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HIBERNATE - Relational
Persistence for Idiomatic Java
Hibernate Reference Documentation
3.3.1
HIBERNATE - Relational Persistence for Idiomatic Java
Copyright © 2004 Red Hat Middleware, LLC.
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Preface ......................................................................................................... xi
1. Introduction to Hibernate ...................................................................... 1
1.1. Preface .......................................................................................... 1
1.2. Part 1 - The first Hibernate Application ......................................... 1
1.2.1. The first class ..................................................................... 2
1.2.2. The mapping file ................................................................. 4
1.2.3. Hibernate configuration ....................................................... 6
1.2.4. Building with Ant ................................................................. 8
1.2.5. Startup and helpers .......................................................... 10
1.2.6. Loading and storing objects .............................................. 11
1.3. Part 2 - Mapping associations ..................................................... 15
1.3.1. Mapping the Person class ................................................ 15
1.3.2. A unidirectional Set-based association ............................. 16
1.3.3. Working the association .................................................... 17
1.3.4. Collection of values ........................................................... 19
1.3.5. Bi-directional associations ................................................. 21
1.3.6. Working bi-directional links ............................................... 22
1.4. Part 3 - The EventManager web application ................................ 23
1.4.1. Writing the basic servlet ................................................... 23
1.4.2. Processing and rendering ................................................. 24
1.4.3. Deploying and testing ....................................................... 26
1.5. Summary ..................................................................................... 27
2. Architecture .......................................................................................... 29
2.1. Overview ...................................................................................... 29
2.2. Instance states ............................................................................ 32
2.3. JMX Integration ........................................................................... 32
2.4. JCA Support ................................................................................ 33
2.5. Contextual Sessions .................................................................... 33
3. Configuration ........................................................................................ 35
3.1. Programmatic configuration ......................................................... 35
3.2. Obtaining a SessionFactory ......................................................... 36
3.3. JDBC connections ....................................................................... 36
3.4. Optional configuration properties ................................................. 38
3.4.1. SQL Dialects ..................................................................... 43
3.4.2. Outer Join Fetching .......................................................... 43
3.4.3. Binary Streams ................................................................. 44
3.4.4. Second-level and query cache .......................................... 44
3.4.5. Query Language Substitution ............................................ 44
3.4.6. Hibernate statistics ............................................................ 44
3.5. Logging ........................................................................................ 45
3.6. Implementing a NamingStrategy ................................................... 45
3.7. XML configuration file .................................................................. 46
3.8. J2EE Application Server integration ............................................ 47
Hibernate 3.3.1
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3.8.1. Transaction strategy configuration ....................................
3.8.2. JNDI-bound SessionFactory ..............................................
3.8.3. Current Session context management with JTA ...............
3.8.4. JMX deployment ...............................................................
4. Persistent Classes ...............................................................................
4.1. A simple POJO example .............................................................
4.1.1. Implement a no-argument constructor ..............................
4.1.2. Provide an identifier property (optional) ............................
4.1.3. Prefer non-final classes (optional) .....................................
4.1.4. Declare accessors and mutators for persistent fields
(optional) .....................................................................................
4.2. Implementing inheritance .............................................................
4.3. Implementing equals() and hashCode() .......................................
4.4. Dynamic models ..........................................................................
4.5. Tuplizers ......................................................................................
4.6. Extentsions ..................................................................................
5. Basic O/R Mapping ..............................................................................
5.1. Mapping declaration ....................................................................
5.1.1. Doctype .............................................................................
5.1.1.1. EntityResolver ........................................................
5.1.2. hibernate-mapping ............................................................
5.1.3. class ..................................................................................
5.1.4. id .......................................................................................
5.1.4.1. Generator ...............................................................
5.1.4.2. Hi/lo algorithm ........................................................
5.1.4.3. UUID algorithm .......................................................
5.1.4.4. Identity columns and sequences ............................
5.1.4.5. Assigned identifiers ................................................
5.1.4.6. Primary keys assigned by triggers .........................
5.1.5. Enhanced identifier generators .........................................
5.1.6. Identifier generator optimization ........................................
5.1.7. composite-id ......................................................................
5.1.8. discriminator ......................................................................
5.1.9. version (optional) ..............................................................
5.1.10. timestamp (optional) .......................................................
5.1.11. property ...........................................................................
5.1.12. many-to-one ....................................................................
5.1.13. one-to-one .......................................................................
5.1.14. natural-id .........................................................................
5.1.15. component, dynamic-component ....................................
5.1.16. properties ........................................................................
5.1.17. subclass ..........................................................................
5.1.18. joined-subclass ...............................................................
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5.1.19. union-subclass ................................................................ 93
5.1.20. join .................................................................................. 94
5.1.21. key .................................................................................. 95
5.1.22. column and formula elements ......................................... 96
5.1.23. import .............................................................................. 97
5.1.24. any .................................................................................. 97
5.2. Hibernate Types .......................................................................... 98
5.2.1. Entities and values ........................................................... 98
5.2.2. Basic value types .............................................................. 99
5.2.3. Custom value types ........................................................ 101
5.3. Mapping a class more than once .............................................. 102
5.4. SQL quoted identifiers ............................................................... 103
5.5. Metadata alternatives ................................................................ 103
5.5.1. Using XDoclet markup .................................................... 103
5.5.2. Using JDK 5.0 Annotations ............................................. 106
5.6. Generated Properties ................................................................ 106
5.7. Auxiliary Database Objects ........................................................ 107
6. Collection Mapping ............................................................................ 109
6.1. Persistent collections ................................................................. 109
6.2. Collection mappings .................................................................. 110
6.2.1. Collection foreign keys .................................................... 112
6.2.2. Collection elements ......................................................... 112
6.2.3. Indexed collections ......................................................... 112
6.2.4. Collections of values and many-to-many associations.... 113
6.2.5. One-to-many associations .............................................. 116
6.3. Advanced collection mappings .................................................. 117
6.3.1. Sorted collections ............................................................ 117
6.3.2. Bidirectional associations ................................................ 118
6.3.3. Bidirectional associations with indexed collections .......... 119
6.3.4. Ternary associations ....................................................... 121
6.3.5. Using an <idbag> ............................................................ 121
6.4. Collection examples ................................................................... 122
7. Association Mappings ....................................................................... 127
7.1. Introduction ................................................................................ 127
7.2. Unidirectional associations ........................................................ 127
7.2.1. many to one .................................................................... 127
7.2.2. one to one ...................................................................... 128
7.2.3. one to many .................................................................... 129
7.3. Unidirectional associations with join tables ................................ 129
7.3.1. one to many .................................................................... 129
7.3.2. many to one .................................................................... 130
7.3.3. one to one ...................................................................... 130
7.3.4. many to many ................................................................. 131
Hibernate 3.3.1
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7.4. Bidirectional associations ........................................................... 132
7.4.1. one to many / many to one ............................................. 132
7.4.2. one to one ...................................................................... 133
7.5. Bidirectional associations with join tables .................................. 134
7.5.1. one to many / many to one ............................................. 134
7.5.2. one to one ...................................................................... 135
7.5.3. many to many ................................................................. 135
7.6. More complex association mappings ......................................... 136
8. Component Mapping .......................................................................... 139
8.1. Dependent objects ..................................................................... 139
8.2. Collections of dependent objects ............................................... 141
8.3. Components as Map indices ..................................................... 142
8.4. Components as composite identifiers ........................................ 143
8.5. Dynamic components ................................................................ 144
9. Inheritance Mapping ........................................................................... 147
9.1. The Three Strategies ................................................................. 147
9.1.1. Table per class hierarchy ................................................ 148
9.1.2. Table per subclass .......................................................... 148
9.1.3. Table per subclass, using a discriminator ....................... 149
9.1.4. Mixing table per class hierarchy with table per subclass
.................................................................................................... 150
9.1.5. Table per concrete class ................................................. 150
9.1.6. Table per concrete class, using implicit polymorphism.... 151
9.1.7. Mixing implicit polymorphism with other inheritance
mappings ................................................................................... 152
9.2. Limitations .................................................................................. 153
10. Working with objects ....................................................................... 155
10.1. Hibernate object states ............................................................ 155
10.2. Making objects persistent ........................................................ 156
10.3. Loading an object .................................................................... 157
10.4. Querying .................................................................................. 158
10.4.1. Executing queries ......................................................... 158
10.4.1.1. Iterating results ................................................... 159
10.4.1.2. Queries that return tuples ................................... 159
10.4.1.3. Scalar results ..................................................... 160
10.4.1.4. Bind parameters ................................................. 160
10.4.1.5. Pagination .......................................................... 161
10.4.1.6. Scrollable iteration .............................................. 161
10.4.1.7. Externalizing named queries .............................. 162
10.4.2. Filtering collections ....................................................... 162
10.4.3. Criteria queries .............................................................. 163
10.4.4. Queries in native SQL .................................................. 163
10.5. Modifying persistent objects .................................................... 164
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Hibernate 3.3.1
11.
12.
13.
14.
10.6. Modifying detached objects .....................................................
10.7. Automatic state detection ........................................................
10.8. Deleting persistent objects .......................................................
10.9. Replicating object between two different datastores ................
10.10. Flushing the Session .............................................................
10.11. Transitive persistence ............................................................
10.12. Using metadata ......................................................................
Transactions And Concurrency ......................................................
11.1. Session and transaction scopes ..............................................
11.1.1. Unit of work ...................................................................
11.1.2. Long conversations .......................................................
11.1.3. Considering object identity ............................................
11.1.4. Common issues ............................................................
11.2. Database transaction demarcation ..........................................
11.2.1. Non-managed environment ...........................................
11.2.2. Using JTA .....................................................................
11.2.3. Exception handling ........................................................
11.2.4. Transaction timeout ......................................................
11.3. Optimistic concurrency control .................................................
11.3.1. Application version checking .........................................
11.3.2. Extended session and automatic versioning .................
11.3.3. Detached objects and automatic versioning ..................
11.3.4. Customizing automatic versioning .................................
11.4. Pessimistic Locking .................................................................
11.5. Connection Release Modes .....................................................
Interceptors and events ...................................................................
12.1. Interceptors ..............................................................................
12.2. Event system ...........................................................................
12.3. Hibernate declarative security ..................................................
Batch processing .............................................................................
13.1. Batch inserts ............................................................................
13.2. Batch updates ..........................................................................
13.3. The StatelessSession interface ...............................................
13.4. DML-style operations ...............................................................
HQL: The Hibernate Query Language ............................................
14.1. Case Sensitivity .......................................................................
14.2. The from clause .......................................................................
14.3. Associations and joins .............................................................
14.4. Forms of join syntax ................................................................
14.5. Refering to identifier property ..................................................
14.6. The select clause ....................................................................
14.7. Aggregate functions .................................................................
14.8. Polymorphic queries ................................................................
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14.9. The where clause .................................................................... 208
14.10. Expressions ........................................................................... 210
14.11. The order by clause ............................................................... 214
14.12. The group by clause .............................................................. 214
14.13. Subqueries ............................................................................. 215
14.14. HQL examples ....................................................................... 215
14.15. Bulk update and delete .......................................................... 218
14.16. Tips & Tricks .......................................................................... 218
14.17. Components ........................................................................... 219
14.18. Row value constructor syntax ................................................ 219
Criteria Queries ................................................................................ 221
15.1. Creating a Criteria instance ................................................... 221
15.2. Narrowing the result set .......................................................... 221
15.3. Ordering the results ................................................................. 222
15.4. Associations ............................................................................. 222
15.5. Dynamic association fetching .................................................. 223
15.6. Example queries ...................................................................... 223
15.7. Projections, aggregation and grouping .................................... 224
15.8. Detached queries and subqueries ........................................... 226
15.9. Queries by natural identifier ..................................................... 227
Native SQL ........................................................................................ 229
16.1. Using a SQLQuery ..................................................................... 229
16.1.1. Scalar queries ............................................................... 229
16.1.2. Entity queries ................................................................ 230
16.1.3. Handling associations and collections ........................... 231
16.1.4. Returning multiple entities ............................................. 231
16.1.4.1. Alias and property references ............................ 232
16.1.5. Returning non-managed entities ................................... 233
16.1.6. Handling inheritance ..................................................... 234
16.1.7. Parameters .................................................................... 234
16.2. Named SQL queries ................................................................ 234
16.2.1. Using return-property to explicitly specify column/alias
names ....................................................................................... 236
16.2.2. Using stored procedures for querying ........................... 237
16.2.2.1. Rules/limitations for using stored procedures ..... 237
16.3. Custom SQL for create, update and delete ............................. 238
16.4. Custom SQL for loading .......................................................... 239
Filtering data ..................................................................................... 241
17.1. Hibernate filters ....................................................................... 241
XML Mapping .................................................................................... 245
18.1. Working with XML data ........................................................... 245
18.1.1. Specifying XML and class mapping together ................ 245
18.1.2. Specifying only an XML mapping .................................. 246
Hibernate 3.3.1
18.2. XML mapping metadata ..........................................................
18.3. Manipulating XML data ............................................................
19. Improving performance ....................................................................
19.1. Fetching strategies ..................................................................
19.1.1. Working with lazy associations .....................................
19.1.2. Tuning fetch strategies ..................................................
19.1.3. Single-ended association proxies ..................................
19.1.4. Initializing collections and proxies .................................
19.1.5. Using batch fetching .....................................................
19.1.6. Using subselect fetching ...............................................
19.1.7. Using lazy property fetching ..........................................
19.2. The Second Level Cache ........................................................
19.2.1. Cache mappings ...........................................................
19.2.2. Strategy: read only ........................................................
19.2.3. Strategy: read/write .......................................................
19.2.4. Strategy: nonstrict read/write ........................................
19.2.5. Strategy: transactional ..................................................
19.2.6. Cache-provider/concurrency-strategy compatibility .......
19.3. Managing the caches ..............................................................
19.4. The Query Cache ....................................................................
19.5. Understanding Collection performance ....................................
19.5.1. Taxonomy .....................................................................
19.5.2. Lists, maps, idbags and sets are the most efficient
collections to update .................................................................
19.5.3. Bags and lists are the most efficient inverse collections
....................................................................................................
19.5.4. One shot delete ............................................................
19.6. Monitoring performance ...........................................................
19.6.1. Monitoring a SessionFactory .........................................
19.6.2. Metrics ..........................................................................
20. Toolset Guide ...................................................................................
20.1. Automatic schema generation .................................................
20.1.1. Customizing the schema ...............................................
20.1.2. Running the tool ...........................................................
20.1.3. Properties ......................................................................
20.1.4. Using Ant ......................................................................
20.1.5. Incremental schema updates ........................................
20.1.6. Using Ant for incremental schema updates ...................
20.1.7. Schema validation .........................................................
20.1.8. Using Ant for schema validation ...................................
21. Example: Parent/Child .....................................................................
21.1. A note about collections ..........................................................
21.2. Bidirectional one-to-many ........................................................
Hibernate 3.3.1
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21.3. Cascading life cycle .................................................................
