson JSONP Support”.
• A new lifecycle option is available for intercepting @ResponseBody and ResponseEntity methods
just after the controller method returns and before the response is written. To take advantage declare
an @ControllerAdvice bean that implements ResponseBodyAdvice. The built-in support for
@JsonView and JSONP take advantage of this. See the section called “Intercepting requests with
a HandlerInterceptor”.
• There are three new HttpMessageConverter options:
• Gson — lighter footprint than Jackson; has already been in use in Spring Android.
• Google Protocol Buffers — efficient and effective as an inter-service communication data protocol
within an enterprise but can also be exposed as JSON and XML for browsers.
• Jackson based XML serialization is now supported through the jackson-dataformat-xml extension.
When using @EnableWebMvc or <mvc:annotation-driven/>, this is used by default instead
of JAXB2 if jackson-dataformat-xml is in the classpath.
• Views such as JSPs can now build links to controllers by referring to controller mappings by name. A
default name is assigned to every @RequestMapping. For example FooController with method
handleFoo is named "FC#handleFoo". The naming strategy is pluggable. It is also possible to name
an @RequestMapping explicitly through its name attribute. A new mvcUrl function in the Spring JSP
tag library makes this easy to use in JSP pages. See the section called “Building URIs to Controllers
and methods from views”.
• ResponseEntity provides a builder-style API to guide controller methods towards the preparation
of server-side responses, e.g. ResponseEntity.ok().
• RequestEntity is a new type that provides a builder-style API to guide client-side REST code
towards the preparation of HTTP requests.
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• MVC Java config and XML namespace:
• View resolvers can now be configured including support for content negotiation, see the section
called “View Resolvers”.
• View controllers now have built-in support for redirects and for setting the response status. An
application can use this to configure redirect URLs, render 404 responses with a view, send "no
content" responses, etc. Some use cases are listed here.
• Path matching customizations are frequently used and now built-in. See the section called “Path
Matching”.
• Groovy markup template support (based on Groovy 2.3). See the GroovyMarkupConfigurer and
respecitve ViewResolver and `View' implementations.
4.4 WebSocket Messaging Improvements
• SockJS (Java) client-side support. See SockJsClient and classes in same package.
• New application context events SessionSubscribeEvent and SessionUnsubscribeEvent
published when STOMP clients subscribe and unsubscribe.
• New "websocket" scope. See the section called “WebSocket Scope”.
• @SendToUser can target only a single session and does not require an authenticated user.
• @MessageMapping methods can use dot "." instead of slash "/" as path separator. See SPR-11660.
• STOMP/WebSocket monitoring info collected and logged. See the section called “Runtime
Monitoring”.
• Significantly optimized and improved logging that should remain very readable and compact even at
DEBUG level.
• Optimized message creation including support for temporary message mutability and avoiding
automatic message id and timestamp creation. See Javadoc of MessageHeaderAccessor.
• Close STOMP/WebSocket connections that have no activity within 60 seconds after the WebSocket
session is established. See SPR-11884.
4.5 Testing Improvements
• Groovy scripts can now be used to configure the ApplicationContext loaded for integration tests
in the TestContext framework.
• See the section called “Context configuration with Groovy scripts” for details.
• Test-managed transactions can now be programmatically started and ended within transactional test
methods via the new TestTransaction API.
• See the section called “Programmatic transaction management” for details.
• SQL script execution can now be configured declaratively via the new @Sql and @SqlConfig
annotations on a per-class or per-method basis.
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• See the section called “Executing SQL scripts” for details.
• Test property sources which automatically override system and application property sources can be
configured via the new @TestPropertySource annotation.
• See the section called “Context configuration with test property sources” for details.
• Default TestExecutionListeners can now be automatically discovered.
• See the section called “Automatic discovery of default TestExecutionListeners” for details.
• Custom TestExecutionListeners can now be automatically merged with the default listeners.
• See the section called “Merging TestExecutionListeners” for details.
• The documentation for transactional testing support in the TestContext framework has been improved
with more thorough explanations and additional examples.
• See the section called “Transaction management” for details.
• Various improvements to MockServletContext, MockHttpServletRequest, and other Servlet
API mocks.
• AssertThrows has been refactored to support Throwable instead of Exception.
• In Spring MVC Test, JSON responses can be asserted with JSON Assert as an extra option to using
JSONPath much like it has been possible to do for XML with XMLUnit.
• MockMvcBuilder recipes can now be created with the help of MockMvcConfigurer. This was
added to make it easy to apply Spring Security setup but can be used to encapsulate common setup
for any 3rd party framework or within a project.
• MockRestServiceServer now supports the AsyncRestTemplate for client-side testing.
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5. New Features and Enhancements in Spring
Framework 4.2
5.1 Core Container Improvements
• Annotations such as @Bean get detected and processed on Java 8 default methods as well, allowing
for composing a configuration class from interfaces with default @Bean methods.
• Configuration classes may declare @Import with regular component classes now, allowing for a mix
of imported configuration classes and component classes.
• Configuration classes may declare an @Order value, getting processed in a corresponding order (e.g.
for overriding beans by name) even when detected through classpath scanning.
• @Resource injection points support an @Lazy declaration, analogous to @Autowired, receiving a
lazy-initializing proxy for the requested target bean.
• The application event infrastructure now offers an annotation-based model as well as the ability to
publish any arbitrary event.
• Any public method in a managed bean can be annotated with @EventListener to consume
events.
• @TransactionalEventListener provides transaction-bound event support.
• Spring Framework 4.2 introduces first-class support for declaring and looking up aliases for annotation
attributes. The new @AliasFor annotation can be used to declare a pair of aliased attributes within
a single annotation or to declare an alias from one attribute in a custom composed annotation to an
attribute in a meta-annotation.
• The
following
annotations
have
been
retrofitted
with
@AliasFor
support
in
order
to
provide
meaningful
aliases
for
their
value
attributes:
@Cacheable, @CacheEvict, @CachePut, @ComponentScan, @ComponentScan.Filter,
@ImportResource, @Scope, @ManagedResource, @Header, @Payload, @SendToUser,
@ActiveProfiles, @ContextConfiguration, @Sql, @TestExecutionListeners,
@TestPropertySource,
@Transactional,
@ControllerAdvice,
@CookieValue,
@CrossOrigin,
@MatrixVariable,
@RequestHeader,
@RequestMapping,
@RequestParam,
@RequestPart,
@ResponseStatus,
@SessionAttributes,
@ActionMapping, @RenderMapping, @EventListener, @TransactionalEventListener.
• For example, @ContextConfiguration from the spring-test module is now declared as
follows:
public @interface ContextConfiguration {
@AliasFor("locations")
String[] value() default {};
@AliasFor("value")
String[] locations() default {};
// ...
}
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• Similarly, composed annotations that override attributes from meta-annotations can now use
@AliasFor for fine-grained control over exactly which attributes are overridden within an
annotation hierarchy. In fact, it is now possible to declare an alias for the value attribute of a metaannotation.
• For example, one can now develop a composed annotation with a custom attribute override as
follows.
@ContextConfiguration
public @interface MyTestConfig {
@AliasFor(annotation = ContextConfiguration.class, attribute = "value")
String[] xmlFiles();
// ...
}
• See Spring Annotation Programming Model.
• Numerous improvements to Spring’s search algorithms used for finding meta-annotations. For
example, locally declared composed annotations are now favored over inherited annotations.
• Composed annotations that override attributes from meta-annotations can now be discovered on
interfaces and on abstract, bridge, & interface methods as well as on classes, standard methods,
constructors, and fields.
• Maps representing annotation attributes (and AnnotationAttributes instances) can be
synthesized (i.e., converted) into an annotation.
• The features of field-based data binding (DirectFieldAccessor) have been aligned with the
current property-based data binding (BeanWrapper). In particular, field-based binding now supports
navigation for Collections, Arrays, and Maps.
• DefaultConversionService now provides out-of-the-box converters for Stream, Charset,
Currency, and TimeZone. Such converters can be added individually to any arbitrary
ConversionService as well.
• DefaultFormattingConversionService comes with out-of-the-box support for the value types
in JSR-354 Money & Currency (if the 'javax.money' API is present on the classpath): namely,
MonetaryAmount and CurrencyUnit. This includes support for applying @NumberFormat.
• @NumberFormat can now be used as a meta-annotation.
• JavaMailSenderImpl has a new testConnection() method for checking connectivity to the
server.
• ScheduledTaskRegistrar exposes scheduled tasks.
• Apache commons-pool2 is now supported for a pooling AOP CommonsPool2TargetSource.
• Introduced StandardScriptFactory as a JSR-223 based mechanism for scripted beans,
exposed through the lang:std element in XML. Supports e.g. JavaScript and JRuby. (Note:
JRubyScriptFactory and lang:jruby are deprecated now, in favor of using JSR-223.)
5.2 Data Access Improvements
• javax.transaction.Transactional is now supported via AspectJ.
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• SimpleJdbcCallOperations now supports named binding.
• Full support for Hibernate ORM 5.0: as a JPA provider (automatically adapted) as well as through its
native API (covered by the new org.springframework.orm.hibernate5 package).
• Embedded databases can now be automatically assigned unique names, and <jdbc:embeddeddatabase> supports a new database-name attribute. See "Testing Improvements" below for further
details.
5.3 JMS Improvements
• The autoStartup attribute can be controlled via JmsListenerContainerFactory.
• The type of the reply Destination can now be configured per listener container.
• The value of the @SendTo annotation can now use a SpEL expression.
• The response destination can be computed at runtime using JmsResponse
• @JmsListener is now a repeatable annotation to declare several JMS containers on the same
method (use the newly introduced @JmsListeners if you’re not using Java8 yet).
5.4 Web Improvements
• HTTP Streaming and Server-Sent Events support, see the section called “HTTP Streaming”.
• Built-in support for CORS including global (MVC Java config and XML namespace) and local (e.g.
@CrossOrigin) configuration. See Chapter 26, CORS Support for details.
• HTTP caching updates:
• new CacheControl builder; plugged into ResponseEntity, WebContentGenerator,
ResourceHttpRequestHandler.
• improved ETag/Last-Modified support in WebRequest.
• Custom mapping annotations, using @RequestMapping as a meta-annotation.
• Public methods in AbstractHandlerMethodMapping to register and unregister request mappings
at runtime.
• Protected
createDispatcherServlet
method
in
AbstractDispatcherServletInitializer to further customize the DispatcherServlet
instance to use.
• HandlerMethod as a method argument on @ExceptionHandler methods, especially handy in
@ControllerAdvice components.
• java.util.concurrent.CompletableFuture as an @Controller method return value type.
• Byte-range request support in HttpHeaders and for serving static resources.
• @ResponseStatus detected on nested exceptions.
• UriTemplateHandler extension point in the RestTemplate.
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• DefaultUriTemplateHandler exposes baseUrl property and path segment encoding options.
• the extension point can also be used to plug in any URI template library.
• OkHTTP integration with the RestTemplate.
• Custom baseUrl alternative for methods in MvcUriComponentsBuilder.
• Serialization/deserialization exception messages are now logged at WARN level.
• Default JSON prefix has been changed from "{} && " to the safer ")]}', " one.
• New RequestBodyAdvice extension point and built-in implementation to support Jackson’s
@JsonView on @RequestBody method arguments.
• When using GSON or Jackson 2.6+, the handler method return type is used to improve serialization
of parameterized types like List<Foo>.
• Introduced ScriptTemplateView as a JSR-223 based mechanism for scripted web views, with a
focus on JavaScript view templating on Nashorn (JDK 8).
5.5 WebSocket Messaging Improvements
• Expose presence information about connected users and subscriptions:
• new SimpUserRegistry exposed as a bean named "userRegistry".
• sharing of presence information across cluster of servers (see broker relay config options).
• Resolve user destinations across cluster of servers (see broker relay config options).
• StompSubProtocolErrorHandler extension point to customize and control STOMP ERROR
frames to clients.
• Global @MessageExceptionHandler methods via @ControllerAdvice components.
• Heart-beats
and
a
SpEL
expression
SimpleBrokerMessageHandler.
'selector'
header
for
subscriptions
with
• STOMP client for use over TCP and WebSocket; see the section called “STOMP Client”.
• @SendTo and @SendToUser can contain destination variable placeholders.
• Jackson’s @JsonView supported
@SubscribeMapping methods.
for
return
values
on
@MessageMapping
and
• ListenableFuture and CompletableFuture as return value types from @MessageMapping
and @SubscribeMapping methods.
• MarshallingMessageConverter for XML payloads.
5.6 Testing Improvements
• JUnit-based integration tests can now be executed with JUnit rules instead of the
SpringJUnit4ClassRunner. This allows Spring-based integration tests to be run with alternative
runners like JUnit’s Parameterized or third-party runners such as the MockitoJUnitRunner.
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• See the section called “Spring JUnit Rules” for details.
• The Spring MVC Test framework now provides first-class support for HtmlUnit, including integration
with Selenium’s WebDriver, allowing for page-based web application testing without the need to
deploy to a Servlet container.
• See the section called “HtmlUnit Integration” for details.
• AopTestUtils is a new testing utility that allows developers to obtain a reference to the underlying
target object hidden behind one or more Spring proxies.
• See the section called “General testing utilities” for details.
• ReflectionTestUtils now supports setting and getting static fields, including constants.
• The original ordering of bean definition profiles declared via @ActiveProfiles is now retained in
order to support use cases such as Spring Boot’s ConfigFileApplicationListener which loads
configuration files based on the names of active profiles.
• @DirtiesContext
supports
new
BEFORE_METHOD,
BEFORE_CLASS,
and
BEFORE_EACH_TEST_METHOD modes for closing the ApplicationContext before a test — for
example, if some rogue (i.e., yet to be determined) test within a large test suite has corrupted the
original configuration for the ApplicationContext.
• @Commit is a new annotation that may be used as a direct replacement for @Rollback(false).
• @Rollback may now be used to configure class-level default rollback semantics.
• Consequently, @TransactionConfiguration is now deprecated and will be removed in a
subsequent release.
• @Sql now supports execution of inlined SQL statements via a new statements attribute.
• The ContextCache that is used for caching ApplicationContexts between tests is now a public API
with a default implementation that can be replaced for custom caching needs.
• DefaultTestContext,
DefaultBootstrapContext,
and
DefaultCacheAwareContextLoaderDelegate are now public classes in the support
subpackage, allowing for custom extensions.
• TestContextBootstrappers are now responsible for building the TestContext.
• In the Spring MVC Test framework, MvcResult details can now be logged at DEBUG level or
written to a custom OutputStream or Writer. See the new log(), print(OutputStream), and
print(Writer) methods in MockMvcResultHandlers for details.
• The JDBC XML namespace supports a new database-name attribute in <jdbc:embeddeddatabase>, allowing developers to set unique names for embedded databases –- for example, via
a SpEL expression or a property placeholder that is influenced by the current active bean definition
profiles.
• Embedded databases can now be automatically assigned a unique name, allowing common test
database configuration to be reused in different ApplicationContexts within a test suite.
• See the section called “Generating unique names for embedded databases” for details.
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• MockHttpServletRequest and MockHttpServletResponse now provide better support for
date header formatting via the getDateHeader and setDateHeader methods.
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Part III. Core Technologies
This part of the reference documentation covers all of those technologies that are absolutely integral
to the Spring Framework.
Foremost amongst these is the Spring Framework’s Inversion of Control (IoC) container. A thorough
treatment of the Spring Framework’s IoC container is closely followed by comprehensive coverage of
Spring’s Aspect-Oriented Programming (AOP) technologies. The Spring Framework has its own AOP
framework, which is conceptually easy to understand, and which successfully addresses the 80% sweet
spot of AOP requirements in Java enterprise programming.
Coverage of Spring’s integration with AspectJ (currently the richest - in terms of features - and certainly
most mature AOP implementation in the Java enterprise space) is also provided.
• Chapter 6, The IoC container
• Chapter 7, Resources
• Chapter 8, Validation, Data Binding, and Type Conversion
• Chapter 9, Spring Expression Language (SpEL)
• Chapter 10, Aspect Oriented Programming with Spring
• Chapter 11, Spring AOP APIs
Spring Framework Reference Documentation
6. The IoC container
6.1 Introduction to the Spring IoC container and beans
1
This chapter covers the Spring Framework implementation of the Inversion of Control (IoC) principle.
IoC is also known as dependency injection (DI). It is a process whereby objects define their
dependencies, that is, the other objects they work with, only through constructor arguments, arguments
to a factory method, or properties that are set on the object instance after it is constructed or returned
from a factory method. The container then injects those dependencies when it creates the bean. This
process is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself
controlling the instantiation or location of its dependencies by using direct construction of classes, or a
mechanism such as the Service Locator pattern.
The org.springframework.beans and org.springframework.context packages are the
basis for Spring Framework’s IoC container. The BeanFactory interface provides an advanced
configuration mechanism capable of managing any type of object. ApplicationContext is a subinterface of BeanFactory. It adds easier integration with Spring’s AOP features; message resource
handling (for use in internationalization), event publication; and application-layer specific contexts such
as the WebApplicationContext for use in web applications.
In short, the BeanFactory provides the configuration framework and basic functionality, and the
ApplicationContext adds more enterprise-specific functionality. The ApplicationContext is
a complete superset of the BeanFactory, and is used exclusively in this chapter in descriptions
of Spring’s IoC container. For more information on using the BeanFactory instead of the
ApplicationContext, refer to Section 6.16, “The BeanFactory”.
In Spring, the objects that form the backbone of your application and that are managed by the Spring IoC
container are called beans. A bean is an object that is instantiated, assembled, and otherwise managed
by a Spring IoC container. Otherwise, a bean is simply one of many objects in your application. Beans,
and the dependencies among them, are reflected in the configuration metadata used by a container.
6.2 Container overview
The interface org.springframework.context.ApplicationContext represents the Spring IoC
container and is responsible for instantiating, configuring, and assembling the aforementioned beans.
The container gets its instructions on what objects to instantiate, configure, and assemble by reading
configuration metadata. The configuration metadata is represented in XML, Java annotations, or Java
code. It allows you to express the objects that compose your application and the rich interdependencies
between such objects.
Several implementations of the ApplicationContext interface are supplied out-of-thebox with Spring. In standalone applications it is common to create an instance of
ClassPathXmlApplicationContext or FileSystemXmlApplicationContext. While XML has
been the traditional format for defining configuration metadata you can instruct the container to use
Java annotations or code as the metadata format by providing a small amount of XML configuration to
declaratively enable support for these additional metadata formats.
In most application scenarios, explicit user code is not required to instantiate one or more instances
of a Spring IoC container. For example, in a web application scenario, a simple eight (or so) lines
1
See Background
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of boilerplate web descriptor XML in the web.xml file of the application will typically suffice (see the
section called “Convenient ApplicationContext instantiation for web applications”). If you are using the
Spring Tool Suite Eclipse-powered development environment this boilerplate configuration can be easily
created with few mouse clicks or keystrokes.
The following diagram is a high-level view of how Spring works. Your application classes are combined
with configuration metadata so that after the ApplicationContext is created and initialized, you have
a fully configured and executable system or application.
Figure 6.1. The Spring IoC container
Configuration metadata
As the preceding diagram shows, the Spring IoC container consumes a form of configuration metadata;
this configuration metadata represents how you as an application developer tell the Spring container to
instantiate, configure, and assemble the objects in your application.
Configuration metadata is traditionally supplied in a simple and intuitive XML format, which is what most
of this chapter uses to convey key concepts and features of the Spring IoC container.
Note
XML-based metadata is not the only allowed form of configuration metadata. The Spring IoC
container itself is totally decoupled from the format in which this configuration metadata is
actually written. These days many developers choose Java-based configuration for their Spring
applications.
For information about using other forms of metadata with the Spring container, see:
• Annotation-based configuration: Spring 2.5 introduced support for annotation-based configuration
metadata.
• Java-based configuration: Starting with Spring 3.0, many features provided by the Spring JavaConfig
project became part of the core Spring Framework. Thus you can define beans external to your
application classes by using Java rather than XML files. To use these new features, see the
@Configuration, @Bean, @Import and @DependsOn annotations.
Spring configuration consists of at least one and typically more than one bean definition that the
container must manage. XML-based configuration metadata shows these beans configured as <bean/
> elements inside a top-level <beans/> element. Java configuration typically uses @Bean annotated
methods within a @Configuration class.
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These bean definitions correspond to the actual objects that make up your application. Typically you
define service layer objects, data access objects (DAOs), presentation objects such as Struts Action
instances, infrastructure objects such as Hibernate SessionFactories, JMS Queues, and so forth.
Typically one does not configure fine-grained domain objects in the container, because it is usually the
responsibility of DAOs and business logic to create and load domain objects. However, you can use
Spring’s integration with AspectJ to configure objects that have been created outside the control of an
IoC container. See Using AspectJ to dependency-inject domain objects with Spring.
The following example shows the basic structure of XML-based configuration metadata:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="..." class="...">
<!-- collaborators and configuration for this bean go here -->
</bean>
<bean id="..." class="...">
<!-- collaborators and configuration for this bean go here -->
</bean>
<!-- more bean definitions go here -->
</beans>
The id attribute is a string that you use to identify the individual bean definition. The class attribute
defines the type of the bean and uses the fully qualified classname. The value of the id attribute refers
to collaborating objects. The XML for referring to collaborating objects is not shown in this example; see
Dependencies for more information.
Instantiating a container
Instantiating a Spring IoC container is straightforward. The location path or paths supplied to an
ApplicationContext constructor are actually resource strings that allow the container to load
configuration metadata from a variety of external resources such as the local file system, from the Java
CLASSPATH, and so on.
ApplicationContext context =
new ClassPathXmlApplicationContext(new String[] {"services.xml", "daos.xml"});
Note
After you learn about Spring’s IoC container, you may want to know more about Spring’s
Resource abstraction, as described in Chapter 7, Resources, which provides a convenient
mechanism for reading an InputStream from locations defined in a URI syntax. In particular,
Resource paths are used to construct applications contexts as described in Section 7.7,
“Application contexts and Resource paths”.
The following example shows the service layer objects (services.xml) configuration file:
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<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<!-- services -->
<bean id="petStore" class="org.springframework.samples.jpetstore.services.PetStoreServiceImpl">
<property name="accountDao" ref="accountDao"/>
<property name="itemDao" ref="itemDao"/>
<!-- additional collaborators and configuration for this bean go here -->
</bean>
<!-- more bean definitions for services go here -->
</beans>
The following example shows the data access objects daos.xml file:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="accountDao"
class="org.springframework.samples.jpetstore.dao.jpa.JpaAccountDao">
<!-- additional collaborators and configuration for this bean go here -->
</bean>
<bean id="itemDao" class="org.springframework.samples.jpetstore.dao.jpa.JpaItemDao">
<!-- additional collaborators and configuration for this bean go here -->
</bean>
<!-- more bean definitions for data access objects go here -->
</beans>
In the preceding example, the service layer consists of the class PetStoreServiceImpl, and two data
access objects of the type JpaAccountDao and JpaItemDao (based on the JPA Object/Relational
mapping standard). The property name element refers to the name of the JavaBean property,
and the ref element refers to the name of another bean definition. This linkage between id and
ref elements expresses the dependency between collaborating objects. For details of configuring an
object’s dependencies, see Dependencies.
Composing XML-based configuration metadata
It can be useful to have bean definitions span multiple XML files. Often each individual XML configuration
file represents a logical layer or module in your architecture.
You can use the application context constructor to load bean definitions from all these XML fragments.
This constructor takes multiple Resource locations, as was shown in the previous section. Alternatively,
use one or more occurrences of the <import/> element to load bean definitions from another file or
files. For example:
<beans>
<import resource="services.xml"/>
<import resource="resources/messageSource.xml"/>
<import resource="/resources/themeSource.xml"/>
<bean id="bean1" class="..."/>
<bean id="bean2" class="..."/>
</beans>
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In the preceding example, external bean definitions are loaded from three files: services.xml,
messageSource.xml, and themeSource.xml. All location paths are relative to the definition file
doing the importing, so services.xml must be in the same directory or classpath location as the file
doing the importing, while messageSource.xml and themeSource.xml must be in a resources
location below the location of the importing file. As you can see, a leading slash is ignored, but given
that these paths are relative, it is better form not to use the slash at all. The contents of the files being
imported, including the top level <beans/> element, must be valid XML bean definitions according to
the Spring Schema.
Note
It is possible, but not recommended, to reference files in parent directories using a relative
"../" path. Doing so creates a dependency on a file that is outside the current application. In
particular, this reference is not recommended for "classpath:" URLs (for example, "classpath:../
services.xml"), where the runtime resolution process chooses the "nearest" classpath root and
then looks into its parent directory. Classpath configuration changes may lead to the choice of a
different, incorrect directory.
You can always use fully qualified resource locations instead of relative paths: for example, "file:C:/
config/services.xml" or "classpath:/config/services.xml". However, be aware that you are coupling
your application’s configuration to specific absolute locations. It is generally preferable to keep an
indirection for such absolute locations, for example, through "${…}" placeholders that are resolved
against JVM system properties at runtime.
Using the container
The ApplicationContext is the interface for an advanced factory capable of maintaining a registry
of different beans and their dependencies. Using the method T getBean(String name, Class<T>
requiredType) you can retrieve instances of your beans.
The ApplicationContext enables you to read bean definitions and access them as follows:
// create and configure beans
ApplicationContext context =
new ClassPathXmlApplicationContext(new String[] {"services.xml", "daos.xml"});
// retrieve configured instance
PetStoreService service = context.getBean("petStore", PetStoreService.class);
// use configured instance
List<String> userList = service.getUsernameList();
You use getBean() to retrieve instances of your beans. The ApplicationContext interface has a
few other methods for retrieving beans, but ideally your application code should never use them. Indeed,
your application code should have no calls to the getBean() method at all, and thus no dependency
on Spring APIs at all. For example, Spring’s integration with web frameworks provides for dependency
injection for various web framework classes such as controllers and JSF-managed beans.
6.3 Bean overview
A Spring IoC container manages one or more beans. These beans are created with the configuration
metadata that you supply to the container, for example, in the form of XML <bean/> definitions.
Within the container itself, these bean definitions are represented as BeanDefinition objects, which
contain (among other information) the following metadata:
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• A package-qualified class name: typically the actual implementation class of the bean being defined.
• Bean behavioral configuration elements, which state how the bean should behave in the container
(scope, lifecycle callbacks, and so forth).
• References to other beans that are needed for the bean to do its work; these references are also
called collaborators or dependencies.
• Other configuration settings to set in the newly created object, for example, the number of connections
to use in a bean that manages a connection pool, or the size limit of the pool.
This metadata translates to a set of properties that make up each bean definition.
Table 6.1. The bean definition
Property
Explained in…
class
the section called “Instantiating beans”
name
the section called “Naming beans”
scope
Section 6.5, “Bean scopes”
constructor arguments
the section called “Dependency Injection”
properties
the section called “Dependency Injection”
autowiring mode
the section called “Autowiring collaborators”
lazy-initialization mode
the section called “Lazy-initialized beans”
initialization method
the section called “Initialization callbacks”
destruction method
the section called “Destruction callbacks”
In addition to bean definitions that contain information on how to create a specific bean, the
ApplicationContext implementations also permit the registration of existing objects that are
created outside the container, by users. This is done by accessing the ApplicationContext’s
BeanFactory via the method getBeanFactory() which returns the BeanFactory implementation
DefaultListableBeanFactory. DefaultListableBeanFactory supports this registration
through the methods registerSingleton(..) and registerBeanDefinition(..). However,
typical applications work solely with beans defined through metadata bean definitions.
Note
Bean metadata and manually supplied singleton instances need to be registered as early as
possible, in order for the container to properly reason about them during autowiring and other
introspection steps. While overriding of existing metadata and existing singleton instances is
supported to some degree, the registration of new beans at runtime (concurrently with live
access to factory) is not officially supported and may lead to concurrent access exceptions and/
or inconsistent state in the bean container.
Naming beans
Every bean has one or more identifiers. These identifiers must be unique within the container that hosts
the bean. A bean usually has only one identifier, but if it requires more than one, the extra ones can
be considered aliases.
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In XML-based configuration metadata, you use the id and/or name attributes to specify the bean
identifier(s). The id attribute allows you to specify exactly one id. Conventionally these names are
alphanumeric ('myBean', 'fooService', etc.), but may contain special characters as well. If you want to
introduce other aliases to the bean, you can also specify them in the name attribute, separated by a
comma (,), semicolon (;), or white space. As a historical note, in versions prior to Spring 3.1, the id
attribute was defined as an xsd:ID type, which constrained possible characters. As of 3.1, it is defined
as an xsd:string type. Note that bean id uniqueness is still enforced by the container, though no
longer by XML parsers.
You are not required to supply a name or id for a bean. If no name or id is supplied explicitly, the container
generates a unique name for that bean. However, if you want to refer to that bean by name, through the
use of the ref element or Service Locator style lookup, you must provide a name. Motivations for not
supplying a name are related to using inner beans and autowiring collaborators.
Bean Naming Conventions
The convention is to use the standard Java convention for instance field names when naming
beans. That is, bean names start with a lowercase letter, and are camel-cased from then on.
Examples of such names would be (without quotes) 'accountManager', 'accountService',
'userDao', 'loginController', and so forth.
Naming beans consistently makes your configuration easier to read and understand, and if you
are using Spring AOP it helps a lot when applying advice to a set of beans related by name.
Note
With component scanning in the classpath, Spring generates bean names for unnamed
components, following the rules above: essentially, taking the simple class name and
turning its initial character to lower-case. However, in the (unusual) special case when
there is more than one character and both the first and second characters are upper
case, the original casing gets preserved. These are the same rules as defined by
java.beans.Introspector.decapitalize (which Spring is using here).
Aliasing a bean outside the bean definition
In a bean definition itself, you can supply more than one name for the bean, by using a combination
of up to one name specified by the id attribute, and any number of other names in the name attribute.
These names can be equivalent aliases to the same bean, and are useful for some situations, such as
allowing each component in an application to refer to a common dependency by using a bean name
that is specific to that component itself.
Specifying all aliases where the bean is actually defined is not always adequate, however. It is
sometimes desirable to introduce an alias for a bean that is defined elsewhere. This is commonly the
case in large systems where configuration is split amongst each subsystem, each subsystem having its
own set of object definitions. In XML-based configuration metadata, you can use the <alias/> element
to accomplish this.
<alias name="fromName" alias="toName"/>
In this case, a bean in the same container which is named fromName, may also, after the use of this
alias definition, be referred to as toName.
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For example, the configuration metadata for subsystem A may refer to a DataSource via the name
subsystemA-dataSource. The configuration metadata for subsystem B may refer to a DataSource
via the name subsystemB-dataSource. When composing the main application that uses both these
subsystems the main application refers to the DataSource via the name myApp-dataSource. To have
all three names refer to the same object you add to the MyApp configuration metadata the following
aliases definitions:
<alias name="subsystemA-dataSource" alias="subsystemB-dataSource"/>
<alias name="subsystemA-dataSource" alias="myApp-dataSource" />
Now each component and the main application can refer to the dataSource through a name that is
unique and guaranteed not to clash with any other definition (effectively creating a namespace), yet
they refer to the same bean.
Java-configuration
If you are using Java-configuration, the @Bean annotation can be used to provide aliases see the
section called “Using the @Bean annotation” for details.
Instantiating beans
A bean definition essentially is a recipe for creating one or more objects. The container looks at the
recipe for a named bean when asked, and uses the configuration metadata encapsulated by that bean
definition to create (or acquire) an actual object.
If you use XML-based configuration metadata, you specify the type (or class) of object that is to be
instantiated in the class attribute of the <bean/> element. This class attribute, which internally is a
Class property on a BeanDefinition instance, is usually mandatory. (For exceptions, see the section
called “Instantiation using an instance factory method” and Section 6.7, “Bean definition inheritance”.)
You use the Class property in one of two ways:
• Typically, to specify the bean class to be constructed in the case where the container itself directly
creates the bean by calling its constructor reflectively, somewhat equivalent to Java code using the
new operator.
• To specify the actual class containing the static factory method that will be invoked to create the
object, in the less common case where the container invokes a static factory method on a class
to create the bean. The object type returned from the invocation of the static factory method may
be the same class or another class entirely.
Inner class names.
If you want to configure a bean definition for a static nested class, you
have to use the binary name of the nested class.
For example, if you have a class called Foo in the com.example package, and this Foo class
has a static nested class called Bar, the value of the 'class' attribute on a bean definition
would be…
com.example.Foo$Bar
Notice the use of the $ character in the name to separate the nested class name from the outer
class name.
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Instantiation with a constructor
When you create a bean by the constructor approach, all normal classes are usable by and compatible
with Spring. That is, the class being developed does not need to implement any specific interfaces or
to be coded in a specific fashion. Simply specifying the bean class should suffice. However, depending
on what type of IoC you use for that specific bean, you may need a default (empty) constructor.
The Spring IoC container can manage virtually any class you want it to manage; it is not limited to
managing true JavaBeans. Most Spring users prefer actual JavaBeans with only a default (no-argument)
constructor and appropriate setters and getters modeled after the properties in the container. You can
also have more exotic non-bean-style classes in your container. If, for example, you need to use a legacy
connection pool that absolutely does not adhere to the JavaBean specification, Spring can manage it
as well.
With XML-based configuration metadata you can specify your bean class as follows:
<bean id="exampleBean" class="examples.ExampleBean"/>
<bean name="anotherExample" class="examples.ExampleBeanTwo"/>
For details about the mechanism for supplying arguments to the constructor (if required) and setting
object instance properties after the object is constructed, see Injecting Dependencies.
Instantiation with a static factory method
When defining a bean that you create with a static factory method, you use the class attribute to specify
the class containing the static factory method and an attribute named factory-method to specify
the name of the factory method itself. You should be able to call this method (with optional arguments as
described later) and return a live object, which subsequently is treated as if it had been created through
a constructor. One use for such a bean definition is to call static factories in legacy code.
The following bean definition specifies that the bean will be created by calling a factory-method. The
definition does not specify the type (class) of the returned object, only the class containing the factory
method. In this example, the createInstance() method must be a static method.
<bean id="clientService"
class="examples.ClientService"
factory-method="createInstance"/>
public class ClientService {
private static ClientService clientService = new ClientService();
private ClientService() {}
public static ClientService createInstance() {
return clientService;
}
}
For details about the mechanism for supplying (optional) arguments to the factory method and
setting object instance properties after the object is returned from the factory, see Dependencies and
configuration in detail.
Instantiation using an instance factory method
Similar to instantiation through a static factory method, instantiation with an instance factory method
invokes a non-static method of an existing bean from the container to create a new bean. To use this
mechanism, leave the class attribute empty, and in the factory-bean attribute, specify the name of a
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bean in the current (or parent/ancestor) container that contains the instance method that is to be invoked
to create the object. Set the name of the factory method itself with the factory-method attribute.
<!-- the factory bean, which contains a method called createInstance() -->
<bean id="serviceLocator" class="examples.DefaultServiceLocator">
<!-- inject any dependencies required by this locator bean -->
</bean>
<!-- the bean to be created via the factory bean -->
<bean id="clientService"
factory-bean="serviceLocator"
factory-method="createClientServiceInstance"/>
public class DefaultServiceLocator {
private static ClientService clientService = new ClientServiceImpl();
private DefaultServiceLocator() {}
public ClientService createClientServiceInstance() {
return clientService;
}
}
One factory class can also hold more than one factory method as shown here:
<bean id="serviceLocator" class="examples.DefaultServiceLocator">
<!-- inject any dependencies required by this locator bean -->
</bean>
<bean id="clientService"
factory-bean="serviceLocator"
factory-method="createClientServiceInstance"/>
<bean id="accountService"
factory-bean="serviceLocator"
factory-method="createAccountServiceInstance"/>
public class DefaultServiceLocator {
private static ClientService clientService = new ClientServiceImpl();
private static AccountService accountService = new AccountServiceImpl();
private DefaultServiceLocator() {}
public ClientService createClientServiceInstance() {
return clientService;
}
public AccountService createAccountServiceInstance() {
return accountService;
}
}
This approach shows that the factory bean itself can be managed and configured through dependency
injection (DI). See Dependencies and configuration in detail.
Note
In Spring documentation, factory bean refers to a bean that is configured in the Spring container
that will create objects through an instance or static factory method. By contrast, FactoryBean
(notice the capitalization) refers to a Spring-specific FactoryBean.
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6.4 Dependencies
A typical enterprise application does not consist of a single object (or bean in the Spring parlance). Even
the simplest application has a few objects that work together to present what the end-user sees as a
coherent application. This next section explains how you go from defining a number of bean definitions
that stand alone to a fully realized application where objects collaborate to achieve a goal.
Dependency Injection
Dependency injection (DI) is a process whereby objects define their dependencies, that is, the other
objects they work with, only through constructor arguments, arguments to a factory method, or properties
that are set on the object instance after it is constructed or returned from a factory method. The container
then injects those dependencies when it creates the bean. This process is fundamentally the inverse,
hence the name Inversion of Control (IoC), of the bean itself controlling the instantiation or location of
its dependencies on its own by using direct construction of classes, or the Service Locator pattern.
Code is cleaner with the DI principle and decoupling is more effective when objects are provided with
their dependencies. The object does not look up its dependencies, and does not know the location
or class of the dependencies. As such, your classes become easier to test, in particular when the
dependencies are on interfaces or abstract base classes, which allow for stub or mock implementations
to be used in unit tests.
DI exists in two major variants, Constructor-based dependency injection and Setter-based dependency
injection.
Constructor-based dependency injection
Constructor-based DI is accomplished by the container invoking a constructor with a number of
arguments, each representing a dependency. Calling a static factory method with specific arguments
to construct the bean is nearly equivalent, and this discussion treats arguments to a constructor and to
a static factory method similarly. The following example shows a class that can only be dependencyinjected with constructor injection. Notice that there is nothing special about this class, it is a POJO that
has no dependencies on container specific interfaces, base classes or annotations.
public class SimpleMovieLister {
// the SimpleMovieLister has a dependency on a MovieFinder
private MovieFinder movieFinder;
// a constructor so that the Spring container can inject a MovieFinder
public SimpleMovieLister(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// business logic that actually uses the injected MovieFinder is omitted...
}
Constructor argument resolution
Constructor argument resolution matching occurs using the argument’s type. If no potential ambiguity
exists in the constructor arguments of a bean definition, then the order in which the constructor
arguments are defined in a bean definition is the order in which those arguments are supplied to the
appropriate constructor when the bean is being instantiated. Consider the following class:
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package x.y;
public class Foo {
public Foo(Bar bar, Baz baz) {
// ...
}
}
No potential ambiguity exists, assuming that Bar and Baz classes are not related by inheritance. Thus
the following configuration works fine, and you do not need to specify the constructor argument indexes
and/or types explicitly in the <constructor-arg/> element.
<beans>
<bean id="foo" class="x.y.Foo">
<constructor-arg ref="bar"/>
<constructor-arg ref="baz"/>
</bean>
<bean id="bar" class="x.y.Bar"/>
<bean id="baz" class="x.y.Baz"/>
</beans>
When another bean is referenced, the type is known, and matching can occur (as was the case with
the preceding example). When a simple type is used, such as <value>true</value>, Spring cannot
determine the type of the value, and so cannot match by type without help. Consider the following class:
package examples;
public class ExampleBean {
// Number of years to calculate the Ultimate Answer
private int years;
// The Answer to Life, the Universe, and Everything
private String ultimateAnswer;
public ExampleBean(int years, String ultimateAnswer) {
this.years = years;
this.ultimateAnswer = ultimateAnswer;
}
}
In the preceding scenario, the container can use type matching with simple types if you explicitly specify
the type of the constructor argument using the type attribute. For example:
<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg type="int" value="7500000"/>
<constructor-arg type="java.lang.String" value="42"/>
</bean>
Use the index attribute to specify explicitly the index of constructor arguments. For example:
<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg index="0" value="7500000"/>
<constructor-arg index="1" value="42"/>
</bean>
In addition to resolving the ambiguity of multiple simple values, specifying an index resolves ambiguity
where a constructor has two arguments of the same type. Note that the index is 0 based.
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You can also use the constructor parameter name for value disambiguation:
<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg name="years" value="7500000"/>
<constructor-arg name="ultimateAnswer" value="42"/>
</bean>
Keep in mind that to make this work out of the box your code must be compiled with the debug flag
enabled so that Spring can look up the parameter name from the constructor. If you can’t compile your
code with debug flag (or don’t want to) you can use @ConstructorProperties JDK annotation to explicitly
name your constructor arguments. The sample class would then have to look as follows:
package examples;
public class ExampleBean {
// Fields omitted
@ConstructorProperties({"years", "ultimateAnswer"})
public ExampleBean(int years, String ultimateAnswer) {
this.years = years;
this.ultimateAnswer = ultimateAnswer;
}
}
Setter-based dependency injection
Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a
no-argument constructor or no-argument static factory method to instantiate your bean.
The following example shows a class that can only be dependency-injected using pure setter injection.
This class is conventional Java. It is a POJO that has no dependencies on container specific interfaces,
base classes or annotations.
public class SimpleMovieLister {
// the SimpleMovieLister has a dependency on the MovieFinder
private MovieFinder movieFinder;
// a setter method so that the Spring container can inject a MovieFinder
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// business logic that actually uses the injected MovieFinder is omitted...
}
The ApplicationContext supports constructor-based and setter-based DI for the beans it manages.
It also supports setter-based DI after some dependencies have already been injected through the
constructor approach. You configure the dependencies in the form of a BeanDefinition, which
you use in conjunction with PropertyEditor instances to convert properties from one format to
another. However, most Spring users do not work with these classes directly (i.e., programmatically) but
rather with XML bean definitions, annotated components (i.e., classes annotated with @Component,
@Controller, etc.), or @Bean methods in Java-based @Configuration classes. These sources are
then converted internally into instances of BeanDefinition and used to load an entire Spring IoC
container instance.
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Constructor-based or setter-based DI?
Since you can mix constructor-based and setter-based DI, it is a good rule of thumb to use
constructors for mandatory dependencies and setter methods or configuration methods for
optional dependencies. Note that use of the @Required annotation on a setter method can be
used to make the property a required dependency.
The Spring team generally advocates constructor injection as it enables one to implement
application components as immutable objects and to ensure that required dependencies are not
null. Furthermore constructor-injected components are always returned to client (calling) code
in a fully initialized state. As a side note, a large number of constructor arguments is a bad code
smell, implying that the class likely has too many responsibilities and should be refactored to better
address proper separation of concerns.
Setter injection should primarily only be used for optional dependencies that can be assigned
reasonable default values within the class. Otherwise, not-null checks must be performed
everywhere the code uses the dependency. One benefit of setter injection is that setter methods
make objects of that class amenable to reconfiguration or re-injection later. Management through
JMX MBeans is therefore a compelling use case for setter injection.
Use the DI style that makes the most sense for a particular class. Sometimes, when dealing with
third-party classes for which you do not have the source, the choice is made for you. For example,
if a third-party class does not expose any setter methods, then constructor injection may be the
only available form of DI.
Dependency resolution process
The container performs bean dependency resolution as follows:
• The ApplicationContext is created and initialized with configuration metadata that describes all
the beans. Configuration metadata can be specified via XML, Java code, or annotations.
• For each bean, its dependencies are expressed in the form of properties, constructor arguments, or
arguments to the static-factory method if you are using that instead of a normal constructor. These
dependencies are provided to the bean, when the bean is actually created.
• Each property or constructor argument is an actual definition of the value to set, or a reference to
another bean in the container.
• Each property or constructor argument which is a value is converted from its specified format to the
actual type of that property or constructor argument. By default Spring can convert a value supplied
in string format to all built-in types, such as int, long, String, boolean, etc.
The Spring container validates the configuration of each bean as the container is created. However,
the bean properties themselves are not set until the bean is actually created. Beans that are singletonscoped and set to be pre-instantiated (the default) are created when the container is created. Scopes
are defined in Section 6.5, “Bean scopes”. Otherwise, the bean is created only when it is requested.
Creation of a bean potentially causes a graph of beans to be created, as the bean’s dependencies and
its dependencies' dependencies (and so on) are created and assigned. Note that resolution mismatches
among those dependencies may show up late, i.e. on first creation of the affected bean.
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Circular dependencies
If you use predominantly constructor injection, it is possible to create an unresolvable circular
dependency scenario.
For example: Class A requires an instance of class B through constructor injection, and class B
requires an instance of class A through constructor injection. If you configure beans for classes
A and B to be injected into each other, the Spring IoC container detects this circular reference at
runtime, and throws a BeanCurrentlyInCreationException.
One possible solution is to edit the source code of some classes to be configured by setters
rather than constructors. Alternatively, avoid constructor injection and use setter injection only. In
other words, although it is not recommended, you can configure circular dependencies with setter
injection.
Unlike the typical case (with no circular dependencies), a circular dependency between bean A
and bean B forces one of the beans to be injected into the other prior to being fully initialized itself
(a classic chicken/egg scenario).
You can generally trust Spring to do the right thing. It detects configuration problems, such as references
to non-existent beans and circular dependencies, at container load-time. Spring sets properties and
resolves dependencies as late as possible, when the bean is actually created. This means that a Spring
container which has loaded correctly can later generate an exception when you request an object if there
is a problem creating that object or one of its dependencies. For example, the bean throws an exception
as a result of a missing or invalid property. This potentially delayed visibility of some configuration issues
is why ApplicationContext implementations by default pre-instantiate singleton beans. At the cost
of some upfront time and memory to create these beans before they are actually needed, you discover
configuration issues when the ApplicationContext is created, not later. You can still override this
default behavior so that singleton beans will lazy-initialize, rather than be pre-instantiated.
If no circular dependencies exist, when one or more collaborating beans are being injected into a
dependent bean, each collaborating bean is totally configured prior to being injected into the dependent
bean. This means that if bean A has a dependency on bean B, the Spring IoC container completely
configures bean B prior to invoking the setter method on bean A. In other words, the bean is instantiated
(if not a pre-instantiated singleton), its dependencies are set, and the relevant lifecycle methods (such
as a configured init method or the InitializingBean callback method) are invoked.
Examples of dependency injection
The following example uses XML-based configuration metadata for setter-based DI. A small part of a
Spring XML configuration file specifies some bean definitions:
<bean id="exampleBean" class="examples.ExampleBean">
<!-- setter injection using the nested ref element -->
<property name="beanOne">
<ref bean="anotherExampleBean"/>
</property>
<!-- setter injection using the neater ref attribute -->
<property name="beanTwo" ref="yetAnotherBean"/>
<property name="integerProperty" value="1"/>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
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public class ExampleBean {
private AnotherBean beanOne;
private YetAnotherBean beanTwo;
private int i;
public void setBeanOne(AnotherBean beanOne) {
this.beanOne = beanOne;
}
public void setBeanTwo(YetAnotherBean beanTwo) {
this.beanTwo = beanTwo;
}
public void setIntegerProperty(int i) {
this.i = i;
}
}
In the preceding example, setters are declared to match against the properties specified in the XML file.
The following example uses constructor-based DI:
<bean id="exampleBean" class="examples.ExampleBean">
<!-- constructor injection using the nested ref element -->
<constructor-arg>
<ref bean="anotherExampleBean"/>
</constructor-arg>
<!-- constructor injection using the neater ref attribute -->
<constructor-arg ref="yetAnotherBean"/>
<constructor-arg type="int" value="1"/>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {
private AnotherBean beanOne;
private YetAnotherBean beanTwo;
private int i;
public ExampleBean(
AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
this.beanOne = anotherBean;
this.beanTwo = yetAnotherBean;
this.i = i;
}
}
The constructor arguments specified in the bean definition will be used as arguments to the constructor
of the ExampleBean.
Now consider a variant of this example, where instead of using a constructor, Spring is told to call a
static factory method to return an instance of the object:
<bean id="exampleBean" class="examples.ExampleBean" factory-method="createInstance">
<constructor-arg ref="anotherExampleBean"/>
<constructor-arg ref="yetAnotherBean"/>
<constructor-arg value="1"/>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
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public class ExampleBean {
// a private constructor
private ExampleBean(...) {
...
}
// a static factory method; the arguments to this method can be
// considered the dependencies of the bean that is returned,
// regardless of how those arguments are actually used.
public static ExampleBean createInstance (
AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
ExampleBean eb = new ExampleBean (...);
// some other operations...
return eb;
}
}
Arguments to the static factory method are supplied via <constructor-arg/> elements, exactly
the same as if a constructor had actually been used. The type of the class being returned by the factory
method does not have to be of the same type as the class that contains the static factory method,
although in this example it is. An instance (non-static) factory method would be used in an essentially
identical fashion (aside from the use of the factory-bean attribute instead of the class attribute),
so details will not be discussed here.
Dependencies and configuration in detail
As mentioned in the previous section, you can define bean properties and constructor arguments as
references to other managed beans (collaborators), or as values defined inline. Spring’s XML-based
configuration metadata supports sub-element types within its <property/> and <constructorarg/> elements for this purpose.
Straight values (primitives, Strings, and so on)
The value attribute of the <property/> element specifies a property or constructor argument as a
human-readable string representation. Spring’s conversion service is used to convert these values from
a String to the actual type of the property or argument.
<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<!-- results in a setDriverClassName(String) call -->
<property name="driverClassName" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/mydb"/>
<property name="username" value="root"/>
<property name="password" value="masterkaoli"/>
</bean>
The following example uses the p-namespace for even more succinct XML configuration.
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<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource"
destroy-method="close"
p:driverClassName="com.mysql.jdbc.Driver"
p:url="jdbc:mysql://localhost:3306/mydb"
p:username="root"
p:password="masterkaoli"/>
</beans>
The preceding XML is more succinct; however, typos are discovered at runtime rather than design time,
unless you use an IDE such as IntelliJ IDEA or the Spring Tool Suite (STS) that support automatic
property completion when you create bean definitions. Such IDE assistance is highly recommended.
You can also configure a java.util.Properties instance as:
<bean id="mappings"
class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<!-- typed as a java.util.Properties -->
<property name="properties">
<value>
jdbc.driver.className=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mydb
</value>
</property>
</bean>
The Spring container converts the text inside the <value/> element into a java.util.Properties
instance by using the JavaBeans PropertyEditor mechanism. This is a nice shortcut, and is one of
a few places where the Spring team do favor the use of the nested <value/> element over the value
attribute style.
The idref element
The idref element is simply an error-proof way to pass the id (string value - not a reference) of another
bean in the container to a <constructor-arg/> or <property/> element.
<bean id="theTargetBean" class="..."/>
<bean id="theClientBean" class="...">
<property name="targetName">
<idref bean="theTargetBean" />
</property>
</bean>
The above bean definition snippet is exactly equivalent (at runtime) to the following snippet:
<bean id="theTargetBean" class="..." />
<bean id="client" class="...">
<property name="targetName" value="theTargetBean" />
</bean>
The first form is preferable to the second, because using the idref tag allows the container to validate at
deployment time that the referenced, named bean actually exists. In the second variation, no validation
is performed on the value that is passed to the targetName property of the client bean. Typos are
only discovered (with most likely fatal results) when the client bean is actually instantiated. If the
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client bean is a prototype bean, this typo and the resulting exception may only be discovered long
after the container is deployed.
Note
The local attribute on the idref element is no longer supported in the 4.0 beans xsd since
it does not provide value over a regular bean reference anymore. Simply change your existing
idref local references to idref bean when upgrading to the 4.0 schema.
A common place (at least in versions earlier than Spring 2.0) where the <idref/> element brings value
is in the configuration of AOP interceptors in a ProxyFactoryBean bean definition. Using <idref/>
elements when you specify the interceptor names prevents you from misspelling an interceptor id.
References to other beans (collaborators)
The ref element is the final element inside a <constructor-arg/> or <property/> definition
element. Here you set the value of the specified property of a bean to be a reference to another
bean (a collaborator) managed by the container. The referenced bean is a dependency of the bean
whose property will be set, and it is initialized on demand as needed before the property is set. (If
the collaborator is a singleton bean, it may be initialized already by the container.) All references are
ultimately a reference to another object. Scoping and validation depend on whether you specify the id/
name of the other object through the bean, local, or parent attributes.
Specifying the target bean through the bean attribute of the <ref/> tag is the most general form, and
allows creation of a reference to any bean in the same container or parent container, regardless of
whether it is in the same XML file. The value of the bean attribute may be the same as the id attribute
of the target bean, or as one of the values in the name attribute of the target bean.
<ref bean="someBean"/>
Specifying the target bean through the parent attribute creates a reference to a bean that is in a parent
container of the current container. The value of the parent attribute may be the same as either the id
attribute of the target bean, or one of the values in the name attribute of the target bean, and the target
bean must be in a parent container of the current one. You use this bean reference variant mainly when
you have a hierarchy of containers and you want to wrap an existing bean in a parent container with a
proxy that will have the same name as the parent bean.
<!-- in the parent context -->
<bean id="accountService" class="com.foo.SimpleAccountService">
<!-- insert dependencies as required as here -->
</bean>
<!-- in the child (descendant) context -->
<bean id="accountService" <!-- bean name is the same as the parent bean -->
class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="target">
<ref parent="accountService"/> <!-- notice how we refer to the parent bean -->
</property>
<!-- insert other configuration and dependencies as required here -->
</bean>
Note
The local attribute on the ref element is no longer supported in the 4.0 beans xsd since it
does not provide value over a regular bean reference anymore. Simply change your existing ref
local references to ref bean when upgrading to the 4.0 schema.
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Inner beans
A <bean/> element inside the <property/> or <constructor-arg/> elements defines a so-called
inner bean.
<bean id="outer" class="...">
<!-- instead of using a reference to a target bean, simply define the target bean inline -->
<property name="target">
<bean class="com.example.Person"> <!-- this is the inner bean -->
<property name="name" value="Fiona Apple"/>
<property name="age" value="25"/>
</bean>
</property>
</bean>
An inner bean definition does not require a defined id or name; if specified, the container does not use
such a value as an identifier. The container also ignores the scope flag on creation: Inner beans are
always anonymous and they are always created with the outer bean. It is not possible to inject inner
beans into collaborating beans other than into the enclosing bean or to access them independently.
As a corner case, it is possible to receive destruction callbacks from a custom scope, e.g. for a requestscoped inner bean contained within a singleton bean: The creation of the inner bean instance will be tied
to its containing bean, but destruction callbacks allow it to participate in the request scope’s lifecycle.
This is not a common scenario; inner beans typically simply share their containing bean’s scope.
Collections
In the <list/>, <set/>, <map/>, and <props/> elements, you set the properties and arguments of
the Java Collection types List, Set, Map, and Properties, respectively.
<bean id="moreComplexObject" class="example.ComplexObject">
<!-- results in a setAdminEmails(java.util.Properties) call -->
<property name="adminEmails">
<props>
<prop key="administrator">administrator@example.org</prop>
<prop key="support">support@example.org</prop>
<prop key="development">development@example.org</prop>
</props>
</property>
<!-- results in a setSomeList(java.util.List) call -->
<property name="someList">
<list>
<value>a list element followed by a reference</value>
<ref bean="myDataSource" />
</list>
</property>
<!-- results in a setSomeMap(java.util.Map) call -->
<property name="someMap">
<map>
<entry key="an entry" value="just some string"/>
<entry key ="a ref" value-ref="myDataSource"/>
</map>
</property>
<!-- results in a setSomeSet(java.util.Set) call -->
<property name="someSet">
<set>
<value>just some string</value>
<ref bean="myDataSource" />
</set>
</property>
</bean>
The value of a map key or value, or a set value, can also again be any of the following elements:
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bean | ref | idref | list | set | map | props | value | null
Collection merging
The Spring container also supports the merging of collections. An application developer can define a
parent-style <list/>, <map/>, <set/> or <props/> element, and have child-style <list/>, <map/
>, <set/> or <props/> elements inherit and override values from the parent collection. That is, the
child collection’s values are the result of merging the elements of the parent and child collections, with
the child’s collection elements overriding values specified in the parent collection.
This section on merging discusses the parent-child bean mechanism. Readers unfamiliar with parent
and child bean definitions may wish to read the relevant section before continuing.
The following example demonstrates collection merging:
<beans>
<bean id="parent" abstract="true" class="example.ComplexObject">
<property name="adminEmails">
<props>
<prop key="administrator">administrator@example.com</prop>
<prop key="support">support@example.com</prop>
</props>
</property>
</bean>
<bean id="child" parent="parent">
<property name="adminEmails">
<!-- the merge is specified on the child collection definition -->
<props merge="true">
<prop key="sales">sales@example.com</prop>
<prop key="support">support@example.co.uk</prop>
</props>
</property>
</bean>
<beans>
Notice the use of the merge=true attribute on the <props/> element of the adminEmails property
of the child bean definition. When the child bean is resolved and instantiated by the container, the
resulting instance has an adminEmails Properties collection that contains the result of the merging
of the child’s adminEmails collection with the parent’s adminEmails collection.
administrator=administrator@example.com
sales=sales@example.com
support=support@example.co.uk
The child Properties collection’s value set inherits all property elements from the parent <props/>,
and the child’s value for the support value overrides the value in the parent collection.
This merging behavior applies similarly to the <list/>, <map/>, and <set/> collection types. In the
specific case of the <list/> element, the semantics associated with the List collection type, that is,
the notion of an ordered collection of values, is maintained; the parent’s values precede all of the child
list’s values. In the case of the Map, Set, and Properties collection types, no ordering exists. Hence
no ordering semantics are in effect for the collection types that underlie the associated Map, Set, and
Properties implementation types that the container uses internally.
Limitations of collection merging
You cannot merge different collection types (such as a Map and a List), and if you do attempt to do
so an appropriate Exception is thrown. The merge attribute must be specified on the lower, inherited,
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child definition; specifying the merge attribute on a parent collection definition is redundant and will not
result in the desired merging.
Strongly-typed collection
With the introduction of generic types in Java 5, you can use strongly typed collections. That is, it is
possible to declare a Collection type such that it can only contain String elements (for example).
If you are using Spring to dependency-inject a strongly-typed Collection into a bean, you can
take advantage of Spring’s type-conversion support such that the elements of your strongly-typed
Collection instances are converted to the appropriate type prior to being added to the Collection.
public class Foo {
private Map<String, Float> accounts;
public void setAccounts(Map<String, Float> accounts) {
this.accounts = accounts;
}
}
<beans>
<bean id="foo" class="x.y.Foo">
<property name="accounts">
<map>
<entry key="one" value="9.99"/>
<entry key="two" value="2.75"/>
<entry key="six" value="3.99"/>
</map>
</property>
</bean>
</beans>
When the accounts property of the foo bean is prepared for injection, the generics information about
the element type of the strongly-typed Map<String, Float> is available by reflection. Thus Spring’s
type conversion infrastructure recognizes the various value elements as being of type Float, and the
string values 9.99, 2.75, and 3.99 are converted into an actual Float type.
Null and empty string values
Spring treats empty arguments for properties and the like as empty Strings. The following XML-based
configuration metadata snippet sets the email property to the empty String value ("").
<bean class="ExampleBean">
<property name="email" value=""/>
</bean>
The preceding example is equivalent to the following Java code:
exampleBean.setEmail("")
The <null/> element handles null values. For example:
<bean class="ExampleBean">
<property name="email">
<null/>
</property>
</bean>
The above configuration is equivalent to the following Java code:
exampleBean.setEmail(null)
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XML shortcut with the p-namespace
The p-namespace enables you to use the bean element’s attributes, instead of nested <property/>
elements, to describe your property values and/or collaborating beans.
Spring supports extensible configuration formats with namespaces, which are based on an XML Schema
definition. The beans configuration format discussed in this chapter is defined in an XML Schema
document. However, the p-namespace is not defined in an XSD file and exists only in the core of Spring.
The following example shows two XML snippets that resolve to the same result: The first uses standard
XML format and the second uses the p-namespace.
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean name="classic" class="com.example.ExampleBean">
<property name="email" value="foo@bar.com"/>
</bean>
<bean name="p-namespace" class="com.example.ExampleBean"
p:email="foo@bar.com"/>
</beans>
The example shows an attribute in the p-namespace called email in the bean definition. This tells Spring
to include a property declaration. As previously mentioned, the p-namespace does not have a schema
definition, so you can set the name of the attribute to the property name.
This next example includes two more bean definitions that both have a reference to another bean:
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean name="john-classic" class="com.example.Person">
<property name="name" value="John Doe"/>
<property name="spouse" ref="jane"/>
</bean>
<bean name="john-modern"
class="com.example.Person"
p:name="John Doe"
p:spouse-ref="jane"/>
<bean name="jane" class="com.example.Person">
<property name="name" value="Jane Doe"/>
</bean>
</beans>
As you can see, this example includes not only a property value using the p-namespace, but also uses
a special format to declare property references. Whereas the first bean definition uses <property
name="spouse" ref="jane"/> to create a reference from bean john to bean jane, the second
bean definition uses p:spouse-ref="jane" as an attribute to do the exact same thing. In this case
spouse is the property name, whereas the -ref part indicates that this is not a straight value but rather
a reference to another bean.
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Note
The p-namespace is not as flexible as the standard XML format. For example, the format for
declaring property references clashes with properties that end in Ref, whereas the standard XML
format does not. We recommend that you choose your approach carefully and communicate this
to your team members, to avoid producing XML documents that use all three approaches at the
same time.
XML shortcut with the c-namespace
Similar to the the section called “XML shortcut with the p-namespace”, the c-namespace, newly
introduced in Spring 3.1, allows usage of inlined attributes for configuring the constructor arguments
rather then nested constructor-arg elements.
Let’s review the examples from the section called “Constructor-based dependency injection” with the
c: namespace:
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:c="http://www.springframework.org/schema/c"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="bar" class="x.y.Bar"/>
<bean id="baz" class="x.y.Baz"/>
<!-- traditional declaration -->
<bean id="foo" class="x.y.Foo">
<constructor-arg ref="bar"/>
<constructor-arg ref="baz"/>
<constructor-arg value="foo@bar.com"/>
</bean>
<!-- c-namespace declaration -->
<bean id="foo" class="x.y.Foo" c:bar-ref="bar" c:baz-ref="baz" c:email="foo@bar.com"/>
</beans>
The c: namespace uses the same conventions as the p: one (trailing -ref for bean references) for
setting the constructor arguments by their names. And just as well, it needs to be declared even though
it is not defined in an XSD schema (but it exists inside the Spring core).
For the rare cases where the constructor argument names are not available (usually if the bytecode was
compiled without debugging information), one can use fallback to the argument indexes:
<!-- c-namespace index declaration -->
<bean id="foo" class="x.y.Foo" c:_0-ref="bar" c:_1-ref="baz"/>
Note
Due to the XML grammar, the index notation requires the presence of the leading _ as XML
attribute names cannot start with a number (even though some IDE allow it).
In practice, the constructor resolution mechanism is quite efficient in matching arguments so unless one
really needs to, we recommend using the name notation through-out your configuration.
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Compound property names
You can use compound or nested property names when you set bean properties, as long as all
components of the path except the final property name are not null. Consider the following bean
definition.
<bean id="foo" class="foo.Bar">
<property name="fred.bob.sammy" value="123" />
</bean>
The foo bean has a fred property, which has a bob property, which has a sammy property, and that final
sammy property is being set to the value 123. In order for this to work, the fred property of foo, and the
bob property of fred must not be null after the bean is constructed, or a NullPointerException
is thrown.
Using depends-on
If a bean is a dependency of another that usually means that one bean is set as a property of another.
Typically you accomplish this with the <ref/> element in XML-based configuration metadata. However,
sometimes dependencies between beans are less direct; for example, a static initializer in a class needs
to be triggered, such as database driver registration. The depends-on attribute can explicitly force one
or more beans to be initialized before the bean using this element is initialized. The following example
uses the depends-on attribute to express a dependency on a single bean:
<bean id="beanOne" class="ExampleBean" depends-on="manager"/>
<bean id="manager" class="ManagerBean" />
To express a dependency on multiple beans, supply a list of bean names as the value of the dependson attribute, with commas, whitespace and semicolons, used as valid delimiters:
<bean id="beanOne" class="ExampleBean" depends-on="manager,accountDao">
<property name="manager" ref="manager" />
</bean>
<bean id="manager" class="ManagerBean" />
<bean id="accountDao" class="x.y.jdbc.JdbcAccountDao" />
Note
The depends-on attribute in the bean definition can specify both an initialization time dependency
and, in the case of singleton beans only, a corresponding destroy time dependency. Dependent
beans that define a depends-on relationship with a given bean are destroyed first, prior to the
given bean itself being destroyed. Thus depends-on can also control shutdown order.
Lazy-initialized beans
By default, ApplicationContext implementations eagerly create and configure all singleton beans
as part of the initialization process. Generally, this pre-instantiation is desirable, because errors in the
configuration or surrounding environment are discovered immediately, as opposed to hours or even
days later. When this behavior is not desirable, you can prevent pre-instantiation of a singleton bean by
marking the bean definition as lazy-initialized. A lazy-initialized bean tells the IoC container to create a
bean instance when it is first requested, rather than at startup.
In XML, this behavior is controlled by the lazy-init attribute on the <bean/> element; for example:
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<bean id="lazy" class="com.foo.ExpensiveToCreateBean" lazy-init="true"/>
<bean name="not.lazy" class="com.foo.AnotherBean"/>
When the preceding configuration is consumed by an ApplicationContext, the bean named lazy
is not eagerly pre-instantiated when the ApplicationContext is starting up, whereas the not.lazy
bean is eagerly pre-instantiated.
However, when a lazy-initialized bean is a dependency of a singleton bean that is not lazy-initialized,
the ApplicationContext creates the lazy-initialized bean at startup, because it must satisfy the
singleton’s dependencies. The lazy-initialized bean is injected into a singleton bean elsewhere that is
not lazy-initialized.
You can also control lazy-initialization at the container level by using the default-lazy-init attribute
on the <beans/> element; for example:
<beans default-lazy-init="true">
<!-- no beans will be pre-instantiated... -->
</beans>
Autowiring collaborators
The Spring container can autowire relationships between collaborating beans. You can allow Spring
to resolve collaborators (other beans) automatically for your bean by inspecting the contents of the
ApplicationContext. Autowiring has the following advantages:
• Autowiring can significantly reduce the need to specify properties or constructor arguments. (Other
mechanisms such as a bean template discussed elsewhere in this chapter are also valuable in this
regard.)
• Autowiring can update a configuration as your objects evolve. For example, if you need to add a
dependency to a class, that dependency can be satisfied automatically without you needing to modify
the configuration. Thus autowiring can be especially useful during development, without negating the
option of switching to explicit wiring when the code base becomes more stable.
10
When using XML-based configuration metadata , you specify autowire mode for a bean definition
with the autowire attribute of the <bean/> element. The autowiring functionality has four modes. You
specify autowiring per bean and thus can choose which ones to autowire.
Table 6.2. Autowiring modes
Mode
Explanation
no
(Default) No autowiring. Bean references must
be defined via a ref element. Changing the
default setting is not recommended for larger
deployments, because specifying collaborators
explicitly gives greater control and clarity. To
some extent, it documents the structure of a
system.
byName
Autowiring by property name. Spring looks for
a bean with the same name as the property
10
See the section called “Dependency Injection”
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Mode
Explanation
that needs to be autowired. For example, if a
bean definition is set to autowire by name, and
it contains a master property (that is, it has a
setMaster(..) method), Spring looks for a bean
definition named master, and uses it to set the
property.
byType
Allows a property to be autowired if exactly one
bean of the property type exists in the container.
If more than one exists, a fatal exception is
thrown, which indicates that you may not use
byType autowiring for that bean. If there are no
matching beans, nothing happens; the property
is not set.
constructor
Analogous to byType, but applies to constructor
arguments. If there is not exactly one bean of
the constructor argument type in the container, a
fatal error is raised.
With byType or constructor autowiring mode, you can wire arrays and typed-collections. In such cases
all autowire candidates within the container that match the expected type are provided to satisfy the
dependency. You can autowire strongly-typed Maps if the expected key type is String. An autowired
Maps values will consist of all bean instances that match the expected type, and the Maps keys will
contain the corresponding bean names.
You can combine autowire behavior with dependency checking, which is performed after autowiring
completes.
Limitations and disadvantages of autowiring
Autowiring works best when it is used consistently across a project. If autowiring is not used in general,
it might be confusing to developers to use it to wire only one or two bean definitions.
Consider the limitations and disadvantages of autowiring:
• Explicit dependencies in property and constructor-arg settings always override autowiring.
You cannot autowire so-called simple properties such as primitives, Strings, and Classes (and
arrays of such simple properties). This limitation is by-design.
• Autowiring is less exact than explicit wiring. Although, as noted in the above table, Spring is careful
to avoid guessing in case of ambiguity that might have unexpected results, the relationships between
your Spring-managed objects are no longer documented explicitly.
• Wiring information may not be available to tools that may generate documentation from a Spring
container.
• Multiple bean definitions within the container may match the type specified by the setter method
or constructor argument to be autowired. For arrays, collections, or Maps, this is not necessarily
a problem. However for dependencies that expect a single value, this ambiguity is not arbitrarily
resolved. If no unique bean definition is available, an exception is thrown.
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In the latter scenario, you have several options:
• Abandon autowiring in favor of explicit wiring.
• Avoid autowiring for a bean definition by setting its autowire-candidate attributes to false as
described in the next section.
• Designate a single bean definition as the primary candidate by setting the primary attribute of its
<bean/> element to true.
• Implement the more fine-grained control available with annotation-based configuration, as described
in Section 6.9, “Annotation-based container configuration”.
Excluding a bean from autowiring
On a per-bean basis, you can exclude a bean from autowiring. In Spring’s XML format, set the
autowire-candidate attribute of the <bean/> element to false; the container makes that specific
bean definition unavailable to the autowiring infrastructure (including annotation style configurations
such as @Autowired).
You can also limit autowire candidates based on pattern-matching against bean names. The toplevel <beans/> element accepts one or more patterns within its default-autowire-candidates
attribute. For example, to limit autowire candidate status to any bean whose name ends with Repository,
provide a value of *Repository. To provide multiple patterns, define them in a comma-separated list. An
explicit value of true or false for a bean definitions autowire-candidate attribute always takes
precedence, and for such beans, the pattern matching rules do not apply.
These techniques are useful for beans that you never want to be injected into other beans by autowiring.
It does not mean that an excluded bean cannot itself be configured using autowiring. Rather, the bean
itself is not a candidate for autowiring other beans.
Method injection
In most application scenarios, most beans in the container are singletons. When a singleton bean needs
to collaborate with another singleton bean, or a non-singleton bean needs to collaborate with another
non-singleton bean, you typically handle the dependency by defining one bean as a property of the
other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use
non-singleton (prototype) bean B, perhaps on each method invocation on A. The container only creates
the singleton bean A once, and thus only gets one opportunity to set the properties. The container cannot
provide bean A with a new instance of bean B every time one is needed.
A solution is to forego some inversion of control. You can make bean A aware of the container by
implementing the ApplicationContextAware interface, and by making a getBean("B") call to the
container ask for (a typically new) bean B instance every time bean A needs it. The following is an
example of this approach:
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// a class that uses a stateful Command-style class to perform some processing
package fiona.apple;
// Spring-API imports
import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
public class CommandManager implements ApplicationContextAware {
private ApplicationContext applicationContext;
public Object process(Map commandState) {
// grab a new instance of the appropriate Command
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
protected Command createCommand() {
// notice the Spring API dependency!
return this.applicationContext.getBean("command", Command.class);
}
public void setApplicationContext(
ApplicationContext applicationContext) throws BeansException {
this.applicationContext = applicationContext;
}
}
The preceding is not desirable, because the business code is aware of and coupled to the Spring
Framework. Method Injection, a somewhat advanced feature of the Spring IoC container, allows this
use case to be handled in a clean fashion.
You can read more about the motivation for Method Injection in this blog entry.
Lookup method injection
Lookup method injection is the ability of the container to override methods on container managed beans,
to return the lookup result for another named bean in the container. The lookup typically involves a
prototype bean as in the scenario described in the preceding section. The Spring Framework implements
this method injection by using bytecode generation from the CGLIB library to generate dynamically a
subclass that overrides the method.
Note
• For this dynamic subclassing to work, the class that the Spring bean container will subclass
cannot be final, and the method to be overridden cannot be final either.
• Unit-testing a class that has an abstract method requires you to subclass the class yourself
and to supply a stub implementation of the abstract method.
• Concrete methods are also necessary for component scanning which requires concrete classes
to pick up.
• A further key limitation is that lookup methods won’t work with factory methods and in particular
not with @Bean methods in configuration classes, since the container is not in charge of creating
the instance in that case and therefore cannot create a runtime-generated subclass on the fly.
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• Finally, objects that have been the target of method injection cannot be serialized.
Looking at the CommandManager class in the previous code snippet, you see that the Spring
container will dynamically override the implementation of the createCommand() method. Your
CommandManager class will not have any Spring dependencies, as can be seen in the reworked
example:
package fiona.apple;
// no more Spring imports!
public abstract class CommandManager {
public Object process(Object commandState) {
// grab a new instance of the appropriate Command interface
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// okay... but where is the implementation of this method?
protected abstract Command createCommand();
}
In the client class containing the method to be injected (the CommandManager in this case), the method
to be injected requires a signature of the following form:
<public|protected> [abstract] <return-type> theMethodName(no-arguments);
If the method is abstract, the dynamically-generated subclass implements the method. Otherwise,
the dynamically-generated subclass overrides the concrete method defined in the original class. For
example:
<!-- a stateful bean deployed as a prototype (non-singleton) -->
<bean id="command" class="fiona.apple.AsyncCommand" scope="prototype">
<!-- inject dependencies here as required -->
</bean>
<!-- commandProcessor uses statefulCommandHelper -->
<bean id="commandManager" class="fiona.apple.CommandManager">
<lookup-method name="createCommand" bean="command"/>
</bean>
The bean identified as commandManager calls its own method createCommand() whenever it needs
a new instance of the command bean. You must be careful to deploy the command bean as a prototype,
if that is actually what is needed. If it is deployed as a singleton, the same instance of the command
bean is returned each time.
Tip
The interested reader may also find the ServiceLocatorFactoryBean (in the
org.springframework.beans.factory.config package) to be of use. The approach
used in ServiceLocatorFactoryBean is similar to that of another utility class,
ObjectFactoryCreatingFactoryBean, but it allows you to specify your own lookup interface
as opposed to a Spring-specific lookup interface. Consult the javadocs of these classes for
additional information.