21.4. Cascades and unsaved-value ..................................................
21.5. Conclusion ...............................................................................
22. Example: Weblog Application .........................................................
22.1. Persistent Classes ...................................................................
22.2. Hibernate Mappings .................................................................
22.3. Hibernate Code .......................................................................
23. Example: Various Mappings ............................................................
23.1. Employer/Employee .................................................................
23.2. Author/Work .............................................................................
23.3. Customer/Order/Product ..........................................................
23.4. Miscellaneous example mappings ...........................................
23.4.1. "Typed" one-to-one association ....................................
23.4.2. Composite key example ................................................
23.4.3. Many-to-many with shared composite key attribute .......
23.4.4. Content based discrimination ........................................
23.4.5. Associations on alternate keys ......................................
24. Best Practices ...................................................................................
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Hibernate 3.3.1
Preface
Working with object-oriented software and a relational database can be
cumbersome and time consuming in today's enterprise environments.
Hibernate is an object/relational mapping tool for Java environments. The
term object/relational mapping (ORM) refers to the technique of mapping a
data representation from an object model to a relational data model with a
SQL-based schema.
Hibernate not only takes care of the mapping from Java classes to database
tables (and from Java data types to SQL data types), but also provides data
query and retrieval facilities and can significantly reduce development time
otherwise spent with manual data handling in SQL and JDBC.
Hibernates goal is to relieve the developer from 95 percent of common
data persistence related programming tasks. Hibernate may not be the
best solution for data-centric applications that only use stored-procedures
to implement the business logic in the database, it is most useful with
object-oriented domain models and business logic in the Java-based
middle-tier. However, Hibernate can certainly help you to remove or
encapsulate vendor-specific SQL code and will help with the common task of
result set translation from a tabular representation to a graph of objects.
If you are new to Hibernate and Object/Relational Mapping or even Java,
please follow these steps:
1. Read Chapter 1, Introduction to Hibernate for a tutorial with step-by-step
instructions. The source code for the tutorial is included in the distribution
in the doc/reference/tutorial/ directory.
2. Read Chapter 2, Architecture to understand the environments where
Hibernate can be used.
3. Have a look at the eg/ directory in the Hibernate distribution, it contains a
simple standalone application. Copy your JDBC driver to the lib/ directory
and edit etc/hibernate.properties, specifying correct values for your
database. From a command prompt in the distribution directory, type ant
eg (using Ant), or under Windows, type build eg.
4. Use this reference documentation as your primary source of
information. Consider reading Java Persistence with Hibernate
(http://www.manning.com/bauer2) if you need more help with
application design or if you prefer a step-by-step tutorial. Also visit
http://caveatemptor.hibernate.org and download the example application
for Java Persistence with Hibernate.
Hibernate 3.3.1
xi
5. FAQs are answered on the Hibernate website.
6. Third party demos, examples, and tutorials are linked on the Hibernate
website.
7. The Community Area on the Hibernate website is a good resource for
design patterns and various integration solutions (Tomcat, JBoss AS,
Struts, EJB, etc.).
If you have questions, use the user forum linked on the Hibernate website.
We also provide a JIRA issue trackings system for bug reports and feature
requests. If you are interested in the development of Hibernate, join the
developer mailing list. If you are interested in translating this documentation
into your language, contact us on the developer mailing list.
Commercial development support, production support, and
training for Hibernate is available through JBoss Inc. (see
http://www.hibernate.org/SupportTraining/). Hibernate is a Professional Open
Source project and a critical component of the JBoss Enterprise Middleware
System (JEMS) suite of products.
xii
Hibernate 3.3.1
Chapter 1. Introduction to Hibernate
1.1. Preface
This chapter is an introduction to Hibernate by way of a tutorial, intended
for new users of Hibernate. We start with a simple application using an
in-memory database. We build the application in small, easy to understand
steps. The tutorial is based on another, earlier one developed by Michael
Gloegl. All code is contained in the tutorials/web directory of the project
source.
Important
This tutorial expects the user have knowledge of both Java and SQL.
If you are new or uncomfortable with either, it is advised that you start
with a good introduction to that technology prior to attempting to learn
Hibernate. It will save time and effort in the long run.
Note
There is another tutorial/example application in the /tutorials/eg
directory of the project source. That example is console based and
as such would not have the dependency on a servlet container to
execute. The basic setup is the same as the instructions below.
1.2. Part 1 - The first Hibernate Application
Let's assume we need a small database application that can store events
we want to attend, and information about the host(s) of these events. We
will use an in-memory, Java database named HSQLDB to avoid describing
installation/setup of any particular database servers. Feel free to tweak this
tutorial to use whatever database you feel comfortable using.
The first thing we need to do is set up our development environment, and
specifically to setup all the required dependencies to Hibernate as well as
other libraries. Hibernate is built using Maven which amongst other features
provides dependecy management; moreover it provides transitive dependecy
management which simply means that to use Hibernate we can simply define
our dependency on Hibernate, Hibernate itself defines the dependencies it
needs which then become transitive dependencies of our project.
.
<project xmlns="http://maven.apache.org/POM/4.0.0"
Hibernate 3.3.1
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Chapter 1. Introduction to Hibernate
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
http://maven.apache.org/xsd/maven-4.0.0.xsd">
...
<dependencies>
<dependency>
<groupId>${groupId}</groupId>
<artifactId>hibernate-core</artifactId>
</dependency>
<!-- Because this is a web app, we also have a dependency on
the servlet api. -->
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
</dependency>
</dependencies>
</project>
Note
Essentially we are describing here the /tutorials/web/pom.xml file.
See the Maven [http://maven.org] site for more information.
Tip
While not strictly necessary, most IDEs have integration with Maven
to read these POM files and automatically set up a project for you
which can save lots of time and effort.
Next we create a class that represents the event we want to store in
database.
1.2.1. The first class
Our first persistent class is a simple JavaBean class with some properties:
package org.hibernate.tutorial.domain;
import java.util.Date;
public class Event {
private Long id;
private String title;
private Date date;
2
Hibernate 3.3.1
The first class
public Event() {}
public Long getId() {
return id;
}
private void setId(Long id) {
this.id = id;
}
public Date getDate() {
return date;
}
public void setDate(Date date) {
this.date = date;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
}
You can see that this class uses standard JavaBean naming conventions for
property getter and setter methods, as well as private visibility for the fields.
This is a recommended design - but not required. Hibernate can also access
fields directly, the benefit of accessor methods is robustness for refactoring.
The no-argument constructor is required to instantiate an object of this class
through reflection.
The id property holds a unique identifier value for a particular event. All
persistent entity classes (there are less important dependent classes as
well) will need such an identifier property if we want to use the full feature
set of Hibernate. In fact, most applications (esp. web applications) need to
distinguish objects by identifier, so you should consider this a feature rather
than a limitation. However, we usually don't manipulate the identity of an
object, hence the setter method should be private. Only Hibernate will assign
identifiers when an object is saved. You can see that Hibernate can access
public, private, and protected accessor methods, as well as (public, private,
protected) fields directly. The choice is up to you and you can match it to fit
your application design.
The no-argument constructor is a requirement for all persistent classes;
Hibernate has to create objects for you, using Java Reflection. The
constructor can be private, however, package visibility is required for
Hibernate 3.3.1
3
Chapter 1. Introduction to Hibernate
runtime proxy generation and efficient data retrieval without bytecode
instrumentation.
Place this Java source file in a directory called src in the development folder,
and in its correct package. The directory should now look like this:
.
+lib
<Hibernate and third-party libraries>
+src
+events
Event.java
In the next step, we tell Hibernate about this persistent class.
1.2.2. The mapping file
Hibernate needs to know how to load and store objects of the persistent
class. This is where the Hibernate mapping file comes into play. The
mapping file tells Hibernate what table in the database it has to access, and
what columns in that table it should use.
The basic structure of a mapping file looks like this:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
[...]
</hibernate-mapping>
Note that the Hibernate DTD is very sophisticated. You can use it for
auto-completion of XML mapping elements and attributes in your editor or
IDE. You also should open up the DTD file in your text editor - it's the easiest
way to get an overview of all elements and attributes and to see the defaults,
as well as some comments. Note that Hibernate will not load the DTD file
from the web, but first look it up from the classpath of the application. The
DTD file is included in hibernate3.jar as well as in the src/ directory of the
Hibernate distribution.
We will omit the DTD declaration in future examples to shorten the code. It is
of course not optional.
Between the two hibernate-mapping tags, include a class element. All
persistent entity classes (again, there might be dependent classes later on,
4
Hibernate 3.3.1
The mapping file
which are not first-class entities) need such a mapping, to a table in the SQL
database:
<hibernate-mapping>
<class name="events.Event" table="EVENTS">
</class>
</hibernate-mapping>
So far we told Hibernate how to persist and load object of class Event to
the table EVENTS, each instance represented by a row in that table. Now we
continue with a mapping of the unique identifier property to the tables primary
key. In addition, as we don't want to care about handling this identifier, we
configure Hibernate's identifier generation strategy for a surrogate primary
key column:
<hibernate-mapping>
<class name="events.Event" table="EVENTS">
<id name="id" column="EVENT_ID">
<generator class="native"/>
</id>
</class>
</hibernate-mapping>
The id element is the declaration of the identifier property, name="id"
declares the name of the Java property - Hibernate will use the getter and
setter methods to access the property. The column attribute tells Hibernate
which column of the EVENTS table we use for this primary key. The nested
generator element specifies the identifier generation strategy, in this case
we used native, which picks the best strategy depending on the configured
database (dialect). Hibernate supports database generated, globally unique,
as well as application assigned identifiers (or any strategy you have written
an extension for).
Finally we include declarations for the persistent properties of the class in the
mapping file. By default, no properties of the class are considered persistent:
<hibernate-mapping>
<class name="events.Event" table="EVENTS">
<id name="id" column="EVENT_ID">
<generator class="native"/>
</id>
<property name="date" type="timestamp" column="EVENT_DATE"/>
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Chapter 1. Introduction to Hibernate
<property name="title"/>
</class>
</hibernate-mapping>
Just as with the id element, the name attribute of the property element tells
Hibernate which getter and setter methods to use. So, in this case, Hibernate
will look for getDate()/setDate(), as well as getTitle()/setTitle().
Why does the date property mapping include the column attribute, but the
title doesn't? Without the column attribute Hibernate by default uses the
property name as the column name. This works fine for title. However, date
is a reserved keyword in most database, so we better map it to a different
name.
The next interesting thing is that the title mapping also lacks a type
attribute. The types we declare and use in the mapping files are not, as you
might expect, Java data types. They are also not SQL database types. These
types are so called Hibernate mapping types, converters which can translate
from Java to SQL data types and vice versa. Again, Hibernate will try to
determine the correct conversion and mapping type itself if the type attribute
is not present in the mapping. In some cases this automatic detection (using
Reflection on the Java class) might not have the default you expect or need.
This is the case with the date property. Hibernate can't know if the property
(which is of java.util.Date) should map to a SQL date, timestamp, or time
column. We preserve full date and time information by mapping the property
with a timestamp converter.
This mapping file should be saved as Event.hbm.xml, right in the directory
next to the Event Java class source file. The naming of mapping files can
be arbitrary, however the hbm.xml suffix is a convention in the Hibernate
developer community. The directory structure should now look like this:
.
+lib
<Hibernate and third-party libraries>
+src
+events
Event.java
Event.hbm.xml
We continue with the main configuration of Hibernate.
1.2.3. Hibernate configuration
We now have a persistent class and its mapping file in place. It is time to
configure Hibernate. Before we do this, we will need a database. HSQL
6
Hibernate 3.3.1
Hibernate configuration
DB, a java-based SQL DBMS, can be downloaded from the HSQL DB
website(http://hsqldb.org/). Actually, you only need the hsqldb.jar from this
download. Place this file in the lib/ directory of the development folder.
Create a directory called data in the root of the development directory this is where HSQL DB will store its data files. Now start the database by
running java -classpath ../lib/hsqldb.jar org.hsqldb.Server in this data
directory. You can see it start up and bind to a TCP/IP socket, this is where
our application will connect later. If you want to start with a fresh database
during this tutorial, shutdown HSQL DB (press CTRL + C in the window),
delete all files in the data/ directory, and start HSQL DB again.
Hibernate is the layer in your application which connects to this database, so
it needs connection information. The connections are made through a JDBC
connection pool, which we also have to configure. The Hibernate distribution
contains several open source JDBC connection pooling tools, but will use the
Hibernate built-in connection pool for this tutorial. Note that you have to copy
the required library into your classpath and use different connection pooling
settings if you want to use a production-quality third party JDBC pooling
software.
For Hibernate's configuration, we can use a simple hibernate.properties
file, a slightly more sophisticated hibernate.cfg.xml file, or even complete
programmatic setup. Most users prefer the XML configuration file:
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- Database connection settings -->
<property
name="connection.driver_class">org.hsqldb.jdbcDriver</property>
<property
name="connection.url">jdbc:hsqldb:hsql://localhost</property>
<property name="connection.username">sa</property>
<property name="connection.password"></property>
<!-- JDBC connection pool (use the built-in) -->
<property name="connection.pool_size">1</property>
<!-- SQL dialect -->
<property
name="dialect">org.hibernate.dialect.HSQLDialect</property>
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Chapter 1. Introduction to Hibernate
<!-- Enable Hibernate's automatic session context management
-->
<property
name="current_session_context_class">thread</property>
<!-- Disable the second-level cache -->
<property
name="cache.provider_class">org.hibernate.cache.NoCacheProvider</
property>
<!-- Echo all executed SQL to stdout -->
<property name="show_sql">true</property>
<!-- Drop and re-create the database schema on startup -->
<property name="hbm2ddl.auto">create</property>
<mapping resource="events/Event.hbm.xml"/>
</session-factory>
</hibernate-configuration>
Note that this XML configuration uses a different DTD. We configure
Hibernate's SessionFactory - a global factory responsible for a particular
database. If you have several databases, use several <session-factory>
configurations, usually in several configuration files (for easier startup).
The first four property elements contain the necessary configuration for the
JDBC connection. The dialect property element specifies the particular SQL
variant Hibernate generates. Hibernate's automatic session management
for persistence contexts will come in handy as you will soon see. The
hbm2ddl.auto option turns on automatic generation of database schemas directly into the database. This can of course also be turned off (by removing
the config option) or redirected to a file with the help of the SchemaExport
Ant task. Finally, we add the mapping file(s) for persistent classes to the
configuration.
Copy this file into the source directory, so it will end up in the root of the
classpath. Hibernate automatically looks for a file called hibernate.cfg.xml in
the root of the classpath, on startup.
1.2.4. Building with Ant
We'll now build the tutorial with Ant. You will need to have Ant installed get it from the Ant download page [http://ant.apache.org/bindownload.cgi].
How to install Ant will not be covered here. Please refer to the Ant manual
[http://ant.apache.org/manual/index.html]. After you have installed Ant, we
can start to create the buildfile. It will be called build.xml and placed directly
in the development directory.
8
Hibernate 3.3.1
Building with Ant
A basic build file looks like this:
<project name="hibernate-tutorial" default="compile">
<property name="sourcedir" value="${basedir}/src"/>
<property name="targetdir" value="${basedir}/bin"/>
<property name="librarydir" value="${basedir}/lib"/>
<path id="libraries">
<fileset dir="${librarydir}">
<include name="*.jar"/>
</fileset>
</path>
<target name="clean">
<delete dir="${targetdir}"/>
<mkdir dir="${targetdir}"/>
</target>
<target name="compile" depends="clean, copy-resources">
<javac srcdir="${sourcedir}"
destdir="${targetdir}"
classpathref="libraries"/>
</target>
<target name="copy-resources">
<copy todir="${targetdir}">
<fileset dir="${sourcedir}">
<exclude name="**/*.java"/>
</fileset>
</copy>
</target>
</project>
This will tell Ant to add all files in the lib directory ending with .jar to the
classpath used for compilation. It will also copy all non-Java source files to
the target directory, e.g. configuration and Hibernate mapping files. If you
now run Ant, you should get this output:
C:\hibernateTutorial\>ant
Buildfile: build.xml
copy-resources:
[copy] Copying 2 files to C:\hibernateTutorial\bin
compile:
[javac] Compiling 1 source file to C:\hibernateTutorial\bin
BUILD SUCCESSFUL
Total time: 1 second
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9
Chapter 1. Introduction to Hibernate
1.2.5. Startup and helpers
It's time to load and store some Event objects, but first we have to complete
the setup with some infrastructure code. We have to startup Hibernate.
This startup includes building a global SessionFactory object and to store it
somewhere for easy access in application code. A SessionFactory can open
up new Session's. A Session represents a single-threaded unit of work, the
SessionFactory is a thread-safe global object, instantiated once.
We'll create a HibernateUtil helper class which takes care of startup and
makes accessing a SessionFactory convenient. Let's have a look at the
implementation:
package util;
import org.hibernate.*;
import org.hibernate.cfg.*;
public class HibernateUtil {
private static final SessionFactory sessionFactory;
static {
try {
// Create the SessionFactory from hibernate.cfg.xml
sessionFactory = new
Configuration().configure().buildSessionFactory();
} catch (Throwable ex) {
// Make sure you log the exception, as it might be
swallowed
System.err.println("Initial SessionFactory creation
failed." + ex);
throw new ExceptionInInitializerError(ex);
}
}
public static SessionFactory getSessionFactory() {
return sessionFactory;
}
}
This class does not only produce the global SessionFactory in its static
initializer (called once by the JVM when the class is loaded), but also
hides the fact that it uses a static singleton. It might as well lookup the
SessionFactory from JNDI in an application server.
If you give the SessionFactory a name in your configuration file, Hibernate
will in fact try to bind it to JNDI after it has been built. To avoid this code
completely you could also use JMX deployment and let the JMX-capable
10
Hibernate 3.3.1
Loading and storing objects
container instantiate and bind a HibernateService to JNDI. These advanced
options are discussed in the Hibernate reference documentation.
Place HibernateUtil.java in the development source directory, in a package
next to events:
.
+lib
<Hibernate and third-party libraries>
+src
+events
Event.java
Event.hbm.xml
+util
HibernateUtil.java
hibernate.cfg.xml
+data
build.xml
This should again compile without problems. We finally need to configure a
logging system - Hibernate uses commons logging and leaves you the choice
between Log4j and JDK 1.4 logging. Most developers prefer Log4j: copy
log4j.properties from the Hibernate distribution (it's in the etc/ directory)
to your src directory, next to hibernate.cfg.xml. Have a look at the example
configuration and change the settings if you like to have more verbose
output. By default, only Hibernate startup message are shown on stdout.
The tutorial infrastructure is complete - and we are ready to do some real
work with Hibernate.
1.2.6. Loading and storing objects
Finally, we can use Hibernate to load and store objects. We write an
EventManager class with a main() method:
package events;
import org.hibernate.Session;
import java.util.Date;
import util.HibernateUtil;
public class EventManager {
public static void main(String[] args) {
EventManager mgr = new EventManager();
if (args[0].equals("store")) {
mgr.createAndStoreEvent("My Event", new Date());
}
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Chapter 1. Introduction to Hibernate
HibernateUtil.getSessionFactory().close();
}
private void createAndStoreEvent(String title, Date theDate) {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Event theEvent = new Event();
theEvent.setTitle(title);
theEvent.setDate(theDate);
session.save(theEvent);
session.getTransaction().commit();
}
}
We create a new Event object, and hand it over to Hibernate. Hibernate now
takes care of the SQL and executes INSERTs on the database. Let's have a
look at the Session and Transaction-handling code before we run this.
A Session is a single unit of work. For now we'll keep things simple and
assume a one-to-one granularity between a Hibernate Session and a
database transaction. To shield our code from the actual underlying
transaction system (in this case plain JDBC, but it could also run with JTA)
we use the Transaction API that is available on the Hibernate Session.
What does sessionFactory.getCurrentSession() do? First, you can call it as
many times and anywhere you like, once you get hold of your SessionFactory
(easy thanks to HibernateUtil). The getCurrentSession() method always
returns the "current" unit of work. Remember that we switched the
configuration option for this mechanism to "thread" in hibernate.cfg.xml?
Hence, the current unit of work is bound to the current Java thread that
executes our application. However, this is not the full picture, you also have
to consider scope, when a unit of work begins and when it ends.
A Session begins when it is first needed, when the first call to
getCurrentSession() is made. It is then bound by Hibernate to the current
thread. When the transaction ends, either through commit or rollback,
Hibernate automatically unbinds the Session from the thread and closes it for
you. If you call getCurrentSession() again, you get a new Session and can
start a new unit of work. This thread-bound programming model is the most
popular way of using Hibernate, as it allows flexible layering of your code
12
Hibernate 3.3.1
Loading and storing objects
(transaction demarcation code can be separated from data access code, we'll
do this later in this tutorial).
Related to the unit of work scope, should the Hibernate Session be used
to execute one or several database operations? The above example uses
one Session for one operation. This is pure coincidence, the example is
just not complex enough to show any other approach. The scope of a
Hibernate Session is flexible but you should never design your application
to use a new Hibernate Session for every database operation. So even if
you see it a few more times in the following (very trivial) examples, consider
session-per-operation an anti-pattern. A real (web) application is shown later
in this tutorial.
Have a look at Chapter 11, Transactions And Concurrency for more
information about transaction handling and demarcation. We also skipped
any error handling and rollback in the previous example.
To run this first routine we have to add a callable target to the Ant build file:
<target name="run" depends="compile">
<java fork="true" classname="events.EventManager"
classpathref="libraries">
<classpath path="${targetdir}"/>
<arg value="${action}"/>
</java>
</target>
The value of the action argument is set on the command line when calling
the target:
C:\hibernateTutorial\>ant run -Daction=store
You should see, after compilation, Hibernate starting up and, depending on
your configuration, lots of log output. At the end you will find the following
line:
[java] Hibernate: insert into EVENTS (EVENT_DATE, title, EVENT_ID)
values (?, ?, ?)
This is the INSERT executed by Hibernate, the question marks represent
JDBC bind parameters. To see the values bound as arguments, or to reduce
the verbosity of the log, check your log4j.properties.
Now we'd like to list stored events as well, so we add an option to the main
method:
if (args[0].equals("store")) {
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Chapter 1. Introduction to Hibernate
mgr.createAndStoreEvent("My Event", new Date());
}
else if (args[0].equals("list")) {
List events = mgr.listEvents();
for (int i = 0; i < events.size(); i++) {
Event theEvent = (Event) events.get(i);
System.out.println("Event: " + theEvent.getTitle() +
" Time: " + theEvent.getDate());
}
}
We also add a new listEvents() method:
private List listEvents() {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
List result = session.createQuery("from Event").list();
session.getTransaction().commit();
return result;
}
What we do here is use an HQL (Hibernate Query Language) query to load
all existing Event objects from the database. Hibernate will generate the
appropriate SQL, send it to the database and populate Event objects with the
data. You can create more complex queries with HQL, of course.
Now, to execute and test all of this, follow these steps:
• Run ant run -Daction=store to store something into the database and, of
course, to generate the database schema before through hbm2ddl.
• Now disable hbm2ddl by commenting out the property in your
hibernate.cfg.xml file. Usually you only leave it turned on in continuous
unit testing, but another run of hbm2ddl would drop everything you have
stored - the create configuration setting actually translates into "drop all
tables from the schema, then re-create all tables, when the SessionFactory
is build".
If you now call Ant with -Daction=list, you should see the events you have
stored so far. You can of course also call the store action a few times more.
Note: Most new Hibernate users fail at this point and we see questions
about Table not found error messages regularly. However, if you follow the
14
Hibernate 3.3.1
Part 2 - Mapping associations
steps outlined above you will not have this problem, as hbm2ddl creates the
database schema on the first run, and subsequent application restarts will
use this schema. If you change the mapping and/or database schema, you
have to re-enable hbm2ddl once again.
1.3. Part 2 - Mapping associations
We mapped a persistent entity class to a table. Let's build on this and add
some class associations. First we'll add people to our application, and store a
list of events they participate in.
1.3.1. Mapping the Person class
The first cut of the Person class is simple:
package events;
public class Person {
private
private
private
private
Long id;
int age;
String firstname;
String lastname;
public Person() {}
// Accessor methods for all properties, private setter for 'id'
}
Create a new mapping file called Person.hbm.xml (don't forget the DTD
reference at the top):
<hibernate-mapping>
<class name="events.Person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="native"/>
</id>
<property name="age"/>
<property name="firstname"/>
<property name="lastname"/>
</class>
</hibernate-mapping>
Finally, add the new mapping to Hibernate's configuration:
<mapping resource="events/Event.hbm.xml"/>
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Chapter 1. Introduction to Hibernate
<mapping resource="events/Person.hbm.xml"/>
We'll now create an association between these two entities. Obviously,
persons can participate in events, and events have participants. The design
questions we have to deal with are: directionality, multiplicity, and collection
behavior.
1.3.2. A unidirectional Set-based association
We'll add a collection of events to the Person class. That way we can easily
navigate to the events for a particular person, without executing an explicit
query - by calling aPerson.getEvents(). We use a Java collection, a Set,
because the collection will not contain duplicate elements and the ordering is
not relevant for us.
We need a unidirectional, many-valued associations, implemented with a Set.
Let's write the code for this in the Java classes and then map it:
public class Person {
private Set events = new HashSet();
public Set getEvents() {
return events;
}
public void setEvents(Set events) {
this.events = events;
}
}
Before we map this association, think about the other side. Clearly,
we could just keep this unidirectional. Or, we could create another
collection on the Event, if we want to be able to navigate it bi-directional,
i.e. anEvent.getParticipants(). This is not necessary, from a functional
perspective. You could always execute an explicit query to retrieve the
participants for a particular event. This is a design choice left to you, but
what is clear from this discussion is the multiplicity of the association: "many"
valued on both sides, we call this a many-to-many association. Hence, we
use Hibernate's many-to-many mapping:
<class name="events.Person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="native"/>
</id>
<property name="age"/>
<property name="firstname"/>
<property name="lastname"/>
16
Hibernate 3.3.1
Working the association
<set name="events" table="PERSON_EVENT">
<key column="PERSON_ID"/>
<many-to-many column="EVENT_ID" class="events.Event"/>
</set>
</class>
Hibernate supports all kinds of collection mappings, a <set> being most
common. For a many-to-many association (or n:m entity relationship), an
association table is needed. Each row in this table represents a link between
a person and an event. The table name is configured with the table attribute
of the set element. The identifier column name in the association, for the
person's side, is defined with the <key> element, the column name for the
event's side with the column attribute of the <many-to-many>. You also have to
tell Hibernate the class of the objects in your collection (correct: the class on
the other side of the collection of references).
The database schema for this mapping is therefore:
_____________
__________________
|
|
|
|
_____________
|
EVENTS
|
|
PERSON_EVENT
|
|
|
|_____________|
|__________________|
|
PERSON
|
|
|
|
|
|_____________|
| *EVENT_ID
| <--> | *EVENT_ID
|
|
|
| EVENT_DATE |
| *PERSON_ID
| <--> | *PERSON_ID |
| TITLE
|
|__________________|
| AGE
|
|_____________|
| FIRSTNAME |
| LASTNAME
|
|_____________|
1.3.3. Working the association
Let's bring some people and events together in a new method in
EventManager:
private void addPersonToEvent(Long personId, Long eventId) {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Person aPerson = (Person) session.load(Person.class, personId);
Event anEvent = (Event) session.load(Event.class, eventId);
aPerson.getEvents().add(anEvent);
session.getTransaction().commit();
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Chapter 1. Introduction to Hibernate
}
After loading a Person and an Event, simply modify the collection using
the normal collection methods. As you can see, there is no explicit call
to update() or save(), Hibernate automatically detects that the collection
has been modified and needs to be updated. This is called automatic
dirty checking, and you can also try it by modifying the name or the date
property of any of your objects. As long as they are in persistent state, that
is, bound to a particular Hibernate Session (i.e. they have been just loaded
or saved in a unit of work), Hibernate monitors any changes and executes
SQL in a write-behind fashion. The process of synchronizing the memory
state with the database, usually only at the end of a unit of work, is called
flushing. In our code, the unit of work ends with a commit (or rollback) of the
database transaction - as defined by the thread configuration option for the
CurrentSessionContext class.
You might of course load person and event in different units of work. Or you
modify an object outside of a Session, when it is not in persistent state (if it
was persistent before, we call this state detached). You can even modify a
collection when it is detached:
private void addPersonToEvent(Long personId, Long eventId) {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Person aPerson = (Person) session
.createQuery("select p from Person p left join fetch
p.events where p.id = :pid")
.setParameter("pid", personId)
.uniqueResult(); // Eager fetch the collection so we can
use it detached
Event anEvent = (Event) session.load(Event.class, eventId);
session.getTransaction().commit();
// End of first unit of work
aPerson.getEvents().add(anEvent); // aPerson (and its
collection) is detached
// Begin second unit of work
Session session2 =
HibernateUtil.getSessionFactory().getCurrentSession();
session2.beginTransaction();
session2.update(aPerson); // Reattachment of aPerson
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Hibernate 3.3.1
Collection of values
session2.getTransaction().commit();
}
The call to update makes a detached object persistent again, you could say
it binds it to a new unit of work, so any modifications you made to it while
detached can be saved to the database. This includes any modifications
(additions/deletions) you made to a collection of that entity object.
Well, this is not much use in our current situation, but it's an important
concept you can design into your own application. For now, complete this
exercise by adding a new action to the EventManager's main method and call
it from the command line. If you need the identifiers of a person and an event
- the save() method returns it (you might have to modify some of the previous
methods to return that identifier):
else if (args[0].equals("addpersontoevent")) {
Long eventId = mgr.createAndStoreEvent("My Event", new Date());
Long personId = mgr.createAndStorePerson("Foo", "Bar");
mgr.addPersonToEvent(personId, eventId);
System.out.println("Added person " + personId + " to event " +
eventId);
}
This was an example of an association between two equally important
classes, two entities. As mentioned earlier, there are other classes and types
in a typical model, usually "less important". Some you have already seen, like
an int or a String. We call these classes value types, and their instances
depend on a particular entity. Instances of these types don't have their own
identity, nor are they shared between entities (two persons don't reference
the same firstname object, even if they have the same first name). Of
course, value types can not only be found in the JDK (in fact, in a Hibernate
application all JDK classes are considered value types), but you can also
write dependent classes yourself, Address or MonetaryAmount, for example.