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Arbitrary method replacement
A less useful form of method injection than lookup method injection is the ability to replace arbitrary
methods in a managed bean with another method implementation. Users may safely skip the rest of
this section until the functionality is actually needed.
With XML-based configuration metadata, you can use the replaced-method element to replace an
existing method implementation with another, for a deployed bean. Consider the following class, with
a method computeValue, which we want to override:
public class MyValueCalculator {
public String computeValue(String input) {
// some real code...
}
// some other methods...
}
A class implementing the org.springframework.beans.factory.support.MethodReplacer
interface provides the new method definition.
/**
* meant to be used to override the existing computeValue(String)
* implementation in MyValueCalculator
*/
public class ReplacementComputeValue implements MethodReplacer {
public Object reimplement(Object o, Method m, Object[] args) throws Throwable {
// get the input value, work with it, and return a computed result
String input = (String) args[0];
...
return ...;
}
}
The bean definition to deploy the original class and specify the method override would look like this:
<bean id="myValueCalculator" class="x.y.z.MyValueCalculator">
<!-- arbitrary method replacement -->
<replaced-method name="computeValue" replacer="replacementComputeValue">
<arg-type>String</arg-type>
</replaced-method>
</bean>
<bean id="replacementComputeValue" class="a.b.c.ReplacementComputeValue"/>
You can use one or more contained <arg-type/> elements within the <replaced-method/>
element to indicate the method signature of the method being overridden. The signature for the
arguments is necessary only if the method is overloaded and multiple variants exist within the class.
For convenience, the type string for an argument may be a substring of the fully qualified type name.
For example, the following all match java.lang.String:
java.lang.String
String
Str
Because the number of arguments is often enough to distinguish between each possible choice, this
shortcut can save a lot of typing, by allowing you to type only the shortest string that will match an
argument type.
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6.5 Bean scopes
When you create a bean definition, you create a recipe for creating actual instances of the class defined
by that bean definition. The idea that a bean definition is a recipe is important, because it means that,
as with a class, you can create many object instances from a single recipe.
You can control not only the various dependencies and configuration values that are to be plugged into
an object that is created from a particular bean definition, but also the scope of the objects created from
a particular bean definition. This approach is powerful and flexible in that you can choose the scope
of the objects you create through configuration instead of having to bake in the scope of an object at
the Java class level. Beans can be defined to be deployed in one of a number of scopes: out of the
box, the Spring Framework supports seven scopes, five of which are available only if you use a webaware ApplicationContext.
The following scopes are supported out of the box. You can also create a custom scope.
Table 6.3. Bean scopes
Scope
Description
singleton
(Default) Scopes a single bean definition to a
single object instance per Spring IoC container.
prototype
Scopes a single bean definition to any number of
object instances.
request
Scopes a single bean definition to the lifecycle
of a single HTTP request; that is, each HTTP
request has its own instance of a bean created
off the back of a single bean definition. Only
valid in the context of a web-aware Spring
ApplicationContext.
session
Scopes a single bean definition to the lifecycle of
an HTTP Session. Only valid in the context of a
web-aware Spring ApplicationContext.
globalSession
Scopes a single bean definition to the lifecycle
of a global HTTP Session. Typically only
valid when used in a Portlet context. Only
valid in the context of a web-aware Spring
ApplicationContext.
application
Scopes a single bean definition to the lifecycle of
a ServletContext. Only valid in the context of
a web-aware Spring ApplicationContext.
websocket
Scopes a single bean definition to the lifecycle
of a WebSocket. Only valid in the context of a
web-aware Spring ApplicationContext.
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Note
As of Spring 3.0, a thread scope is available, but is not registered by default. For more information,
see the documentation for SimpleThreadScope. For instructions on how to register this or any
other custom scope, see the section called “Using a custom scope”.
The singleton scope
Only one shared instance of a singleton bean is managed, and all requests for beans with an id or
ids matching that bean definition result in that one specific bean instance being returned by the Spring
container.
To put it another way, when you define a bean definition and it is scoped as a singleton, the Spring IoC
container creates exactly one instance of the object defined by that bean definition. This single instance
is stored in a cache of such singleton beans, and all subsequent requests and references for that named
bean return the cached object.
Spring’s concept of a singleton bean differs from the Singleton pattern as defined in the Gang of Four
(GoF) patterns book. The GoF Singleton hard-codes the scope of an object such that one and only
one instance of a particular class is created per ClassLoader. The scope of the Spring singleton is best
described as per container and per bean. This means that if you define one bean for a particular class
in a single Spring container, then the Spring container creates one and only one instance of the class
defined by that bean definition. The singleton scope is the default scope in Spring. To define a bean as
a singleton in XML, you would write, for example:
<bean id="accountService" class="com.foo.DefaultAccountService"/>
<!-- the following is equivalent, though redundant (singleton scope is the default) -->
<bean id="accountService" class="com.foo.DefaultAccountService" scope="singleton"/>
The prototype scope
The non-singleton, prototype scope of bean deployment results in the creation of a new bean instance
every time a request for that specific bean is made. That is, the bean is injected into another bean or
you request it through a getBean() method call on the container. As a rule, use the prototype scope
for all stateful beans and the singleton scope for stateless beans.
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The following diagram illustrates the Spring prototype scope. A data access object (DAO) is not typically
configured as a prototype, because a typical DAO does not hold any conversational state; it was just
easier for this author to reuse the core of the singleton diagram.
The following example defines a bean as a prototype in XML:
<bean id="accountService" class="com.foo.DefaultAccountService" scope="prototype"/>
In contrast to the other scopes, Spring does not manage the complete lifecycle of a prototype bean: the
container instantiates, configures, and otherwise assembles a prototype object, and hands it to the client,
with no further record of that prototype instance. Thus, although initialization lifecycle callback methods
are called on all objects regardless of scope, in the case of prototypes, configured destruction lifecycle
callbacks are not called. The client code must clean up prototype-scoped objects and release expensive
resources that the prototype bean(s) are holding. To get the Spring container to release resources held
by prototype-scoped beans, try using a custom bean post-processor, which holds a reference to beans
that need to be cleaned up.
In some respects, the Spring container’s role in regard to a prototype-scoped bean is a replacement
for the Java new operator. All lifecycle management past that point must be handled by the client. (For
details on the lifecycle of a bean in the Spring container, see the section called “Lifecycle callbacks”.)
Singleton beans with prototype-bean dependencies
When you use singleton-scoped beans with dependencies on prototype beans, be aware that
dependencies are resolved at instantiation time. Thus if you dependency-inject a prototype-scoped bean
into a singleton-scoped bean, a new prototype bean is instantiated and then dependency-injected into
the singleton bean. The prototype instance is the sole instance that is ever supplied to the singletonscoped bean.
However, suppose you want the singleton-scoped bean to acquire a new instance of the prototypescoped bean repeatedly at runtime. You cannot dependency-inject a prototype-scoped bean into your
singleton bean, because that injection occurs only once, when the Spring container is instantiating the
singleton bean and resolving and injecting its dependencies. If you need a new instance of a prototype
bean at runtime more than once, see the section called “Method injection”
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Request, session, global session, application, and WebSocket scopes
The request, session, globalSession, application, and websocket scopes are only
available if you use a web-aware Spring ApplicationContext implementation (such as
XmlWebApplicationContext). If you use these scopes with regular Spring IoC containers
such as the ClassPathXmlApplicationContext, an IllegalStateException will be thrown
complaining about an unknown bean scope.
Initial web configuration
To support the scoping of beans at the request, session, globalSession, application, and
websocket levels (web-scoped beans), some minor initial configuration is required before you define
your beans. (This initial setup is not required for the standard scopes, singleton and prototype.)
How you accomplish this initial setup depends on your particular Servlet environment.
If you access scoped beans within Spring Web MVC, in effect, within a request that is processed
by the Spring DispatcherServlet or DispatcherPortlet, then no special setup is necessary:
DispatcherServlet and DispatcherPortlet already expose all relevant state.
If you use a Servlet 2.5 web container, with requests processed outside of
Spring’s DispatcherServlet (for example, when using JSF or Struts), you need
to register the org.springframework.web.context.request.RequestContextListener
ServletRequestListener. For Servlet 3.0+, this can be done programmatically via the
WebApplicationInitializer interface. Alternatively, or for older containers, add the following
declaration to your web application’s web.xml file:
<web-app>
...
<listener>
<listener-class>
org.springframework.web.context.request.RequestContextListener
</listener-class>
</listener>
...
</web-app>
Alternatively, if there are issues with your listener setup, consider using Spring’s
RequestContextFilter. The filter mapping depends on the surrounding web application
configuration, so you have to change it as appropriate.
<web-app>
...
<filter>
<filter-name>requestContextFilter</filter-name>
<filter-class>org.springframework.web.filter.RequestContextFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>requestContextFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
...
</web-app>
DispatcherServlet, RequestContextListener, and RequestContextFilter all do exactly
the same thing, namely bind the HTTP request object to the Thread that is servicing that request. This
makes beans that are request- and session-scoped available further down the call chain.
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Request scope
Consider the following XML configuration for a bean definition:
<bean id="loginAction" class="com.foo.LoginAction" scope="request"/>
The Spring container creates a new instance of the LoginAction bean by using the loginAction
bean definition for each and every HTTP request. That is, the loginAction bean is scoped at the
HTTP request level. You can change the internal state of the instance that is created as much as you
want, because other instances created from the same loginAction bean definition will not see these
changes in state; they are particular to an individual request. When the request completes processing,
the bean that is scoped to the request is discarded.
Session scope
Consider the following XML configuration for a bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>
The Spring container creates a new instance of the UserPreferences bean by using the
userPreferences bean definition for the lifetime of a single HTTP Session. In other words, the
userPreferences bean is effectively scoped at the HTTP Session level. As with request-scoped
beans, you can change the internal state of the instance that is created as much as you want,
knowing that other HTTP Session instances that are also using instances created from the same
userPreferences bean definition do not see these changes in state, because they are particular to an
individual HTTP Session. When the HTTP Session is eventually discarded, the bean that is scoped
to that particular HTTP Session is also discarded.
Global session scope
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="globalSession"/>
The globalSession scope is similar to the standard HTTP Session scope (described above), and
applies only in the context of portlet-based web applications. The portlet specification defines the notion
of a global Session that is shared among all portlets that make up a single portlet web application.
Beans defined at the globalSession scope are scoped (or bound) to the lifetime of the global portlet
Session.
If you write a standard Servlet-based web application and you define one or more beans as having
globalSession scope, the standard HTTP Session scope is used, and no error is raised.
Application scope
Consider the following XML configuration for a bean definition:
<bean id="appPreferences" class="com.foo.AppPreferences" scope="application"/>
The Spring container creates a new instance of the AppPreferences bean by using the
appPreferences bean definition once for the entire web application. That is, the appPreferences
bean is scoped at the ServletContext level, stored as a regular ServletContext attribute. This
is somewhat similar to a Spring singleton bean but differs in two important ways: It is a singleton per
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ServletContext, not per Spring 'ApplicationContext' (for which there may be several in any given
web application), and it is actually exposed and therefore visible as a ServletContext attribute.
Scoped beans as dependencies
The Spring IoC container manages not only the instantiation of your objects (beans), but also the wiring
up of collaborators (or dependencies). If you want to inject (for example) an HTTP request scoped bean
into another bean of a longer-lived scope, you may choose to inject an AOP proxy in place of the scoped
bean. That is, you need to inject a proxy object that exposes the same public interface as the scoped
object but that can also retrieve the real target object from the relevant scope (such as an HTTP request)
and delegate method calls onto the real object.
Note
You may also use <aop:scoped-proxy/> between beans that are scoped as singleton, with
the reference then going through an intermediate proxy that is serializable and therefore able to
re-obtain the target singleton bean on deserialization.
When declaring <aop:scoped-proxy/> against a bean of scope prototype, every method
call on the shared proxy will lead to the creation of a new target instance which the call is then
being forwarded to.
Also, scoped proxies are not the only way to access beans from shorter scopes in a lifecycle-safe
fashion. You may also simply declare your injection point (i.e. the constructor/setter argument
or autowired field) as ObjectFactory<MyTargetBean>, allowing for a getObject() call to
retrieve the current instance on demand every time it is needed - without holding on to the instance
or storing it separately.
The JSR-330 variant of this is called Provider, used with a Provider<MyTargetBean>
declaration and a corresponding get() call for every retrieval attempt. See here for more details
on JSR-330 overall.
The configuration in the following example is only one line, but it is important to understand the "why"
as well as the "how" behind it.
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/aop
http://www.springframework.org/schema/aop/spring-aop.xsd">
<!-- an HTTP Session-scoped bean exposed as a proxy -->
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
<!-- instructs the container to proxy the surrounding bean -->
<aop:scoped-proxy/>
</bean>
<!-- a singleton-scoped bean injected with a proxy to the above bean -->
<bean id="userService" class="com.foo.SimpleUserService">
<!-- a reference to the proxied userPreferences bean -->
<property name="userPreferences" ref="userPreferences"/>
</bean>
</beans>
To create such a proxy, you insert a child <aop:scoped-proxy/> element into a scoped bean
definition (see the section called “Choosing the type of proxy to create” and Chapter 40, XML Schema-
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based configuration). Why do definitions of beans scoped at the request, session, globalSession
and custom-scope levels require the <aop:scoped-proxy/> element? Let’s examine the following
singleton bean definition and contrast it with what you need to define for the aforementioned scopes
(note that the following userPreferences bean definition as it stands is incomplete).
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
In the preceding example, the singleton bean userManager is injected with a reference to the HTTP
Session-scoped bean userPreferences. The salient point here is that the userManager bean is a
singleton: it will be instantiated exactly once per container, and its dependencies (in this case only one,
the userPreferences bean) are also injected only once. This means that the userManager bean
will only operate on the exact same userPreferences object, that is, the one that it was originally
injected with.
This is not the behavior you want when injecting a shorter-lived scoped bean into a longerlived scoped bean, for example injecting an HTTP Session-scoped collaborating bean as a
dependency into singleton bean. Rather, you need a single userManager object, and for the
lifetime of an HTTP Session, you need a userPreferences object that is specific to said HTTP
Session. Thus the container creates an object that exposes the exact same public interface as
the UserPreferences class (ideally an object that is a UserPreferences instance) which can
fetch the real UserPreferences object from the scoping mechanism (HTTP request, Session,
etc.). The container injects this proxy object into the userManager bean, which is unaware that this
UserPreferences reference is a proxy. In this example, when a UserManager instance invokes
a method on the dependency-injected UserPreferences object, it actually is invoking a method on
the proxy. The proxy then fetches the real UserPreferences object from (in this case) the HTTP
Session, and delegates the method invocation onto the retrieved real UserPreferences object.
Thus you need the following, correct and complete, configuration when injecting request-, session-,
and globalSession-scoped beans into collaborating objects:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
<aop:scoped-proxy/>
</bean>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
Choosing the type of proxy to create
By default, when the Spring container creates a proxy for a bean that is marked up with the
<aop:scoped-proxy/> element, a CGLIB-based class proxy is created.
Note
CGLIB proxies only intercept public method calls! Do not call non-public methods on such a proxy;
they will not be delegated to the actual scoped target object.
Alternatively, you can configure the Spring container to create standard JDK interface-based proxies
for such scoped beans, by specifying false for the value of the proxy-target-class attribute of
the <aop:scoped-proxy/> element. Using JDK interface-based proxies means that you do not need
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additional libraries in your application classpath to effect such proxying. However, it also means that the
class of the scoped bean must implement at least one interface, and that all collaborators into which the
scoped bean is injected must reference the bean through one of its interfaces.
<!-- DefaultUserPreferences implements the UserPreferences interface -->
<bean id="userPreferences" class="com.foo.DefaultUserPreferences" scope="session">
<aop:scoped-proxy proxy-target-class="false"/>
</bean>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
For more detailed information about choosing class-based or interface-based proxying, see
Section 10.6, “Proxying mechanisms”.
Custom scopes
The bean scoping mechanism is extensible; You can define your own scopes, or even redefine existing
scopes, although the latter is considered bad practice and you cannot override the built-in singleton
and prototype scopes.
Creating a custom scope
To integrate your custom scope(s) into the Spring container, you need to implement the
org.springframework.beans.factory.config.Scope interface, which is described in this
section. For an idea of how to implement your own scopes, see the Scope implementations that are
supplied with the Spring Framework itself and the Scope javadocs, which explains the methods you
need to implement in more detail.
The Scope interface has four methods to get objects from the scope, remove them from the scope,
and allow them to be destroyed.
The following method returns the object from the underlying scope. The session scope implementation,
for example, returns the session-scoped bean (and if it does not exist, the method returns a new instance
of the bean, after having bound it to the session for future reference).
Object get(String name, ObjectFactory objectFactory)
The following method removes the object from the underlying scope. The session scope implementation
for example, removes the session-scoped bean from the underlying session. The object should be
returned, but you can return null if the object with the specified name is not found.
Object remove(String name)
The following method registers the callbacks the scope should execute when it is destroyed or when
the specified object in the scope is destroyed. Refer to the javadocs or a Spring scope implementation
for more information on destruction callbacks.
void registerDestructionCallback(String name, Runnable destructionCallback)
The following method obtains the conversation identifier for the underlying scope. This identifier is
different for each scope. For a session scoped implementation, this identifier can be the session
identifier.
String getConversationId()
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Using a custom scope
After you write and test one or more custom Scope implementations, you need to make the Spring
container aware of your new scope(s). The following method is the central method to register a new
Scope with the Spring container:
void registerScope(String scopeName, Scope scope);
This method is declared on the ConfigurableBeanFactory interface, which is available on most
of the concrete ApplicationContext implementations that ship with Spring via the BeanFactory
property.
The first argument to the registerScope(..) method is the unique name associated with a
scope; examples of such names in the Spring container itself are singleton and prototype. The
second argument to the registerScope(..) method is an actual instance of the custom Scope
implementation that you wish to register and use.
Suppose that you write your custom Scope implementation, and then register it as below.
Note
The example below uses SimpleThreadScope which is included with Spring, but not registered
by default. The instructions would be the same for your own custom Scope implementations.
Scope threadScope = new SimpleThreadScope();
beanFactory.registerScope("thread", threadScope);
You then create bean definitions that adhere to the scoping rules of your custom Scope:
<bean id="..." class="..." scope="thread">
With a custom Scope implementation, you are not limited to programmatic registration of the scope.
You can also do the Scope registration declaratively, using the CustomScopeConfigurer class:
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<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/aop
http://www.springframework.org/schema/aop/spring-aop.xsd">
<bean class="org.springframework.beans.factory.config.CustomScopeConfigurer">
<property name="scopes">
<map>
<entry key="thread">
<bean class="org.springframework.context.support.SimpleThreadScope"/>
</entry>
</map>
</property>
</bean>
<bean id="bar" class="x.y.Bar" scope="thread">
<property name="name" value="Rick"/>
<aop:scoped-proxy/>
</bean>
<bean id="foo" class="x.y.Foo">
<property name="bar" ref="bar"/>
</bean>
</beans>
Note
When you place <aop:scoped-proxy/> in a FactoryBean implementation, it is the factory
bean itself that is scoped, not the object returned from getObject().
6.6 Customizing the nature of a bean
Lifecycle callbacks
To interact with the container’s management of the bean lifecycle, you can implement
the Spring InitializingBean and DisposableBean interfaces. The container calls
afterPropertiesSet() for the former and destroy() for the latter to allow the bean to perform
certain actions upon initialization and destruction of your beans.
Tip
The JSR-250 @PostConstruct and @PreDestroy annotations are generally considered best
practice for receiving lifecycle callbacks in a modern Spring application. Using these annotations
means that your beans are not coupled to Spring specific interfaces. For details see the section
called “@PostConstruct and @PreDestroy”.
If you don’t want to use the JSR-250 annotations but you are still looking to remove coupling
consider the use of init-method and destroy-method object definition metadata.
Internally, the Spring Framework uses BeanPostProcessor implementations to process any callback
interfaces it can find and call the appropriate methods. If you need custom features or other lifecycle
behavior Spring does not offer out-of-the-box, you can implement a BeanPostProcessor yourself.
For more information, see Section 6.8, “Container Extension Points”.
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In addition to the initialization and destruction callbacks, Spring-managed objects may also implement
the Lifecycle interface so that those objects can participate in the startup and shutdown process as
driven by the container’s own lifecycle.
The lifecycle callback interfaces are described in this section.
Initialization callbacks
The org.springframework.beans.factory.InitializingBean interface allows a bean to
perform initialization work after all necessary properties on the bean have been set by the container.
The InitializingBean interface specifies a single method:
void afterPropertiesSet() throws Exception;
It is recommended that you do not use the InitializingBean interface because it unnecessarily
couples the code to Spring. Alternatively, use the @PostConstruct annotation or specify a POJO
initialization method. In the case of XML-based configuration metadata, you use the init-method
attribute to specify the name of the method that has a void no-argument signature. With Java config,
you use the initMethod attribute of @Bean, see the section called “Receiving lifecycle callbacks”. For
example, the following:
<bean id="exampleInitBean" class="examples.ExampleBean" init-method="init"/>
public class ExampleBean {
public void init() {
// do some initialization work
}
}
…is exactly the same as…
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements InitializingBean {
public void afterPropertiesSet() {
// do some initialization work
}
}
but does not couple the code to Spring.
Destruction callbacks
Implementing the org.springframework.beans.factory.DisposableBean interface allows a
bean to get a callback when the container containing it is destroyed. The DisposableBean interface
specifies a single method:
void destroy() throws Exception;
It is recommended that you do not use the DisposableBean callback interface because it
unnecessarily couples the code to Spring. Alternatively, use the @PreDestroy annotation or specify
a generic method that is supported by bean definitions. With XML-based configuration metadata, you
use the destroy-method attribute on the <bean/>. With Java config, you use the destroyMethod
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attribute of @Bean, see the section called “Receiving lifecycle callbacks”. For example, the following
definition:
<bean id="exampleInitBean" class="examples.ExampleBean" destroy-method="cleanup"/>
public class ExampleBean {
public void cleanup() {
// do some destruction work (like releasing pooled connections)
}
}
is exactly the same as:
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements DisposableBean {
public void destroy() {
// do some destruction work (like releasing pooled connections)
}
}
but does not couple the code to Spring.
Tip
The destroy-method attribute of a <bean> element can be assigned a special (inferred)
value which instructs Spring to automatically detect a public close or shutdown method
on the specific bean class (any class that implements java.lang.AutoCloseable or
java.io.Closeable would therefore match). This special (inferred) value can also be set
on the default-destroy-method attribute of a <beans> element to apply this behavior to an
entire set of beans (see the section called “Default initialization and destroy methods”). Note that
this is the default behavior with Java config.
Default initialization and destroy methods
When you write initialization and destroy method callbacks that do not use the Spring-specific
InitializingBean and DisposableBean callback interfaces, you typically write methods with
names such as init(), initialize(), dispose(), and so on. Ideally, the names of such lifecycle
callback methods are standardized across a project so that all developers use the same method names
and ensure consistency.
You can configure the Spring container to look for named initialization and destroy callback method
names on every bean. This means that you, as an application developer, can write your application
classes and use an initialization callback called init(), without having to configure an initmethod="init" attribute with each bean definition. The Spring IoC container calls that method
when the bean is created (and in accordance with the standard lifecycle callback contract described
previously). This feature also enforces a consistent naming convention for initialization and destroy
method callbacks.
Suppose that your initialization callback methods are named init() and destroy callback methods are
named destroy(). Your class will resemble the class in the following example.
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public class DefaultBlogService implements BlogService {
private BlogDao blogDao;
public void setBlogDao(BlogDao blogDao) {
this.blogDao = blogDao;
}
// this is (unsurprisingly) the initialization callback method
public void init() {
if (this.blogDao == null) {
throw new IllegalStateException("The [blogDao] property must be set.");
}
}
}
<beans default-init-method="init">
<bean id="blogService" class="com.foo.DefaultBlogService">
<property name="blogDao" ref="blogDao" />
</bean>
</beans>
The presence of the default-init-method attribute on the top-level <beans/> element attribute
causes the Spring IoC container to recognize a method called init on beans as the initialization method
callback. When a bean is created and assembled, if the bean class has such a method, it is invoked
at the appropriate time.
You configure destroy method callbacks similarly (in XML, that is) by using the default-destroymethod attribute on the top-level <beans/> element.
Where existing bean classes already have callback methods that are named at variance with the
convention, you can override the default by specifying (in XML, that is) the method name using the
init-method and destroy-method attributes of the <bean/> itself.
The Spring container guarantees that a configured initialization callback is called immediately after
a bean is supplied with all dependencies. Thus the initialization callback is called on the raw bean
reference, which means that AOP interceptors and so forth are not yet applied to the bean. A target
bean is fully created first, then an AOP proxy (for example) with its interceptor chain is applied. If the
target bean and the proxy are defined separately, your code can even interact with the raw target bean,
bypassing the proxy. Hence, it would be inconsistent to apply the interceptors to the init method, because
doing so would couple the lifecycle of the target bean with its proxy/interceptors and leave strange
semantics when your code interacts directly to the raw target bean.
Combining lifecycle mechanisms
As of Spring 2.5, you have three options for controlling bean lifecycle behavior: the InitializingBean
and DisposableBean callback interfaces; custom init() and destroy() methods; and the
@PostConstruct and @PreDestroy annotations. You can combine these mechanisms to control a
given bean.
Note
If multiple lifecycle mechanisms are configured for a bean, and each mechanism is configured
with a different method name, then each configured method is executed in the order listed below.
However, if the same method name is configured - for example, init() for an initialization
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method - for more than one of these lifecycle mechanisms, that method is executed once, as
explained in the preceding section.
Multiple lifecycle mechanisms configured for the same bean, with different initialization methods, are
called as follows:
• Methods annotated with @PostConstruct
• afterPropertiesSet() as defined by the InitializingBean callback interface
• A custom configured init() method
Destroy methods are called in the same order:
• Methods annotated with @PreDestroy
• destroy() as defined by the DisposableBean callback interface
• A custom configured destroy() method
Startup and shutdown callbacks
The Lifecycle interface defines the essential methods for any object that has its own lifecycle
requirements (e.g. starts and stops some background process):
public interface Lifecycle {
void start();
void stop();
boolean isRunning();
}
Any Spring-managed object may implement that interface. Then, when the ApplicationContext
itself receives start and stop signals, e.g. for a stop/restart scenario at runtime, it will cascade those
calls to all Lifecycle implementations defined within that context. It does this by delegating to a
LifecycleProcessor:
public interface LifecycleProcessor extends Lifecycle {
void onRefresh();
void onClose();
}
Notice that the LifecycleProcessor is itself an extension of the Lifecycle interface. It also adds
two other methods for reacting to the context being refreshed and closed.
Tip
Note that the regular org.springframework.context.Lifecycle interface is just a plain
contract for explicit start/stop notifications and does NOT imply auto-startup at context refresh
time. Consider implementing org.springframework.context.SmartLifecycle instead
for fine-grained control over auto-startup of a specific bean (including startup phases). Also, please
note that stop notifications are not guaranteed to come before destruction: On regular shutdown,
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all Lifecycle beans will first receive a stop notification before the general destruction callbacks
are being propagated; however, on hot refresh during a context’s lifetime or on aborted refresh
attempts, only destroy methods will be called.
The order of startup and shutdown invocations can be important. If a "depends-on" relationship exists
between any two objects, the dependent side will start after its dependency, and it will stop before its
dependency. However, at times the direct dependencies are unknown. You may only know that objects
of a certain type should start prior to objects of another type. In those cases, the SmartLifecycle
interface defines another option, namely the getPhase() method as defined on its super-interface,
Phased.
public interface Phased {
int getPhase();
}
public interface SmartLifecycle extends Lifecycle, Phased {
boolean isAutoStartup();
void stop(Runnable callback);
}
When starting, the objects with the lowest phase start first, and when stopping, the reverse order is
followed. Therefore, an object that implements SmartLifecycle and whose getPhase() method
returns Integer.MIN_VALUE would be among the first to start and the last to stop. At the other end of
the spectrum, a phase value of Integer.MAX_VALUE would indicate that the object should be started
last and stopped first (likely because it depends on other processes to be running). When considering
the phase value, it’s also important to know that the default phase for any "normal" Lifecycle object
that does not implement SmartLifecycle would be 0. Therefore, any negative phase value would
indicate that an object should start before those standard components (and stop after them), and vice
versa for any positive phase value.
As you can see the stop method defined by SmartLifecycle accepts a callback. Any implementation
must invoke that callback’s run() method after that implementation’s shutdown process is complete.
That enables asynchronous shutdown where necessary since the default implementation of the
LifecycleProcessor interface, DefaultLifecycleProcessor, will wait up to its timeout value
for the group of objects within each phase to invoke that callback. The default per-phase timeout
is 30 seconds. You can override the default lifecycle processor instance by defining a bean named
"lifecycleProcessor" within the context. If you only want to modify the timeout, then defining the following
would be sufficient:
<bean id="lifecycleProcessor" class="org.springframework.context.support.DefaultLifecycleProcessor">
<!-- timeout value in milliseconds -->
<property name="timeoutPerShutdownPhase" value="10000"/>
</bean>
As mentioned, the LifecycleProcessor interface defines callback methods for the refreshing and
closing of the context as well. The latter will simply drive the shutdown process as if stop() had
been called explicitly, but it will happen when the context is closing. The 'refresh' callback on the
other hand enables another feature of SmartLifecycle beans. When the context is refreshed (after
all objects have been instantiated and initialized), that callback will be invoked, and at that point the
default lifecycle processor will check the boolean value returned by each SmartLifecycle object’s
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isAutoStartup() method. If "true", then that object will be started at that point rather than waiting for
an explicit invocation of the context’s or its own start() method (unlike the context refresh, the context
start does not happen automatically for a standard context implementation). The "phase" value as well
as any "depends-on" relationships will determine the startup order in the same way as described above.
Shutting down the Spring IoC container gracefully in non-web applications
Note
This section applies only to non-web applications. Spring’s web-based ApplicationContext
implementations already have code in place to shut down the Spring IoC container gracefully
when the relevant web application is shut down.
If you are using Spring’s IoC container in a non-web application environment; for example, in a rich
client desktop environment; you register a shutdown hook with the JVM. Doing so ensures a graceful
shutdown and calls the relevant destroy methods on your singleton beans so that all resources are
released. Of course, you must still configure and implement these destroy callbacks correctly.
To register a shutdown hook, you call the registerShutdownHook() method that is declared on the
ConfigurableApplicationContext interface:
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public final class Boot {
public static void main(final String[] args) throws Exception {
ConfigurableApplicationContext ctx = new ClassPathXmlApplicationContext(
new String []{"beans.xml"});
// add a shutdown hook for the above context...
ctx.registerShutdownHook();
// app runs here...
// main method exits, hook is called prior to the app shutting down...
}
}
ApplicationContextAware and BeanNameAware
When an ApplicationContext creates an object instance that implements the
org.springframework.context.ApplicationContextAware interface, the instance is
provided with a reference to that ApplicationContext.
public interface ApplicationContextAware {
void setApplicationContext(ApplicationContext applicationContext) throws BeansException;
}
Thus beans can manipulate programmatically the ApplicationContext that created them, through
the ApplicationContext interface, or by casting the reference to a known subclass of this interface,
such as ConfigurableApplicationContext, which exposes additional functionality. One use
would be the programmatic retrieval of other beans. Sometimes this capability is useful; however, in
general you should avoid it, because it couples the code to Spring and does not follow the Inversion
of Control style, where collaborators are provided to beans as properties. Other methods of the
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ApplicationContext provide access to file resources, publishing application events, and accessing
a MessageSource. These additional features are described in Section 6.15, “Additional Capabilities
of the ApplicationContext”
As of Spring 2.5, autowiring is another alternative to obtain reference to the ApplicationContext.
The "traditional" constructor and byType autowiring modes (as described in the section called
“Autowiring collaborators”) can provide a dependency of type ApplicationContext for a constructor
argument or setter method parameter, respectively. For more flexibility, including the ability to autowire
fields and multiple parameter methods, use the new annotation-based autowiring features. If you do,
the ApplicationContext is autowired into a field, constructor argument, or method parameter that
is expecting the ApplicationContext type if the field, constructor, or method in question carries the
@Autowired annotation. For more information, see the section called “@Autowired”.
When
an
ApplicationContext
creates
a
class
that
implements
the
org.springframework.beans.factory.BeanNameAware interface, the class is provided with a
reference to the name defined in its associated object definition.
public interface BeanNameAware {
void setBeanName(String name) throws BeansException;
}
The callback is invoked after population of normal bean properties but before an initialization callback
such as InitializingBean afterPropertiesSet or a custom init-method.
Other Aware interfaces
Besides ApplicationContextAware and BeanNameAware discussed above, Spring offers a range
of Aware interfaces that allow beans to indicate to the container that they require a certain infrastructure
dependency. The most important Aware interfaces are summarized below - as a general rule, the name
is a good indication of the dependency type:
Table 6.4. Aware interfaces
Name
Injected Dependency
Explained in…
ApplicationContextAware
Declaring
ApplicationContext
the section called
“ApplicationContextAware and
BeanNameAware”
ApplicationEventPublisherAware
Event publisher of the enclosing Section 6.15, “Additional
ApplicationContext
Capabilities of the
ApplicationContext”
BeanClassLoaderAware
Class loader used to load the
bean classes.
the section called “Instantiating
beans”
BeanFactoryAware
Declaring BeanFactory
the section called
“ApplicationContextAware and
BeanNameAware”
BeanNameAware
Name of the declaring bean
the section called
“ApplicationContextAware and
BeanNameAware”
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Name
Injected Dependency
Explained in…
BootstrapContextAware
Resource adapter
BootstrapContext the
container runs in. Typically
available only in JCA aware
ApplicationContexts
Chapter 31, JCA CCI
LoadTimeWeaverAware
Defined weaver for processing
class definition at load time
the section called “Load-time
weaving with AspectJ in the
Spring Framework”
MessageSourceAware
Configured strategy for
resolving messages (with
support for parametrization and
internationalization)
Section 6.15, “Additional
Capabilities of the
ApplicationContext”
NotificationPublisherAware
Spring JMX notification
publisher
Section 30.7, “Notifications”
PortletConfigAware
Current PortletConfig
the container runs in. Valid
only in a web-aware Spring
ApplicationContext
Chapter 24, Portlet MVC
Framework
PortletContextAware
Current PortletContext
the container runs in. Valid
only in a web-aware Spring
ApplicationContext
Chapter 24, Portlet MVC
Framework
ResourceLoaderAware
Configured loader for low-level
access to resources
Chapter 7, Resources
ServletConfigAware
Current ServletConfig
the container runs in. Valid
only in a web-aware Spring
ApplicationContext
Chapter 21, Web MVC
framework
ServletContextAware
Current ServletContext
the container runs in. Valid
only in a web-aware Spring
ApplicationContext
Chapter 21, Web MVC
framework
Note again that usage of these interfaces ties your code to the Spring API and does not follow
the Inversion of Control style. As such, they are recommended for infrastructure beans that require
programmatic access to the container.
6.7 Bean definition inheritance
A bean definition can contain a lot of configuration information, including constructor arguments, property
values, and container-specific information such as initialization method, static factory method name,
and so on. A child bean definition inherits configuration data from a parent definition. The child definition
can override some values, or add others, as needed. Using parent and child bean definitions can save
a lot of typing. Effectively, this is a form of templating.
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If you work with an ApplicationContext interface programmatically, child bean definitions
are represented by the ChildBeanDefinition class. Most users do not work with
them on this level, instead configuring bean definitions declaratively in something like the
ClassPathXmlApplicationContext. When you use XML-based configuration metadata, you
indicate a child bean definition by using the parent attribute, specifying the parent bean as the value
of this attribute.
<bean id="inheritedTestBean" abstract="true"
class="org.springframework.beans.TestBean">
<property name="name" value="parent"/>
<property name="age" value="1"/>
</bean>
<bean id="inheritsWithDifferentClass"
class="org.springframework.beans.DerivedTestBean"
parent="inheritedTestBean" init-method="initialize">
<property name="name" value="override"/>
<!-- the age property value of 1 will be inherited from parent -->
</bean>
A child bean definition uses the bean class from the parent definition if none is specified, but can also
override it. In the latter case, the child bean class must be compatible with the parent, that is, it must
accept the parent’s property values.
A child bean definition inherits scope, constructor argument values, property values, and method
overrides from the parent, with the option to add new values. Any scope, initialization method, destroy
method, and/or static factory method settings that you specify will override the corresponding parent
settings.
The remaining settings are always taken from the child definition: depends on, autowire mode,
dependency check, singleton, lazy init.
The preceding example explicitly marks the parent bean definition as abstract by using the abstract
attribute. If the parent definition does not specify a class, explicitly marking the parent bean definition
as abstract is required, as follows:
<bean id="inheritedTestBeanWithoutClass" abstract="true">
<property name="name" value="parent"/>
<property name="age" value="1"/>
</bean>
<bean id="inheritsWithClass" class="org.springframework.beans.DerivedTestBean"
parent="inheritedTestBeanWithoutClass" init-method="initialize">
<property name="name" value="override"/>
<!-- age will inherit the value of 1 from the parent bean definition-->
</bean>
The parent bean cannot be instantiated on its own because it is incomplete, and it is also explicitly
marked as abstract. When a definition is abstract like this, it is usable only as a pure template
bean definition that serves as a parent definition for child definitions. Trying to use such an
abstract parent bean on its own, by referring to it as a ref property of another bean or doing an
explicit getBean() call with the parent bean id, returns an error. Similarly, the container’s internal
preInstantiateSingletons() method ignores bean definitions that are defined as abstract.
Note
ApplicationContext pre-instantiates all singletons by default. Therefore, it is important (at
least for singleton beans) that if you have a (parent) bean definition which you intend to use only
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as a template, and this definition specifies a class, you must make sure to set the abstract attribute
to true, otherwise the application context will actually (attempt to) pre-instantiate the abstract
bean.
6.8 Container Extension Points
Typically, an application developer does not need to subclass ApplicationContext implementation
classes. Instead, the Spring IoC container can be extended by plugging in implementations of special
integration interfaces. The next few sections describe these integration interfaces.
Customizing beans using a BeanPostProcessor
The BeanPostProcessor interface defines callback methods that you can implement to provide your
own (or override the container’s default) instantiation logic, dependency-resolution logic, and so forth. If
you want to implement some custom logic after the Spring container finishes instantiating, configuring,
and initializing a bean, you can plug in one or more BeanPostProcessor implementations.
You can configure multiple BeanPostProcessor instances, and you can control the order in which
these BeanPostProcessors execute by setting the order property. You can set this property only if the
BeanPostProcessor implements the Ordered interface; if you write your own BeanPostProcessor
you should consider implementing the Ordered interface too. For further details, consult the javadocs
of the BeanPostProcessor and Ordered interfaces. See also the note below on programmatic
registration of BeanPostProcessors.
Note
BeanPostProcessors operate on bean (or object) instances; that is to say, the Spring IoC container
instantiates a bean instance and then BeanPostProcessors do their work.
BeanPostProcessors are scoped per-container. This is only relevant if you are using container
hierarchies. If you define a BeanPostProcessor in one container, it will only post-process the
beans in that container. In other words, beans that are defined in one container are not postprocessed by a BeanPostProcessor defined in another container, even if both containers are
part of the same hierarchy.
To change the actual bean definition (i.e., the blueprint that defines the bean), you instead
need to use a BeanFactoryPostProcessor as described in the section called “Customizing
configuration metadata with a BeanFactoryPostProcessor”.
The org.springframework.beans.factory.config.BeanPostProcessor interface consists
of exactly two callback methods. When such a class is registered as a post-processor with the container,
for each bean instance that is created by the container, the post-processor gets a callback from the
container both before container initialization methods (such as InitializingBean’s afterPropertiesSet()
and any declared init method) are called as well as after any bean initialization callbacks. The postprocessor can take any action with the bean instance, including ignoring the callback completely. A
bean post-processor typically checks for callback interfaces or may wrap a bean with a proxy. Some
Spring AOP infrastructure classes are implemented as bean post-processors in order to provide proxywrapping logic.
An ApplicationContext automatically detects any beans that are defined in the configuration
metadata which implement the BeanPostProcessor interface. The ApplicationContext registers
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these beans as post-processors so that they can be called later upon bean creation. Bean postprocessors can be deployed in the container just like any other beans.
Note that when declaring a BeanPostProcessor using an @Bean factory method on a configuration
class, the return type of the factory method should be the implementation class itself or at least
the org.springframework.beans.factory.config.BeanPostProcessor interface, clearly
indicating the post-processor nature of that bean. Otherwise, the ApplicationContext won’t be able
to autodetect it by type before fully creating it. Since a BeanPostProcessor needs to be instantiated early
in order to apply to the initialization of other beans in the context, this early type detection is critical.
Programmatically registering BeanPostProcessors
While the recommended approach for BeanPostProcessor registration is through
ApplicationContext auto-detection (as described above), it is also possible to register them
programmatically against a ConfigurableBeanFactory using the addBeanPostProcessor
method. This can be useful when needing to evaluate conditional logic before registration,
or even for copying bean post processors across contexts in a hierarchy. Note however
that BeanPostProcessors added programmatically do not respect the Ordered interface.
Here it is the order of registration that dictates the order of execution. Note also that
BeanPostProcessors registered programmatically are always processed before those
registered through auto-detection, regardless of any explicit ordering.
BeanPostProcessors and AOP auto-proxying
Classes that implement the BeanPostProcessor interface are special and are treated differently
by the container. All BeanPostProcessors and beans that they reference directly are
instantiated on startup, as part of the special startup phase of the ApplicationContext. Next,
all BeanPostProcessors are registered in a sorted fashion and applied to all further beans in the
container. Because AOP auto-proxying is implemented as a BeanPostProcessor itself, neither
BeanPostProcessors nor the beans they reference directly are eligible for auto-proxying, and
thus do not have aspects woven into them.
For any such bean, you should see an informational log message: "Bean foo is not eligible
for getting processed by all BeanPostProcessor interfaces (for example: not eligible for autoproxying)".
Note that if you have beans wired into your BeanPostProcessor using autowiring or
@Resource (which may fall back to autowiring), Spring might access unexpected beans when
searching for type-matching dependency candidates, and therefore make them ineligible for
auto-proxying or other kinds of bean post-processing. For example, if you have a dependency
annotated with @Resource where the field/setter name does not directly correspond to the
declared name of a bean and no name attribute is used, then Spring will access other beans for
matching them by type.
The following examples show how to write, register, and use BeanPostProcessors in an
ApplicationContext.
Example: Hello World, BeanPostProcessor-style
This first example illustrates basic usage. The example shows a custom BeanPostProcessor
implementation that invokes the toString() method of each bean as it is created by the container
and prints the resulting string to the system console.
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Find below the custom BeanPostProcessor implementation class definition:
package scripting;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.beans.BeansException;
public class InstantiationTracingBeanPostProcessor implements BeanPostProcessor {
// simply return the instantiated bean as-is
public Object postProcessBeforeInitialization(Object bean,
String beanName) throws BeansException {
return bean; // we could potentially return any object reference here...
}
public Object postProcessAfterInitialization(Object bean,
String beanName) throws BeansException {
System.out.println("Bean '" + beanName + "' created : " + bean.toString());
return bean;
}
}
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:lang="http://www.springframework.org/schema/lang"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/lang
http://www.springframework.org/schema/lang/spring-lang.xsd">
<lang:groovy id="messenger"
script-source="classpath:org/springframework/scripting/groovy/Messenger.groovy">
<lang:property name="message" value="Fiona Apple Is Just So Dreamy."/>
</lang:groovy>
<!-when the above bean (messenger) is instantiated, this custom
BeanPostProcessor implementation will output the fact to the system console
-->
<bean class="scripting.InstantiationTracingBeanPostProcessor"/>
</beans>
Notice how the InstantiationTracingBeanPostProcessor is simply defined. It does not even
have a name, and because it is a bean it can be dependency-injected just like any other bean. (The
preceding configuration also defines a bean that is backed by a Groovy script. The Spring dynamic
language support is detailed in the chapter entitled Chapter 34, Dynamic language support.)
The following simple Java application executes the preceding code and configuration:
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.scripting.Messenger;
public final class Boot {
public static void main(final String[] args) throws Exception {
ApplicationContext ctx = new ClassPathXmlApplicationContext("scripting/beans.xml");
Messenger messenger = (Messenger) ctx.getBean("messenger");
System.out.println(messenger);
}
}
The output of the preceding application resembles the following:
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Bean 'messenger' created : org.springframework.scripting.groovy.GroovyMessenger@272961
org.springframework.scripting.groovy.GroovyMessenger@272961
Example: The RequiredAnnotationBeanPostProcessor
Using callback interfaces or annotations in conjunction with a custom BeanPostProcessor
implementation is a common means of extending the Spring IoC container. An example is Spring’s
RequiredAnnotationBeanPostProcessor - a BeanPostProcessor implementation that ships
with the Spring distribution which ensures that JavaBean properties on beans that are marked with an
(arbitrary) annotation are actually (configured to be) dependency-injected with a value.
Customizing configuration metadata with a BeanFactoryPostProcessor
The
next
extension
point
that
we
will
look
at
is
the org.springframework.beans.factory.config.BeanFactoryPostProcessor. The
semantics of this interface are similar to those of the BeanPostProcessor, with one major difference:
BeanFactoryPostProcessor operates on the bean configuration metadata; that is, the Spring IoC
container allows a BeanFactoryPostProcessor to read the configuration metadata and potentially
change it before the container instantiates any beans other than BeanFactoryPostProcessors.
You can configure multiple BeanFactoryPostProcessors, and you can control the order in which
these BeanFactoryPostProcessors execute by setting the order property. However, you can only
set this property if the BeanFactoryPostProcessor implements the Ordered interface. If you write
your own BeanFactoryPostProcessor, you should consider implementing the Ordered interface
too. Consult the javadocs of the BeanFactoryPostProcessor and Ordered interfaces for more
details.
Note
If you want to change the actual bean instances (i.e., the objects that are created from the
configuration metadata), then you instead need to use a BeanPostProcessor (described
above in the section called “Customizing beans using a BeanPostProcessor”). While it is
technically possible to work with bean instances within a BeanFactoryPostProcessor (e.g.,
using BeanFactory.getBean()), doing so causes premature bean instantiation, violating the
standard container lifecycle. This may cause negative side effects such as bypassing bean post
processing.
Also, BeanFactoryPostProcessors are scoped per-container. This is only relevant if you are
using container hierarchies. If you define a BeanFactoryPostProcessor in one container, it
will only be applied to the bean definitions in that container. Bean definitions in one container
will not be post-processed by BeanFactoryPostProcessors in another container, even if both
containers are part of the same hierarchy.
A bean factory post-processor is executed automatically when it is declared inside an
ApplicationContext, in order to apply changes to the configuration metadata that
define the container. Spring includes a number of predefined bean factory post-processors,
such as PropertyOverrideConfigurer and PropertyPlaceholderConfigurer. A custom
BeanFactoryPostProcessor can also be used, for example, to register custom property editors.
An ApplicationContext automatically detects any beans that are deployed into it that implement
the BeanFactoryPostProcessor interface. It uses these beans as bean factory post-processors, at
the appropriate time. You can deploy these post-processor beans as you would any other bean.
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Note
As with BeanPostProcessors , you typically do not want to configure BeanFactoryPostProcessors
for lazy initialization. If no other bean references a Bean(Factory)PostProcessor, that postprocessor will not get instantiated at all. Thus, marking it for lazy initialization will be ignored, and
the Bean(Factory)PostProcessor will be instantiated eagerly even if you set the defaultlazy-init attribute to true on the declaration of your <beans /> element.
Example: the Class name substitution PropertyPlaceholderConfigurer
You use the PropertyPlaceholderConfigurer to externalize property values from a bean
definition in a separate file using the standard Java Properties format. Doing so enables the person
deploying an application to customize environment-specific properties such as database URLs and
passwords, without the complexity or risk of modifying the main XML definition file or files for the
container.
Consider the following XML-based configuration metadata fragment, where a DataSource with
placeholder values is defined. The example shows properties configured from an external Properties
file. At runtime, a PropertyPlaceholderConfigurer is applied to the metadata that will replace
some properties of the DataSource. The values to replace are specified as placeholders of the form
${property-name} which follows the Ant / log4j / JSP EL style.
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations" value="classpath:com/foo/jdbc.properties"/>
</bean>
<bean id="dataSource" destroy-method="close"
class="org.apache.commons.dbcp.BasicDataSource">
<property name="driverClassName" value="${jdbc.driverClassName}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</bean>
The actual values come from another file in the standard Java Properties format:
jdbc.driverClassName=org.hsqldb.jdbcDriver
jdbc.url=jdbc:hsqldb:hsql://production:9002
jdbc.username=sa
jdbc.password=root
Therefore, the string ${jdbc.username} is replaced at runtime with the value 'sa', and
the same applies for other placeholder values that match keys in the properties file. The
PropertyPlaceholderConfigurer checks for placeholders in most properties and attributes of a
bean definition. Furthermore, the placeholder prefix and suffix can be customized.
With the context namespace introduced in Spring 2.5, it is possible to configure property placeholders
with a dedicated configuration element. One or more locations can be provided as a comma-separated
list in the location attribute.
<context:property-placeholder location="classpath:com/foo/jdbc.properties"/>
The PropertyPlaceholderConfigurer not only looks for properties in the Properties file you
specify. By default it also checks against the Java System properties if it cannot find a property in the
specified properties files. You can customize this behavior by setting the systemPropertiesMode
property of the configurer with one of the following three supported integer values:
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• never (0): Never check system properties
• fallback (1): Check system properties if not resolvable in the specified properties files. This is the
default.
• override (2): Check system properties first, before trying the specified properties files. This allows
system properties to override any other property source.
Consult the PropertyPlaceholderConfigurer javadocs for more information.
Tip
You can use the PropertyPlaceholderConfigurer to substitute class names, which is
sometimes useful when you have to pick a particular implementation class at runtime. For
example:
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<value>classpath:com/foo/strategy.properties</value>
</property>
<property name="properties">
<value>custom.strategy.class=com.foo.DefaultStrategy</value>
</property>
</bean>
<bean id="serviceStrategy" class="${custom.strategy.class}"/>
If the class cannot be resolved at runtime to a valid class, resolution of the bean fails when
it is about to be created, which is during the preInstantiateSingletons() phase of an
ApplicationContext for a non-lazy-init bean.
Example: the PropertyOverrideConfigurer
The PropertyOverrideConfigurer, another bean factory post-processor, resembles the
PropertyPlaceholderConfigurer, but unlike the latter, the original definitions can have default
values or no values at all for bean properties. If an overriding Properties file does not have an entry
for a certain bean property, the default context definition is used.
Note that the bean definition is not aware of being overridden, so it is not immediately obvious
from the XML definition file that the override configurer is being used. In case of multiple
PropertyOverrideConfigurer instances that define different values for the same bean property,
the last one wins, due to the overriding mechanism.
Properties file configuration lines take this format:
beanName.property=value
For example:
dataSource.driverClassName=com.mysql.jdbc.Driver
dataSource.url=jdbc:mysql:mydb
This example file can be used with a container definition that contains a bean called dataSource, which
has driver and url properties.
Compound property names are also supported, as long as every component of the path except the
final property being overridden is already non-null (presumably initialized by the constructors). In this
example…
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foo.fred.bob.sammy=123
i. the sammy property of the bob property of the fred property of the foo bean is set to the scalar
value 123.
Note
Specified override values are always literal values; they are not translated into bean references.
This convention also applies when the original value in the XML bean definition specifies a bean
reference.
With the context namespace introduced in Spring 2.5, it is possible to configure property overriding
with a dedicated configuration element:
<context:property-override location="classpath:override.properties"/>
Customizing instantiation logic with a FactoryBean
Implement the org.springframework.beans.factory.FactoryBean interface for objects that
are themselves factories.
The FactoryBean interface is a point of pluggability into the Spring IoC container’s instantiation logic.
If you have complex initialization code that is better expressed in Java as opposed to a (potentially)
verbose amount of XML, you can create your own FactoryBean, write the complex initialization inside
that class, and then plug your custom FactoryBean into the container.
The FactoryBean interface provides three methods:
• Object getObject(): returns an instance of the object this factory creates. The instance can
possibly be shared, depending on whether this factory returns singletons or prototypes.
• boolean isSingleton(): returns true if this FactoryBean returns singletons, false otherwise.
• Class getObjectType(): returns the object type returned by the getObject() method or null
if the type is not known in advance.
The FactoryBean concept and interface is used in a number of places within the Spring Framework;
more than 50 implementations of the FactoryBean interface ship with Spring itself.
When you need to ask a container for an actual FactoryBean instance itself instead of the bean
it produces, preface the bean’s id with the ampersand symbol ( &) when calling the getBean()
method of the ApplicationContext. So for a given FactoryBean with an id of myBean, invoking
getBean("myBean") on the container returns the product of the FactoryBean; whereas, invoking
getBean("&myBean") returns the FactoryBean instance itself.
6.9 Annotation-based container configuration
Are annotations better than XML for configuring Spring?
The introduction of annotation-based configurations raised the question of whether this approach
is 'better' than XML. The short answer is it depends. The long answer is that each approach has
its pros and cons, and usually it is up to the developer to decide which strategy suits them better.
Due to the way they are defined, annotations provide a lot of context in their declaration, leading
to shorter and more concise configuration. However, XML excels at wiring up components without
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touching their source code or recompiling them. Some developers prefer having the wiring close
to the source while others argue that annotated classes are no longer POJOs and, furthermore,
that the configuration becomes decentralized and harder to control.
No matter the choice, Spring can accommodate both styles and even mix them together. It’s worth
pointing out that through its JavaConfig option, Spring allows annotations to be used in a noninvasive way, without touching the target components source code and that in terms of tooling, all
configuration styles are supported by the Spring Tool Suite.
An alternative to XML setups is provided by annotation-based configuration which rely on the bytecode
metadata for wiring up components instead of angle-bracket declarations. Instead of using XML to
describe a bean wiring, the developer moves the configuration into the component class itself by using
annotations on the relevant class, method, or field declaration. As mentioned in the section called
“Example: The RequiredAnnotationBeanPostProcessor”, using a BeanPostProcessor in conjunction
with annotations is a common means of extending the Spring IoC container. For example, Spring
2.0 introduced the possibility of enforcing required properties with the @Required annotation. Spring
2.5 made it possible to follow that same general approach to drive Spring’s dependency injection.
Essentially, the @Autowired annotation provides the same capabilities as described in the section
called “Autowiring collaborators” but with more fine-grained control and wider applicability. Spring 2.5
also added support for JSR-250 annotations such as @PostConstruct, and @PreDestroy. Spring
3.0 added support for JSR-330 (Dependency Injection for Java) annotations contained in the javax.inject
package such as @Inject and @Named. Details about those annotations can be found in the relevant
section.
Note
Annotation injection is performed before XML injection, thus the latter configuration will override
the former for properties wired through both approaches.
As always, you can register them as individual bean definitions, but they can also be implicitly registered
by including the following tag in an XML-based Spring configuration (notice the inclusion of the context
namespace):
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<context:annotation-config/>
</beans>
(The implicitly registered post-processors include AutowiredAnnotationBeanPostProcessor,
CommonAnnotationBeanPostProcessor, PersistenceAnnotationBeanPostProcessor, as
well as the aforementioned RequiredAnnotationBeanPostProcessor.)
Note
<context:annotation-config/> only looks for annotations on beans in the same application
context in which it is defined. This means that, if you put <context:annotation-config/>
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in a WebApplicationContext for a DispatcherServlet, it only checks for @Autowired
beans in your controllers, and not your services. See Section 21.2, “The DispatcherServlet” for
more information.
@Required
The @Required annotation applies to bean property setter methods, as in the following example:
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Required
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// ...
}
This annotation simply indicates that the affected bean property must be populated at configuration time,
through an explicit property value in a bean definition or through autowiring. The container throws an
exception if the affected bean property has not been populated; this allows for eager and explicit failure,
avoiding NullPointerExceptions or the like later on. It is still recommended that you put assertions into
the bean class itself, for example, into an init method. Doing so enforces those required references and
values even when you use the class outside of a container.
@Autowired
As expected, you can apply the @Autowired annotation to "traditional" setter methods:
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Autowired
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// ...
}
Note
JSR 330’s @Inject annotation can be used in place of Spring’s @Autowired annotation in the
examples below. See here for more details.
You can also apply the annotation to methods with arbitrary names and/or multiple arguments:
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public class MovieRecommender {
private MovieCatalog movieCatalog;
private CustomerPreferenceDao customerPreferenceDao;
@Autowired
public void prepare(MovieCatalog movieCatalog,
CustomerPreferenceDao customerPreferenceDao) {
this.movieCatalog = movieCatalog;
this.customerPreferenceDao = customerPreferenceDao;
}
// ...
}
You can apply @Autowired to fields as well and even mix it with constructors:
public class MovieRecommender {
private final CustomerPreferenceDao customerPreferenceDao;
@Autowired
private MovieCatalog movieCatalog;
@Autowired
public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
this.customerPreferenceDao = customerPreferenceDao;
}
// ...
}
It is also possible to provide all beans of a particular type from the ApplicationContext by adding
the annotation to a field or method that expects an array of that type:
public class MovieRecommender {
@Autowired
private MovieCatalog[] movieCatalogs;
// ...
}
The same applies for typed collections:
public class MovieRecommender {
private Set<MovieCatalog> movieCatalogs;
@Autowired
public void setMovieCatalogs(Set<MovieCatalog> movieCatalogs) {
this.movieCatalogs = movieCatalogs;
}
// ...
}
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Tip
Your beans can implement the org.springframework.core.Ordered interface or either use
the @Order or standard @Priority annotation if you want items in the array or list to be sorted
into a specific order.
Even typed Maps can be autowired as long as the expected key type is String. The Map values will
contain all beans of the expected type, and the keys will contain the corresponding bean names:
public class MovieRecommender {
private Map<String, MovieCatalog> movieCatalogs;
@Autowired
public void setMovieCatalogs(Map<String, MovieCatalog> movieCatalogs) {
this.movieCatalogs = movieCatalogs;
}
// ...
}
By default, the autowiring fails whenever zero candidate beans are available; the default behavior is to
treat annotated methods, constructors, and fields as indicating required dependencies. This behavior
can be changed as demonstrated below.
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Autowired(required=false)
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// ...
}
Note
Only one annotated constructor per-class can be marked as required, but multiple non-required
constructors can be annotated. In that case, each is considered among the candidates and Spring
uses the greediest constructor whose dependencies can be satisfied, that is the constructor that
has the largest number of arguments.
@Autowired’s required attribute is recommended over the `@Required
annotation. The required attribute indicates that the property is not required for autowiring
purposes, the property is ignored if it cannot be autowired. @Required, on the other hand, is
stronger in that it enforces the property that was set by any means supported by the container. If
no value is injected, a corresponding exception is raised.
You can also use @Autowired for interfaces that are well-known resolvable
dependencies: BeanFactory, ApplicationContext, Environment, ResourceLoader,
ApplicationEventPublisher, and MessageSource. These interfaces and their extended
interfaces, such as ConfigurableApplicationContext or ResourcePatternResolver, are
automatically resolved, with no special setup necessary.
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public class MovieRecommender {
@Autowired
private ApplicationContext context;
public MovieRecommender() {
}
// ...
}
Note
@Autowired, @Inject, @Resource, and @Value annotations are handled by Spring
BeanPostProcessor implementations which in turn means that you cannot apply these
annotations within your own BeanPostProcessor or BeanFactoryPostProcessor types (if
any). These types must be 'wired up' explicitly via XML or using a Spring @Bean method.
Fine-tuning annotation-based autowiring with @Primary
Because autowiring by type may lead to multiple candidates, it is often necessary to have more
control over the selection process. One way to accomplish this is with Spring’s @Primary annotation.
@Primary indicates that a particular bean should be given preference when multiple beans are
candidates to be autowired to a single-valued dependency. If exactly one 'primary' bean exists among
the candidates, it will be the autowired value.
Let’s assume we have the following configuration that defines firstMovieCatalog as the primary
MovieCatalog.
@Configuration
public class MovieConfiguration {
@Bean
@Primary
public MovieCatalog firstMovieCatalog() { ... }
@Bean
public MovieCatalog secondMovieCatalog() { ... }
// ...
}
With such configuration,
firstMovieCatalog.
the
following
MovieRecommender
will
be
autowired
with
the
public class MovieRecommender {
@Autowired
private MovieCatalog movieCatalog;
// ...
}
The corresponding bean definitions appear as follows.
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<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<context:annotation-config/>
<bean class="example.SimpleMovieCatalog" primary="true">
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<!-- inject any dependencies required by this bean -->
</bean>
<bean id="movieRecommender" class="example.MovieRecommender"/>
</beans>
Fine-tuning annotation-based autowiring with qualifiers
@Primary is an effective way to use autowiring by type with several instances when one primary
candidate can be determined. When more control over the selection process is required, Spring’s
@Qualifier annotation can be used. You can associate qualifier values with specific arguments,
narrowing the set of type matches so that a specific bean is chosen for each argument. In the simplest
case, this can be a plain descriptive value:
public class MovieRecommender {
@Autowired
@Qualifier("main")
private MovieCatalog movieCatalog;
// ...
}
The @Qualifier annotation can also be specified on individual constructor arguments or method
parameters:
public class MovieRecommender {
private MovieCatalog movieCatalog;
private CustomerPreferenceDao customerPreferenceDao;
@Autowired
public void prepare(@Qualifier("main")MovieCatalog movieCatalog,
CustomerPreferenceDao customerPreferenceDao) {
this.movieCatalog = movieCatalog;
this.customerPreferenceDao = customerPreferenceDao;
}
// ...
}
The corresponding bean definitions appear as follows. The bean with qualifier value "main" is wired with
the constructor argument that is qualified with the same value.
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<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<context:annotation-config/>
<bean class="example.SimpleMovieCatalog">
<qualifier value="main"/>
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<qualifier value="action"/>
<!-- inject any dependencies required by this bean -->
</bean>
<bean id="movieRecommender" class="example.MovieRecommender"/>
</beans>
For a fallback match, the bean name is considered a default qualifier value. Thus you can define the bean
with an id "main" instead of the nested qualifier element, leading to the same matching result. However,
although you can use this convention to refer to specific beans by name, @Autowired is fundamentally
about type-driven injection with optional semantic qualifiers. This means that qualifier values, even with
the bean name fallback, always have narrowing semantics within the set of type matches; they do not
semantically express a reference to a unique bean id. Good qualifier values are "main" or "EMEA" or
"persistent", expressing characteristics of a specific component that are independent from the bean id,
which may be auto-generated in case of an anonymous bean definition like the one in the preceding
example.
Qualifiers also apply to typed collections, as discussed above, for example, to Set<MovieCatalog>. In
this case, all matching beans according to the declared qualifiers are injected as a collection. This implies
that qualifiers do not have to be unique; they rather simply constitute filtering criteria. For example, you
can define multiple MovieCatalog beans with the same qualifier value "action", all of which would be
injected into a Set<MovieCatalog> annotated with @Qualifier("action").
Tip
If you intend to express annotation-driven injection by name, do not primarily use @Autowired,
even if is technically capable of referring to a bean name through @Qualifier values. Instead,
use the JSR-250 @Resource annotation, which is semantically defined to identify a specific target
component by its unique name, with the declared type being irrelevant for the matching process.
As a specific consequence of this semantic difference, beans that are themselves defined as a
collection or map type cannot be injected through @Autowired, because type matching is not
properly applicable to them. Use @Resource for such beans, referring to the specific collection
or map bean by unique name.
@Autowired applies to fields, constructors, and multi-argument methods, allowing for narrowing
through qualifier annotations at the parameter level. By contrast, @Resource is supported only
for fields and bean property setter methods with a single argument. As a consequence, stick with
qualifiers if your injection target is a constructor or a multi-argument method.
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You can create your own custom qualifier annotations. Simply define an annotation and provide the
@Qualifier annotation within your definition:
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface Genre {
String value();
}
Then you can provide the custom qualifier on autowired fields and parameters:
public class MovieRecommender {
@Autowired
@Genre("Action")
private MovieCatalog actionCatalog;
private MovieCatalog comedyCatalog;
@Autowired
public void setComedyCatalog(@Genre("Comedy") MovieCatalog comedyCatalog) {
this.comedyCatalog = comedyCatalog;
}
// ...
}
Next, provide the information for the candidate bean definitions. You can add <qualifier/> tags as
sub-elements of the <bean/> tag and then specify the type and value to match your custom qualifier
annotations. The type is matched against the fully-qualified class name of the annotation. Or, as a
convenience if no risk of conflicting names exists, you can use the short class name. Both approaches
are demonstrated in the following example.
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<context:annotation-config/>
<bean class="example.SimpleMovieCatalog">
<qualifier type="Genre" value="Action"/>
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<qualifier type="example.Genre" value="Comedy"/>
<!-- inject any dependencies required by this bean -->
</bean>
<bean id="movieRecommender" class="example.MovieRecommender"/>
</beans>
In Section 6.10, “Classpath scanning and managed components”, you will see an annotation-based
alternative to providing the qualifier metadata in XML. Specifically, see the section called “Providing
qualifier metadata with annotations”.
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In some cases, it may be sufficient to use an annotation without a value. This may be useful when
the annotation serves a more generic purpose and can be applied across several different types of
dependencies. For example, you may provide an offline catalog that would be searched when no Internet
connection is available. First define the simple annotation:
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface Offline {
}
Then add the annotation to the field or property to be autowired:
public class MovieRecommender {
@Autowired
@Offline
private MovieCatalog offlineCatalog;
// ...
}
Now the bean definition only needs a qualifier type:
<bean class="example.SimpleMovieCatalog">
<qualifier type="Offline"/>
<!-- inject any dependencies required by this bean -->
</bean>
You can also define custom qualifier annotations that accept named attributes in addition to or instead
of the simple value attribute. If multiple attribute values are then specified on a field or parameter
to be autowired, a bean definition must match all such attribute values to be considered an autowire
candidate. As an example, consider the following annotation definition:
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface MovieQualifier {
String genre();
Format format();
}
In this case Format is an enum:
public enum Format {
VHS, DVD, BLURAY
}
The fields to be autowired are annotated with the custom qualifier and include values for both attributes:
genre and format.
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public class MovieRecommender {
@Autowired
@MovieQualifier(format=Format.VHS, genre="Action")
private MovieCatalog actionVhsCatalog;
@Autowired
@MovieQualifier(format=Format.VHS, genre="Comedy")
private MovieCatalog comedyVhsCatalog;
@Autowired
@MovieQualifier(format=Format.DVD, genre="Action")
private MovieCatalog actionDvdCatalog;
@Autowired
@MovieQualifier(format=Format.BLURAY, genre="Comedy")
private MovieCatalog comedyBluRayCatalog;
// ...
}
Finally, the bean definitions should contain matching qualifier values. This example also demonstrates
that bean meta attributes may be used instead of the <qualifier/> sub-elements. If available, the
<qualifier/> and its attributes take precedence, but the autowiring mechanism falls back on the
values provided within the <meta/> tags if no such qualifier is present, as in the last two bean definitions
in the following example.
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<context:annotation-config/>
<bean class="example.SimpleMovieCatalog">
<qualifier type="MovieQualifier">
<attribute key="format" value="VHS"/>
<attribute key="genre" value="Action"/>
</qualifier>
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<qualifier type="MovieQualifier">
<attribute key="format" value="VHS"/>
<attribute key="genre" value="Comedy"/>
</qualifier>
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<meta key="format" value="DVD"/>
<meta key="genre" value="Action"/>
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<meta key="format" value="BLURAY"/>
<meta key="genre" value="Comedy"/>
<!-- inject any dependencies required by this bean -->
</bean>
</beans>
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Using generics as autowiring qualifiers
In addition to the @Qualifier annotation, it is also possible to use Java generic types as an implicit
form of qualification. For example, suppose you have the following configuration:
@Configuration
public class MyConfiguration {
@Bean
public StringStore stringStore() {
return new StringStore();
}
@Bean
public IntegerStore integerStore() {
return new IntegerStore();
}
}
Assuming that beans above implement a generic interface, i.e. Store<String> and
Store<Integer>, you can @Autowire the Store interface and the generic will be used as a qualifier:
@Autowired
private Store<String> s1; // <String> qualifier, injects the stringStore bean
@Autowired
private Store<Integer> s2; // <Integer> qualifier, injects the integerStore bean
Generic qualifiers also apply when autowiring Lists, Maps and Arrays:
// Inject all Store beans as long as they have an <Integer> generic
// Store<String> beans will not appear in this list
@Autowired
private List<Store<Integer>> s;
CustomAutowireConfigurer
The CustomAutowireConfigurer is a BeanFactoryPostProcessor that enables you to register
your own custom qualifier annotation types even if they are not annotated with Spring’s @Qualifier
annotation.
<bean id="customAutowireConfigurer"
class="org.springframework.beans.factory.annotation.CustomAutowireConfigurer">
<property name="customQualifierTypes">
<set>
<value>example.CustomQualifier</value>
</set>
</property>
</bean>
The AutowireCandidateResolver determines autowire candidates by:
• the autowire-candidate value of each bean definition
• any default-autowire-candidates pattern(s) available on the <beans/> element
• the presence of @Qualifier annotations and any custom annotations registered with the
CustomAutowireConfigurer
When multiple beans qualify as autowire candidates, the determination of a "primary" is the following:
if exactly one bean definition among the candidates has a primary attribute set to true, it will be
selected.
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@Resource
Spring also supports injection using the JSR-250 @Resource annotation on fields or bean property
setter methods. This is a common pattern in Java EE 5 and 6, for example in JSF 1.2 managed beans
or JAX-WS 2.0 endpoints. Spring supports this pattern for Spring-managed objects as well.
@Resource takes a name attribute, and by default Spring interprets that value as the bean name to be
injected. In other words, it follows by-name semantics, as demonstrated in this example:
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Resource(name="myMovieFinder")
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
}
If no name is specified explicitly, the default name is derived from the field name or setter method. In
case of a field, it takes the field name; in case of a setter method, it takes the bean property name. So
the following example is going to have the bean with name "movieFinder" injected into its setter method:
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Resource
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
}
Note
The name provided with the annotation is resolved as a bean name by the
ApplicationContext of which the CommonAnnotationBeanPostProcessor is aware. The
names can be resolved through JNDI if you configure Spring’s SimpleJndiBeanFactory
explicitly. However, it is recommended that you rely on the default behavior and simply use
Spring’s JNDI lookup capabilities to preserve the level of indirection.
In the exclusive case of @Resource usage with no explicit name specified, and similar to @Autowired,
@Resource finds a primary type match instead of a specific named bean and resolves wellknown resolvable dependencies: the BeanFactory, ApplicationContext, ResourceLoader,
ApplicationEventPublisher, and MessageSource interfaces.
Thus in the following example, the customerPreferenceDao field first looks for a bean
named customerPreferenceDao, then falls back to a primary type match for the type
CustomerPreferenceDao. The "context" field is injected based on the known resolvable dependency
type ApplicationContext.
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public class MovieRecommender {
@Resource
private CustomerPreferenceDao customerPreferenceDao;
@Resource
private ApplicationContext context;
public MovieRecommender() {
}
// ...
}
@PostConstruct and @PreDestroy
The CommonAnnotationBeanPostProcessor not only recognizes the @Resource annotation
but also the JSR-250 lifecycle annotations. Introduced in Spring 2.5, the support for these
annotations offers yet another alternative to those described in initialization callbacks and destruction
callbacks. Provided that the CommonAnnotationBeanPostProcessor is registered within the Spring
ApplicationContext, a method carrying one of these annotations is invoked at the same point in the
lifecycle as the corresponding Spring lifecycle interface method or explicitly declared callback method.
In the example below, the cache will be pre-populated upon initialization and cleared upon destruction.
public class CachingMovieLister {
@PostConstruct
public void populateMovieCache() {
// populates the movie cache upon initialization...
}
@PreDestroy
public void clearMovieCache() {
// clears the movie cache upon destruction...
}
}
Note
For details about the effects of combining various lifecycle mechanisms, see the section called
“Combining lifecycle mechanisms”.
6.10 Classpath scanning and managed components
Most examples in this chapter use XML to specify the configuration metadata that produces each
BeanDefinition within the Spring container. The previous section (Section 6.9, “Annotation-based
container configuration”) demonstrates how to provide a lot of the configuration metadata through
source-level annotations. Even in those examples, however, the "base" bean definitions are explicitly
defined in the XML file, while the annotations only drive the dependency injection. This section describes
an option for implicitly detecting the candidate components by scanning the classpath. Candidate
components are classes that match against a filter criteria and have a corresponding bean definition
registered with the container. This removes the need to use XML to perform bean registration; instead
you can use annotations (for example @Component), AspectJ type expressions, or your own custom
filter criteria to select which classes will have bean definitions registered with the container.
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Note
Starting with Spring 3.0, many features provided by the Spring JavaConfig project are part of
the core Spring Framework. This allows you to define beans using Java rather than using the
traditional XML files. Take a look at the @Configuration, @Bean, @Import, and @DependsOn
annotations for examples of how to use these new features.
@Component and further stereotype annotations
The @Repository annotation is a marker for any class that fulfills the role or stereotype of a repository
(also known as Data Access Object or DAO). Among the uses of this marker is the automatic translation
of exceptions as described in the section called “Exception translation”.
Spring provides further stereotype annotations: @Component, @Service, and @Controller.
@Component is a generic stereotype for any Spring-managed component. @Repository, @Service,
and @Controller are specializations of @Component for more specific use cases, for example,
in the persistence, service, and presentation layers, respectively. Therefore, you can annotate your
component classes with @Component, but by annotating them with @Repository, @Service, or
@Controller instead, your classes are more properly suited for processing by tools or associating
with aspects. For example, these stereotype annotations make ideal targets for pointcuts. It is also
possible that @Repository, @Service, and @Controller may carry additional semantics in future
releases of the Spring Framework. Thus, if you are choosing between using @Component or @Service
for your service layer, @Service is clearly the better choice. Similarly, as stated above, @Repository
is already supported as a marker for automatic exception translation in your persistence layer.