You can also design a collection of value types. This is conceptually very
different from a collection of references to other entities, but looks almost the
same in Java.
1.3.4. Collection of values
We add a collection of value typed objects to the Person entity. We want to
store email addresses, so the type we use is String, and the collection is
again a Set:
private Set emailAddresses = new HashSet();
public Set getEmailAddresses() {
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Chapter 1. Introduction to Hibernate
return emailAddresses;
}
public void setEmailAddresses(Set emailAddresses) {
this.emailAddresses = emailAddresses;
}
The mapping of this Set:
<set name="emailAddresses" table="PERSON_EMAIL_ADDR">
<key column="PERSON_ID"/>
<element type="string" column="EMAIL_ADDR"/>
</set>
The difference compared with the earlier mapping is the element part, which
tells Hibernate that the collection does not contain references to another
entity, but a collection of elements of type String (the lowercase name tells
you it's a Hibernate mapping type/converter). Once again, the table attribute
of the set element determines the table name for the collection. The key
element defines the foreign-key column name in the collection table. The
column attribute in the element element defines the column name where the
String values will actually be stored.
Have a look at the updated schema:
_____________
__________________
|
|
|
_____________
EVENTS
|
|
PERSON_EVENT
|
|
|
___________________
|_____________|
|__________________|
|
PERSON
|
|
|
|
|
|
|
|_____________|
| PERSON_EMAIL_ADDR |
| *EVENT_ID
| <--> | *EVENT_ID
|
|
|
|___________________|
| EVENT_DATE |
| *PERSON_ID
| <--> | *PERSON_ID | <-->
| *PERSON_ID
|
| TITLE
|
|__________________|
| AGE
|
| *EMAIL_ADDR
|
|_____________|
| FIRSTNAME |
|___________________|
| LASTNAME
|
|_____________|
|
|
You can see that the primary key of the collection table is in fact a composite
key, using both columns. This also implies that there can't be duplicate email
addresses per person, which is exactly the semantics we need for a set in
Java.
20
Hibernate 3.3.1
Bi-directional associations
You can now try and add elements to this collection, just like we did before
by linking persons and events. It's the same code in Java:
private void addEmailToPerson(Long personId, String emailAddress) {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Person aPerson = (Person) session.load(Person.class, personId);
// The getEmailAddresses() might trigger a lazy load of the
collection
aPerson.getEmailAddresses().add(emailAddress);
session.getTransaction().commit();
}
This time we didn't use a fetch query to initialize the collection. Hence, the
call to its getter method will trigger an additional select to initialize it, so we
can add an element to it. Monitor the SQL log and try to optimize this with an
eager fetch.
1.3.5. Bi-directional associations
Next we are going to map a bi-directional association - making the
association between person and event work from both sides in Java. Of
course, the database schema doesn't change, we still have many-to-many
multiplicity. A relational database is more flexible than a network
programming language, so it doesn't need anything like a navigation direction
- data can be viewed and retrieved in any possible way.
First, add a collection of participants to the Event Event class:
private Set participants = new HashSet();
public Set getParticipants() {
return participants;
}
public void setParticipants(Set participants) {
this.participants = participants;
}
Now map this side of the association too, in Event.hbm.xml.
<set name="participants" table="PERSON_EVENT" inverse="true">
<key column="EVENT_ID"/>
<many-to-many column="PERSON_ID" class="events.Person"/>
</set>
Hibernate 3.3.1
21
Chapter 1. Introduction to Hibernate
As you see, these are normal set mappings in both mapping documents.
Notice that the column names in key and many-to-many are swapped in both
mapping documents. The most important addition here is the inverse="true"
attribute in the set element of the Event's collection mapping.
What this means is that Hibernate should take the other side - the Person
class - when it needs to find out information about the link between the two.
This will be a lot easier to understand once you see how the bi-directional link
between our two entities is created .
1.3.6. Working bi-directional links
First, keep in mind that Hibernate does not affect normal Java semantics.
How did we create a link between a Person and an Event in the unidirectional
example? We added an instance of Event to the collection of event
references, of an instance of Person. So, obviously, if we want to make this
link working bi-directional, we have to do the same on the other side - adding
a Person reference to the collection in an Event. This "setting the link on both
sides" is absolutely necessary and you should never forget doing it.
Many developers program defensively and create link management methods
to correctly set both sides, e.g. in Person:
protected Set getEvents() {
return events;
}
protected void setEvents(Set events) {
this.events = events;
}
public void addToEvent(Event event) {
this.getEvents().add(event);
event.getParticipants().add(this);
}
public void removeFromEvent(Event event) {
this.getEvents().remove(event);
event.getParticipants().remove(this);
}
Notice that the get and set methods for the collection are now protected this allows classes in the same package and subclasses to still access the
methods, but prevents everybody else from messing with the collections
directly (well, almost). You should probably do the same with the collection
on the other side.
What about the inverse mapping attribute? For you, and for Java, a
bi-directional link is simply a matter of setting the references on both sides
22
Hibernate 3.3.1
Part 3 - The EventManager web application
correctly. Hibernate however doesn't have enough information to correctly
arrange SQL INSERT and UPDATE statements (to avoid constraint violations),
and needs some help to handle bi-directional associations properly. Making
one side of the association inverse tells Hibernate to basically ignore it, to
consider it a mirror of the other side. That's all that is necessary for Hibernate
to work out all of the issues when transformation a directional navigation
model to a SQL database schema. The rules you have to remember are
straightforward: All bi-directional associations need one side as inverse.
In a one-to-many association it has to be the many-side, in many-to-many
association you can pick either side, there is no difference.
1.4. Part 3 - The EventManager web application
Let's turn the following discussion into a small web application...
A Hibernate web application uses Session and Transaction almost like a
standalone application. However, some common patterns are useful. We
now write an EventManagerServlet. This servlet can list all events stored in
the database, and it provides an HTML form to enter new events.
1.4.1. Writing the basic servlet
Create a new class in your source directory, in the events package:
package events;
// Imports
public class EventManagerServlet extends HttpServlet {
// Servlet code
}
The servlet handles HTTP GET requests only, hence, the method we
implement is doGet():
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
SimpleDateFormat dateFormatter = new
SimpleDateFormat("dd.MM.yyyy");
try {
// Begin unit of work
HibernateUtil.getSessionFactory()
.getCurrentSession().beginTransaction();
// Process request and render page...
Hibernate 3.3.1
23
Chapter 1. Introduction to Hibernate
// End unit of work
HibernateUtil.getSessionFactory()
.getCurrentSession().getTransaction().commit();
} catch (Exception ex) {
HibernateUtil.getSessionFactory()
.getCurrentSession().getTransaction().rollback();
throw new ServletException(ex);
}
}
The pattern we are applying here is called session-per-request. When a
request hits the servlet, a new Hibernate Session is opened through the
first call to getCurrentSession() on the SessionFactory. Then a database
transaction is started-all data access as to occur inside a transaction, no
matter if data is read or written (we don't use the auto-commit mode in
applications).
Do not use a new Hibernate Session for every database operation.
Use one Hibernate Session that is scoped to the whole request. Use
getCurrentSession(), so that it is automatically bound to the current Java
thread.
Next, the possible actions of the request are processed and the response
HTML is rendered. We'll get to that part soon.
Finally, the unit of work ends when processing and rendering is complete.
If any problem occurred during processing or rendering, an exception will
be thrown and the database transaction rolled back. This completes the
session-per-request pattern. Instead of the transaction demarcation code in
every servlet you could also write a servlet filter. See the Hibernate website
and Wiki for more information about this pattern, called Open Session in
View-you'll need it as soon as you consider rendering your view in JSP, not in
a servlet.
1.4.2. Processing and rendering
Let's implement the processing of the request and rendering of the page.
// Write HTML header
PrintWriter out = response.getWriter();
out.println("<html><head><title>Event
Manager</title></head><body>");
// Handle actions
if ( "store".equals(request.getParameter("action")) ) {
24
Hibernate 3.3.1
Processing and rendering
String eventTitle = request.getParameter("eventTitle");
String eventDate = request.getParameter("eventDate");
if ( "".equals(eventTitle) || "".equals(eventDate) ) {
out.println("<b><i>Please enter event title and
date.</i></b>");
} else {
createAndStoreEvent(eventTitle,
dateFormatter.parse(eventDate));
out.println("<b><i>Added event.</i></b>");
}
}
// Print page
printEventForm(out);
listEvents(out, dateFormatter);
// Write HTML footer
out.println("</body></html>");
out.flush();
out.close();
Granted, this coding style with a mix of Java and HTML would not scale in
a more complex application-keep in mind that we are only illustrating basic
Hibernate concepts in this tutorial. The code prints an HTML header and a
footer. Inside this page, an HTML form for event entry and a list of all events
in the database are printed. The first method is trivial and only outputs HTML:
private void printEventForm(PrintWriter out) {
out.println("<h2>Add new event:</h2>");
out.println("<form>");
out.println("Title: <input name='eventTitle'
length='50'/><br/>");
out.println("Date (e.g. 24.12.2009): <input name='eventDate'
length='10'/><br/>");
out.println("<input type='submit' name='action'
value='store'/>");
out.println("</form>");
}
The listEvents() method uses the Hibernate Session bound to the current
thread to execute a query:
private void listEvents(PrintWriter out, SimpleDateFormat
dateFormatter) {
List result = HibernateUtil.getSessionFactory()
.getCurrentSession().createCriteria(Event.class).list();
if (result.size() > 0) {
out.println("<h2>Events in database:</h2>");
out.println("<table border='1'>");
Hibernate 3.3.1
25
Chapter 1. Introduction to Hibernate
out.println("<tr>");
out.println("<th>Event title</th>");
out.println("<th>Event date</th>");
out.println("</tr>");
for (Iterator it = result.iterator(); it.hasNext();) {
Event event = (Event) it.next();
out.println("<tr>");
out.println("<td>" + event.getTitle() + "</td>");
out.println("<td>" +
dateFormatter.format(event.getDate()) + "</td>");
out.println("</tr>");
}
out.println("</table>");
}
}
Finally, the store action is dispatched to the createAndStoreEvent() method,
which also uses the Session of the current thread:
protected void createAndStoreEvent(String title, Date theDate) {
Event theEvent = new Event();
theEvent.setTitle(title);
theEvent.setDate(theDate);
HibernateUtil.getSessionFactory()
.getCurrentSession().save(theEvent);
}
That's it, the servlet is complete. A request to the servlet will be processed
in a single Session and Transaction. As earlier in the standalone application,
Hibernate can automatically bind these objects to the current thread of
execution. This gives you the freedom to layer your code and access the
SessionFactory in any way you like. Usually you'd use a more sophisticated
design and move the data access code into data access objects (the DAO
pattern). See the Hibernate Wiki for more examples.
1.4.3. Deploying and testing
To deploy this application you have to create a web archive, a WAR. Add the
following Ant target to your build.xml:
<target name="war" depends="compile">
<war destfile="hibernate-tutorial.war" webxml="web.xml">
<lib dir="${librarydir}">
<exclude name="jsdk*.jar"/>
</lib>
<classes dir="${targetdir}"/>
</war>
</target>
26
Hibernate 3.3.1
Summary
This target creates a file called hibernate-tutorial.war in your project
directory. It packages all libraries and the web.xml descriptor, which is
expected in the base directory of your project:
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<servlet>
<servlet-name>Event Manager</servlet-name>
<servlet-class>events.EventManagerServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Event Manager</servlet-name>
<url-pattern>/eventmanager</url-pattern>
</servlet-mapping>
</web-app>
Before you compile and deploy the web application, note that an additional
library is required: jsdk.jar. This is the Java servlet development kit, if you
don't have this library already, get it from the Sun website and copy it to your
library directory. However, it will be only used for compilation and excluded
from the WAR package.
To build and deploy call ant war in your project directory and copy the
hibernate-tutorial.war file into your Tomcat webapp directory. If you don't
have Tomcat installed, download it and follow the installation instructions.
You don't have to change any Tomcat configuration to deploy this application
though.
Once deployed and Tomcat is running, access the application at
http://localhost:8080/hibernate-tutorial/eventmanager. Make sure you
watch the Tomcat log to see Hibernate initialize when the first request hits
your servlet (the static initializer in HibernateUtil is called) and to get the
detailed output if any exceptions occurs.
1.5. Summary
This tutorial covered the basics of writing a simple standalone Hibernate
application and a small web application.
If you already feel confident with Hibernate, continue browsing through the
reference documentation table of contents for topics you find interesting most asked are transactional processing (Chapter 11, Transactions And
Hibernate 3.3.1
27
Chapter 1. Introduction to Hibernate
Concurrency), fetch performance (Chapter 19, Improving performance),
or the usage of the API (Chapter 10, Working with objects) and the query
features (Section 10.4, “Querying”).
Don't forget to check the Hibernate website for more (specialized) tutorials.
28
Hibernate 3.3.1
Chapter 2. Architecture
2.1. Overview
A (very) high-level view of the Hibernate architecture:
This diagram shows Hibernate using the database and configuration data to
provide persistence services (and persistent objects) to the application.
We would like to show a more detailed view of the runtime architecture.
Unfortunately, Hibernate is flexible and supports several approaches. We will
show the two extremes. The "lite" architecture has the application provide
its own JDBC connections and manage its own transactions. This approach
uses a minimal subset of Hibernate's APIs:
Hibernate 3.3.1
29
Chapter 2. Architecture
The "full cream" architecture abstracts the application away from the
underlying JDBC/JTA APIs and lets Hibernate take care of the details.
Heres some definitions of the objects in the diagrams:
30
Hibernate 3.3.1
Overview
SessionFactory (org.hibernate.SessionFactory)
A threadsafe (immutable) cache of compiled mappings for a single
database. A factory for Session and a client of ConnectionProvider. Might
hold an optional (second-level) cache of data that is reusable between
transactions, at a process- or cluster-level.
Session (org.hibernate.Session)
A single-threaded, short-lived object representing a conversation between
the application and the persistent store. Wraps a JDBC connection.
Factory for Transaction. Holds a mandatory (first-level) cache of
persistent objects, used when navigating the object graph or looking up
objects by identifier.
Persistent objects and collections
Short-lived, single threaded objects containing persistent state and
business function. These might be ordinary JavaBeans/POJOs, the only
special thing about them is that they are currently associated with (exactly
one) Session. As soon as the Session is closed, they will be detached and
free to use in any application layer (e.g. directly as data transfer objects
to and from presentation).
Transient and detached objects and collections
Instances of persistent classes that are not currently associated with a
Session. They may have been instantiated by the application and not (yet)
persisted or they may have been instantiated by a closed Session.
Transaction (org.hibernate.Transaction)
(Optional) A single-threaded, short-lived object used by the application to
specify atomic units of work. Abstracts application from underlying JDBC,
JTA or CORBA transaction. A Session might span several Transactions
in some cases. However, transaction demarcation, either using the
underlying API or Transaction, is never optional!
ConnectionProvider (org.hibernate.connection.ConnectionProvider)
(Optional) A factory for (and pool of) JDBC connections. Abstracts
application from underlying Datasource or DriverManager. Not exposed to
application, but can be extended/implemented by the developer.
TransactionFactory (org.hibernate.TransactionFactory)
(Optional) A factory for Transaction instances. Not exposed to the
application, but can be extended/implemented by the developer.
Extension Interfaces
Hibernate offers many optional extension interfaces you can implement
to customize the behavior of your persistence layer. See the API
documentation for details.
Hibernate 3.3.1
31
Chapter 2. Architecture
Given a "lite" architecture, the application bypasses the
Transaction/TransactionFactory and/or ConnectionProvider APIs to talk to
JTA or JDBC directly.
2.2. Instance states
An instance of a persistent classes may be in one of three different states,
which are defined with respect to a persistence context. The Hibernate
Session object is the persistence context:
transient
The instance is not, and has never been associated with any persistence
context. It has no persistent identity (primary key value).
persistent
The instance is currently associated with a persistence context. It has
a persistent identity (primary key value) and, perhaps, a corresponding
row in the database. For a particular persistence context, Hibernate
guarantees that persistent identity is equivalent to Java identity
(in-memory location of the object).
detached
The instance was once associated with a persistence context, but that
context was closed, or the instance was serialized to another process.
It has a persistent identity and, perhaps, a corresponding row in the
database. For detached instances, Hibernate makes no guarantees about
the relationship between persistent identity and Java identity.
2.3. JMX Integration
JMX is the J2EE standard for management of Java components. Hibernate
may be managed via a JMX standard service. We provide an MBean
implementation in the distribution, org.hibernate.jmx.HibernateService.
For an example how to deploy Hibernate as a JMX service on the JBoss
Application Server, please see the JBoss User Guide. On JBoss AS, you
also get these benefits if you deploy using JMX:
• Session Management: The Hibernate Session's life cycle can be
automatically bound to the scope of a JTA transaction. This means you no
longer have to manually open and close the Session, this becomes the job
of a JBoss EJB interceptor. You also don't have to worry about transaction
demarcation in your code anymore (unless you'd like to write a portable
persistence layer of course, use the optional Hibernate Transaction API for
this). You call the HibernateContext to access a Session.
32
Hibernate 3.3.1
JCA Support
• HAR deployment: Usually you deploy the Hibernate JMX service using
a JBoss service deployment descriptor (in an EAR and/or SAR file), it
supports all the usual configuration options of a Hibernate SessionFactory.
However, you still have to name all your mapping files in the deployment
descriptor. If you decide to use the optional HAR deployment, JBoss will
automatically detect all mapping files in your HAR file.
Consult the JBoss AS user guide for more information about these options.
Another feature available as a JMX service are runtime Hibernate statistics.
See Section 3.4.6, “Hibernate statistics”.
2.4. JCA Support
Hibernate may also be configured as a JCA connector. Please see the
website for more details. Please note that Hibernate JCA support is still
considered experimental.
2.5. Contextual Sessions
Most applications using Hibernate need some form of "contextual" sessions,
where a given session is in effect throughout the scope of a given context.
However, across applications the definition of what constitutes a context is
typically different; and different contexts define different scopes to the notion
of current. Applications using Hibernate prior to version 3.0 tended to utilize
either home-grown ThreadLocal-based contextual sessions, helper classes
such as HibernateUtil, or utilized third-party frameworks (such as Spring or
Pico) which provided proxy/interception-based contextual sessions.
Starting with version 3.0.1, Hibernate added the
SessionFactory.getCurrentSession() method. Initially, this assumed usage
of JTA transactions, where the JTA transaction defined both the scope
and context of a current session. The Hibernate team maintains that,
given the maturity of the numerous stand-alone JTA TransactionManager
implementations out there, most (if not all) applications should be using
JTA transaction management whether or not they are deployed into a J2EE
container. Based on that, the JTA-based contextual sessions is all you should
ever need to use.
However, as of version 3.1, the processing behind
SessionFactory.getCurrentSession() is now pluggable. To that end, a new
extension interface (org.hibernate.context.CurrentSessionContext) and a
new configuration parameter (hibernate.current_session_context_class)
have been added to allow pluggability of the scope and context of defining
current sessions.
Hibernate 3.3.1
33
Chapter 2. Architecture
See the Javadocs for the org.hibernate.context.CurrentSessionContext
interface for a detailed discussion of its contract. It defines a single method,
currentSession(), by which the implementation is responsible for tracking
the current contextual session. Out-of-the-box, Hibernate comes with three
implementations of this interface.
• org.hibernate.context.JTASessionContext - current sessions are tracked
and scoped by a JTA transaction. The processing here is exactly the same
as in the older JTA-only approach. See the Javadocs for details.
• org.hibernate.context.ThreadLocalSessionContext - current sessions are
tracked by thread of execution. Again, see the Javadocs for details.
• org.hibernate.context.ManagedSessionContext - current sessions are
tracked by thread of execution. However, you are responsible to bind and
unbind a Session instance with static methods on this class, it does never
open, flush, or close a Session.
The first two implementations provide a "one session - one
database transaction" programming model, also known and used as
session-per-request. The beginning and end of a Hibernate session is
defined by the duration of a database transaction. If you use programmatic
transaction demarcation in plain JSE without JTA, you are advised to use
the Hibernate Transaction API to hide the underlying transaction system
from your code. If you use JTA, use the JTA interfaces to demarcate
transactions. If you execute in an EJB container that supports CMT,
transaction boundaries are defined declaratively and you don't need any
transaction or session demarcation operations in your code. Refer to
Chapter 11, Transactions And Concurrency for more information and code
examples.
The hibernate.current_session_context_class configuration parameter
defines which org.hibernate.context.CurrentSessionContext implementation
should be used. Note that for backwards compatibility, if this config param
is not set but a org.hibernate.transaction.TransactionManagerLookup is
configured, Hibernate will use the org.hibernate.context.JTASessionContext.
Typically, the value of this parameter would just name the implementation
class to use; for the three out-of-the-box implementations, however, there are
three corresponding short names, "jta", "thread", and "managed".
34
Hibernate 3.3.1
Chapter 3. Configuration
Because Hibernate is designed to operate in many different environments,
there are a large number of configuration parameters. Fortunately, most
have sensible default values and Hibernate is distributed with an example
hibernate.properties file in etc/ that shows the various options. Just put the
example file in your classpath and customize it.
3.1. Programmatic configuration
An instance of org.hibernate.cfg.Configuration represents an entire
set of mappings of an application's Java types to an SQL database.
The org.hibernate.cfg.Configuration is used to build an (immutable)
org.hibernate.SessionFactory. The mappings are compiled from various
XML mapping files.
You may obtain a org.hibernate.cfg.Configuration instance by instantiating
it directly and specifying XML mapping documents. If the mapping files are in
the classpath, use addResource():
Configuration cfg = new Configuration()
.addResource("Item.hbm.xml")
.addResource("Bid.hbm.xml");
An alternative (sometimes better) way is to specify the mapped class, and let
Hibernate find the mapping document for you:
Configuration cfg = new Configuration()
.addClass(org.hibernate.auction.Item.class)
.addClass(org.hibernate.auction.Bid.class);
Then Hibernate will look for mapping files named
/org/hibernate/auction/Item.hbm.xml and
/org/hibernate/auction/Bid.hbm.xml in the classpath. This approach
eliminates any hardcoded filenames.
A org.hibernate.cfg.Configuration also allows you to specify configuration
properties:
Configuration cfg = new Configuration()
.addClass(org.hibernate.auction.Item.class)
.addClass(org.hibernate.auction.Bid.class)
.setProperty("hibernate.dialect",
"org.hibernate.dialect.MySQLInnoDBDialect")
.setProperty("hibernate.connection.datasource",
"java:comp/env/jdbc/test")
Hibernate 3.3.1
35
Chapter 3. Configuration
.setProperty("hibernate.order_updates", "true");
This is not the only way to pass configuration properties to Hibernate. The
various options include:
1. Pass an instance of java.util.Properties to
Configuration.setProperties().
2. Place a file named hibernate.properties in a root directory of the
classpath.
3. Set System properties using java -Dproperty=value.
4. Include <property> elements in hibernate.cfg.xml (discussed later).
hibernate.properties
is the easiest approach if you want to get started
quickly.
The org.hibernate.cfg.Configuration is intended as a startup-time object, to
be discarded once a SessionFactory is created.
3.2. Obtaining a SessionFactory
When all mappings have been parsed by the
org.hibernate.cfg.Configuration, the application must obtain a factory for
org.hibernate.Session instances. This factory is intended to be shared by all
application threads:
SessionFactory sessions = cfg.buildSessionFactory();
Hibernate does allow your application to instantiate more than one
org.hibernate.SessionFactory. This is useful if you are using more than one
database.
3.3. JDBC connections
Usually, you want to have the org.hibernate.SessionFactory create and
pool JDBC connections for you. If you take this approach, opening a
org.hibernate.Session is as simple as:
Session session = sessions.openSession(); // open a new Session
As soon as you do something that requires access to the database, a JDBC
connection will be obtained from the pool.
For this to work, we need to pass some JDBC connection properties to
Hibernate. All Hibernate property names and semantics are defined on
36
Hibernate 3.3.1
JDBC connections
the class org.hibernate.cfg.Environment. We will now describe the most
important settings for JDBC connection configuration.
Hibernate will obtain (and pool) connections using java.sql.DriverManager if
you set the following properties:
Table 3.1. Hibernate JDBC Properties
Property name
Purpose
hibernate.connection.driver_class
JDBC driver class
hibernate.connection.url
JDBC URL
hibernate.connection.username
database user
hibernate.connection.password
database user password
hibernate.connection.pool_size
maximum number of pooled
connections
Hibernate's own connection pooling algorithm is however quite rudimentary.
It is intended to help you get started and is not intended for use in a
production system or even for performance testing. You should use
a third party pool for best performance and stability. Just replace the
hibernate.connection.pool_size property with connection pool specific
settings. This will turn off Hibernate's internal pool. For example, you might
like to use C3P0.
C3P0 is an open source JDBC connection pool distributed
along with Hibernate in the lib directory. Hibernate will use its
org.hibernate.connection.C3P0ConnectionProvider for connection pooling
if you set hibernate.c3p0.* properties. If you'd like to use Proxool refer to
the packaged hibernate.properties and the Hibernate web site for more
information.
Here is an example hibernate.properties file for C3P0:
hibernate.connection.driver_class = org.postgresql.Driver
hibernate.connection.url = jdbc:postgresql://localhost/mydatabase
hibernate.connection.username = myuser
hibernate.connection.password = secret
hibernate.c3p0.min_size=5
hibernate.c3p0.max_size=20
hibernate.c3p0.timeout=1800
hibernate.c3p0.max_statements=50
hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect
For use inside an application server, you should almost always
configure Hibernate to obtain connections from an application server
Hibernate 3.3.1
37
hibernate.transaction.factory_class = \
hibernate.connection.username
database user (optional)
Table
3.2. Hibernate Datasource
Properties
hibernate.default_catalog
Qualify
unqualified table names with
org.hibernate.transaction.JTATransactionFactory
hibernate.connection.password
database
user
(optional)
hibernate.transaction.manager_lookup_class
\ password
the given =catalog
in generated
SQL.
Chapter 3.
org.hibernate.transaction.JBossTransactionManagerLookup
hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect
Configuration
eg. CATALOG_NAME
hibernate.session_factory_name
The org.hibernate.SessionFactory
will be automatically bound to this
javax.sql.Datasource registered in JNDI. You'll need to set at least one of
name in JNDI after it has been
the following properties:
created.
eg. jndi/composite/name
hibernate.max_fetch_depth
Set a maximum "depth" for the
outer join fetch tree for single-ended
associations (one-to-one,
many-to-one). A 0 disables default
outer join fetching.
eg. recommended values between 0
and 3
hibernate.default_batch_fetch_size
Set a default size for Hibernate batch
fetching of associations.
eg. recommended values 4, 8, 16
hibernate.default_entity_mode
Set a default mode for entity
representation for all sessions
opened from this SessionFactory
dynamic-map, dom4j, pojo
hibernate.order_updates
Force Hibernate to order SQL
updates by the primary key value of
the items being updated. This will
result in fewer transaction deadlocks
in highly concurrent systems.
eg. true | false
hibernate.generate_statistics
If enabled, Hibernate will collect
statistics useful for performance
tuning.
eg. true | false
hibernate.use_identifier_rollback
If enabled, generated identifier
properties will be reset to default
values when objects are deleted.
eg. true | false
hibernate.use_sql_comments
If turned on, Hibernate will generate
comments inside the SQL, for easier
debugging, defaults to false.
eg. true | false
38
Hibernate 3.3.1
eg. classname.of.ConnectionProvider
hibernate.connection.isolation
Set the JDBC transaction isolation
level. Check java.sql.Connection for
Optional
configuration
properties
meaningful
values
but note that
most
databases do not support all isolation
levels and some define additional,
Table 3.4. Hibernate JDBC andnon-standard
Connection
Properties
isolations.
eg. 1, 2, 4, 8
hibernate.connection.autocommit
Enables autocommit for JDBC pooled
connections (not recommended).
eg. true | false
hibernate.connection.release_mode
Specify when Hibernate should
release JDBC connections. By
default, a JDBC connection is
held until the session is explicitly
closed or disconnected. For an
application server JTA datasource,
you should use after_statement to
aggressively release connections
after every JDBC call. For a non-JTA
connection, it often makes sense
to release the connection at the
end of each transaction, by using
after_transaction. auto will choose
after_statement for the JTA and
CMT transaction strategies and
after_transaction for the JDBC
transaction strategy.
eg. auto (default) | on_close |
after_transaction | after_statement
Note that this setting only
affects Sessions returned from
SessionFactory.openSession.
For Sessions obtained through
SessionFactory.getCurrentSession,
the CurrentSessionContext
implementation configured for use
controls the connection release mode
for those Sessions. See Section 2.5,
“Contextual Sessions”
hibernate.connection.<propertyName>Pass the JDBC property
<propertyName> to
DriverManager.getConnection().
hibernate.jndi.<propertyName>
Hibernate 3.3.1
Pass the property <propertyName> to
the JNDI InitialContextFactory.
39
Chapter 3. Configuration
Table 3.5. Hibernate Cache Properties
Property name
Purpose
hibernate.cache.provider_class
The classname of a custom
CacheProvider.
eg. classname.of.CacheProvider
hibernate.cache.use_minimal_puts
Optimize second-level cache
operation to minimize writes, at the
cost of more frequent reads. This
setting is most useful for clustered
caches and, in Hibernate3, is enabled
by default for clustered cache
implementations.
eg. true|false
hibernate.cache.use_query_cache
Enable the query cache, individual
queries still have to be set cachable.
eg. true|false
hibernate.cache.use_second_level_cache
May
be used to completely disable
the second level cache, which is
enabled by default for classes which
specify a <cache> mapping.
eg. true|false
hibernate.cache.query_cache_factory
The classname of a custom
QueryCache interface, defaults to the
built-in StandardQueryCache.
eg. classname.of.QueryCache
hibernate.cache.region_prefix
A prefix to use for second-level cache
region names.
eg. prefix
hibernate.cache.use_structured_entries
Forces
Hibernate to store data in
the second-level cache in a more
human-friendly format.
eg. true|false
40
Hibernate 3.3.1
Optional configuration properties
Table 3.6. Hibernate Transaction Properties
Property name
Purpose
hibernate.transaction.factory_class
The classname of a
to use with
Hibernate Transaction API (defaults
to JDBCTransactionFactory).