Meta-annotations
Many of the annotations provided by Spring can be used as meta-annotations in your own code. A
meta-annotation is simply an annotation that can be applied to another annotation. For example, the
@Service annotation mentioned above is meta-annotated with @Component:
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component // Spring will see this and treat @Service in the same way as @Component
public @interface Service {
// ....
}
Meta-annotations can also be combined to create composed annotations. For example,
the @RestController annotation from Spring MVC is composed of @Controller and
@ResponseBody.
In addition, composed annotations may optionally redeclare attributes from meta-annotations to allow
user customization. This can be particularly useful when you want to only expose a subset of the metaannotation’s attributes. For example, the following is a custom @Scope annotation that hardcodes the
scope name to session but still allows customization of the proxyMode.
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Scope("session")
public @interface SessionScope {
ScopedProxyMode proxyMode() default ScopedProxyMode.DEFAULT;
}
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@SessionScope can then be used without declaring the proxyMode as follows:
@Service
@SessionScope
public class SessionScopedUserService implements UserService {
// ...
}
Or with an overridden value for the proxyMode as follows:
@Service
@SessionScope(proxyMode = ScopedProxyMode.TARGET_CLASS)
public class SessionScopedService {
// ...
}
For further details, consult the Spring Annotation Programming Model.
Automatically detecting classes and registering bean definitions
Spring can automatically detect stereotyped classes and register corresponding BeanDefinitions with
the ApplicationContext. For example, the following two classes are eligible for such autodetection:
@Service
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Autowired
public SimpleMovieLister(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
}
@Repository
public class JpaMovieFinder implements MovieFinder {
// implementation elided for clarity
}
To autodetect these classes and register the corresponding beans, you need to add @ComponentScan
to your @Configuration class, where the basePackages attribute is a common parent package for
the two classes. (Alternatively, you can specify a comma/semicolon/space-separated list that includes
the parent package of each class.)
@Configuration
@ComponentScan(basePackages = "org.example")
public class AppConfig {
...
}
Note
for concision, the above may have used the value attribute of the annotation, i.e.
ComponentScan("org.example")
The following is an alternative using XML
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<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<context:component-scan base-package="org.example"/>
</beans>
Tip
The use of <context:component-scan> implicitly enables the functionality of
<context:annotation-config>. There is usually no need to include the
<context:annotation-config> element when using <context:component-scan>.
Note
The scanning of classpath packages requires the presence of corresponding directory entries
in the classpath. When you build JARs with Ant, make sure that you do not activate the filesonly switch of the JAR task. Also, classpath directories may not get exposed based on security
policies in some environments, e.g. standalone apps on JDK 1.7.0_45 and higher (which requires
'Trusted-Library' setup in your manifests; see http://stackoverflow.com/questions/19394570/javajre-7u45-breaks-classloader-getresources).
Furthermore,
the
AutowiredAnnotationBeanPostProcessor
and
CommonAnnotationBeanPostProcessor are both included implicitly when you use the componentscan element. That means that the two components are autodetected and wired together - all without
any bean configuration metadata provided in XML.
Note
You can disable the registration of AutowiredAnnotationBeanPostProcessor and
CommonAnnotationBeanPostProcessor by including the annotation-config attribute with a
value of false.
Using filters to customize scanning
By default, classes annotated with @Component, @Repository, @Service, @Controller, or
a custom annotation that itself is annotated with @Component are the only detected candidate
components. However, you can modify and extend this behavior simply by applying custom filters. Add
them as includeFilters or excludeFilters parameters of the @ComponentScan annotation (or as includefilter or exclude-filter sub-elements of the component-scan element). Each filter element requires the
type and expression attributes. The following table describes the filtering options.
Table 6.5. Filter Types
Filter Type
Example Expression
annotation (default)
org.example.SomeAnnotation
An annotation to be present
at the type level in target
components.
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Filter Type
Example Expression
Description
assignable
org.example.SomeClass
A class (or interface) that
the target components
are assignable to (extend/
implement).
aspectj
org.example..*Service+
An AspectJ type expression
to be matched by the target
components.
regex
org\.example\.Default.*
A regex expression to be
matched by the target
components class names.
custom
org.example.MyTypeFilter A custom implementation of the
org.springframework.core.type .TypeFi
interface.
The following example shows the configuration ignoring all @Repository annotations and using "stub"
repositories instead.
@Configuration
@ComponentScan(basePackages = "org.example",
includeFilters = @Filter(type = FilterType.REGEX, pattern = ".*Stub.*Repository"),
excludeFilters = @Filter(Repository.class))
public class AppConfig {
...
}
and the equivalent using XML
<beans>
<context:component-scan base-package="org.example">
<context:include-filter type="regex"
expression=".*Stub.*Repository"/>
<context:exclude-filter type="annotation"
expression="org.springframework.stereotype.Repository"/>
</context:component-scan>
</beans>
Note
You can also disable the default filters by setting useDefaultFilters=false on the annotation
or providing use-default-filters="false" as an attribute of the <component-scan/>
element. This will in effect disable automatic detection of classes annotated with @Component,
@Repository, @Service, @Controller, or @Configuration.
Defining bean metadata within components
Spring components can also contribute bean definition metadata to the container. You do this with the
same @Bean annotation used to define bean metadata within @Configuration annotated classes.
Here is a simple example:
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@Component
public class FactoryMethodComponent {
@Bean
@Qualifier("public")
public TestBean publicInstance() {
return new TestBean("publicInstance");
}
public void doWork() {
// Component method implementation omitted
}
}
This class is a Spring component that has application-specific code contained in its doWork()
method. However, it also contributes a bean definition that has a factory method referring to the
method publicInstance(). The @Bean annotation identifies the factory method and other bean
definition properties, such as a qualifier value through the @Qualifier annotation. Other method level
annotations that can be specified are @Scope, @Lazy, and custom qualifier annotations.
Tip
In addition to its role for component initialization, the @Lazy annotation may also be placed on
injection points marked with @Autowired or @Inject. In this context, it leads to the injection
of a lazy-resolution proxy.
Autowired fields and methods are supported as previously discussed, with additional support for
autowiring of @Bean methods:
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@Component
public class FactoryMethodComponent {
private static int i;
@Bean
@Qualifier("public")
public TestBean publicInstance() {
return new TestBean("publicInstance");
}
// use of a custom qualifier and autowiring of method parameters
@Bean
protected TestBean protectedInstance(
@Qualifier("public") TestBean spouse,
@Value("#{privateInstance.age}") String country) {
TestBean tb = new TestBean("protectedInstance", 1);
tb.setSpouse(spouse);
tb.setCountry(country);
return tb;
}
@Bean
@Scope(BeanDefinition.SCOPE_SINGLETON)
private TestBean privateInstance() {
return new TestBean("privateInstance", i++);
}
@Bean
@Scope(value = WebApplicationContext.SCOPE_SESSION, proxyMode = ScopedProxyMode.TARGET_CLASS)
public TestBean requestScopedInstance() {
return new TestBean("requestScopedInstance", 3);
}
}
The example autowires the String method parameter country to the value of the Age property on
another bean named privateInstance. A Spring Expression Language element defines the value
of the property through the notation #{ <expression> }. For @Value annotations, an expression
resolver is preconfigured to look for bean names when resolving expression text.
The @Bean methods in a Spring component are processed differently than their counterparts inside
a Spring @Configuration class. The difference is that @Component classes are not enhanced
with CGLIB to intercept the invocation of methods and fields. CGLIB proxying is the means by which
invoking methods or fields within @Bean methods in @Configuration classes creates bean metadata
references to collaborating objects; such methods are not invoked with normal Java semantics but
rather go through the container in order to provide the usual lifecycle management and proxying of
Spring beans even when referring to other beans via programmatic calls to @Bean methods. In contrast,
invoking a method or field in an @Bean method within a plain @Component class has standard Java
semantics, with no special CGLIB processing or other constraints applying.
Note
You may declare @Bean methods as static, allowing for them to be called without creating their
containing configuration class as an instance. This makes particular sense when defining postprocessor beans, e.g. of type BeanFactoryPostProcessor or BeanPostProcessor, since
such beans will get initialized early in the container lifecycle and should avoid triggering other
parts of the configuration at that point.
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Note that calls to static @Bean methods will never get intercepted by the container, not even within
@Configuration classes (see above). This is due to technical limitations: CGLIB subclassing
can only override non-static methods. As a consequence, a direct call to another @Bean method
will have standard Java semantics, resulting in an independent instance being returned straight
from the factory method itself.
The Java language visibility of @Bean methods does not have an immediate impact on the
resulting bean definition in Spring’s container. You may freely declare your factory methods as
you see fit in non-@Configuration classes and also for static methods anywhere. However,
regular @Bean methods in @Configuration classes need to be overridable, i.e. they must not
be declared as private or final.
@Bean methods will also be discovered on base classes of a given component or configuration
class, as well as on Java 8 default methods declared in interfaces implemented by the component
or configuration class. This allows for a lot of flexibility in composing complex configuration
arrangements, with even multiple inheritance being possible through Java 8 default methods as
of Spring 4.2.
Finally, note that a single class may hold multiple @Bean methods for the same bean, as an
arrangement of multiple factory methods to use depending on available dependencies at runtime.
This is the same algorithm as for choosing the "greediest" constructor or factory method in
other configuration scenarios: The variant with the largest number of satisfiable dependencies
will be picked at construction time, analogous to how the container selects between multiple
@Autowired constructors.
Naming autodetected components
When a component is autodetected as part of the scanning process, its bean name is generated by the
BeanNameGenerator strategy known to that scanner. By default, any Spring stereotype annotation
(@Component, @Repository, @Service, and @Controller) that contains a name value will
thereby provide that name to the corresponding bean definition.
If such an annotation contains no name value or for any other detected component (such as those
discovered by custom filters), the default bean name generator returns the uncapitalized non-qualified
class name. For example, if the following two components were detected, the names would be
myMovieLister and movieFinderImpl:
@Service("myMovieLister")
public class SimpleMovieLister {
// ...
}
@Repository
public class MovieFinderImpl implements MovieFinder {
// ...
}
Note
If you do not want to rely on the default bean-naming strategy, you can provide a custom beannaming strategy. First, implement the BeanNameGenerator interface, and be sure to include
a default no-arg constructor. Then, provide the fully-qualified class name when configuring the
scanner:
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@Configuration
@ComponentScan(basePackages = "org.example", nameGenerator = MyNameGenerator.class)
public class AppConfig {
...
}
<beans>
<context:component-scan base-package="org.example"
name-generator="org.example.MyNameGenerator" />
</beans>
As a general rule, consider specifying the name with the annotation whenever other components may be
making explicit references to it. On the other hand, the auto-generated names are adequate whenever
the container is responsible for wiring.
Providing a scope for autodetected components
As with Spring-managed components in general, the default and most common scope for autodetected
components is singleton. However, sometimes you need a different scope which can be specified
via the @Scope annotation. Simply provide the name of the scope within the annotation:
@Scope("prototype")
@Repository
public class MovieFinderImpl implements MovieFinder {
// ...
}
Note
To provide a custom strategy for scope resolution rather than relying on the annotation-based
approach, implement the ScopeMetadataResolver interface, and be sure to include a default
no-arg constructor. Then, provide the fully-qualified class name when configuring the scanner:
@Configuration
@ComponentScan(basePackages = "org.example", scopeResolver = MyScopeResolver.class)
public class AppConfig {
...
}
<beans>
<context:component-scan base-package="org.example"
scope-resolver="org.example.MyScopeResolver" />
</beans>
When using certain non-singleton scopes, it may be necessary to generate proxies for the scoped
objects. The reasoning is described in the section called “Scoped beans as dependencies”. For this
purpose, a scoped-proxy attribute is available on the component-scan element. The three possible
values are: no, interfaces, and targetClass. For example, the following configuration will result in
standard JDK dynamic proxies:
@Configuration
@ComponentScan(basePackages = "org.example", scopedProxy = ScopedProxyMode.INTERFACES)
public class AppConfig {
...
}
<beans>
<context:component-scan base-package="org.example"
scoped-proxy="interfaces" />
</beans>
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Providing qualifier metadata with annotations
The @Qualifier annotation is discussed in the section called “Fine-tuning annotation-based
autowiring with qualifiers”. The examples in that section demonstrate the use of the @Qualifier
annotation and custom qualifier annotations to provide fine-grained control when you resolve autowire
candidates. Because those examples were based on XML bean definitions, the qualifier metadata was
provided on the candidate bean definitions using the qualifier or meta sub-elements of the bean
element in the XML. When relying upon classpath scanning for autodetection of components, you
provide the qualifier metadata with type-level annotations on the candidate class. The following three
examples demonstrate this technique:
@Component
@Qualifier("Action")
public class ActionMovieCatalog implements MovieCatalog {
// ...
}
@Component
@Genre("Action")
public class ActionMovieCatalog implements MovieCatalog {
// ...
}
@Component
@Offline
public class CachingMovieCatalog implements MovieCatalog {
// ...
}
Note
As with most annotation-based alternatives, keep in mind that the annotation metadata is bound
to the class definition itself, while the use of XML allows for multiple beans of the same type
to provide variations in their qualifier metadata, because that metadata is provided per-instance
rather than per-class.
6.11 Using JSR 330 Standard Annotations
Starting with Spring 3.0, Spring offers support for JSR-330 standard annotations (Dependency
Injection). Those annotations are scanned in the same way as the Spring annotations. You just need
to have the relevant jars in your classpath.
Note
If you are using Maven, the javax.inject artifact is available in the standard Maven
repository ( http://repo1.maven.org/maven2/javax/inject/javax.inject/1/). You can add the following
dependency to your file pom.xml:
<dependency>
<groupId>javax.inject</groupId>
<artifactId>javax.inject</artifactId>
<version>1</version>
</dependency>
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Dependency Injection with @Inject and @Named
Instead of @Autowired, @javax.inject.Inject may be used as follows:
import javax.inject.Inject;
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Inject
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
public void listMovies() {
this.movieFinder.findMovies(...);
...
}
}
As with @Autowired, it is possible to use @Inject at the field level, method level and constructorargument level. Furthermore, you may declare your injection point as a Provider, allowing for ondemand access to beans of shorter scopes or lazy access to other beans through a Provider.get()
call. As a variant of the example above:
import javax.inject.Inject;
import javax.inject.Provider;
public class SimpleMovieLister {
private Provider<MovieFinder> movieFinder;
public void listMovies() {
this.movieFinder.get().findMovies(...);
...
}
}
If you would like to use a qualified name for the dependency that should be injected, you should use
the @Named annotation as follows:
import javax.inject.Inject;
import javax.inject.Named;
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Inject
public void setMovieFinder(@Named("main") MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// ...
}
@Named: a standard equivalent to the @Component annotation
Instead of @Component, @javax.inject.Named may be used as follows:
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import javax.inject.Inject;
import javax.inject.Named;
@Named("movieListener")
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Inject
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// ...
}
It is very common to use @Component without specifying a name for the component. @Named can be
used in a similar fashion:
import javax.inject.Inject;
import javax.inject.Named;
@Named
public class SimpleMovieLister {
private MovieFinder movieFinder;
@Inject
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// ...
}
When using @Named, it is possible to use component scanning in the exact same way as when using
Spring annotations:
@Configuration
@ComponentScan(basePackages = "org.example")
public class AppConfig {
...
}
Note
In contrast to @Component, the JSR-330 @Named annotation is not composable. Please use
Spring’s stereotype model for building custom component annotations.
Limitations of JSR-330 standard annotations
When working with standard annotations, it is important to know that some significant features are not
available as shown in the table below:
Table 6.6. Spring component model elements vs. JSR-330 variants
Spring
javax.inject.*
javax.inject restrictions /
comments
@Autowired
@Inject
@Inject has no 'required'
attribute; can be used with Java
8’s Optional instead.
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Spring
javax.inject.*
javax.inject restrictions /
comments
@Component
@Named
JSR-330 does not provide a
composable model, just a way
to identify named components.
@Scope("singleton")
@Singleton
The JSR-330 default scope
is like Spring’s prototype.
However, in order to keep
it consistent with Spring’s
general defaults, a JSR-330
bean declared in the Spring
container is a singleton by
default. In order to use a scope
other than singleton, you
should use Spring’s @Scope
annotation. javax.inject
also provides a @Scope
annotation. Nevertheless, this
one is only intended to be
used for creating your own
annotations.
@Qualifier
@Qualifier / @Named
javax.inject.Qualifier
is just a meta-annotation
for building custom
qualifiers. Concrete String
qualifiers (like Spring’s
@Qualifier with a value)
can be associated through
javax.inject.Named.
@Value
-
no equivalent
@Required
-
no equivalent
@Lazy
-
no equivalent
ObjectFactory
Provider
javax.inject.Provider is
a direct alternative to Spring’s
ObjectFactory, just with
a shorter get() method
name. It can also be used
in combination with Spring’s
@Autowired or with nonannotated constructors and
setter methods.
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6.12 Java-based container configuration
Basic concepts: @Bean and @Configuration
The central artifacts in Spring’s new Java-configuration support are @Configuration-annotated
classes and @Bean-annotated methods.
The @Bean annotation is used to indicate that a method instantiates, configures and initializes a
new object to be managed by the Spring IoC container. For those familiar with Spring’s <beans/>
XML configuration the @Bean annotation plays the same role as the <bean/> element. You can use
@Bean annotated methods with any Spring @Component, however, they are most often used with
@Configuration beans.
Annotating a class with @Configuration indicates that its primary purpose is as a source of bean
definitions. Furthermore, @Configuration classes allow inter-bean dependencies to be defined by
simply calling other @Bean methods in the same class. The simplest possible @Configuration class
would read as follows:
@Configuration
public class AppConfig {
@Bean
public MyService myService() {
return new MyServiceImpl();
}
}
The AppConfig class above would be equivalent to the following Spring <beans/> XML:
<beans>
<bean id="myService" class="com.acme.services.MyServiceImpl"/>
</beans>
Full @Configuration vs 'lite' @Beans mode?
When @Bean methods are declared within classes that are not annotated with @Configuration
they are referred to as being processed in a 'lite' mode. For example, bean methods declared in
a @Component or even in a plain old class will be considered 'lite'.
Unlike full @Configuration, lite @Bean methods cannot easily declare inter-bean dependencies.
Usually one @Bean method should not invoke another @Bean method when operating in 'lite' mode.
Only using @Bean methods within @Configuration classes is a recommended approach of
ensuring that 'full' mode is always used. This will prevent the same @Bean method from accidentally
being invoked multiple times and helps to reduce subtle bugs that can be hard to track down when
operating in 'lite' mode.
The @Bean and @Configuration annotations will be discussed in depth in the sections below. First,
however, we’ll cover the various ways of creating a spring container using Java-based configuration.
Instantiating the Spring container using
AnnotationConfigApplicationContext
The sections below document Spring’s AnnotationConfigApplicationContext, new in Spring
3.0. This versatile ApplicationContext implementation is capable of accepting not only
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@Configuration classes as input, but also plain @Component classes and classes annotated with
JSR-330 metadata.
When @Configuration classes are provided as input, the @Configuration class itself is registered
as a bean definition, and all declared @Bean methods within the class are also registered as bean
definitions.
When @Component and JSR-330 classes are provided, they are registered as bean definitions, and it
is assumed that DI metadata such as @Autowired or @Inject are used within those classes where
necessary.
Simple construction
In much the same way that Spring XML files are used as input when instantiating a
ClassPathXmlApplicationContext, @Configuration classes may be used as input when
instantiating an AnnotationConfigApplicationContext. This allows for completely XML-free
usage of the Spring container:
public static void main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
MyService myService = ctx.getBean(MyService.class);
myService.doStuff();
}
As mentioned above, AnnotationConfigApplicationContext is not limited to working only with
@Configuration classes. Any @Component or JSR-330 annotated class may be supplied as input
to the constructor. For example:
public static void main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(MyServiceImpl.class,
Dependency1.class, Dependency2.class);
MyService myService = ctx.getBean(MyService.class);
myService.doStuff();
}
The above assumes that MyServiceImpl, Dependency1 and Dependency2 use Spring dependency
injection annotations such as @Autowired.
Building the container programmatically using register(Class<?>…)
An AnnotationConfigApplicationContext may be instantiated using a no-arg constructor
and then configured using the register() method. This approach is particularly useful when
programmatically building an AnnotationConfigApplicationContext.
public static void main(String[] args) {
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
ctx.register(AppConfig.class, OtherConfig.class);
ctx.register(AdditionalConfig.class);
ctx.refresh();
MyService myService = ctx.getBean(MyService.class);
myService.doStuff();
}
Enabling component scanning with scan(String…)
To enable component scanning, just annotate your @Configuration class as follows:
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@Configuration
@ComponentScan(basePackages = "com.acme")
public class AppConfig {
...
}
Tip
Experienced Spring users will be familiar with the XML declaration equivalent from Spring’s
context: namespace
<beans>
<context:component-scan base-package="com.acme"/>
</beans>
In the example above, the com.acme package will be scanned, looking for any @Componentannotated classes, and those classes will be registered as Spring bean definitions within the container.
AnnotationConfigApplicationContext exposes the scan(String…) method to allow for the
same component-scanning functionality:
public static void main(String[] args) {
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
ctx.scan("com.acme");
ctx.refresh();
MyService myService = ctx.getBean(MyService.class);
}
Note
Remember that @Configuration classes are meta-annotated with @Component, so they
are candidates for component-scanning! In the example above, assuming that AppConfig is
declared within the com.acme package (or any package underneath), it will be picked up during
the call to scan(), and upon refresh() all its @Bean methods will be processed and registered
as bean definitions within the container.
Support for web applications with AnnotationConfigWebApplicationContext
A WebApplicationContext variant of AnnotationConfigApplicationContext is available
with AnnotationConfigWebApplicationContext. This implementation may be used
when configuring the Spring ContextLoaderListener servlet listener, Spring MVC
DispatcherServlet, etc. What follows is a web.xml snippet that configures a typical Spring MVC
web application. Note the use of the contextClass context-param and init-param:
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<web-app>
<!-- Configure ContextLoaderListener to use AnnotationConfigWebApplicationContext
instead of the default XmlWebApplicationContext -->
<context-param>
<param-name>contextClass</param-name>
<param-value>
org.springframework.web.context.support.AnnotationConfigWebApplicationContext
</param-value>
</context-param>
<!-- Configuration locations must consist of one or more comma- or space-delimited
fully-qualified @Configuration classes. Fully-qualified packages may also be
specified for component-scanning -->
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>com.acme.AppConfig</param-value>
</context-param>
<!-- Bootstrap the root application context as usual using ContextLoaderListener -->
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<!-- Declare a Spring MVC DispatcherServlet as usual -->
<servlet>
<servlet-name>dispatcher</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<!-- Configure DispatcherServlet to use AnnotationConfigWebApplicationContext
instead of the default XmlWebApplicationContext -->
<init-param>
<param-name>contextClass</param-name>
<param-value>
org.springframework.web.context.support.AnnotationConfigWebApplicationContext
</param-value>
</init-param>
<!-- Again, config locations must consist of one or more comma- or space-delimited
and fully-qualified @Configuration classes -->
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>com.acme.web.MvcConfig</param-value>
</init-param>
</servlet>
<!-- map all requests for /app/* to the dispatcher servlet -->
<servlet-mapping>
<servlet-name>dispatcher</servlet-name>
<url-pattern>/app/*</url-pattern>
</servlet-mapping>
</web-app>
Using the @Bean annotation
@Bean is a method-level annotation and a direct analog of the XML <bean/> element. The annotation
supports some of the attributes offered by <bean/>, such as: init-method, destroy-method, autowiring
and name.
You can use the @Bean annotation in a @Configuration-annotated or in a @Component-annotated
class.
Declaring a bean
To declare a bean, simply annotate a method with the @Bean annotation. You use this method to register
a bean definition within an ApplicationContext of the type specified as the method’s return value.
By default, the bean name will be the same as the method name. The following is a simple example
of a @Bean method declaration:
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@Configuration
public class AppConfig {
@Bean
public TransferService transferService() {
return new TransferServiceImpl();
}
}
The preceding configuration is exactly equivalent to the following Spring XML:
<beans>
<bean id="transferService" class="com.acme.TransferServiceImpl"/>
</beans>
Both declarations make a bean named transferService available in the ApplicationContext,
bound to an object instance of type TransferServiceImpl:
transferService -> com.acme.TransferServiceImpl
Bean dependencies
A @Bean annotated method can have an arbitrary number of parameters describing the dependencies
required to build that bean. For instance if our TransferService requires an AccountRepository
we can materialize that dependency via a method parameter:
@Configuration
public class AppConfig {
@Bean
public TransferService transferService(AccountRepository accountRepository) {
return new TransferServiceImpl(accountRepository);
}
}
The resolution mechanism is pretty much identical to constructor-based dependency injection, see the
relevant section for more details.
Receiving lifecycle callbacks
Any classes defined with the @Bean annotation support the regular lifecycle callbacks and can use the
@PostConstruct and @PreDestroy annotations from JSR-250, see JSR-250 annotations for further
details.
The regular Spring lifecycle callbacks are fully supported as well. If a bean implements
InitializingBean, DisposableBean, or Lifecycle, their respective methods are called by the
container.
The standard set of *Aware interfaces such as BeanFactoryAware, BeanNameAware,
MessageSourceAware, ApplicationContextAware, and so on are also fully supported.
The @Bean annotation supports specifying arbitrary initialization and destruction callback methods,
much like Spring XML’s init-method and destroy-method attributes on the bean element:
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public class Foo {
public void init() {
// initialization logic
}
}
public class Bar {
public void cleanup() {
// destruction logic
}
}
@Configuration
public class AppConfig {
@Bean(initMethod = "init")
public Foo foo() {
return new Foo();
}
@Bean(destroyMethod = "cleanup")
public Bar bar() {
return new Bar();
}
}
Note
By default, beans defined using Java config that have a public close or shutdown method
are automatically enlisted with a destruction callback. If you have a public close or shutdown
method and you do not wish for it to be called when the container shuts down, simply add
@Bean(destroyMethod="") to your bean definition to disable the default (inferred) mode.
You may want to do that by default for a resource that you acquire via JNDI as its lifecycle is
managed outside the application. In particular, make sure to always do it for a DataSource as
it is known to be problematic on Java EE application servers.
@Bean(destroyMethod="")
public DataSource dataSource() throws NamingException {
return (DataSource) jndiTemplate.lookup("MyDS");
}
Also, with @Bean methods, you will typically choose to use programmatic JNDI lookups:
either using Spring’s JndiTemplate/JndiLocatorDelegate helpers or straight JNDI
InitialContext usage, but not the JndiObjectFactoryBean variant which would force you
to declare the return type as the FactoryBean type instead of the actual target type, making it
harder to use for cross-reference calls in other @Bean methods that intend to refer to the provided
resource here.
Of course, in the case of Foo above, it would be equally as valid to call the init() method directly
during construction:
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@Configuration
public class AppConfig {
@Bean
public Foo foo() {
Foo foo = new Foo();
foo.init();
return foo;
}
// ...
}
Tip
When you work directly in Java, you can do anything you like with your objects and do not always
need to rely on the container lifecycle!
Specifying bean scope
Using the @Scope annotation
You can specify that your beans defined with the @Bean annotation should have a specific scope. You
can use any of the standard scopes specified in the Bean Scopes section.
The default scope is singleton, but you can override this with the @Scope annotation:
@Configuration
public class MyConfiguration {
@Bean
@Scope("prototype")
public Encryptor encryptor() {
// ...
}
}
@Scope and scoped-proxy
Spring offers a convenient way of working with scoped dependencies through scoped proxies. The
easiest way to create such a proxy when using the XML configuration is the <aop:scoped-proxy/
> element. Configuring your beans in Java with a @Scope annotation offers equivalent support with
the proxyMode attribute. The default is no proxy ( ScopedProxyMode.NO), but you can specify
ScopedProxyMode.TARGET_CLASS or ScopedProxyMode.INTERFACES.
If you port the scoped proxy example from the XML reference documentation (see preceding link) to
our @Bean using Java, it would look like the following:
// an HTTP Session-scoped bean exposed as a proxy
@Bean
@Scope(value = "session", proxyMode = ScopedProxyMode.TARGET_CLASS)
public UserPreferences userPreferences() {
return new UserPreferences();
}
@Bean
public Service userService() {
UserService service = new SimpleUserService();
// a reference to the proxied userPreferences bean
service.setUserPreferences(userPreferences());
return service;
}
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Customizing bean naming
By default, configuration classes use a @Bean method’s name as the name of the resulting bean. This
functionality can be overridden, however, with the name attribute.
@Configuration
public class AppConfig {
@Bean(name = "myFoo")
public Foo foo() {
return new Foo();
}
}
Bean aliasing
As discussed in the section called “Naming beans”, it is sometimes desirable to give a single bean
multiple names, otherwise known asbean aliasing. The name attribute of the @Bean annotation accepts
a String array for this purpose.
@Configuration
public class AppConfig {
@Bean(name = { "dataSource", "subsystemA-dataSource", "subsystemB-dataSource" })
public DataSource dataSource() {
// instantiate, configure and return DataSource bean...
}
}
Bean description
Sometimes it is helpful to provide a more detailed textual description of a bean. This can be particularly
useful when beans are exposed (perhaps via JMX) for monitoring purposes.
To add a description to a @Bean the @Description annotation can be used:
@Configuration
public class AppConfig {
@Bean
@Description("Provides a basic example of a bean")
public Foo foo() {
return new Foo();
}
}
Using the @Configuration annotation
@Configuration is a class-level annotation indicating that an object is a source of bean definitions.
@Configuration classes declare beans via public @Bean annotated methods. Calls to @Bean
methods on @Configuration classes can also be used to define inter-bean dependencies. See the
section called “Basic concepts: @Bean and @Configuration” for a general introduction.
Injecting inter-bean dependencies
When @Beans have dependencies on one another, expressing that dependency is as simple as having
one bean method call another:
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@Configuration
public class AppConfig {
@Bean
public Foo foo() {
return new Foo(bar());
}
@Bean
public Bar bar() {
return new Bar();
}
}
In the example above, the foo bean receives a reference to bar via constructor injection.
Note
This method of declaring inter-bean dependencies only works when the @Bean method is declared
within a @Configuration class. You cannot declare inter-bean dependencies using plain
@Component classes.
Lookup method injection
As noted earlier, lookup method injection is an advanced feature that you should use rarely. It is useful
in cases where a singleton-scoped bean has a dependency on a prototype-scoped bean. Using Java
for this type of configuration provides a natural means for implementing this pattern.
public abstract class CommandManager {
public Object process(Object commandState) {
// grab a new instance of the appropriate Command interface
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// okay... but where is the implementation of this method?
protected abstract Command createCommand();
}
Using Java-configuration support , you can create a subclass of CommandManager where the abstract
createCommand() method is overridden in such a way that it looks up a new (prototype) command
object:
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@Bean
@Scope("prototype")
public AsyncCommand asyncCommand() {
AsyncCommand command = new AsyncCommand();
// inject dependencies here as required
return command;
}
@Bean
public CommandManager commandManager() {
// return new anonymous implementation of CommandManager with command() overridden
// to return a new prototype Command object
return new CommandManager() {
protected Command createCommand() {
return asyncCommand();
}
}
}
Further information about how Java-based configuration works internally
The following example shows a @Bean annotated method being called twice:
@Configuration
public class AppConfig {
@Bean
public ClientService clientService1() {
ClientServiceImpl clientService = new ClientServiceImpl();
clientService.setClientDao(clientDao());
return clientService;
}
@Bean
public ClientService clientService2() {
ClientServiceImpl clientService = new ClientServiceImpl();
clientService.setClientDao(clientDao());
return clientService;
}
@Bean
public ClientDao clientDao() {
return new ClientDaoImpl();
}
}
clientDao() has been called once in clientService1() and once in clientService2(). Since
this method creates a new instance of ClientDaoImpl and returns it, you would normally expect
having 2 instances (one for each service). That definitely would be problematic: in Spring, instantiated
beans have a singleton scope by default. This is where the magic comes in: All @Configuration
classes are subclassed at startup-time with CGLIB. In the subclass, the child method checks the
container first for any cached (scoped) beans before it calls the parent method and creates a new
instance. Note that as of Spring 3.2, it is no longer necessary to add CGLIB to your classpath because
CGLIB classes have been repackaged under org.springframework and included directly within the
spring-core JAR.
Note
The behavior could be different according to the scope of your bean. We are talking about
singletons here.
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Note
There are a few restrictions due to the fact that CGLIB dynamically adds features at startup-time:
• Configuration classes should not be final
• They should have a constructor with no arguments
Composing Java-based configurations
Using the @Import annotation
Much as the <import/> element is used within Spring XML files to aid in modularizing configurations,
the @Import annotation allows for loading @Bean definitions from another configuration class:
@Configuration
public class ConfigA {
@Bean
public A a() {
return new A();
}
}
@Configuration
@Import(ConfigA.class)
public class ConfigB {
@Bean
public B b() {
return new B();
}
}
Now, rather than needing to specify both ConfigA.class and ConfigB.class when instantiating
the context, only ConfigB needs to be supplied explicitly:
public static void main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(ConfigB.class);
// now both beans A and B will be available...
A a = ctx.getBean(A.class);
B b = ctx.getBean(B.class);
}
This approach simplifies container instantiation, as only one class needs to be dealt with, rather than
requiring the developer to remember a potentially large number of @Configuration classes during
construction.
Injecting dependencies on imported @Bean definitions
The example above works, but is simplistic. In most practical scenarios, beans will have dependencies
on one another across configuration classes. When using XML, this is not an issue, per se, because
there is no compiler involved, and one can simply declare ref="someBean" and trust that Spring will
work it out during container initialization. Of course, when using @Configuration classes, the Java
compiler places constraints on the configuration model, in that references to other beans must be valid
Java syntax.
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Fortunately, solving this problem is simple. As we already discussed, @Bean method can have an
arbitrary number of parameters describing the bean dependencies. Let’s consider a more real-world
scenario with several @Configuration classes, each depending on beans declared in the others:
@Configuration
public class ServiceConfig {
@Bean
public TransferService transferService(AccountRepository accountRepository) {
return new TransferServiceImpl(accountRepository);
}
}
@Configuration
public class RepositoryConfig {
@Bean
public AccountRepository accountRepository(DataSource dataSource) {
return new JdbcAccountRepository(dataSource);
}
}
@Configuration
@Import({ServiceConfig.class, RepositoryConfig.class})
public class SystemTestConfig {
@Bean
public DataSource dataSource() {
// return new DataSource
}
}
public static void main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
// everything wires up across configuration classes...
TransferService transferService = ctx.getBean(TransferService.class);
transferService.transfer(100.00, "A123", "C456");
}
There is another way to achieve the same result. Remember that @Configuration classes are
ultimately just another bean in the container: This means that they can take advantage of @Autowired
and @Value injection etc just like any other bean!
Warning
Make sure that the dependencies you inject that way are of the simplest kind only.
@Configuration classes are processed quite early during the initialization of the context and
forcing a dependency to be injected this way may lead to unexpected early initialization. Whenever
possible, resort to parameter-based injection as in the example above.
Also, be particularly careful with BeanPostProcessor and BeanFactoryPostProcessor
definitions via @Bean. Those should usually be declared as static @Bean methods, not
triggering the instantiation of their containing configuration class. Otherwise, @Autowired and
@Value won’t work on the configuration class itself since it is being created as a bean instance
too early.
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@Configuration
public class ServiceConfig {
@Autowired
private AccountRepository accountRepository;
@Bean
public TransferService transferService() {
return new TransferServiceImpl(accountRepository);
}
}
@Configuration
public class RepositoryConfig {
@Autowired
private DataSource dataSource;
@Bean
public AccountRepository accountRepository() {
return new JdbcAccountRepository(dataSource);
}
}
@Configuration
@Import({ServiceConfig.class, RepositoryConfig.class})
public class SystemTestConfig {
@Bean
public DataSource dataSource() {
// return new DataSource
}
}
public static void main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
// everything wires up across configuration classes...
TransferService transferService = ctx.getBean(TransferService.class);
transferService.transfer(100.00, "A123", "C456");
}
In the scenario above, using @Autowired works well and provides the desired modularity, but
determining exactly where the autowired bean definitions are declared is still somewhat ambiguous. For
example, as a developer looking at ServiceConfig, how do you know exactly where the @Autowired
AccountRepository bean is declared? It’s not explicit in the code, and this may be just fine.
Remember that the Spring Tool Suite provides tooling that can render graphs showing how everything
is wired up - that may be all you need. Also, your Java IDE can easily find all declarations and uses of
the AccountRepository type, and will quickly show you the location of @Bean methods that return
that type.
In cases where this ambiguity is not acceptable and you wish to have direct navigation from within
your IDE from one @Configuration class to another, consider autowiring the configuration classes
themselves:
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@Configuration
public class ServiceConfig {
@Autowired
private RepositoryConfig repositoryConfig;
@Bean
public TransferService transferService() {
// navigate 'through' the config class to the @Bean method!
return new TransferServiceImpl(repositoryConfig.accountRepository());
}
}
In the situation above, it is completely explicit where AccountRepository is defined. However,
ServiceConfig is now tightly coupled to RepositoryConfig; that’s the tradeoff. This tight coupling
can be somewhat mitigated by using interface-based or abstract class-based @Configuration
classes. Consider the following:
@Configuration
public class ServiceConfig {
@Autowired
private RepositoryConfig repositoryConfig;
@Bean
public TransferService transferService() {
return new TransferServiceImpl(repositoryConfig.accountRepository());
}
}
@Configuration
public interface RepositoryConfig {
@Bean
AccountRepository accountRepository();
}
@Configuration
public class DefaultRepositoryConfig implements RepositoryConfig {
@Bean
public AccountRepository accountRepository() {
return new JdbcAccountRepository(...);
}
}
@Configuration
@Import({ServiceConfig.class, DefaultRepositoryConfig.class}) // import the concrete config!
public class SystemTestConfig {
@Bean
public DataSource dataSource() {
// return DataSource
}
}
public static void main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
TransferService transferService = ctx.getBean(TransferService.class);
transferService.transfer(100.00, "A123", "C456");
}
Now ServiceConfig is loosely coupled with respect to the concrete DefaultRepositoryConfig,
and built-in IDE tooling is still useful: it will be easy for the developer to get a type hierarchy of
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RepositoryConfig implementations. In this way, navigating @Configuration classes and their
dependencies becomes no different than the usual process of navigating interface-based code.
Conditionally include @Configuration classes or @Bean methods
It is often useful to conditionally enable or disable a complete @Configuration class, or even
individual @Bean methods, based on some arbitrary system state. One common example of this is to
use the @Profile annotation to activate beans only when a specific profile has been enabled in the
Spring Environment (see the section called “Bean definition profiles” for details).
The @Profile annotation is actually implemented using a much more flexible
annotation called @Conditional. The @Conditional annotation indicates specific
org.springframework.context.annotation.Condition implementations that should be
consulted before a @Bean is registered.
Implementations of the Condition interface simply provide a matches(…) method that returns true
or false. For example, here is the actual Condition implementation used for @Profile:
@Override
public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
if (context.getEnvironment() != null) {
// Read the @Profile annotation attributes
MultiValueMap<String, Object> attrs =
metadata.getAllAnnotationAttributes(Profile.class.getName());
if (attrs != null) {
for (Object value : attrs.get("value")) {
if (context.getEnvironment().acceptsProfiles(((String[]) value))) {
return true;
}
}
return false;
}
}
return true;
}
See the @Conditional javadocs for more detail.
Combining Java and XML configuration
Spring’s @Configuration class support does not aim to be a 100% complete replacement for Spring
XML. Some facilities such as Spring XML namespaces remain an ideal way to configure the container.
In cases where XML is convenient or necessary, you have a choice: either instantiate the container in an
"XML-centric" way using, for example, ClassPathXmlApplicationContext, or in a "Java-centric"
fashion using AnnotationConfigApplicationContext and the @ImportResource annotation to
import XML as needed.
XML-centric use of @Configuration classes
It may be preferable to bootstrap the Spring container from XML and include @Configuration classes
in an ad-hoc fashion. For example, in a large existing codebase that uses Spring XML, it will be easier to
create @Configuration classes on an as-needed basis and include them from the existing XML files.
Below you’ll find the options for using @Configuration classes in this kind of "XML-centric" situation.
Remember that @Configuration classes are ultimately just bean definitions in the container. In this
example, we create a @Configuration class named AppConfig and include it within system-testconfig.xml as a <bean/> definition. Because <context:annotation-config/> is switched
on, the container will recognize the @Configuration annotation and process the @Bean methods
declared in AppConfig properly.
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@Configuration
public class AppConfig {
@Autowired
private DataSource dataSource;
@Bean
public AccountRepository accountRepository() {
return new JdbcAccountRepository(dataSource);
}
@Bean
public TransferService transferService() {
return new TransferService(accountRepository());
}
}
system-test-config.xml:
<beans>
<!-- enable processing of annotations such as @Autowired and @Configuration -->
<context:annotation-config/>
<context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>
<bean class="com.acme.AppConfig"/>
<bean class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</bean>
</beans>
jdbc.properties:
jdbc.url=jdbc:hsqldb:hsql://localhost/xdb
jdbc.username=sa
jdbc.password=
public static void main(String[] args) {
ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath:/com/acme/system-testconfig.xml");
TransferService transferService = ctx.getBean(TransferService.class);
// ...
}
Note
In system-test-config.xml above, the AppConfig <bean/> does not declare an id
element. While it would be acceptable to do so, it is unnecessary given that no other bean will ever
refer to it, and it is unlikely that it will be explicitly fetched from the container by name. Likewise
with the DataSource bean - it is only ever autowired by type, so an explicit bean id is not strictly
required.
Because @Configuration is meta-annotated with @Component, @Configuration-annotated
classes are automatically candidates for component scanning. Using the same scenario as above,
we can redefine system-test-config.xml to take advantage of component-scanning. Note that
in this case, we don’t need to explicitly declare <context:annotation-config/>, because
<context:component-scan/> enables the same functionality.
system-test-config.xml:
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<beans>
<!-- picks up and registers AppConfig as a bean definition -->
<context:component-scan base-package="com.acme"/>
<context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>
<bean class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</bean>
</beans>
@Configuration class-centric use of XML with @ImportResource
In applications where @Configuration classes are the primary mechanism for configuring the
container, it will still likely be necessary to use at least some XML. In these scenarios, simply use
@ImportResource and define only as much XML as is needed. Doing so achieves a "Java-centric"
approach to configuring the container and keeps XML to a bare minimum.
@Configuration
@ImportResource("classpath:/com/acme/properties-config.xml")
public class AppConfig {
@Value("${jdbc.url}")
private String url;
@Value("${jdbc.username}")
private String username;
@Value("${jdbc.password}")
private String password;
@Bean
public DataSource dataSource() {
return new DriverManagerDataSource(url, username, password);
}
}
properties-config.xml
<beans>
<context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>
</beans>
jdbc.properties
jdbc.url=jdbc:hsqldb:hsql://localhost/xdb
jdbc.username=sa
jdbc.password=
public static void main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
TransferService transferService = ctx.getBean(TransferService.class);
// ...
}
6.13 Environment abstraction
The Environment is an abstraction integrated in the container that models two key aspects of the
application environment: profiles and properties.
A profile is a named, logical group of bean definitions to be registered with the container only if the given
profile is active. Beans may be assigned to a profile whether defined in XML or via annotations. The role
of the Environment object with relation to profiles is in determining which profiles (if any) are currently
active, and which profiles (if any) should be active by default.
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Properties play an important role in almost all applications, and may originate from a variety of
sources: properties files, JVM system properties, system environment variables, JNDI, servlet context
parameters, ad-hoc Properties objects, Maps, and so on. The role of the Environment object with
relation to properties is to provide the user with a convenient service interface for configuring property
sources and resolving properties from them.
Bean definition profiles
Bean definition profiles is a mechanism in the core container that allows for registration of different beans
in different environments. The word environment can mean different things to different users and this
feature can help with many use cases, including:
• working against an in-memory datasource in development vs looking up that same datasource from
JNDI when in QA or production
• registering monitoring infrastructure only when deploying an application into a performance
environment
• registering customized implementations of beans for customer A vs. customer B deployments
Let’s consider the first use case in a practical application that requires a DataSource. In a test
environment, the configuration may look like this:
@Bean
public DataSource dataSource() {
return new EmbeddedDatabaseBuilder()
.setType(EmbeddedDatabaseType.HSQL)
.addScript("my-schema.sql")
.addScript("my-test-data.sql")
.build();
}
Let’s now consider how this application will be deployed into a QA or production environment, assuming
that the datasource for the application will be registered with the production application server’s JNDI
directory. Our dataSource bean now looks like this:
@Bean(destroyMethod="")
public DataSource dataSource() throws Exception {
Context ctx = new InitialContext();
return (DataSource) ctx.lookup("java:comp/env/jdbc/datasource");
}
The problem is how to switch between using these two variations based on the current environment.
Over time, Spring users have devised a number of ways to get this done, usually relying on a combination
of system environment variables and XML <import/> statements containing ${placeholder}
tokens that resolve to the correct configuration file path depending on the value of an environment
variable. Bean definition profiles is a core container feature that provides a solution to this problem.
If we generalize the example use case above of environment-specific bean definitions, we end up with
the need to register certain bean definitions in certain contexts, while not in others. You could say that
you want to register a certain profile of bean definitions in situation A, and a different profile in situation
B. Let’s first see how we can update our configuration to reflect this need.
@Profile
The @Profile annotation allows you to indicate that a component is eligible for registration when
one or more specified profiles are active. Using our example above, we can rewrite the dataSource
configuration as follows:
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@Configuration
@Profile("dev")
public class StandaloneDataConfig {
@Bean
public DataSource dataSource() {
return new EmbeddedDatabaseBuilder()
.setType(EmbeddedDatabaseType.HSQL)
.addScript("classpath:com/bank/config/sql/schema.sql")
.addScript("classpath:com/bank/config/sql/test-data.sql")
.build();
}
}
@Configuration
@Profile("production")
public class JndiDataConfig {
@Bean(destroyMethod="")
public DataSource dataSource() throws Exception {
Context ctx = new InitialContext();
return (DataSource) ctx.lookup("java:comp/env/jdbc/datasource");
}
}
Note
As mentioned before, with @Bean methods, you will typically choose to use programmatic JNDI
lookups: either using Spring’s JndiTemplate/JndiLocatorDelegate helpers or the straight
JNDI InitialContext usage shown above, but not the JndiObjectFactoryBean variant
which would force you to declare the return type as the FactoryBean type.
@Profile can be used as a meta-annotation for the purpose of creating a custom composed
annotation. The following example defines a custom @Production annotation that can be used as a
drop-in replacement for @Profile("production"):
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Profile("production")
public @interface Production {
}
@Profile can also be declared at the method level to include only one particular bean of a configuration
class:
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@Configuration
public class AppConfig {
@Bean
@Profile("dev")
public DataSource devDataSource() {
return new EmbeddedDatabaseBuilder()
.setType(EmbeddedDatabaseType.HSQL)
.addScript("classpath:com/bank/config/sql/schema.sql")
.addScript("classpath:com/bank/config/sql/test-data.sql")
.build();
}
@Bean
@Profile("production")
public DataSource productionDataSource() throws Exception {
Context ctx = new InitialContext();
return (DataSource) ctx.lookup("java:comp/env/jdbc/datasource");
}
}
Tip
If a @Configuration class is marked with @Profile, all of the @Bean methods and @Import
annotations associated with that class will be bypassed unless one or more of the specified profiles
are active. If a @Component or @Configuration class is marked with @Profile({"p1",
"p2"}), that class will not be registered/processed unless profiles 'p1' and/or 'p2' have been
activated. If a given profile is prefixed with the NOT operator (!), the annotated element will
be registered if the profile is not active. For example, given @Profile({"p1", "!p2"}),
registration will occur if profile 'p1' is active or if profile 'p2' is not active.
XML bean definition profiles
The XML counterpart is the profile attribute of the <beans> element. Our sample configuration above
can be rewritten in two XML files as follows:
<beans profile="dev"
xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:jdbc="http://www.springframework.org/schema/jdbc"
xsi:schemaLocation="...">
<jdbc:embedded-database id="dataSource">
<jdbc:script location="classpath:com/bank/config/sql/schema.sql"/>
<jdbc:script location="classpath:com/bank/config/sql/test-data.sql"/>
</jdbc:embedded-database>
</beans>
<beans profile="production"
xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:jee="http://www.springframework.org/schema/jee"
xsi:schemaLocation="...">
<jee:jndi-lookup id="dataSource" jndi-name="java:comp/env/jdbc/datasource"/>
</beans>
It is also possible to avoid that split and nest <beans/> elements within the same file:
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<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:jdbc="http://www.springframework.org/schema/jdbc"
xmlns:jee="http://www.springframework.org/schema/jee"
xsi:schemaLocation="...">
<!-- other bean definitions -->
<beans profile="dev">
<jdbc:embedded-database id="dataSource">
<jdbc:script location="classpath:com/bank/config/sql/schema.sql"/>
<jdbc:script location="classpath:com/bank/config/sql/test-data.sql"/>
</jdbc:embedded-database>
</beans>
<beans profile="production">
<jee:jndi-lookup id="dataSource" jndi-name="java:comp/env/jdbc/datasource"/>
</beans>
</beans>
The spring-bean.xsd has been constrained to allow such elements only as the last ones in the file.
This should help provide flexibility without incurring clutter in the XML files.
Activating a profile
Now that we have updated our configuration, we still need to instruct Spring which profile is active. If
we started our sample application right now, we would see a NoSuchBeanDefinitionException
thrown, because the container could not find the Spring bean named dataSource.
Activating a profile can be done in several ways, but the most straightforward is to do it programmatically
against the Environment API which is available via an ApplicationContext:
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
ctx.getEnvironment().setActiveProfiles("dev");
ctx.register(SomeConfig.class, StandaloneDataConfig.class, JndiDataConfig.class);
ctx.refresh();
In addition, profiles may also be activated declaratively through the spring.profiles.active
property which may be specified through system environment variables, JVM system properties, servlet
context parameters in web.xml, or even as an entry in JNDI (see the section called “PropertySource
abstraction”). In integration tests, active profiles can be declared via the @ActiveProfiles annotation
in the spring-test module (see the section called “Context configuration with environment profiles”).
Note that profiles are not an "either-or" proposition; it is possible to activate multiple profiles at once.
Programmatically, simply provide multiple profile names to the setActiveProfiles() method, which
accepts String… varargs:
ctx.getEnvironment().setActiveProfiles("profile1", "profile2");
Declaratively, spring.profiles.active may accept a comma-separated list of profile names:
-Dspring.profiles.active="profile1,profile2"
Default profile
The default profile represents the profile that is enabled by default. Consider the following:
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@Configuration
@Profile("default")
public class DefaultDataConfig {
@Bean
public DataSource dataSource() {
return new EmbeddedDatabaseBuilder()
.setType(EmbeddedDatabaseType.HSQL)
.addScript("classpath:com/bank/config/sql/schema.sql")
.build();
}
}
If no profile is active, the dataSource above will be created; this can be seen as a way to provide a
default definition for one or more beans. If any profile is enabled, the default profile will not apply.
The name of the default profile can be changed using setDefaultProfiles() on the Environment
or declaratively using the spring.profiles.default property.
PropertySource abstraction
Spring’s Environment abstraction provides search operations over a configurable hierarchy of
property sources. To explain fully, consider the following:
ApplicationContext ctx = new GenericApplicationContext();
Environment env = ctx.getEnvironment();
boolean containsFoo = env.containsProperty("foo");
System.out.println("Does my environment contain the 'foo' property? " + containsFoo);
In the snippet above, we see a high-level way of asking Spring whether the foo property is defined for the
current environment. To answer this question, the Environment object performs a search over a set of
PropertySource objects. A PropertySource is a simple abstraction over any source of key-value
pairs, and Spring’s StandardEnvironment is configured with two PropertySource objects — one
representing the set of JVM system properties (a la System.getProperties()) and one representing
the set of system environment variables (a la System.getenv()).
Note
These default property sources are present for StandardEnvironment, for use in
standalone applications. StandardServletEnvironment is populated with additional
default property sources including servlet config and servlet context parameters.
StandardPortletEnvironment similarly has access to portlet config and portlet context
parameters as property sources. Both can optionally enable a JndiPropertySource. See the
javadocs for details.
Concretely, when using the StandardEnvironment, the call to env.containsProperty("foo")
will return true if a foo system property or foo environment variable is present at runtime.
Tip
The search performed is hierarchical. By default, system properties have precedence over
environment variables, so if the foo property happens to be set in both places during a call to
env.getProperty("foo"), the system property value will 'win' and be returned preferentially
over the environment variable. Note that property values will not get merged but rather completely
overridden by a preceding entry.
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For a common StandardServletEnvironment, the full hierarchy looks as follows, with the
highest-precedence entries at the top: * ServletConfig parameters (if applicable, e.g. in case of a
DispatcherServlet context) * ServletContext parameters (web.xml context-param entries) *
JNDI environment variables ("java:comp/env/" entries) * JVM system properties ("-D" commandline arguments) * JVM system environment (operating system environment variables)
Most importantly, the entire mechanism is configurable. Perhaps you have a custom source of properties
that you’d like to integrate into this search. No problem — simply implement and instantiate your own
PropertySource and add it to the set of PropertySources for the current Environment:
ConfigurableApplicationContext ctx = new GenericApplicationContext();
MutablePropertySources sources = ctx.getEnvironment().getPropertySources();
sources.addFirst(new MyPropertySource());
In the code above, MyPropertySource has been added with highest precedence in the search. If
it contains a foo property, it will be detected and returned ahead of any foo property in any other
PropertySource. The MutablePropertySources API exposes a number of methods that allow for
precise manipulation of the set of property sources.
@PropertySource
The @PropertySource annotation provides a convenient and declarative mechanism for adding a
PropertySource to Spring’s Environment.
Given a file "app.properties" containing the key/value pair testbean.name=myTestBean, the
following @Configuration class uses @PropertySource in such a way that a call to
testBean.getName() will return "myTestBean".
@Configuration
@PropertySource("classpath:/com/myco/app.properties")
public class AppConfig {
@Autowired
Environment env;
@Bean
public TestBean testBean() {
TestBean testBean = new TestBean();
testBean.setName(env.getProperty("testbean.name"));
return testBean;
}
}
Any ${…} placeholders present in a @PropertySource resource location will be resolved against the
set of property sources already registered against the environment. For example:
@Configuration
@PropertySource("classpath:/com/${my.placeholder:default/path}/app.properties")
public class AppConfig {
@Autowired
Environment env;
@Bean
public TestBean testBean() {
TestBean testBean = new TestBean();
testBean.setName(env.getProperty("testbean.name"));
return testBean;
}
}
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Assuming that "my.placeholder" is present in one of the property sources already registered, e.g. system
properties or environment variables, the placeholder will be resolved to the corresponding value. If not,
then "default/path" will be used as a default. If no default is specified and a property cannot be resolved,
an IllegalArgumentException will be thrown.
Placeholder resolution in statements
Historically, the value of placeholders in elements could be resolved only against JVM system properties
or environment variables. No longer is this the case. Because the Environment abstraction is integrated
throughout the container, it’s easy to route resolution of placeholders through it. This means that you
may configure the resolution process in any way you like: change the precedence of searching through
system properties and environment variables, or remove them entirely; add your own property sources
to the mix as appropriate.
Concretely, the following statement works regardless of where the customer property is defined, as
long as it is available in the Environment:
<beans>
<import resource="com/bank/service/${customer}-config.xml"/>
</beans>
6.14 Registering a LoadTimeWeaver
The LoadTimeWeaver is used by Spring to dynamically transform classes as they are loaded into the
Java virtual machine (JVM).
To enable load-time weaving add the @EnableLoadTimeWeaving to one of your @Configuration
classes:
@Configuration
@EnableLoadTimeWeaving
public class AppConfig {
}
Alternatively for XML configuration use the context:load-time-weaver element:
<beans>
<context:load-time-weaver/>
</beans>
Once configured for the ApplicationContext. Any bean within that ApplicationContext
may implement LoadTimeWeaverAware, thereby receiving a reference to the load-time
weaver instance. This is particularly useful in combination with Spring’s JPA support
where load-time weaving may be necessary for JPA class transformation. Consult the
LocalContainerEntityManagerFactoryBean javadocs for more detail. For more on AspectJ
load-time weaving, see the section called “Load-time weaving with AspectJ in the Spring Framework”.
6.15 Additional Capabilities of the ApplicationContext
As was discussed in the chapter introduction, the org.springframework.beans.factory package
provides basic functionality for managing and manipulating beans, including in a programmatic
way. The org.springframework.context package adds the ApplicationContext interface,
which extends the BeanFactory interface, in addition to extending other interfaces to provide
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additional functionality in a more application framework-oriented style. Many people use the
ApplicationContext in a completely declarative fashion, not even creating it programmatically,
but instead relying on support classes such as ContextLoader to automatically instantiate an
ApplicationContext as part of the normal startup process of a Java EE web application.
To enhance BeanFactory functionality in a more framework-oriented style the context package also
provides the following functionality:
• Access to messages in i18n-style, through the MessageSource interface.
• Access to resources, such as URLs and files, through the ResourceLoader interface.
• Event publication to namely beans implementing the ApplicationListener interface, through the
use of the ApplicationEventPublisher interface.
• Loading of multiple (hierarchical) contexts, allowing each to be focused on one particular layer, such
as the web layer of an application, through the HierarchicalBeanFactory interface.
Internationalization using MessageSource
The ApplicationContext interface extends an interface called MessageSource, and
therefore provides internationalization (i18n) functionality. Spring also provides the interface
HierarchicalMessageSource, which can resolve messages hierarchically. Together these
interfaces provide the foundation upon which Spring effects message resolution. The methods defined
on these interfaces include:
• String getMessage(String code, Object[] args, String default, Locale loc):
The basic method used to retrieve a message from the MessageSource. When no message is found
for the specified locale, the default message is used. Any arguments passed in become replacement
values, using the MessageFormat functionality provided by the standard library.
• String getMessage(String code, Object[] args, Locale loc): Essentially the same
as the previous method, but with one difference: no default message can be specified; if the message
cannot be found, a NoSuchMessageException is thrown.
• String getMessage(MessageSourceResolvable resolvable, Locale locale):
All properties used in the preceding methods are also wrapped in a class named
MessageSourceResolvable, which you can use with this method.
When an ApplicationContext is loaded, it automatically searches for a MessageSource bean
defined in the context. The bean must have the name messageSource. If such a bean is found, all
calls to the preceding methods are delegated to the message source. If no message source is found,
the ApplicationContext attempts to find a parent containing a bean with the same name. If it does,
it uses that bean as the MessageSource. If the ApplicationContext cannot find any source for
messages, an empty DelegatingMessageSource is instantiated in order to be able to accept calls
to the methods defined above.
Spring provides two MessageSource implementations, ResourceBundleMessageSource and
StaticMessageSource. Both implement HierarchicalMessageSource in order to do nested
messaging. The StaticMessageSource is rarely used but provides programmatic ways to add
messages to the source. The ResourceBundleMessageSource is shown in the following example:
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<beans>
<bean id="messageSource"
class="org.springframework.context.support.ResourceBundleMessageSource">
<property name="basenames">
<list>
<value>format</value>
<value>exceptions</value>
<value>windows</value>
</list>
</property>
</bean>
</beans>
In the example it is assumed you have three resource bundles defined in your classpath called format,
exceptions and windows. Any request to resolve a message will be handled in the JDK standard
way of resolving messages through ResourceBundles. For the purposes of the example, assume the
contents of two of the above resource bundle files are…
# in format.properties
message=Alligators rock!
# in exceptions.properties
argument.required=The {0} argument is required.
A program to execute the MessageSource functionality is shown in the next example. Remember that
all ApplicationContext implementations are also MessageSource implementations and so can be
cast to the MessageSource interface.
public static void main(String[] args) {
MessageSource resources = new ClassPathXmlApplicationContext("beans.xml");
String message = resources.getMessage("message", null, "Default", null);
System.out.println(message);
}
The resulting output from the above program will be…
Alligators rock!
So to summarize, the MessageSource is defined in a file called beans.xml, which exists at the root of
your classpath. The messageSource bean definition refers to a number of resource bundles through
its basenames property. The three files that are passed in the list to the basenames property exist as
files at the root of your classpath and are called format.properties, exceptions.properties,
and windows.properties respectively.
The next example shows arguments passed to the message lookup; these arguments will be converted
into Strings and inserted into placeholders in the lookup message.
<beans>
<!-- this MessageSource is being used in a web application -->
<bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource">
<property name="basename" value="exceptions"/>
</bean>
<!-- lets inject the above MessageSource into this POJO -->
<bean id="example" class="com.foo.Example">
<property name="messages" ref="messageSource"/>
</bean>
</beans>
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public class Example {
private MessageSource messages;
public void setMessages(MessageSource messages) {
this.messages = messages;
}
public void execute() {
String message = this.messages.getMessage("argument.required",
new Object [] {"userDao"}, "Required", null);
System.out.println(message);
}
}
The resulting output from the invocation of the execute() method will be…
The userDao argument is required.
With regard to internationalization (i18n), Spring’s various MessageSource implementations follow
the same locale resolution and fallback rules as the standard JDK ResourceBundle. In short, and
continuing with the example messageSource defined previously, if you want to resolve messages
against the British (en-GB) locale, you would create files called format_en_GB.properties,
exceptions_en_GB.properties, and windows_en_GB.properties respectively.
Typically, locale resolution is managed by the surrounding environment of the application. In this
example, the locale against which (British) messages will be resolved is specified manually.
# in exceptions_en_GB.properties
argument.required=Ebagum lad, the {0} argument is required, I say, required.
public static void main(final String[] args) {
MessageSource resources = new ClassPathXmlApplicationContext("beans.xml");
String message = resources.getMessage("argument.required",
new Object [] {"userDao"}, "Required", Locale.UK);
System.out.println(message);
}
The resulting output from the running of the above program will be…
Ebagum lad, the 'userDao' argument is required, I say, required.
You can also use the MessageSourceAware interface to acquire a reference to any MessageSource
that has been defined. Any bean that is defined in an ApplicationContext that implements the
MessageSourceAware interface is injected with the application context’s MessageSource when the
bean is created and configured.
Note
As
an
alternative
to
ResourceBundleMessageSource,
Spring
provides
a
ReloadableResourceBundleMessageSource class. This variant supports the same bundle
file format but is more flexible than the standard JDK based ResourceBundleMessageSource
implementation. In particular, it allows for reading files from any Spring resource location (not just
from the classpath) and supports hot reloading of bundle property files (while efficiently caching
them in between). Check out the ReloadableResourceBundleMessageSource javadocs for
details.
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Standard and Custom Events
Event handling in the ApplicationContext is provided through the ApplicationEvent class
and ApplicationListener interface. If a bean that implements the ApplicationListener
interface is deployed into the context, every time an ApplicationEvent gets published to the
ApplicationContext, that bean is notified. Essentially, this is the standard Observer design pattern.
Tip
As of Spring 4.2, the event infrastructure has been significantly improved and offer an annotationbased model as well as the ability to publish any arbitrary event, that is an object that does not
necessarily extend from ApplicationEvent. When such an object is published we wrap it in
a PayloadApplicationEvent for you.
Spring provides the following standard events:
Table 6.7. Built-in Events
Event
Explanation
ContextRefreshedEvent
Published when the ApplicationContext
is initialized or refreshed, for example,
using the refresh() method on the
ConfigurableApplicationContext
interface. "Initialized" here means that all
beans are loaded, post-processor beans are
detected and activated, singletons are preinstantiated, and the ApplicationContext
object is ready for use. As long as the
context has not been closed, a refresh can
be triggered multiple times, provided that
the chosen ApplicationContext actually
supports such "hot" refreshes. For example,
XmlWebApplicationContext supports hot
refreshes, but GenericApplicationContext
does not.
ContextStartedEvent
Published when the ApplicationContext
is started, using the start() method on
the ConfigurableApplicationContext
interface. "Started" here means that all
Lifecycle beans receive an explicit start
signal. Typically this signal is used to restart
beans after an explicit stop, but it may also
be used to start components that have not
been configured for autostart , for example,
components that have not already started on
initialization.
ContextStoppedEvent
Published when the ApplicationContext
is stopped, using the stop() method on
the ConfigurableApplicationContext
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Event
Explanation
interface. "Stopped" here means that all
Lifecycle beans receive an explicit stop
signal. A stopped context may be restarted
through a start() call.
ContextClosedEvent
Published when the ApplicationContext
is closed, using the close() method on the
ConfigurableApplicationContext
interface. "Closed" here means that all singleton
beans are destroyed. A closed context reaches
its end of life; it cannot be refreshed or restarted.
RequestHandledEvent
A web-specific event telling all beans that an
HTTP request has been serviced. This event
is published after the request is complete. This
event is only applicable to web applications
using Spring’s DispatcherServlet.
You can also create and publish your own custom events. This example demonstrates a simple class
that extends Spring’s ApplicationEvent base class:
public class BlackListEvent extends ApplicationEvent {
private final String address;
private final String test;
public BlackListEvent(Object source, String address, String test) {
super(source);
this.address = address;
this.test = test;
}
// accessor and other methods...
}
To publish a custom ApplicationEvent, call the publishEvent() method on an
ApplicationEventPublisher. Typically this is done by creating a class that implements
ApplicationEventPublisherAware and registering it as a Spring bean. The following example
demonstrates such a class:
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public class EmailService implements ApplicationEventPublisherAware {
private List<String> blackList;
private ApplicationEventPublisher publisher;
public void setBlackList(List<String> blackList) {
this.blackList = blackList;
}
public void setApplicationEventPublisher(ApplicationEventPublisher publisher) {
this.publisher = publisher;
}
public void sendEmail(String address, String text) {
if (blackList.contains(address)) {
BlackListEvent event = new BlackListEvent(this, address, text);
publisher.publishEvent(event);
return;
}
// send email...
}
}
At
configuration
time,
the
Spring
container
will
detect
that
EmailService
implements
ApplicationEventPublisherAware
and
will
automatically
call
setApplicationEventPublisher(). In reality, the parameter passed in will be the Spring container
itself; you’re simply interacting with the application context via its ApplicationEventPublisher
interface.
To receive the custom ApplicationEvent, create a class that implements ApplicationListener
and register it as a Spring bean. The following example demonstrates such a class:
public class BlackListNotifier implements ApplicationListener<BlackListEvent> {
private String notificationAddress;
public void setNotificationAddress(String notificationAddress) {
this.notificationAddress = notificationAddress;
}
public void onApplicationEvent(BlackListEvent event) {
// notify appropriate parties via notificationAddress...
}
}
Notice that ApplicationListener is generically parameterized with the type of your custom event,
BlackListEvent. This means that the onApplicationEvent() method can remain type-safe,
avoiding any need for downcasting. You may register as many event listeners as you wish, but note that
by default event listeners receive events synchronously. This means the publishEvent() method
blocks until all listeners have finished processing the event. One advantage of this synchronous and
single-threaded approach is that when a listener receives an event, it operates inside the transaction
context of the publisher if a transaction context is available. If another strategy for event publication
becomes necessary, refer to the JavaDoc for Spring’s ApplicationEventMulticaster interface.
The following example shows the bean definitions used to register and configure each of the classes
above:
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<bean id="emailService" class="example.EmailService">
<property name="blackList">
<list>
<value>known.spammer@example.org</value>
<value>known.hacker@example.org</value>
<value>john.doe@example.org</value>
</list>
</property>
</bean>
<bean id="blackListNotifier" class="example.BlackListNotifier">
<property name="notificationAddress" value="blacklist@example.org"/>
</bean>
Putting it all together, when the sendEmail() method of the emailService bean is called, if there
are any emails that should be blacklisted, a custom event of type BlackListEvent is published.
The blackListNotifier bean is registered as an ApplicationListener and thus receives the
BlackListEvent, at which point it can notify appropriate parties.
Note
Spring’s eventing mechanism is designed for simple communication between Spring beans within
the same application context. However, for more sophisticated enterprise integration needs,
the separately-maintained Spring Integration project provides complete support for building
lightweight, pattern-oriented, event-driven architectures that build upon the well-known Spring
programming model.
Annotation-based Event Listeners
As of Spring 4.2, an event listener can be registered on any public method of a managed bean via the
EventListener annotation. The BlackListNotifier can be rewritten as follows:
public class BlackListNotifier {
private String notificationAddress;
public void setNotificationAddress(String notificationAddress) {
this.notificationAddress = notificationAddress;
}
@EventListener
public void processBlackListEvent(BlackListEvent event) {
// notify appropriate parties via notificationAddress...
}
}
As you can see above, the method signature actually infer which even type it listens to. This also works
for nested generics as long as the actual event resolves the generics parameter you would filter on.
If your method should listen to several events or if you want to define it with no parameter at all, the
event type(s) can also be specified on the annotation itself:
@EventListener({ContextStartedEvent.class, ContextRefreshedEvent.class})
public void handleContextStart() {
}
It is also possible to add additional runtime filtering via the condition attribute of the annotation that
defines a SpEL expression that should match to actually invoke the method for a particular event.
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For instance, our notifier can be rewritten to be only invoked if the test attribute of the event is equal
to foo:
@EventListener(condition = "#event.test == 'foo'")
public void processBlackListEvent(BlackListEvent event) {
// notify appropriate parties via notificationAddress...
}
Each SpEL expression evaluates again a dedicated context. The next table lists the items made
available to the context so one can use them for conditional event processing:
Table 6.8. Event SpEL available metadata
Name
Location
Description
Example
event
root object
The actual
ApplicationEvent
#root.event
args
root object
The arguments (as
#root.args[0]
array) used for invoking
the target
argument name
evaluation context
Name of any of the
method arguments.
If for some reason
the names are not
available (e.g. no
debug information),
the argument names
are also available
under the #a<#arg>
where #arg stands for
the argument index
(starting from 0).
#iban or #a0 (one
can also use #p0 or
#p<#arg> notation as
an alias).
Note that #root.event allows you to access to the underlying event, even if your method signature
actually refers to an arbitrary object that was published.
If you need to publish an event as the result of processing another, just change the method signature
to return the event that should be published, something like:
@EventListener
public ListUpdateEvent handleBlackListEvent(BlackListEvent event) {
// notify appropriate parties via notificationAddress and
// then publish a ListUpdateEvent...
}
This new method will publish a new ListUpdateEvent for every BlackListEvent handled by the
method above. If you need to publish several events, just return a Collection of events instead.
Finally if you need the listener to be invoked before another one, just add the @Order annotation to
the method declaration:
@EventListener
@Order(42)
public void processBlackListEvent(BlackListEvent event) {
// notify appropriate parties via notificationAddress...
}
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Generic Events
You may also use generics to further define the structure of your event. Consider an
EntityCreatedEvent<T> where T is the type of the actual entity that got created. You can create
the following listener definition to only receive EntityCreatedEvent for a Person:
@EventListener
public void onPersonCreated(EntityCreatedEvent<Person> event) {
...
}
Due to type erasure, this will only work if the event that is fired resolves the generic parameter(s) on
which the event listener filters on (that is something like class PersonCreatedEvent extends
EntityCreatedEvent<Person> { … }).
In certain circumstances, this may become quite tedious if all events follow the same structure
(as it should be the case for the event above). In such a case, you can implement
ResolvableTypeProvider to guide the framework beyond what the runtime environment provides:
public class EntityCreatedEvent<T>
extends ApplicationEvent implements ResolvableTypeProvider {
public EntityCreatedEvent(T entity) {
super(entity);
}
@Override
public ResolvableType getResolvableType() {
return ResolvableType.forClassWithGenerics(getClass(),
ResolvableType.forInstance(getSource()));
}
}
Tip
This works not only for ApplicationEvent but any arbitrary object that you’d send as an event.
Convenient access to low-level resources
For optimal usage and understanding of application contexts, users should generally familiarize
themselves with Spring’s Resource abstraction, as described in the chapter Chapter 7, Resources.
An application context is a ResourceLoader, which can be used to load Resources. A Resource is
essentially a more feature rich version of the JDK class java.net.URL, in fact, the implementations
of the Resource wrap an instance of java.net.URL where appropriate. A Resource can obtain
low-level resources from almost any location in a transparent fashion, including from the classpath,
a filesystem location, anywhere describable with a standard URL, and some other variations. If the
resource location string is a simple path without any special prefixes, where those resources come from
is specific and appropriate to the actual application context type.
You can configure a bean deployed into the application context to implement the special callback
interface, ResourceLoaderAware, to be automatically called back at initialization time with the
application context itself passed in as the ResourceLoader. You can also expose properties of type
Resource, to be used to access static resources; they will be injected into it like any other properties.
You can specify those Resource properties as simple String paths, and rely on a special JavaBean
PropertyEditor that is automatically registered by the context, to convert those text strings to actual
Resource objects when the bean is deployed.
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The location path or paths supplied to an ApplicationContext constructor are actually resource
strings, and in simple form are treated appropriately to the specific context implementation.
ClassPathXmlApplicationContext treats a simple location path as a classpath location. You can
also use location paths (resource strings) with special prefixes to force loading of definitions from the
classpath or a URL, regardless of the actual context type.
Convenient ApplicationContext instantiation for web applications
You can create ApplicationContext instances declaratively by using, for example, a
ContextLoader. Of course you can also create ApplicationContext instances programmatically
by using one of the ApplicationContext implementations.
You can register an ApplicationContext using the ContextLoaderListener as follows:
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/daoContext.xml /WEB-INF/applicationContext.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
The listener inspects the contextConfigLocation parameter. If the parameter does not exist, the
listener uses /WEB-INF/applicationContext.xml as a default. When the parameter does exist,
the listener separates the String by using predefined delimiters (comma, semicolon and whitespace)
and uses the values as locations where application contexts will be searched. Ant-style path patterns
are supported as well. Examples are /WEB-INF/*Context.xml for all files with names ending with
"Context.xml", residing in the "WEB-INF" directory, and /WEB-INF/**/*Context.xml, for all such
files in any subdirectory of "WEB-INF".
Deploying a Spring ApplicationContext as a Java EE RAR file
It is possible to deploy a Spring ApplicationContext as a RAR file, encapsulating the context and all of
its required bean classes and library JARs in a Java EE RAR deployment unit. This is the equivalent
of bootstrapping a standalone ApplicationContext, just hosted in Java EE environment, being able
to access the Java EE servers facilities. RAR deployment is more natural alternative to scenario of
deploying a headless WAR file, in effect, a WAR file without any HTTP entry points that is used only for
bootstrapping a Spring ApplicationContext in a Java EE environment.
RAR deployment is ideal for application contexts that do not need HTTP entry points but rather
consist only of message endpoints and scheduled jobs. Beans in such a context can use application
server resources such as the JTA transaction manager and JNDI-bound JDBC DataSources and JMS
ConnectionFactory instances, and may also register with the platform’s JMX server - all through Spring’s
standard transaction management and JNDI and JMX support facilities. Application components
can also interact with the application server’s JCA WorkManager through Spring’s TaskExecutor
abstraction.
Check out the JavaDoc of the SpringContextResourceAdapter class for the configuration details
involved in RAR deployment.
For a simple deployment of a Spring ApplicationContext as a Java EE RAR file: package all application
classes into a RAR file, which is a standard JAR file with a different file extension. Add all required library
JARs into the root of the RAR archive. Add a "META-INF/ra.xml" deployment descriptor (as shown in
SpringContextResourceAdapter's JavaDoc) and the corresponding Spring XML bean definition file(s)
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(typically "META-INF/applicationContext.xml"), and drop the resulting RAR file into your application
server’s deployment directory.
Note
Such RAR deployment units are usually self-contained; they do not expose components to the
outside world, not even to other modules of the same application. Interaction with a RAR-based
ApplicationContext usually occurs through JMS destinations that it shares with other modules. A
RAR-based ApplicationContext may also, for example, schedule some jobs, reacting to new files
in the file system (or the like). If it needs to allow synchronous access from the outside, it could
for example export RMI endpoints, which of course may be used by other application modules
on the same machine.
6.16 The BeanFactory
The BeanFactory provides the underlying basis for Spring’s IoC functionality but it is only used
directly in integration with other third-party frameworks and is now largely historical in nature for
most users of Spring. The BeanFactory and related interfaces, such as BeanFactoryAware,
InitializingBean, DisposableBean, are still present in Spring for the purposes of backward
compatibility with the large number of third-party frameworks that integrate with Spring. Often third-party
components that can not use more modern equivalents such as @PostConstruct or @PreDestroy
in order to remain compatible with JDK 1.4 or to avoid a dependency on JSR-250.
This section provides additional background into the differences between the BeanFactory and
ApplicationContext and how one might access the IoC container directly through a classic singleton
lookup.
BeanFactory or ApplicationContext?
Use an ApplicationContext unless you have a good reason for not doing so.
Because the ApplicationContext includes all functionality of the BeanFactory, it is generally
recommended over the BeanFactory, except for a few situations such as in embedded applications
running on resource-constrained devices where memory consumption might be critical and a few
extra kilobytes might make a difference. However, for most typical enterprise applications and
systems, the ApplicationContext is what you will want to use. Spring makes heavy use of
the BeanPostProcessor extension point (to effect proxying and so on). If you use only a plain
BeanFactory, a fair amount of support such as transactions and AOP will not take effect, at least not
without some extra steps on your part. This situation could be confusing because nothing is actually
wrong with the configuration.
The following table lists features provided by the BeanFactory and ApplicationContext interfaces
and implementations.
Table 6.9. Feature Matrix
Feature
BeanFactory
ApplicationContext
Bean instantiation/wiring
Yes
Yes
Automatic
BeanPostProcessor
registration
No
Yes
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Feature
BeanFactory
ApplicationContext
Automatic
No
BeanFactoryPostProcessor
registration
Yes
Convenient MessageSource
access (for i18n)
No
Yes
ApplicationEvent
publication
No
Yes
To explicitly register a bean post-processor with a BeanFactory implementation, you need to write
code like this:
DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
// populate the factory with bean definitions
// now register any needed BeanPostProcessor instances
MyBeanPostProcessor postProcessor = new MyBeanPostProcessor();
factory.addBeanPostProcessor(postProcessor);
// now start using the factory
To explicitly register a BeanFactoryPostProcessor when using a BeanFactory implementation,
you must write code like this:
DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
reader.loadBeanDefinitions(new FileSystemResource("beans.xml"));
// bring in some property values from a Properties file
PropertyPlaceholderConfigurer cfg = new PropertyPlaceholderConfigurer();
cfg.setLocation(new FileSystemResource("jdbc.properties"));
// now actually do the replacement
cfg.postProcessBeanFactory(factory);
In both cases, the explicit registration step is inconvenient, which is one reason why
the various ApplicationContext implementations are preferred above plain BeanFactory
implementations in the vast majority of Spring-backed applications, especially when using
BeanFactoryPostProcessors and BeanPostProcessors. These mechanisms implement
important functionality such as property placeholder replacement and AOP.
Glue code and the evil singleton
It is best to write most application code in a dependency-injection (DI) style, where that code is served
out of a Spring IoC container, has its own dependencies supplied by the container when it is created, and
is completely unaware of the container. However, for the small glue layers of code that are sometimes
needed to tie other code together, you sometimes need a singleton (or quasi-singleton) style access
to a Spring IoC container. For example, third-party code may try to construct new objects directly (
Class.forName() style), without the ability to get these objects out of a Spring IoC container.If the
object constructed by the third-party code is a small stub or proxy, which then uses a singleton style
access to a Spring IoC container to get a real object to delegate to, then inversion of control has still been
achieved for the majority of the code (the object coming out of the container). Thus most code is still
unaware of the container or how it is accessed, and remains decoupled from other code, with all ensuing
benefits. EJBs may also use this stub/proxy approach to delegate to a plain Java implementation object,
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retrieved from a Spring IoC container. While the Spring IoC container itself ideally does not have to be
a singleton, it may be unrealistic in terms of memory usage or initialization times (when using beans in
the Spring IoC container such as a Hibernate SessionFactory) for each bean to use its own, nonsingleton Spring IoC container.
Looking up the application context in a service locator style is sometimes the only option for accessing
shared Spring-managed components, such as in an EJB 2.1 environment, or when you want to share
a single ApplicationContext as a parent to WebApplicationContexts across WAR files. In this case
you should look into using the utility class ContextSingletonBeanFactoryLocator locator that is
described in this Spring team blog entry.
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7. Resources
7.1 Introduction
Java’s standard java.net.URL class and standard handlers for various URL prefixes unfortunately are
not quite adequate enough for all access to low-level resources. For example, there is no standardized
URL implementation that may be used to access a resource that needs to be obtained from the classpath,
or relative to a ServletContext. While it is possible to register new handlers for specialized URL
prefixes (similar to existing handlers for prefixes such as http:), this is generally quite complicated, and
the URL interface still lacks some desirable functionality, such as a method to check for the existence
of the resource being pointed to.
7.2 The Resource interface
Spring’s Resource interface is meant to be a more capable interface for abstracting access to lowlevel resources.
public interface Resource extends InputStreamSource {
boolean exists();
boolean isOpen();
URL getURL() throws IOException;
File getFile() throws IOException;
Resource createRelative(String relativePath) throws IOException;
String getFilename();
String getDescription();
}
public interface InputStreamSource {
InputStream getInputStream() throws IOException;
}
Some of the most important methods from the Resource interface are:
• getInputStream(): locates and opens the resource, returning an InputStream for reading from
the resource. It is expected that each invocation returns a fresh InputStream. It is the responsibility
of the caller to close the stream.
• exists(): returns a boolean indicating whether this resource actually exists in physical form.
• isOpen(): returns a boolean indicating whether this resource represents a handle with an open
stream. If true, the InputStream cannot be read multiple times, and must be read once only and
then closed to avoid resource leaks. Will be false for all usual resource implementations, with the
exception of InputStreamResource.
• getDescription(): returns a description for this resource, to be used for error output when working
with the resource. This is often the fully qualified file name or the actual URL of the resource.
Other methods allow you to obtain an actual URL or File object representing the resource (if the
underlying implementation is compatible, and supports that functionality).
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The Resource abstraction is used extensively in Spring itself, as an argument type in many method
signatures when a resource is needed. Other methods in some Spring APIs (such as the constructors to
various ApplicationContext implementations), take a String which in unadorned or simple form
is used to create a Resource appropriate to that context implementation, or via special prefixes on
the String path, allow the caller to specify that a specific Resource implementation must be created
and used.
While the Resource interface is used a lot with Spring and by Spring, it’s actually very useful to use as
a general utility class by itself in your own code, for access to resources, even when your code doesn’t
know or care about any other parts of Spring. While this couples your code to Spring, it really only
couples it to this small set of utility classes, which are serving as a more capable replacement for URL,
and can be considered equivalent to any other library you would use for this purpose.
It is important to note that the Resource abstraction does not replace functionality: it wraps it where
possible. For example, a UrlResource wraps a URL, and uses the wrapped URL to do its work.
7.3 Built-in Resource implementations
There are a number of Resource implementations that come supplied straight out of the box in Spring:
UrlResource
The UrlResource wraps a java.net.URL, and may be used to access any object that is normally
accessible via a URL, such as files, an HTTP target, an FTP target, etc. All URLs have a standardized
String representation, such that appropriate standardized prefixes are used to indicate one URL type
from another. This includes file: for accessing filesystem paths, http: for accessing resources via
the HTTP protocol, ftp: for accessing resources via FTP, etc.
A UrlResource is created by Java code explicitly using the UrlResource constructor, but will often
be created implicitly when you call an API method which takes a String argument which is meant
to represent a path. For the latter case, a JavaBeans PropertyEditor will ultimately decide which
type of Resource to create. If the path string contains a few well-known (to it, that is) prefixes such as
classpath:, it will create an appropriate specialized Resource for that prefix. However, if it doesn’t
recognize the prefix, it will assume the this is just a standard URL string, and will create a UrlResource.
ClassPathResource
This class represents a resource which should be obtained from the classpath. This uses either the
thread context class loader, a given class loader, or a given class for loading resources.
This Resource implementation supports resolution as java.io.File if the class path resource
resides in the file system, but not for classpath resources which reside in a jar and have not been
expanded (by the servlet engine, or whatever the environment is) to the filesystem. To address this the
various Resource implementations always support resolution as a java.net.URL.
A ClassPathResource is created by Java code explicitly using the ClassPathResource
constructor, but will often be created implicitly when you call an API method which takes a String
argument which is meant to represent a path. For the latter case, a JavaBeans PropertyEditor will
recognize the special prefix classpath: on the string path, and create a ClassPathResource in
that case.
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FileSystemResource
This is a Resource implementation for java.io.File handles. It obviously supports resolution as a
File, and as a URL.
ServletContextResource
This is a Resource implementation for ServletContext resources, interpreting relative paths within
the relevant web application’s root directory.
This always supports stream access and URL access, but only allows java.io.File access when
the web application archive is expanded and the resource is physically on the filesystem. Whether or
not it’s expanded and on the filesystem like this, or accessed directly from the JAR or somewhere else
like a DB (it’s conceivable) is actually dependent on the Servlet container.
InputStreamResource
A Resource implementation for a given InputStream. This should only be used if no specific
Resource implementation is applicable. In particular, prefer ByteArrayResource or any of the filebased Resource implementations where possible.
In contrast to other Resource implementations, this is a descriptor for an already opened resource therefore returning true from isOpen(). Do not use it if you need to keep the resource descriptor
somewhere, or if you need to read a stream multiple times.
ByteArrayResource
This is a Resource implementation for a given byte array. It creates a ByteArrayInputStream for
the given byte array.
It’s useful for loading content from any given byte array, without having to resort to a single-use
InputStreamResource.
7.4 The ResourceLoader
The ResourceLoader interface is meant to be implemented by objects that can return (i.e. load)
Resource instances.
public interface ResourceLoader {
Resource getResource(String location);
}
All application contexts implement the ResourceLoader interface, and therefore all application
contexts may be used to obtain Resource instances.
When you call getResource() on a specific application context, and the location path specified
doesn’t have a specific prefix, you will get back a Resource type that is appropriate to that particular
application context. For example, assume the following snippet of code was executed against a
ClassPathXmlApplicationContext instance:
Resource template = ctx.getResource("some/resource/path/myTemplate.txt");
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What would be returned would be a ClassPathResource; if the same method was executed against
a FileSystemXmlApplicationContext instance, you’d get back a FileSystemResource. For a
WebApplicationContext, you’d get back a ServletContextResource, and so on.
As such, you can load resources in a fashion appropriate to the particular application context.
On the other hand, you may also force ClassPathResource to be used, regardless of the application
context type, by specifying the special classpath: prefix:
Resource template = ctx.getResource("classpath:some/resource/path/myTemplate.txt");
Similarly, one can force a UrlResource to be used by specifying any of the standard java.net.URL
prefixes:
Resource template = ctx.getResource("file:///some/resource/path/myTemplate.txt");
Resource template = ctx.getResource("http://myhost.com/resource/path/myTemplate.txt");
The following table summarizes the strategy for converting Strings to Resources:
Table 7.1. Resource strings
Prefix
Example
Explanation
classpath:
classpath:com/myapp/
config.xml
Loaded from the classpath.
file:
file:///data/config.xml
Loaded as a URL, from the
1
filesystem.
http:
http://myserver/
logo.png
Loaded as a URL.
(none)
/data/config.xml
Depends on the underlying
ApplicationContext.
1
But see also the section called “FileSystemResource caveats”.
7.5 The ResourceLoaderAware interface
The ResourceLoaderAware interface is a special marker interface, identifying objects that expect to
be provided with a ResourceLoader reference.
public interface ResourceLoaderAware {
void setResourceLoader(ResourceLoader resourceLoader);
}
When a class implements ResourceLoaderAware and is deployed into an application context (as a
Spring-managed bean), it is recognized as ResourceLoaderAware by the application context. The
application context will then invoke the setResourceLoader(ResourceLoader), supplying itself as
the argument (remember, all application contexts in Spring implement the ResourceLoader interface).
Of course, since an ApplicationContext is a ResourceLoader, the bean could also implement
the ApplicationContextAware interface and use the supplied application context directly to load
resources, but in general, it’s better to use the specialized ResourceLoader interface if that’s all that’s
needed. The code would just be coupled to the resource loading interface, which can be considered a
utility interface, and not the whole Spring ApplicationContext interface.
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As of Spring 2.5, you can rely upon autowiring of the ResourceLoader as an alternative to
implementing the ResourceLoaderAware interface. The "traditional" constructor and byType
autowiring modes (as described in the section called “Autowiring collaborators”) are now capable
of providing a dependency of type ResourceLoader for either a constructor argument or setter
method parameter respectively. For more flexibility (including the ability to autowire fields and multiple
parameter methods), consider using the new annotation-based autowiring features. In that case, the
ResourceLoader will be autowired into a field, constructor argument, or method parameter that is
expecting the ResourceLoader type as long as the field, constructor, or method in question carries
the @Autowired annotation. For more information, see the section called “@Autowired”.
7.6 Resources as dependencies
If the bean itself is going to determine and supply the resource path through some sort of dynamic
process, it probably makes sense for the bean to use the ResourceLoader interface to load resources.
Consider as an example the loading of a template of some sort, where the specific resource that is
needed depends on the role of the user. If the resources are static, it makes sense to eliminate the use
of the ResourceLoader interface completely, and just have the bean expose the Resource properties
it needs, and expect that they will be injected into it.
What makes it trivial to then inject these properties, is that all application contexts register and use a
special JavaBeans PropertyEditor which can convert String paths to Resource objects. So if
myBean has a template property of type Resource, it can be configured with a simple string for that
resource, as follows:
<bean id="myBean" class="...">
<property name="template" value="some/resource/path/myTemplate.txt"/>
</bean>
Note that the resource path has no prefix, so because the application context itself is going to
be used as the ResourceLoader, the resource itself will be loaded via a ClassPathResource,
FileSystemResource, or ServletContextResource (as appropriate) depending on the exact type
of the context.
If there is a need to force a specific Resource type to be used, then a prefix may be used. The following
two examples show how to force a ClassPathResource and a UrlResource (the latter being used
to access a filesystem file).
<property name="template" value="classpath:some/resource/path/myTemplate.txt">
<property name="template" value="file:///some/resource/path/myTemplate.txt"/>
7.7 Application contexts and Resource paths
Constructing application contexts
An application context constructor (for a specific application context type) generally takes a string or
array of strings as the location path(s) of the resource(s) such as XML files that make up the definition
of the context.
When such a location path doesn’t have a prefix, the specific Resource type built from that path and
used to load the bean definitions, depends on and is appropriate to the specific application context. For
example, if you create a ClassPathXmlApplicationContext as follows:
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ApplicationContext ctx = new ClassPathXmlApplicationContext("conf/appContext.xml");
The bean definitions will be loaded from the classpath, as a ClassPathResource will be used. But if
you create a FileSystemXmlApplicationContext as follows:
ApplicationContext ctx =
new FileSystemXmlApplicationContext("conf/appContext.xml");
The bean definition will be loaded from a filesystem location, in this case relative to the current working
directory.
Note that the use of the special classpath prefix or a standard URL prefix on the location
path will override the default type of Resource created to load the definition. So this
FileSystemXmlApplicationContext…
ApplicationContext ctx =
new FileSystemXmlApplicationContext("classpath:conf/appContext.xml");
i. will actually load its bean definitions from the classpath. However, it is still a
FileSystemXmlApplicationContext. If it is subsequently used as a ResourceLoader, any
unprefixed paths will still be treated as filesystem paths.
Constructing ClassPathXmlApplicationContext instances - shortcuts
The ClassPathXmlApplicationContext exposes a number of constructors to enable convenient
instantiation. The basic idea is that one supplies merely a string array containing just the filenames of
the XML files themselves (without the leading path information), and one also supplies a Class; the
ClassPathXmlApplicationContext will derive the path information from the supplied class.
An example will hopefully make this clear. Consider a directory layout that looks like this:
com/
foo/
services.xml
daos.xml
MessengerService.class
A ClassPathXmlApplicationContext instance composed of the beans defined in the
'services.xml' and 'daos.xml' could be instantiated like so…
ApplicationContext ctx = new ClassPathXmlApplicationContext(
new String[] {"services.xml", "daos.xml"}, MessengerService.class);
Please do consult the ClassPathXmlApplicationContext javadocs for details on the various
constructors.
Wildcards in application context constructor resource paths
The resource paths in application context constructor values may be a simple path (as shown
above) which has a one-to-one mapping to a target Resource, or alternately may contain the special
"classpath*:" prefix and/or internal Ant-style regular expressions (matched using Spring’s PathMatcher
utility). Both of the latter are effectively wildcards
One use for this mechanism is when doing component-style application assembly. All components can
'publish' context definition fragments to a well-known location path, and when the final application context
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is created using the same path prefixed via classpath*:, all component fragments will be picked up
automatically.
Note that this wildcarding is specific to use of resource paths in application context constructors (or
when using the PathMatcher utility class hierarchy directly), and is resolved at construction time. It
has nothing to do with the Resource type itself. It’s not possible to use the classpath*: prefix to
construct an actual Resource, as a resource points to just one resource at a time.
Ant-style Patterns
When the path location contains an Ant-style pattern, for example:
/WEB-INF/*-context.xml
com/mycompany/**/applicationContext.xml
file:C:/some/path/*-context.xml
classpath:com/mycompany/**/applicationContext.xml
i. the resolver follows a more complex but defined procedure to try to resolve the wildcard. It produces
a Resource for the path up to the last non-wildcard segment and obtains a URL from it. If this URL
is not a "jar:" URL or container-specific variant (e.g. " zip:`" in WebLogic, " `wsjar`"
in WebSphere, etc.), then a `java.io.File is obtained from it and used to resolve
the wildcard by traversing the filesystem. In the case of a jar URL, the resolver either gets a
java.net.JarURLConnection from it or manually parses the jar URL and then traverses the
contents of the jar file to resolve the wildcards.
Implications on portability
If the specified path is already a file URL (either explicitly, or implicitly because the base
ResourceLoader is a filesystem one, then wildcarding is guaranteed to work in a completely portable
fashion.
If the specified path is a classpath location, then the resolver must obtain the last non-wildcard path
segment URL via a Classloader.getResource() call. Since this is just a node of the path (not the
file at the end) it is actually undefined (in the ClassLoader javadocs) exactly what sort of a URL is
returned in this case. In practice, it is always a java.io.File representing the directory, where the
classpath resource resolves to a filesystem location, or a jar URL of some sort, where the classpath
resource resolves to a jar location. Still, there is a portability concern on this operation.
If a jar URL is obtained for the last non-wildcard segment, the resolver must be able to get a
java.net.JarURLConnection from it, or manually parse the jar URL, to be able to walk the contents
of the jar, and resolve the wildcard. This will work in most environments, but will fail in others, and it is
strongly recommended that the wildcard resolution of resources coming from jars be thoroughly tested
in your specific environment before you rely on it.
The Classpath*: portability classpath*: prefix
When constructing an XML-based application context, a location string may use the special
classpath*: prefix:
ApplicationContext ctx =
new ClassPathXmlApplicationContext("classpath*:conf/appContext.xml");
This special prefix specifies that all classpath resources that match the given name must be obtained
(internally, this essentially happens via a ClassLoader.getResources(…) call), and then merged
to form the final application context definition.
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Note
The wildcard classpath relies on the getResources() method of the underlying classloader.
As most application servers nowadays supply their own classloader implementation, the
behavior might differ especially when dealing with jar files. A simple test to check if
classpath* works is to use the classloader to load a file from within a jar on the classpath:
getClass().getClassLoader().getResources("<someFileInsideTheJar>"). Try
this test with files that have the same name but are placed inside two different locations. In case
an inappropriate result is returned, check the application server documentation for settings that
might affect the classloader behavior.
The " classpath*:`" prefix can also be combined with a `PathMatcher pattern
in the rest of the location path, for example " `classpath*:META-INF/*-beans.xml`". In this case, the
resolution strategy is fairly simple: a ClassLoader.getResources() call is used on the last non-wildcard
path segment to get all the matching resources in the class loader hierarchy, and then off each resource
the same PathMatcher resolution strategy described above is used for the wildcard subpath.
Other notes relating to wildcards
Please note that classpath*: when combined with Ant-style patterns will only work reliably with at
least one root directory before the pattern starts, unless the actual target files reside in the file system.
This means that a pattern like " classpath*:*.xml`" will not retrieve files from the
root of jar files but rather only from the root of expanded directories. This
originates from a limitation in the JDK’s `ClassLoader.getResources() method
which only returns file system locations for a passed-in empty string (indicating potential roots to search).
Ant-style patterns with " `classpath:`" resources are not guaranteed to find matching resources if the
root package to search is available in multiple class path locations. This is because a resource such as
com/mycompany/package1/service-context.xml
may be in only one location, but when a path such as
classpath:com/mycompany/**/service-context.xml
is used to try to resolve it, the resolver will work off the (first) URL returned by getResource("com/
mycompany");. If this base package node exists in multiple classloader locations, the actual end
resource may not be underneath. Therefore, preferably, use " `classpath*:`" with the same Ant-style
pattern in such a case, which will search all class path locations that contain the root package.
FileSystemResource caveats
A FileSystemResource that is not attached to a FileSystemApplicationContext (that is,
a FileSystemApplicationContext is not the actual ResourceLoader) will treat absolute vs.
relative paths as you would expect. Relative paths are relative to the current working directory, while
absolute paths are relative to the root of the filesystem.
For
backwards
compatibility
(historical)
reasons
however,
this
changes
when
the
FileSystemApplicationContext
is
the
ResourceLoader.
The
FileSystemApplicationContext simply forces all attached FileSystemResource instances to
treat all location paths as relative, whether they start with a leading slash or not. In practice, this means
the following are equivalent:
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ApplicationContext ctx =
new FileSystemXmlApplicationContext("conf/context.xml");
ApplicationContext ctx =
new FileSystemXmlApplicationContext("/conf/context.xml");
As are the following: (Even though it would make sense for them to be different, as one case is relative
and the other absolute.)
FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("some/resource/path/myTemplate.txt");
FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("/some/resource/path/myTemplate.txt");
In practice, if true absolute filesystem paths are needed, it is better to forgo the use of absolute paths
with FileSystemResource / FileSystemXmlApplicationContext, and just force the use of a
UrlResource, by using the file: URL prefix.
// actual context type doesn't matter, the Resource will always be UrlResource
ctx.getResource("file:///some/resource/path/myTemplate.txt");
// force this FileSystemXmlApplicationContext to load its definition via a UrlResource
ApplicationContext ctx =
new FileSystemXmlApplicationContext("file:///conf/context.xml");
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8. Validation, Data Binding, and Type Conversion
8.1 Introduction
JSR-303/JSR-349 Bean Validation
Spring Framework 4.0 supports Bean Validation 1.0 (JSR-303) and Bean Validation 1.1 (JSR-349)
in terms of setup support, also adapting it to Spring’s Validator interface.
An application can choose to enable Bean Validation once globally, as described in Section 8.8,
“Spring Validation”, and use it exclusively for all validation needs.
An application can also register additional Spring Validator instances per DataBinder
instance, as described in the section called “Configuring a DataBinder”. This may be useful for
plugging in validation logic without the use of annotations.
There are pros and cons for considering validation as business logic, and Spring offers a design for
validation (and data binding) that does not exclude either one of them. Specifically validation should
not be tied to the web tier, should be easy to localize and it should be possible to plug in any validator
available. Considering the above, Spring has come up with a Validator interface that is both basic
and eminently usable in every layer of an application.
Data binding is useful for allowing user input to be dynamically bound to the domain model of
an application (or whatever objects you use to process user input). Spring provides the so-called
DataBinder to do exactly that. The Validator and the DataBinder make up the validation
package, which is primarily used in but not limited to the MVC framework.
The BeanWrapper is a fundamental concept in the Spring Framework and is used in a lot of places.
However, you probably will not have the need to use the BeanWrapper directly. Because this is
reference documentation however, we felt that some explanation might be in order. We will explain the
BeanWrapper in this chapter since, if you were going to use it at all, you would most likely do so when
trying to bind data to objects.
Spring’s DataBinder and the lower-level BeanWrapper both use PropertyEditors to parse and format
property values. The PropertyEditor concept is part of the JavaBeans specification, and is also
explained in this chapter. Spring 3 introduces a "core.convert" package that provides a general type
conversion facility, as well as a higher-level "format" package for formatting UI field values. These new
packages may be used as simpler alternatives to PropertyEditors, and will also be discussed in this
chapter.
8.2 Validation using Spring’s Validator interface
Spring features a Validator interface that you can use to validate objects. The Validator interface
works using an Errors object so that while validating, validators can report validation failures to the
Errors object.
Let’s consider a small data object:
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public class Person {
private String name;
private int age;
// the usual getters and setters...
}
We’re going to provide validation behavior for the Person class by implementing the following two
methods of the org.springframework.validation.Validator interface:
• supports(Class) - Can this Validator validate instances of the supplied Class?
• validate(Object, org.springframework.validation.Errors) - validates the given
object and in case of validation errors, registers those with the given Errors object
Implementing a Validator is fairly straightforward, especially when you know of the
ValidationUtils helper class that the Spring Framework also provides.
public class PersonValidator implements Validator {
/**
* This Validator validates *just* Person instances
*/
public boolean supports(Class clazz) {
return Person.class.equals(clazz);
}
public void validate(Object obj, Errors e) {
ValidationUtils.rejectIfEmpty(e, "name", "name.empty");
Person p = (Person) obj;
if (p.getAge() < 0) {
e.rejectValue("age", "negativevalue");
} else if (p.getAge() > 110) {
e.rejectValue("age", "too.darn.old");
}
}
}
As you can see, the static rejectIfEmpty(..) method on the ValidationUtils class is used
to reject the 'name' property if it is null or the empty string. Have a look at the ValidationUtils
javadocs to see what functionality it provides besides the example shown previously.
While it is certainly possible to implement a single Validator class to validate each of the nested
objects in a rich object, it may be better to encapsulate the validation logic for each nested class
of object in its own Validator implementation. A simple example of a 'rich' object would be a
Customer that is composed of two String properties (a first and second name) and a complex
Address object. Address objects may be used independently of Customer objects, and so a distinct
AddressValidator has been implemented. If you want your CustomerValidator to reuse the
logic contained within the AddressValidator class without resorting to copy-and-paste, you can
dependency-inject or instantiate an AddressValidator within your CustomerValidator, and use
it like so:
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public class CustomerValidator implements Validator {
private final Validator addressValidator;
public CustomerValidator(Validator addressValidator) {
if (addressValidator == null) {
throw new IllegalArgumentException("The supplied [Validator] is " +
"required and must not be null.");
}
if (!addressValidator.supports(Address.class)) {
throw new IllegalArgumentException("The supplied [Validator] must " +
support the validation of [Address] instances.");
}
this.addressValidator = addressValidator;
}
/**
* This Validator validates Customer instances, and any subclasses of Customer too
*/
public boolean supports(Class clazz) {
return Customer.class.isAssignableFrom(clazz);
}
public void validate(Object target, Errors errors) {
ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "field.required");
ValidationUtils.rejectIfEmptyOrWhitespace(errors, "surname", "field.required");
Customer customer = (Customer) target;
try {
errors.pushNestedPath("address");
ValidationUtils.invokeValidator(this.addressValidator, customer.getAddress(), errors);
} finally {
errors.popNestedPath();
}
}
}
Validation errors are reported to the Errors object passed to the validator. In case of Spring Web MVC
you can use <spring:bind/> tag to inspect the error messages, but of course you can also inspect
the errors object yourself. More information about the methods it offers can be found in the javadocs.
8.3 Resolving codes to error messages
We’ve talked about databinding and validation. Outputting messages corresponding to validation errors
is the last thing we need to discuss. In the example we’ve shown above, we rejected the name and
the age field. If we’re going to output the error messages by using a MessageSource, we will do
so using the error code we’ve given when rejecting the field ('name' and 'age' in this case). When
you call (either directly, or indirectly, using for example the ValidationUtils class) rejectValue
or one of the other reject methods from the Errors interface, the underlying implementation will
not only register the code you’ve passed in, but also a number of additional error codes. What
error codes it registers is determined by the MessageCodesResolver that is used. By default, the
DefaultMessageCodesResolver is used, which for example not only registers a message with
the code you gave, but also messages that include the field name you passed to the reject method.
So in case you reject a field using rejectValue("age", "too.darn.old"), apart from the
too.darn.old code, Spring will also register too.darn.old.age and too.darn.old.age.int
(so the first will include the field name and the second will include the type of the field); this is done as
a convenience to aid developers in targeting error messages and suchlike.
More information on the MessageCodesResolver and the default strategy can be found online in the
javadocs of MessageCodesResolver and DefaultMessageCodesResolver, respectively.
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8.4 Bean manipulation and the BeanWrapper
The org.springframework.beans package adheres to the JavaBeans standard provided by
Oracle. A JavaBean is simply a class with a default no-argument constructor, which follows a naming
convention where (by way of an example) a property named bingoMadness would have a setter
method setBingoMadness(..) and a getter method getBingoMadness(). For more information
about JavaBeans and the specification, please refer to Oracle’s website ( javabeans).
One quite important class in the beans package is the BeanWrapper interface and its corresponding
implementation ( BeanWrapperImpl). As quoted from the javadocs, the BeanWrapper offers
functionality to set and get property values (individually or in bulk), get property descriptors, and to
query properties to determine if they are readable or writable. Also, the BeanWrapper offers support
for nested properties, enabling the setting of properties on sub-properties to an unlimited depth. Then,
the BeanWrapper supports the ability to add standard JavaBeans PropertyChangeListeners and
VetoableChangeListeners, without the need for supporting code in the target class. Last but not
least, the BeanWrapper provides support for the setting of indexed properties. The BeanWrapper
usually isn’t used by application code directly, but by the DataBinder and the BeanFactory.
The way the BeanWrapper works is partly indicated by its name: it wraps a bean to perform actions
on that bean, like setting and retrieving properties.
Setting and getting basic and nested properties
Setting and getting properties is done using the setPropertyValue(s) and
getPropertyValue(s) methods that both come with a couple of overloaded variants. They’re all
described in more detail in the javadocs Spring comes with. What’s important to know is that there are
a couple of conventions for indicating properties of an object. A couple of examples:
Table 8.1. Examples of properties
Expression
Explanation
name
Indicates the property name corresponding to
the methods getName() or isName() and
setName(..)
account.name
Indicates the nested property name of the
property account corresponding e.g. to the
methods getAccount().setName() or
getAccount().getName()
account[2]
Indicates the third element of the indexed
property account. Indexed properties can be
of type array, list or other naturally ordered
collection
account[COMPANYNAME]
Indicates the value of the map entry indexed by
the key COMPANYNAME of the Map property
account
Below you’ll find some examples of working with the BeanWrapper to get and set properties.
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(This next section is not vitally important to you if you’re not planning to work with the BeanWrapper
directly. If you’re just using the DataBinder and the BeanFactory and their out-of-the-box
implementation, you should skip ahead to the section about PropertyEditors.)
Consider the following two classes:
public class Company {
private String name;
private Employee managingDirector;
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public Employee getManagingDirector() {
return this.managingDirector;
}
public void setManagingDirector(Employee managingDirector) {
this.managingDirector = managingDirector;
}
}
public class Employee {
private String name;
private float salary;
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public float getSalary() {
return salary;
}
public void setSalary(float salary) {
this.salary = salary;
}
}
The following code snippets show some examples of how to retrieve and manipulate some of the
properties of instantiated Companies and Employees:
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BeanWrapper company = new BeanWrapperImpl(new Company());
// setting the company name..
company.setPropertyValue("name", "Some Company Inc.");
// ... can also be done like this:
PropertyValue value = new PropertyValue("name", "Some Company Inc.");
company.setPropertyValue(value);
// ok, let's create the director and tie it to the company:
BeanWrapper jim = new BeanWrapperImpl(new Employee());
jim.setPropertyValue("name", "Jim Stravinsky");
company.setPropertyValue("managingDirector", jim.getWrappedInstance());
// retrieving the salary of the managingDirector through the company
Float salary = (Float) company.getPropertyValue("managingDirector.salary");
Built-in PropertyEditor implementations
Spring uses the concept of PropertyEditors to effect the conversion between an Object and a
String. If you think about it, it sometimes might be handy to be able to represent properties in a different
way than the object itself. For example, a Date can be represented in a human readable way (as the
String ' 2007-14-09’), while we’re still able to convert the human readable
form back to the original date (or even better: convert any date entered
in a human readable form, back to `Date objects). This behavior can be achieved by
registering custom editors, of type java.beans.PropertyEditor. Registering custom editors on a
BeanWrapper or alternately in a specific IoC container as mentioned in the previous chapter, gives it
the knowledge of how to convert properties to the desired type. Read more about PropertyEditors
in the javadocs of the java.beans package provided by Oracle.
A couple of examples where property editing is used in Spring:
• setting properties on beans is done using PropertyEditors. When mentioning
java.lang.String as the value of a property of some bean you’re declaring in XML file, Spring
will (if the setter of the corresponding property has a Class-parameter) use the ClassEditor to try
to resolve the parameter to a Class object.
• parsing HTTP request parameters in Spring’s MVC framework is done using all kinds of
PropertyEditors that you can manually bind in all subclasses of the CommandController.
Spring has a number of built-in PropertyEditors to make life easy. Each of those is listed below
and they are all located in the org.springframework.beans.propertyeditors package. Most,
but not all (as indicated below), are registered by default by BeanWrapperImpl. Where the property
editor is configurable in some fashion, you can of course still register your own variant to override the
default one:
Table 8.2. Built-in PropertyEditors
Class
Explanation
ByteArrayPropertyEditor
Editor for byte arrays. Strings will simply
be converted to their corresponding byte
representations. Registered by default by
BeanWrapperImpl.
ClassEditor
Parses Strings representing classes to actual
classes and the other way around. When a class
is not found, an IllegalArgumentException
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Class
Explanation
is thrown. Registered by default by
BeanWrapperImpl.
CustomBooleanEditor
Customizable property editor for Boolean
properties. Registered by default by
BeanWrapperImpl, but, can be overridden
by registering custom instance of it as custom
editor.
CustomCollectionEditor
Property editor for Collections, converting
any source Collection to a given target
Collection type.
CustomDateEditor
Customizable property editor for java.util.Date,
supporting a custom DateFormat. NOT
registered by default. Must be user registered as
needed with appropriate format.
CustomNumberEditor
Customizable property editor for any Number
subclass like Integer, Long, Float, Double.
Registered by default by BeanWrapperImpl,
but can be overridden by registering custom
instance of it as a custom editor.
FileEditor
Capable of resolving Strings to java.io.File
objects. Registered by default by
BeanWrapperImpl.
InputStreamEditor
One-way property editor, capable of taking a
text string and producing (via an intermediate
ResourceEditor and Resource) an
InputStream, so InputStream properties
may be directly set as Strings. Note
that the default usage will not close the
InputStream for you! Registered by default by
BeanWrapperImpl.
LocaleEditor
Capable of resolving Strings to Locale
objects and vice versa (the String format is
[country][variant], which is the same thing
the toString() method of Locale provides).
Registered by default by BeanWrapperImpl.
PatternEditor
Capable of resolving Strings to
java.util.regex.Pattern objects and vice
versa.
PropertiesEditor
Capable of converting Strings (formatted
using the format as defined in the javadocs
of the java.util.Properties class) to
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Class
Explanation
Properties objects. Registered by default by
BeanWrapperImpl.
StringTrimmerEditor
Property editor that trims Strings. Optionally
allows transforming an empty string into a null
value. NOT registered by default; must be user
registered as needed.
URLEditor
Capable of resolving a String representation of
a URL to an actual URL object. Registered by
default by BeanWrapperImpl.
Spring uses the java.beans.PropertyEditorManager to set the search path for property
editors that might be needed. The search path also includes sun.bean.editors, which includes
PropertyEditor implementations for types such as Font, Color, and most of the primitive types.
Note also that the standard JavaBeans infrastructure will automatically discover PropertyEditor
classes (without you having to register them explicitly) if they are in the same package as the class
they handle, and have the same name as that class, with 'Editor' appended; for example, one could
have the following class and package structure, which would be sufficient for the FooEditor class to
be recognized and used as the PropertyEditor for Foo-typed properties.
com
chank
pop
Foo
FooEditor // the PropertyEditor for the Foo class
Note that you can also use the standard BeanInfo JavaBeans mechanism here as well (described
in not-amazing-detail here). Find below an example of using the BeanInfo mechanism for explicitly
registering one or more PropertyEditor instances with the properties of an associated class.
com
chank
pop
Foo
FooBeanInfo // the BeanInfo for the Foo class
Here is the Java source code for the referenced FooBeanInfo class. This would associate a
CustomNumberEditor with the age property of the Foo class.
public class FooBeanInfo extends SimpleBeanInfo {
public PropertyDescriptor[] getPropertyDescriptors() {
try {
final PropertyEditor numberPE = new CustomNumberEditor(Integer.class, true);
PropertyDescriptor ageDescriptor = new PropertyDescriptor("age", Foo.class) {
public PropertyEditor createPropertyEditor(Object bean) {
return numberPE;
};
};
return new PropertyDescriptor[] { ageDescriptor };
}
catch (IntrospectionException ex) {
throw new Error(ex.toString());
}
}
}
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Registering additional custom PropertyEditors
When setting bean properties as a string value, a Spring IoC container ultimately uses standard
JavaBeans PropertyEditors to convert these Strings to the complex type of the property. Spring preregisters a number of custom PropertyEditors (for example, to convert a classname expressed as
a string into a real Class object). Additionally, Java’s standard JavaBeans PropertyEditor lookup
mechanism allows a PropertyEditor for a class simply to be named appropriately and placed in the
same package as the class it provides support for, to be found automatically.
If there is a need to register other custom PropertyEditors, there are several mechanisms available.
The most manual approach, which is not normally convenient or recommended, is to simply use the
registerCustomEditor() method of the ConfigurableBeanFactory interface, assuming you
have a BeanFactory reference. Another, slightly more convenient, mechanism is to use a special bean
factory post-processor called CustomEditorConfigurer. Although bean factory post-processors can
be used with BeanFactory implementations, the CustomEditorConfigurer has a nested property
setup, so it is strongly recommended that it is used with the ApplicationContext, where it may be
deployed in similar fashion to any other bean, and automatically detected and applied.
Note that all bean factories and application contexts automatically use a number of built-in property
editors, through their use of something called a BeanWrapper to handle property conversions. The
standard property editors that the BeanWrapper registers are listed in the previous section. Additionally,
ApplicationContexts also override or add an additional number of editors to handle resource
lookups in a manner appropriate to the specific application context type.
Standard JavaBeans PropertyEditor instances are used to convert property values expressed as
strings to the actual complex type of the property. CustomEditorConfigurer, a bean factory postprocessor, may be used to conveniently add support for additional PropertyEditor instances to an
ApplicationContext.
Consider a user class ExoticType, and another class DependsOnExoticType which needs
ExoticType set as a property:
package example;
public class ExoticType {
private String name;
public ExoticType(String name) {
this.name = name;
}
}
public class DependsOnExoticType {
private ExoticType type;
public void setType(ExoticType type) {
this.type = type;
}
}
When things are properly set up, we want to be able to assign the type property as a string, which a
PropertyEditor will behind the scenes convert into an actual ExoticType instance:
<bean id="sample" class="example.DependsOnExoticType">
<property name="type" value="aNameForExoticType"/>
</bean>
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The PropertyEditor implementation could look similar to this:
// converts string representation to ExoticType object
package example;
public class ExoticTypeEditor extends PropertyEditorSupport {
public void setAsText(String text) {
setValue(new ExoticType(text.toUpperCase()));
}
}
Finally, we use CustomEditorConfigurer to register the new PropertyEditor with the
ApplicationContext, which will then be able to use it as needed:
<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
<property name="customEditors">
<map>
<entry key="example.ExoticType" value="example.ExoticTypeEditor"/>
</map>
</property>
</bean>
Using PropertyEditorRegistrars
Another mechanism for registering property editors with the Spring container is to create and
use a PropertyEditorRegistrar. This interface is particularly useful when you need to
use the same set of property editors in several different situations: write a corresponding
registrar and reuse that in each case. PropertyEditorRegistrars work in conjunction with
an interface called PropertyEditorRegistry, an interface that is implemented by the Spring
BeanWrapper (and DataBinder). PropertyEditorRegistrars are particularly convenient when
used in conjunction with the CustomEditorConfigurer (introduced here), which exposes a
property called setPropertyEditorRegistrars(..): PropertyEditorRegistrars added to
a CustomEditorConfigurer in this fashion can easily be shared with DataBinder and Spring
MVC Controllers. Furthermore, it avoids the need for synchronization on custom editors: a
PropertyEditorRegistrar is expected to create fresh PropertyEditor instances for each bean
creation attempt.
Using a PropertyEditorRegistrar is perhaps best illustrated with an example. First off, you need
to create your own PropertyEditorRegistrar implementation:
package com.foo.editors.spring;
public final class CustomPropertyEditorRegistrar implements PropertyEditorRegistrar {
public void registerCustomEditors(PropertyEditorRegistry registry) {
// it is expected that new PropertyEditor instances are created
registry.registerCustomEditor(ExoticType.class, new ExoticTypeEditor());
// you could register as many custom property editors as are required here...
}
}
See also the org.springframework.beans.support.ResourceEditorRegistrar for an
example PropertyEditorRegistrar implementation. Notice how in its implementation of the
registerCustomEditors(..) method it creates new instances of each property editor.
Next we configure a CustomEditorConfigurer
CustomPropertyEditorRegistrar into it:
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<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
<property name="propertyEditorRegistrars">
<list>
<ref bean="customPropertyEditorRegistrar"/>
</list>
</property>
</bean>
<bean id="customPropertyEditorRegistrar"
class="com.foo.editors.spring.CustomPropertyEditorRegistrar"/>
Finally, and in a bit of a departure from the focus of this chapter, for those of you using Spring’s MVC
web framework, using PropertyEditorRegistrars in conjunction with data-binding Controllers
(such as SimpleFormController) can be very convenient. Find below an example of using a
PropertyEditorRegistrar in the implementation of an initBinder(..) method:
public final class RegisterUserController extends SimpleFormController {
private final PropertyEditorRegistrar customPropertyEditorRegistrar;
public RegisterUserController(PropertyEditorRegistrar propertyEditorRegistrar) {
this.customPropertyEditorRegistrar = propertyEditorRegistrar;
}
protected void initBinder(HttpServletRequest request,
ServletRequestDataBinder binder) throws Exception {
this.customPropertyEditorRegistrar.registerCustomEditors(binder);
}
// other methods to do with registering a User
}
This style of PropertyEditor registration can lead to concise code (the implementation of
initBinder(..) is just one line long!), and allows common PropertyEditor registration code to
be encapsulated in a class and then shared amongst as many Controllers as needed.
8.5 Spring Type Conversion
Spring 3 introduces a core.convert package that provides a general type conversion system. The
system defines an SPI to implement type conversion logic, as well as an API to execute type conversions
at runtime. Within a Spring container, this system can be used as an alternative to PropertyEditors to
convert externalized bean property value strings to required property types. The public API may also be
used anywhere in your application where type conversion is needed.
Converter SPI
The SPI to implement type conversion logic is simple and strongly typed:
package org.springframework.core.convert.converter;
public interface Converter<S, T> {
T convert(S source);
}
To create your own converter, simply implement the interface above. Parameterize S as the type
you are converting from, and T as the type you are converting to. Such a converter can also be
applied transparently if a collection or array of S needs to be converted to an array or collection
of T, provided that a delegating array/collection converter has been registered as well (which
DefaultConversionService does by default).
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For each call to convert(S), the source argument is guaranteed to be NOT null. Your Converter
may throw any unchecked exception if conversion fails; specifically, an IllegalArgumentException
should be thrown to report an invalid source value. Take care to ensure that your Converter
implementation is thread-safe.
Several converter implementations are provided in the core.convert.support package as a
convenience. These include converters from Strings to Numbers and other common types. Consider
StringToInteger as an example for a typical Converter implementation:
package org.springframework.core.convert.support;
final class StringToInteger implements Converter<String, Integer> {
public Integer convert(String source) {
return Integer.valueOf(source);
}
}
ConverterFactory
When you need to centralize the conversion logic for an entire class hierarchy, for example, when
converting from String to java.lang.Enum objects, implement ConverterFactory:
package org.springframework.core.convert.converter;
public interface ConverterFactory<S, R> {
<T extends R> Converter<S, T> getConverter(Class<T> targetType);
}
Parameterize S to be the type you are converting from and R to be the base type defining the range of
classes you can convert to. Then implement getConverter(Class<T>), where T is a subclass of R.
Consider the StringToEnum ConverterFactory as an example:
package org.springframework.core.convert.support;
final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {
public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
return new StringToEnumConverter(targetType);
}
private final class StringToEnumConverter<T extends Enum> implements Converter<String, T> {
private Class<T> enumType;
public StringToEnumConverter(Class<T> enumType) {
this.enumType = enumType;
}
public T convert(String source) {
return (T) Enum.valueOf(this.enumType, source.trim());
}
}
}
GenericConverter
When you require a sophisticated Converter implementation, consider the GenericConverter interface.
With a more flexible but less strongly typed signature, a GenericConverter supports converting between
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multiple source and target types. In addition, a GenericConverter makes available source and target field
context you can use when implementing your conversion logic. Such context allows a type conversion
to be driven by a field annotation, or generic information declared on a field signature.
package org.springframework.core.convert.converter;
public interface GenericConverter {
public Set<ConvertiblePair> getConvertibleTypes();
Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
}
To implement a GenericConverter, have getConvertibleTypes() return the supported source#target type
pairs. Then implement convert(Object, TypeDescriptor, TypeDescriptor) to implement your conversion
logic. The source TypeDescriptor provides access to the source field holding the value being converted.
The target TypeDescriptor provides access to the target field where the converted value will be set.
A good example of a GenericConverter is a converter that converts between a Java Array and a
Collection. Such an ArrayToCollectionConverter introspects the field that declares the target Collection
type to resolve the Collection’s element type. This allows each element in the source array to be
converted to the Collection element type before the Collection is set on the target field.
Note
Because GenericConverter is a more complex SPI interface, only use it when you need it. Favor
Converter or ConverterFactory for basic type conversion needs.
ConditionalGenericConverter
Sometimes you only want a Converter to execute if a specific condition holds true. For example,
you might only want to execute a Converter if a specific annotation is present on the target
field. Or you might only want to execute a Converter if a specific method, such as a static
valueOf method, is defined on the target class. ConditionalGenericConverter is the union of the
GenericConverter and ConditionalConverter interfaces that allows you to define such custom
matching criteria:
public interface ConditionalGenericConverter
extends GenericConverter, ConditionalConverter {
boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType);
}
A good example of a ConditionalGenericConverter is an EntityConverter that converts between
an persistent entity identifier and an entity reference. Such a EntityConverter might only match if the
target entity type declares a static finder method e.g. findAccount(Long). You would perform such
a finder method check in the implementation of matches(TypeDescriptor, TypeDescriptor).
ConversionService API
The ConversionService defines a unified API for executing type conversion logic at runtime. Converters
are often executed behind this facade interface:
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package org.springframework.core.convert;
public interface ConversionService {
boolean canConvert(Class<?> sourceType, Class<?> targetType);
<T> T convert(Object source, Class<T> targetType);
boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType);
Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
}
Most ConversionService implementations also implement ConverterRegistry, which provides an
SPI for registering converters. Internally, a ConversionService implementation delegates to its registered
converters to carry out type conversion logic.
A robust ConversionService implementation is provided in the core.convert.support package.
GenericConversionService is the general-purpose implementation suitable for use in most
environments. ConversionServiceFactory provides a convenient factory for creating common
ConversionService configurations.
Configuring a ConversionService
A ConversionService is a stateless object designed to be instantiated at application startup, then shared
between multiple threads. In a Spring application, you typically configure a ConversionService instance
per Spring container (or ApplicationContext). That ConversionService will be picked up by Spring and
then used whenever a type conversion needs to be performed by the framework. You may also inject
this ConversionService into any of your beans and invoke it directly.
Note
If no ConversionService is registered with Spring, the original PropertyEditor-based system is
used.
To register a default ConversionService with Spring, add the following bean definition with id
conversionService:
<bean id="conversionService"
class="org.springframework.context.support.ConversionServiceFactoryBean"/>
A default ConversionService can convert between strings, numbers, enums, collections, maps, and
other common types. To supplement or override the default converters with your own custom
converter(s), set the converters property. Property values may implement either of the Converter,
ConverterFactory, or GenericConverter interfaces.
<bean id="conversionService"
class="org.springframework.context.support.ConversionServiceFactoryBean">
<property name="converters">
<set>
<bean class="example.MyCustomConverter"/>
</set>
</property>
</bean>
It is also common to use a ConversionService within a Spring MVC application. See the section called
“Conversion and Formatting” in the Spring MVC chapter.
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In certain situations you may wish to apply formatting during conversion. See the section called
“FormatterRegistry SPI” for details on using FormattingConversionServiceFactoryBean.
Using a ConversionService programmatically
To work with a ConversionService instance programmatically, simply inject a reference to it like you
would for any other bean:
@Service
public class MyService {
@Autowired
public MyService(ConversionService conversionService) {
this.conversionService = conversionService;
}
public void doIt() {
this.conversionService.convert(...)
}
}
For most use cases, the convert method specifying the targetType can be used but it will not work with
more complex types such as a collection of a parameterized element. If you want to convert a List of
Integer to a List of String programmatically, for instance, you need to provide a formal definition
of the source and target types.
Fortunately, TypeDescriptor provides various options to make that straightforward:
DefaultConversionService cs = new DefaultConversionService();
List<Integer> input = ....
cs.convert(input,
TypeDescriptor.forObject(input), // List<Integer> type descriptor
TypeDescriptor.collection(List.class, TypeDescriptor.valueOf(String.class)));
Note that DefaultConversionService registers converters automatically which are appropriate for
most environments. This includes collection converters, scalar converters, and also basic Object to
String converters. The same converters can be registered with any ConverterRegistry using the
static addDefaultConverters method on the DefaultConversionService class.
Converters for value types will be reused for arrays and collections, so there is no need to create a
specific converter to convert from a Collection of S to a Collection of T, assuming that standard
collection handling is appropriate.
8.6 Spring Field Formatting
As discussed in the previous section, core.convert is a general-purpose type conversion system. It
provides a unified ConversionService API as well as a strongly-typed Converter SPI for implementing
conversion logic from one type to another. A Spring Container uses this system to bind bean property
values. In addition, both the Spring Expression Language (SpEL) and DataBinder use this system
to bind field values. For example, when SpEL needs to coerce a Short to a Long to complete
an expression.setValue(Object bean, Object value) attempt, the core.convert system
performs the coercion.
Now consider the type conversion requirements of a typical client environment such as a web or desktop
application. In such environments, you typically convert from String to support the client postback
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process, as well as back to String to support the view rendering process. In addition, you often need to
localize String values. The more general core.convert Converter SPI does not address such formatting
requirements directly. To directly address them, Spring 3 introduces a convenient Formatter SPI that
provides a simple and robust alternative to PropertyEditors for client environments.
In general, use the Converter SPI when you need to implement general-purpose type conversion logic;
for example, for converting between a java.util.Date and and java.lang.Long. Use the Formatter SPI
when you’re working in a client environment, such as a web application, and need to parse and print
localized field values. The ConversionService provides a unified type conversion API for both SPIs.
Formatter SPI
The Formatter SPI to implement field formatting logic is simple and strongly typed:
package org.springframework.format;
public interface Formatter<T> extends Printer<T>, Parser<T> {
}
Where Formatter extends from the Printer and Parser building-block interfaces:
public interface Printer<T> {
String print(T fieldValue, Locale locale);
}
import java.text.ParseException;
public interface Parser<T> {
T parse(String clientValue, Locale locale) throws ParseException;
}
To create your own Formatter, simply implement the Formatter interface above. Parameterize T to be the
type of object you wish to format, for example, java.util.Date. Implement the print() operation
to print an instance of T for display in the client locale. Implement the parse() operation to parse an
instance of T from the formatted representation returned from the client locale. Your Formatter should
throw a ParseException or IllegalArgumentException if a parse attempt fails. Take care to ensure your
Formatter implementation is thread-safe.
Several Formatter implementations are provided in format subpackages as a convenience. The
number package provides a NumberFormatter, CurrencyFormatter, and PercentFormatter
to format java.lang.Number objects using a java.text.NumberFormat. The datetime package
provides a DateFormatter to format java.util.Date objects with a java.text.DateFormat.
The datetime.joda package provides comprehensive datetime formatting support based on the Joda
Time library.
Consider DateFormatter as an example Formatter implementation:
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package org.springframework.format.datetime;
public final class DateFormatter implements Formatter<Date> {
private String pattern;
public DateFormatter(String pattern) {
this.pattern = pattern;
}
public String print(Date date, Locale locale) {
if (date == null) {
return "";
}
return getDateFormat(locale).format(date);
}
public Date parse(String formatted, Locale locale) throws ParseException {
if (formatted.length() == 0) {
return null;
}
return getDateFormat(locale).parse(formatted);
}
protected DateFormat getDateFormat(Locale locale) {
DateFormat dateFormat = new SimpleDateFormat(this.pattern, locale);
dateFormat.setLenient(false);
return dateFormat;
}
}
The Spring team welcomes community-driven Formatter contributions; see jira.spring.io to contribute.
Annotation-driven Formatting
As you will see, field formatting can be configured by field type or annotation. To bind an Annotation to
a formatter, implement AnnotationFormatterFactory:
package org.springframework.format;
public interface AnnotationFormatterFactory<A extends Annotation> {
Set<Class<?>> getFieldTypes();
Printer<?> getPrinter(A annotation, Class<?> fieldType);
Parser<?> getParser(A annotation, Class<?> fieldType);
}
Parameterize A to be the field annotationType you wish to associate formatting logic
with, for example org.springframework.format.annotation.DateTimeFormat. Have
getFieldTypes() return the types of fields the annotation may be used on. Have getPrinter()
return a Printer to print the value of an annotated field. Have getParser() return a Parser to parse
a clientValue for an annotated field.
The example AnnotationFormatterFactory implementation below binds the @NumberFormat Annotation
to a formatter. This annotation allows either a number style or pattern to be specified:
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public final class NumberFormatAnnotationFormatterFactory
implements AnnotationFormatterFactory<NumberFormat> {
public Set<Class<?>> getFieldTypes() {
return new HashSet<Class<?>>(asList(new Class<?>[] {
Short.class, Integer.class, Long.class, Float.class,
Double.class, BigDecimal.class, BigInteger.class }));
}
public Printer<Number> getPrinter(NumberFormat annotation, Class<?> fieldType) {
return configureFormatterFrom(annotation, fieldType);
}
public Parser<Number> getParser(NumberFormat annotation, Class<?> fieldType) {
return configureFormatterFrom(annotation, fieldType);
}
private Formatter<Number> configureFormatterFrom(NumberFormat annotation,
Class<?> fieldType) {
if (!annotation.pattern().isEmpty()) {
return new NumberFormatter(annotation.pattern());
} else {
Style style = annotation.style();
if (style == Style.PERCENT) {
return new PercentFormatter();
} else if (style == Style.CURRENCY) {
return new CurrencyFormatter();
} else {
return new NumberFormatter();
}
}
}
}
To trigger formatting, simply annotate fields with @NumberFormat:
public class MyModel {
@NumberFormat(style=Style.CURRENCY)
private BigDecimal decimal;
}
Format Annotation API
A portable format annotation API exists in the org.springframework.format.annotation
package. Use @NumberFormat to format java.lang.Number fields. Use @DateTimeFormat to format
java.util.Date, java.util.Calendar, java.util.Long, or Joda Time fields.
The example below uses @DateTimeFormat to format a java.util.Date as a ISO Date (yyyy-MM-dd):
public class MyModel {
@DateTimeFormat(iso=ISO.DATE)
private Date date;
}
FormatterRegistry SPI
The
FormatterRegistry
is
an
SPI
for
registering
formatters
and
converters.
FormattingConversionService is an implementation of FormatterRegistry suitable for most
environments. This implementation may be configured programmatically or declaratively as a Spring
bean using FormattingConversionServiceFactoryBean. Because this implementation also
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implements ConversionService, it can be directly configured for use with Spring’s DataBinder and
the Spring Expression Language (SpEL).
Review the FormatterRegistry SPI below:
package org.springframework.format;
public interface FormatterRegistry extends ConverterRegistry {
void addFormatterForFieldType(Class<?> fieldType, Printer<?> printer, Parser<?> parser);
void addFormatterForFieldType(Class<?> fieldType, Formatter<?> formatter);
void addFormatterForFieldType(Formatter<?> formatter);
void addFormatterForAnnotation(AnnotationFormatterFactory<?, ?> factory);
}
As shown above, Formatters can be registered by fieldType or annotation.
The FormatterRegistry SPI allows you to configure Formatting rules centrally, instead of duplicating such
configuration across your Controllers. For example, you might want to enforce that all Date fields are
formatted a certain way, or fields with a specific annotation are formatted in a certain way. With a shared
FormatterRegistry, you define these rules once and they are applied whenever formatting is needed.
FormatterRegistrar SPI
The FormatterRegistrar is an SPI for registering formatters and converters through the
FormatterRegistry:
package org.springframework.format;
public interface FormatterRegistrar {
void registerFormatters(FormatterRegistry registry);
}
A FormatterRegistrar is useful when registering multiple related converters and formatters for a given
formatting category, such as Date formatting. It can also be useful where declarative registration is
insufficient. For example when a formatter needs to be indexed under a specific field type different from
its own <T> or when registering a Printer/Parser pair. The next section provides more information on
converter and formatter registration.
Configuring Formatting in Spring MVC
See the section called “Conversion and Formatting” in the Spring MVC chapter.
8.7 Configuring a global date & time format
By default, date and time fields that are not annotated with @DateTimeFormat are converted from
strings using the the DateFormat.SHORT style. If you prefer, you can change this by defining your
own global format.
You
will
need
to
ensure
that
Spring
does
not
register
default
formatters,
and
instead
you
should
register
all
formatters
manually.
Use
the org.springframework.format.datetime.joda.JodaTimeFormatterRegistrar or
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org.springframework.format.datetime.DateFormatterRegistrar class depending on
whether you use the Joda Time library.
For example, the following Java configuration will register a global ' `yyyyMMdd’ format. This example
does not depend on the Joda Time library:
@Configuration
public class AppConfig {
@Bean
public FormattingConversionService conversionService() {
// Use the DefaultFormattingConversionService but do not register defaults
DefaultFormattingConversionService conversionService = new
DefaultFormattingConversionService(false);
// Ensure @NumberFormat is still supported
conversionService.addFormatterForFieldAnnotation(new NumberFormatAnnotationFormatterFactory());
// Register date conversion with a specific global format
DateFormatterRegistrar registrar = new DateFormatterRegistrar();
registrar.setFormatter(new DateFormatter("yyyyMMdd"));
registrar.registerFormatters(conversionService);
return conversionService;
}
}
If
you
prefer
XML
based
configuration
you
can
use
a
FormattingConversionServiceFactoryBean. Here is the same example, this time using Joda
Time:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd>
<bean
id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
<property name="registerDefaultFormatters" value="false" />
<property name="formatters">
<set>
<bean class="org.springframework.format.number.NumberFormatAnnotationFormatterFactory" /
>
</set>
</property>
<property name="formatterRegistrars">
<set>
<bean class="org.springframework.format.datetime.joda.JodaTimeFormatterRegistrar">
<property name="dateFormatter">
<bean class="org.springframework.format.datetime.joda.DateTimeFormatterFactoryBean">
<property name="pattern" value="yyyyMMdd"/>
</bean>
</property>
</bean>
</set>
</property>
</bean>
</beans>
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Note
Joda Time provides separate distinct types to represent date, time and date-time
values. The dateFormatter, timeFormatter and dateTimeFormatter properties of the
JodaTimeFormatterRegistrar should be used to configure the different formats for each
type. The DateTimeFormatterFactoryBean provides a convenient way to create formatters.
If you are using Spring MVC remember to explicitly configure the conversion service that is used. For
Java based @Configuration this means extending the WebMvcConfigurationSupport class and
overriding the mvcConversionService() method. For XML you should use the 'conversionservice' attribute of the mvc:annotation-driven element. See the section called “Conversion
and Formatting” for details.
8.8 Spring Validation
Spring 3 introduces several enhancements to its validation support. First, the JSR-303 Bean Validation
API is now fully supported. Second, when used programmatically, Spring’s DataBinder can now validate
objects as well as bind to them. Third, Spring MVC now has support for declaratively validating
@Controller inputs.
Overview of the JSR-303 Bean Validation API
JSR-303 standardizes validation constraint declaration and metadata for the Java platform. Using this
API, you annotate domain model properties with declarative validation constraints and the runtime
enforces them. There are a number of built-in constraints you can take advantage of. You may also
define your own custom constraints.
To illustrate, consider a simple PersonForm model with two properties:
public class PersonForm {
private String name;
private int age;
}
JSR-303 allows you to define declarative validation constraints against such properties:
public class PersonForm {
@NotNull
@Size(max=64)
private String name;
@Min(0)
private int age;
}
When an instance of this class is validated by a JSR-303 Validator, these constraints will be enforced.
For general information on JSR-303/JSR-349, see the Bean Validation website. For information on the
specific capabilities of the default reference implementation, see the Hibernate Validator documentation.
To learn how to setup a Bean Validation provider as a Spring bean, keep reading.
Configuring a Bean Validation Provider
Spring provides full support for the Bean Validation API. This includes convenient support for
bootstrapping a JSR-303/JSR-349 Bean Validation provider as a Spring bean. This allows for
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a javax.validation.ValidatorFactory or javax.validation.Validator to be injected
wherever validation is needed in your application.
Use the LocalValidatorFactoryBean to configure a default Validator as a Spring bean:
<bean id="validator"
class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean"/>
The basic configuration above will trigger Bean Validation to initialize using its default bootstrap
mechanism. A JSR-303/JSR-349 provider, such as Hibernate Validator, is expected to be present in
the classpath and will be detected automatically.
Injecting a Validator
LocalValidatorFactoryBean implements both javax.validation.ValidatorFactory and
javax.validation.Validator,
as
well
as
Spring’s
org.springframework.validation.Validator. You may inject a reference to either of these
interfaces into beans that need to invoke validation logic.
Inject a reference to javax.validation.Validator if you prefer to work with the Bean Validation
API directly:
import javax.validation.Validator;
@Service
public class MyService {
@Autowired
private Validator validator;
Inject a reference to org.springframework.validation.Validator if your bean requires the
Spring Validation API:
import org.springframework.validation.Validator;
@Service
public class MyService {
@Autowired
private Validator validator;
}
Configuring Custom Constraints
Each Bean Validation constraint consists of two parts. First, a @Constraint annotation
that declares the constraint and its configurable properties. Second, an implementation of
the javax.validation.ConstraintValidator interface that implements the constraint’s
behavior. To associate a declaration with an implementation, each @Constraint annotation
references a corresponding ValidationConstraint implementation class. At runtime, a
ConstraintValidatorFactory instantiates the referenced implementation when the constraint
annotation is encountered in your domain model.
By
default,
the
LocalValidatorFactoryBean
configures
a
SpringConstraintValidatorFactory that uses Spring to create ConstraintValidator instances.
This allows your custom ConstraintValidators to benefit from dependency injection like any other Spring
bean.
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Shown below is an example of a custom @Constraint declaration, followed by an associated
ConstraintValidator implementation that uses Spring for dependency injection:
@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy=MyConstraintValidator.class)
public @interface MyConstraint {
}
import javax.validation.ConstraintValidator;
public class MyConstraintValidator implements ConstraintValidator {
@Autowired;
private Foo aDependency;
...
}
As you can see, a ConstraintValidator implementation may have its dependencies @Autowired like any
other Spring bean.
Spring-driven Method Validation
The method validation feature supported by Bean Validation 1.1, and as a custom extension
also by Hibernate Validator 4.3, can be integrated into a Spring context through a
MethodValidationPostProcessor bean definition:
<bean class="org.springframework.validation.beanvalidation.MethodValidationPostProcessor"/>
In order to be eligible for Spring-driven method validation, all target classes need to be annotated
with Spring’s @Validated annotation, optionally declaring the validation groups to use. Check out the
MethodValidationPostProcessor javadocs for setup details with Hibernate Validator and Bean
Validation 1.1 providers.
Additional Configuration Options
The default LocalValidatorFactoryBean configuration should prove sufficient for most cases.
There are a number of configuration options for various Bean Validation constructs, from message
interpolation to traversal resolution. See the LocalValidatorFactoryBean javadocs for more
information on these options.
Configuring a DataBinder
Since Spring 3, a DataBinder instance can be configured with a Validator. Once configured, the Validator
may be invoked by calling binder.validate(). Any validation Errors are automatically added to the
binder’s BindingResult.
When working with the DataBinder programmatically, this can be used to invoke validation logic after
binding to a target object:
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Foo target = new Foo();
DataBinder binder = new DataBinder(target);
binder.setValidator(new FooValidator());
// bind to the target object
binder.bind(propertyValues);
// validate the target object
binder.validate();
// get BindingResult that includes any validation errors
BindingResult results = binder.getBindingResult();
A DataBinder can also be configured with multiple Validator instances via
dataBinder.addValidators and dataBinder.replaceValidators. This is useful when
combining globally configured Bean Validation with a Spring Validator configured locally on a
DataBinder instance. See ???.
Spring MVC 3 Validation
See the section called “Validation” in the Spring MVC chapter.
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9. Spring Expression Language (SpEL)
9.1 Introduction
The Spring Expression Language (SpEL for short) is a powerful expression language that supports
querying and manipulating an object graph at runtime. The language syntax is similar to Unified EL but
offers additional features, most notably method invocation and basic string templating functionality.
While there are several other Java expression languages available, OGNL, MVEL, and JBoss EL, to
name a few, the Spring Expression Language was created to provide the Spring community with a single
well supported expression language that can be used across all the products in the Spring portfolio.
Its language features are driven by the requirements of the projects in the Spring portfolio, including
tooling requirements for code completion support within the eclipse based Spring Tool Suite. That said,
SpEL is based on a technology agnostic API allowing other expression language implementations to
be integrated should the need arise.
While SpEL serves as the foundation for expression evaluation within the Spring portfolio, it is not directly
tied to Spring and can be used independently. In order to be self contained, many of the examples in
this chapter use SpEL as if it were an independent expression language. This requires creating a few
bootstrapping infrastructure classes such as the parser. Most Spring users will not need to deal with this
infrastructure and will instead only author expression strings for evaluation. An example of this typical
use is the integration of SpEL into creating XML or annotated based bean definitions as shown in the
section Expression support for defining bean definitions.
This chapter covers the features of the expression language, its API, and its language syntax. In several
places an Inventor and Inventor’s Society class are used as the target objects for expression evaluation.
These class declarations and the data used to populate them are listed at the end of the chapter.
9.2 Feature Overview
The expression language supports the following functionality
• Literal expressions
• Boolean and relational operators
• Regular expressions
• Class expressions
• Accessing properties, arrays, lists, maps
• Method invocation
• Relational operators
• Assignment
• Calling constructors
• Bean references
• Array construction
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• Inline lists
• Inline maps
• Ternary operator
• Variables
• User defined functions
• Collection projection
• Collection selection
• Templated expressions
9.3 Expression Evaluation using Spring’s Expression Interface
This section introduces the simple use of SpEL interfaces and its expression language. The complete
language reference can be found in the section Language Reference.
The following code introduces the SpEL API to evaluate the literal string expression 'Hello World'.
ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("'Hello World'");
String message = (String) exp.getValue();
The value of the message variable is simply 'Hello World'.
The SpEL classes and interfaces you are most likely to use are located in the packages
org.springframework.expression and its sub packages and spel.support.
The interface ExpressionParser is responsible for parsing an expression string. In this example
the expression string is a string literal denoted by the surrounding single quotes. The interface
Expression is responsible for evaluating the previously defined expression string. There are
two exceptions that can be thrown, ParseException and EvaluationException when calling
parser.parseExpression and exp.getValue respectively.
SpEL supports a wide range of features, such as calling methods, accessing properties, and calling
constructors.
As an example of method invocation, we call the concat method on the string literal.
ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("'Hello World'.concat('!')");
String message = (String) exp.getValue();
The value of message is now 'Hello World!'.
As an example of calling a JavaBean property, the String property Bytes can be called as shown below.
ExpressionParser parser = new SpelExpressionParser();
// invokes 'getBytes()'
Expression exp = parser.parseExpression("'Hello World'.bytes");
byte[] bytes = (byte[]) exp.getValue();
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SpEL also supports nested properties using standard dot notation, i.e. prop1.prop2.prop3 and the setting
of property values
Public fields may also be accessed.
ExpressionParser parser = new SpelExpressionParser();
// invokes 'getBytes().length'
Expression exp = parser.parseExpression("'Hello World'.bytes.length");
int length = (Integer) exp.getValue();
The String’s constructor can be called instead of using a string literal.
ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("new String('hello world').toUpperCase()");
String message = exp.getValue(String.class);
Note the use of the generic method public <T> T getValue(Class<T> desiredResultType).
Using this method removes the need to cast the value of the expression to the desired result type. An
EvaluationException will be thrown if the value cannot be cast to the type T or converted using
the registered type converter.
The more common usage of SpEL is to provide an expression string that is evaluated against a specific
object instance (called the root object). There are two options here and which to choose depends on
whether the object against which the expression is being evaluated will be changing with each call to
evaluate the expression. In the following example we retrieve the name property from an instance of
the Inventor class.
// Create and set a calendar
GregorianCalendar c = new GregorianCalendar();
c.set(1856, 7, 9);
// The constructor arguments are name, birthday, and nationality.
Inventor tesla = new Inventor("Nikola Tesla", c.getTime(), "Serbian");
ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("name");
EvaluationContext context = new StandardEvaluationContext(tesla);
String name = (String) exp.getValue(context);
In the last line, the value of the string variable name will be set to "Nikola Tesla". The class
StandardEvaluationContext is where you can specify which object the "name" property will be evaluated
against. This is the mechanism to use if the root object is unlikely to change, it can simply be set once
in the evaluation context. If the root object is likely to change repeatedly, it can be supplied on each call
to getValue, as this next example shows:
/ Create and set a calendar
GregorianCalendar c = new GregorianCalendar();
c.set(1856, 7, 9);
// The constructor arguments are name, birthday, and nationality.
Inventor tesla = new Inventor("Nikola Tesla", c.getTime(), "Serbian");
ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("name");
String name = (String) exp.getValue(tesla);
In this case the inventor tesla has been supplied directly to getValue and the expression evaluation
infrastructure creates and manages a default evaluation context internally - it did not require one to be
supplied.
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The StandardEvaluationContext is relatively expensive to construct and during repeated usage it builds
up cached state that enables subsequent expression evaluations to be performed more quickly. For
this reason it is better to cache and reuse them where possible, rather than construct a new one for
each expression evaluation.
In some cases it can be desirable to use a configured evaluation context and yet still supply a different
root object on each call to getValue. getValue allows both to be specified on the same call. In these
situations the root object passed on the call is considered to override any (which maybe null) specified
on the evaluation context.
Note
In standalone usage of SpEL there is a need to create the parser, parse expressions and perhaps
provide evaluation contexts and a root context object. However, more common usage is to provide
only the SpEL expression string as part of a configuration file, for example for Spring bean or
Spring Web Flow definitions. In this case, the parser, evaluation context, root object and any
predefined variables are all set up implicitly, requiring the user to specify nothing other than the
expressions.
As a final introductory example, the use of a boolean operator is shown using the Inventor object in
the previous example.
Expression exp = parser.parseExpression("name == 'Nikola Tesla'");
boolean result = exp.getValue(context, Boolean.class); // evaluates to true
The EvaluationContext interface
The interface EvaluationContext is used when evaluating an expression to
resolve properties, methods, fields, and to help perform type conversion. The outof-the-box implementation, StandardEvaluationContext, uses reflection to manipulate
the object, caching java.lang.reflect.Method, java.lang.reflect.Field, and
java.lang.reflect.Constructor instances for increased performance.
The StandardEvaluationContext is where you may specify the root object to evaluate against
via the method setRootObject() or passing the root object into the constructor. You can also
specify variables and functions that will be used in the expression using the methods setVariable()
and registerFunction(). The use of variables and functions are described in the language
reference sections Variables and Functions. The StandardEvaluationContext is also where you
can register custom ConstructorResolvers, MethodResolvers, and PropertyAccessors to extend how
SpEL evaluates expressions. Please refer to the JavaDoc of these classes for more details.