TransactionFactory
eg. classname.of.TransactionFactory
jta.UserTransaction
A JNDI name used by
to obtain
the JTA UserTransaction from the
application server.
JTATransactionFactory
eg. jndi/composite/name
hibernate.transaction.manager_lookup_class
The classname
of a
- required
when JVM-level caching is enabled
or when using hilo generator in a JTA
environment.
TransactionManagerLookup
eg. classname.of.TransactionManagerLookup
hibernate.transaction.flush_before_completion
If enabled,
the session will be
automatically flushed during the
before completion phase of the
transaction. Built-in and automatic
session context management
is preferred, see Section 2.5,
“Contextual Sessions”.
eg. true | false
hibernate.transaction.auto_close_session
If enabled,
the session will be
automatically closed during the after
completion phase of the transaction.
Built-in and utomatic session context
management is preferred, see
Section 2.5, “Contextual Sessions”.
eg. true | false
Hibernate 3.3.1
41
Chapter 3. Configuration
Table 3.7. Miscellaneous Properties
Property name
Purpose
hibernate.current_session_context_class
Supply
a (custom) strategy for the
scoping of the "current" Session. See
Section 2.5, “Contextual Sessions”
for more information about the built-in
strategies.
eg. jta | thread | managed |
custom.Class
hibernate.query.factory_class
Chooses the HQL parser
implementation.
eg. org.hibernate.hql.ast.ASTQueryTranslatorFactory
or org.hibernate.hql.classic.ClassicQueryTranslatorF
hibernate.query.substitutions
Mapping from tokens in Hibernate
queries to SQL tokens (tokens might
be function or literal names, for
example).
eg. hqlLiteral=SQL_LITERAL,
hqlFunction=SQLFUNC
hibernate.hbm2ddl.auto
Automatically validate or export
schema DDL to the database when
the SessionFactory is created.
With create-drop, the database
schema will be dropped when the
SessionFactory is closed explicitly.
eg. validate | update | create |
create-drop
hibernate.cglib.use_reflection_optimizer
Enables
use of CGLIB instead of
runtime reflection (System-level
property). Reflection can sometimes
be useful when troubleshooting,
note that Hibernate always requires
CGLIB even if you turn off the
optimizer. You can not set this
property in hibernate.cfg.xml.
eg. true | false
42
Hibernate 3.3.1
SQL Dialects
3.4.1. SQL Dialects
You should always set the hibernate.dialect property to the correct
org.hibernate.dialect.Dialect subclass for your database. If you specify a
dialect, Hibernate will use sensible defaults for some of the other properties
listed above, saving you the effort of specifying them manually.
Table 3.8. Hibernate SQL Dialects (hibernate.dialect)
RDBMS
Dialect
DB2
org.hibernate.dialect.DB2Dialect
DB2 AS/400
org.hibernate.dialect.DB2400Dialect
DB2 OS390
org.hibernate.dialect.DB2390Dialect
PostgreSQL
org.hibernate.dialect.PostgreSQLDialect
MySQL
org.hibernate.dialect.MySQLDialect
MySQL with InnoDB
org.hibernate.dialect.MySQLInnoDBDialect
MySQL with MyISAM
org.hibernate.dialect.MySQLMyISAMDialect
Oracle (any version)
org.hibernate.dialect.OracleDialect
Oracle 9i/10g
org.hibernate.dialect.Oracle9Dialect
Sybase
org.hibernate.dialect.SybaseDialect
Sybase Anywhere
org.hibernate.dialect.SybaseAnywhereDialect
Microsoft SQL Server
org.hibernate.dialect.SQLServerDialect
SAP DB
org.hibernate.dialect.SAPDBDialect
Informix
org.hibernate.dialect.InformixDialect
HypersonicSQL
org.hibernate.dialect.HSQLDialect
Ingres
org.hibernate.dialect.IngresDialect
Progress
org.hibernate.dialect.ProgressDialect
Mckoi SQL
org.hibernate.dialect.MckoiDialect
Interbase
org.hibernate.dialect.InterbaseDialect
Pointbase
org.hibernate.dialect.PointbaseDialect
FrontBase
org.hibernate.dialect.FrontbaseDialect
Firebird
org.hibernate.dialect.FirebirdDialect
3.4.2. Outer Join Fetching
If your database supports ANSI, Oracle or Sybase style outer joins, outer
join fetching will often increase performance by limiting the number of round
Hibernate 3.3.1
43
Chapter 3. Configuration
trips to and from the database (at the cost of possibly more work performed
by the database itself). Outer join fetching allows a whole graph of objects
connected by many-to-one, one-to-many, many-to-many and one-to-one
associations to be retrieved in a single SQL SELECT.
Outer join fetching may be disabled globally by setting the property
hibernate.max_fetch_depth to 0. A setting of 1 or higher enables outer join
fetching for one-to-one and many-to-one associations which have been
mapped with fetch="join".
See Section 19.1, “Fetching strategies” for more information.
3.4.3. Binary Streams
Oracle limits the size of byte arrays that may be passed to/from its JDBC
driver. If you wish to use large instances of binary or serializable type,
you should enable hibernate.jdbc.use_streams_for_binary. This is a
system-level setting only.
3.4.4. Second-level and query cache
The properties prefixed by hibernate.cache allow you to use a process
or cluster scoped second-level cache system with Hibernate. See the
Section 19.2, “The Second Level Cache” for more details.
3.4.5. Query Language Substitution
You may define new Hibernate query tokens using
hibernate.query.substitutions. For example:
hibernate.query.substitutions true=1, false=0
would cause the tokens true and false to be translated to integer literals in
the generated SQL.
hibernate.query.substitutions toLowercase=LOWER
would allow you to rename the SQL LOWER function.
3.4.6. Hibernate statistics
If you enable hibernate.generate_statistics, Hibernate will expose
a number of metrics that are useful when tuning a running system via
SessionFactory.getStatistics(). Hibernate can even be configured to
expose these statistics via JMX. Read the Javadoc of the interfaces in
org.hibernate.stats for more information.
44
Hibernate 3.3.1
Logging
3.5. Logging
Hibernate utilizes Simple Logging Facade for Java [http://www.slf4j.org/]
(SLF4J) in order to log various system events. SLF4J can direct your logging
output to several logging frameworks (NOP, Simple, log4j version 1.2, JDK
1.4 logging, JCL or logback) depending on your chosen binding. In order to
setup logging properly you will need slf4j-api.jar in your classpath together
with the jar file for your preferred binding - slf4j-log4j12.jar in the case of
Log4J. See the SLF4J documentation [http://www.slf4j.org/manual.html] for
more detail. To use Log4j you will also need to place a log4j.properties file
in your classpath, an example properties file is distributed with Hibernate in
the src/ directory.
We strongly recommend that you familiarize yourself with Hibernate's
log messages. A lot of work has been put into making the Hibernate log
as detailed as possible, without making it unreadable. It is an essential
troubleshooting device. The most interesting log categories are the following:
Table 3.9. Hibernate Log Categories
Category
Function
org.hibernate.SQL
Log all SQL DML statements as they are executed
org.hibernate.type
Log all JDBC parameters
org.hibernate.tool.hbm2ddl
Log all
SQL DDL statements as they are executed
org.hibernate.pretty Log
the state of all entities (max 20 entities)
associated with the session at flush time
org.hibernate.cache
Log all second-level cache activity
org.hibernate.transaction
Log transaction
org.hibernate.jdbc
Log all JDBC resource acquisition
org.hibernate.hql.ast.AST
Log HQL
org.hibernate.secure Log
org.hibernate
related activity
and SQL ASTs during query parsing
all JAAS authorization requests
Log everything (a lot of information, but very useful for
troubleshooting)
When developing applications with Hibernate, you should almost always
work with debug enabled for the category org.hibernate.SQL, or, alternatively,
the property hibernate.show_sql enabled.
3.6. Implementing a NamingStrategy
The interface org.hibernate.cfg.NamingStrategy allows you to specify a
"naming standard" for database objects and schema elements.
Hibernate 3.3.1
45
Chapter 3. Configuration
You may provide rules for automatically generating database identifiers from
Java identifiers or for processing "logical" column and table names given in
the mapping file into "physical" table and column names. This feature helps
reduce the verbosity of the mapping document, eliminating repetitive noise
(TBL_ prefixes, for example). The default strategy used by Hibernate is quite
minimal.
You may specify a different strategy by calling
Configuration.setNamingStrategy() before adding mappings:
SessionFactory sf = new Configuration()
.setNamingStrategy(ImprovedNamingStrategy.INSTANCE)
.addFile("Item.hbm.xml")
.addFile("Bid.hbm.xml")
.buildSessionFactory();
is a built-in strategy that might be
a useful starting point for some applications.
org.hibernate.cfg.ImprovedNamingStrategy
3.7. XML configuration file
An alternative approach to configuration is to specify a full configuration in a
file named hibernate.cfg.xml. This file can be used as a replacement for the
hibernate.properties file or, if both are present, to override properties.
The XML configuration file is by default expected to be in the root o your
CLASSPATH. Here is an example:
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<!-- a SessionFactory instance listed as /jndi/name -->
<session-factory
name="java:hibernate/SessionFactory">
<!-- properties -->
<property
name="connection.datasource">java:/comp/env/jdbc/MyDB</property>
<property
name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="show_sql">false</property>
<property name="transaction.factory_class">
org.hibernate.transaction.JTATransactionFactory
</property>
46
Hibernate 3.3.1
J2EE Application Server integration
<property
name="jta.UserTransaction">java:comp/UserTransaction</property>
<!-- mapping files -->
<mapping resource="org/hibernate/auction/Item.hbm.xml"/>
<mapping resource="org/hibernate/auction/Bid.hbm.xml"/>
<!-- cache settings -->
<class-cache class="org.hibernate.auction.Item"
usage="read-write"/>
<class-cache class="org.hibernate.auction.Bid"
usage="read-only"/>
<collection-cache
collection="org.hibernate.auction.Item.bids" usage="read-write"/>
</session-factory>
</hibernate-configuration>
As you can see, the advantage of this approach is the externalization of the
mapping file names to configuration. The hibernate.cfg.xml is also more
convenient once you have to tune the Hibernate cache. Note that is your
choice to use either hibernate.properties or hibernate.cfg.xml, both are
equivalent, except for the above mentioned benefits of using the XML syntax.
With the XML configuration, starting Hibernate is then as simple as
SessionFactory sf = new
Configuration().configure().buildSessionFactory();
You can pick a different XML configuration file using
SessionFactory sf = new Configuration()
.configure("catdb.cfg.xml")
.buildSessionFactory();
3.8. J2EE Application Server integration
Hibernate has the following integration points for J2EE infrastructure:
• Container-managed datasources: Hibernate can use JDBC connections
managed by the container and provided through JNDI. Usually, a JTA
compatible TransactionManager and a ResourceManager take care of
transaction management (CMT), esp. distributed transaction handling
across several datasources. You may of course also demarcate
transaction boundaries programmatically (BMT) or you might want to use
the optional Hibernate Transaction API for this to keep your code portable.
• Automatic JNDI binding: Hibernate can bind its SessionFactory to JNDI
after startup.
Hibernate 3.3.1
47
Chapter 3. Configuration
• JTA Session binding: The Hibernate Session may be automatically bound
to the scope of JTA transactions. Simply lookup the SessionFactory
from JNDI and get the current Session. Let Hibernate take care of
flushing and closing the Session when your JTA transaction completes.
Transaction demarcation is either declarative (CMT) or programmatic
(BMT/UserTransaction).
• JMX deployment: If you have a JMX capable application server (e.g.
JBoss AS), you can chose to deploy Hibernate as a managed MBean.
This saves you the one line startup code to build your SessionFactory
from a Configuration. The container will startup your HibernateService,
and ideally also take care of service dependencies (Datasource has to be
available before Hibernate starts, etc).
Depending on your environment, you might have to set the configuration
option hibernate.connection.aggressive_release to true if your application
server shows "connection containment" exceptions.
3.8.1. Transaction strategy configuration
The Hibernate Session API is independent of any transaction demarcation
system in your architecture. If you let Hibernate use JDBC directly, through
a connection pool, you may begin and end your transactions by calling the
JDBC API. If you run in a J2EE application server, you might want to use
bean-managed transactions and call the JTA API and UserTransaction when
needed.
To keep your code portable between these two (and other) environments
we recommend the optional Hibernate Transaction API, which wraps
and hides the underlying system. You have to specify a factory class for
Transaction instances by setting the Hibernate configuration property
hibernate.transaction.factory_class.
There are three standard (built-in) choices:
org.hibernate.transaction.JDBCTransactionFactory
delegates to database (JDBC) transactions (default)
org.hibernate.transaction.JTATransactionFactory
delegates to container-managed transaction if an existing transaction is
underway in this context (e.g. EJB session bean method), otherwise a
new transaction is started and bean-managed transaction are used.
org.hibernate.transaction.CMTTransactionFactory
delegates to container-managed JTA transactions
48
Hibernate 3.3.1
JNDI-bound SessionFactory
You may also define your own transaction strategies (for a CORBA
transaction service, for example).
Some features in Hibernate (i.e. the second level cache, Contextual Sessions
with JTA, etc.) require access to the JTA TransactionManager in a managed
environment. In an application server you have to specify how Hibernate
should obtain a reference to the TransactionManager, since J2EE does not
standardize a single mechanism:
Table 3.10. JTA TransactionManagers
Transaction Factory
Application Server
org.hibernate.transaction.JBossTransactionManagerLookup
JBoss
org.hibernate.transaction.WeblogicTransactionManagerLookup
Weblogic
org.hibernate.transaction.WebSphereTransactionManagerLookup
WebSphere
org.hibernate.transaction.WebSphereExtendedJTATransactionLookup
WebSphere
org.hibernate.transaction.OrionTransactionManagerLookup
Orion
org.hibernate.transaction.ResinTransactionManagerLookup
Resin
org.hibernate.transaction.JOTMTransactionManagerLookup
JOTM
org.hibernate.transaction.JOnASTransactionManagerLookup
JOnAS
org.hibernate.transaction.JRun4TransactionManagerLookup
JRun4
org.hibernate.transaction.BESTransactionManagerLookup
6
Borland ES
3.8.2. JNDI-bound SessionFactory
A JNDI bound Hibernate SessionFactory can simplify the lookup of the factory
and the creation of new Sessions. Note that this is not related to a JNDI
bound Datasource, both simply use the same registry!
If you wish to have the SessionFactory bound to a JNDI namespace,
specify a name (eg. java:hibernate/SessionFactory) using the property
hibernate.session_factory_name. If this property is omitted, the
SessionFactory will not be bound to JNDI. (This is especially useful in
environments with a read-only JNDI default implementation, e.g. Tomcat.)
When binding the SessionFactory to JNDI, Hibernate will use the values of
hibernate.jndi.url, hibernate.jndi.class to instantiate an initial context. If
they are not specified, the default InitialContext will be used.
Hibernate will automatically place the SessionFactory in JNDI after you call
cfg.buildSessionFactory(). This means you will at least have this call in
some startup code (or utility class) in your application, unless you use JMX
deployment with the HibernateService (discussed later).