Type Conversion
By
default
SpEL
uses
the
conversion
service
available
in
Spring
core
(
org.springframework.core.convert.ConversionService). This conversion service comes
with many converters built in for common conversions but is also fully extensible so custom conversions
between types can be added. Additionally it has the key capability that it is generics aware. This means
that when working with generic types in expressions, SpEL will attempt conversions to maintain type
correctness for any objects it encounters.
What does this mean in practice? Suppose assignment, using setValue(), is being used to set a List
property. The type of the property is actually List<Boolean>. SpEL will recognize that the elements
of the list need to be converted to Boolean before being placed in it. A simple example:
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class Simple {
public List<Boolean> booleanList = new ArrayList<Boolean>();
}
Simple simple = new Simple();
simple.booleanList.add(true);
StandardEvaluationContext simpleContext = new StandardEvaluationContext(simple);
// false is passed in here as a string. SpEL and the conversion service will
// correctly recognize that it needs to be a Boolean and convert it
parser.parseExpression("booleanList[0]").setValue(simpleContext, "false");
// b will be false
Boolean b = simple.booleanList.get(0);
Parser configuration
It is possible to configure the SpEL expression parser using a parser configuration object
(org.springframework.expression.spel.SpelParserConfiguration). The configuration
object controls the behavior of some of the expression components. For example, if indexing into an
array or collection and the element at the specified index is null it is possible to automatically create the
element. This is useful when using expressions made up of a chain of property references. If indexing
into an array or list and specifying an index that is beyond the end of the current size of the array or list
it is possible to automatically grow the array or list to accommodate that index.
class Demo {
public List<String> list;
}
// Turn on:
// - auto null reference initialization
// - auto collection growing
SpelParserConfiguration config = new SpelParserConfiguration(true,true);
ExpressionParser parser = new SpelExpressionParser(config);
Expression expression = parser.parseExpression("list[3]");
Demo demo = new Demo();
Object o = expression.getValue(demo);
// demo.list will now be a real collection of 4 entries
// Each entry is a new empty String
It is also possible to configure the behaviour of the SpEL expression compiler.
SpEL compilation
Spring Framework 4.1 includes a basic expression compiler. Expressions are usually interpreted which
provides a lot of dynamic flexibility during evaluation but does not provide the optimum performance. For
occasional expression usage this is fine, but when used by other components like Spring Integration,
performance can be very important and there is no real need for the dynamism.
The new SpEL compiler is intended to address this need. The compiler will generate a real Java
class on the fly during evaluation that embodies the expression behavior and use that to achieve
much faster expression evaluation. Due to the lack of typing around expressions the compiler uses
information gathered during the interpreted evaluations of an expression when performing compilation.
For example, it does not know the type of a property reference purely from the expression but during the
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first interpreted evaluation it will find out what it is. Of course, basing the compilation on this information
could cause trouble later if the types of the various expression elements change over time. For this
reason compilation is best suited to expressions whose type information is not going to change on
repeated evaluations.
For a basic expression like this:
someArray[0].someProperty.someOtherProperty < 0.1
which involves array access, some property derefencing and numeric operations, the performance gain
can be very noticeable. In an example micro benchmark run of 50000 iterations, it was taking 75ms to
evaluate using only the interpreter and just 3ms using the compiled version of the expression.
Compiler configuration
The compiler is not turned on by default, but there are two ways to turn it on. It can be turned on using the
parser configuration process discussed earlier or via a system property when SpEL usage is embedded
inside another component. This section discusses both of these options.
Is is important to understand that there are a few modes the compiler can operate in, captured in an enum
(org.springframework.expression.spel.SpelCompilerMode). The modes are as follows:
• OFF - The compiler is switched off; this is the default.
• IMMEDIATE - In immediate mode the expressions are compiled as soon as possible. This is typically
after the first interpreted evaluation. If the compiled expression fails (typically due to a type changing,
as described above) then the caller of the expression evaluation will receive an exception.
• MIXED - In mixed mode the expressions silently switch between interpreted and compiled mode over
time. After some number of interpreted runs they will switch to compiled form and if something goes
wrong with the compiled form (like a type changing, as described above) then the expression will
automatically switch back to interpreted form again. Sometime later it may generate another compiled
form and switch to it. Basically the exception that the user gets in IMMEDIATE mode is instead handled
internally.
IMMEDIATE mode exists because MIXED mode could cause issues for expressions that have side
effects. If a compiled expression blows up after partially succeeding it may have already done something
that has affected the state of the system. If this has happened the caller may not want it to silently rerun in interpreted mode since part of the expression may be running twice.
After selecting a mode, use the SpelParserConfiguration to configure the parser:
SpelParserConfiguration config = new SpelParserConfiguration(SpelCompilerMode.IMMEDIATE,
this.getClass().getClassLoader());
SpelExpressionParser parser = new SpelExpressionParser(config);
Expression expr = parser.parseExpression("payload");
MyMessage message = new MyMessage();
Object payload = expr.getValue(message);
When specifying the compiler mode it is also possible to specify a classloader (passing null is allowed).
Compiled expressions will be defined in a child classloader created under any that is supplied. It is
important to ensure if a classloader is specified it can see all the types involved in the expression
evaluation process. If none is specified then a default classloader will be used (typically the context
classloader for the thread that is running during expression evaluation).
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The second way to configure the compiler is for use when SpEL is embedded inside some other
component and it may not be possible to configure via a configuration object. In these cases it is possible
to use a system property. The property spring.expression.compiler.mode can be set to one of
the SpelCompilerMode enum values (off, immediate, or mixed).
Compiler limitations
With Spring Framework 4.1 the basic compilation framework is in place. However, the framework
does not yet support compiling every kind of expression. The initial focus has been on the common
expressions that are likely to be used in performance critical contexts. These kinds of expression cannot
be compiled at the moment:
• expressions involving assignment
• expressions relying on the conversion service
• expressions using custom resolvers or accessors
• expressions using selection or projection
More and more types of expression will be compilable in the future.
9.4 Expression support for defining bean definitions
SpEL expressions can be used with XML or annotation-based configuration metadata for defining
BeanDefinitions. In both cases the syntax to define the expression is of the form #{ <expression
string> }.
XML based configuration
A property or constructor-arg value can be set using expressions as shown below.
<bean id="numberGuess" class="org.spring.samples.NumberGuess">
<property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>
<!-- other properties -->
</bean>
The variable systemProperties is predefined, so you can use it in your expressions as shown below.
Note that you do not have to prefix the predefined variable with the # symbol in this context.
<bean id="taxCalculator" class="org.spring.samples.TaxCalculator">
<property name="defaultLocale" value="#{ systemProperties['user.region'] }"/>
<!-- other properties -->
</bean>
You can also refer to other bean properties by name, for example.
<bean id="numberGuess" class="org.spring.samples.NumberGuess">
<property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>
<!-- other properties -->
</bean>
<bean id="shapeGuess" class="org.spring.samples.ShapeGuess">
<property name="initialShapeSeed" value="#{ numberGuess.randomNumber }"/>
<!-- other properties -->
</bean>
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Annotation-based configuration
The @Value annotation can be placed on fields, methods and method/constructor parameters to specify
a default value.
Here is an example to set the default value of a field variable.
public static class FieldValueTestBean
@Value("#{ systemProperties['user.region'] }")
private String defaultLocale;
public void setDefaultLocale(String defaultLocale) {
this.defaultLocale = defaultLocale;
}
public String getDefaultLocale() {
return this.defaultLocale;
}
}
The equivalent but on a property setter method is shown below.
public static class PropertyValueTestBean
private String defaultLocale;
@Value("#{ systemProperties['user.region'] }")
public void setDefaultLocale(String defaultLocale) {
this.defaultLocale = defaultLocale;
}
public String getDefaultLocale() {
return this.defaultLocale;
}
}
Autowired methods and constructors can also use the @Value annotation.
public class SimpleMovieLister {
private MovieFinder movieFinder;
private String defaultLocale;
@Autowired
public void configure(MovieFinder movieFinder,
@Value("#{ systemProperties['user.region'] }") String defaultLocale) {
this.movieFinder = movieFinder;
this.defaultLocale = defaultLocale;
}
// ...
}
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public class MovieRecommender {
private String defaultLocale;
private CustomerPreferenceDao customerPreferenceDao;
@Autowired
public MovieRecommender(CustomerPreferenceDao customerPreferenceDao,
@Value("#{systemProperties['user.country']}") String defaultLocale) {
this.customerPreferenceDao = customerPreferenceDao;
this.defaultLocale = defaultLocale;
}
// ...
}
9.5 Language Reference
Literal expressions
The types of literal expressions supported are strings, dates, numeric values (int, real, and hex), boolean
and null. Strings are delimited by single quotes. To put a single quote itself in a string use two single
quote characters. The following listing shows simple usage of literals. Typically they would not be used
in isolation like this, but as part of a more complex expression, for example using a literal on one side
of a logical comparison operator.
ExpressionParser parser = new SpelExpressionParser();
// evals to "Hello World"
String helloWorld = (String) parser.parseExpression("'Hello World'").getValue();
double avogadrosNumber = (Double) parser.parseExpression("6.0221415E+23").getValue();
// evals to 2147483647
int maxValue = (Integer) parser.parseExpression("0x7FFFFFFF").getValue();
boolean trueValue = (Boolean) parser.parseExpression("true").getValue();
Object nullValue = parser.parseExpression("null").getValue();
Numbers support the use of the negative sign, exponential notation, and decimal points. By default real
numbers are parsed using Double.parseDouble().
Properties, Arrays, Lists, Maps, Indexers
Navigating with property references is easy: just use a period to indicate a nested property value. The
instances of the Inventor class, pupin, and tesla, were populated with data listed in the section Classes
used in the examples. To navigate "down" and get Tesla’s year of birth and Pupin’s city of birth the
following expressions are used.
// evals to 1856
int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context);
String city = (String) parser.parseExpression("placeOfBirth.City").getValue(context);
Case insensitivity is allowed for the first letter of property names. The contents of arrays and lists are
obtained using square bracket notation.
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ExpressionParser parser = new SpelExpressionParser();
// Inventions Array
StandardEvaluationContext teslaContext = new StandardEvaluationContext(tesla);
// evaluates to "Induction motor"
String invention = parser.parseExpression("inventions[3]").getValue(
teslaContext, String.class);
// Members List
StandardEvaluationContext societyContext = new StandardEvaluationContext(ieee);
// evaluates to "Nikola Tesla"
String name = parser.parseExpression("Members[0].Name").getValue(
societyContext, String.class);
// List and Array navigation
// evaluates to "Wireless communication"
String invention = parser.parseExpression("Members[0].Inventions[6]").getValue(
societyContext, String.class);
The contents of maps are obtained by specifying the literal key value within the brackets. In this case,
because keys for the Officers map are strings, we can specify string literals.
// Officer's Dictionary
Inventor pupin = parser.parseExpression("Officers['president']").getValue(
societyContext, Inventor.class);
// evaluates to "Idvor"
String city = parser.parseExpression("Officers['president'].PlaceOfBirth.City").getValue(
societyContext, String.class);
// setting values
parser.parseExpression("Officers['advisors'][0].PlaceOfBirth.Country").setValue(
societyContext, "Croatia");
Inline lists
Lists can be expressed directly in an expression using {} notation.
// evaluates to a Java list containing the four numbers
List numbers = (List) parser.parseExpression("{1,2,3,4}").getValue(context);
List listOfLists = (List) parser.parseExpression("{{'a','b'},{'x','y'}}").getValue(context);
{} by itself means an empty list. For performance reasons, if the list is itself entirely composed of fixed
literals then a constant list is created to represent the expression, rather than building a new list on
each evaluation.
Inline Maps
Maps can also be expressed directly in an expression using {key:value} notation.
// evaluates to a Java map containing the two entries
Map inventorInfo = (Map) parser.parseExpression("{name:'Nikola',dob:'10-July-1856'}").getValue(context);
Map mapOfMaps = (Map) parser.parseExpression("{name:{first:'Nikola',last:'Tesla'},dob:
{day:10,month:'July',year:1856}}").getValue(context);
{:} by itself means an empty map. For performance reasons, if the map is itself composed of fixed
literals or other nested constant structures (lists or maps) then a constant map is created to represent
the expression, rather than building a new map on each evaluation. Quoting of the map keys is optional,
the examples above are not using quoted keys.
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Array construction
Arrays can be built using the familiar Java syntax, optionally supplying an initializer to have the array
populated at construction time.
int[] numbers1 = (int[]) parser.parseExpression("new int[4]").getValue(context);
// Array with initializer
int[] numbers2 = (int[]) parser.parseExpression("new int[]{1,2,3}").getValue(context);
// Multi dimensional array
int[][] numbers3 = (int[][]) parser.parseExpression("new int[4][5]").getValue(context);
It is not currently allowed to supply an initializer when constructing a multi-dimensional array.
Methods
Methods are invoked using typical Java programming syntax. You may also invoke methods on literals.
Varargs are also supported.
// string literal, evaluates to "bc"
String c = parser.parseExpression("'abc'.substring(2, 3)").getValue(String.class);
// evaluates to true
boolean isMember = parser.parseExpression("isMember('Mihajlo Pupin')").getValue(
societyContext, Boolean.class);
Operators
Relational operators
The relational operators; equal, not equal, less than, less than or equal, greater than, and greater than
or equal are supported using standard operator notation.
// evaluates to true
boolean trueValue = parser.parseExpression("2 == 2").getValue(Boolean.class);
// evaluates to false
boolean falseValue = parser.parseExpression("2 < -5.0").getValue(Boolean.class);
// evaluates to true
boolean trueValue = parser.parseExpression("'black' < 'block'").getValue(Boolean.class);
In addition to standard relational operators SpEL supports the instanceof and regular expression
based matches operator.
// evaluates to false
boolean falseValue = parser.parseExpression(
"'xyz' instanceof T(int)").getValue(Boolean.class);
// evaluates to true
boolean trueValue = parser.parseExpression(
"'5.00' matches '\^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class);
//evaluates to false
boolean falseValue = parser.parseExpression(
"'5.0067' matches '\^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class);
Each symbolic operator can also be specified as a purely alphabetic equivalent. This avoids problems
where the symbols used have special meaning for the document type in which the expression is
embedded (eg. an XML document). The textual equivalents are shown here: lt (<), gt (>), le (#), ge
(>=), eq (==), ne (!=), div (/), mod (%), not (!). These are case insensitive.
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Logical operators
The logical operators that are supported are and, or, and not. Their use is demonstrated below.
// -- AND -// evaluates to false
boolean falseValue = parser.parseExpression("true and false").getValue(Boolean.class);
// evaluates to true
String expression = "isMember('Nikola Tesla') and isMember('Mihajlo Pupin')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);
// -- OR -// evaluates to true
boolean trueValue = parser.parseExpression("true or false").getValue(Boolean.class);
// evaluates to true
String expression = "isMember('Nikola Tesla') or isMember('Albert Einstein')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);
// -- NOT -// evaluates to false
boolean falseValue = parser.parseExpression("!true").getValue(Boolean.class);
// -- AND and NOT -String expression = "isMember('Nikola Tesla') and !isMember('Mihajlo Pupin')";
boolean falseValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);
Mathematical operators
The addition operator can be used on both numbers and strings. Subtraction, multiplication and
division can be used only on numbers. Other mathematical operators supported are modulus (%) and
exponential power (^). Standard operator precedence is enforced. These operators are demonstrated
below.
// Addition
int two = parser.parseExpression("1 + 1").getValue(Integer.class); // 2
String testString = parser.parseExpression(
"'test' + ' ' + 'string'").getValue(String.class); // 'test string'
// Subtraction
int four = parser.parseExpression("1 - -3").getValue(Integer.class); // 4
double d = parser.parseExpression("1000.00 - 1e4").getValue(Double.class); // -9000
// Multiplication
int six = parser.parseExpression("-2 * -3").getValue(Integer.class); // 6
double twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double.class); // 24.0
// Division
int minusTwo = parser.parseExpression("6 / -3").getValue(Integer.class); // -2
double one = parser.parseExpression("8.0 / 4e0 / 2").getValue(Double.class); // 1.0
// Modulus
int three = parser.parseExpression("7 % 4").getValue(Integer.class); // 3
int one = parser.parseExpression("8 / 5 % 2").getValue(Integer.class); // 1
// Operator precedence
int minusTwentyOne = parser.parseExpression("1+2-3*8").getValue(Integer.class); // -21
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Assignment
Setting of a property is done by using the assignment operator. This would typically be done within a
call to setValue but can also be done inside a call to getValue.
Inventor inventor = new Inventor();
StandardEvaluationContext inventorContext = new StandardEvaluationContext(inventor);
parser.parseExpression("Name").setValue(inventorContext, "Alexander Seovic2");
// alternatively
String aleks = parser.parseExpression(
"Name = 'Alexandar Seovic'").getValue(inventorContext, String.class);
Types
The special T operator can be used to specify an instance of java.lang.Class (the type). Static methods
are invoked using this operator as well. The StandardEvaluationContext uses a TypeLocator
to find types and the StandardTypeLocator (which can be replaced) is built with an understanding
of the java.lang package. This means T() references to types within java.lang do not need to be fully
qualified, but all other type references must be.
Class dateClass = parser.parseExpression("T(java.util.Date)").getValue(Class.class);
Class stringClass = parser.parseExpression("T(String)").getValue(Class.class);
boolean trueValue = parser.parseExpression(
"T(java.math.RoundingMode).CEILING < T(java.math.RoundingMode).FLOOR")
.getValue(Boolean.class);
Constructors
Constructors can be invoked using the new operator. The fully qualified class name should be used for
all but the primitive type and String (where int, float, etc, can be used).
Inventor einstein = p.parseExpression(
"new org.spring.samples.spel.inventor.Inventor('Albert Einstein', 'German')")
.getValue(Inventor.class);
//create new inventor instance within add method of List
p.parseExpression(
"Members.add(new org.spring.samples.spel.inventor.Inventor(
'Albert Einstein', 'German'))").getValue(societyContext);
Variables
Variables can be referenced in the expression using the syntax #variableName. Variables are set
using the method setVariable on the StandardEvaluationContext.
Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
StandardEvaluationContext context = new StandardEvaluationContext(tesla);
context.setVariable("newName", "Mike Tesla");
parser.parseExpression("Name = #newName").getValue(context);
System.out.println(tesla.getName()) // "Mike Tesla"
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The #this and #root variables
The variable #this is always defined and refers to the current evaluation object (against which unqualified
references are resolved). The variable #root is always defined and refers to the root context object.
Although #this may vary as components of an expression are evaluated, #root always refers to the root.
// create an array of integers
List<Integer> primes = new ArrayList<Integer>();
primes.addAll(Arrays.asList(2,3,5,7,11,13,17));
// create parser and set variable 'primes' as the array of integers
ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setVariable("primes",primes);
// all prime numbers > 10 from the list (using selection ?{...})
// evaluates to [11, 13, 17]
List<Integer> primesGreaterThanTen = (List<Integer>) parser.parseExpression(
"#primes.?[#this>10]").getValue(context);
Functions
You can extend SpEL by registering user defined functions that can be called within the expression
string. The function is registered with the StandardEvaluationContext using the method.
public void registerFunction(String name, Method m)
A reference to a Java Method provides the implementation of the function. For example, a utility method
to reverse a string is shown below.
public abstract class StringUtils {
public static String reverseString(String input) {
StringBuilder backwards = new StringBuilder();
for (int i = 0; i < input.length(); i++)
backwards.append(input.charAt(input.length() - 1 - i));
}
return backwards.toString();
}
}
This method is then registered with the evaluation context and can be used within an expression string.
ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.registerFunction("reverseString",
StringUtils.class.getDeclaredMethod("reverseString", new Class[] { String.class }));
String helloWorldReversed = parser.parseExpression(
"#reverseString('hello')").getValue(context, String.class);
Bean references
If the evaluation context has been configured with a bean resolver it is possible to lookup beans from
an expression using the (@) symbol.
ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setBeanResolver(new MyBeanResolver());
// This will end up calling resolve(context,"foo") on MyBeanResolver during evaluation
Object bean = parser.parseExpression("@foo").getValue(context);
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Ternary Operator (If-Then-Else)
You can use the ternary operator for performing if-then-else conditional logic inside the expression. A
minimal example is:
String falseString = parser.parseExpression(
"false ? 'trueExp' : 'falseExp'").getValue(String.class);
In this case, the boolean false results in returning the string value 'falseExp'. A more realistic example
is shown below.
parser.parseExpression("Name").setValue(societyContext, "IEEE");
societyContext.setVariable("queryName", "Nikola Tesla");
expression = "isMember(#queryName)? #queryName + ' is a member of the ' " +
"+ Name + ' Society' : #queryName + ' is not a member of the ' + Name + ' Society'";
String queryResultString = parser.parseExpression(expression)
.getValue(societyContext, String.class);
// queryResultString = "Nikola Tesla is a member of the IEEE Society"
Also see the next section on the Elvis operator for an even shorter syntax for the ternary operator.
The Elvis Operator
The Elvis operator is a shortening of the ternary operator syntax and is used in the Groovy language.
With the ternary operator syntax you usually have to repeat a variable twice, for example:
String name = "Elvis Presley";
String displayName = name != null ? name : "Unknown";
Instead you can use the Elvis operator, named for the resemblance to Elvis' hair style.
ExpressionParser parser = new SpelExpressionParser();
String name = parser.parseExpression("name?:'Unknown'").getValue(String.class);
System.out.println(name); // 'Unknown'
Here is a more complex example.
ExpressionParser parser = new SpelExpressionParser();
Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
StandardEvaluationContext context = new StandardEvaluationContext(tesla);
String name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, String.class);
System.out.println(name); // Nikola Tesla
tesla.setName(null);
name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, String.class);
System.out.println(name); // Elvis Presley
Safe Navigation operator
The Safe Navigation operator is used to avoid a NullPointerException and comes from the Groovy
language. Typically when you have a reference to an object you might need to verify that it is not null
before accessing methods or properties of the object. To avoid this, the safe navigation operator will
simply return null instead of throwing an exception.
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ExpressionParser parser = new SpelExpressionParser();
Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
tesla.setPlaceOfBirth(new PlaceOfBirth("Smiljan"));
StandardEvaluationContext context = new StandardEvaluationContext(tesla);
String city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class);
System.out.println(city); // Smiljan
tesla.setPlaceOfBirth(null);
city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class);
System.out.println(city); // null - does not throw NullPointerException!!!
Note
The Elvis operator can be used to apply default values in expressions, e.g. in an @Value
expression:
@Value("#{systemProperties['pop3.port'] ?: 25}")
This will inject a system property pop3.port if it is defined or 25 if not.
Collection Selection
Selection is a powerful expression language feature that allows you to transform some source collection
into another by selecting from its entries.
Selection uses the syntax ?[selectionExpression]. This will filter the collection and return a new
collection containing a subset of the original elements. For example, selection would allow us to easily
get a list of Serbian inventors:
List<Inventor> list = (List<Inventor>) parser.parseExpression(
"Members.?[Nationality == 'Serbian']").getValue(societyContext);
Selection is possible upon both lists and maps. In the former case the selection criteria is evaluated
against each individual list element whilst against a map the selection criteria is evaluated against each
map entry (objects of the Java type Map.Entry). Map entries have their key and value accessible as
properties for use in the selection.
This expression will return a new map consisting of those elements of the original map where the entry
value is less than 27.
Map newMap = parser.parseExpression("map.?[value<27]").getValue();
In addition to returning all the selected elements, it is possible to retrieve just the first or the last value.
To obtain the first entry matching the selection the syntax is ^[…] whilst to obtain the last matching
selection the syntax is $[…].
Collection Projection
Projection allows a collection to drive the evaluation of a sub-expression and the result is a new
collection. The syntax for projection is ![projectionExpression]. Most easily understood by
example, suppose we have a list of inventors but want the list of cities where they were born. Effectively
we want to evaluate 'placeOfBirth.city' for every entry in the inventor list. Using projection:
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// returns ['Smiljan', 'Idvor' ]
List placesOfBirth = (List)parser.parseExpression("Members.![placeOfBirth.city]");
A map can also be used to drive projection and in this case the projection expression is evaluated
against each entry in the map (represented as a Java Map.Entry). The result of a projection across a
map is a list consisting of the evaluation of the projection expression against each map entry.
Expression templating
Expression templates allow a mixing of literal text with one or more evaluation blocks. Each evaluation
block is delimited with prefix and suffix characters that you can define, a common choice is to use #{ }
as the delimiters. For example,
String randomPhrase = parser.parseExpression(
"random number is #{T(java.lang.Math).random()}",
new TemplateParserContext()).getValue(String.class);
// evaluates to "random number is 0.7038186818312008"
The string is evaluated by concatenating the literal text 'random number is ' with the result of evaluating
the expression inside the #{ } delimiter, in this case the result of calling that random() method. The second
argument to the method parseExpression() is of the type ParserContext. The ParserContext
interface is used to influence how the expression is parsed in order to support the expression templating
functionality. The definition of TemplateParserContext is shown below.
public class TemplateParserContext implements ParserContext {
public String getExpressionPrefix() {
return "#{";
}
public String getExpressionSuffix() {
return "}";
}
public boolean isTemplate() {
return true;
}
}
9.6 Classes used in the examples
Inventor.java
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package org.spring.samples.spel.inventor;
import java.util.Date;
import java.util.GregorianCalendar;
public class Inventor {
private
private
private
private
private
String name;
String nationality;
String[] inventions;
Date birthdate;
PlaceOfBirth placeOfBirth;
public Inventor(String name, String nationality) {
GregorianCalendar c= new GregorianCalendar();
this.name = name;
this.nationality = nationality;
this.birthdate = c.getTime();
}
public Inventor(String name, Date birthdate, String nationality) {
this.name = name;
this.nationality = nationality;
this.birthdate = birthdate;
}
public Inventor() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getNationality() {
return nationality;
}
public void setNationality(String nationality) {
this.nationality = nationality;
}
public Date getBirthdate() {
return birthdate;
}
public void setBirthdate(Date birthdate) {
this.birthdate = birthdate;
}
public PlaceOfBirth getPlaceOfBirth() {
return placeOfBirth;
}
public void setPlaceOfBirth(PlaceOfBirth placeOfBirth) {
this.placeOfBirth = placeOfBirth;
}
public void setInventions(String[] inventions) {
this.inventions = inventions;
}
public String[] getInventions() {
return inventions;
}
}
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PlaceOfBirth.java
package org.spring.samples.spel.inventor;
public class PlaceOfBirth {
private String city;
private String country;
public PlaceOfBirth(String city) {
this.city=city;
}
public PlaceOfBirth(String city, String country) {
this(city);
this.country = country;
}
public String getCity() {
return city;
}
public void setCity(String s) {
this.city = s;
}
public String getCountry() {
return country;
}
public void setCountry(String country) {
this.country = country;
}
}
Society.java
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package org.spring.samples.spel.inventor;
import java.util.*;
public class Society {
private String name;
public static String Advisors = "advisors";
public static String President = "president";
private List<Inventor> members = new ArrayList<Inventor>();
private Map officers = new HashMap();
public List getMembers() {
return members;
}
public Map getOfficers() {
return officers;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public boolean isMember(String name) {
for (Inventor inventor : members) {
if (inventor.getName().equals(name)) {
return true;
}
}
return false;
}
}
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10. Aspect Oriented Programming with Spring
10.1 Introduction
Aspect-Oriented Programming (AOP) complements Object-Oriented Programming (OOP) by providing
another way of thinking about program structure. The key unit of modularity in OOP is the class, whereas
in AOP the unit of modularity is the aspect. Aspects enable the modularization of concerns such as
transaction management that cut across multiple types and objects. (Such concerns are often termed
crosscutting concerns in AOP literature.)
One of the key components of Spring is the AOP framework. While the Spring IoC container does not
depend on AOP, meaning you do not need to use AOP if you don’t want to, AOP complements Spring
IoC to provide a very capable middleware solution.
Spring 2.0 AOP
Spring 2.0 introduces a simpler and more powerful way of writing custom aspects using either a
schema-based approach or the @AspectJ annotation style. Both of these styles offer fully typed
advice and use of the AspectJ pointcut language, while still using Spring AOP for weaving.
The Spring 2.0 schema- and @AspectJ-based AOP support is discussed in this chapter. Spring 2.0
AOP remains fully backwards compatible with Spring 1.2 AOP, and the lower-level AOP support
offered by the Spring 1.2 APIs is discussed in the following chapter.
AOP is used in the Spring Framework to…
• … provide declarative enterprise services, especially as a replacement for EJB declarative services.
The most important such service is declarative transaction management.
• … allow users to implement custom aspects, complementing their use of OOP with AOP.
Note
If you are interested only in generic declarative services or other pre-packaged declarative
middleware services such as pooling, you do not need to work directly with Spring AOP, and can
skip most of this chapter.
AOP concepts
Let us begin by defining some central AOP concepts and terminology. These terms are not Springspecific… unfortunately, AOP terminology is not particularly intuitive; however, it would be even more
confusing if Spring used its own terminology.
• Aspect: a modularization of a concern that cuts across multiple classes. Transaction management is
a good example of a crosscutting concern in enterprise Java applications. In Spring AOP, aspects
are implemented using regular classes (the schema-based approach) or regular classes annotated
with the @Aspect annotation (the @AspectJ style).
• Join point: a point during the execution of a program, such as the execution of a method or the handling
of an exception. In Spring AOP, a join point always represents a method execution.
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• Advice: action taken by an aspect at a particular join point. Different types of advice include "around,"
"before" and "after" advice. (Advice types are discussed below.) Many AOP frameworks, including
Spring, model an advice as an interceptor, maintaining a chain of interceptors around the join point.
• Pointcut: a predicate that matches join points. Advice is associated with a pointcut expression and
runs at any join point matched by the pointcut (for example, the execution of a method with a certain
name). The concept of join points as matched by pointcut expressions is central to AOP, and Spring
uses the AspectJ pointcut expression language by default.
• Introduction: declaring additional methods or fields on behalf of a type. Spring AOP allows you to
introduce new interfaces (and a corresponding implementation) to any advised object. For example,
you could use an introduction to make a bean implement an IsModified interface, to simplify
caching. (An introduction is known as an inter-type declaration in the AspectJ community.)
• Target object: object being advised by one or more aspects. Also referred to as the advised object.
Since Spring AOP is implemented using runtime proxies, this object will always be a proxied object.
• AOP proxy: an object created by the AOP framework in order to implement the aspect contracts
(advise method executions and so on). In the Spring Framework, an AOP proxy will be a JDK dynamic
proxy or a CGLIB proxy.
• Weaving: linking aspects with other application types or objects to create an advised object. This can
be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring
AOP, like other pure Java AOP frameworks, performs weaving at runtime.
Types of advice:
• Before advice: Advice that executes before a join point, but which does not have the ability to prevent
execution flow proceeding to the join point (unless it throws an exception).
• After returning advice: Advice to be executed after a join point completes normally: for example, if a
method returns without throwing an exception.
• After throwing advice: Advice to be executed if a method exits by throwing an exception.
• After (finally) advice: Advice to be executed regardless of the means by which a join point exits (normal
or exceptional return).
• Around advice: Advice that surrounds a join point such as a method invocation. This is the most
powerful kind of advice. Around advice can perform custom behavior before and after the method
invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the
advised method execution by returning its own return value or throwing an exception.
Around advice is the most general kind of advice. Since Spring AOP, like AspectJ, provides a full range
of advice types, we recommend that you use the least powerful advice type that can implement the
required behavior. For example, if you need only to update a cache with the return value of a method, you
are better off implementing an after returning advice than an around advice, although an around advice
can accomplish the same thing. Using the most specific advice type provides a simpler programming
model with less potential for errors. For example, you do not need to invoke the proceed() method on
the JoinPoint used for around advice, and hence cannot fail to invoke it.
In Spring 2.0, all advice parameters are statically typed, so that you work with advice parameters of
the appropriate type (the type of the return value from a method execution for example) rather than
Object arrays.
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The concept of join points, matched by pointcuts, is the key to AOP which distinguishes it from
older technologies offering only interception. Pointcuts enable advice to be targeted independently
of the Object-Oriented hierarchy. For example, an around advice providing declarative transaction
management can be applied to a set of methods spanning multiple objects (such as all business
operations in the service layer).
Spring AOP capabilities and goals
Spring AOP is implemented in pure Java. There is no need for a special compilation process. Spring AOP
does not need to control the class loader hierarchy, and is thus suitable for use in a Servlet container
or application server.
Spring AOP currently supports only method execution join points (advising the execution of methods
on Spring beans). Field interception is not implemented, although support for field interception could be
added without breaking the core Spring AOP APIs. If you need to advise field access and update join
points, consider a language such as AspectJ.
Spring AOP’s approach to AOP differs from that of most other AOP frameworks. The aim is not to provide
the most complete AOP implementation (although Spring AOP is quite capable); it is rather to provide
a close integration between AOP implementation and Spring IoC to help solve common problems in
enterprise applications.
Thus, for example, the Spring Framework’s AOP functionality is normally used in conjunction with the
Spring IoC container. Aspects are configured using normal bean definition syntax (although this allows
powerful "autoproxying" capabilities): this is a crucial difference from other AOP implementations. There
are some things you cannot do easily or efficiently with Spring AOP, such as advise very fine-grained
objects (such as domain objects typically): AspectJ is the best choice in such cases. However, our
experience is that Spring AOP provides an excellent solution to most problems in enterprise Java
applications that are amenable to AOP.
Spring AOP will never strive to compete with AspectJ to provide a comprehensive AOP solution. We
believe that both proxy-based frameworks like Spring AOP and full-blown frameworks such as AspectJ
are valuable, and that they are complementary, rather than in competition. Spring seamlessly integrates
Spring AOP and IoC with AspectJ, to enable all uses of AOP to be catered for within a consistent Springbased application architecture. This integration does not affect the Spring AOP API or the AOP Alliance
API: Spring AOP remains backward-compatible. See the following chapter for a discussion of the Spring
AOP APIs.
Note
One of the central tenets of the Spring Framework is that of non-invasiveness; this is the idea
that you should not be forced to introduce framework-specific classes and interfaces into your
business/domain model. However, in some places the Spring Framework does give you the option
to introduce Spring Framework-specific dependencies into your codebase: the rationale in giving
you such options is because in certain scenarios it might be just plain easier to read or code some
specific piece of functionality in such a way. The Spring Framework (almost) always offers you
the choice though: you have the freedom to make an informed decision as to which option best
suits your particular use case or scenario.
One such choice that is relevant to this chapter is that of which AOP framework (and which AOP
style) to choose. You have the choice of AspectJ and/or Spring AOP, and you also have the choice
of either the @AspectJ annotation-style approach or the Spring XML configuration-style approach.
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The fact that this chapter chooses to introduce the @AspectJ-style approach first should not be
taken as an indication that the Spring team favors the @AspectJ annotation-style approach over
the Spring XML configuration-style.
See Section 10.4, “Choosing which AOP declaration style to use” for a more complete discussion
of the whys and wherefores of each style.
AOP Proxies
Spring AOP defaults to using standard JDK dynamic proxies for AOP proxies. This enables any interface
(or set of interfaces) to be proxied.
Spring AOP can also use CGLIB proxies. This is necessary to proxy classes rather than interfaces.
CGLIB is used by default if a business object does not implement an interface. As it is good practice
to program to interfaces rather than classes; business classes normally will implement one or more
business interfaces. It is possible to force the use of CGLIB, in those (hopefully rare) cases where you
need to advise a method that is not declared on an interface, or where you need to pass a proxied object
to a method as a concrete type.
It is important to grasp the fact that Spring AOP is proxy-based. See the section called “Understanding
AOP proxies” for a thorough examination of exactly what this implementation detail actually means.
10.2 @AspectJ support
@AspectJ refers to a style of declaring aspects as regular Java classes annotated with annotations.
The @AspectJ style was introduced by the AspectJ project as part of the AspectJ 5 release. Spring
interprets the same annotations as AspectJ 5, using a library supplied by AspectJ for pointcut parsing
and matching. The AOP runtime is still pure Spring AOP though, and there is no dependency on the
AspectJ compiler or weaver.
Note
Using the AspectJ compiler and weaver enables use of the full AspectJ language, and is discussed
in Section 10.8, “Using AspectJ with Spring applications”.
Enabling @AspectJ Support
To use @AspectJ aspects in a Spring configuration you need to enable Spring support for configuring
Spring AOP based on @AspectJ aspects, and autoproxying beans based on whether or not they are
advised by those aspects. By autoproxying we mean that if Spring determines that a bean is advised by
one or more aspects, it will automatically generate a proxy for that bean to intercept method invocations
and ensure that advice is executed as needed.
The @AspectJ support can be enabled with XML or Java style configuration. In either case you will
also need to ensure that AspectJ’s aspectjweaver.jar library is on the classpath of your application
(version 1.6.8 or later). This library is available in the 'lib' directory of an AspectJ distribution or via
the Maven Central repository.
Enabling @AspectJ Support with Java configuration
To enable @AspectJ support with Java @Configuration add the @EnableAspectJAutoProxy
annotation:
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@Configuration
@EnableAspectJAutoProxy
public class AppConfig {
}
Enabling @AspectJ Support with XML configuration
To enable @AspectJ support with XML based configuration use the aop:aspectj-autoproxy
element:
<aop:aspectj-autoproxy/>
This assumes that you are using schema support as described in Chapter 40, XML Schema-based
configuration. See the section called “the aop schema” for how to import the tags in the aop namespace.
Declaring an aspect
With the @AspectJ support enabled, any bean defined in your application context with a class that is
an @AspectJ aspect (has the @Aspect annotation) will be automatically detected by Spring and used
to configure Spring AOP. The following example shows the minimal definition required for a not-veryuseful aspect:
A regular bean definition in the application context, pointing to a bean class that has the @Aspect
annotation:
<bean id="myAspect" class="org.xyz.NotVeryUsefulAspect">
<!-- configure properties of aspect here as normal -->
</bean>
And
the
NotVeryUsefulAspect
class
org.aspectj.lang.annotation.Aspect annotation;
definition,
annotated
with
package org.xyz;
import org.aspectj.lang.annotation.Aspect;
@Aspect
public class NotVeryUsefulAspect {
}
Aspects (classes annotated with @Aspect) may have methods and fields just like any other class. They
may also contain pointcut, advice, and introduction (inter-type) declarations.
Autodetecting aspects through component scanning
You may register aspect classes as regular beans in your Spring XML configuration, or autodetect
them through classpath scanning - just like any other Spring-managed bean. However, note that
the @Aspect annotation is not sufficient for autodetection in the classpath: For that purpose, you
need to add a separate @Component annotation (or alternatively a custom stereotype annotation
that qualifies, as per the rules of Spring’s component scanner).
Advising aspects with other aspects?
In Spring AOP, it is not possible to have aspects themselves be the target of advice from other
aspects. The @Aspect annotation on a class marks it as an aspect, and hence excludes it from
auto-proxying.
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Declaring a pointcut
Recall that pointcuts determine join points of interest, and thus enable us to control when advice
executes. Spring AOP only supports method execution join points for Spring beans, so you can think of
a pointcut as matching the execution of methods on Spring beans. A pointcut declaration has two parts:
a signature comprising a name and any parameters, and a pointcut expression that determines exactly
which method executions we are interested in. In the @AspectJ annotation-style of AOP, a pointcut
signature is provided by a regular method definition, and the pointcut expression is indicated using the
@Pointcut annotation (the method serving as the pointcut signature must have a void return type).
An example will help make this distinction between a pointcut signature and a pointcut expression clear.
The following example defines a pointcut named 'anyOldTransfer' that will match the execution of
any method named 'transfer':
@Pointcut("execution(* transfer(..))")// the pointcut expression
private void anyOldTransfer() {}// the pointcut signature
The pointcut expression that forms the value of the @Pointcut annotation is a regular AspectJ 5
pointcut expression. For a full discussion of AspectJ’s pointcut language, see the AspectJ Programming
Guide (and for extensions, the AspectJ 5 Developers Notebook) or one of the books on AspectJ such
as "Eclipse AspectJ" by Colyer et. al. or "AspectJ in Action" by Ramnivas Laddad.
Supported Pointcut Designators
Spring AOP supports the following AspectJ pointcut designators (PCD) for use in pointcut expressions:
Other pointcut types
The full AspectJ pointcut language supports additional pointcut designators that are not supported
in Spring. These are: call, get, set, preinitialization, staticinitialization,
initialization, handler, adviceexecution, withincode, cflow, cflowbelow,
if, @this, and @withincode. Use of these pointcut designators in pointcut expressions
interpreted by Spring AOP will result in an IllegalArgumentException being thrown.
The set of pointcut designators supported by Spring AOP may be extended in future releases to
support more of the AspectJ pointcut designators.
• execution - for matching method execution join points, this is the primary pointcut designator you will
use when working with Spring AOP
• within - limits matching to join points within certain types (simply the execution of a method declared
within a matching type when using Spring AOP)
• this - limits matching to join points (the execution of methods when using Spring AOP) where the bean
reference (Spring AOP proxy) is an instance of the given type
• target - limits matching to join points (the execution of methods when using Spring AOP) where the
target object (application object being proxied) is an instance of the given type
• args - limits matching to join points (the execution of methods when using Spring AOP) where the
arguments are instances of the given types
• @target - limits matching to join points (the execution of methods when using Spring AOP) where the
class of the executing object has an annotation of the given type
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• @args - limits matching to join points (the execution of methods when using Spring AOP) where the
runtime type of the actual arguments passed have annotations of the given type(s)
• @within - limits matching to join points within types that have the given annotation (the execution of
methods declared in types with the given annotation when using Spring AOP)
• @annotation - limits matching to join points where the subject of the join point (method being executed
in Spring AOP) has the given annotation
Because Spring AOP limits matching to only method execution join points, the discussion of the pointcut
designators above gives a narrower definition than you will find in the AspectJ programming guide. In
addition, AspectJ itself has type-based semantics and at an execution join point both 'this' and 'target'
refer to the same object - the object executing the method. Spring AOP is a proxy-based system and
differentiates between the proxy object itself (bound to 'this') and the target object behind the proxy
(bound to 'target').
Note
Due to the proxy-based nature of Spring’s AOP framework, protected methods are by definition
not intercepted, neither for JDK proxies (where this isn’t applicable) nor for CGLIB proxies (where
this is technically possible but not recommendable for AOP purposes). As a consequence, any
given pointcut will be matched against public methods only!
If your interception needs include protected/private methods or even constructors, consider the
use of Spring-driven native AspectJ weaving instead of Spring’s proxy-based AOP framework.
This constitutes a different mode of AOP usage with different characteristics, so be sure to make
yourself familiar with weaving first before making a decision.
Spring AOP also supports an additional PCD named 'bean'. This PCD allows you to limit the matching of
join points to a particular named Spring bean, or to a set of named Spring beans (when using wildcards).
The 'bean' PCD has the following form:
bean(idOrNameOfBean)
The 'idOrNameOfBean' token can be the name of any Spring bean: limited wildcard support using the '*'
character is provided, so if you establish some naming conventions for your Spring beans you can quite
easily write a 'bean' PCD expression to pick them out. As is the case with other pointcut designators,
the 'bean' PCD can be &&'ed, ||'ed, and ! (negated) too.
Note
Please note that the 'bean' PCD is only supported in Spring AOP - and not in native AspectJ
weaving. It is a Spring-specific extension to the standard PCDs that AspectJ defines and therefore
not available for aspects declared in the @Aspect model.
The 'bean' PCD operates at the instance level (building on the Spring bean name concept) rather
than at the type level only (which is what weaving-based AOP is limited to). Instance-based
pointcut designators are a special capability of Spring’s proxy-based AOP framework and its close
integration with the Spring bean factory, where it is natural and straightforward to identify specific
beans by name.
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Combining pointcut expressions
Pointcut expressions can be combined using '&&', '||' and '!'. It is also possible to refer
to pointcut expressions by name. The following example shows three pointcut expressions:
anyPublicOperation (which matches if a method execution join point represents the execution of
any public method); inTrading (which matches if a method execution is in the trading module), and
tradingOperation (which matches if a method execution represents any public method in the trading
module).
@Pointcut("execution(public * *(..))")
private void anyPublicOperation() {}
@Pointcut("within(com.xyz.someapp.trading..*)")
private void inTrading() {}
@Pointcut("anyPublicOperation() && inTrading()")
private void tradingOperation() {}
It is a best practice to build more complex pointcut expressions out of smaller named components as
shown above. When referring to pointcuts by name, normal Java visibility rules apply (you can see
private pointcuts in the same type, protected pointcuts in the hierarchy, public pointcuts anywhere and
so on). Visibility does not affect pointcut matching.
Sharing common pointcut definitions
When working with enterprise applications, you often want to refer to modules of the application
and particular sets of operations from within several aspects. We recommend defining a
"SystemArchitecture" aspect that captures common pointcut expressions for this purpose. A typical such
aspect would look as follows:
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package com.xyz.someapp;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
@Aspect
public class SystemArchitecture {
/**
* A join point is in the web layer if the method is defined
* in a type in the com.xyz.someapp.web package or any sub-package
* under that.
*/
@Pointcut("within(com.xyz.someapp.web..*)")
public void inWebLayer() {}
/**
* A join point is in the service layer if the method is defined
* in a type in the com.xyz.someapp.service package or any sub-package
* under that.
*/
@Pointcut("within(com.xyz.someapp.service..*)")
public void inServiceLayer() {}
/**
* A join point is in the data access layer if the method is defined
* in a type in the com.xyz.someapp.dao package or any sub-package
* under that.
*/
@Pointcut("within(com.xyz.someapp.dao..*)")
public void inDataAccessLayer() {}
/**
* A business service is the execution of any method defined on a service
* interface. This definition assumes that interfaces are placed in the
* "service" package, and that implementation types are in sub-packages.
*
* If you group service interfaces by functional area (for example,
* in packages com.xyz.someapp.abc.service and com.xyz.someapp.def.service) then
* the pointcut expression "execution(* com.xyz.someapp..service.*.*(..))"
* could be used instead.
*
* Alternatively, you can write the expression using the 'bean'
* PCD, like so "bean(*Service)". (This assumes that you have
* named your Spring service beans in a consistent fashion.)
*/
@Pointcut("execution(* com.xyz.someapp..service.*.*(..))")
public void businessService() {}
/**
* A data access operation is the execution of any method defined on a
* dao interface. This definition assumes that interfaces are placed in the
* "dao" package, and that implementation types are in sub-packages.
*/
@Pointcut("execution(* com.xyz.someapp.dao.*.*(..))")
public void dataAccessOperation() {}
}
The pointcuts defined in such an aspect can be referred to anywhere that you need a pointcut
expression. For example, to make the service layer transactional, you could write:
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<aop:config>
<aop:advisor
pointcut="com.xyz.someapp.SystemArchitecture.businessService()"
advice-ref="tx-advice"/>
</aop:config>
<tx:advice id="tx-advice">
<tx:attributes>
<tx:method name="*" propagation="REQUIRED"/>
</tx:attributes>
</tx:advice>
The <aop:config> and <aop:advisor> elements are discussed in Section 10.3, “Schema-based
AOP support”. The transaction elements are discussed in Chapter 16, Transaction Management.
Examples
Spring AOP users are likely to use the execution pointcut designator the most often. The format of
an execution expression is:
execution(modifiers-pattern? ret-type-pattern declaring-type-pattern?name-pattern(param-pattern)
throws-pattern?)
All parts except the returning type pattern (ret-type-pattern in the snippet above), name pattern, and
parameters pattern are optional. The returning type pattern determines what the return type of the
method must be in order for a join point to be matched. Most frequently you will use * as the returning
type pattern, which matches any return type. A fully-qualified type name will match only when the method
returns the given type. The name pattern matches the method name. You can use the * wildcard as all
or part of a name pattern. If specifying a declaring type pattern then include a trailing . to join it to the
name pattern component. The parameters pattern is slightly more complex: () matches a method that
takes no parameters, whereas (..) matches any number of parameters (zero or more). The pattern
(*) matches a method taking one parameter of any type, (*,String) matches a method taking two
parameters, the first can be of any type, the second must be a String. Consult the Language Semantics
section of the AspectJ Programming Guide for more information.
Some examples of common pointcut expressions are given below.
• the execution of any public method:
execution(public * *(..))
• the execution of any method with a name beginning with "set":
execution(* set*(..))
• the execution of any method defined by the AccountService interface:
execution(* com.xyz.service.AccountService.*(..))
• the execution of any method defined in the service package:
execution(* com.xyz.service.*.*(..))
• the execution of any method defined in the service package or a sub-package:
execution(* com.xyz.service..*.*(..))
• any join point (method execution only in Spring AOP) within the service package:
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within(com.xyz.service.*)
• any join point (method execution only in Spring AOP) within the service package or a sub-package:
within(com.xyz.service..*)
• any join point (method execution only in Spring AOP) where the proxy implements the
AccountService interface:
this(com.xyz.service.AccountService)
Note
'this' is more commonly used in a binding form :- see the following section on advice for how to
make the proxy object available in the advice body.
• any join point (method execution only in Spring AOP) where the target object implements the
AccountService interface:
target(com.xyz.service.AccountService)
Note
'target' is more commonly used in a binding form :- see the following section on advice for how
to make the target object available in the advice body.
• any join point (method execution only in Spring AOP) which takes a single parameter, and where the
argument passed at runtime is Serializable:
args(java.io.Serializable)
Note
'args' is more commonly used in a binding form :- see the following section on advice for how to
make the method arguments available in the advice body.
Note that the pointcut given in this example is different to execution(*
*(java.io.Serializable)): the args version matches if the argument passed at runtime is
Serializable, the execution version matches if the method signature declares a single parameter of type
Serializable.
• any join point (method execution only in Spring AOP) where the target object has an
@Transactional annotation:
@target(org.springframework.transaction.annotation.Transactional)
Note
'@target' can also be used in a binding form :- see the following section on advice for how to make
the annotation object available in the advice body.
• any join point (method execution only in Spring AOP) where the declared type of the target object
has an @Transactional annotation:
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@within(org.springframework.transaction.annotation.Transactional)
Note
'@within' can also be used in a binding form :- see the following section on advice for how to make
the annotation object available in the advice body.
• any join point (method execution only in Spring AOP) where the executing method has an
@Transactional annotation:
@annotation(org.springframework.transaction.annotation.Transactional)
Note
'@annotation' can also be used in a binding form :- see the following section on advice for how
to make the annotation object available in the advice body.
• any join point (method execution only in Spring AOP) which takes a single parameter, and where the
runtime type of the argument passed has the @Classified annotation:
@args(com.xyz.security.Classified)
Note
'@args' can also be used in a binding form :- see the following section on advice for how to make
the annotation object(s) available in the advice body.
• any join point (method execution only in Spring AOP) on a Spring bean named 'tradeService':
bean(tradeService)
• any join point (method execution only in Spring AOP) on Spring beans having names that match the
wildcard expression '*Service':
bean(*Service)
Writing good pointcuts
During compilation, AspectJ processes pointcuts in order to try and optimize matching performance.
Examining code and determining if each join point matches (statically or dynamically) a given pointcut
is a costly process. (A dynamic match means the match cannot be fully determined from static analysis
and a test will be placed in the code to determine if there is an actual match when the code is running).
On first encountering a pointcut declaration, AspectJ will rewrite it into an optimal form for the matching
process. What does this mean? Basically pointcuts are rewritten in DNF (Disjunctive Normal Form) and
the components of the pointcut are sorted such that those components that are cheaper to evaluate are
checked first. This means you do not have to worry about understanding the performance of various
pointcut designators and may supply them in any order in a pointcut declaration.
However, AspectJ can only work with what it is told, and for optimal performance of matching you
should think about what they are trying to achieve and narrow the search space for matches as much
as possible in the definition. The existing designators naturally fall into one of three groups: kinded,
scoping and context:
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• Kinded designators are those which select a particular kind of join point. For example: execution, get,
set, call, handler
• Scoping designators are those which select a group of join points of interest (of probably many kinds).
For example: within, withincode
• Contextual designators are those that match (and optionally bind) based on context. For example:
this, target, @annotation
A well written pointcut should try and include at least the first two types (kinded and scoping), whilst
the contextual designators may be included if wishing to match based on join point context, or bind that
context for use in the advice. Supplying either just a kinded designator or just a contextual designator will
work but could affect weaving performance (time and memory used) due to all the extra processing and
analysis. Scoping designators are very fast to match and their usage means AspectJ can very quickly
dismiss groups of join points that should not be further processed - that is why a good pointcut should
always include one if possible.
Declaring advice
Advice is associated with a pointcut expression, and runs before, after, or around method executions
matched by the pointcut. The pointcut expression may be either a simple reference to a named pointcut,
or a pointcut expression declared in place.
Before advice
Before advice is declared in an aspect using the @Before annotation:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
@Aspect
public class BeforeExample {
@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
public void doAccessCheck() {
// ...
}
}
If using an in-place pointcut expression we could rewrite the above example as:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
@Aspect
public class BeforeExample {
@Before("execution(* com.xyz.myapp.dao.*.*(..))")
public void doAccessCheck() {
// ...
}
}
After returning advice
After returning advice runs when a matched method execution returns normally. It is declared using the
@AfterReturning annotation:
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import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;
@Aspect
public class AfterReturningExample {
@AfterReturning("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
public void doAccessCheck() {
// ...
}
}
Note
Note: it is of course possible to have multiple advice declarations, and other members as well,
all inside the same aspect. We’re just showing a single advice declaration in these examples to
focus on the issue under discussion at the time.
Sometimes you need access in the advice body to the actual value that was returned. You can use the
form of @AfterReturning that binds the return value for this:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;
@Aspect
public class AfterReturningExample {
@AfterReturning(
pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
returning="retVal")
public void doAccessCheck(Object retVal) {
// ...
}
}
The name used in the returning attribute must correspond to the name of a parameter in the advice
method. When a method execution returns, the return value will be passed to the advice method as
the corresponding argument value. A returning clause also restricts matching to only those method
executions that return a value of the specified type ( Object in this case, which will match any return
value).
Please note that it is not possible to return a totally different reference when using after-returning advice.
After throwing advice
After throwing advice runs when a matched method execution exits by throwing an exception. It is
declared using the @AfterThrowing annotation:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;
@Aspect
public class AfterThrowingExample {
@AfterThrowing("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
public void doRecoveryActions() {
// ...
}
}
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Often you want the advice to run only when exceptions of a given type are thrown, and you also often
need access to the thrown exception in the advice body. Use the throwing attribute to both restrict
matching (if desired, use Throwable as the exception type otherwise) and bind the thrown exception
to an advice parameter.
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;
@Aspect
public class AfterThrowingExample {
@AfterThrowing(
pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
throwing="ex")
public void doRecoveryActions(DataAccessException ex) {
// ...
}
}
The name used in the throwing attribute must correspond to the name of a parameter in the advice
method. When a method execution exits by throwing an exception, the exception will be passed to the
advice method as the corresponding argument value. A throwing clause also restricts matching to
only those method executions that throw an exception of the specified type ( DataAccessException
in this case).
After (finally) advice
After (finally) advice runs however a matched method execution exits. It is declared using the @After
annotation. After advice must be prepared to handle both normal and exception return conditions. It is
typically used for releasing resources, etc.
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.After;
@Aspect
public class AfterFinallyExample {
@After("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
public void doReleaseLock() {
// ...
}
}
Around advice
The final kind of advice is around advice. Around advice runs "around" a matched method execution.
It has the opportunity to do work both before and after the method executes, and to determine when,
how, and even if, the method actually gets to execute at all. Around advice is often used if you need to
share state before and after a method execution in a thread-safe manner (starting and stopping a timer
for example). Always use the least powerful form of advice that meets your requirements (i.e. don’t use
around advice if simple before advice would do).
Around advice is declared using the @Around annotation. The first parameter of the advice method
must be of type ProceedingJoinPoint. Within the body of the advice, calling proceed() on the
ProceedingJoinPoint causes the underlying method to execute. The proceed method may also
be called passing in an Object[] - the values in the array will be used as the arguments to the method
execution when it proceeds.
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Note
The behavior of proceed when called with an Object[] is a little different than the behavior of
proceed for around advice compiled by the AspectJ compiler. For around advice written using
the traditional AspectJ language, the number of arguments passed to proceed must match the
number of arguments passed to the around advice (not the number of arguments taken by the
underlying join point), and the value passed to proceed in a given argument position supplants
the original value at the join point for the entity the value was bound to (Don’t worry if this doesn’t
make sense right now!). The approach taken by Spring is simpler and a better match to its proxybased, execution only semantics. You only need to be aware of this difference if you are compiling
@AspectJ aspects written for Spring and using proceed with arguments with the AspectJ compiler
and weaver. There is a way to write such aspects that is 100% compatible across both Spring
AOP and AspectJ, and this is discussed in the following section on advice parameters.
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.ProceedingJoinPoint;
@Aspect
public class AroundExample {
@Around("com.xyz.myapp.SystemArchitecture.businessService()")
public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
// start stopwatch
Object retVal = pjp.proceed();
// stop stopwatch
return retVal;
}
}
The value returned by the around advice will be the return value seen by the caller of the method. A
simple caching aspect for example could return a value from a cache if it has one, and invoke proceed()
if it does not. Note that proceed may be invoked once, many times, or not at all within the body of the
around advice, all of these are quite legal.
Advice parameters
Spring offers fully typed advice - meaning that you declare the parameters you need in the advice
signature (as we saw for the returning and throwing examples above) rather than work with Object[]
arrays all the time. We’ll see how to make argument and other contextual values available to the advice
body in a moment. First let’s take a look at how to write generic advice that can find out about the method
the advice is currently advising.
Access to the current JoinPoint
Any advice method may declare as its first parameter, a parameter of type
org.aspectj.lang.JoinPoint (please note that around advice is required to declare a first
parameter of type ProceedingJoinPoint, which is a subclass of JoinPoint. The JoinPoint
interface provides a number of useful methods such as getArgs() (returns the method arguments),
getThis() (returns the proxy object), getTarget() (returns the target object), getSignature()
(returns a description of the method that is being advised) and toString() (prints a useful description
of the method being advised). Please do consult the javadocs for full details.
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Passing parameters to advice
We’ve already seen how to bind the returned value or exception value (using after returning and after
throwing advice). To make argument values available to the advice body, you can use the binding form
of args. If a parameter name is used in place of a type name in an args expression, then the value
of the corresponding argument will be passed as the parameter value when the advice is invoked. An
example should make this clearer. Suppose you want to advise the execution of dao operations that
take an Account object as the first parameter, and you need access to the account in the advice body.
You could write the following:
@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
public void validateAccount(Account account) {
// ...
}
The args(account,..) part of the pointcut expression serves two purposes: firstly, it restricts
matching to only those method executions where the method takes at least one parameter, and the
argument passed to that parameter is an instance of Account; secondly, it makes the actual Account
object available to the advice via the account parameter.
Another way of writing this is to declare a pointcut that "provides" the Account object value when it
matches a join point, and then just refer to the named pointcut from the advice. This would look as
follows:
@Pointcut("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
private void accountDataAccessOperation(Account account) {}
@Before("accountDataAccessOperation(account)")
public void validateAccount(Account account) {
// ...
}
The interested reader is once more referred to the AspectJ programming guide for more details.
The proxy object ( this), target object ( target), and annotations ( @within, @target,
@annotation, @args) can all be bound in a similar fashion. The following example shows how you
could match the execution of methods annotated with an @Auditable annotation, and extract the audit
code.
First the definition of the @Auditable annotation:
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Auditable {
AuditCode value();
}
And then the advice that matches the execution of @Auditable methods:
@Before("com.xyz.lib.Pointcuts.anyPublicMethod() && @annotation(auditable)")
public void audit(Auditable auditable) {
AuditCode code = auditable.value();
// ...
}
Advice parameters and generics
Spring AOP can handle generics used in class declarations and method parameters. Suppose you have
a generic type like this:
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public interface Sample<T> {
void sampleGenericMethod(T param);
void sampleGenericCollectionMethod(Collection<T> param);
}
You can restrict interception of method types to certain parameter types by simply typing the advice
parameter to the parameter type you want to intercept the method for:
@Before("execution(* ..Sample+.sampleGenericMethod(*)) && args(param)")
public void beforeSampleMethod(MyType param) {
// Advice implementation
}
That this works is pretty obvious as we already discussed above. However, it’s worth pointing out that
this won’t work for generic collections. So you cannot define a pointcut like this:
@Before("execution(* ..Sample+.sampleGenericCollectionMethod(*)) && args(param)")
public void beforeSampleMethod(Collection<MyType> param) {
// Advice implementation
}
To make this work we would have to inspect every element of the collection, which is not reasonable
as we also cannot decide how to treat null values in general. To achieve something similar to this you
have to type the parameter to Collection<?> and manually check the type of the elements.
Determining argument names
The parameter binding in advice invocations relies on matching names used in pointcut expressions
to declared parameter names in (advice and pointcut) method signatures. Parameter names are not
available through Java reflection, so Spring AOP uses the following strategies to determine parameter
names:
• If the parameter names have been specified by the user explicitly, then the specified parameter names
are used: both the advice and the pointcut annotations have an optional "argNames" attribute which
can be used to specify the argument names of the annotated method - these argument names are
available at runtime. For example:
@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)",
argNames="bean,auditable")
public void audit(Object bean, Auditable auditable) {
AuditCode code = auditable.value();
// ... use code and bean
}
If the first parameter is of the JoinPoint, ProceedingJoinPoint, or JoinPoint.StaticPart
type, you may leave out the name of the parameter from the value of the "argNames" attribute. For
example, if you modify the preceding advice to receive the join point object, the "argNames" attribute
need not include it:
@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)",
argNames="bean,auditable")
public void audit(JoinPoint jp, Object bean, Auditable auditable) {
AuditCode code = auditable.value();
// ... use code, bean, and jp
}
The special treatment given to the first parameter of the JoinPoint, ProceedingJoinPoint, and
JoinPoint.StaticPart types is particularly convenient for advice that do not collect any other join
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point context. In such situations, you may simply omit the "argNames" attribute. For example, the
following advice need not declare the "argNames" attribute:
@Before("com.xyz.lib.Pointcuts.anyPublicMethod()")
public void audit(JoinPoint jp) {
// ... use jp
}
• Using the 'argNames' attribute is a little clumsy, so if the 'argNames' attribute has not been
specified, then Spring AOP will look at the debug information for the class and try to determine the
parameter names from the local variable table. This information will be present as long as the classes
have been compiled with debug information ( '-g:vars' at a minimum). The consequences of
compiling with this flag on are: (1) your code will be slightly easier to understand (reverse engineer), (2)
the class file sizes will be very slightly bigger (typically inconsequential), (3) the optimization to remove
unused local variables will not be applied by your compiler. In other words, you should encounter no
difficulties building with this flag on.
Note
If an @AspectJ aspect has been compiled by the AspectJ compiler (ajc) even without the debug
information then there is no need to add the argNames attribute as the compiler will retain the
needed information.
• If the code has been compiled without the necessary debug information, then Spring AOP will
attempt to deduce the pairing of binding variables to parameters (for example, if only one variable
is bound in the pointcut expression, and the advice method only takes one parameter, the pairing
is obvious!). If the binding of variables is ambiguous given the available information, then an
AmbiguousBindingException will be thrown.
• If all of the above strategies fail then an IllegalArgumentException will be thrown.
Proceeding with arguments
We remarked earlier that we would describe how to write a proceed call with arguments that works
consistently across Spring AOP and AspectJ. The solution is simply to ensure that the advice signature
binds each of the method parameters in order. For example:
@Around("execution(List<Account> find*(..)) && " +
"com.xyz.myapp.SystemArchitecture.inDataAccessLayer() && " +
"args(accountHolderNamePattern)")
public Object preProcessQueryPattern(ProceedingJoinPoint pjp,
String accountHolderNamePattern) throws Throwable {
String newPattern = preProcess(accountHolderNamePattern);
return pjp.proceed(new Object[] {newPattern});
}
In many cases you will be doing this binding anyway (as in the example above).
Advice ordering
What happens when multiple pieces of advice all want to run at the same join point? Spring AOP
follows the same precedence rules as AspectJ to determine the order of advice execution. The highest
precedence advice runs first "on the way in" (so given two pieces of before advice, the one with highest
precedence runs first). "On the way out" from a join point, the highest precedence advice runs last (so
given two pieces of after advice, the one with the highest precedence will run second).
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When two pieces of advice defined in different aspects both need to run at the same join point,
unless you specify otherwise the order of execution is undefined. You can control the order of
execution by specifying precedence. This is done in the normal Spring way by either implementing the
org.springframework.core.Ordered interface in the aspect class or annotating it with the Order
annotation. Given two aspects, the aspect returning the lower value from Ordered.getValue() (or
the annotation value) has the higher precedence.
When two pieces of advice defined in the same aspect both need to run at the same join point, the
ordering is undefined (since there is no way to retrieve the declaration order via reflection for javaccompiled classes). Consider collapsing such advice methods into one advice method per join point in
each aspect class, or refactor the pieces of advice into separate aspect classes - which can be ordered
at the aspect level.
Introductions
Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare that advised
objects implement a given interface, and to provide an implementation of that interface on behalf of
those objects.
An introduction is made using the @DeclareParents annotation. This annotation is used to
declare that matching types have a new parent (hence the name). For example, given an interface
UsageTracked, and an implementation of that interface DefaultUsageTracked, the following
aspect declares that all implementors of service interfaces also implement the UsageTracked interface.
(In order to expose statistics via JMX for example.)
@Aspect
public class UsageTracking {
@DeclareParents(value="com.xzy.myapp.service.*+", defaultImpl=DefaultUsageTracked.class)
public static UsageTracked mixin;
@Before("com.xyz.myapp.SystemArchitecture.businessService() && this(usageTracked)")
public void recordUsage(UsageTracked usageTracked) {
usageTracked.incrementUseCount();
}
}
The interface to be implemented is determined by the type of the annotated field. The value attribute
of the @DeclareParents annotation is an AspectJ type pattern :- any bean of a matching type will
implement the UsageTracked interface. Note that in the before advice of the above example, service
beans can be directly used as implementations of the UsageTracked interface. If accessing a bean
programmatically you would write the following:
UsageTracked usageTracked = (UsageTracked) context.getBean("myService");
Aspect instantiation models
Note
(This is an advanced topic, so if you are just starting out with AOP you can safely skip it until later.)
By default there will be a single instance of each aspect within the application context. AspectJ calls
this the singleton instantiation model. It is possible to define aspects with alternate lifecycles :- Spring
supports AspectJ’s perthis and pertarget instantiation models ( percflow, percflowbelow,
and pertypewithin are not currently supported).
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A "perthis" aspect is declared by specifying a perthis clause in the @Aspect annotation. Let’s look
at an example, and then we’ll explain how it works.
@Aspect("perthis(com.xyz.myapp.SystemArchitecture.businessService())")
public class MyAspect {
private int someState;
@Before(com.xyz.myapp.SystemArchitecture.businessService())
public void recordServiceUsage() {
// ...
}
}
The effect of the 'perthis' clause is that one aspect instance will be created for each unique service
object executing a business service (each unique object bound to 'this' at join points matched by the
pointcut expression). The aspect instance is created the first time that a method is invoked on the service
object. The aspect goes out of scope when the service object goes out of scope. Before the aspect
instance is created, none of the advice within it executes. As soon as the aspect instance has been
created, the advice declared within it will execute at matched join points, but only when the service object
is the one this aspect is associated with. See the AspectJ programming guide for more information on
per-clauses.
The 'pertarget' instantiation model works in exactly the same way as perthis, but creates one aspect
instance for each unique target object at matched join points.
Example
Now that you have seen how all the constituent parts work, let’s put them together to do something
useful!
The execution of business services can sometimes fail due to concurrency issues (for example, deadlock
loser). If the operation is retried, it is quite likely to succeed next time round. For business services
where it is appropriate to retry in such conditions (idempotent operations that don’t need to go back to
the user for conflict resolution), we’d like to transparently retry the operation to avoid the client seeing
a PessimisticLockingFailureException. This is a requirement that clearly cuts across multiple
services in the service layer, and hence is ideal for implementing via an aspect.
Because we want to retry the operation, we will need to use around advice so that we can call proceed
multiple times. Here’s how the basic aspect implementation looks:
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@Aspect
public class ConcurrentOperationExecutor implements Ordered {
private static final int DEFAULT_MAX_RETRIES = 2;
private int maxRetries = DEFAULT_MAX_RETRIES;
private int order = 1;
public void setMaxRetries(int maxRetries) {
this.maxRetries = maxRetries;
}
public int getOrder() {
return this.order;
}
public void setOrder(int order) {
this.order = order;
}
@Around("com.xyz.myapp.SystemArchitecture.businessService()")
public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
int numAttempts = 0;
PessimisticLockingFailureException lockFailureException;
do {
numAttempts++;
try {
return pjp.proceed();
}
catch(PessimisticLockingFailureException ex) {
lockFailureException = ex;
}
} while(numAttempts <= this.maxRetries);
throw lockFailureException;
}
}
Note that the aspect implements the Ordered interface so we can set the precedence of the
aspect higher than the transaction advice (we want a fresh transaction each time we retry).
The maxRetries and order properties will both be configured by Spring. The main action
happens in the doConcurrentOperation around advice. Notice that for the moment we’re
applying the retry logic to all businessService()s. We try to proceed, and if we fail with an
PessimisticLockingFailureException we simply try again unless we have exhausted all of our
retry attempts.
The corresponding Spring configuration is:
<aop:aspectj-autoproxy/>
<bean id="concurrentOperationExecutor" class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor">
<property name="maxRetries" value="3"/>
<property name="order" value="100"/>
</bean>
To refine the aspect so that it only retries idempotent operations, we might define an Idempotent
annotation:
@Retention(RetentionPolicy.RUNTIME)
public @interface Idempotent {
// marker annotation
}
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and use the annotation to annotate the implementation of service operations. The change to the
aspect to only retry idempotent operations simply involves refining the pointcut expression so that only
@Idempotent operations match:
@Around("com.xyz.myapp.SystemArchitecture.businessService() && " +
"@annotation(com.xyz.myapp.service.Idempotent)")
public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
...
}
10.3 Schema-based AOP support
If you prefer an XML-based format, then Spring also offers support for defining aspects using the new
"aop" namespace tags. The exact same pointcut expressions and advice kinds are supported as when
using the @AspectJ style, hence in this section we will focus on the new syntax and refer the reader
to the discussion in the previous section (Section 10.2, “@AspectJ support”) for an understanding of
writing pointcut expressions and the binding of advice parameters.
To use the aop namespace tags described in this section, you need to import the spring-aop schema
as described in Chapter 40, XML Schema-based configuration. See the section called “the aop schema”
for how to import the tags in the aop namespace.
Within your Spring configurations, all aspect and advisor elements must be placed within an
<aop:config> element (you can have more than one <aop:config> element in an application
context configuration). An <aop:config> element can contain pointcut, advisor, and aspect elements
(note these must be declared in that order).
Warning
The <aop:config> style of configuration makes heavy use of Spring’s auto-proxying
mechanism. This can cause issues (such as advice not being woven) if you are already
using explicit auto-proxying via the use of BeanNameAutoProxyCreator or suchlike. The
recommended usage pattern is to use either just the <aop:config> style, or just the
AutoProxyCreator style.
Declaring an aspect
Using the schema support, an aspect is simply a regular Java object defined as a bean in your Spring
application context. The state and behavior is captured in the fields and methods of the object, and the
pointcut and advice information is captured in the XML.
An aspect is declared using the <aop:aspect> element, and the backing bean is referenced using the
ref attribute:
<aop:config>
<aop:aspect id="myAspect" ref="aBean">
...
</aop:aspect>
</aop:config>
<bean id="aBean" class="...">
...
</bean>
The bean backing the aspect (" `aBean`" in this case) can of course be configured and dependency
injected just like any other Spring bean.
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Declaring a pointcut
A named pointcut can be declared inside an <aop:config> element, enabling the pointcut definition to
be shared across several aspects and advisors.
A pointcut representing the execution of any business service in the service layer could be defined as
follows:
<aop:config>
<aop:pointcut id="businessService"
expression="execution(* com.xyz.myapp.service.*.*(..))"/>
</aop:config>
Note that the pointcut expression itself is using the same AspectJ pointcut expression language as
described in Section 10.2, “@AspectJ support”. If you are using the schema based declaration style,
you can refer to named pointcuts defined in types (@Aspects) within the pointcut expression. Another
way of defining the above pointcut would be:
<aop:config>
<aop:pointcut id="businessService"
expression="com.xyz.myapp.SystemArchitecture.businessService()"/>
</aop:config>
Assuming you have a SystemArchitecture aspect as described in the section called “Sharing
common pointcut definitions”.
Declaring a pointcut inside an aspect is very similar to declaring a top-level pointcut:
<aop:config>
<aop:aspect id="myAspect" ref="aBean">
<aop:pointcut id="businessService"
expression="execution(* com.xyz.myapp.service.*.*(..))"/>
...
</aop:aspect>
</aop:config>
Much the same way in an @AspectJ aspect, pointcuts declared using the schema based definition style
may collect join point context. For example, the following pointcut collects the 'this' object as the join
point context and passes it to advice:
<aop:config>
<aop:aspect id="myAspect" ref="aBean">
<aop:pointcut id="businessService"
expression="execution(* com.xyz.myapp.service.*.*(..)) &amp;&amp; this(service)"/>
<aop:before pointcut-ref="businessService" method="monitor"/>
...
</aop:aspect>
</aop:config>
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The advice must be declared to receive the collected join point context by including parameters of the
matching names:
public void monitor(Object service) {
...
}
When combining pointcut sub-expressions, '&&' is awkward within an XML document, and so the
keywords 'and', 'or' and 'not' can be used in place of '&&', '||' and '!' respectively. For example, the
previous pointcut may be better written as:
<aop:config>
<aop:aspect id="myAspect" ref="aBean">
<aop:pointcut id="businessService"
expression="execution(* com.xyz.myapp.service.*.*(..)) **and** this(service)"/>
<aop:before pointcut-ref="businessService" method="monitor"/>
...
</aop:aspect>
</aop:config>
Note that pointcuts defined in this way are referred to by their XML id and cannot be used as named
pointcuts to form composite pointcuts. The named pointcut support in the schema based definition style
is thus more limited than that offered by the @AspectJ style.
Declaring advice
The same five advice kinds are supported as for the @AspectJ style, and they have exactly the same
semantics.
Before advice
Before advice runs before a matched method execution. It is declared inside an <aop:aspect> using
the <aop:before> element.
<aop:aspect id="beforeExample" ref="aBean">
<aop:before
pointcut-ref="dataAccessOperation"
method="doAccessCheck"/>
...
</aop:aspect>
Here dataAccessOperation is the id of a pointcut defined at the top ( <aop:config>) level. To
define the pointcut inline instead, replace the pointcut-ref attribute with a pointcut attribute:
<aop:aspect id="beforeExample" ref="aBean">
<aop:before
pointcut="execution(* com.xyz.myapp.dao.*.*(..))"
method="doAccessCheck"/>
...
</aop:aspect>
As we noted in the discussion of the @AspectJ style, using named pointcuts can significantly improve
the readability of your code.
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The method attribute identifies a method ( doAccessCheck) that provides the body of the advice. This
method must be defined for the bean referenced by the aspect element containing the advice. Before a
data access operation is executed (a method execution join point matched by the pointcut expression),
the "doAccessCheck" method on the aspect bean will be invoked.
After returning advice
After returning advice runs when a matched method execution completes normally. It is declared inside
an <aop:aspect> in the same way as before advice. For example:
<aop:aspect id="afterReturningExample" ref="aBean">
<aop:after-returning
pointcut-ref="dataAccessOperation"
method="doAccessCheck"/>
...
</aop:aspect>
Just as in the @AspectJ style, it is possible to get hold of the return value within the advice body. Use
the returning attribute to specify the name of the parameter to which the return value should be passed:
<aop:aspect id="afterReturningExample" ref="aBean">
<aop:after-returning
pointcut-ref="dataAccessOperation"
returning="retVal"
method="doAccessCheck"/>
...
</aop:aspect>
The doAccessCheck method must declare a parameter named retVal. The type of this parameter
constrains matching in the same way as described for @AfterReturning. For example, the method
signature may be declared as:
public void doAccessCheck(Object retVal) {...
After throwing advice
After throwing advice executes when a matched method execution exits by throwing an exception. It is
declared inside an <aop:aspect> using the after-throwing element:
<aop:aspect id="afterThrowingExample" ref="aBean">
<aop:after-throwing
pointcut-ref="dataAccessOperation"
method="doRecoveryActions"/>
...
</aop:aspect>
Just as in the @AspectJ style, it is possible to get hold of the thrown exception within the advice body.
Use the throwing attribute to specify the name of the parameter to which the exception should be passed:
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<aop:aspect id="afterThrowingExample" ref="aBean">
<aop:after-throwing
pointcut-ref="dataAccessOperation"
throwing="dataAccessEx"
method="doRecoveryActions"/>
...
</aop:aspect>
The doRecoveryActions method must declare a parameter named dataAccessEx. The type of this
parameter constrains matching in the same way as described for @AfterThrowing. For example, the
method signature may be declared as:
public void doRecoveryActions(DataAccessException dataAccessEx) {...
After (finally) advice
After (finally) advice runs however a matched method execution exits. It is declared using the after
element:
<aop:aspect id="afterFinallyExample" ref="aBean">
<aop:after
pointcut-ref="dataAccessOperation"
method="doReleaseLock"/>
...
</aop:aspect>
Around advice
The final kind of advice is around advice. Around advice runs "around" a matched method execution.
It has the opportunity to do work both before and after the method executes, and to determine when,
how, and even if, the method actually gets to execute at all. Around advice is often used if you need
to share state before and after a method execution in a thread-safe manner (starting and stopping a
timer for example). Always use the least powerful form of advice that meets your requirements; don’t
use around advice if simple before advice would do.
Around advice is declared using the aop:around element. The first parameter of the advice method
must be of type ProceedingJoinPoint. Within the body of the advice, calling proceed() on the
ProceedingJoinPoint causes the underlying method to execute. The proceed method may also
be calling passing in an Object[] - the values in the array will be used as the arguments to the method
execution when it proceeds. See the section called “Around advice” for notes on calling proceed with
an Object[].
<aop:aspect id="aroundExample" ref="aBean">
<aop:around
pointcut-ref="businessService"
method="doBasicProfiling"/>
...
</aop:aspect>
The implementation of the doBasicProfiling advice would be exactly the same as in the @AspectJ
example (minus the annotation of course):
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public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
// start stopwatch
Object retVal = pjp.proceed();
// stop stopwatch
return retVal;
}
Advice parameters
The schema based declaration style supports fully typed advice in the same way as described for the
@AspectJ support - by matching pointcut parameters by name against advice method parameters. See
the section called “Advice parameters” for details. If you wish to explicitly specify argument names for
the advice methods (not relying on the detection strategies previously described) then this is done using
the arg-names attribute of the advice element, which is treated in the same manner to the "argNames"
attribute in an advice annotation as described in the section called “Determining argument names”. For
example:
<aop:before
pointcut="com.xyz.lib.Pointcuts.anyPublicMethod() and @annotation(auditable)"
method="audit"
arg-names="auditable"/>
The arg-names attribute accepts a comma-delimited list of parameter names.
Find below a slightly more involved example of the XSD-based approach that illustrates some around
advice used in conjunction with a number of strongly typed parameters.
package x.y.service;
public interface FooService {
Foo getFoo(String fooName, int age);
}
public class DefaultFooService implements FooService {
public Foo getFoo(String name, int age) {
return new Foo(name, age);
}
}
Next up is the aspect. Notice the fact that the profile(..) method accepts a number of stronglytyped parameters, the first of which happens to be the join point used to proceed with the method call:
the presence of this parameter is an indication that the profile(..) is to be used as around advice:
package x.y;
import org.aspectj.lang.ProceedingJoinPoint;
import org.springframework.util.StopWatch;
public class SimpleProfiler {
public Object profile(ProceedingJoinPoint call, String name, int age) throws Throwable {
StopWatch clock = new StopWatch("Profiling for '" + name + "' and '" + age + "'");
try {
clock.start(call.toShortString());
return call.proceed();
} finally {
clock.stop();
System.out.println(clock.prettyPrint());
}
}
}
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Finally, here is the XML configuration that is required to effect the execution of the above advice for
a particular join point:
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/springbeans.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/springaop.xsd">
<!-- this is the object that will be proxied by Spring's AOP infrastructure -->
<bean id="fooService" class="x.y.service.DefaultFooService"/>
<!-- this is the actual advice itself -->
<bean id="profiler" class="x.y.SimpleProfiler"/>
<aop:config>
<aop:aspect ref="profiler">
<aop:pointcut id="theExecutionOfSomeFooServiceMethod"
expression="execution(* x.y.service.FooService.getFoo(String,int))
and args(name, age)"/>
<aop:around pointcut-ref="theExecutionOfSomeFooServiceMethod"
method="profile"/>
</aop:aspect>
</aop:config>
</beans>
If we had the following driver script, we would get output something like this on standard output:
import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import x.y.service.FooService;
public final class Boot {
public static void main(final String[] args) throws Exception {
BeanFactory ctx = new ClassPathXmlApplicationContext("x/y/plain.xml");
FooService foo = (FooService) ctx.getBean("fooService");
foo.getFoo("Pengo", 12);
}
}
StopWatch 'Profiling for 'Pengo' and '12'': running time (millis) = 0
----------------------------------------ms
%
Task name
----------------------------------------00000 ? execution(getFoo)
Advice ordering
When multiple advice needs to execute at the same join point (executing method) the ordering rules are
as described in the section called “Advice ordering”. The precedence between aspects is determined
by either adding the Order annotation to the bean backing the aspect or by having the bean implement
the Ordered interface.
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Introductions
Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare that advised
objects implement a given interface, and to provide an implementation of that interface on behalf of
those objects.
An introduction is made using the aop:declare-parents element inside an aop:aspect This
element is used to declare that matching types have a new parent (hence the name). For example, given
an interface UsageTracked, and an implementation of that interface DefaultUsageTracked, the
following aspect declares that all implementors of service interfaces also implement the UsageTracked
interface. (In order to expose statistics via JMX for example.)
<aop:aspect id="usageTrackerAspect" ref="usageTracking">
<aop:declare-parents
types-matching="com.xzy.myapp.service.*+"
implement-interface="com.xyz.myapp.service.tracking.UsageTracked"
default-impl="com.xyz.myapp.service.tracking.DefaultUsageTracked"/>
<aop:before
pointcut="com.xyz.myapp.SystemArchitecture.businessService()
and this(usageTracked)"
method="recordUsage"/>
</aop:aspect>
The class backing the usageTracking bean would contain the method:
public void recordUsage(UsageTracked usageTracked) {
usageTracked.incrementUseCount();
}
The interface to be implemented is determined by implement-interface attribute. The value of the
types-matching attribute is an AspectJ type pattern :- any bean of a matching type will implement the
UsageTracked interface. Note that in the before advice of the above example, service beans can be
directly used as implementations of the UsageTracked interface. If accessing a bean programmatically
you would write the following:
UsageTracked usageTracked = (UsageTracked) context.getBean("myService");
Aspect instantiation models
The only supported instantiation model for schema-defined aspects is the singleton model. Other
instantiation models may be supported in future releases.
Advisors
The concept of "advisors" is brought forward from the AOP support defined in Spring 1.2 and does not
have a direct equivalent in AspectJ. An advisor is like a small self-contained aspect that has a single
piece of advice. The advice itself is represented by a bean, and must implement one of the advice
interfaces described in the section called “Advice types in Spring”. Advisors can take advantage of
AspectJ pointcut expressions though.
Spring supports the advisor concept with the <aop:advisor> element. You will most commonly see
it used in conjunction with transactional advice, which also has its own namespace support in Spring.
Here’s how it looks:
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<aop:config>
<aop:pointcut id="businessService"
expression="execution(* com.xyz.myapp.service.*.*(..))"/>
<aop:advisor
pointcut-ref="businessService"
advice-ref="tx-advice"/>
</aop:config>
<tx:advice id="tx-advice">
<tx:attributes>
<tx:method name="*" propagation="REQUIRED"/>
</tx:attributes>
</tx:advice>
As well as the pointcut-ref attribute used in the above example, you can also use the pointcut
attribute to define a pointcut expression inline.
To define the precedence of an advisor so that the advice can participate in ordering, use the order
attribute to define the Ordered value of the advisor.
Example
Let’s see how the concurrent locking failure retry example from the section called “Example” looks when
rewritten using the schema support.
The execution of business services can sometimes fail due to concurrency issues (for example, deadlock
loser). If the operation is retried, it is quite likely it will succeed next time round. For business services
where it is appropriate to retry in such conditions (idempotent operations that don’t need to go back to
the user for conflict resolution), we’d like to transparently retry the operation to avoid the client seeing
a PessimisticLockingFailureException. This is a requirement that clearly cuts across multiple
services in the service layer, and hence is ideal for implementing via an aspect.
Because we want to retry the operation, we’ll need to use around advice so that we can call proceed
multiple times. Here’s how the basic aspect implementation looks (it’s just a regular Java class using
the schema support):
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public class ConcurrentOperationExecutor implements Ordered {
private static final int DEFAULT_MAX_RETRIES = 2;
private int maxRetries = DEFAULT_MAX_RETRIES;
private int order = 1;
public void setMaxRetries(int maxRetries) {
this.maxRetries = maxRetries;
}
public int getOrder() {
return this.order;
}
public void setOrder(int order) {
this.order = order;
}
public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
int numAttempts = 0;
PessimisticLockingFailureException lockFailureException;
do {
numAttempts++;
try {
return pjp.proceed();
}
catch(PessimisticLockingFailureException ex) {
lockFailureException = ex;
}
} while(numAttempts <= this.maxRetries);
throw lockFailureException;
}
}
Note that the aspect implements the Ordered interface so we can set the precedence of the
aspect higher than the transaction advice (we want a fresh transaction each time we retry). The
maxRetries and order properties will both be configured by Spring. The main action happens
in the doConcurrentOperation around advice method. We try to proceed, and if we fail with a
PessimisticLockingFailureException we simply try again unless we have exhausted all of our
retry attempts.
Note
This class is identical to the one used in the @AspectJ example, but with the annotations removed.
The corresponding Spring configuration is:
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<aop:config>
<aop:aspect id="concurrentOperationRetry" ref="concurrentOperationExecutor">
<aop:pointcut id="idempotentOperation"
expression="execution(* com.xyz.myapp.service.*.*(..))"/>
<aop:around
pointcut-ref="idempotentOperation"
method="doConcurrentOperation"/>
</aop:aspect>
</aop:config>
<bean id="concurrentOperationExecutor"
class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor">
<property name="maxRetries" value="3"/>
<property name="order" value="100"/>
</bean>
Notice that for the time being we assume that all business services are idempotent. If this is not the
case we can refine the aspect so that it only retries genuinely idempotent operations, by introducing
an Idempotent annotation:
@Retention(RetentionPolicy.RUNTIME)
public @interface Idempotent {
// marker annotation
}
and using the annotation to annotate the implementation of service operations. The change to the
aspect to retry only idempotent operations simply involves refining the pointcut expression so that only
@Idempotent operations match:
<aop:pointcut id="idempotentOperation"
expression="execution(* com.xyz.myapp.service.*.*(..)) and
@annotation(com.xyz.myapp.service.Idempotent)"/>
10.4 Choosing which AOP declaration style to use
Once you have decided that an aspect is the best approach for implementing a given requirement, how
do you decide between using Spring AOP or AspectJ, and between the Aspect language (code) style,
@AspectJ annotation style, or the Spring XML style? These decisions are influenced by a number of
factors including application requirements, development tools, and team familiarity with AOP.
Spring AOP or full AspectJ?
Use the simplest thing that can work. Spring AOP is simpler than using full AspectJ as there is no
requirement to introduce the AspectJ compiler / weaver into your development and build processes.
If you only need to advise the execution of operations on Spring beans, then Spring AOP is the right
choice. If you need to advise objects not managed by the Spring container (such as domain objects
typically), then you will need to use AspectJ. You will also need to use AspectJ if you wish to advise join
points other than simple method executions (for example, field get or set join points, and so on).
When using AspectJ, you have the choice of the AspectJ language syntax (also known as the "code
style") or the @AspectJ annotation style. Clearly, if you are not using Java 5+ then the choice has been
made for you… use the code style. If aspects play a large role in your design, and you are able to use
the AspectJ Development Tools (AJDT) plugin for Eclipse, then the AspectJ language syntax is the
preferred option: it is cleaner and simpler because the language was purposefully designed for writing
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aspects. If you are not using Eclipse, or have only a few aspects that do not play a major role in your
application, then you may want to consider using the @AspectJ style and sticking with a regular Java
compilation in your IDE, and adding an aspect weaving phase to your build script.
@AspectJ or XML for Spring AOP?
If you have chosen to use Spring AOP, then you have a choice of @AspectJ or XML style. There are
various tradeoffs to consider.
The XML style will be most familiar to existing Spring users and it is backed by genuine POJOs. When
using AOP as a tool to configure enterprise services then XML can be a good choice (a good test
is whether you consider the pointcut expression to be a part of your configuration you might want to
change independently). With the XML style arguably it is clearer from your configuration what aspects
are present in the system.
The XML style has two disadvantages. Firstly it does not fully encapsulate the implementation of the
requirement it addresses in a single place. The DRY principle says that there should be a single,
unambiguous, authoritative representation of any piece of knowledge within a system. When using the
XML style, the knowledge of how a requirement is implemented is split across the declaration of the
backing bean class, and the XML in the configuration file. When using the @AspectJ style there is a
single module - the aspect - in which this information is encapsulated. Secondly, the XML style is slightly
more limited in what it can express than the @AspectJ style: only the "singleton" aspect instantiation
model is supported, and it is not possible to combine named pointcuts declared in XML. For example,
in the @AspectJ style you can write something like:
@Pointcut(execution(* get*()))
public void propertyAccess() {}
@Pointcut(execution(org.xyz.Account+ *(..))
public void operationReturningAnAccount() {}
@Pointcut(propertyAccess() && operationReturningAnAccount())
public void accountPropertyAccess() {}
In the XML style I can declare the first two pointcuts:
<aop:pointcut id="propertyAccess"
expression="execution(* get*())"/>
<aop:pointcut id="operationReturningAnAccount"
expression="execution(org.xyz.Account+ *(..))"/>
The downside of the XML approach is that you cannot define the 'accountPropertyAccess' pointcut by
combining these definitions.
The @AspectJ style supports additional instantiation models, and richer pointcut composition. It has the
advantage of keeping the aspect as a modular unit. It also has the advantage the @AspectJ aspects
can be understood (and thus consumed) both by Spring AOP and by AspectJ - so if you later decide
you need the capabilities of AspectJ to implement additional requirements then it is very easy to migrate
to an AspectJ-based approach. On balance the Spring team prefer the @AspectJ style whenever you
have aspects that do more than simple "configuration" of enterprise services.
10.5 Mixing aspect types
It is perfectly possible to mix @AspectJ style aspects using the autoproxying support, schema-defined
<aop:aspect> aspects, <aop:advisor> declared advisors and even proxies and interceptors
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defined using the Spring 1.2 style in the same configuration. All of these are implemented using the
same underlying support mechanism and will co-exist without any difficulty.
10.6 Proxying mechanisms
Spring AOP uses either JDK dynamic proxies or CGLIB to create the proxy for a given target object.
(JDK dynamic proxies are preferred whenever you have a choice).
If the target object to be proxied implements at least one interface then a JDK dynamic proxy will be
used. All of the interfaces implemented by the target type will be proxied. If the target object does not
implement any interfaces then a CGLIB proxy will be created.
If you want to force the use of CGLIB proxying (for example, to proxy every method defined for the
target object, not just those implemented by its interfaces) you can do so. However, there are some
issues to consider:
• final methods cannot be advised, as they cannot be overridden.
• As of Spring 3.2, it is no longer necessary to add CGLIB to your project classpath, as CGLIB classes
are repackaged under org.springframework and included directly in the spring-core JAR. This means
that CGLIB-based proxy support 'just works' in the same way that JDK dynamic proxies always have.
• As of Spring 4.0, the constructor of your proxied object will NOT be called twice anymore since the
CGLIB proxy instance will be created via Objenesis. Only if your JVM does not allow for constructor
bypassing, you might see double invocations and corresponding debug log entries from Spring’s AOP
support.
To force the use of CGLIB proxies set the value of the proxy-target-class attribute of the
<aop:config> element to true:
<aop:config proxy-target-class="true">
<!-- other beans defined here... -->
</aop:config>
To force CGLIB proxying when using the @AspectJ autoproxy support, set the 'proxy-targetclass' attribute of the <aop:aspectj-autoproxy> element to true:
<aop:aspectj-autoproxy proxy-target-class="true"/>
Note
Multiple <aop:config/> sections are collapsed into a single unified auto-proxy creator
at runtime, which applies the strongest proxy settings that any of the <aop:config/>
sections (typically from different XML bean definition files) specified. This also applies to the
<tx:annotation-driven/> and <aop:aspectj-autoproxy/> elements.
To be clear: using 'proxy-target-class="true"' on <tx:annotation-driven/>,
<aop:aspectj-autoproxy/> or <aop:config/> elements will force the use of CGLIB
proxies for all three of them.
Understanding AOP proxies
Spring AOP is proxy-based. It is vitally important that you grasp the semantics of what that last statement
actually means before you write your own aspects or use any of the Spring AOP-based aspects supplied
with the Spring Framework.
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Consider first the scenario where you have a plain-vanilla, un-proxied, nothing-special-about-it, straight
object reference, as illustrated by the following code snippet.
public class SimplePojo implements Pojo {
public void foo() {
// this next method invocation is a direct call on the 'this' reference
this.bar();
}
public void bar() {
// some logic...
}
}
If you invoke a method on an object reference, the method is invoked directly on that object reference,
as can be seen below.
public class Main {
public static void main(String[] args) {
Pojo pojo = new SimplePojo();
// this is a direct method call on the 'pojo' reference
pojo.foo();
}
}
Things change slightly when the reference that client code has is a proxy. Consider the following diagram
and code snippet.
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public class Main {
public static void main(String[] args) {
ProxyFactory factory = new ProxyFactory(new SimplePojo());
factory.addInterface(Pojo.class);
factory.addAdvice(new RetryAdvice());
Pojo pojo = (Pojo) factory.getProxy();
// this is a method call on the proxy!
pojo.foo();
}
}
The key thing to understand here is that the client code inside the main(..) of the Main class
has a reference to the proxy. This means that method calls on that object reference will be calls on
the proxy, and as such the proxy will be able to delegate to all of the interceptors (advice) that are
relevant to that particular method call. However, once the call has finally reached the target object, the
SimplePojo reference in this case, any method calls that it may make on itself, such as this.bar() or
this.foo(), are going to be invoked against the this reference, and not the proxy. This has important
implications. It means that self-invocation is not going to result in the advice associated with a method
invocation getting a chance to execute.
Okay, so what is to be done about this? The best approach (the term best is used loosely here) is to
refactor your code such that the self-invocation does not happen. For sure, this does entail some work
on your part, but it is the best, least-invasive approach. The next approach is absolutely horrendous,
and I am almost reticent to point it out precisely because it is so horrendous. You can (choke!) totally
tie the logic within your class to Spring AOP by doing this:
public class SimplePojo implements Pojo {
public void foo() {
// this works, but... gah!
((Pojo) AopContext.currentProxy()).bar();
}
public void bar() {
// some logic...
}
}
This totally couples your code to Spring AOP, and it makes the class itself aware of the fact that it is being
used in an AOP context, which flies in the face of AOP. It also requires some additional configuration
when the proxy is being created:
public class Main {
public static void main(String[] args) {
ProxyFactory factory = new ProxyFactory(new SimplePojo());
factory.adddInterface(Pojo.class);
factory.addAdvice(new RetryAdvice());
factory.setExposeProxy(true);
Pojo pojo = (Pojo) factory.getProxy();
// this is a method call on the proxy!
pojo.foo();
}
}
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Finally, it must be noted that AspectJ does not have this self-invocation issue because it is not a proxybased AOP framework.
10.7 Programmatic creation of @AspectJ Proxies
In addition to declaring aspects in your configuration using either <aop:config> or <aop:aspectjautoproxy>, it is also possible programmatically to create proxies that advise target objects. For the full
details of Spring’s AOP API, see the next chapter. Here we want to focus on the ability to automatically
create proxies using @AspectJ aspects.
The class org.springframework.aop.aspectj.annotation.AspectJProxyFactory can be
used to create a proxy for a target object that is advised by one or more @AspectJ aspects. Basic usage
for this class is very simple, as illustrated below. See the javadocs for full information.
// create a factory that can generate a proxy for the given target object
AspectJProxyFactory factory = new AspectJProxyFactory(targetObject);
// add an aspect, the class must be an @AspectJ aspect
// you can call this as many times as you need with different aspects
factory.addAspect(SecurityManager.class);
// you can also add existing aspect instances, the type of the object supplied must be an @AspectJ
aspect
factory.addAspect(usageTracker);
// now get the proxy object...
MyInterfaceType proxy = factory.getProxy();
10.8 Using AspectJ with Spring applications
Everything we’ve covered so far in this chapter is pure Spring AOP. In this section, we’re going to look
at how you can use the AspectJ compiler/weaver instead of, or in addition to, Spring AOP if your needs
go beyond the facilities offered by Spring AOP alone.
Spring ships with a small AspectJ aspect library, which is available standalone in your distribution as
spring-aspects.jar; you’ll need to add this to your classpath in order to use the aspects in it. the
section called “Using AspectJ to dependency inject domain objects with Spring” and the section called
“Other Spring aspects for AspectJ” discuss the content of this library and how you can use it. the section
called “Configuring AspectJ aspects using Spring IoC” discusses how to dependency inject AspectJ
aspects that are woven using the AspectJ compiler. Finally, the section called “Load-time weaving with
AspectJ in the Spring Framework” provides an introduction to load-time weaving for Spring applications
using AspectJ.
Using AspectJ to dependency inject domain objects with Spring
The Spring container instantiates and configures beans defined in your application context. It is also
possible to ask a bean factory to configure a pre-existing object given the name of a bean definition
containing the configuration to be applied. The spring-aspects.jar contains an annotation-driven
aspect that exploits this capability to allow dependency injection of any object. The support is intended
to be used for objects created outside of the control of any container. Domain objects often fall into this
category because they are often created programmatically using the new operator, or by an ORM tool
as a result of a database query.
The @Configurable annotation marks a class as eligible for Spring-driven configuration. In the
simplest case it can be used just as a marker annotation:
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package com.xyz.myapp.domain;
import org.springframework.beans.factory.annotation.Configurable;
@Configurable
public class Account {
// ...
}
When used as a marker interface in this way, Spring will configure new instances of the annotated type
( Account in this case) using a bean definition (typically prototype-scoped) with the same name as the
fully-qualified type name ( com.xyz.myapp.domain.Account). Since the default name for a bean
is the fully-qualified name of its type, a convenient way to declare the prototype definition is simply to
omit the id attribute:
<bean class="com.xyz.myapp.domain.Account" scope="prototype">
<property name="fundsTransferService" ref="fundsTransferService"/>
</bean>
If you want to explicitly specify the name of the prototype bean definition to use, you can do so directly
in the annotation:
package com.xyz.myapp.domain;
import org.springframework.beans.factory.annotation.Configurable;
@Configurable("account")
public class Account {
// ...
}
Spring will now look for a bean definition named "account" and use that as the definition to configure
new Account instances.
You can also use autowiring to avoid having to specify a dedicated bean
definition at all. To have Spring apply autowiring use the 'autowire' property of the
@Configurable annotation: specify either @Configurable(autowire=Autowire.BY_TYPE) or
@Configurable(autowire=Autowire.BY_NAME for autowiring by type or by name respectively. As
an alternative, as of Spring 2.5 it is preferable to specify explicit, annotation-driven dependency injection
for your @Configurable beans by using @Autowired or @Inject at the field or method level (see
Section 6.9, “Annotation-based container configuration” for further details).
Finally you can enable Spring dependency checking for the object references in the
newly created and configured object by using the dependencyCheck attribute (for example:
@Configurable(autowire=Autowire.BY_NAME,dependencyCheck=true)). If this attribute is
set to true, then Spring will validate after configuration that all properties (which are not primitives or
collections) have been set.
Using
the
annotation
on
its
own
does
nothing
of
course.
It
is
the
AnnotationBeanConfigurerAspect in spring-aspects.jar that acts on the presence of the
annotation. In essence the aspect says "after returning from the initialization of a new object of a type
annotated with @Configurable, configure the newly created object using Spring in accordance with
the properties of the annotation". In this context, initialization refers to newly instantiated objects (e.g.,
objects instantiated with the 'new' operator) as well as to Serializable objects that are undergoing
deserialization (e.g., via readResolve()).
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Note
One of the key phrases in the above paragraph is 'in essence'. For most cases, the exact
semantics of 'after returning from the initialization of a new object' will be fine… in this context, 'after
initialization' means that the dependencies will be injected after the object has been constructed
- this means that the dependencies will not be available for use in the constructor bodies of the
class. If you want the dependencies to be injected before the constructor bodies execute, and
thus be available for use in the body of the constructors, then you need to define this on the
@Configurable declaration like so:
@Configurable(preConstruction=true)
You can find out more information about the language semantics of the various pointcut types in
AspectJ in this appendix of the AspectJ Programming Guide.
For this to work the annotated types must be woven with the AspectJ weaver - you can either use a buildtime Ant or Maven task to do this (see for example the AspectJ Development Environment Guide) or
load-time weaving (see the section called “Load-time weaving with AspectJ in the Spring Framework”).
The AnnotationBeanConfigurerAspect itself needs configuring by Spring (in order to obtain a
reference to the bean factory that is to be used to configure new objects). If you are using Java based
configuration simply add @EnableSpringConfigured to any @Configuration class.
@Configuration
@EnableSpringConfigured
public class AppConfig {
}
If you prefer XML based configuration, the Spring context namespace defines a convenient
context:spring-configured element:
<context:spring-configured/>
Instances of @Configurable objects created before the aspect has been configured will result in a
message being issued to the debug log and no configuration of the object taking place. An example
might be a bean in the Spring configuration that creates domain objects when it is initialized by Spring.
In this case you can use the "depends-on" bean attribute to manually specify that the bean depends
on the configuration aspect.
<bean id="myService"
class="com.xzy.myapp.service.MyService"
depends-on="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect">
<!-- ... -->
</bean>
Note
Do not activate @Configurable processing through the bean configurer aspect unless you
really mean to rely on its semantics at runtime. In particular, make sure that you do not
use @Configurable on bean classes which are registered as regular Spring beans with the
container: You would get double initialization otherwise, once through the container and once
through the aspect.
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Unit testing @Configurable objects
One of the goals of the @Configurable support is to enable independent unit testing of domain objects
without the difficulties associated with hard-coded lookups. If @Configurable types have not been
woven by AspectJ then the annotation has no affect during unit testing, and you can simply set mock
or stub property references in the object under test and proceed as normal. If @Configurable types
have been woven by AspectJ then you can still unit test outside of the container as normal, but you will
see a warning message each time that you construct an @Configurable object indicating that it has
not been configured by Spring.
Working with multiple application contexts
The AnnotationBeanConfigurerAspect used to implement the @Configurable support is an
AspectJ singleton aspect. The scope of a singleton aspect is the same as the scope of static
members, that is to say there is one aspect instance per classloader that defines the type. This
means that if you define multiple application contexts within the same classloader hierarchy you need
to consider where to define the @EnableSpringConfigured bean and where to place springaspects.jar on the classpath.
Consider a typical Spring web-app configuration with a shared parent application context defining
common business services and everything needed to support them, and one child application context
per servlet containing definitions particular to that servlet. All of these contexts will co-exist within
the same classloader hierarchy, and so the AnnotationBeanConfigurerAspect can only hold a
reference to one of them. In this case we recommend defining the @EnableSpringConfigured bean
in the shared (parent) application context: this defines the services that you are likely to want to inject
into domain objects. A consequence is that you cannot configure domain objects with references to
beans defined in the child (servlet-specific) contexts using the @Configurable mechanism (probably not
something you want to do anyway!).
When deploying multiple web-apps within the same container, ensure that each web-application loads
the types in spring-aspects.jar using its own classloader (for example, by placing springaspects.jar in 'WEB-INF/lib'). If spring-aspects.jar is only added to the container wide
classpath (and hence loaded by the shared parent classloader), all web applications will share the same
aspect instance which is probably not what you want.
Other Spring aspects for AspectJ
In addition to the @Configurable aspect, spring-aspects.jar contains an AspectJ aspect
that can be used to drive Spring’s transaction management for types and methods annotated with
the @Transactional annotation. This is primarily intended for users who want to use the Spring
Framework’s transaction support outside of the Spring container.
The aspect that interprets @Transactional annotations is the AnnotationTransactionAspect.
When using this aspect, you must annotate the implementation class (and/or methods within that class),
not the interface (if any) that the class implements. AspectJ follows Java’s rule that annotations on
interfaces are not inherited.
A @Transactional annotation on a class specifies the default transaction semantics for the execution
of any public operation in the class.
A @Transactional annotation on a method within the class overrides the default transaction
semantics given by the class annotation (if present). Methods of any visibility may be annotated,
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including private methods. Annotating non-public methods directly is the only way to get transaction
demarcation for the execution of such methods.
Tip
Since Spring Framework 4.2, spring-aspects provides a similar aspect that offers the exact
same features for the standard javax.transaction.Transactional annotation. Check
JtaAnnotationTransactionAspect for more details.
For AspectJ programmers that want to use the Spring configuration and transaction management
support but don’t want to (or cannot) use annotations, spring-aspects.jar also contains
abstract aspects you can extend to provide your own pointcut definitions. See the sources for
the AbstractBeanConfigurerAspect and AbstractTransactionAspect aspects for more
information. As an example, the following excerpt shows how you could write an aspect to configure
all instances of objects defined in the domain model using prototype bean definitions that match the
fully-qualified class names:
public aspect DomainObjectConfiguration extends AbstractBeanConfigurerAspect {
public DomainObjectConfiguration() {
setBeanWiringInfoResolver(new ClassNameBeanWiringInfoResolver());
}
// the creation of a new bean (any object in the domain model)
protected pointcut beanCreation(Object beanInstance) :
initialization(new(..)) &&
SystemArchitecture.inDomainModel() &&
this(beanInstance);
}
Configuring AspectJ aspects using Spring IoC
When using AspectJ aspects with Spring applications, it is natural to both want and expect to be able to
configure such aspects using Spring. The AspectJ runtime itself is responsible for aspect creation, and
the means of configuring the AspectJ created aspects via Spring depends on the AspectJ instantiation
model (the 'per-xxx' clause) used by the aspect.
The majority of AspectJ aspects are singleton aspects. Configuration of these aspects is very easy:
simply create a bean definition referencing the aspect type as normal, and include the bean attribute
'factory-method="aspectOf"'. This ensures that Spring obtains the aspect instance by asking
AspectJ for it rather than trying to create an instance itself. For example:
<bean id="profiler" class="com.xyz.profiler.Profiler"
factory-method="aspectOf">
<property name="profilingStrategy" ref="jamonProfilingStrategy"/>
</bean>
Non-singleton aspects are harder to configure: however it is possible to do so by creating prototype
bean definitions and using the @Configurable support from spring-aspects.jar to configure the
aspect instances once they have bean created by the AspectJ runtime.
If you have some @AspectJ aspects that you want to weave with AspectJ (for example, using load-time
weaving for domain model types) and other @AspectJ aspects that you want to use with Spring AOP,
and these aspects are all configured using Spring, then you will need to tell the Spring AOP @AspectJ
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autoproxying support which exact subset of the @AspectJ aspects defined in the configuration should
be used for autoproxying. You can do this by using one or more <include/> elements inside the
<aop:aspectj-autoproxy/> declaration. Each <include/> element specifies a name pattern, and
only beans with names matched by at least one of the patterns will be used for Spring AOP autoproxy
configuration:
<aop:aspectj-autoproxy>
<aop:include name="thisBean"/>
<aop:include name="thatBean"/>
</aop:aspectj-autoproxy>
Note
Do not be misled by the name of the <aop:aspectj-autoproxy/> element: using it will result
in the creation of Spring AOP proxies. The @AspectJ style of aspect declaration is just being used
here, but the AspectJ runtime is not involved.
Load-time weaving with AspectJ in the Spring Framework
Load-time weaving (LTW) refers to the process of weaving AspectJ aspects into an application’s class
files as they are being loaded into the Java virtual machine (JVM). The focus of this section is on
configuring and using LTW in the specific context of the Spring Framework: this section is not an
introduction to LTW though. For full details on the specifics of LTW and configuring LTW with just AspectJ
(with Spring not being involved at all), see the LTW section of the AspectJ Development Environment
Guide.
The value-add that the Spring Framework brings to AspectJ LTW is in enabling much finer-grained
control over the weaving process. 'Vanilla' AspectJ LTW is effected using a Java (5+) agent, which is
switched on by specifying a VM argument when starting up a JVM. It is thus a JVM-wide setting, which
may be fine in some situations, but often is a little too coarse. Spring-enabled LTW enables you to
switch on LTW on a per-ClassLoader basis, which obviously is more fine-grained and which can make
more sense in a 'single-JVM-multiple-application' environment (such as is found in a typical application
server environment).
Further, in certain environments, this support enables load-time weaving without making any
modifications to the application server’s launch script that will be needed to add -javaagent:path/
to/aspectjweaver.jar or (as we describe later in this section) -javaagent:path/to/
org.springframework.instrument-{version}.jar (previously named spring-agent.jar).
Developers simply modify one or more files that form the application context to enable load-time weaving
instead of relying on administrators who typically are in charge of the deployment configuration such
as the launch script.
Now that the sales pitch is over, let us first walk through a quick example of AspectJ LTW using Spring,
followed by detailed specifics about elements introduced in the following example. For a complete
example, please see the Petclinic sample application.
A first example
Let us assume that you are an application developer who has been tasked with diagnosing the cause of
some performance problems in a system. Rather than break out a profiling tool, what we are going to do
is switch on a simple profiling aspect that will enable us to very quickly get some performance metrics,
so that we can then apply a finer-grained profiling tool to that specific area immediately afterwards.
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Note
The example presented here uses XML style configuration, it is also possible to configure and
use @AspectJ with Java Configuration. Specifically the @EnableLoadTimeWeaving annotation
can be used as an alternative to <context:load-time-weaver/> (see below for details).
Here is the profiling aspect. Nothing too fancy, just a quick-and-dirty time-based profiler, using the
@AspectJ-style of aspect declaration.
package foo;
import
import
import
import
import
import
org.aspectj.lang.ProceedingJoinPoint;
org.aspectj.lang.annotation.Aspect;
org.aspectj.lang.annotation.Around;
org.aspectj.lang.annotation.Pointcut;
org.springframework.util.StopWatch;
org.springframework.core.annotation.Order;
@Aspect
public class ProfilingAspect {
@Around("methodsToBeProfiled()")
public Object profile(ProceedingJoinPoint pjp) throws Throwable {
StopWatch sw = new StopWatch(getClass().getSimpleName());
try {
sw.start(pjp.getSignature().getName());
return pjp.proceed();
} finally {
sw.stop();
System.out.println(sw.prettyPrint());
}
}
@Pointcut("execution(public * foo..*.*(..))")
public void methodsToBeProfiled(){}
}
We will also need to create an 'META-INF/aop.xml' file, to inform the AspectJ weaver that we want to
weave our ProfilingAspect into our classes. This file convention, namely the presence of a file (or
files) on the Java classpath called 'META-INF/aop.xml' is standard AspectJ.
<!DOCTYPE aspectj PUBLIC "-//AspectJ//DTD//EN" "http://www.eclipse.org/aspectj/dtd/aspectj.dtd">
<aspectj>
<weaver>
<!-- only weave classes in our application-specific packages -->
<include within="foo.*"/>
</weaver>
<aspects>
<!-- weave in just this aspect -->
<aspect name="foo.ProfilingAspect"/>
</aspects>
</aspectj>
Now to the Spring-specific portion of the configuration. We need to configure a LoadTimeWeaver
(all explained later, just take it on trust for now). This load-time weaver is the essential component
responsible for weaving the aspect configuration in one or more 'META-INF/aop.xml' files into the
classes in your application. The good thing is that it does not require a lot of configuration, as can be
seen below (there are some more options that you can specify, but these are detailed later).
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<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<!-- a service object; we will be profiling its methods -->
<bean id="entitlementCalculationService"
class="foo.StubEntitlementCalculationService"/>
<!-- this switches on the load-time weaving -->
<context:load-time-weaver/>
</beans>
Now that all the required artifacts are in place - the aspect, the 'META-INF/aop.xml' file, and the Spring
configuration -, let us create a simple driver class with a main(..) method to demonstrate the LTW
in action.
package foo;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public final class Main {
public static void main(String[] args) {
ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml", Main.class);
EntitlementCalculationService entitlementCalculationService
= (EntitlementCalculationService) ctx.getBean("entitlementCalculationService");
// the profiling aspect is 'woven' around this method execution
entitlementCalculationService.calculateEntitlement();
}
}
There is one last thing to do. The introduction to this section did say that one could switch on LTW
selectively on a per- ClassLoader basis with Spring, and this is true. However, just for this example,
we are going to use a Java agent (supplied with Spring) to switch on the LTW. This is the command
line we will use to run the above Main class:
java -javaagent:C:/projects/foo/lib/global/spring-instrument.jar foo.Main
The '-javaagent' is a flag for specifying and enabling agents to instrument programs running on the
JVM. The Spring Framework ships with such an agent, the InstrumentationSavingAgent, which
is packaged in the spring-instrument.jar that was supplied as the value of the -javaagent
argument in the above example.
The output from the execution of the Main program will look something like that below. (I have introduced
a Thread.sleep(..) statement into the calculateEntitlement() implementation so that the
profiler actually captures something other than 0 milliseconds - the 01234 milliseconds is not an
overhead introduced by the AOP :) )
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Calculating entitlement
StopWatch 'ProfilingAspect': running time (millis) = 1234
------ ----- ---------------------------ms
%
Task name
------ ----- ---------------------------01234 100% calculateEntitlement
Since this LTW is effected using full-blown AspectJ, we are not just limited to advising Spring beans;
the following slight variation on the Main program will yield the same result.
package foo;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public final class Main {
public static void main(String[] args) {
new ClassPathXmlApplicationContext("beans.xml", Main.class);
EntitlementCalculationService entitlementCalculationService =
new StubEntitlementCalculationService();
// the profiling aspect will be 'woven' around this method execution
entitlementCalculationService.calculateEntitlement();
}
}
Notice how in the above program we are simply bootstrapping the Spring container, and then creating a
new instance of the StubEntitlementCalculationService totally outside the context of Spring…
the profiling advice still gets woven in.
The example admittedly is simplistic… however the basics of the LTW support in Spring have all been
introduced in the above example, and the rest of this section will explain the 'why' behind each bit of
configuration and usage in detail.
Note
The ProfilingAspect used in this example may be basic, but it is quite useful. It is a nice
example of a development-time aspect that developers can use during development (of course),
and then quite easily exclude from builds of the application being deployed into UAT or production.
Aspects
The aspects that you use in LTW have to be AspectJ aspects. They can be written in either the AspectJ
language itself or you can write your aspects in the @AspectJ-style. It means that your aspects are
then both valid AspectJ and Spring AOP aspects. Furthermore, the compiled aspect classes need to
be available on the classpath.
'META-INF/aop.xml'
The AspectJ LTW infrastructure is configured using one or more 'META-INF/aop.xml' files, that are on
the Java classpath (either directly, or more typically in jar files).
The structure and contents of this file is detailed in the main AspectJ reference documentation, and the
interested reader is referred to that resource. (I appreciate that this section is brief, but the 'aop.xml' file
is 100% AspectJ - there is no Spring-specific information or semantics that apply to it, and so there is no
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extra value that I can contribute either as a result), so rather than rehash the quite satisfactory section
that the AspectJ developers wrote, I am just directing you there.)
Required libraries (JARS)
At a minimum you will need the following libraries to use the Spring Framework’s support for AspectJ
LTW:
• spring-aop.jar (version 2.5 or later, plus all mandatory dependencies)
• aspectjweaver.jar (version 1.6.8 or later)
If you are using the Spring-provided agent to enable instrumentation, you will also need:
• spring-instrument.jar
Spring configuration
The key component in Spring’s LTW support is the LoadTimeWeaver interface (in
the org.springframework.instrument.classloading package), and the numerous
implementations of it that ship with the Spring distribution. A LoadTimeWeaver is responsible for adding
one or more java.lang.instrument.ClassFileTransformers to a ClassLoader at runtime,
which opens the door to all manner of interesting applications, one of which happens to be the LTW
of aspects.
Tip
If you are unfamiliar with the idea of runtime class file transformation, you are encouraged to read
the javadoc API documentation for the java.lang.instrument package before continuing.
This is not a huge chore because there is - rather annoyingly - precious little documentation
there… the key interfaces and classes will at least be laid out in front of you for reference as you
read through this section.
Configuring a LoadTimeWeaver for a particular ApplicationContext can be as easy as adding one
line. (Please note that you almost certainly will need to be using an ApplicationContext as your
Spring container - typically a BeanFactory will not be enough because the LTW support makes use
of BeanFactoryPostProcessors.)
To enable the Spring Framework’s LTW support, you need to configure a LoadTimeWeaver, which
typically is done using the @EnableLoadTimeWeaving annotation.
@Configuration
@EnableLoadTimeWeaving
public class AppConfig {
}
Alternatively, if you prefer XML based configuration, use the <context:load-time-weaver/>
element. Note that the element is defined in the 'context' namespace.
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<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<context:load-time-weaver/>
</beans>
The above configuration will define and register a number of LTW-specific infrastructure beans for
you automatically, such as a LoadTimeWeaver and an AspectJWeavingEnabler. The default
LoadTimeWeaver is the DefaultContextLoadTimeWeaver class, which attempts to decorate
an automatically detected LoadTimeWeaver: the exact type of LoadTimeWeaver that will be
'automatically detected' is dependent upon your runtime environment (summarized in the following
table).
Table 10.1. DefaultContextLoadTimeWeaver LoadTimeWeavers
Runtime Environment
LoadTimeWeaver implementation
Running in Oracle’s WebLogic
WebLogicLoadTimeWeaver
Running in Oracle’s GlassFish
GlassFishLoadTimeWeaver
Running in Apache Tomcat
TomcatLoadTimeWeaver
Running in Red Hat’s JBoss AS or WildFly
JBossLoadTimeWeaver
Running in IBM’s WebSphere
WebSphereLoadTimeWeaver
JVM started with Spring
InstrumentationSavingAgent (java javaagent:path/to/spring-instrument.jar)
InstrumentationLoadTimeWeaver
Fallback, expecting the underlying ClassLoader
ReflectiveLoadTimeWeaver
to follow common conventions (e.g. applicable to
TomcatInstrumentableClassLoader and
Resin)
Note that these are just the LoadTimeWeavers that are autodetected when using
the DefaultContextLoadTimeWeaver: it is of course possible to specify exactly which
LoadTimeWeaver implementation that you wish to use.
To specify a specific LoadTimeWeaver with Java configuration implement
LoadTimeWeavingConfigurer interface and override the getLoadTimeWeaver() method:
the
@Configuration
@EnableLoadTimeWeaving
public class AppConfig implements LoadTimeWeavingConfigurer {
@Override
public LoadTimeWeaver getLoadTimeWeaver() {
return new ReflectiveLoadTimeWeaver();
}
}
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If you are using XML based configuration you can specify the fully-qualified classname as the value of
the 'weaver-class' attribute on the <context:load-time-weaver/> element:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<context:load-time-weaver
weaver-class="org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver"/>
</beans>
The LoadTimeWeaver that is defined and registered by the configuration can be later retrieved
from the Spring container using the well-known name 'loadTimeWeaver'. Remember that the
LoadTimeWeaver exists just as a mechanism for Spring’s LTW infrastructure to add one or
more ClassFileTransformers. The actual ClassFileTransformer that does the LTW is the
ClassPreProcessorAgentAdapter (from the org.aspectj.weaver.loadtime package) class.
See the class-level javadocs of the ClassPreProcessorAgentAdapter class for further details,
because the specifics of how the weaving is actually effected is beyond the scope of this section.
There is one final attribute of the configuration left to discuss: the 'aspectjWeaving' attribute (or 'aspectjweaving' if you are using XML). This is a simple attribute that controls whether LTW is enabled or not; it
is as simple as that. It accepts one of three possible values, summarized below, with the default value
being 'autodetect' if the attribute is not present.
Table 10.2. AspectJ weaving attribute values
Annotation Value
XML Value
Explanation
ENABLED
on
AspectJ weaving is on, and
aspects will be woven at loadtime as appropriate.
DISABLED
off
LTW is off… no aspect will be
woven at load-time.
AUTODETECT
autodetect
If the Spring LTW infrastructure
can find at least one 'METAINF/aop.xml' file, then AspectJ
weaving is on, else it is off. This
is the default value.
Environment-specific configuration
This last section contains any additional settings and configuration that you will need when using
Spring’s LTW support in environments such as application servers and web containers.
Tomcat
Historically, Apache Tomcat's default class loader did not support class transformation which
is why Spring provides an enhanced implementation that addresses this need. Named
TomcatInstrumentableClassLoader, the loader works on Tomcat 6.0 and above.
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Tip
Do not define TomcatInstrumentableClassLoader anymore on Tomcat 8.0 and higher.
Instead, let Spring automatically use Tomcat’s new native InstrumentableClassLoader
facility through the TomcatLoadTimeWeaver strategy.
If you still need to use TomcatInstrumentableClassLoader, it can be registered individually for
each web application as follows:
• Copy org.springframework.instrument.tomcat.jar into $CATALINA_HOME/lib, where
$CATALINA_HOME represents the root of the Tomcat installation)
• Instruct Tomcat to use the custom class loader (instead of the default) by editing the web application
context file:
<Context path="/myWebApp" docBase="/my/webApp/location">
<Loader
loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader"/>
</Context>
Apache Tomcat (6.0+) supports several context locations:
• server configuration file - $CATALINA_HOME/conf/server.xml
• default context configuration - $CATALINA_HOME/conf/context.xml - that affects all deployed web
applications
• per-web application configuration which can be deployed either on the server-side at
$CATALINA_HOME/conf/[enginename]/[hostname]/[webapp]-context.xml or embedded inside the
web-app archive at META-INF/context.xml
For efficiency, the embedded per-web-app configuration style is recommended because it will impact
only applications that use the custom class loader and does not require any changes to the server
configuration. See the Tomcat 6.0.x documentation for more details about available context locations.
Alternatively, consider the use of the Spring-provided generic VM agent, to be specified in Tomcat’s
launch script (see above). This will make instrumentation available to all deployed web applications, no
matter what ClassLoader they happen to run on.
WebLogic, WebSphere, Resin, GlassFish, JBoss
Recent versions of WebLogic Server (version 10 and above), IBM WebSphere Application Server
(version 7 and above), Resin (3.1 and above) and JBoss (6.x or above) provide a ClassLoader
that is capable of local instrumentation. Spring’s native LTW leverages such ClassLoaders to enable
AspectJ weaving. You can enable LTW by simply activating load-time weaving as described earlier.
Specifically, you do not need to modify the launch script to add -javaagent:path/to/springinstrument.jar.
Note that GlassFish instrumentation-capable ClassLoader is available only in its EAR environment. For
GlassFish web applications, follow the Tomcat setup instructions as outlined above.
Note that on JBoss 6.x, the app server scanning needs to be disabled to prevent it from loading the
classes before the application actually starts. A quick workaround is to add to your artifact a file named
WEB-INF/jboss-scanning.xml with the following content:
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<scanning xmlns="urn:jboss:scanning:1.0"/>
Generic Java applications
When class instrumentation is required in environments that do not support or are not supported
by the existing LoadTimeWeaver implementations, a JDK agent can be the only solution. For
such cases, Spring provides InstrumentationLoadTimeWeaver, which requires a Spring-specific
(but very general) VM agent, org.springframework.instrument-{version}.jar (previously
named spring-agent.jar).
To use it, you must start the virtual machine with the Spring agent, by supplying the following JVM
options:
-javaagent:/path/to/org.springframework.instrument-{version}.jar
Note that this requires modification of the VM launch script which may prevent you from using this in
application server environments (depending on your operation policies). Additionally, the JDK agent will
instrument the entire VM which can prove expensive.
For performance reasons, it is recommended to use this configuration only if your target environment
(such as Jetty) does not have (or does not support) a dedicated LTW.
10.9 Further Resources
More information on AspectJ can be found on the AspectJ website.
The book Eclipse AspectJ by Adrian Colyer et. al. (Addison-Wesley, 2005) provides a comprehensive
introduction and reference for the AspectJ language.
The book AspectJ in Action, Second Edition by Ramnivas Laddad (Manning, 2009) comes highly
recommended; the focus of the book is on AspectJ, but a lot of general AOP themes are explored (in
some depth).
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11. Spring AOP APIs
11.1 Introduction
The previous chapter described the Spring’s support for AOP using @AspectJ and schema-based
aspect definitions. In this chapter we discuss the lower-level Spring AOP APIs and the AOP support
used in Spring 1.2 applications. For new applications, we recommend the use of the Spring 2.0 and later
AOP support described in the previous chapter, but when working with existing applications, or when
reading books and articles, you may come across Spring 1.2 style examples. Spring 4.0 is backwards
compatible with Spring 1.2 and everything described in this chapter is fully supported in Spring 4.0.
11.2 Pointcut API in Spring
Let’s look at how Spring handles the crucial pointcut concept.
Concepts
Spring’s pointcut model enables pointcut reuse independent of advice types. It’s possible to target
different advice using the same pointcut.
The org.springframework.aop.Pointcut interface is the central interface, used to target advices
to particular classes and methods. The complete interface is shown below:
public interface Pointcut {
ClassFilter getClassFilter();
MethodMatcher getMethodMatcher();
}
Splitting the Pointcut interface into two parts allows reuse of class and method matching parts, and
fine-grained composition operations (such as performing a "union" with another method matcher).
The ClassFilter interface is used to restrict the pointcut to a given set of target classes. If the
matches() method always returns true, all target classes will be matched:
public interface ClassFilter {
boolean matches(Class clazz);
}
The MethodMatcher interface is normally more important. The complete interface is shown below:
public interface MethodMatcher {
boolean matches(Method m, Class targetClass);
boolean isRuntime();
boolean matches(Method m, Class targetClass, Object[] args);
}
The matches(Method, Class) method is used to test whether this pointcut will ever match a given
method on a target class. This evaluation can be performed when an AOP proxy is created, to avoid the
need for a test on every method invocation. If the 2-argument matches method returns true for a given
method, and the isRuntime() method for the MethodMatcher returns true, the 3-argument matches
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method will be invoked on every method invocation. This enables a pointcut to look at the arguments
passed to the method invocation immediately before the target advice is to execute.
Most MethodMatchers are static, meaning that their isRuntime() method returns false. In this case,
the 3-argument matches method will never be invoked.
Tip
If possible, try to make pointcuts static, allowing the AOP framework to cache the results of pointcut
evaluation when an AOP proxy is created.
Operations on pointcuts
Spring supports operations on pointcuts: notably, union and intersection.
• Union means the methods that either pointcut matches.
• Intersection means the methods that both pointcuts match.
• Union is usually more useful.
• Pointcuts
can
be
composed
using
the
static
methods
in
the
org.springframework.aop.support.Pointcuts class, or using the ComposablePointcut class in the same
package. However, using AspectJ pointcut expressions is usually a simpler approach.
AspectJ expression pointcuts
Since
2.0,
the
most
important
type
of
pointcut
used
by
Spring
is
org.springframework.aop.aspectj.AspectJExpressionPointcut. This is a pointcut that
uses an AspectJ supplied library to parse an AspectJ pointcut expression string.
See the previous chapter for a discussion of supported AspectJ pointcut primitives.
Convenience pointcut implementations
Spring provides several convenient pointcut implementations. Some can be used out of the box; others
are intended to be subclassed in application-specific pointcuts.
Static pointcuts
Static pointcuts are based on method and target class, and cannot take into account the method’s
arguments. Static pointcuts are sufficient - and best - for most usages. It’s possible for Spring to evaluate
a static pointcut only once, when a method is first invoked: after that, there is no need to evaluate the
pointcut again with each method invocation.
Let’s consider some static pointcut implementations included with Spring.
Regular expression pointcuts
One obvious way to specify static pointcuts is regular expressions. Several AOP frameworks besides
Spring make this possible. org.springframework.aop.support.JdkRegexpMethodPointcut
is a generic regular expression pointcut, using the regular expression support in JDK 1.4+.
Using the JdkRegexpMethodPointcut class, you can provide a list of pattern Strings. If any of these
is a match, the pointcut will evaluate to true. (So the result is effectively the union of these pointcuts.)
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The usage is shown below:
<bean id="settersAndAbsquatulatePointcut"
class="org.springframework.aop.support.JdkRegexpMethodPointcut">
<property name="patterns">
<list>
<value>.*set.*</value>
<value>.*absquatulate</value>
</list>
</property>
</bean>
Spring provides a convenience class, RegexpMethodPointcutAdvisor, that allows us to
also reference an Advice (remember that an Advice can be an interceptor, before advice,
throws advice etc.). Behind the scenes, Spring will use a JdkRegexpMethodPointcut. Using
RegexpMethodPointcutAdvisor simplifies wiring, as the one bean encapsulates both pointcut and
advice, as shown below:
<bean id="settersAndAbsquatulateAdvisor"
class="org.springframework.aop.support.RegexpMethodPointcutAdvisor">
<property name="advice">
<ref bean="beanNameOfAopAllianceInterceptor"/>
</property>
<property name="patterns">
<list>
<value>.*set.*</value>
<value>.*absquatulate</value>
</list>
</property>
</bean>
RegexpMethodPointcutAdvisor can be used with any Advice type.
Attribute-driven pointcuts
An important type of static pointcut is a metadata-driven pointcut. This uses the values of metadata
attributes: typically, source-level metadata.
Dynamic pointcuts
Dynamic pointcuts are costlier to evaluate than static pointcuts. They take into account method
arguments, as well as static information. This means that they must be evaluated with every method
invocation; the result cannot be cached, as arguments will vary.
The main example is the control flow pointcut.
Control flow pointcuts
Spring control flow pointcuts are conceptually similar to AspectJ cflow pointcuts, although less
powerful. (There is currently no way to specify that a pointcut executes below a join point
matched by another pointcut.) A control flow pointcut matches the current call stack. For
example, it might fire if the join point was invoked by a method in the com.mycompany.web
package, or by the SomeCaller class. Control flow pointcuts are specified using the
org.springframework.aop.support.ControlFlowPointcut class.
Note
Control flow pointcuts are significantly more expensive to evaluate at runtime than even other
dynamic pointcuts. In Java 1.4, the cost is about 5 times that of other dynamic pointcuts.
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Pointcut superclasses
Spring provides useful pointcut superclasses to help you to implement your own pointcuts.
Because static pointcuts are most useful, you’ll probably subclass StaticMethodMatcherPointcut, as
shown below. This requires implementing just one abstract method (although it’s possible to override
other methods to customize behavior):
class TestStaticPointcut extends StaticMethodMatcherPointcut {
public boolean matches(Method m, Class targetClass) {
// return true if custom criteria match
}
}
There are also superclasses for dynamic pointcuts.
You can use custom pointcuts with any advice type in Spring 1.0 RC2 and above.
Custom pointcuts
Because pointcuts in Spring AOP are Java classes, rather than language features (as in AspectJ)
it’s possible to declare custom pointcuts, whether static or dynamic. Custom pointcuts in Spring can
be arbitrarily complex. However, using the AspectJ pointcut expression language is recommended if
possible.
Note
Later versions of Spring may offer support for "semantic pointcuts" as offered by JAC: for example,
"all methods that change instance variables in the target object."
11.3 Advice API in Spring
Let’s now look at how Spring AOP handles advice.
Advice lifecycles
Each advice is a Spring bean. An advice instance can be shared across all advised objects, or unique
to each advised object. This corresponds to per-class or per-instance advice.
Per-class advice is used most often. It is appropriate for generic advice such as transaction advisors.
These do not depend on the state of the proxied object or add new state; they merely act on the method
and arguments.
Per-instance advice is appropriate for introductions, to support mixins. In this case, the advice adds
state to the proxied object.
It’s possible to use a mix of shared and per-instance advice in the same AOP proxy.
Advice types in Spring
Spring provides several advice types out of the box, and is extensible to support arbitrary advice types.
Let us look at the basic concepts and standard advice types.
Interception around advice
The most fundamental advice type in Spring is interception around advice.
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Spring is compliant with the AOP Alliance interface for around advice using method interception.
MethodInterceptors implementing around advice should implement the following interface:
public interface MethodInterceptor extends Interceptor {
Object invoke(MethodInvocation invocation) throws Throwable;
}
The MethodInvocation argument to the invoke() method exposes the method being invoked; the
target join point; the AOP proxy; and the arguments to the method. The invoke() method should return
the invocation’s result: the return value of the join point.
A simple MethodInterceptor implementation looks as follows:
public class DebugInterceptor implements MethodInterceptor {
public Object invoke(MethodInvocation invocation) throws Throwable {
System.out.println("Before: invocation=[" + invocation + "]");
Object rval = invocation.proceed();
System.out.println("Invocation returned");
return rval;
}
}
Note the call to the MethodInvocation’s proceed() method. This proceeds down the interceptor chain
towards the join point. Most interceptors will invoke this method, and return its return value. However,
a MethodInterceptor, like any around advice, can return a different value or throw an exception rather
than invoke the proceed method. However, you don’t want to do this without good reason!
Note
MethodInterceptors offer interoperability with other AOP Alliance-compliant AOP
implementations. The other advice types discussed in the remainder of this section implement
common AOP concepts, but in a Spring-specific way. While there is an advantage in using the
most specific advice type, stick with MethodInterceptor around advice if you are likely to want
to run the aspect in another AOP framework. Note that pointcuts are not currently interoperable
between frameworks, and the AOP Alliance does not currently define pointcut interfaces.
Before advice
A simpler advice type is a before advice. This does not need a MethodInvocation object, since it will
only be called before entering the method.
The main advantage of a before advice is that there is no need to invoke the proceed() method, and
therefore no possibility of inadvertently failing to proceed down the interceptor chain.
The MethodBeforeAdvice interface is shown below. (Spring’s API design would allow for field before
advice, although the usual objects apply to field interception and it’s unlikely that Spring will ever
implement it).
public interface MethodBeforeAdvice extends BeforeAdvice {
void before(Method m, Object[] args, Object target) throws Throwable;
}
Note the return type is void. Before advice can insert custom behavior before the join point executes, but
cannot change the return value. If a before advice throws an exception, this will abort further execution
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of the interceptor chain. The exception will propagate back up the interceptor chain. If it is unchecked,
or on the signature of the invoked method, it will be passed directly to the client; otherwise it will be
wrapped in an unchecked exception by the AOP proxy.
An example of a before advice in Spring, which counts all method invocations:
public class CountingBeforeAdvice implements MethodBeforeAdvice {
private int count;
public void before(Method m, Object[] args, Object target) throws Throwable {
++count;
}
public int getCount() {
return count;
}
}
Tip
Before advice can be used with any pointcut.
Throws advice
Throws advice is invoked after the return of the join point if the join point threw an exception. Spring offers
typed throws advice. Note that this means that the org.springframework.aop.ThrowsAdvice
interface does not contain any methods: It is a tag interface identifying that the given object implements
one or more typed throws advice methods. These should be in the form of:
afterThrowing([Method, args, target], subclassOfThrowable)
Only the last argument is required. The method signatures may have either one or four arguments,
depending on whether the advice method is interested in the method and arguments. The following
classes are examples of throws advice.
The advice below is invoked if a RemoteException is thrown (including subclasses):
public class RemoteThrowsAdvice implements ThrowsAdvice {
public void afterThrowing(RemoteException ex) throws Throwable {
// Do something with remote exception
}
}
The following advice is invoked if a ServletException is thrown. Unlike the above advice, it declares
4 arguments, so that it has access to the invoked method, method arguments and target object:
public class ServletThrowsAdviceWithArguments implements ThrowsAdvice {
public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) {
// Do something with all arguments
}
}
The final example illustrates how these two methods could be used in a single class, which handles
both RemoteException and ServletException. Any number of throws advice methods can be
combined in a single class.
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public static class CombinedThrowsAdvice implements ThrowsAdvice {
public void afterThrowing(RemoteException ex) throws Throwable {
// Do something with remote exception
}
public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) {
// Do something with all arguments
}
}
Note
If a throws-advice method throws an exception itself, it will override the original exception
(i.e. change the exception thrown to the user). The overriding exception will typically be a
RuntimeException; this is compatible with any method signature. However, if a throws-advice
method throws a checked exception, it will have to match the declared exceptions of the target
method and is hence to some degree coupled to specific target method signatures. Do not throw
an undeclared checked exception that is incompatible with the target method’s signature!
Tip
Throws advice can be used with any pointcut.
After Returning advice
An after returning advice in Spring must implement the org.springframework.aop.AfterReturningAdvice
interface, shown below:
public interface AfterReturningAdvice extends Advice {
void afterReturning(Object returnValue, Method m, Object[] args, Object target)
throws Throwable;
}
An after returning advice has access to the return value (which it cannot modify), invoked method,
methods arguments and target.
The following after returning advice counts all successful method invocations that have not thrown
exceptions:
public class CountingAfterReturningAdvice implements AfterReturningAdvice {
private int count;
public void afterReturning(Object returnValue, Method m, Object[] args, Object target)
throws Throwable {
++count;
}
public int getCount() {
return count;
}
}
This advice doesn’t change the execution path. If it throws an exception, this will be thrown up the
interceptor chain instead of the return value.
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Tip
After returning advice can be used with any pointcut.
Introduction advice
Spring treats introduction advice as a special kind of interception advice.
Introduction requires an IntroductionAdvisor,
implementing the following interface:
and
an
IntroductionInterceptor,
public interface IntroductionInterceptor extends MethodInterceptor {
boolean implementsInterface(Class intf);
}
The invoke() method inherited from the AOP Alliance MethodInterceptor interface must
implement the introduction: that is, if the invoked method is on an introduced interface, the introduction
interceptor is responsible for handling the method call - it cannot invoke proceed().
Introduction advice cannot be used with any pointcut, as it applies only at class, rather than method,
level. You can only use introduction advice with the IntroductionAdvisor, which has the following
methods:
public interface IntroductionAdvisor extends Advisor, IntroductionInfo {
ClassFilter getClassFilter();
void validateInterfaces() throws IllegalArgumentException;
}
public interface IntroductionInfo {
Class[] getInterfaces();
}
There is no MethodMatcher, and hence no Pointcut, associated with introduction advice. Only class
filtering is logical.
The getInterfaces() method returns the interfaces introduced by this advisor.
The validateInterfaces() method is used internally to see whether or not the introduced interfaces
can be implemented by the configured IntroductionInterceptor.
Let’s look at a simple example from the Spring test suite. Let’s suppose we want to introduce the
following interface to one or more objects:
public interface Lockable {
void lock();
void unlock();
boolean locked();
}
This illustrates a mixin. We want to be able to cast advised objects to Lockable, whatever their type,
and call lock and unlock methods. If we call the lock() method, we want all setter methods to throw a
LockedException. Thus we can add an aspect that provides the ability to make objects immutable,
without them having any knowledge of it: a good example of AOP.
Firstly, we’ll need an IntroductionInterceptor that does the heavy lifting. In this case, we
extend the org.springframework.aop.support.DelegatingIntroductionInterceptor
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convenience class. We could implement IntroductionInterceptor
DelegatingIntroductionInterceptor is best for most cases.
directly,
but
using
The DelegatingIntroductionInterceptor is designed to delegate an introduction to an actual
implementation of the introduced interface(s), concealing the use of interception to do so. The
delegate can be set to any object using a constructor argument; the default delegate (when the
no-arg constructor is used) is this. Thus in the example below, the delegate is the LockMixin
subclass of DelegatingIntroductionInterceptor. Given a delegate (by default itself), a
DelegatingIntroductionInterceptor instance looks for all interfaces implemented by the
delegate (other than IntroductionInterceptor), and will support introductions against any of them. It’s
possible for subclasses such as LockMixin to call the suppressInterface(Class intf) method
to suppress interfaces that should not be exposed. However, no matter how many interfaces an
IntroductionInterceptor is prepared to support, the IntroductionAdvisor used will control
which interfaces are actually exposed. An introduced interface will conceal any implementation of the
same interface by the target.
Thus LockMixin extends DelegatingIntroductionInterceptor and implements Lockable
itself. The superclass automatically picks up that Lockable can be supported for introduction, so we
don’t need to specify that. We could introduce any number of interfaces in this way.
Note the use of the locked instance variable. This effectively adds additional state to that held in the
target object.
public class LockMixin extends DelegatingIntroductionInterceptor implements Lockable {
private boolean locked;
public void lock() {
this.locked = true;
}
public void unlock() {
this.locked = false;
}
public boolean locked() {
return this.locked;
}
public Object invoke(MethodInvocation invocation) throws Throwable {
if (locked() && invocation.getMethod().getName().indexOf("set") == 0) {
throw new LockedException();
}
return super.invoke(invocation);
}
}
Often
it
isn’t
necessary
to
override
the
invoke()
method:
the
DelegatingIntroductionInterceptor implementation - which calls the delegate method if the
method is introduced, otherwise proceeds towards the join point - is usually sufficient. In the present
case, we need to add a check: no setter method can be invoked if in locked mode.
The introduction advisor required is simple. All it needs to do is hold a distinct LockMixin instance, and
specify the introduced interfaces - in this case, just Lockable. A more complex example might take a
reference to the introduction interceptor (which would be defined as a prototype): in this case, there’s
no configuration relevant for a LockMixin, so we simply create it using new.
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public class LockMixinAdvisor extends DefaultIntroductionAdvisor {
public LockMixinAdvisor() {
super(new LockMixin(), Lockable.class);
}
}
We can apply this advisor very simply: it requires no configuration. (However, it is necessary: It’s
impossible to use an IntroductionInterceptor without an IntroductionAdvisor.) As usual with
introductions, the advisor must be per-instance, as it is stateful. We need a different instance of
LockMixinAdvisor, and hence LockMixin, for each advised object. The advisor comprises part of
the advised object’s state.
We can apply this advisor programmatically, using the Advised.addAdvisor() method, or (the
recommended way) in XML configuration, like any other advisor. All proxy creation choices discussed
below, including "auto proxy creators," correctly handle introductions and stateful mixins.
11.4 Advisor API in Spring
In Spring, an Advisor is an aspect that contains just a single advice object associated with a pointcut
expression.
Apart from the special case of introductions, any advisor can be used with any advice.
org.springframework.aop.support.DefaultPointcutAdvisor is the most commonly used
advisor class. For example, it can be used with a MethodInterceptor, BeforeAdvice or
ThrowsAdvice.
It is possible to mix advisor and advice types in Spring in the same AOP proxy. For example, you could
use a interception around advice, throws advice and before advice in one proxy configuration: Spring
will automatically create the necessary interceptor chain.
11.5 Using the ProxyFactoryBean to create AOP proxies
If you’re using the Spring IoC container (an ApplicationContext or BeanFactory) for your business objects
- and you should be! - you will want to use one of Spring’s AOP FactoryBeans. (Remember that a factory
bean introduces a layer of indirection, enabling it to create objects of a different type.)
Note
The Spring AOP support also uses factory beans under the covers.
The
basic
way
to
create
an
AOP
proxy
in
Spring
is
to
use
the
org.springframework.aop.framework.ProxyFactoryBean. This gives complete control over the pointcuts
and advice that will apply, and their ordering. However, there are simpler options that are preferable if
you don’t need such control.
Basics
The ProxyFactoryBean, like other Spring FactoryBean implementations, introduces a level of
indirection. If you define a ProxyFactoryBean with name foo, what objects referencing foo see
is not the ProxyFactoryBean instance itself, but an object created by the ProxyFactoryBean’s
implementation of the `getObject() method. This method will create an AOP proxy wrapping
a target object.
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One of the most important benefits of using a ProxyFactoryBean or another IoC-aware class to create
AOP proxies, is that it means that advices and pointcuts can also be managed by IoC. This is a powerful
feature, enabling certain approaches that are hard to achieve with other AOP frameworks. For example,
an advice may itself reference application objects (besides the target, which should be available in any
AOP framework), benefiting from all the pluggability provided by Dependency Injection.
JavaBean properties
In common with most FactoryBean implementations provided with Spring, the ProxyFactoryBean
class is itself a JavaBean. Its properties are used to:
• Specify the target you want to proxy.
• Specify whether to use CGLIB (see below and also the section called “JDK- and CGLIB-based
proxies”).
Some key properties are inherited from org.springframework.aop.framework.ProxyConfig
(the superclass for all AOP proxy factories in Spring). These key properties include:
• proxyTargetClass: true if the target class is to be proxied, rather than the target class' interfaces.
If this property value is set to true, then CGLIB proxies will be created (but see also the section
called “JDK- and CGLIB-based proxies”).
• optimize: controls whether or not aggressive optimizations are applied to proxies created via CGLIB.
One should not blithely use this setting unless one fully understands how the relevant AOP proxy
handles optimization. This is currently used only for CGLIB proxies; it has no effect with JDK dynamic
proxies.
• frozen: if a proxy configuration is frozen, then changes to the configuration are no longer allowed.
This is useful both as a slight optimization and for those cases when you don’t want callers to be able
to manipulate the proxy (via the Advised interface) after the proxy has been created. The default
value of this property is false, so changes such as adding additional advice are allowed.
• exposeProxy: determines whether or not the current proxy should be exposed in a ThreadLocal
so that it can be accessed by the target. If a target needs to obtain the proxy and the exposeProxy
property is set to true, the target can use the AopContext.currentProxy() method.
Other properties specific to ProxyFactoryBean include:
• proxyInterfaces: array of String interface names. If this isn’t supplied, a CGLIB proxy for the target
class will be used (but see also the section called “JDK- and CGLIB-based proxies”).
• interceptorNames: String array of Advisor, interceptor or other advice names to apply. Ordering
is significant, on a first come-first served basis. That is to say that the first interceptor in the list will
be the first to be able to intercept the invocation.
The names are bean names in the current factory, including bean names from ancestor factories. You
can’t mention bean references here since doing so would result in the ProxyFactoryBean ignoring
the singleton setting of the advice.
You can append an interceptor name with an asterisk ( *). This will result in the application of all advisor
beans with names starting with the part before the asterisk to be applied. An example of using this
feature can be found in the section called “Using 'global' advisors”.
• singleton: whether or not the factory should return a single object, no matter how often the
getObject() method is called. Several FactoryBean implementations offer such a method. The
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default value is true. If you want to use stateful advice - for example, for stateful mixins - use prototype
advices along with a singleton value of false.
JDK- and CGLIB-based proxies
This section serves as the definitive documentation on how the ProxyFactoryBean chooses to create
one of either a JDK- and CGLIB-based proxy for a particular target object (that is to be proxied).
Note
The behavior of the ProxyFactoryBean with regard to creating JDK- or CGLIB-based
proxies changed between versions 1.2.x and 2.0 of Spring. The ProxyFactoryBean
now exhibits similar semantics with regard to auto-detecting interfaces as those of the
TransactionProxyFactoryBean class.
If the class of a target object that is to be proxied (hereafter simply referred to as the target class) doesn’t
implement any interfaces, then a CGLIB-based proxy will be created. This is the easiest scenario,
because JDK proxies are interface based, and no interfaces means JDK proxying isn’t even possible.
One simply plugs in the target bean, and specifies the list of interceptors via the interceptorNames
property. Note that a CGLIB-based proxy will be created even if the proxyTargetClass property of
the ProxyFactoryBean has been set to false. (Obviously this makes no sense, and is best removed
from the bean definition because it is at best redundant, and at worst confusing.)
If the target class implements one (or more) interfaces, then the type of proxy that is created depends
on the configuration of the ProxyFactoryBean.
If the proxyTargetClass property of the ProxyFactoryBean has been set to true, then a CGLIBbased proxy will be created. This makes sense, and is in keeping with the principle of least surprise.