Hibernate 3.3.1
49
Chapter 3. Configuration
If you use a JNDI SessionFactory, an EJB or any other class may obtain the
SessionFactory using a JNDI lookup.
We recommend that you bind the SessionFactory to JNDI in a managed
environment and use a static singleton otherwise. To shield your
application code from these details, we also recommend to hide the
actual lookup code for a SessionFactory in a helper class, such as
HibernateUtil.getSessionFactory(). Note that such a class is also a
convenient way to startup Hibernatesee chapter 1.
3.8.3. Current Session context management with JTA
The easiest way to handle Sessions and transactions is Hibernates automatic
"current" Session management. See the discussion of current sessions.
Using the "jta" session context, if there is no Hibernate Session associated
with the current JTA transaction, one will be started and associated with that
JTA transaction the first time you call sessionFactory.getCurrentSession().
The Sessions retrieved via getCurrentSession() in "jta" context will be
set to automatically flush before the transaction completes, close after the
transaction completes, and aggressively release JDBC connections after
each statement. This allows the Sessions to be managed by the life cycle
of the JTA transaction to which it is associated, keeping user code clean of
such management concerns. Your code can either use JTA programmatically
through UserTransaction, or (recommended for portable code) use the
Hibernate Transaction API to set transaction boundaries. If you run in an EJB
container, declarative transaction demarcation with CMT is preferred.
3.8.4. JMX deployment
The line cfg.buildSessionFactory() still has to be executed somewhere to
get a SessionFactory into JNDI. You can do this either in a static initializer
block (like the one in HibernateUtil) or you deploy Hibernate as a managed
service.
Hibernate is distributed with org.hibernate.jmx.HibernateService for
deployment on an application server with JMX capabilities, such as JBoss
AS. The actual deployment and configuration is vendor specific. Here is an
example jboss-service.xml for JBoss 4.0.x:
<?xml version="1.0"?>
<server>
<mbean code="org.hibernate.jmx.HibernateService"
name="jboss.jca:service=HibernateFactory,name=HibernateFactory">
<!-- Required services -->
50
Hibernate 3.3.1
JMX deployment
<depends>jboss.jca:service=RARDeployer</depends>
<depends>jboss.jca:service=LocalTxCM,name=HsqlDS</depends>
<!-- Bind the Hibernate service to JNDI -->
<attribute
name="JndiName">java:/hibernate/SessionFactory</attribute>
<!-- Datasource settings -->
<attribute name="Datasource">java:HsqlDS</attribute>
<attribute
name="Dialect">org.hibernate.dialect.HSQLDialect</attribute>
<!-- Transaction integration -->
<attribute name="TransactionStrategy">
org.hibernate.transaction.JTATransactionFactory</attribute>
<attribute name="TransactionManagerLookupStrategy">
org.hibernate.transaction.JBossTransactionManagerLookup</attribute>
<attribute name="FlushBeforeCompletionEnabled">true</attribute>
<attribute name="AutoCloseSessionEnabled">true</attribute>
<!-- Fetching options -->
<attribute name="MaximumFetchDepth">5</attribute>
<!-- Second-level caching -->
<attribute name="SecondLevelCacheEnabled">true</attribute>
<attribute
name="CacheProviderClass">org.hibernate.cache.EhCacheProvider</
attribute>
<attribute name="QueryCacheEnabled">true</attribute>
<!-- Logging -->
<attribute name="ShowSqlEnabled">true</attribute>
<!-- Mapping files -->
<attribute
name="MapResources">auction/Item.hbm.xml,auction/Category.hbm.xml</
attribute>
</mbean>
</server>
This file is deployed in a directory called META-INF and packaged in a JAR
file with the extension .sar (service archive). You also need to package
Hibernate, its required third-party libraries, your compiled persistent
classes, as well as your mapping files in the same archive. Your enterprise
beans (usually session beans) may be kept in their own JAR file, but you
may include this EJB JAR file in the main service archive to get a single
(hot-)deployable unit. Consult the JBoss AS documentation for more
information about JMX service and EJB deployment.
Hibernate 3.3.1
51
52
Hibernate 3.3.1
Chapter 4. Persistent Classes
Persistent classes are classes in an application that implement the entities
of the business problem (e.g. Customer and Order in an E-commerce
application). Not all instances of a persistent class are considered to be in the
persistent state - an instance may instead be transient or detached.
Hibernate works best if these classes follow some simple rules, also known
as the Plain Old Java Object (POJO) programming model. However, none
of these rules are hard requirements. Indeed, Hibernate3 assumes very
little about the nature of your persistent objects. You may express a domain
model in other ways: using trees of Map instances, for example.
4.1. A simple POJO example
Most Java applications require a persistent class representing felines.
package eg;
import java.util.Set;
import java.util.Date;
public class Cat {
private Long id; // identifier
private
private
private
private
private
Date birthdate;
Color color;
char sex;
float weight;
int litterId;
private Cat mother;
private Set kittens = new HashSet();
private void setId(Long id) {
this.id=id;
}
public Long getId() {
return id;
}
void setBirthdate(Date date) {
birthdate = date;
}
public Date getBirthdate() {
return birthdate;
}
void setWeight(float weight) {
this.weight = weight;
}
Hibernate 3.3.1
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Chapter 4. Persistent Classes
public float getWeight() {
return weight;
}
public Color getColor() {
return color;
}
void setColor(Color color) {
this.color = color;
}
void setSex(char sex) {
this.sex=sex;
}
public char getSex() {
return sex;
}
void setLitterId(int id) {
this.litterId = id;
}
public int getLitterId() {
return litterId;
}
void setMother(Cat mother) {
this.mother = mother;
}
public Cat getMother() {
return mother;
}
void setKittens(Set kittens) {
this.kittens = kittens;
}
public Set getKittens() {
return kittens;
}
// addKitten not needed by Hibernate
public void addKitten(Cat kitten) {
kitten.setMother(this);
kitten.setLitterId( kittens.size() );
kittens.add(kitten);
}
}
There are four main rules to follow here:
4.1.1. Implement a no-argument constructor
has a no-argument constructor. All persistent classes must have a default
constructor (which may be non-public) so that Hibernate can instantiate them
using Constructor.newInstance(). We strongly recommend having a default
Cat
54
Hibernate 3.3.1
Provide an identifier property (optional)
constructor with at least package visibility for runtime proxy generation in
Hibernate.
4.1.2. Provide an identifier property (optional)
has a property called id. This property maps to the primary key column
of a database table. The property might have been called anything, and
its type might have been any primitive type, any primitive "wrapper" type,
java.lang.String or java.util.Date. (If your legacy database table has
composite keys, you can even use a user-defined class with properties of
these types - see the section on composite identifiers later.)
Cat
The identifier property is strictly optional. You can leave them off and let
Hibernate keep track of object identifiers internally. We do not recommend
this, however.
In fact, some functionality is available only to classes which declare an
identifier property:
• Transitive reattachment for detached objects (cascade update or cascade
merge) - see Section 10.11, “Transitive persistence”
• Session.saveOrUpdate()
• Session.merge()
We recommend you declare consistently-named identifier properties on
persistent classes. We further recommend that you use a nullable (ie.
non-primitive) type.
4.1.3. Prefer non-final classes (optional)
A central feature of Hibernate, proxies, depends upon the persistent class
being either non-final, or the implementation of an interface that declares all
public methods.
You can persist final classes that do not implement an interface with
Hibernate, but you won't be able to use proxies for lazy association fetching which will limit your options for performance tuning.
You should also avoid declaring public final methods on the non-final
classes. If you want to use a class with a public final method, you must
explicitly disable proxying by setting lazy="false".
4.1.4. Declare accessors and mutators for persistent
fields (optional)
declares accessor methods for all its persistent fields. Many other ORM
tools directly persist instance variables. We believe it is better to provide
Cat
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Chapter 4. Persistent Classes
an indirection between the relational schema and internal data structures
of the class. By default, Hibernate persists JavaBeans style properties, and
recognizes method names of the form getFoo, isFoo and setFoo. You may
switch to direct field access for particular properties, if needed.
Properties need not be declared public - Hibernate can persist a property
with a default, protected or private get / set pair.
4.2. Implementing inheritance
A subclass must also observe the first and second rules. It inherits its
identifier property from the superclass, Cat.
package eg;
public class DomesticCat extends Cat {
private String name;
public String getName() {
return name;
}
protected void setName(String name) {
this.name=name;
}
}
4.3. Implementing equals() and hashCode()
You have to override the equals() and hashCode() methods if you
• intend to put instances of persistent classes in a Set (the recommended
way to represent many-valued associations) and
• intend to use reattachment of detached instances
Hibernate guarantees equivalence of persistent identity (database row) and
Java identity only inside a particular session scope. So as soon as we mix
instances retrieved in different sessions, we must implement equals() and
hashCode() if we wish to have meaningful semantics for Sets.
The most obvious way is to implement equals()/hashCode() by comparing
the identifier value of both objects. If the value is the same, both must be
the same database row, they are therefore equal (if both are added to a
Set, we will only have one element in the Set). Unfortunately, we can't use
that approach with generated identifiers! Hibernate will only assign identifier
values to objects that are persistent, a newly created instance will not have
any identifier value! Furthermore, if an instance is unsaved and currently
in a Set, saving it will assign an identifier value to the object. If equals()
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Dynamic models
and hashCode() are based on the identifier value, the hash code would
change, breaking the contract of the Set. See the Hibernate website for a full
discussion of this problem. Note that this is not a Hibernate issue, but normal
Java semantics of object identity and equality.
We recommend implementing equals() and hashCode() using Business key
equality. Business key equality means that the equals() method compares
only the properties that form the business key, a key that would identify our
instance in the real world (a natural candidate key):
public class Cat {
...
public boolean equals(Object other) {
if (this == other) return true;
if ( !(other instanceof Cat) ) return false;
final Cat cat = (Cat) other;
if ( !cat.getLitterId().equals( getLitterId() ) ) return
false;
if ( !cat.getMother().equals( getMother() ) ) return false;
return true;
}
public int hashCode() {
int result;
result = getMother().hashCode();
result = 29 * result + getLitterId();
return result;
}
}
Note that a business key does not have to be as solid as a database primary
key candidate (see Section 11.1.3, “Considering object identity”). Immutable
or unique properties are usually good candidates for a business key.
4.4. Dynamic models
Note that the following features are currently considered experimental and
may change in the near future.
Persistent entities don't necessarily have to be represented as POJO classes
or as JavaBean objects at runtime. Hibernate also supports dynamic models
(using Maps of Maps at runtime) and the representation of entities as DOM4J
trees. With this approach, you don't write persistent classes, only mapping
files.
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Chapter 4. Persistent Classes
By default, Hibernate works in normal POJO mode. You may set a default
entity representation mode for a particular SessionFactory using the
default_entity_mode configuration option (see Table 3.3, “Hibernate
Configuration Properties”.
The following examples demonstrates the representation using Maps. First, in
the mapping file, an entity-name has to be declared instead of (or in addition
to) a class name:
<hibernate-mapping>
<class entity-name="Customer">
<id name="id"
type="long"
column="ID">
<generator class="sequence"/>
</id>
<property name="name"
column="NAME"
type="string"/>
<property name="address"
column="ADDRESS"
type="string"/>
<many-to-one name="organization"
column="ORGANIZATION_ID"
class="Organization"/>
<bag name="orders"
inverse="true"
lazy="false"
cascade="all">
<key column="CUSTOMER_ID"/>
<one-to-many class="Order"/>
</bag>
</class>
</hibernate-mapping>
Note that even though associations are declared using target class names,
the target type of an associations may also be a dynamic entity instead of a
POJO.
After setting the default entity mode to dynamic-map for the SessionFactory,
we can at runtime work with Maps of Maps:
Session s = openSession();
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Hibernate 3.3.1
Dynamic models
Transaction tx = s.beginTransaction();
Session s = openSession();
// Create a customer
Map david = new HashMap();
david.put("name", "David");
// Create an organization
Map foobar = new HashMap();
foobar.put("name", "Foobar Inc.");
// Link both
david.put("organization", foobar);
// Save both
s.save("Customer", david);
s.save("Organization", foobar);
tx.commit();
s.close();
The advantages of a dynamic mapping are quick turnaround time for
prototyping without the need for entity class implementation. However,
you lose compile-time type checking and will very likely deal with many
exceptions at runtime. Thanks to the Hibernate mapping, the database
schema can easily be normalized and sound, allowing to add a proper
domain model implementation on top later on.
Entity representation modes can also be set on a per Session basis:
Session dynamicSession = pojoSession.getSession(EntityMode.MAP);
// Create a customer
Map david = new HashMap();
david.put("name", "David");
dynamicSession.save("Customer", david);
...
dynamicSession.flush();
dynamicSession.close()
...
// Continue on pojoSession
Please note that the call to getSession() using an EntityMode is on the
Session API, not the SessionFactory. That way, the new Session shares the
underlying JDBC connection, transaction, and other context information. This
means you don't have tocall flush() and close() on the secondary Session,
and also leave the transaction and connection handling to the primary unit of
work.
More information about the XML representation capabilities can be found in
Chapter 18, XML Mapping.
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4.5. Tuplizers
org.hibernate.tuple.Tuplizer,
and its sub-interfaces, are responsible
for managing a particular representation of a piece of data, given that
representation's org.hibernate.EntityMode. If a given piece of data
is thought of as a data structure, then a tuplizer is the thing which
knows how to create such a data structure and how to extract values
from and inject values into such a data structure. For example, for the
POJO entity mode, the correpsonding tuplizer knows how create the
POJO through its constructor and how to access the POJO properties
using the defined property accessors. There are two high-level types of
Tuplizers, represented by the org.hibernate.tuple.entity.EntityTuplizer
and org.hibernate.tuple.component.ComponentTuplizer interfaces.
EntityTuplizers are responsible for managing the above mentioned
contracts in regards to entities, while ComponentTuplizers do the same for
components.
Users may also plug in their own tuplizers. Perhaps you require that a
java.util.Map implementation other than java.util.HashMap be used while
in the dynamic-map entity-mode; or perhaps you need to define a different
proxy generation strategy than the one used by default. Both would be
achieved by defining a custom tuplizer implementation. Tuplizers definitions
are attached to the entity or component mapping they are meant to manage.
Going back to the example of our customer entity:
<hibernate-mapping>
<class entity-name="Customer">
<!-Override the dynamic-map entity-mode
tuplizer for the customer entity
-->
<tuplizer entity-mode="dynamic-map"
class="CustomMapTuplizerImpl"/>
<id name="id" type="long" column="ID">
<generator class="sequence"/>
</id>
<!-- other properties -->
...
</class>
</hibernate-mapping>
public class CustomMapTuplizerImpl
extends org.hibernate.tuple.entity.DynamicMapEntityTuplizer
{
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Hibernate 3.3.1
Extentsions
// override the buildInstantiator() method to plug in our custom
map...
protected final Instantiator buildInstantiator(
org.hibernate.mapping.PersistentClass mappingInfo) {
return new CustomMapInstantiator( mappingInfo );
}
private static final class CustomMapInstantiator
extends org.hibernate.tuple.DynamicMapInstantitor {
// override the generateMap() method to return our custom
map...
protected final Map generateMap() {
return new CustomMap();
}
}
}
4.6. Extentsions
TODO: Document user-extension framework in the property and proxy
packages
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Chapter 5. Basic O/R Mapping
5.1. Mapping declaration
Object/relational mappings are usually defined in an XML document. The
mapping document is designed to be readable and hand-editable. The
mapping language is Java-centric, meaning that mappings are constructed
around persistent class declarations, not table declarations.