Even if the proxyInterfaces property of the ProxyFactoryBean has been set to one or more fully
qualified interface names, the fact that the proxyTargetClass property is set to true will cause
CGLIB-based proxying to be in effect.
If the proxyInterfaces property of the ProxyFactoryBean has been set to one or more fully
qualified interface names, then a JDK-based proxy will be created. The created proxy will implement all
of the interfaces that were specified in the proxyInterfaces property; if the target class happens to
implement a whole lot more interfaces than those specified in the proxyInterfaces property, that is
all well and good but those additional interfaces will not be implemented by the returned proxy.
If the proxyInterfaces property of the ProxyFactoryBean has not been set, but the target class
does implement one (or more) interfaces, then the ProxyFactoryBean will auto-detect the fact that the
target class does actually implement at least one interface, and a JDK-based proxy will be created. The
interfaces that are actually proxied will be all of the interfaces that the target class implements; in effect,
this is the same as simply supplying a list of each and every interface that the target class implements
to the proxyInterfaces property. However, it is significantly less work, and less prone to typos.
Proxying interfaces
Let’s look at a simple example of ProxyFactoryBean in action. This example involves:
• A target bean that will be proxied. This is the "personTarget" bean definition in the example below.
• An Advisor and an Interceptor used to provide advice.
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• An AOP proxy bean definition specifying the target object (the personTarget bean) and the interfaces
to proxy, along with the advices to apply.
<bean id="personTarget" class="com.mycompany.PersonImpl">
<property name="name" value="Tony"/>
<property name="age" value="51"/>
</bean>
<bean id="myAdvisor" class="com.mycompany.MyAdvisor">
<property name="someProperty" value="Custom string property value"/>
</bean>
<bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor">
</bean>
<bean id="person"
class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="proxyInterfaces" value="com.mycompany.Person"/>
<property name="target" ref="personTarget"/>
<property name="interceptorNames">
<list>
<value>myAdvisor</value>
<value>debugInterceptor</value>
</list>
</property>
</bean>
Note that the interceptorNames property takes a list of String: the bean names of the interceptor or
advisors in the current factory. Advisors, interceptors, before, after returning and throws advice objects
can be used. The ordering of advisors is significant.
Note
You might be wondering why the list doesn’t hold bean references. The reason for this is that if
the ProxyFactoryBean’s singleton property is set to false, it must be able to return independent
proxy instances. If any of the advisors is itself a prototype, an independent instance would need
to be returned, so it’s necessary to be able to obtain an instance of the prototype from the factory;
holding a reference isn’t sufficient.
The "person" bean definition above can be used in place of a Person implementation, as follows:
Person person = (Person) factory.getBean("person");
Other beans in the same IoC context can express a strongly typed dependency on it, as with an ordinary
Java object:
<bean id="personUser" class="com.mycompany.PersonUser">
<property name="person"><ref bean="person"/></property>
</bean>
The PersonUser class in this example would expose a property of type Person. As far as it’s concerned,
the AOP proxy can be used transparently in place of a "real" person implementation. However, its class
would be a dynamic proxy class. It would be possible to cast it to the Advised interface (discussed
below).
It’s possible to conceal the distinction between target and proxy using an anonymous inner bean,
as follows. Only the ProxyFactoryBean definition is different; the advice is included only for
completeness:
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<bean id="myAdvisor" class="com.mycompany.MyAdvisor">
<property name="someProperty" value="Custom string property value"/>
</bean>
<bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"/>
<bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="proxyInterfaces" value="com.mycompany.Person"/>
<!-- Use inner bean, not local reference to target -->
<property name="target">
<bean class="com.mycompany.PersonImpl">
<property name="name" value="Tony"/>
<property name="age" value="51"/>
</bean>
</property>
<property name="interceptorNames">
<list>
<value>myAdvisor</value>
<value>debugInterceptor</value>
</list>
</property>
</bean>
This has the advantage that there’s only one object of type Person: useful if we want to prevent users
of the application context from obtaining a reference to the un-advised object, or need to avoid any
ambiguity with Spring IoC autowiring. There’s also arguably an advantage in that the ProxyFactoryBean
definition is self-contained. However, there are times when being able to obtain the un-advised target
from the factory might actually be an advantage: for example, in certain test scenarios.
Proxying classes
What if you need to proxy a class, rather than one or more interfaces?
Imagine that in our example above, there was no Person interface: we needed to advise a class called
Person that didn’t implement any business interface. In this case, you can configure Spring to use
CGLIB proxying, rather than dynamic proxies. Simply set the proxyTargetClass property on the
ProxyFactoryBean above to true. While it’s best to program to interfaces, rather than classes, the ability
to advise classes that don’t implement interfaces can be useful when working with legacy code. (In
general, Spring isn’t prescriptive. While it makes it easy to apply good practices, it avoids forcing a
particular approach.)
If you want to, you can force the use of CGLIB in any case, even if you do have interfaces.
CGLIB proxying works by generating a subclass of the target class at runtime. Spring configures this
generated subclass to delegate method calls to the original target: the subclass is used to implement
the Decorator pattern, weaving in the advice.
CGLIB proxying should generally be transparent to users. However, there are some issues to consider:
• Final methods can’t be advised, as they can’t be overridden.
• There is no need to add CGLIB to your classpath. As of Spring 3.2, CGLIB is repackaged and included
in the spring-core JAR. In other words, CGLIB-based AOP will work "out of the box" just as do JDK
dynamic proxies.
There’s little performance difference between CGLIB proxying and dynamic proxies. As of Spring 1.0,
dynamic proxies are slightly faster. However, this may change in the future. Performance should not be
a decisive consideration in this case.
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Using 'global' advisors
By appending an asterisk to an interceptor name, all advisors with bean names matching the part before
the asterisk, will be added to the advisor chain. This can come in handy if you need to add a standard
set of 'global' advisors:
<bean id="proxy" class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="target" ref="service"/>
<property name="interceptorNames">
<list>
<value>global*</value>
</list>
</property>
</bean>
<bean id="global_debug" class="org.springframework.aop.interceptor.DebugInterceptor"/>
<bean id="global_performance" class="org.springframework.aop.interceptor.PerformanceMonitorInterceptor"/
>
11.6 Concise proxy definitions
Especially when defining transactional proxies, you may end up with many similar proxy definitions. The
use of parent and child bean definitions, along with inner bean definitions, can result in much cleaner
and more concise proxy definitions.
First a parent, template, bean definition is created for the proxy:
<bean id="txProxyTemplate" abstract="true"
class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean">
<property name="transactionManager" ref="transactionManager"/>
<property name="transactionAttributes">
<props>
<prop key="*">PROPAGATION_REQUIRED</prop>
</props>
</property>
</bean>
This will never be instantiated itself, so may actually be incomplete. Then each proxy which needs to be
created is just a child bean definition, which wraps the target of the proxy as an inner bean definition,
since the target will never be used on its own anyway.
<bean id="myService" parent="txProxyTemplate">
<property name="target">
<bean class="org.springframework.samples.MyServiceImpl">
</bean>
</property>
</bean>
It is of course possible to override properties from the parent template, such as in this case, the
transaction propagation settings:
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<bean id="mySpecialService" parent="txProxyTemplate">
<property name="target">
<bean class="org.springframework.samples.MySpecialServiceImpl">
</bean>
</property>
<property name="transactionAttributes">
<props>
<prop key="get*">PROPAGATION_REQUIRED,readOnly</prop>
<prop key="find*">PROPAGATION_REQUIRED,readOnly</prop>
<prop key="load*">PROPAGATION_REQUIRED,readOnly</prop>
<prop key="store*">PROPAGATION_REQUIRED</prop>
</props>
</property>
</bean>
Note that in the example above, we have explicitly marked the parent bean definition as abstract by
using the abstract attribute, as described previously, so that it may not actually ever be instantiated.
Application contexts (but not simple bean factories) will by default pre-instantiate all singletons. It is
therefore important (at least for singleton beans) that if you have a (parent) bean definition which you
intend to use only as a template, and this definition specifies a class, you must make sure to set the
abstract attribute to true, otherwise the application context will actually try to pre-instantiate it.
11.7 Creating AOP proxies programmatically with the
ProxyFactory
It’s easy to create AOP proxies programmatically using Spring. This enables you to use Spring AOP
without dependency on Spring IoC.
The following listing shows creation of a proxy for a target object, with one interceptor and one advisor.
The interfaces implemented by the target object will automatically be proxied:
ProxyFactory factory = new ProxyFactory(myBusinessInterfaceImpl);
factory.addAdvice(myMethodInterceptor);
factory.addAdvisor(myAdvisor);
MyBusinessInterface tb = (MyBusinessInterface) factory.getProxy();
The
first
step
is
to
construct
an
object
of
type
org.springframework.aop.framework.ProxyFactory. You can create this with a target object,
as in the above example, or specify the interfaces to be proxied in an alternate constructor.
You can add advices (with interceptors as a specialized kind of advice) and/or advisors, and manipulate
them for the life of the ProxyFactory. If you add an IntroductionInterceptionAroundAdvisor, you can
cause the proxy to implement additional interfaces.
There are also convenience methods on ProxyFactory (inherited from AdvisedSupport) which allow
you to add other advice types such as before and throws advice. AdvisedSupport is the superclass of
both ProxyFactory and ProxyFactoryBean.
Tip
Integrating AOP proxy creation with the IoC framework is best practice in most applications. We
recommend that you externalize configuration from Java code with AOP, as in general.
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11.8 Manipulating advised objects
However
you
create
AOP
proxies,
you
can
manipulate
them
using
the
org.springframework.aop.framework.Advised interface. Any AOP proxy can be cast to this
interface, whichever other interfaces it implements. This interface includes the following methods:
Advisor[] getAdvisors();
void addAdvice(Advice advice) throws AopConfigException;
void addAdvice(int pos, Advice advice) throws AopConfigException;
void addAdvisor(Advisor advisor) throws AopConfigException;
void addAdvisor(int pos, Advisor advisor) throws AopConfigException;
int indexOf(Advisor advisor);
boolean removeAdvisor(Advisor advisor) throws AopConfigException;
void removeAdvisor(int index) throws AopConfigException;
boolean replaceAdvisor(Advisor a, Advisor b) throws AopConfigException;
boolean isFrozen();
The getAdvisors() method will return an Advisor for every advisor, interceptor or other advice
type that has been added to the factory. If you added an Advisor, the returned advisor at this
index will be the object that you added. If you added an interceptor or other advice type, Spring
will have wrapped this in an advisor with a pointcut that always returns true. Thus if you added a
MethodInterceptor, the advisor returned for this index will be an DefaultPointcutAdvisor
returning your MethodInterceptor and a pointcut that matches all classes and methods.
The addAdvisor() methods can be used to add any Advisor. Usually the advisor holding pointcut and
advice will be the generic DefaultPointcutAdvisor, which can be used with any advice or pointcut
(but not for introductions).
By default, it’s possible to add or remove advisors or interceptors even once a proxy has been created.
The only restriction is that it’s impossible to add or remove an introduction advisor, as existing proxies
from the factory will not show the interface change. (You can obtain a new proxy from the factory to
avoid this problem.)
A simple example of casting an AOP proxy to the Advised interface and examining and manipulating
its advice:
Advised advised = (Advised) myObject;
Advisor[] advisors = advised.getAdvisors();
int oldAdvisorCount = advisors.length;
System.out.println(oldAdvisorCount + " advisors");
// Add an advice like an interceptor without a pointcut
// Will match all proxied methods
// Can use for interceptors, before, after returning or throws advice
advised.addAdvice(new DebugInterceptor());
// Add selective advice using a pointcut
advised.addAdvisor(new DefaultPointcutAdvisor(mySpecialPointcut, myAdvice));
assertEquals("Added two advisors", oldAdvisorCount + 2, advised.getAdvisors().length);
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Note
It’s questionable whether it’s advisable (no pun intended) to modify advice on a business object
in production, although there are no doubt legitimate usage cases. However, it can be very useful
in development: for example, in tests. I have sometimes found it very useful to be able to add test
code in the form of an interceptor or other advice, getting inside a method invocation I want to test.
(For example, the advice can get inside a transaction created for that method: for example, to run
SQL to check that a database was correctly updated, before marking the transaction for roll back.)
Depending on how you created the proxy, you can usually set a frozen flag, in which case the Advised
isFrozen() method will return true, and any attempts to modify advice through addition or removal
will result in an AopConfigException. The ability to freeze the state of an advised object is useful in
some cases, for example, to prevent calling code removing a security interceptor. It may also be used
in Spring 1.1 to allow aggressive optimization if runtime advice modification is known not to be required.
11.9 Using the "auto-proxy" facility
So far we’ve considered explicit creation of AOP proxies using a ProxyFactoryBean or similar factory
bean.
Spring also allows us to use "auto-proxy" bean definitions, which can automatically proxy selected bean
definitions. This is built on Spring "bean post processor" infrastructure, which enables modification of
any bean definition as the container loads.
In this model, you set up some special bean definitions in your XML bean definition file to configure the
auto proxy infrastructure. This allows you just to declare the targets eligible for auto-proxying: you don’t
need to use ProxyFactoryBean.
There are two ways to do this:
• Using an auto-proxy creator that refers to specific beans in the current context.
• A special case of auto-proxy creation that deserves to be considered separately; auto-proxy creation
driven by source-level metadata attributes.
Autoproxy bean definitions
The org.springframework.aop.framework.autoproxy package provides the following
standard auto-proxy creators.
BeanNameAutoProxyCreator
The BeanNameAutoProxyCreator class is a BeanPostProcessor that automatically creates AOP
proxies for beans with names matching literal values or wildcards.
<bean class="org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator">
<property name="beanNames" value="jdk*,onlyJdk"/>
<property name="interceptorNames">
<list>
<value>myInterceptor</value>
</list>
</property>
</bean>
As with ProxyFactoryBean, there is an interceptorNames property rather than a list of
interceptors, to allow correct behavior for prototype advisors. Named "interceptors" can be advisors or
any advice type.
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As with auto proxying in general, the main point of using BeanNameAutoProxyCreator is to apply the
same configuration consistently to multiple objects, with minimal volume of configuration. It is a popular
choice for applying declarative transactions to multiple objects.
Bean definitions whose names match, such as "jdkMyBean" and "onlyJdk" in the above example, are
plain old bean definitions with the target class. An AOP proxy will be created automatically by the
BeanNameAutoProxyCreator. The same advice will be applied to all matching beans. Note that if
advisors are used (rather than the interceptor in the above example), the pointcuts may apply differently
to different beans.
DefaultAdvisorAutoProxyCreator
A more general and extremely powerful auto proxy creator is DefaultAdvisorAutoProxyCreator.
This will automagically apply eligible advisors in the current context, without the need to include
specific bean names in the auto-proxy advisor’s bean definition. It offers the same merit of consistent
configuration and avoidance of duplication as BeanNameAutoProxyCreator.
Using this mechanism involves:
• Specifying a DefaultAdvisorAutoProxyCreator bean definition.
• Specifying any number of Advisors in the same or related contexts. Note that these must be Advisors,
not just interceptors or other advices. This is necessary because there must be a pointcut to evaluate,
to check the eligibility of each advice to candidate bean definitions.
The DefaultAdvisorAutoProxyCreator will automatically evaluate the pointcut contained in each
advisor, to see what (if any) advice it should apply to each business object (such as "businessObject1"
and "businessObject2" in the example).
This means that any number of advisors can be applied automatically to each business object. If no
pointcut in any of the advisors matches any method in a business object, the object will not be proxied.
As bean definitions are added for new business objects, they will automatically be proxied if necessary.
Autoproxying in general has the advantage of making it impossible for callers or dependencies to obtain
an un-advised object. Calling getBean("businessObject1") on this ApplicationContext will return an AOP
proxy, not the target business object. (The "inner bean" idiom shown earlier also offers this benefit.)
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/>
<bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor">
<property name="transactionInterceptor" ref="transactionInterceptor"/>
</bean>
<bean id="customAdvisor" class="com.mycompany.MyAdvisor"/>
<bean id="businessObject1" class="com.mycompany.BusinessObject1">
<!-- Properties omitted -->
</bean>
<bean id="businessObject2" class="com.mycompany.BusinessObject2"/>
The DefaultAdvisorAutoProxyCreator is very useful if you want to apply the same advice
consistently to many business objects. Once the infrastructure definitions are in place, you can simply
add new business objects without including specific proxy configuration. You can also drop in additional
aspects very easily - for example, tracing or performance monitoring aspects - with minimal change to
configuration.
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The DefaultAdvisorAutoProxyCreator offers support for filtering (using a naming convention
so that only certain advisors are evaluated, allowing use of multiple, differently configured,
AdvisorAutoProxyCreators in the same factory) and ordering. Advisors can implement the
org.springframework.core.Ordered interface to ensure correct ordering if this is an issue. The
TransactionAttributeSourceAdvisor used in the above example has a configurable order value; the
default setting is unordered.
AbstractAdvisorAutoProxyCreator
This is the superclass of DefaultAdvisorAutoProxyCreator. You can create your own auto-proxy creators
by subclassing this class, in the unlikely event that advisor definitions offer insufficient customization to
the behavior of the framework DefaultAdvisorAutoProxyCreator.
Using metadata-driven auto-proxying
A particularly important type of auto-proxying is driven by metadata. This produces a similar
programming model to .NET ServicedComponents. Instead of defining metadata in XML descriptors,
configuration for transaction management and other enterprise services is held in source-level attributes.
In this case, you use the DefaultAdvisorAutoProxyCreator, in combination with Advisors that
understand metadata attributes. The metadata specifics are held in the pointcut part of the candidate
advisors, rather than in the auto-proxy creation class itself.
This is really a special case of the DefaultAdvisorAutoProxyCreator, but deserves consideration
on its own. (The metadata-aware code is in the pointcuts contained in the advisors, not the AOP
framework itself.)
The /attributes directory of the JPetStore sample application shows the use of attributedriven auto-proxying. In this case, there’s no need to use the TransactionProxyFactoryBean.
Simply defining transactional attributes on business objects is sufficient, because of the use
of metadata-aware pointcuts. The bean definitions include the following code, in /WEB-INF/
declarativeServices.xml. Note that this is generic, and can be used outside the JPetStore:
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/>
<bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor">
<property name="transactionInterceptor" ref="transactionInterceptor"/>
</bean>
<bean id="transactionInterceptor"
class="org.springframework.transaction.interceptor.TransactionInterceptor">
<property name="transactionManager" ref="transactionManager"/>
<property name="transactionAttributeSource">
<bean class="org.springframework.transaction.interceptor.AttributesTransactionAttributeSource">
<property name="attributes" ref="attributes"/>
</bean>
</property>
</bean>
<bean id="attributes" class="org.springframework.metadata.commons.CommonsAttributes"/>
The DefaultAdvisorAutoProxyCreator bean definition (the name is not significant, hence it
can even be omitted) will pick up all eligible pointcuts in the current application context. In this
case, the "transactionAdvisor" bean definition, of type TransactionAttributeSourceAdvisor, will
apply to classes or methods carrying a transaction attribute. The TransactionAttributeSourceAdvisor
depends on a TransactionInterceptor, via constructor dependency. The example resolves this via
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autowiring. The AttributesTransactionAttributeSource depends on an implementation of the
org.springframework.metadata.Attributes interface. In this fragment, the "attributes" bean
satisfies this, using the Jakarta Commons Attributes API to obtain attribute information. (The application
code must have been compiled using the Commons Attributes compilation task.)
The /annotation directory of the JPetStore sample application contains an analogous example for
auto-proxying driven by JDK 1.5+ annotations. The following configuration enables automatic detection
of Spring’s Transactional annotation, leading to implicit proxies for beans containing that annotation:
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/>
<bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor">
<property name="transactionInterceptor" ref="transactionInterceptor"/>
</bean>
<bean id="transactionInterceptor"
class="org.springframework.transaction.interceptor.TransactionInterceptor">
<property name="transactionManager" ref="transactionManager"/>
<property name="transactionAttributeSource">
<bean class="org.springframework.transaction.annotation.AnnotationTransactionAttributeSource"/>
</property>
</bean>
The TransactionInterceptor defined here depends on a PlatformTransactionManager
definition, which is not included in this generic file (although it could be) because it will be specific to the
application’s transaction requirements (typically JTA, as in this example, or Hibernate, JDO or JDBC):
<bean id="transactionManager"
class="org.springframework.transaction.jta.JtaTransactionManager"/>
Tip
If you require only declarative transaction management, using these generic XML definitions will
result in Spring automatically proxying all classes or methods with transaction attributes. You
won’t need to work directly with AOP, and the programming model is similar to that of .NET
ServicedComponents.
This mechanism is extensible. It’s possible to do auto-proxying based on custom attributes. You need to:
• Define your custom attribute.
• Specify an Advisor with the necessary advice, including a pointcut that is triggered by the presence
of the custom attribute on a class or method. You may be able to use an existing advice, merely
implementing a static pointcut that picks up the custom attribute.
It’s possible for such advisors to be unique to each advised class (for example, mixins): they simply
need to be defined as prototype, rather than singleton, bean definitions. For example, the LockMixin
introduction interceptor from the Spring test suite, shown above, could be used in conjunction with a
generic DefaultIntroductionAdvisor:
<bean id="lockMixin" class="test.mixin.LockMixin" scope="prototype"/>
<bean id="lockableAdvisor" class="org.springframework.aop.support.DefaultIntroductionAdvisor"
scope="prototype">
<constructor-arg ref="lockMixin"/>
</bean>
Note that both lockMixin and lockableAdvisor are defined as prototypes.
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11.10 Using TargetSources
Spring
offers
the
concept
of
a
TargetSource,
expressed
in
the
org.springframework.aop.TargetSource interface. This interface is responsible for returning
the "target object" implementing the join point. The TargetSource implementation is asked for a target
instance each time the AOP proxy handles a method invocation.
Developers using Spring AOP don’t normally need to work directly with TargetSources, but this provides
a powerful means of supporting pooling, hot swappable and other sophisticated targets. For example, a
pooling TargetSource can return a different target instance for each invocation, using a pool to manage
instances.
If you do not specify a TargetSource, a default implementation is used that wraps a local object. The
same target is returned for each invocation (as you would expect).
Let’s look at the standard target sources provided with Spring, and how you can use them.
Tip
When using a custom target source, your target will usually need to be a prototype rather than a
singleton bean definition. This allows Spring to create a new target instance when required.
Hot swappable target sources
The org.springframework.aop.target.HotSwappableTargetSource exists to allow the
target of an AOP proxy to be switched while allowing callers to keep their references to it.
Changing the target source’s target takes effect immediately. The HotSwappableTargetSource is
threadsafe.
You can change the target via the swap() method on HotSwappableTargetSource as follows:
HotSwappableTargetSource swapper = (HotSwappableTargetSource) beanFactory.getBean("swapper");
Object oldTarget = swapper.swap(newTarget);
The XML definitions required look as follows:
<bean id="initialTarget" class="mycompany.OldTarget"/>
<bean id="swapper" class="org.springframework.aop.target.HotSwappableTargetSource">
<constructor-arg ref="initialTarget"/>
</bean>
<bean id="swappable" class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="targetSource" ref="swapper"/>
</bean>
The above swap() call changes the target of the swappable bean. Clients who hold a reference to that
bean will be unaware of the change, but will immediately start hitting the new target.
Although this example doesn’t add any advice - and it’s not necessary to add advice to use a
TargetSource - of course any TargetSource can be used in conjunction with arbitrary advice.
Pooling target sources
Using a pooling target source provides a similar programming model to stateless session EJBs, in which
a pool of identical instances is maintained, with method invocations going to free objects in the pool.
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A crucial difference between Spring pooling and SLSB pooling is that Spring pooling can be applied to
any POJO. As with Spring in general, this service can be applied in a non-invasive way.
Spring provides out-of-the-box support for Commons Pool 2.2, which provides
a fairly efficient pooling implementation. You’ll need the commons-pool Jar on
your application’s classpath to use this feature. It’s also possible to subclass
org.springframework.aop.target.AbstractPoolingTargetSource to support any other
pooling API.
Note
Commons Pool 1.5+ is also supported but deprecated as of Spring Framework 4.2.
Sample configuration is shown below:
<bean id="businessObjectTarget" class="com.mycompany.MyBusinessObject"
scope="prototype">
... properties omitted
</bean>
<bean id="poolTargetSource" class="org.springframework.aop.target.CommonsPool2TargetSource">
<property name="targetBeanName" value="businessObjectTarget"/>
<property name="maxSize" value="25"/>
</bean>
<bean id="businessObject" class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="targetSource" ref="poolTargetSource"/>
<property name="interceptorNames" value="myInterceptor"/>
</bean>
Note that the target object - "businessObjectTarget" in the example - must be a prototype. This allows
the PoolingTargetSource implementation to create new instances of the target to grow the pool
as necessary. See the javadocs of AbstractPoolingTargetSource and the concrete subclass you
wish to use for information about its properties: "maxSize" is the most basic, and always guaranteed
to be present.
In this case, "myInterceptor" is the name of an interceptor that would need to be defined in the same
IoC context. However, it isn’t necessary to specify interceptors to use pooling. If you want only pooling,
and no other advice, don’t set the interceptorNames property at all.
It’s possible to configure Spring so as to be able to cast any pooled object to the
org.springframework.aop.target.PoolingConfig interface, which exposes information
about the configuration and current size of the pool through an introduction. You’ll need to define an
advisor like this:
<bean id="poolConfigAdvisor" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
<property name="targetObject" ref="poolTargetSource"/>
<property name="targetMethod" value="getPoolingConfigMixin"/>
</bean>
This advisor is obtained by calling a convenience method on the AbstractPoolingTargetSource
class, hence the use of MethodInvokingFactoryBean. This advisor’s name ("poolConfigAdvisor" here)
must be in the list of interceptors names in the ProxyFactoryBean exposing the pooled object.
The cast will look as follows:
PoolingConfig conf = (PoolingConfig) beanFactory.getBean("businessObject");
System.out.println("Max pool size is " + conf.getMaxSize());
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Note
Pooling stateless service objects is not usually necessary. We don’t believe it should be the default
choice, as most stateless objects are naturally thread safe, and instance pooling is problematic
if resources are cached.
Simpler pooling is available using auto-proxying. It’s possible to set the TargetSources used by any
auto-proxy creator.
Prototype target sources
Setting up a "prototype" target source is similar to a pooling TargetSource. In this case, a new instance
of the target will be created on every method invocation. Although the cost of creating a new object isn’t
high in a modern JVM, the cost of wiring up the new object (satisfying its IoC dependencies) may be
more expensive. Thus you shouldn’t use this approach without very good reason.
To do this, you could modify the poolTargetSource definition shown above as follows. (I’ve also
changed the name, for clarity.)
<bean id="prototypeTargetSource" class="org.springframework.aop.target.PrototypeTargetSource">
<property name="targetBeanName" ref="businessObjectTarget"/>
</bean>
There’s only one property: the name of the target bean. Inheritance is used in the TargetSource
implementations to ensure consistent naming. As with the pooling target source, the target bean must
be a prototype bean definition.
ThreadLocal target sources
ThreadLocal target sources are useful if you need an object to be created for each incoming request
(per thread that is). The concept of a ThreadLocal provide a JDK-wide facility to transparently store
resource alongside a thread. Setting up a ThreadLocalTargetSource is pretty much the same as
was explained for the other types of target source:
<bean id="threadlocalTargetSource" class="org.springframework.aop.target.ThreadLocalTargetSource">
<property name="targetBeanName" value="businessObjectTarget"/>
</bean>
Note
ThreadLocals come with serious issues (potentially resulting in memory leaks) when incorrectly
using them in a multi-threaded and multi-classloader environments. One should always consider
wrapping a threadlocal in some other class and never directly use the ThreadLocal itself (except
of course in the wrapper class). Also, one should always remember to correctly set and unset
(where the latter simply involved a call to ThreadLocal.set(null)) the resource local to the
thread. Unsetting should be done in any case since not unsetting it might result in problematic
behavior. Spring’s ThreadLocal support does this for you and should always be considered in
favor of using ThreadLocals without other proper handling code.
11.11 Defining new Advice types
Spring AOP is designed to be extensible. While the interception implementation strategy is presently
used internally, it is possible to support arbitrary advice types in addition to the out-of-the-box
interception around advice, before, throws advice and after returning advice.
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The org.springframework.aop.framework.adapter package is an SPI package allowing
support for new custom advice types to be added without changing the core framework. The only
constraint on a custom Advice type is that it must implement the org.aopalliance.aop.Advice
marker interface.
Please refer to the org.springframework.aop.framework.adapter javadocs for further
information.
11.12 Further resources
Please refer to the Spring sample applications for further examples of Spring AOP:
• The JPetStore’s default configuration illustrates the use of the TransactionProxyFactoryBean
for declarative transaction management.
• The /attributes directory of the JPetStore illustrates the use of attribute-driven declarative
transaction management.
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Part IV. Testing
The adoption of the test-driven-development (TDD) approach to software development is certainly
advocated by the Spring team, and so coverage of Spring’s support for integration testing is covered
(alongside best practices for unit testing). The Spring team has found that the correct use of IoC
certainly does make both unit and integration testing easier (in that the presence of setter methods
and appropriate constructors on classes makes them easier to wire together in a test without having
to set up service locator registries and suchlike)… the chapter dedicated solely to testing will hopefully
convince you of this as well.
Spring Framework Reference Documentation
12. Introduction to Spring Testing
Testing is an integral part of enterprise software development. This chapter focuses on the value-add
of the IoC principle to unit testing and on the benefits of the Spring Framework’s support for integration
testing. (A thorough treatment of testing in the enterprise is beyond the scope of this reference manual.)
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13. Unit Testing
Dependency Injection should make your code less dependent on the container than it would be with
traditional Java EE development. The POJOs that make up your application should be testable in JUnit
or TestNG tests, with objects simply instantiated using the new operator, without Spring or any other
container. You can use mock objects (in conjunction with other valuable testing techniques) to test your
code in isolation. If you follow the architecture recommendations for Spring, the resulting clean layering
and componentization of your codebase will facilitate easier unit testing. For example, you can test
service layer objects by stubbing or mocking DAO or Repository interfaces, without needing to access
persistent data while running unit tests.
True unit tests typically run extremely quickly, as there is no runtime infrastructure to set up. Emphasizing
true unit tests as part of your development methodology will boost your productivity. You may not need
this section of the testing chapter to help you write effective unit tests for your IoC-based applications.
For certain unit testing scenarios, however, the Spring Framework provides the following mock objects
and testing support classes.
13.1 Mock Objects
Environment
The org.springframework.mock.env package contains mock implementations of the
Environment and PropertySource abstractions (see the section called “Bean definition profiles” and
the section called “PropertySource abstraction”). MockEnvironment and MockPropertySource are
useful for developing out-of-container tests for code that depends on environment-specific properties.
JNDI
The org.springframework.mock.jndi package contains an implementation of the JNDI SPI,
which you can use to set up a simple JNDI environment for test suites or stand-alone applications.
If, for example, JDBC DataSources get bound to the same JNDI names in test code as within a
Java EE container, you can reuse both application code and configuration in testing scenarios without
modification.
Servlet API
The org.springframework.mock.web package contains a comprehensive set of Servlet API mock
objects, which are useful for testing web contexts, controllers, and filters. These mock objects are
targeted at usage with Spring’s Web MVC framework and are generally more convenient to use than
dynamic mock objects such as EasyMock or alternative Servlet API mock objects such as MockObjects.
As of Spring Framework 4.0, the set of mocks in the org.springframework.mock.web package is
based on the Servlet 3.0 API.
For thorough integration testing of your Spring MVC and REST Controllers in conjunction with your
WebApplicationContext configuration for Spring MVC, see the Spring MVC Test Framework.
Portlet API
The org.springframework.mock.web.portlet package contains a set of Portlet API mock
objects, targeted at usage with Spring’s Portlet MVC framework.
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13.2 Unit Testing support Classes
General testing utilities
The org.springframework.test.util package contains several general purpose utilities for use
in unit and integration testing.
ReflectionTestUtils is a collection of reflection-based utility methods. Developers use these
methods in testing scenarios where they need to change the value of a constant, set a non-public
field, invoke a non-public setter method, or invoke a non-public configuration or lifecycle callback
method when testing application code involving use cases such as the following.
• ORM frameworks such as JPA and Hibernate that condone private or protected field access as
opposed to public setter methods for properties in a domain entity.
• Spring’s support for annotations such as @Autowired, @Inject, and @Resource, which provides
dependency injection for private or protected fields, setter methods, and configuration methods.
• Use of annotations such as @PostConstruct and @PreDestroy for lifecycle callback methods.
AopTestUtils is a collection of AOP-related utility methods. These methods can be used to obtain
a reference to the underlying target object hidden behind one or more Spring proxies. For example,
if you have configured a bean as a dynamic mock using a library like EasyMock or Mockito and the
mock is wrapped in a Spring proxy, you may need direct access to the underlying mock in order to
configure expectations on it and perform verifications. For Spring’s core AOP utilities, see AopUtils
and AopProxyUtils.
Spring MVC
The org.springframework.test.web package contains ModelAndViewAssert, which you can
use in combination with JUnit, TestNG, or any other testing framework for unit tests dealing with Spring
MVC ModelAndView objects.
Unit testing Spring MVC Controllers
To unit test your Spring MVC Controllers as POJOs, use ModelAndViewAssert combined with
MockHttpServletRequest, MockHttpSession, and so on from Spring’s Servlet API mocks.
For thorough integration testing of your Spring MVC and REST Controllers in conjunction with your
WebApplicationContext configuration for Spring MVC, use the Spring MVC Test Framework
instead.
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14. Integration Testing
14.1 Overview
It is important to be able to perform some integration testing without requiring deployment to your
application server or connecting to other enterprise infrastructure. This will enable you to test things
such as:
• The correct wiring of your Spring IoC container contexts.
• Data access using JDBC or an ORM tool. This would include such things as the correctness of SQL
statements, Hibernate queries, JPA entity mappings, etc.
The Spring Framework provides first-class support for integration testing in the spring-test
module. The name of the actual JAR file might include the release version and might also
be in the long org.springframework.test form, depending on where you get it from
(see the section on Dependency Management for an explanation). This library includes the
org.springframework.test package, which contains valuable classes for integration testing with
a Spring container. This testing does not rely on an application server or other deployment environment.
Such tests are slower to run than unit tests but much faster than the equivalent Selenium tests or remote
tests that rely on deployment to an application server.
In Spring 2.5 and later, unit and integration testing support is provided in the form of the annotation-driven
Spring TestContext Framework. The TestContext framework is agnostic of the actual testing framework
in use, thus allowing instrumentation of tests in various environments including JUnit, TestNG, and so on.
14.2 Goals of Integration Testing
Spring’s integration testing support has the following primary goals:
• To manage Spring IoC container caching between test execution.
• To provide Dependency Injection of test fixture instances.
• To provide transaction management appropriate to integration testing.
• To supply Spring-specific base classes that assist developers in writing integration tests.
The next few sections describe each goal and provide links to implementation and configuration details.
Context management and caching
The Spring TestContext Framework provides consistent loading of Spring ApplicationContexts and
WebApplicationContexts as well as caching of those contexts. Support for the caching of loaded contexts
is important, because startup time can become an issue — not because of the overhead of Spring itself,
but because the objects instantiated by the Spring container take time to instantiate. For example, a
project with 50 to 100 Hibernate mapping files might take 10 to 20 seconds to load the mapping files,
and incurring that cost before running every test in every test fixture leads to slower overall test runs
that reduce developer productivity.
Test classes typically declare either an array of resource locations for XML configuration
metadata — often in the classpath — or an array of annotated classes that is used to configure the
application. These locations or classes are the same as or similar to those specified in web.xml or
other deployment configuration files.
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By default, once loaded, the configured ApplicationContext is reused for each test. Thus the
setup cost is incurred only once per test suite, and subsequent test execution is much faster. In this
context, the term test suite means all tests run in the same JVM — for example, all tests run from an
Ant, Maven, or Gradle build for a given project or module. In the unlikely case that a test corrupts the
application context and requires reloading — for example, by modifying a bean definition or the state of
an application object — the TestContext framework can be configured to reload the configuration and
rebuild the application context before executing the next test.
See the section called “Context management” and the section called “Context caching” with the
TestContext framework.
Dependency Injection of test fixtures
When the TestContext framework loads your application context, it can optionally configure instances
of your test classes via Dependency Injection. This provides a convenient mechanism for setting up
test fixtures using preconfigured beans from your application context. A strong benefit here is that you
can reuse application contexts across various testing scenarios (e.g., for configuring Spring-managed
object graphs, transactional proxies, DataSources, etc.), thus avoiding the need to duplicate complex
test fixture setup for individual test cases.
As an example, consider the scenario where we have a class, HibernateTitleRepository, that
implements data access logic for a Title domain entity. We want to write integration tests that test
the following areas:
• The Spring configuration: basically, is everything related to the configuration of the
HibernateTitleRepository bean correct and present?
• The Hibernate mapping file configuration: is everything mapped correctly, and are the correct lazyloading settings in place?
• The logic of the HibernateTitleRepository: does the configured instance of this class perform
as anticipated?
See dependency injection of test fixtures with the TestContext framework.
Transaction management
One common issue in tests that access a real database is their effect on the state of the persistence
store. Even when you’re using a development database, changes to the state may affect future tests.
Also, many operations — such as inserting or modifying persistent data — cannot be performed (or
verified) outside a transaction.
The TestContext framework addresses this issue. By default, the framework will create and roll back a
transaction for each test. You simply write code that can assume the existence of a transaction. If you
call transactionally proxied objects in your tests, they will behave correctly, according to their configured
transactional semantics. In addition, if a test method deletes the contents of selected tables while running
within the transaction managed for the test, the transaction will roll back by default, and the database
will return to its state prior to execution of the test. Transactional support is provided to a test via a
PlatformTransactionManager bean defined in the test’s application context.
If you want a transaction to commit — unusual, but occasionally useful when you want a particular
test to populate or modify the database — the TestContext framework can be instructed to cause the
transaction to commit instead of roll back via the @Commit annotation.
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See transaction management with the TestContext framework.
Support classes for integration testing
The Spring TestContext Framework provides several abstract support classes that simplify the writing
of integration tests. These base test classes provide well-defined hooks into the testing framework as
well as convenient instance variables and methods, which enable you to access:
• The ApplicationContext, for performing explicit bean lookups or testing the state of the context
as a whole.
• A JdbcTemplate, for executing SQL statements to query the database. Such queries can be used
to confirm database state both prior to and after execution of database-related application code, and
Spring ensures that such queries run in the scope of the same transaction as the application code.
When used in conjunction with an ORM tool, be sure to avoid false positives.
In addition, you may want to create your own custom, application-wide superclass with instance
variables and methods specific to your project.
See support classes for the TestContext framework.
14.3 JDBC Testing Support
The org.springframework.test.jdbc package contains JdbcTestUtils, which is a collection
of JDBC related utility functions intended to simplify standard database testing scenarios. Specifically,
JdbcTestUtils provides the following static utility methods.
• countRowsInTable(..): counts the number of rows in the given table
• countRowsInTableWhere(..): counts the number of rows in the given table, using the provided
WHERE clause
• deleteFromTables(..): deletes all rows from the specified tables
• deleteFromTableWhere(..): deletes rows from the given table, using the provided WHERE clause
• dropTables(..): drops the specified tables
Note
that
AbstractTransactionalJUnit4SpringContextTests
and
AbstractTransactionalTestNGSpringContextTests provide convenience methods which
delegate to the aforementioned methods in JdbcTestUtils.
The spring-jdbc module provides support for configuring and launching an embedded database
which can be used in integration tests that interact with a database. For details, see Section 18.8,
“Embedded database support” and the section called “Testing data access logic with an embedded
database”.
14.4 Annotations
Spring Testing Annotations
The Spring Framework provides the following set of Spring-specific annotations that you can use in your
unit and integration tests in conjunction with the TestContext framework. Refer to the corresponding
javadocs for further information, including default attribute values, attribute aliases, and so on.
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• @ContextConfiguration
Defines class-level metadata that is used to determine how to load and configure an
ApplicationContext for integration tests. Specifically, @ContextConfiguration declares the
application context resource locations or the annotated classes that will be used to load the
context.
Resource locations are typically XML configuration files located in the classpath; whereas, annotated
classes are typically @Configuration classes. However, resource locations can also refer to files
in the file system, and annotated classes can be component classes, etc.
@ContextConfiguration("/test-config.xml")
public class XmlApplicationContextTests {
// class body...
}
@ContextConfiguration(classes = TestConfig.class)
public class ConfigClassApplicationContextTests {
// class body...
}
As an alternative or in addition to declaring resource locations or annotated classes,
@ContextConfiguration may be used to declare ApplicationContextInitializer
classes.
@ContextConfiguration(initializers = CustomContextIntializer.class)
public class ContextInitializerTests {
// class body...
}
@ContextConfiguration may optionally be used to declare the ContextLoader strategy as well.
Note, however, that you typically do not need to explicitly configure the loader since the default loader
supports either resource locations or annotated classes as well as initializers.
@ContextConfiguration(locations = "/test-context.xml", loader = CustomContextLoader.class)
public class CustomLoaderXmlApplicationContextTests {
// class body...
}
Note
@ContextConfiguration provides support for inheriting resource locations or configuration
classes as well as context initializers declared by superclasses by default.
See the section called “Context management” and the @ContextConfiguration javadocs for
further details.
• @WebAppConfiguration
A class-level annotation that is used to declare that the ApplicationContext loaded
for an integration test should be a WebApplicationContext. The mere presence of
@WebAppConfiguration on a test class ensures that a WebApplicationContext will be loaded
for the test, using the default value of "file:src/main/webapp" for the path to the root of
the web application (i.e., the resource base path). The resource base path is used behind the
scenes to create a MockServletContext which serves as the ServletContext for the test’s
WebApplicationContext.
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@ContextConfiguration
@WebAppConfiguration
public class WebAppTests {
// class body...
}
To override the default, specify a different base resource path via the implicit value attribute. Both
classpath: and file: resource prefixes are supported. If no resource prefix is supplied the path
is assumed to be a file system resource.
@ContextConfiguration
@WebAppConfiguration("classpath:test-web-resources")
public class WebAppTests {
// class body...
}
Note that @WebAppConfiguration must be used in conjunction with @ContextConfiguration,
either within a single test class or within a test class hierarchy. See the @WebAppConfiguration
javadocs for further details.
• @ContextHierarchy
A class-level annotation that is used to define a hierarchy of ApplicationContexts for integration tests.
@ContextHierarchy should be declared with a list of one or more @ContextConfiguration
instances, each of which defines a level in the context hierarchy. The following examples demonstrate
the use of @ContextHierarchy within a single test class; however, @ContextHierarchy can also
be used within a test class hierarchy.
@ContextHierarchy({
@ContextConfiguration("/parent-config.xml"),
@ContextConfiguration("/child-config.xml")
})
public class ContextHierarchyTests {
// class body...
}
@WebAppConfiguration
@ContextHierarchy({
@ContextConfiguration(classes = AppConfig.class),
@ContextConfiguration(classes = WebConfig.class)
})
public class WebIntegrationTests {
// class body...
}
If you need to merge or override the configuration for a given level of the context hierarchy within
a test class hierarchy, you must explicitly name that level by supplying the same value to the name
attribute in @ContextConfiguration at each corresponding level in the class hierarchy. See the
section called “Context hierarchies” and the @ContextHierarchy javadocs for further examples.
• @ActiveProfiles
A class-level annotation that is used to declare which bean definition profiles should be active when
loading an ApplicationContext for test classes.
@ContextConfiguration
@ActiveProfiles("dev")
public class DeveloperTests {
// class body...
}
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@ContextConfiguration
@ActiveProfiles({"dev", "integration"})
public class DeveloperIntegrationTests {
// class body...
}
Note
@ActiveProfiles provides support for inheriting active bean definition profiles declared
by superclasses by default. It is also possible to resolve active bean definition profiles
programmatically by implementing a custom ActiveProfilesResolver and registering it
via the resolver attribute of @ActiveProfiles.
See the section called “Context configuration with environment profiles” and the @ActiveProfiles
javadocs for examples and further details.
• @TestPropertySource
A class-level annotation that is used to configure the locations of properties files and inlined properties
to be added to the set of PropertySources in the Environment for an ApplicationContext
loaded for an integration test.
Test property sources have higher precedence than those loaded from the operating system’s
environment or Java system properties as well as property sources added by the application
declaratively via @PropertySource or programmatically. Thus, test property sources can be used
to selectively override properties defined in system and application property sources. Furthermore,
inlined properties have higher precedence than properties loaded from resource locations.
The following example demonstrates how to declare a properties file from the classpath.
@ContextConfiguration
@TestPropertySource("/test.properties")
public class MyIntegrationTests {
// class body...
}
The following example demonstrates how to declare inlined properties.
@ContextConfiguration
@TestPropertySource(properties = { "timezone = GMT", "port: 4242" })
public class MyIntegrationTests {
// class body...
}
• @DirtiesContext
Indicates that the underlying Spring ApplicationContext has been dirtied during the execution of
a test (i.e., modified or corrupted in some manner — for example, by changing the state of a singleton
bean) and should be closed. When an application context is marked dirty, it is removed from the
testing framework’s cache and closed. As a consequence, the underlying Spring container will be
rebuilt for any subsequent test that requires a context with the same configuration metadata.
@DirtiesContext can be used as both a class-level and method-level annotation within the same
class or class hierarchy. In such scenarios, the ApplicationContext is marked as dirty before or
after any such annotated method as well as before or after the current test class, depending on the
configured methodMode and classMode.
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The following examples explain when the context would be dirtied for various configuration scenarios:
• Before the current test class, when declared on a class with class mode set to BEFORE_CLASS.
@DirtiesContext(classMode = BEFORE_CLASS)
public class FreshContextTests {
// some tests that require a new Spring container
}
• After the current test class, when declared on a class with class mode set to AFTER_CLASS (i.e.,
the default class mode).
@DirtiesContext
public class ContextDirtyingTests {
// some tests that result in the Spring container being dirtied
}
• Before each test method in the current test class, when declared on a class with class mode set
to BEFORE_EACH_TEST_METHOD.
@DirtiesContext(classMode = BEFORE_EACH_TEST_METHOD)
public class FreshContextTests {
// some tests that require a new Spring container
}
• After each test method in the current test class, when declared on a class with class mode set to
AFTER_EACH_TEST_METHOD.
@DirtiesContext(classMode = AFTER_EACH_TEST_METHOD)
public class ContextDirtyingTests {
// some tests that result in the Spring container being dirtied
}
• Before the current test, when declared on a method with the method mode set to BEFORE_METHOD.
@DirtiesContext(methodMode = BEFORE_METHOD)
@Test
public void testProcessWhichRequiresFreshAppCtx() {
// some logic that requires a new Spring container
}
• After the current test, when declared on a method with the method mode set to AFTER_METHOD
(i.e., the default method mode).
@DirtiesContext
@Test
public void testProcessWhichDirtiesAppCtx() {
// some logic that results in the Spring container being dirtied
}
If @DirtiesContext is used in a test whose context is configured as part of a context hierarchy via
@ContextHierarchy, the hierarchyMode flag can be used to control how the context cache is
cleared. By default an exhaustive algorithm will be used that clears the context cache including not
only the current level but also all other context hierarchies that share an ancestor context common
to the current test; all ApplicationContexts that reside in a sub-hierarchy of the common ancestor
context will be removed from the context cache and closed. If the exhaustive algorithm is overkill for
a particular use case, the simpler current level algorithm can be specified instead, as seen below.
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@ContextHierarchy({
@ContextConfiguration("/parent-config.xml"),
@ContextConfiguration("/child-config.xml")
})
public class BaseTests {
// class body...
}
public class ExtendedTests extends BaseTests {
@Test
@DirtiesContext(hierarchyMode = CURRENT_LEVEL)
public void test() {
// some logic that results in the child context being dirtied
}
}
For further details regarding the EXHAUSTIVE and CURRENT_LEVEL algorithms see the
DirtiesContext.HierarchyMode javadocs.
• @TestExecutionListeners
Defines class-level metadata for configuring which TestExecutionListeners should be registered with
the TestContextManager. Typically, @TestExecutionListeners is used in conjunction with
@ContextConfiguration.
@ContextConfiguration
@TestExecutionListeners({CustomTestExecutionListener.class, AnotherTestExecutionListener.class})
public class CustomTestExecutionListenerTests {
// class body...
}
@TestExecutionListeners supports inherited listeners by default. See the javadocs for an
example and further details.
• @Commit
Indicates that the transaction for a transactional test method should be committed after the test method
has completed. @Commit can be used as a direct replacement for @Rollback(false) in order
to more explicitly convey the intent of the code. Analogous to @Rollback, @Commit may also be
declared as a class-level or method-level annotation.
@Commit
@Test
public void testProcessWithoutRollback() {
// ...
}
• @Rollback
Indicates whether the transaction for a transactional test method should be rolled back after the test
method has completed. If true, the transaction is rolled back; otherwise, the transaction is committed
(see also @Commit). Rollback semantics for integration tests in the Spring TestContext Framework
default to true even if @Rollback is not explicitly declared.
When declared as a class-level annotation, @Rollback defines the default rollback semantics
for all test methods within the test class hierarchy. When declared as a method-level annotation,
@Rollback defines rollback semantics for the specific test method, potentially overriding class-level
@Rollback or @Commit semantics.
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@Rollback(false)
@Test
public void testProcessWithoutRollback() {
// ...
}
• @BeforeTransaction
Indicates that the annotated public void method should be executed before a transaction is started
for test methods configured to run within a transaction via the @Transactional annotation.
@BeforeTransaction
public void beforeTransaction() {
// logic to be executed before a transaction is started
}
• @AfterTransaction
Indicates that the annotated public void method should be executed after a transaction has ended
for test methods configured to run within a transaction via the @Transactional annotation.
@AfterTransaction
public void afterTransaction() {
// logic to be executed after a transaction has ended
}
• @Sql
Used to annotate a test class or test method to configure SQL scripts to be executed against a given
database during integration tests.
@Test
@Sql({"/test-schema.sql", "/test-user-data.sql"})
public void userTest {
// execute code that relies on the test schema and test data
}
See the section called “Executing SQL scripts declaratively with @Sql” for further details.
• @SqlConfig
Defines metadata that is used to determine how to parse and execute SQL scripts configured via the
@Sql annotation.
@Test
@Sql(
scripts = "/test-user-data.sql",
config = @SqlConfig(commentPrefix = "`", separator = "@@")
)
public void userTest {
// execute code that relies on the test data
}
• @SqlGroup
A container annotation that aggregates several @Sql annotations. Can be used natively, declaring
several nested @Sql annotations. Can also be used in conjunction with Java 8’s support for repeatable
annotations, where @Sql can simply be declared several times on the same class or method, implicitly
generating this container annotation.
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@Test
@SqlGroup({
@Sql(scripts = "/test-schema.sql", config = @SqlConfig(commentPrefix = "`")),
@Sql("/test-user-data.sql")
)}
public void userTest {
// execute code that uses the test schema and test data
}
Standard Annotation Support
The following annotations are supported with standard semantics for all configurations of the Spring
TestContext Framework. Note that these annotations are not specific to tests and can be used anywhere
in the Spring Framework.
• @Autowired
• @Qualifier
• @Resource (javax.annotation) if JSR-250 is present
• @Inject (javax.inject) if JSR-330 is present
• @Named (javax.inject) if JSR-330 is present
• @PersistenceContext (javax.persistence) if JPA is present
• @PersistenceUnit (javax.persistence) if JPA is present
• @Required
• @Transactional
JSR-250 Lifecycle Annotations
In the Spring TestContext Framework @PostConstruct and @PreDestroy may be used with
standard semantics on any application components configured in the ApplicationContext;
however, these lifecycle annotations have limited usage within an actual test class.
If a method within a test class is annotated with @PostConstruct, that method will be executed
before any before methods of the underlying test framework (e.g., methods annotated with JUnit’s
@Before), and that will apply for every test method in the test class. On the other hand, if a
method within a test class is annotated with @PreDestroy, that method will never be executed.
Within a test class it is therefore recommended to use test lifecycle callbacks from the underlying
test framework instead of @PostConstruct and @PreDestroy.
Spring JUnit Testing Annotations
The following annotations are only supported when used in conjunction
SpringJUnit4ClassRunner, Spring’s JUnit rules, or Spring’s JUnit support classes.
with
the
• @IfProfileValue
Indicates that the annotated test is enabled for a specific testing environment. If the configured
ProfileValueSource returns a matching value for the provided name, the test is enabled.
Otherwise, the test will be disabled and effectively ignored.
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@IfProfileValue can be applied at the class level, the method level, or both. Class-level usage of
@IfProfileValue takes precedence over method-level usage for any methods within that class or
its subclasses. Specifically, a test is enabled if it is enabled both at the class level and at the method
level; the absence of @IfProfileValue means the test is implicitly enabled. This is analogous to
the semantics of JUnit’s @Ignore annotation, except that the presence of @Ignore always disables
a test.
@IfProfileValue(name="java.vendor", value="Oracle Corporation")
@Test
public void testProcessWhichRunsOnlyOnOracleJvm() {
// some logic that should run only on Java VMs from Oracle Corporation
}
Alternatively, you can configure @IfProfileValue with a list of values (with OR semantics) to
achieve TestNG-like support for test groups in a JUnit environment. Consider the following example:
@IfProfileValue(name="test-groups", values={"unit-tests", "integration-tests"})
@Test
public void testProcessWhichRunsForUnitOrIntegrationTestGroups() {
// some logic that should run only for unit and integration test groups
}
• @ProfileValueSourceConfiguration
Class-level
annotation
that
specifies
what
type
of
ProfileValueSource
to
use when retrieving profile values configured through the @IfProfileValue
annotation. If @ProfileValueSourceConfiguration is not declared for a test,
SystemProfileValueSource is used by default.
@ProfileValueSourceConfiguration(CustomProfileValueSource.class)
public class CustomProfileValueSourceTests {
// class body...
}
• @Timed
Indicates that the annotated test method must finish execution in a specified time period (in
milliseconds). If the text execution time exceeds the specified time period, the test fails.
The time period includes execution of the test method itself, any repetitions of the test (see @Repeat),
as well as any set up or tear down of the test fixture.
@Timed(millis=1000)
public void testProcessWithOneSecondTimeout() {
// some logic that should not take longer than 1 second to execute
}
Spring’s @Timed annotation has different semantics than JUnit’s @Test(timeout=…) support.
Specifically, due to the manner in which JUnit handles test execution timeouts (that is, by executing
the test method in a separate Thread), @Test(timeout=…) preemptively fails the test if the test
takes too long. Spring’s @Timed, on the other hand, does not preemptively fail the test but rather
waits for the test to complete before failing.
• @Repeat
Indicates that the annotated test method must be executed repeatedly. The number of times that the
test method is to be executed is specified in the annotation.
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The scope of execution to be repeated includes execution of the test method itself as well as any set
up or tear down of the test fixture.
@Repeat(10)
@Test
public void testProcessRepeatedly() {
// ...
}
Meta-Annotation Support for Testing
As of Spring Framework 4.0, it is possible to use test-related annotations as meta-annotations in order
to create custom composed annotations and reduce configuration duplication across a test suite.
Each of the following may be used as meta-annotations in conjunction with the TestContext framework.
• @ContextConfiguration
• @ContextHierarchy
• @ActiveProfiles
• @TestPropertySource
• @DirtiesContext
• @WebAppConfiguration
• @TestExecutionListeners
• @Transactional
• @BeforeTransaction
• @AfterTransaction
• @Rollback
• @Sql
• @SqlConfig
• @SqlGroup
• @Repeat
• @Timed
• @IfProfileValue
• @ProfileValueSourceConfiguration
For example, if we discover that we are repeating the following configuration across our JUnit-based
test suite…
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@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration({"/app-config.xml", "/test-data-access-config.xml"})
@ActiveProfiles("dev")
@Transactional
public class OrderRepositoryTests { }
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration({"/app-config.xml", "/test-data-access-config.xml"})
@ActiveProfiles("dev")
@Transactional
public class UserRepositoryTests { }
We can reduce the above duplication by introducing a custom composed annotation that centralizes
the common test configuration like this:
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@ContextConfiguration({"/app-config.xml", "/test-data-access-config.xml"})
@ActiveProfiles("dev")
@Transactional
public @interface TransactionalDevTest { }
Then we can use our custom @TransactionalDevTest annotation to simplify the configuration of
individual test classes as follows:
@RunWith(SpringJUnit4ClassRunner.class)
@TransactionalDevTest
public class OrderRepositoryTests { }
@RunWith(SpringJUnit4ClassRunner.class)
@TransactionalDevTest
public class UserRepositoryTests { }
For further details, consult the Spring Annotation Programming Model.
14.5 Spring TestContext Framework
The Spring TestContext Framework (located in the org.springframework.test.context
package) provides generic, annotation-driven unit and integration testing support that is agnostic of
the testing framework in use. The TestContext framework also places a great deal of importance on
convention over configuration with reasonable defaults that can be overridden through annotation-based
configuration.
In addition to generic testing infrastructure, the TestContext framework provides explicit support for
JUnit and TestNG in the form of abstract support classes. For JUnit, Spring also provides a custom
JUnit Runner and custom JUnit Rules that allow one to write so-called POJO test classes. POJO test
classes are not required to extend a particular class hierarchy.
The following section provides an overview of the internals of the TestContext framework. If you are
only interested in using the framework and not necessarily interested in extending it with your own
custom listeners or custom loaders, feel free to go directly to the configuration (context management,
dependency injection, transaction management), support classes, and annotation support sections.
Key abstractions
The core of the framework consists of the TestContext and TestContextManager classes
and the TestExecutionListener, ContextLoader, and SmartContextLoader interfaces. A
TestContextManager is created on a per-test basis (e.g., for the execution of a single test
method in JUnit). The TestContextManager in turn manages a TestContext that holds the
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context of the current test. The TestContextManager also updates the state of the TestContext
as the test progresses and delegates to TestExecutionListeners, which instrument the actual test
execution by providing dependency injection, managing transactions, and so on. A ContextLoader
(or SmartContextLoader) is responsible for loading an ApplicationContext for a given test
class. Consult the javadocs and the Spring test suite for further information and examples of various
implementations.
• TestContext: Encapsulates the context in which a test is executed, agnostic of the actual
testing framework in use, and provides context management and caching support for the test
instance for which it is responsible. The TestContext also delegates to a ContextLoader (or
SmartContextLoader) to load an ApplicationContext if requested.
• TestContextManager: The main entry point into the Spring TestContext Framework, which
manages a single TestContext and signals events to all registered TestExecutionListeners at welldefined test execution points:
• prior to any before class methods of a particular testing framework
• test instance preparation
• prior to any before methods of a particular testing framework
• after any after methods of a particular testing framework
• after any after class methods of a particular testing framework
• TestExecutionListener: Defines a listener API for reacting to test execution events published
by the TestContextManager with which the listener is registered. See the section called
“TestExecutionListener configuration”.
• ContextLoader: Strategy interface introduced in Spring 2.5 for loading an ApplicationContext
for an integration test managed by the Spring TestContext Framework.
Implement SmartContextLoader instead of this interface in order to provide support for
annotated classes, active bean definition profiles, test property sources, context hierarchies, and
WebApplicationContexts.
• SmartContextLoader: Extension of the ContextLoader interface introduced in Spring 3.1.
The SmartContextLoader SPI supersedes the ContextLoader SPI that was introduced in Spring
2.5. Specifically, a SmartContextLoader can choose to process resource locations, annotated
classes, or context initializers. Furthermore, a SmartContextLoader can set active bean
definition profiles and test property sources in the context that it loads.
Spring provides the following implementations:
• DelegatingSmartContextLoader: one of two default loaders which delegates internally
to an AnnotationConfigContextLoader, a GenericXmlContextLoader, or a
GenericGroovyXmlContextLoader depending either on the configuration declared for the test
class or on the presence of default locations or default configuration classes. Groovy support is
only enabled if Groovy is on the classpath.
• WebDelegatingSmartContextLoader: one of two default loaders which delegates internally
to an AnnotationConfigWebContextLoader, a GenericXmlWebContextLoader, or a
GenericGroovyXmlWebContextLoader depending either on the configuration declared for
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the test class or on the presence of default locations or default configuration classes. A web
ContextLoader will only be used if @WebAppConfiguration is present on the test class.
Groovy support is only enabled if Groovy is on the classpath.
• AnnotationConfigContextLoader: loads a standard ApplicationContext from annotated
classes.
• AnnotationConfigWebContextLoader: loads a WebApplicationContext from annotated
classes.
• GenericGroovyXmlContextLoader: loads a standard ApplicationContext from resource
locations that are either Groovy scripts or XML configuration files.
• GenericGroovyXmlWebContextLoader: loads a WebApplicationContext from resource
locations that are either Groovy scripts or XML configuration files.
• GenericXmlContextLoader: loads a standard ApplicationContext from XML resource
locations.
• GenericXmlWebContextLoader: loads a WebApplicationContext from XML resource
locations.
• GenericPropertiesContextLoader: loads a standard ApplicationContext from Java
Properties files.
The following sections explain how to configure the TestContext framework through annotations and
provide working examples of how to write unit and integration tests with the framework.
TestExecutionListener configuration
Spring provides the following TestExecutionListener implementations that are registered by
default, exactly in this order.
• ServletTestExecutionListener:
WebApplicationContext
configures
Servlet
API
mocks
for
a
• DependencyInjectionTestExecutionListener: provides dependency injection for the test
instance
• DirtiesContextTestExecutionListener: handles the @DirtiesContext annotation
• TransactionalTestExecutionListener: provides transactional test execution with default
rollback semantics
• SqlScriptsTestExecutionListener: executes SQL scripts configured via the @Sql annotation
Registering custom TestExecutionListeners
Custom TestExecutionListeners can be registered for a test class and its subclasses via
the @TestExecutionListeners annotation. See annotation support and the javadocs for
@TestExecutionListeners for details and examples.
Automatic discovery of default TestExecutionListeners
Registering custom TestExecutionListeners via @TestExecutionListeners is suitable for custom
listeners that are used in limited testing scenarios; however, it can become cumbersome if a
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custom listener needs to be used across a test suite. To address this issue, Spring Framework
4.1 supports automatic discovery of default TestExecutionListener implementations via the
SpringFactoriesLoader mechanism.
Specifically, the spring-test module declares all core default TestExecutionListeners under
the org.springframework.test.context.TestExecutionListener key in its META-INF/
spring.factories properties file. Third-party frameworks and developers can contribute their own
TestExecutionListeners to the list of default listeners in the same manner via their own META-INF/
spring.factories properties file.
Ordering TestExecutionListeners
When the TestContext framework discovers default TestExecutionListeners via the aforementioned
SpringFactoriesLoader mechanism, the instantiated listeners are sorted using Spring’s
AnnotationAwareOrderComparator which honors Spring’s Ordered interface and @Order
annotation for ordering. AbstractTestExecutionListener and all default TestExecutionListeners
provided by Spring implement Ordered with appropriate values. Third-party frameworks and
developers should therefore make sure that their default TestExecutionListeners are registered in the
proper order by implementing Ordered or declaring @Order. Consult the javadocs for the getOrder()
methods of the core default TestExecutionListeners for details on what values are assigned to each
core listener.
Merging TestExecutionListeners
If a custom TestExecutionListener is registered via @TestExecutionListeners, the default
listeners will not be registered. In most common testing scenarios, this effectively forces the developer
to manually declare all default listeners in addition to any custom listeners. The following listing
demonstrates this style of configuration.
@ContextConfiguration
@TestExecutionListeners({
MyCustomTestExecutionListener.class,
ServletTestExecutionListener.class,
DependencyInjectionTestExecutionListener.class,
DirtiesContextTestExecutionListener.class,
TransactionalTestExecutionListener.class,
SqlScriptsTestExecutionListener.class
})
public class MyTest {
// class body...
}
The challenge with this approach is that it requires that the developer know exactly which
listeners are registered by default. Moreover, the set of default listeners can change from
release to release — for example, SqlScriptsTestExecutionListener was introduced in Spring
Framework 4.1. Furthermore, third-party frameworks like Spring Security register their own default
TestExecutionListeners via the aforementioned automatic discovery mechanism.
To avoid having to be aware of and re-declare all default listeners, the mergeMode
attribute of @TestExecutionListeners can be set to MergeMode.MERGE_WITH_DEFAULTS.
MERGE_WITH_DEFAULTS indicates that locally declared listeners should be merged with the default
listeners. The merging algorithm ensures that duplicates are removed from the list and that the resulting
set of merged listeners is sorted according to the semantics of AnnotationAwareOrderComparator
as described in the section called “Ordering TestExecutionListeners”. If a listener implements Ordered
or is annotated with @Order it can influence the position in which it is merged with the defaults;
otherwise, locally declared listeners will simply be appended to the list of default listeners when merged.
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For example, if the MyCustomTestExecutionListener class in the previous example configures its
order value (for example, 500) to be less than the order of the ServletTestExecutionListener
(which happens to be 1000), the MyCustomTestExecutionListener can then be automatically
merged with the list of defaults in front of the ServletTestExecutionListener, and the previous
example could be replaced with the following.
@ContextConfiguration
@TestExecutionListeners(
listeners = MyCustomTestExecutionListener.class,
mergeMode = MERGE_WITH_DEFAULTS
)
public class MyTest {
// class body...
}
Context management
Each TestContext provides context management and caching support for the test instance
it is responsible for. Test instances do not automatically receive access to the configured
ApplicationContext. However, if a test class implements the ApplicationContextAware
interface, a reference to the ApplicationContext is supplied to the test instance.
Note that AbstractJUnit4SpringContextTests and AbstractTestNGSpringContextTests
implement ApplicationContextAware and therefore provide access to the ApplicationContext
automatically.
@Autowired ApplicationContext
As an alternative to implementing the ApplicationContextAware interface, you can inject the
application context for your test class through the @Autowired annotation on either a field or
setter method. For example:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
public class MyTest {
@Autowired
private ApplicationContext applicationContext;
// class body...
}
Similarly, if your test is configured to load a WebApplicationContext, you can inject the web
application context into your test as follows:
@RunWith(SpringJUnit4ClassRunner.class)
@WebAppConfiguration
@ContextConfiguration
public class MyWebAppTest {
@Autowired
private WebApplicationContext wac;
// class body...
}
Dependency
injection
via
@Autowired
is
provided
by
the
DependencyInjectionTestExecutionListener which is configured by default (see the
section called “Dependency injection of test fixtures”).
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Test classes that use the TestContext framework do not need to extend any particular class or implement
a specific interface to configure their application context. Instead, configuration is achieved simply
by declaring the @ContextConfiguration annotation at the class level. If your test class does
not explicitly declare application context resource locations or annotated classes, the configured
ContextLoader determines how to load a context from a default location or default configuration
classes. In addition to context resource locations and annotated classes, an application context
can also be configured via application context initializers.
The following sections explain how to configure an ApplicationContext via XML configuration
files, annotated classes (typically @Configuration classes), or context initializers using Spring’s
@ContextConfiguration annotation. Alternatively, you can implement and configure your own
custom SmartContextLoader for advanced use cases.
Context configuration with XML resources
To load an ApplicationContext for your tests using XML configuration files, annotate your test
class with @ContextConfiguration and configure the locations attribute with an array that
contains the resource locations of XML configuration metadata. A plain or relative path — for example
"context.xml" — will be treated as a classpath resource that is relative to the package in which the
test class is defined. A path starting with a slash is treated as an absolute classpath location, for example
"/org/example/config.xml". A path which represents a resource URL (i.e., a path prefixed with
classpath:, file:, http:, etc.) will be used as is.
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from "/app-config.xml" and
// "/test-config.xml" in the root of the classpath
@ContextConfiguration(locations={"/app-config.xml", "/test-config.xml"})
public class MyTest {
// class body...
}
@ContextConfiguration supports an alias for the locations attribute through the standard Java
value attribute. Thus, if you do not need to declare additional attributes in @ContextConfiguration,
you can omit the declaration of the locations attribute name and declare the resource locations by
using the shorthand format demonstrated in the following example.
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration({"/app-config.xml", "/test-config.xml"})
public class MyTest {
// class body...
}
If you omit both the locations and value attributes from the @ContextConfiguration
annotation, the TestContext framework will attempt to detect a default XML resource location.
Specifically, GenericXmlContextLoader and GenericXmlWebContextLoader detect a default
location based on the name of the test class. If your class is named com.example.MyTest,
GenericXmlContextLoader loads your application context from "classpath:com/example/
MyTest-context.xml".
package com.example;
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from
// "classpath:com/example/MyTest-context.xml"
@ContextConfiguration
public class MyTest {
// class body...
}
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Context configuration with Groovy scripts
To load an ApplicationContext for your tests using Groovy scripts that utilize the Groovy Bean
Definition DSL, annotate your test class with @ContextConfiguration and configure the locations
or value attribute with an array that contains the resource locations of Groovy scripts. Resource lookup
semantics for Groovy scripts are the same as those described for XML configuration files.
Enabling Groovy script support
Support for using Groovy scripts to load an ApplicationContext in the Spring TestContext
Framework is enabled automatically if Groovy is on the classpath.
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from "/AppConfig.groovy" and
// "/TestConfig.groovy" in the root of the classpath
@ContextConfiguration({"/AppConfig.groovy", "/TestConfig.Groovy"})
public class MyTest {
// class body...
}
If you omit both the locations and value attributes from the @ContextConfiguration
annotation, the TestContext framework will attempt to detect a default Groovy script. Specifically,
GenericGroovyXmlContextLoader and GenericGroovyXmlWebContextLoader detect a
default location based on the name of the test class. If your class is named com.example.MyTest,
the Groovy context loader will load your application context from "classpath:com/example/
MyTestContext.groovy".
package com.example;
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from
// "classpath:com/example/MyTestContext.groovy"
@ContextConfiguration
public class MyTest {
// class body...
}
Declaring XML config and Groovy scripts simultaneously
Both XML configuration files and Groovy scripts can be declared simultaneously via the
locations or value attribute of @ContextConfiguration. If the path to a configured
resource location ends with .xml it will be loaded using an XmlBeanDefinitionReader;
otherwise it will be loaded using a GroovyBeanDefinitionReader.
The following listing demonstrates how to combine both in an integration test.
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from
// "/app-config.xml" and "/TestConfig.groovy"
@ContextConfiguration({ "/app-config.xml", "/TestConfig.groovy" })
public class MyTest {
// class body...
}
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Context configuration with annotated classes
To load an ApplicationContext for your tests using annotated classes (see Section 6.12,
“Java-based container configuration”), annotate your test class with @ContextConfiguration and
configure the classes attribute with an array that contains references to annotated classes.
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from AppConfig and TestConfig
@ContextConfiguration(classes = {AppConfig.class, TestConfig.class})
public class MyTest {
// class body...
}
Annotated Classes
The term annotated class can refer to any of the following.
• A class annotated with @Configuration
• A component (i.e., a class annotated with @Component, @Service, @Repository, etc.)
• A JSR-330 compliant class that is annotated with javax.inject annotations
• Any other class that contains @Bean-methods
Consult the javadocs of @Configuration and @Bean for further information regarding the
configuration and semantics of annotated classes, paying special attention to the discussion of
`@Bean` Lite Mode.
If you omit the classes attribute from the @ContextConfiguration annotation, the TestContext
framework will attempt to detect the presence of default configuration classes. Specifically,
AnnotationConfigContextLoader and AnnotationConfigWebContextLoader will detect all
static nested classes of the test class that meet the requirements for configuration class
implementations as specified in the @Configuration javadocs. In the following example, the
OrderServiceTest class declares a static nested configuration class named Config that will be
automatically used to load the ApplicationContext for the test class. Note that the name of the
configuration class is arbitrary. In addition, a test class can contain more than one static nested
configuration class if desired.
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@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from the
// static nested Config class
@ContextConfiguration
public class OrderServiceTest {
@Configuration
static class Config {
// this bean will be injected into the OrderServiceTest class
@Bean
public OrderService orderService() {
OrderService orderService = new OrderServiceImpl();
// set properties, etc.
return orderService;
}
}
@Autowired
private OrderService orderService;
@Test
public void testOrderService() {
// test the orderService
}
}
Mixing XML, Groovy scripts, and annotated classes
It may sometimes be desirable to mix XML configuration files, Groovy scripts, and annotated classes
(i.e., typically @Configuration classes) to configure an ApplicationContext for your tests.
For example, if you use XML configuration in production, you may decide that you want to use
@Configuration classes to configure specific Spring-managed components for your tests, or vice
versa.
Furthermore, some third-party frameworks (like Spring Boot) provide first-class support for loading an
ApplicationContext from different types of resources simultaneously (e.g., XML configuration files,
Groovy scripts, and @Configuration classes). The Spring Framework historically has not supported
this for standard deployments. Consequently, most of the SmartContextLoader implementations
that the Spring Framework delivers in the spring-test module support only one resource type per
test context; however, this does not mean that you cannot use both. One exception to the general
rule is that the GenericGroovyXmlContextLoader and GenericGroovyXmlWebContextLoader
support both XML configuration files and Groovy scripts simultaneously. Furthermore, thirdparty frameworks may choose to support the declaration of both locations and classes via
@ContextConfiguration, and with the standard testing support in the TestContext framework, you
have the following options.
If you want to use resource locations (e.g., XML or Groovy) and @Configuration classes to configure
your tests, you will have to pick one as the entry point, and that one will have to include or import the
other. For example, in XML or Groovy scripts you can include @Configuration classes via component
scanning or define them as normal Spring beans; whereas, in a @Configuration class you can use
@ImportResource to import XML configuration files. Note that this behavior is semantically equivalent
to how you configure your application in production: in production configuration you will define either
a set of XML or Groovy resource locations or a set of @Configuration classes that your production
ApplicationContext will be loaded from, but you still have the freedom to include or import the other
type of configuration.
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Context configuration with context initializers
To configure an ApplicationContext for your tests using context initializers, annotate your test
class with @ContextConfiguration and configure the initializers attribute with an array that
contains references to classes that implement ApplicationContextInitializer. The declared
context initializers will then be used to initialize the ConfigurableApplicationContext that is
loaded for your tests. Note that the concrete ConfigurableApplicationContext type supported
by each declared initializer must be compatible with the type of ApplicationContext created by
the SmartContextLoader in use (i.e., typically a GenericApplicationContext). Furthermore,
the order in which the initializers are invoked depends on whether they implement Spring’s Ordered
interface, are annotated with Spring’s @Order or the standard @Priority annotation.
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from TestConfig
// and initialized by TestAppCtxInitializer
@ContextConfiguration(
classes = TestConfig.class,
initializers = TestAppCtxInitializer.class)
public class MyTest {
// class body...
}
It is also possible to omit the declaration of XML configuration files or annotated classes in
@ContextConfiguration entirely and instead declare only ApplicationContextInitializer
classes which are then responsible for registering beans in the context — for example, by
programmatically loading bean definitions from XML files or configuration classes.
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be initialized by EntireAppInitializer
// which presumably registers beans in the context
@ContextConfiguration(initializers = EntireAppInitializer.class)
public class MyTest {
// class body...
}
Context configuration inheritance
@ContextConfiguration supports boolean inheritLocations and inheritInitializers
attributes that denote whether resource locations or annotated classes and context initializers declared
by superclasses should be inherited. The default value for both flags is true. This means that a test
class inherits the resource locations or annotated classes as well as the context initializers declared by
any superclasses. Specifically, the resource locations or annotated classes for a test class are appended
to the list of resource locations or annotated classes declared by superclasses. Similarly, the initializers
for a given test class will be added to the set of initializers defined by test superclasses. Thus, subclasses
have the option of extending the resource locations, annotated classes, or context initializers.
If the inheritLocations or inheritInitializers attribute in @ContextConfiguration is set
to false, the resource locations or annotated classes and the context initializers, respectively, for the
test class shadow and effectively replace the configuration defined by superclasses.
In the following example that uses XML resource locations, the ApplicationContext for
ExtendedTest will be loaded from "base-config.xml" and "extended-config.xml", in that order.
Beans defined in "extended-config.xml" may therefore override (i.e., replace) those defined in "baseconfig.xml".
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@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from "/base-config.xml"
// in the root of the classpath
@ContextConfiguration("/base-config.xml")
public class BaseTest {
// class body...
}
// ApplicationContext will be loaded from "/base-config.xml" and
// "/extended-config.xml" in the root of the classpath
@ContextConfiguration("/extended-config.xml")
public class ExtendedTest extends BaseTest {
// class body...
}
Similarly, in the following example that uses annotated classes, the ApplicationContext for
ExtendedTest will be loaded from the BaseConfig and ExtendedConfig classes, in that
order. Beans defined in ExtendedConfig may therefore override (i.e., replace) those defined in
BaseConfig.
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from BaseConfig
@ContextConfiguration(classes = BaseConfig.class)
public class BaseTest {
// class body...
}
// ApplicationContext will be loaded from BaseConfig and ExtendedConfig
@ContextConfiguration(classes = ExtendedConfig.class)
public class ExtendedTest extends BaseTest {
// class body...
}
In the following example that uses context initializers, the ApplicationContext for ExtendedTest
will be initialized using BaseInitializer and ExtendedInitializer. Note, however, that the
order in which the initializers are invoked depends on whether they implement Spring’s Ordered
interface, are annotated with Spring’s @Order or the standard @Priority annotation.
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be initialized by BaseInitializer
@ContextConfiguration(initializers = BaseInitializer.class)
public class BaseTest {
// class body...
}
// ApplicationContext will be initialized by BaseInitializer
// and ExtendedInitializer
@ContextConfiguration(initializers = ExtendedInitializer.class)
public class ExtendedTest extends BaseTest {
// class body...
}
Context configuration with environment profiles
Spring 3.1 introduced first-class support in the framework for the notion of environments and profiles
(a.k.a., bean definition profiles), and integration tests can be configured to activate particular bean
definition profiles for various testing scenarios. This is achieved by annotating a test class with the
@ActiveProfiles annotation and supplying a list of profiles that should be activated when loading
the ApplicationContext for the test.
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Note
@ActiveProfiles may be used with any implementation of the new SmartContextLoader
SPI, but @ActiveProfiles is not supported with implementations of the older ContextLoader
SPI.
Let’s take a look at some examples with XML configuration and @Configuration classes.
<!-- app-config.xml -->
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:jdbc="http://www.springframework.org/schema/jdbc"
xmlns:jee="http://www.springframework.org/schema/jee"
xsi:schemaLocation="...">
<bean id="transferService"
class="com.bank.service.internal.DefaultTransferService">
<constructor-arg ref="accountRepository"/>
<constructor-arg ref="feePolicy"/>
</bean>
<bean id="accountRepository"
class="com.bank.repository.internal.JdbcAccountRepository">
<constructor-arg ref="dataSource"/>
</bean>
<bean id="feePolicy"
class="com.bank.service.internal.ZeroFeePolicy"/>
<beans profile="dev">
<jdbc:embedded-database id="dataSource">
<jdbc:script
location="classpath:com/bank/config/sql/schema.sql"/>
<jdbc:script
location="classpath:com/bank/config/sql/test-data.sql"/>
</jdbc:embedded-database>
</beans>
<beans profile="production">
<jee:jndi-lookup id="dataSource" jndi-name="java:comp/env/jdbc/datasource"/>
</beans>
<beans profile="default">
<jdbc:embedded-database id="dataSource">
<jdbc:script
location="classpath:com/bank/config/sql/schema.sql"/>
</jdbc:embedded-database>
</beans>
</beans>
package com.bank.service;
@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from "classpath:/app-config.xml"
@ContextConfiguration("/app-config.xml")
@ActiveProfiles("dev")
public class TransferServiceTest {
@Autowired
private TransferService transferService;
@Test
public void testTransferService() {
// test the transferService
}
}
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When TransferServiceTest is run, its ApplicationContext will be loaded from the appconfig.xml configuration file in the root of the classpath. If you inspect app-config.xml you’ll
notice that the accountRepository bean has a dependency on a dataSource bean; however,
dataSource is not defined as a top-level bean. Instead, dataSource is defined three times: in the
production profile, the dev profile, and the default profile.
By annotating TransferServiceTest with @ActiveProfiles("dev") we instruct the Spring
TestContext Framework to load the ApplicationContext with the active profiles set to {"dev"}.
As a result, an embedded database will be created and populated with test data, and the
accountRepository bean will be wired with a reference to the development DataSource. And that’s
likely what we want in an integration test.
It is sometimes useful to assign beans to a default profile. Beans within the default profile are only
included when no other profile is specifically activated. This can be used to define fallback beans to be
used in the application’s default state. For example, you may explicitly provide a data source for dev and
production profiles, but define an in-memory data source as a default when neither of these is active.
The following code listings demonstrate how to implement the same configuration and integration test
but using @Configuration classes instead of XML.
@Configuration
@Profile("dev")
public class StandaloneDataConfig {
@Bean
public DataSource dataSource() {
return new EmbeddedDatabaseBuilder()
.setType(EmbeddedDatabaseType.HSQL)
.addScript("classpath:com/bank/config/sql/schema.sql")
.addScript("classpath:com/bank/config/sql/test-data.sql")
.build();
}
}
@Configuration
@Profile("production")
public class JndiDataConfig {
@Bean(destroyMethod="")
public DataSource dataSource() throws Exception {
Context ctx = new InitialContext();
return (DataSource) ctx.lookup("java:comp/env/jdbc/datasource");
}
}
@Configuration
@Profile("default")
public class DefaultDataConfig {
@Bean
public DataSource dataSource() {
return new EmbeddedDatabaseBuilder()
.setType(EmbeddedDatabaseType.HSQL)
.addScript("classpath:com/bank/config/sql/schema.sql")
.build();
}
}
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@Configuration
public class TransferServiceConfig {
@Autowired DataSource dataSource;
@Bean
public TransferService transferService() {
return new DefaultTransferService(accountRepository(), feePolicy());
}
@Bean
public AccountRepository accountRepository() {
return new JdbcAccountRepository(dataSource);
}
@Bean
public FeePolicy feePolicy() {
return new ZeroFeePolicy();
}
}
package com.bank.service;
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = {
TransferServiceConfig.class,
StandaloneDataConfig.class,
JndiDataConfig.class,
DefaultDataConfig.class})
@ActiveProfiles("dev")
public class TransferServiceTest {
@Autowired
private TransferService transferService;
@Test
public void testTransferService() {
// test the transferService
}
}
In this variation, we have split the XML configuration into four independent @Configuration classes:
• TransferServiceConfig: acquires a dataSource via dependency injection using @Autowired
• StandaloneDataConfig: defines a dataSource for an embedded database suitable for developer
tests
• JndiDataConfig: defines a dataSource that is retrieved from JNDI in a production environment
• DefaultDataConfig: defines a dataSource for a default embedded database in case no profile
is active
As with the XML-based configuration example, we still annotate TransferServiceTest with
@ActiveProfiles("dev"), but this time we specify all four configuration classes via the
@ContextConfiguration annotation. The body of the test class itself remains completely
unchanged.
It is often the case that a single set of profiles is used across multiple test classes within a given
project. Thus, to avoid duplicate declarations of the @ActiveProfiles annotation it is possible
to declare @ActiveProfiles once on a base class, and subclasses will automatically inherit the
@ActiveProfiles configuration from the base class. In the following example, the declaration
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of @ActiveProfiles (as well as other annotations) has been moved to an abstract superclass,
AbstractIntegrationTest.
package com.bank.service;
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = {
TransferServiceConfig.class,
StandaloneDataConfig.class,
JndiDataConfig.class,
DefaultDataConfig.class})
@ActiveProfiles("dev")
public abstract class AbstractIntegrationTest {
}
package com.bank.service;
// "dev" profile inherited from superclass
public class TransferServiceTest extends AbstractIntegrationTest {
@Autowired
private TransferService transferService;
@Test
public void testTransferService() {
// test the transferService
}
}
@ActiveProfiles also supports an inheritProfiles attribute that can be used to disable the
inheritance of active profiles.
package com.bank.service;
// "dev" profile overridden with "production"
@ActiveProfiles(profiles = "production", inheritProfiles = false)
public class ProductionTransferServiceTest extends AbstractIntegrationTest {
// test body
}
Furthermore, it is sometimes necessary to resolve active profiles for tests programmatically instead of
declaratively — for example, based on:
• the current operating system
• whether tests are being executed on a continuous integration build server
• the presence of certain environment variables
• the presence of custom class-level annotations
• etc.
To resolve active bean definition profiles programmatically, simply implement a custom
ActiveProfilesResolver and register it via the resolver attribute of @ActiveProfiles.
The following example demonstrates how to implement and register a custom
OperatingSystemActiveProfilesResolver. For further information, refer to the corresponding
javadocs.
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package com.bank.service;
// "dev" profile overridden programmatically via a custom resolver
@ActiveProfiles(
resolver = OperatingSystemActiveProfilesResolver.class,
inheritProfiles = false)
public class TransferServiceTest extends AbstractIntegrationTest {
// test body
}
package com.bank.service.test;
public class OperatingSystemActiveProfilesResolver implements ActiveProfilesResolver {
@Override
String[] resolve(Class<?> testClass) {
String profile = ...;
// determine the value of profile based on the operating system
return new String[] {profile};
}
}
Context configuration with test property sources
Spring 3.1 introduced first-class support in the framework for the notion of an environment with a
hierarchy of property sources, and since Spring 4.1 integration tests can be configured with test-specific
property sources. In contrast to the @PropertySource annotation used on @Configuration classes,
the @TestPropertySource annotation can be declared on a test class to declare resource locations
for test properties files or inlined properties. These test property sources will be added to the set of
PropertySources in the Environment for the ApplicationContext loaded for the annotated
integration test.
Note
@TestPropertySource may be used with any implementation of the SmartContextLoader
SPI, but @TestPropertySource is not supported with implementations of the older
ContextLoader SPI.
Implementations of SmartContextLoader gain access to merged test property source
values via the getPropertySourceLocations() and getPropertySourceProperties()
methods in MergedContextConfiguration.
Declaring test property sources
Test properties files can be configured via the locations
@TestPropertySource as shown in the following example.
or
value
attribute
of
Both traditional and XML-based properties file formats are supported — for example, "classpath:/
com/example/test.properties" or "file:///path/to/file.xml".
Each path will be interpreted as a Spring Resource. A plain path — for example,
"test.properties" — will be treated as a classpath resource that is relative to the package in which
the test class is defined. A path starting with a slash will be treated as an absolute classpath resource,
for example: "/org/example/test.xml". A path which references a URL (e.g., a path prefixed
with classpath:, file:, http:, etc.) will be loaded using the specified resource protocol. Resource
location wildcards (e.g. */.properties) are not permitted: each location must evaluate to exactly one
.properties or .xml resource.
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@ContextConfiguration
@TestPropertySource("/test.properties")
public class MyIntegrationTests {
// class body...
}
Inlined properties in the form of key-value pairs can be configured via the properties attribute of
@TestPropertySource as shown in the following example. All key-value pairs will be added to the
enclosing Environment as a single test PropertySource with the highest precedence.
The supported syntax for key-value pairs is the same as the syntax defined for entries in a Java
properties file:
• "key=value"
• "key:value"
• "key value"
@ContextConfiguration
@TestPropertySource(properties = {"timezone = GMT", "port: 4242"})
public class MyIntegrationTests {
// class body...
}
Default properties file detection
If @TestPropertySource is declared as an empty annotation (i.e., without explicit values for the
locations or properties attributes), an attempt will be made to detect a default properties
file relative to the class that declared the annotation. For example, if the annotated test class is
com.example.MyTest, the corresponding default properties file is "classpath:com/example/
MyTest.properties". If the default cannot be detected, an IllegalStateException will be
thrown.
Precedence
Test property sources have higher precedence than those loaded from the operating system’s
environment or Java system properties as well as property sources added by the application
declaratively via @PropertySource or programmatically. Thus, test property sources can be used to
selectively override properties defined in system and application property sources. Furthermore, inlined
properties have higher precedence than properties loaded from resource locations.
In the following example, the timezone and port properties as well as any properties defined in "/
test.properties" will override any properties of the same name that are defined in system and
application property sources. Furthermore, if the "/test.properties" file defines entries for the
timezone and port properties those will be overridden by the inlined properties declared via the
properties attribute.
@ContextConfiguration
@TestPropertySource(
locations = "/test.properties",
properties = {"timezone = GMT", "port: 4242"}
)
public class MyIntegrationTests {
// class body...
}
Inheriting and overriding test property sources
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@TestPropertySource supports boolean inheritLocations and inheritProperties
attributes that denote whether resource locations for properties files and inlined properties declared by
superclasses should be inherited. The default value for both flags is true. This means that a test class
inherits the locations and inlined properties declared by any superclasses. Specifically, the locations
and inlined properties for a test class are appended to the locations and inlined properties declared by
superclasses. Thus, subclasses have the option of extending the locations and inlined properties. Note
that properties that appear later will shadow (i.e.., override) properties of the same name that appear
earlier. In addition, the aforementioned precedence rules apply for inherited test property sources as
well.
If the inheritLocations or inheritProperties attribute in @TestPropertySource is set to
false, the locations or inlined properties, respectively, for the test class shadow and effectively replace
the configuration defined by superclasses.
In the following example, the ApplicationContext for BaseTest will be loaded using only the
"base.properties" file as a test property source. In contrast, the ApplicationContext for
ExtendedTest will be loaded using the "base.properties" and "extended.properties" files
as test property source locations.
@TestPropertySource("base.properties")
@ContextConfiguration
public class BaseTest {
// ...
}
@TestPropertySource("extended.properties")
@ContextConfiguration
public class ExtendedTest extends BaseTest {
// ...
}
In the following example, the ApplicationContext for BaseTest will be loaded using only the inlined
key1 property. In contrast, the ApplicationContext for ExtendedTest will be loaded using the
inlined key1 and key2 properties.
@TestPropertySource(properties = "key1 = value1")
@ContextConfiguration
public class BaseTest {
// ...
}
@TestPropertySource(properties = "key2 = value2")
@ContextConfiguration
public class ExtendedTest extends BaseTest {
// ...
}
Loading a WebApplicationContext
Spring 3.2 introduced support for loading a WebApplicationContext in integration tests. To
instruct the TestContext framework to load a WebApplicationContext instead of a standard
ApplicationContext, simply annotate the respective test class with @WebAppConfiguration.
The presence of @WebAppConfiguration on your test class instructs the TestContext framework
(TCF) that a WebApplicationContext (WAC) should be loaded for your integration tests. In the
background the TCF makes sure that a MockServletContext is created and supplied to your test’s
WAC. By default the base resource path for your MockServletContext will be set to "src/main/
webapp". This is interpreted as a path relative to the root of your JVM (i.e., normally the path to
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your project). If you’re familiar with the directory structure of a web application in a Maven project,
you’ll know that "src/main/webapp" is the default location for the root of your WAR. If you need to
override this default, simply provide an alternate path to the @WebAppConfiguration annotation (e.g.,
@WebAppConfiguration("src/test/webapp")). If you wish to reference a base resource path
from the classpath instead of the file system, just use Spring’s classpath: prefix.
Please note that Spring’s testing support for WebApplicationContexts is on par with its
support for standard ApplicationContexts. When testing with a WebApplicationContext
you are free to declare XML configuration files, Groovy scripts, or @Configuration classes via
@ContextConfiguration. You are of course also free to use any other test annotations such as
@ActiveProfiles, @TestExecutionListeners, @Sql, @Rollback, etc.
The following examples demonstrate some of the various configuration options for loading a
WebApplicationContext.
Conventions.
@RunWith(SpringJUnit4ClassRunner.class)
// defaults to "file:src/main/webapp"
@WebAppConfiguration
// detects "WacTests-context.xml" in same package
// or static nested @Configuration class
@ContextConfiguration
public class WacTests {
//...
}
The above example demonstrates the TestContext framework’s support for convention over
configuration. If you annotate a test class with @WebAppConfiguration without specifying a
resource base path, the resource path will effectively default to "file:src/main/webapp". Similarly, if
you declare @ContextConfiguration without specifying resource locations, annotated classes,
or context initializers, Spring will attempt to detect the presence of your configuration using
conventions (i.e., "WacTests-context.xml" in the same package as the WacTests class or static nested
@Configuration classes).
Default resource semantics.
@RunWith(SpringJUnit4ClassRunner.class)
// file system resource
@WebAppConfiguration("webapp")
// classpath resource
@ContextConfiguration("/spring/test-servlet-config.xml")
public class WacTests {
//...
}
This example demonstrates how to explicitly declare a resource base path with
@WebAppConfiguration and an XML resource location with @ContextConfiguration.
The important thing to note here is the different semantics for paths with these two
annotations. By default, @WebAppConfiguration resource paths are file system based; whereas,
@ContextConfiguration resource locations are classpath based.
Explicit resource semantics.
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@RunWith(SpringJUnit4ClassRunner.class)
// classpath resource
@WebAppConfiguration("classpath:test-web-resources")
// file system resource
@ContextConfiguration("file:src/main/webapp/WEB-INF/servlet-config.xml")
public class WacTests {
//...
}
In this third example, we see that we can override the default resource semantics for both annotations by
specifying a Spring resource prefix. Contrast the comments in this example with the previous example.
To
provide
comprehensive
web
testing
support,
Spring
3.2
introduced
a
ServletTestExecutionListener that is enabled by default. When testing against a
WebApplicationContext this TestExecutionListener sets up default thread-local state
via Spring Web’s RequestContextHolder before each test method and creates
a MockHttpServletRequest, MockHttpServletResponse, and ServletWebRequest
based
on
the
base
resource
path
configured
via
@WebAppConfiguration.
ServletTestExecutionListener also ensures that the MockHttpServletResponse and
ServletWebRequest can be injected into the test instance, and once the test is complete it cleans
up thread-local state.
Once you have a WebApplicationContext loaded for your test you might find that you need to
interact with the web mocks — for example, to set up your test fixture or to perform assertions after
invoking your web component. The following example demonstrates which mocks can be autowired
into your test instance. Note that the WebApplicationContext and MockServletContext are
both cached across the test suite; whereas, the other mocks are managed per test method by the
ServletTestExecutionListener.
Injecting mocks.
@WebAppConfiguration
@ContextConfiguration
public class WacTests {
@Autowired
WebApplicationContext wac; // cached
@Autowired
MockServletContext servletContext; // cached
@Autowired
MockHttpSession session;
@Autowired
MockHttpServletRequest request;
@Autowired
MockHttpServletResponse response;
@Autowired
ServletWebRequest webRequest;
//...
}
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Context caching
Once the TestContext framework loads an ApplicationContext (or WebApplicationContext)
for a test, that context will be cached and reused for all subsequent tests that declare the same unique
context configuration within the same test suite. To understand how caching works, it is important to
understand what is meant by unique and test suite.
An ApplicationContext can be uniquely identified by the combination of configuration parameters
that are used to load it. Consequently, the unique combination of configuration parameters are used
to generate a key under which the context is cached. The TestContext framework uses the following
configuration parameters to build the context cache key:
• locations (from @ContextConfiguration)
• classes (from @ContextConfiguration)
• contextInitializerClasses (from @ContextConfiguration)
• contextLoader (from @ContextConfiguration)
• parent (from @ContextHierarchy)
• activeProfiles (from @ActiveProfiles)
• propertySourceLocations (from @TestPropertySource)
• propertySourceProperties (from @TestPropertySource)
• resourceBasePath (from @WebAppConfiguration)
For example, if TestClassA specifies {"app-config.xml", "test-config.xml"} for the
locations (or value) attribute of @ContextConfiguration, the TestContext framework will load
the corresponding ApplicationContext and store it in a static context cache under a key
that is based solely on those locations. So if TestClassB also defines {"app-config.xml",
"test-config.xml"} for its locations (either explicitly or implicitly through inheritance) but does
not define @WebAppConfiguration, a different ContextLoader, different active profiles, different
context initializers, different test property sources, or a different parent context, then the same
ApplicationContext will be shared by both test classes. This means that the setup cost for loading
an application context is incurred only once (per test suite), and subsequent test execution is much
faster.
Test suites and forked processes
The Spring TestContext framework stores application contexts in a static cache. This means that
the context is literally stored in a static variable. In other words, if tests execute in separate
processes the static cache will be cleared between each test execution, and this will effectively
disable the caching mechanism.
To benefit from the caching mechanism, all tests must run within the same process or test suite.
This can be achieved by executing all tests as a group within an IDE. Similarly, when executing
tests with a build framework such as Ant, Maven, or Gradle it is important to make sure that
the build framework does not fork between tests. For example, if the forkMode for the Maven
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Surefire plug-in is set to always or pertest, the TestContext framework will not be able to cache
application contexts between test classes and the build process will run significantly slower as
a result.
Since having a large number of application contexts loaded within a given test suite can cause the suite
to take an unnecessarily long time to execute, it is often beneficial to know exactly how many contexts
have been loaded and cached. To view the statistics for the underlying context cache, simply set the
log level for the org.springframework.test.context.cache logging category to DEBUG.
In the unlikely case that a test corrupts the application context and requires reloading — for example, by
modifying a bean definition or the state of an application object — you can annotate your test class or
test method with @DirtiesContext (see the discussion of @DirtiesContext in the section called
“Spring Testing Annotations”). This instructs Spring to remove the context from the cache and rebuild
the application context before executing the next test. Note that support for the @DirtiesContext
annotation is provided by the DirtiesContextTestExecutionListener which is enabled by
default.
Context hierarchies
When writing integration tests that rely on a loaded Spring ApplicationContext, it is often
sufficient to test against a single context; however, there are times when it is beneficial or even
necessary to test against a hierarchy of ApplicationContexts. For example, if you are developing a
Spring MVC web application you will typically have a root WebApplicationContext loaded via
Spring’s ContextLoaderListener and a child WebApplicationContext loaded via Spring’s
DispatcherServlet. This results in a parent-child context hierarchy where shared components and
infrastructure configuration are declared in the root context and consumed in the child context by webspecific components. Another use case can be found in Spring Batch applications where you often have
a parent context that provides configuration for shared batch infrastructure and a child context for the
configuration of a specific batch job.
As of Spring Framework 3.2.2, it is possible to write integration tests that use context hierarchies by
declaring context configuration via the @ContextHierarchy annotation, either on an individual test
class or within a test class hierarchy. If a context hierarchy is declared on multiple classes within a
test class hierarchy it is also possible to merge or override the context configuration for a specific,
named level in the context hierarchy. When merging configuration for a given level in the hierarchy
the configuration resource type (i.e., XML configuration files or annotated classes) must be consistent;
otherwise, it is perfectly acceptable to have different levels in a context hierarchy configured using
different resource types.
The following JUnit-based examples demonstrate common configuration scenarios for integration tests
that require the use of context hierarchies.
ControllerIntegrationTests represents a typical integration testing scenario for a Spring
MVC web application by declaring a context hierarchy consisting of two levels, one for the root
WebApplicationContext (loaded using the TestAppConfig @Configuration class) and one for
the dispatcher servlet WebApplicationContext (loaded using the WebConfig @Configuration
class). The WebApplicationContext that is autowired into the test instance is the one for the child
context (i.e., the lowest context in the hierarchy).
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@RunWith(SpringJUnit4ClassRunner.class)
@WebAppConfiguration
@ContextHierarchy({
@ContextConfiguration(classes = TestAppConfig.class),
@ContextConfiguration(classes = WebConfig.class)
})
public class ControllerIntegrationTests {
@Autowired
private WebApplicationContext wac;
// ...
}
The following test classes define a context hierarchy within a test class hierarchy. AbstractWebTests
declares the configuration for a root WebApplicationContext in a Spring-powered web
application. Note, however, that AbstractWebTests does not declare @ContextHierarchy;
consequently, subclasses of AbstractWebTests can optionally participate in a context hierarchy
or simply follow the standard semantics for @ContextConfiguration. SoapWebServiceTests
and RestWebServiceTests both extend AbstractWebTests and define a context hierarchy
via @ContextHierarchy. The result is that three application contexts will be loaded (one for
each declaration of @ContextConfiguration), and the application context loaded based on the
configuration in AbstractWebTests will be set as the parent context for each of the contexts loaded
for the concrete subclasses.
@RunWith(SpringJUnit4ClassRunner.class)
@WebAppConfiguration
@ContextConfiguration("file:src/main/webapp/WEB-INF/applicationContext.xml")
public abstract class AbstractWebTests {}
@ContextHierarchy(@ContextConfiguration("/spring/soap-ws-config.xml")
public class SoapWebServiceTests extends AbstractWebTests {}
@ContextHierarchy(@ContextConfiguration("/spring/rest-ws-config.xml")
public class RestWebServiceTests extends AbstractWebTests {}
The following classes demonstrate the use of named hierarchy levels in order to merge the configuration
for specific levels in a context hierarchy. BaseTests defines two levels in the hierarchy, parent
and child. ExtendedTests extends BaseTests and instructs the Spring TestContext Framework
to merge the context configuration for the child hierarchy level, simply by ensuring that the names
declared via the name attribute in @ContextConfiguration are both "child". The result is
that three application contexts will be loaded: one for "/app-config.xml", one for "/userconfig.xml", and one for {"/user-config.xml", "/order-config.xml"}. As with the
previous example, the application context loaded from "/app-config.xml" will be set as the parent
context for the contexts loaded from "/user-config.xml" and {"/user-config.xml", "/
order-config.xml"}.
@RunWith(SpringJUnit4ClassRunner.class)
@ContextHierarchy({
@ContextConfiguration(name = "parent", locations = "/app-config.xml"),
@ContextConfiguration(name = "child", locations = "/user-config.xml")
})
public class BaseTests {}
@ContextHierarchy(
@ContextConfiguration(name = "child", locations = "/order-config.xml")
)
public class ExtendedTests extends BaseTests {}
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In contrast to the previous example, this example demonstrates how to override the configuration
for a given named level in a context hierarchy by setting the inheritLocations flag in
@ContextConfiguration to false. Consequently, the application context for ExtendedTests will
be loaded only from "/test-user-config.xml" and will have its parent set to the context loaded
from "/app-config.xml".
@RunWith(SpringJUnit4ClassRunner.class)
@ContextHierarchy({
@ContextConfiguration(name = "parent", locations = "/app-config.xml"),
@ContextConfiguration(name = "child", locations = "/user-config.xml")
})
public class BaseTests {}
@ContextHierarchy(
@ContextConfiguration(
name = "child",
locations = "/test-user-config.xml",
inheritLocations = false
))
public class ExtendedTests extends BaseTests {}
Dirtying a context within a context hierarchy
If @DirtiesContext is used in a test whose context is configured as part of a context hierarchy,
the hierarchyMode flag can be used to control how the context cache is cleared. For further
details consult the discussion of @DirtiesContext in Spring Testing Annotations and the
@DirtiesContext javadocs.
Dependency injection of test fixtures
When you use the DependencyInjectionTestExecutionListener — which is configured by
default — the dependencies of your test instances are injected from beans in the application context that
you configured with @ContextConfiguration. You may use setter injection, field injection, or both,
depending on which annotations you choose and whether you place them on setter methods or fields.
For consistency with the annotation support introduced in Spring 2.5 and 3.0, you can use Spring’s
@Autowired annotation or the @Inject annotation from JSR 300.
Tip
The TestContext framework does not instrument the manner in which a test instance is
instantiated. Thus the use of @Autowired or @Inject for constructors has no effect for test
classes.
Because @Autowired is used to perform autowiring by type, if you have multiple bean definitions
of the same type, you cannot rely on this approach for those particular beans. In that case, you
can use @Autowired in conjunction with @Qualifier. As of Spring 3.0 you may also choose
to use @Inject in conjunction with @Named. Alternatively, if your test class has access to its
ApplicationContext, you can perform an explicit lookup by using (for example) a call to
applicationContext.getBean("titleRepository").
If you do not want dependency injection applied to your test instances, simply do not annotate
fields or setter methods with @Autowired or @Inject. Alternatively, you can disable dependency
injection altogether by explicitly configuring your class with @TestExecutionListeners and omitting
DependencyInjectionTestExecutionListener.class from the list of listeners.
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Consider the scenario of testing a HibernateTitleRepository class, as outlined in the Goals
section. The next two code listings demonstrate the use of @Autowired on fields and setter methods.
The application context configuration is presented after all sample code listings.
Note
The dependency injection behavior in the following code listings is not specific to JUnit. The same
DI techniques can be used in conjunction with any testing framework.
The following examples make calls to static assertion methods such as assertNotNull() but
without prepending the call with Assert. In such cases, assume that the method was properly
imported through an import static declaration that is not shown in the example.
The first code listing shows a JUnit-based implementation of the test class that uses @Autowired for
field injection.
@RunWith(SpringJUnit4ClassRunner.class)
// specifies the Spring configuration to load for this test fixture
@ContextConfiguration("repository-config.xml")
public class HibernateTitleRepositoryTests {
// this instance will be dependency injected by type
@Autowired
private HibernateTitleRepository titleRepository;
@Test
public void findById() {
Title title = titleRepository.findById(new Long(10));
assertNotNull(title);
}
}
Alternatively, you can configure the class to use @Autowired for setter injection as seen below.
@RunWith(SpringJUnit4ClassRunner.class)
// specifies the Spring configuration to load for this test fixture
@ContextConfiguration("repository-config.xml")
public class HibernateTitleRepositoryTests {
// this instance will be dependency injected by type
private HibernateTitleRepository titleRepository;
@Autowired
public void setTitleRepository(HibernateTitleRepository titleRepository) {
this.titleRepository = titleRepository;
}
@Test
public void findById() {
Title title = titleRepository.findById(new Long(10));
assertNotNull(title);
}
}
The preceding code listings use the same XML context file referenced by the
@ContextConfiguration annotation (that is, repository-config.xml), which looks like this:
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<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<!-- this bean will be injected into the HibernateTitleRepositoryTests class -->
<bean id="titleRepository" class="com.foo.repository.hibernate.HibernateTitleRepository">
<property name="sessionFactory" ref="sessionFactory"/>
</bean>
<bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
<!-- configuration elided for brevity -->
</bean>
</beans>
Note
If you are extending from a Spring-provided test base class that happens to use @Autowired
on one of its setter methods, you might have multiple beans of the affected type defined in your
application context: for example, multiple DataSource beans. In such a case, you can override
the setter method and use the @Qualifier annotation to indicate a specific target bean as
follows, but make sure to delegate to the overridden method in the superclass as well.
// ...
@Autowired
@Override
public void setDataSource(@Qualifier("myDataSource") DataSource dataSource) {
super.setDataSource(dataSource);
}
// ...
The specified qualifier value indicates the specific DataSource bean to inject, narrowing the set
of type matches to a specific bean. Its value is matched against <qualifier> declarations within
the corresponding <bean> definitions. The bean name is used as a fallback qualifier value, so
you may effectively also point to a specific bean by name there (as shown above, assuming that
"myDataSource" is the bean id).
Testing request and session scoped beans
Request and session scoped beans have been supported by Spring for several years now, but it’s
always been a bit non-trivial to test them. As of Spring 3.2 it’s a breeze to test your request-scoped and
session-scoped beans by following these steps.
• Ensure that a WebApplicationContext is loaded for your test by annotating your test class with
@WebAppConfiguration.
• Inject the mock request or session into your test instance and prepare your test fixture as appropriate.
• Invoke your web component that you retrieved from the configured WebApplicationContext (i.e.,
via dependency injection).
• Perform assertions against the mocks.
The following code snippet displays the XML configuration for a login use case. Note that the
userService bean has a dependency on a request-scoped loginAction bean. Also, the
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LoginAction is instantiated using SpEL expressions that retrieve the username and password from
the current HTTP request. In our test, we will want to configure these request parameters via the mock
managed by the TestContext framework.
Request-scoped bean configuration.
<beans>
<bean id="userService"
class="com.example.SimpleUserService"
c:loginAction-ref="loginAction" />
<bean id="loginAction" class="com.example.LoginAction"
c:username="{request.getParameter(''user'')}"
c:password="{request.getParameter(''pswd'')}"
scope="request">
<aop:scoped-proxy />
</bean>
</beans>
In RequestScopedBeanTests we inject both the UserService (i.e., the subject under test) and the
MockHttpServletRequest into our test instance. Within our requestScope() test method we set
up our test fixture by setting request parameters in the provided MockHttpServletRequest. When
the loginUser() method is invoked on our userService we are assured that the user service has
access to the request-scoped loginAction for the current MockHttpServletRequest (i.e., the one
we just set parameters in). We can then perform assertions against the results based on the known
inputs for the username and password.
Request-scoped bean test.
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
@WebAppConfiguration
public class RequestScopedBeanTests {
@Autowired UserService userService;
@Autowired MockHttpServletRequest request;
@Test
public void requestScope() {
request.setParameter("user", "enigma");
request.setParameter("pswd", "$pr!ng");
LoginResults results = userService.loginUser();
// assert results
}
}
The following code snippet is similar to the one we saw above for a request-scoped bean; however, this
time the userService bean has a dependency on a session-scoped userPreferences bean. Note
that the UserPreferences bean is instantiated using a SpEL expression that retrieves the theme from
the current HTTP session. In our test, we will need to configure a theme in the mock session managed
by the TestContext framework.
Session-scoped bean configuration.
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<beans>
<bean id="userService"
class="com.example.SimpleUserService"
c:userPreferences-ref="userPreferences" />
<bean id="userPreferences"
class="com.example.UserPreferences"
c:theme="#{session.getAttribute(''theme'')}"
scope="session">
<aop:scoped-proxy />
</bean>
</beans>
In SessionScopedBeanTests we inject the UserService and the MockHttpSession into our test
instance. Within our sessionScope() test method we set up our test fixture by setting the expected
"theme" attribute in the provided MockHttpSession. When the processUserPreferences()
method is invoked on our userService we are assured that the user service has access to the sessionscoped userPreferences for the current MockHttpSession, and we can perform assertions against
the results based on the configured theme.
Session-scoped bean test.
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
@WebAppConfiguration
public class SessionScopedBeanTests {
@Autowired UserService userService;
@Autowired MockHttpSession session;
@Test
public void sessionScope() throws Exception {
session.setAttribute("theme", "blue");
Results results = userService.processUserPreferences();
// assert results
}
}
Transaction management
In
the
TestContext
framework,
transactions
are
managed
by
the
TransactionalTestExecutionListener which is configured by default, even if you do not
explicitly declare @TestExecutionListeners on your test class. To enable support for transactions,
however, you must configure a PlatformTransactionManager bean in the ApplicationContext
that is loaded via @ContextConfiguration semantics (further details are provided below). In
addition, you must declare Spring’s @Transactional annotation either at the class or method level
for your tests.
Test-managed transactions
Test-managed transactions are transactions that are managed declaratively via the
TransactionalTestExecutionListener or programmatically via TestTransaction (see
below). Such transactions should not be confused with Spring-managed transactions (i.e., those
managed directly by Spring within the ApplicationContext loaded for tests) or applicationmanaged transactions (i.e., those managed programmatically within application code that is invoked via
tests). Spring-managed and application-managed transactions will typically participate in test-managed
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transactions; however, caution should be taken if Spring-managed or application-managed transactions
are configured with any propagation type other than REQUIRED or SUPPORTS (see the discussion on
transaction propagation for details).
Enabling and disabling transactions
Annotating a test method with @Transactional causes the test to be run within a transaction that
will, by default, be automatically rolled back after completion of the test. If a test class is annotated
with @Transactional, each test method within that class hierarchy will be run within a transaction.
Test methods that are not annotated with @Transactional (at the class or method level) will not be
run within a transaction. Furthermore, tests that are annotated with @Transactional but have the
propagation type set to NOT_SUPPORTED will not be run within a transaction.
Note
that
AbstractTransactionalJUnit4SpringContextTests
and
AbstractTransactionalTestNGSpringContextTests are preconfigured for transactional
support at the class level.
The following example demonstrates a common scenario for writing an integration test for a
Hibernate-based UserRepository. As explained in the section called “Transaction rollback and
commit behavior”, there is no need to clean up the database after the createUser() method
is executed since any changes made to the database will be automatically rolled back by the
TransactionalTestExecutionListener. See Section 14.7, “PetClinic Example” for an additional
example.
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = TestConfig.class)
@Transactional
public class HibernateUserRepositoryTests {
@Autowired
HibernateUserRepository repository;
@Autowired
SessionFactory sessionFactory;
JdbcTemplate jdbcTemplate;
@Autowired
public void setDataSource(DataSource dataSource) {
this.jdbcTemplate = new JdbcTemplate(dataSource);
}
@Test
public void createUser() {
// track initial state in test database:
final int count = countRowsInTable("user");
User user = new User(...);
repository.save(user);
// Manual flush is required to avoid false positive in test
sessionFactory.getCurrentSession().flush();
assertNumUsers(count + 1);
}
protected int countRowsInTable(String tableName) {
return JdbcTestUtils.countRowsInTable(this.jdbcTemplate, tableName);
}
protected void assertNumUsers(int expected) {
assertEquals("Number of rows in the [user] table.", expected, countRowsInTable("user"));
}
}
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Transaction rollback and commit behavior
By default, test transactions will be automatically rolled back after completion of the test; however,
transactional commit and rollback behavior can be configured declaratively via the @Commit and
@Rollback annotations. See the corresponding entries in the annotation support section for further
details.
Programmatic transaction management
As of Spring Framework 4.1, it is possible to interact with test-managed transactions
programmatically via the static methods in TestTransaction. For example, TestTransaction
may be used within test methods, before methods, and after methods to start or end
the current test-managed transaction or to configure the current test-managed transaction for
rollback or commit. Support for TestTransaction is automatically available whenever the
TransactionalTestExecutionListener is enabled.
The following example demonstrates some of the features of TestTransaction. Consult the javadocs
for TestTransaction for further details.
@ContextConfiguration(classes = TestConfig.class)
public class ProgrammaticTransactionManagementTests extends
AbstractTransactionalJUnit4SpringContextTests {
@Test
public void transactionalTest() {
// assert initial state in test database:
assertNumUsers(2);
deleteFromTables("user");
// changes to the database will be committed!
TestTransaction.flagForCommit();
TestTransaction.end();
assertFalse(TestTransaction.isActive());
assertNumUsers(0);
TestTransaction.start();
// perform other actions against the database that will
// be automatically rolled back after the test completes...
}
protected void assertNumUsers(int expected) {
assertEquals("Number of rows in the [user] table.", expected, countRowsInTable("user"));
}
}
Executing code outside of a transaction
Occasionally you need to execute certain code before or after a transactional test method but outside
the transactional context — for example, to verify the initial database state prior to execution of
your test or to verify expected transactional commit behavior after test execution (if the test was
configured not to roll back the transaction). TransactionalTestExecutionListener supports the
@BeforeTransaction and @AfterTransaction annotations exactly for such scenarios. Simply
annotate any public void method in your test class with one of these annotations, and the
TransactionalTestExecutionListener ensures that your before transaction method or after
transaction method is executed at the appropriate time.
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Tip
Any before methods (such as methods annotated with JUnit’s @Before) and any after methods
(such as methods annotated with JUnit’s @After) are executed within a transaction. In addition,
methods annotated with @BeforeTransaction or @AfterTransaction are naturally not
executed for test methods that are not configured to run within a transaction.
Configuring a transaction manager
TransactionalTestExecutionListener
expects
a
PlatformTransactionManager
bean
to
be
defined
in
the
Spring
ApplicationContext
for
the
test.
In
case
there
are
multiple
instances
of
PlatformTransactionManager
within
the
test’s
ApplicationContext,
a
qualifier
may
be
declared
via
@Transactional("myTxMgr") or @Transactional(transactionManager = "myTxMgr"), or
TransactionManagementConfigurer can be implemented by an @Configuration class. Consult
the javadocs for TestContextTransactionUtils.retrieveTransactionManager() for details
on the algorithm used to look up a transaction manager in the test’s ApplicationContext.
Demonstration of all transaction-related annotations
The following JUnit-based example displays a fictitious integration testing scenario highlighting all
transaction-related annotations. The example is not intended to demonstrate best practices but rather
to demonstrate how these annotations can be used. Consult the annotation support section for further
information and configuration examples. Transaction management for @Sql contains an additional
example using @Sql for declarative SQL script execution with default transaction rollback semantics.
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
@Transactional(transactionManager = "txMgr")
@Commit
public class FictitiousTransactionalTest {
@BeforeTransaction
public void verifyInitialDatabaseState() {
// logic to verify the initial state before a transaction is started
}
@Before
public void setUpTestDataWithinTransaction() {
// set up test data within the transaction
}
@Test
// overrides the class-level @Commit setting
@Rollback
public void modifyDatabaseWithinTransaction() {
// logic which uses the test data and modifies database state
}
@After
public void tearDownWithinTransaction() {
// execute "tear down" logic within the transaction
}
@AfterTransaction
public void verifyFinalDatabaseState() {
// logic to verify the final state after transaction has rolled back
}
}
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Avoid false positives when testing ORM code
When you test application code that manipulates the state of the Hibernate or JPA session, make
sure to flush the underlying session within test methods that execute that code. Failing to flush the
underlying session can produce false positives: your test may pass, but the same code throws an
exception in a live, production environment. In the following Hibernate-based example test case,
one method demonstrates a false positive, and the other method correctly exposes the results of
flushing the session. Note that this applies to any ORM frameworks that maintain an in-memory
unit of work.
// ...
@Autowired
private SessionFactory sessionFactory;
@Test // no expected exception!
public void falsePositive() {
updateEntityInHibernateSession();
// False positive: an exception will be thrown once the Hibernate
// Session is finally flushed (i.e., in production code)
}
@Test(expected = ...)
public void updateWithSessionFlush() {
updateEntityInHibernateSession();
// Manual flush is required to avoid false positive in test
sessionFactory.getCurrentSession().flush();
}
// ...
Or for JPA:
// ...
@Autowired
private EntityManager entityManager;
@Test // no expected exception!
public void falsePositive() {
updateEntityInJpaTransaction();
// False positive: an exception will be thrown once the JPA
// EntityManager is finally flushed (i.e., in production code)
}
@Test(expected = ...)
public void updateWithEntityManagerFlush() {
updateEntityInJpaTransaction();
// Manual flush is required to avoid false positive in test
entityManager.flush();
}
// ...
Executing SQL scripts
When writing integration tests against a relational database, it is often beneficial to execute SQL scripts
to modify the database schema or insert test data into tables. The spring-jdbc module provides
support for initializing an embedded or existing database by executing SQL scripts when the Spring
ApplicationContext is loaded. See Section 18.8, “Embedded database support” and the section
called “Testing data access logic with an embedded database” for details.
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Although it is very useful to initialize a database for testing once when the ApplicationContext is
loaded, sometimes it is essential to be able to modify the database during integration tests. The following
sections explain how to execute SQL scripts programmatically and declaratively during integration tests.
Executing SQL scripts programmatically
Spring provides the following options for executing SQL scripts programmatically within integration test
methods.
• org.springframework.jdbc.datasource.init.ScriptUtils
• org.springframework.jdbc.datasource.init.ResourceDatabasePopulator
• org.springframework.test.context.junit4.AbstractTransactionalJUnit4SpringContextTests
• org.springframework.test.context.testng.AbstractTransactionalTestNGSpringContextTests
ScriptUtils provides a collection of static utility methods for working with SQL scripts and is mainly
intended for internal use within the framework. However, if you require full control over how SQL scripts
are parsed and executed, ScriptUtils may suit your needs better than some of the other alternatives
described below. Consult the javadocs for individual methods in ScriptUtils for further details.
ResourceDatabasePopulator provides a simple object-based API for programmatically populating,
initializing, or cleaning up a database using SQL scripts defined in external resources.
ResourceDatabasePopulator provides options for configuring the character encoding, statement
separator, comment delimiters, and error handling flags used when parsing and executing the scripts,
and each of the configuration options has a reasonable default value. Consult the javadocs for
details on default values. To execute the scripts configured in a ResourceDatabasePopulator,
you can invoke either the populate(Connection) method to execute the populator against a
java.sql.Connection or the execute(DataSource) method to execute the populator against a
javax.sql.DataSource. The following example specifies SQL scripts for a test schema and test
data, sets the statement separator to "@@", and then executes the scripts against a DataSource.
@Test
public void databaseTest {
ResourceDatabasePopulator populator = new ResourceDatabasePopulator();
populator.addScripts(
new ClassPathResource("test-schema.sql"),
new ClassPathResource("test-data.sql"));
populator.setSeparator("@@");
populator.execute(this.dataSource);
// execute code that uses the test schema and data
}
Note that ResourceDatabasePopulator internally delegates to ScriptUtils for
parsing and executing SQL scripts. Similarly, the executeSqlScript(..) methods in
AbstractTransactionalJUnit4SpringContextTests
and
AbstractTransactionalTestNGSpringContextTests
internally
use
a
ResourceDatabasePopulator for executing SQL scripts. Consult the javadocs for the various
executeSqlScript(..) methods for further details.
Executing SQL scripts declaratively with @Sql
In addition to the aforementioned mechanisms for executing SQL scripts programmatically, SQL scripts
can also be configured declaratively in the Spring TestContext Framework. Specifically, the @Sql
annotation can be declared on a test class or test method to configure the resource paths to SQL scripts
that should be executed against a given database either before or after an integration test method. Note
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that method-level declarations override class-level declarations and that support for @Sql is provided
by the SqlScriptsTestExecutionListener which is enabled by default.
Path resource semantics
Each path will be interpreted as a Spring Resource. A plain path — for example, "schema.sql" — will
be treated as a classpath resource that is relative to the package in which the test class is defined.
A path starting with a slash will be treated as an absolute classpath resource, for example: "/org/
example/schema.sql". A path which references a URL (e.g., a path prefixed with classpath:,
file:, http:, etc.) will be loaded using the specified resource protocol.
The following example demonstrates how to use @Sql at the class level and at the method level within
a JUnit-based integration test class.
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
@Sql("/test-schema.sql")
public class DatabaseTests {
@Test
public void emptySchemaTest {
// execute code that uses the test schema without any test data
}
@Test
@Sql({"/test-schema.sql", "/test-user-data.sql"})
public void userTest {
// execute code that uses the test schema and test data
}
}
Default script detection
If no SQL scripts are specified, an attempt will be made to detect a default script depending on where
@Sql is declared. If a default cannot be detected, an IllegalStateException will be thrown.
• class-level declaration: if the annotated test class is com.example.MyTest, the corresponding
default script is "classpath:com/example/MyTest.sql".
• method-level declaration: if the annotated test method is named testMethod() and is defined in
the class com.example.MyTest, the corresponding default script is "classpath:com/example/
MyTest.testMethod.sql".
Declaring multiple @Sql sets
If multiple sets of SQL scripts need to be configured for a given test class or test method but with
different syntax configuration, different error handling rules, or different execution phases per set, it
is possible to declare multiple instances of @Sql. With Java 8, @Sql can be used as a repeatable
annotation. Otherwise, the @SqlGroup annotation can be used as an explicit container for declaring
multiple instances of @Sql.
The following example demonstrates the use of @Sql as a repeatable annotation using Java 8. In this
scenario the test-schema.sql script uses a different syntax for single-line comments.
@Test
@Sql(scripts = "/test-schema.sql", config = @SqlConfig(commentPrefix = "`"))
@Sql("/test-user-data.sql")
public void userTest {
// execute code that uses the test schema and test data
}
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The following example is identical to the above except that the @Sql declarations are grouped together
within @SqlGroup for compatibility with Java 6 and Java 7.
@Test
@SqlGroup({
@Sql(scripts = "/test-schema.sql", config = @SqlConfig(commentPrefix = "`")),
@Sql("/test-user-data.sql")
)}
public void userTest {
// execute code that uses the test schema and test data
}
Script execution phases
By default, SQL scripts will be executed before the corresponding test method. However, if a particular
set of scripts needs to be executed after the test method — for example, to clean up database
state — the executionPhase attribute in @Sql can be used as seen in the following example. Note
that ISOLATED and AFTER_TEST_METHOD are statically imported from Sql.TransactionMode and
Sql.ExecutionPhase respectively.
@Test
@Sql(
scripts = "create-test-data.sql",
config = @SqlConfig(transactionMode = ISOLATED)
)
@Sql(
scripts = "delete-test-data.sql",
config = @SqlConfig(transactionMode = ISOLATED),
executionPhase = AFTER_TEST_METHOD
)
public void userTest {
// execute code that needs the test data to be committed
// to the database outside of the test's transaction
}
Script configuration with @SqlConfig
Configuration for script parsing and error handling can be configured via the @SqlConfig annotation.
When declared as a class-level annotation on an integration test class, @SqlConfig serves as global
configuration for all SQL scripts within the test class hierarchy. When declared directly via the config
attribute of the @Sql annotation, @SqlConfig serves as local configuration for the SQL scripts declared
within the enclosing @Sql annotation. Every attribute in @SqlConfig has an implicit default value which
is documented in the javadocs of the corresponding attribute. Due to the rules defined for annotation
attributes in the Java Language Specification, it is unfortunately not possible to assign a value of
null to an annotation attribute. Thus, in order to support overrides of inherited global configuration,
@SqlConfig attributes have an explicit default value of either "" for Strings or DEFAULT for Enums.
This approach allows local declarations of @SqlConfig to selectively override individual attributes
from global declarations of @SqlConfig by providing a value other than "" or DEFAULT. Global
@SqlConfig attributes are inherited whenever local @SqlConfig attributes do not supply an explicit
value other than "" or DEFAULT. Explicit local configuration therefore overrides global configuration.
The configuration options provided by @Sql and @SqlConfig are equivalent to those supported
by ScriptUtils and ResourceDatabasePopulator but are a superset of those provided by
the <jdbc:initialize-database/> XML namespace element. Consult the javadocs of individual
attributes in @Sql and @SqlConfig for details.
Transaction management for @Sql
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By default, the SqlScriptsTestExecutionListener will infer the desired transaction semantics
for scripts configured via @Sql. Specifically, SQL scripts will be executed without a transaction,
within an existing Spring-managed transaction — for example, a transaction managed by the
TransactionalTestExecutionListener for a test annotated with @Transactional — or
within an isolated transaction, depending on the configured value of the transactionMode
attribute in @SqlConfig and the presence of a PlatformTransactionManager in the test’s
ApplicationContext. As a bare minimum however, a javax.sql.DataSource must be present
in the test’s ApplicationContext.
If the algorithms used by SqlScriptsTestExecutionListener to detect a DataSource and
PlatformTransactionManager and infer the transaction semantics do not suit your needs,
you may specify explicit names via the dataSource and transactionManager attributes
of @SqlConfig. Furthermore, the transaction propagation behavior can be controlled via the
transactionMode attribute of @SqlConfig — for example, if scripts should be executed in
an isolated transaction. Although a thorough discussion of all supported options for transaction
management with @Sql is beyond the scope of this reference manual, the javadocs for @SqlConfig
and SqlScriptsTestExecutionListener provide detailed information, and the following example
demonstrates a typical testing scenario using JUnit and transactional tests with @Sql. Note that there
is no need to clean up the database after the usersTest() method is executed since any changes
made to the database (either within the the test method or within the /test-data.sql script) will
be automatically rolled back by the TransactionalTestExecutionListener (see transaction
management for details).
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = TestDatabaseConfig.class)
@Transactional
public class TransactionalSqlScriptsTests {
protected JdbcTemplate jdbcTemplate;
@Autowired
public void setDataSource(DataSource dataSource) {
this.jdbcTemplate = new JdbcTemplate(dataSource);
}
@Test
@Sql("/test-data.sql")
public void usersTest() {
// verify state in test database:
assertNumUsers(2);
// execute code that uses the test data...
}
protected int countRowsInTable(String tableName) {
return JdbcTestUtils.countRowsInTable(this.jdbcTemplate, tableName);
}
protected void assertNumUsers(int expected) {
assertEquals("Number of rows in the [user] table.", expected, countRowsInTable("user"));
}
}
TestContext Framework support classes
Spring JUnit Runner
The Spring TestContext Framework offers full integration with JUnit 4.9+ through a
custom runner (supported on JUnit 4.9 through 4.12). By annotating test classes with
@RunWith(SpringJUnit4ClassRunner.class), developers can implement standard JUnit-based
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unit and integration tests and simultaneously reap the benefits of the TestContext framework such
as support for loading application contexts, dependency injection of test instances, transactional
test method execution, and so on. If you would like to use the Spring TestContext Framework
with an alternative runner such as JUnit’s Parameterized or third-party runners such as the
MockitoJUnitRunner, you may optionally use Spring’s support for JUnit rules instead.
The following code listing displays the minimal requirements for configuring a test class to run with
the custom Spring Runner. @TestExecutionListeners is configured with an empty list in order to
disable the default listeners, which otherwise would require an ApplicationContext to be configured
through @ContextConfiguration.
@RunWith(SpringJUnit4ClassRunner.class)
@TestExecutionListeners({})
public class SimpleTest {
@Test
public void testMethod() {
// execute test logic...
}
}
Spring JUnit Rules
The org.springframework.test.context.junit4.rules package provides the following JUnit
rules.
• SpringClassRule
• SpringMethodRule
SpringClassRule is a JUnit TestRule that supports class-level features of the Spring TestContext
Framework; whereas, SpringMethodRule is a JUnit MethodRule that supports instance-level and
method-level features of the Spring TestContext Framework.
In contrast to the SpringJUnit4ClassRunner, Spring’s rule-based JUnit support has the advantage
that it is independent of any org.junit.runner.Runner implementation and can therefore be
combined with existing alternative runners like JUnit’s Parameterized or third-party runners such as
the MockitoJUnitRunner.
In order to support the full functionality of the TestContext framework, a SpringClassRule must be
combined with a SpringMethodRule. The following example demonstrates the proper way to declare
these rules in an integration test.
// Optionally specify a non-Spring Runner via @RunWith(...)
@ContextConfiguration
public class IntegrationTest {
@ClassRule
public static final SpringClassRule SPRING_CLASS_RULE = new SpringClassRule();
@Rule
public final SpringMethodRule springMethodRule = new SpringMethodRule();
@Test
public void testMethod() {
// execute test logic...
}
}
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JUnit support classes
The org.springframework.test.context.junit4 package provides the following support
classes for JUnit-based test cases.
• AbstractJUnit4SpringContextTests
• AbstractTransactionalJUnit4SpringContextTests
AbstractJUnit4SpringContextTests is an abstract base test class that integrates the Spring
TestContext Framework with explicit ApplicationContext testing support in a JUnit 4.9+
environment. When you extend AbstractJUnit4SpringContextTests, you can access a
protected applicationContext instance variable that can be used to perform explicit bean
lookups or to test the state of the context as a whole.
AbstractTransactionalJUnit4SpringContextTests is an abstract transactional extension
of AbstractJUnit4SpringContextTests that adds some convenience functionality
for JDBC access. This class expects a javax.sql.DataSource bean and a
PlatformTransactionManager bean to be defined in the ApplicationContext. When you
extend AbstractTransactionalJUnit4SpringContextTests you can access a protected
jdbcTemplate instance variable that can be used to execute SQL statements to query
the database. Such queries can be used to confirm database state both prior to and after
execution of database-related application code, and Spring ensures that such queries run
in the scope of the same transaction as the application code. When used in conjunction
with an ORM tool, be sure to avoid false positives. As mentioned in Section 14.3,
“JDBC Testing Support”, AbstractTransactionalJUnit4SpringContextTests also provides
convenience methods which delegate to methods in JdbcTestUtils using the aforementioned
jdbcTemplate. Furthermore, AbstractTransactionalJUnit4SpringContextTests provides
an executeSqlScript(..) method for executing SQL scripts against the configured DataSource.
Tip
These classes are a convenience for extension. If you do not want your test classes to be tied
to a Spring-specific class hierarchy, you can configure your own custom test classes by using
@RunWith(SpringJUnit4ClassRunner.class) or Spring’s JUnit rules.
TestNG support classes
The org.springframework.test.context.testng package provides the following support
classes for TestNG based test cases.
• AbstractTestNGSpringContextTests
• AbstractTransactionalTestNGSpringContextTests
AbstractTestNGSpringContextTests is an abstract base test class that integrates the Spring
TestContext Framework with explicit ApplicationContext testing support in a TestNG environment.
When you extend AbstractTestNGSpringContextTests, you can access a protected
applicationContext instance variable that can be used to perform explicit bean lookups or to test
the state of the context as a whole.
AbstractTransactionalTestNGSpringContextTests is an abstract transactional extension
of AbstractTestNGSpringContextTests that adds some convenience functionality
for JDBC access. This class expects a javax.sql.DataSource bean and a
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PlatformTransactionManager bean to be defined in the ApplicationContext. When you
extend AbstractTransactionalTestNGSpringContextTests you can access a protected
jdbcTemplate instance variable that can be used to execute SQL statements to query
the database. Such queries can be used to confirm database state both prior to and after
execution of database-related application code, and Spring ensures that such queries run
in the scope of the same transaction as the application code. When used in conjunction
with an ORM tool, be sure to avoid false positives. As mentioned in Section 14.3,
“JDBC Testing Support”, AbstractTransactionalTestNGSpringContextTests also provides
convenience methods which delegate to methods in JdbcTestUtils using the aforementioned
jdbcTemplate. Furthermore, AbstractTransactionalTestNGSpringContextTests provides
an executeSqlScript(..) method for executing SQL scripts against the configured DataSource.
Tip
These classes are a convenience for extension. If you do not want your test classes to be
tied to a Spring-specific class hierarchy, you can configure your own custom test classes
by using @ContextConfiguration, @TestExecutionListeners, and so on, and by
manually instrumenting your test class with a TestContextManager. See the source code of
AbstractTestNGSpringContextTests for an example of how to instrument your test class.
14.6 Spring MVC Test Framework
The Spring MVC Test framework provides first class support for testing Spring MVC code using a fluent
API that can be used with JUnit, TestNG, or any other testing framework. It’s built on the Servlet API
mock objects from the spring-test module and hence does not use a running Servlet container. It
uses the DispatcherServlet to provide full Spring MVC runtime behavior and provides support for
loading actual Spring configuration with the TestContext framework in addition to a standalone mode in
which controllers may be instantiated manually and tested one at a time.
Spring MVC Test also provides client-side support for testing code that uses the RestTemplate. Clientside tests mock the server responses and also do not use a running server.
Tip
Spring Boot provides an option to write full, end-to-end integration tests that include a running
server. If this is your goal please have a look at the Spring Boot reference page. For more
information on the differences between out-of-container and end-to-end integration tests, see the
section called “Differences between Out-of-Container and End-to-End Integration Tests”.
Server-Side Tests
It’s easy to write a plain unit test for a Spring MVC controller using JUnit or TestNG: simply
instantiate the controller, inject it with mocked or stubbed dependencies, and call its methods passing
MockHttpServletRequest, MockHttpServletResponse, etc., as necessary. However, when
writing such a unit test, much remains untested: for example, request mappings, data binding, type
conversion, validation, and much more. Furthermore, other controller methods such as @InitBinder,
@ModelAttribute, and @ExceptionHandler may also be invoked as part of the request processing
lifecycle.
The goal of Spring MVC Test is to provide an effective way for testing controllers by performing requests
and generating responses through the actual DispatcherServlet.
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Spring MVC Test builds on the familiar "mock" implementations of the Servlet API available in the
spring-test module. This allows performing requests and generating responses without the need
for running in a Servlet container. For the most part everything should work as it does at runtime with
a few notable exceptions as explained in the section called “Differences between Out-of-Container and
End-to-End Integration Tests”. Here is a JUnit-based example of using Spring MVC Test:
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
@RunWith(SpringJUnit4ClassRunner.class)
@WebAppConfiguration
@ContextConfiguration("test-servlet-context.xml")
public class ExampleTests {
@Autowired
private WebApplicationContext wac;
private MockMvc mockMvc;
@Before
public void setup() {
this.mockMvc = MockMvcBuilders.webAppContextSetup(this.wac).build();
}
@Test
public void getAccount() throws Exception {
this.mockMvc.perform(get("/accounts/1").accept(MediaType.parseMediaType("application/
json;charset=UTF-8")))
.andExpect(status().isOk())
.andExpect(content().contentType("application/json"))
.andExpect(jsonPath("$.name").value("Lee"));
}
}
The above test relies on the WebApplicationContext support of the TestContext framework for
loading Spring configuration from an XML configuration file located in the same package as the test
class, but Java-based and Groovy-based configuration are also supported. See these sample tests.
The MockMvc instance is used to perform a GET request to "/accounts/1" and verify that the resulting
response has status 200, the content type is "application/json", and the response body has a
JSON property called "name" with the value "Lee". The jsonPath syntax is supported through the
Jayway JsonPath project. There are lots of other options for verifying the result of the performed request
that will be discussed below.
Static Imports
The fluent API in the example above requires a few static imports such as
MockMvcRequestBuilders.*, MockMvcResultMatchers.*, and MockMvcBuilders.*. An
easy way to find these classes is to search for types matching "MockMvc*". If using Eclipse, be sure to
add them as "favorite static members" in the Eclipse preferences under Java # Editor # Content Assist #
Favorites. That will allow use of content assist after typing the first character of the static method name.
Other IDEs (e.g. IntelliJ) may not require any additional configuration. Just check the support for code
completion on static members.
Setup Options
There are two main options for creating an instance of MockMvc. The first is to load Spring MVC
configuration through the TestContext framework, which loads the Spring configuration and injects a
WebApplicationContext into the test to use to build a MockMvc instance:
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@RunWith(SpringJUnit4ClassRunner.class)
@WebAppConfiguration
@ContextConfiguration("my-servlet-context.xml")
public class MyWebTests {
@Autowired
private WebApplicationContext wac;
private MockMvc mockMvc;
@Before
public void setup() {
this.mockMvc = MockMvcBuilders.webAppContextSetup(this.wac).build();
}
// ...
}
The second is to simply create a controller instance manually without loading Spring configuration.
Instead basic default configuration, roughly comparable to that of the MVC JavaConfig or the MVC
namespace, is automatically created and can be customized to a degree:
public class MyWebTests {
private MockMvc mockMvc;
@Before
public void setup() {
this.mockMvc = MockMvcBuilders.standaloneSetup(new AccountController()).build();
}
// ...
}
Which setup option should you use?
The "webAppContextSetup" loads your actual Spring MVC configuration resulting in a more complete
integration test. Since the TestContext framework caches the loaded Spring configuration, it helps keep
tests running fast, even as you introduce more tests in your test suite. Furthermore, you can inject mock
services into controllers through Spring configuration in order to remain focused on testing the web
layer. Here is an example of declaring a mock service with Mockito:
<bean id="accountService" class="org.mockito.Mockito" factory-method="mock">
<constructor-arg value="org.example.AccountService"/>
</bean>
You can then inject the mock service into the test in order set up and verify expectations:
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@RunWith(SpringJUnit4ClassRunner.class)
@WebAppConfiguration
@ContextConfiguration("test-servlet-context.xml")
public class AccountTests {
@Autowired
private WebApplicationContext wac;
private MockMvc mockMvc;
@Autowired
private AccountService accountService;
// ...
}
The "standaloneSetup" on the other hand is a little closer to a unit test. It tests one controller at a
time: the controller can be injected with mock dependencies manually, and it doesn’t involve loading
Spring configuration. Such tests are more focused on style and make it easier to see which controller
is being tested, whether any specific Spring MVC configuration is required to work, and so on. The
"standaloneSetup" is also a very convenient way to write ad-hoc tests to verify specific behavior or to
debug an issue.
Just like with any "integration vs. unit testing" debate, there is no right or wrong answer. However,
using the "standaloneSetup" does imply the need for additional "webAppContextSetup" tests in
order to verify your Spring MVC configuration. Alternatively, you may choose to write all tests with
"webAppContextSetup" in order to always test against your actual Spring MVC configuration.
Performing Requests
It’s easy to perform requests using any HTTP method:
mockMvc.perform(post("/hotels/{id}", 42).accept(MediaType.APPLICATION_JSON));
You can also perform file upload requests that internally use MockMultipartHttpServletRequest
so that there is no actual parsing of a multipart request but rather you have to set it up:
mockMvc.perform(fileUpload("/doc").file("a1", "ABC".getBytes("UTF-8")));
You can specify query parameters in URI template style:
mockMvc.perform(get("/hotels?foo={foo}", "bar"));
Or you can add Servlet request parameters representing either query of form parameters:
mockMvc.perform(get("/hotels").param("foo", "bar"));
If application code relies on Servlet request parameters and doesn’t check the query string explicitly (as
is most often the case) then it doesn’t matter which option you use. Keep in mind however that query
params provided with the URI template will be decoded while request parameters provided through the
param(…) method are expected to already be decoded.
In most cases it’s preferable to leave out the context path and the Servlet path from the request URI. If
you must test with the full request URI, be sure to set the contextPath and servletPath accordingly
so that request mappings will work:
mockMvc.perform(get("/app/main/hotels/{id}").contextPath("/app").servletPath("/main"))
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Looking at the above example, it would be cumbersome to set the contextPath and servletPath with
every performed request. Instead you can set up default request properties:
public class MyWebTests {
private MockMvc mockMvc;
@Before
public void setup() {
mockMvc = standaloneSetup(new AccountController())
.defaultRequest(get("/")
.contextPath("/app").servletPath("/main")
.accept(MediaType.APPLICATION_JSON).build();
}
The above properties will affect every request performed through the MockMvc instance. If the same
property is also specified on a given request, it overrides the default value. That is why the HTTP method
and URI in the default request don’t matter since they must be specified on every request.
Defining Expectations
Expectations can be defined by appending one or more .andExpect(..) calls after performing a
request:
mockMvc.perform(get("/accounts/1")).andExpect(status().isOk());
MockMvcResultMatchers.* provides a number of expectations, some of which are further nested
with more detailed expectations.
Expectations fall in two general categories. The first category of assertions verifies properties of the
response: for example, the response status, headers, and content. These are the most important results
to assert.
The second category of assertions goes beyond the response. These assertions allow one to inspect
Spring MVC specific aspects such as which controller method processed the request, whether an
exception was raised and handled, what the content of the model is, what view was selected, what
flash attributes were added, and so on. They also allow one to inspect Servlet specific aspects such
as request and session attributes.
The following test asserts that binding or validation failed:
mockMvc.perform(post("/persons"))
.andExpect(status().isOk())
.andExpect(model().attributeHasErrors("person"));
Many times when writing tests, it’s useful to dump the results of the performed request. This can be
done as follows, where print() is a static import from MockMvcResultHandlers:
mockMvc.perform(post("/persons"))
.andDo(print())
.andExpect(status().isOk())
.andExpect(model().attributeHasErrors("person"));
As long as request processing does not cause an unhandled exception, the print() method will print
all the available result data to System.out. Spring Framework 4.2 introduces a log() method and two
additional variants of the print() method, one that accepts an OutputStream and one that accepts a
Writer. For example, invoking print(System.err) will print the result data to System.err; while
invoking print(myWriter) will print the result data to a custom writer. If you would like to have the
result data logged instead of printed, simply invoke the log() method which will log the result data as
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a single DEBUG message under the org.springframework.test.web.servlet.result logging
category.
In some cases, you may want to get direct access to the result and verify something that cannot be
verified otherwise. This can be achieved by appending .andReturn() after all other expectations:
MvcResult mvcResult = mockMvc.perform(post("/persons")).andExpect(status().isOk()).andReturn();
// ...
If all tests repeat the same expectations you can set up common expectations once when building the
MockMvc instance:
standaloneSetup(new SimpleController())
.alwaysExpect(status().isOk())
.alwaysExpect(content().contentType("application/json;charset=UTF-8"))
.build()
Note that common expectations are always applied and cannot be overridden without creating a
separate MockMvc instance.
When JSON response content contains hypermedia links created with Spring HATEOAS, the resulting
links can be verified using JsonPath expressions:
mockMvc.perform(get("/people").accept(MediaType.APPLICATION_JSON))
.andExpect(jsonPath("$.links[?(@.rel == ''self'')].href").value("http://localhost:8080/people"));
When XML response content contains hypermedia links created with Spring HATEOAS, the resulting
links can be verified using XPath expressions:
Map<String, String> ns = Collections.singletonMap("ns", "http://www.w3.org/2005/Atom");
mockMvc.perform(get("/handle").accept(MediaType.APPLICATION_XML))
.andExpect(xpath("/person/ns:link[@rel=''self'']/@href", ns).string("http://localhost:8080/
people"));
Filter Registrations
When setting up a MockMvc instance, you can register one or more Servlet Filter instances:
mockMvc = standaloneSetup(new PersonController()).addFilters(new CharacterEncodingFilter()).build();
Registered filters will be invoked through via the MockFilterChain from spring-test, and the last
filter will delegate to the DispatcherServlet.
Differences between Out-of-Container and End-to-End Integration Tests
As mentioned earlier Spring MVC Test is built on the Servlet API mock objects from the spring-test
module and does not use a running Servlet container. Therefore there are some important differences
compared to full end-to-end integration tests with an actual client and server running.
The easiest way to think about this is starting with a blank MockHttpServletRequest. Whatever
you add to it is what the request will be. Things that may catch you by surprise are that there
is no context path by default, no jsessionid cookie, no forwarding, error, or async dispatches,
and therefore no actual JSP rendering. Instead, "forwarded" and "redirected" URLs are saved in the
MockHttpServletResponse and can be asserted with expectations.
This means if you are using JSPs you can verify the JSP page to which the request was forwarded,
but there won’t be any HTML rendered. In other words, the JSP will not be invoked. Note however that
all other rendering technologies which don’t rely on forwarding such as Thymeleaf, Freemarker, and
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Velocity will render HTML to the response body as expected. The same is true for rendering JSON,
XML, and other formats via @ResponseBody methods.
Alternatively you may consider the full end-to-end integration testing support from Spring Boot via
@WebIntegrationTest. See the Spring Boot reference.
There are pros and cons for each approach. The options provided in Spring MVC Test are different stops
on the scale from classic unit testing to full integration testing. To be certain, none of the options in Spring
MVC Test fall under the category of classic unit testing, but they are a little closer to it. For example,
you can isolate the web layer by injecting mocked services into controllers, in which case you’re testing
the web layer only through the DispatcherServlet but with actual Spring configuration, just like you
might test the data access layer in isolation from the layers above. Or you can use the standalone setup
focusing on one controller at a time and manually providing the configuration required to make it work.
Another important distinction when using Spring MVC Test is that conceptually such tests are on the
inside of the server-side so you can check what handler was used, if an exception was handled with a
HandlerExceptionResolver, what the content of the model is, what binding errors there were, etc. That
means it’s easier to write expectations since the server is not a black box as it is when testing it through
an actual HTTP client. This is generally an advantage of classic unit testing, that it’s easier to write,
reason about, and debug but does not replace the need for full integration tests. At the same time it’s
important not to lose sight of the fact that the response is the most important thing to check. In short,
there is room here for multiple styles and strategies of testing even within the same project.
Further Server-Side Test Examples
The framework’s own tests include many sample tests intended to demonstrate how to use Spring MVC
Test. Browse these examples for further ideas. Also the spring-mvc-showcase has full test coverage
based on Spring MVC Test.
HtmlUnit Integration
Spring provides integration between MockMvc and HtmlUnit. This simplifies performing end-to-end
testing when using HTML based views. This integration enables developers to:
• Easily test HTML pages using tools such as HtmlUnit, WebDriver, & Geb without the need to deploy
to a Servlet container
• Test JavaScript within pages
• Optionally test using mock services to speed up testing
• Share logic between in-container end-to-end tests and out-of-container integration tests
Note
MockMvc works with templating technologies that do not rely on a Servlet Container (e.g.,
Thymeleaf, Freemarker, Velocity, etc.), but it does not work with JSPs since they rely on the
Servlet Container.
Why HtmlUnit Integration?
The most obvious question that comes to mind is, "Why do I need this?". The answer is best found by
exploring a very basic sample application. Assume you have a Spring MVC web application that supports
CRUD operations on a Message object. The application also supports paging through all messages.
How would you go about testing it?
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With Spring MVC Test, we can easily test if we are able to create a Message.
MockHttpServletRequestBuilder createMessage = post("/messages/")
.param("summary", "Spring Rocks")
.param("text", "In case you didn't know, Spring Rocks!");
mockMvc.perform(createMessage)
.andExpect(status().is3xxRedirection())
.andExpect(redirectedUrl("/messages/123"));
What if we want to test our form view that allows us to create the message? For example, assume our
form looks like the following snippet:
<form id="messageForm" action="/messages/" method="post">
<div class="pull-right"><a href="/messages/">Messages</a></div>
<label for="summary">Summary</label>
<input type="text" class="required" id="summary" name="summary" value="" />
<label for="text">Message</label>
<textarea id="text" name="text"></textarea>
<div class="form-actions">
<input type="submit" value="