Note that, even though many Hibernate users choose to write the XML by
hand, a number of tools exist to generate the mapping document, including
XDoclet, Middlegen and AndroMDA.
Lets kick off with an example mapping:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class name="Cat"
table="cats"
discriminator-value="C">
<id name="id">
<generator class="native"/>
</id>
<discriminator column="subclass"
type="character"/>
<property name="weight"/>
<property name="birthdate"
type="date"
not-null="true"
update="false"/>
<property name="color"
type="eg.types.ColorUserType"
not-null="true"
update="false"/>
<property name="sex"
not-null="true"
update="false"/>
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<property name="litterId"
column="litterId"
update="false"/>
<many-to-one name="mother"
column="mother_id"
update="false"/>
<set name="kittens"
inverse="true"
order-by="litter_id">
<key column="mother_id"/>
<one-to-many class="Cat"/>
</set>
<subclass name="DomesticCat"
discriminator-value="D">
<property name="name"
type="string"/>
</subclass>
</class>
<class name="Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>
We will now discuss the content of the mapping document. We will only
describe the document elements and attributes that are used by Hibernate at
runtime. The mapping document also contains some extra optional attributes
and elements that affect the database schemas exported by the schema
export tool. (For example the not-null attribute.)
5.1.1. Doctype
All XML mappings should declare the doctype shown. The
actual DTD may be found at the URL above, in the directory
hibernate-x.x.x/src/org/hibernate or in hibernate3.jar. Hibernate will
always look for the DTD in its classpath first. If you experience lookups of the
DTD using an Internet connection, check your DTD declaration against the
contents of your claspath.
5.1.1.1. EntityResolver
As mentioned previously, Hibernate will first attempt to resolve DTDs in
its classpath. The manner in which it does this is by registering a custom
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hibernate-mapping
implementation with the SAXReader it uses to
read in the xml files. This custom EntityResolver recognizes two different
systemId namespaces.
org.xml.sax.EntityResolver
• a hibernate namespace is recognized whenever the resolver encounteres
a systemId starting with http://hibernate.sourceforge.net/; the resolver
attempts to resolve these entities via the classlaoder which loaded the
Hibernate classes.
• a user namespace is recognized whenever the resolver encounteres a
systemId using a classpath:// URL protocol; the resolver will attempt to
resolve these entities via (1) the current thread context classloader and (2)
the classloader which loaded the Hibernate classes.
An example of utilizing user namespacing:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"
[
<!ENTITY types SYSTEM "classpath://your/domain/types.xml">
]>
<hibernate-mapping package="your.domain">
<class name="MyEntity">
<id name="id" type="my-custom-id-type">
...
</id>
<class>
&types;
</hibernate-mapping>
Where types.xml is a resource in the your.domain package and contains a
custom typedef.
5.1.2. hibernate-mapping
This element has several optional attributes. The schema and catalog
attributes specify that tables referred to in this mapping belong to the named
schema and/or catalog. If specified, tablenames will be qualified by the given
schema and catalog names. If missing, tablenames will be unqualified. The
default-cascade attribute specifies what cascade style should be assumed
for properties and collections which do not specify a cascade attribute.
The auto-import attribute lets us use unqualified class names in the query
language, by default.
<hibernate-mapping
schema="schemaName"
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catalog="catalogName"
default-cascade="cascade_style"
default-access="field|property|ClassName"
default-lazy="true|false"
auto-import="true|false"
package="package.name"
(2)
(3)
(4)
(5)
(6)
(7)
/>
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(optional): The name of a database schema.
catalog (optional): The name of a database catalog.
default-cascade (optional - defaults to none): A default cascade style.
default-access (optional - defaults to property): The strategy
Hibernate should use for accessing all properties. Can be a custom
implementation of PropertyAccessor.
default-lazy (optional - defaults to true): The default value for
unspecifed lazy attributes of class and collection mappings.
auto-import (optional - defaults to true): Specifies whether we can
use unqualified class names (of classes in this mapping) in the query
language.
package (optional): Specifies a package prefix to assume for unqualified
class names in the mapping document.
schema
If you have two persistent classes with the same (unqualified) name, you
should set auto-import="false". Hibernate will throw an exception if you
attempt to assign two classes to the same "imported" name.
Note that the hibernate-mapping element allows you to nest several
persistent <class> mappings, as shown above. It is however good practice
(and expected by some tools) to map only a single persistent class
(or a single class hierarchy) in one mapping file and name it after the
persistent superclass, e.g. Cat.hbm.xml, Dog.hbm.xml, or if using inheritance,
Animal.hbm.xml.
5.1.3. class
You may declare a persistent class using the class element:
<class
name="ClassName"
table="tableName"
discriminator-value="discriminator_value"
mutable="true|false"
schema="owner"
catalog="catalog"
proxy="ProxyInterface"
dynamic-update="true|false"
dynamic-insert="true|false"
select-before-update="true|false"
polymorphism="implicit|explicit"
66
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
Hibernate 3.3.1
class
where="arbitrary sql where condition"
persister="PersisterClass"
batch-size="N"
optimistic-lock="none|version|dirty|all"
lazy="true|false"
entity-name="EntityName"
check="arbitrary sql check condition"
rowid="rowid"
subselect="SQL expression"
abstract="true|false"
node="element-name"
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
(21)
/>
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(optional): The fully qualified Java class name of the persistent
class (or interface). If this attribute is missing, it is assumed that the
mapping is for a non-POJO entity.
table (optional - defaults to the unqualified class name): The name of its
database table.
discriminator-value (optional - defaults to the class name): A value
that distiguishes individual subclasses, used for polymorphic behaviour.
Acceptable values include null and not null.
mutable (optional, defaults to true): Specifies that instances of the class
are (not) mutable.
schema (optional): Override the schema name specified by the root
<hibernate-mapping> element.
catalog (optional): Override the catalog name specified by the root
<hibernate-mapping> element.
proxy (optional): Specifies an interface to use for lazy initializing proxies.
You may specify the name of the class itself.
dynamic-update (optional, defaults to false): Specifies that UPDATE SQL
should be generated at runtime and contain only those columns whose
values have changed.
dynamic-insert (optional, defaults to false): Specifies that INSERT SQL
should be generated at runtime and contain only the columns whose
values are not null.
select-before-update (optional, defaults to false): Specifies that
Hibernate should never perform an SQL UPDATE unless it is certain
that an object is actually modified. In certain cases (actually, only
when a transient object has been associated with a new session using
update()), this means that Hibernate will perform an extra SQL SELECT to
determine if an UPDATE is actually required.
polymorphism (optional, defaults to implicit): Determines whether
implicit or explicit query polymorphism is used.
where (optional) specify an arbitrary SQL WHERE condition to be used
when retrieving objects of this class
persister (optional): Specifies a custom ClassPersister.
name
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(optional, defaults to 1) specify a "batch size" for fetching
instances of this class by identifier.
optimistic-lock (optional, defaults to version): Determines the
optimistic locking strategy.
lazy (optional): Lazy fetching may be completely disabled by setting
lazy="false".
entity-name (optional, defaults to the class name): Hibernate3 allows a
class to be mapped multiple times (to different tables, potentially), and
allows entity mappings that are represented by Maps or XML at the Java
level. In these cases, you should provide an explicit arbitrary name for
the entity. See Section 4.4, “Dynamic models” and Chapter 18, XML
Mapping for more information.
check (optional): A SQL expression used to generate a multi-row check
constraint for automatic schema generation.
rowid (optional): Hibernate can use so called ROWIDs on databases
which support. E.g. on Oracle, Hibernate can use the rowid extra
column for fast updates if you set this option to rowid. A ROWID is an
implementation detail and represents the physical location of a stored
tuple.
subselect (optional): Maps an immutable and read-only entity to a
database subselect. Useful if you want to have a view instead of a base
table, but don't. See below for more information.
abstract (optional): Used to mark abstract superclasses in
<union-subclass> hierarchies.
(14) batch-size
(15)
(16)
(17)
(18)
(19)
(20)
(21)
It is perfectly acceptable for the named persistent class to be an interface.
You would then declare implementing classes of that interface using the
<subclass> element. You may persist any static inner class. You should
specify the class name using the standard form ie. eg.Foo$Bar.
Immutable classes, mutable="false", may not be updated or deleted by
the application. This allows Hibernate to make some minor performance
optimizations.
The optional proxy attribute enables lazy initialization of persistent instances
of the class. Hibernate will initially return CGLIB proxies which implement the
named interface. The actual persistent object will be loaded when a method
of the proxy is invoked. See "Initializing collections and proxies" below.
Implicit polymorphism means that instances of the class will be returned by
a query that names any superclass or implemented interface or the class
and that instances of any subclass of the class will be returned by a query
that names the class itself. Explicit polymorphism means that class instances
will be returned only by queries that explicitly name that class and that
queries that name the class will return only instances of subclasses mapped
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class
inside this <class> declaration as a <subclass> or <joined-subclass>. For
most purposes the default, polymorphism="implicit", is appropriate. Explicit
polymorphism is useful when two different classes are mapped to the same
table (this allows a "lightweight" class that contains a subset of the table
columns).
The persister attribute lets you customize the persistence strategy
used for the class. You may, for example, specify your own
subclass of org.hibernate.persister.EntityPersister or you might
even provide a completely new implementation of the interface
org.hibernate.persister.ClassPersister that implements persistence via,
for example, stored procedure calls, serialization to flat files or LDAP. See
org.hibernate.test.CustomPersister for a simple example (of "persistence"
to a Hashtable).
Note that the dynamic-update and dynamic-insert settings are not
inherited by subclasses and so may also be specified on the <subclass>
or <joined-subclass> elements. These settings may increase performance
in some cases, but might actually decrease performance in others. Use
judiciously.
Use of select-before-update will usually decrease performance. It is very
useful to prevent a database update trigger being called unnecessarily if you
reattach a graph of detached instances to a Session.
If you enable dynamic-update, you will have a choice of optimistic locking
strategies:
• version check the version/timestamp columns
• all check all columns
• dirty check the changed columns, allowing some concurrent updates
• none do not use optimistic locking
We very strongly recommend that you use version/timestamp columns for
optimistic locking with Hibernate. This is the optimal strategy with respect
to performance and is the only strategy that correctly handles modifications
made to detached instances (ie. when Session.merge() is used).
There is no difference between a view and a base table for a Hibernate
mapping, as expected this is transparent at the database level (note
that some DBMS don't support views properly, especially with updates).
Sometimes you want to use a view, but can't create one in the database
(ie. with a legacy schema). In this case, you can map an immutable and
read-only entity to a given SQL subselect expression:
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<class name="Summary">
<subselect>
select item.name, max(bid.amount), count(*)
from item
join bid on bid.item_id = item.id
group by item.name
</subselect>
<synchronize table="item"/>
<synchronize table="bid"/>
<id name="name"/>
...
</class>
Declare the tables to synchronize this entity with, ensuring that auto-flush
happens correctly, and that queries against the derived entity do not return
stale data. The <subselect> is available as both as an attribute and a nested
mapping element.
5.1.4. id
Mapped classes must declare the primary key column of the database table.
Most classes will also have a JavaBeans-style property holding the unique
identifier of an instance. The <id> element defines the mapping from that
property to the primary key column.
<id
name="propertyName"
(1)
type="typename"
(2)
column="column_name"
(3)
unsaved-value="null|any|none|undefined|id_value"
(4)
access="field|property|ClassName">
(5)
node="element-name|@attribute-name|element/@attribute|."
<generator class="generatorClass"/>
</id>
name
(2)
type
(3)
(4)
70
(optional): The name of the identifier property.
(optional): A name that indicates the Hibernate type.
column (optional - defaults to the property name): The name of the
primary key column.
unsaved-value (optional - defaults to a "sensible" value): An identifier
property value that indicates that an instance is newly instantiated
(unsaved), distinguishing it from detached instances that were saved or
loaded in a previous session.
(1)
Hibernate 3.3.1
id
(optional - defaults to property): The strategy Hibernate should
use for accessing the property value.
(5)
access
If the name attribute is missing, it is assumed that the class has no identifier
property.
The unsaved-value attribute is almost never needed in Hibernate3.
There is an alternative <composite-id> declaration to allow access to legacy
data with composite keys. We strongly discourage its use for anything else.
5.1.4.1. Generator
The optional <generator> child element names a Java class used to generate
unique identifiers for instances of the persistent class. If any parameters are
required to configure or initialize the generator instance, they are passed
using the <param> element.
<id name="id" type="long" column="cat_id">
<generator class="org.hibernate.id.TableHiLoGenerator">
<param name="table">uid_table</param>
<param name="column">next_hi_value_column</param>
</generator>
</id>
All generators implement the interface
org.hibernate.id.IdentifierGenerator.
This is a very simple interface; some
applications may choose to provide their own specialized implementations.
However, Hibernate provides a range of built-in implementations. There are
shortcut names for the built-in generators:
increment
generates identifiers of type long, short or int that are unique only when
no other process is inserting data into the same table. Do not use in a
cluster.
identity
supports identity columns in DB2, MySQL, MS SQL Server, Sybase and
HypersonicSQL. The returned identifier is of type long, short or int.
sequence
uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a
generator in Interbase. The returned identifier is of type long, short or int
hilo
uses a hi/lo algorithm to efficiently generate identifiers of type long,
short or int, given a table and column (by default hibernate_unique_key
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Chapter 5. Basic O/R Mapping
and next_hi respectively) as a source of hi values. The hi/lo algorithm
generates identifiers that are unique only for a particular database.
seqhilo
uses a hi/lo algorithm to efficiently generate identifiers of type long, short
or int, given a named database sequence.
uuid
uses a 128-bit UUID algorithm to generate identifiers of type string,
unique within a network (the IP address is used). The UUID is encoded
as a string of hexadecimal digits of length 32.
guid
uses a database-generated GUID string on MS SQL Server and MySQL.
native
picks identity, sequence or hilo depending upon the capabilities of the
underlying database.
assigned
lets the application to assign an identifier to the object before save() is
called. This is the default strategy if no <generator> element is specified.
select
retrieves a primary key assigned by a database trigger by selecting the
row by some unique key and retrieving the primary key value.
foreign
uses the identifier of another associated object. Usually used in
conjunction with a <one-to-one> primary key association.
sequence-identity
a specialized sequence generation strategy which utilizes a database
sequence for the actual value generation, but combines this with JDBC3
getGeneratedKeys to actually return the generated identifier value as
part of the insert statement execution. This strategy is only known to be
supported on Oracle 10g drivers targetted for JDK 1.4. Note comments
on these insert statements are disabled due to a bug in the Oracle
drivers.
5.1.4.2. Hi/lo algorithm
The hilo and seqhilo generators provide two alternate implementations
of the hi/lo algorithm, a favorite approach to identifier generation. The first
implementation requires a "special" database table to hold the next available
"hi" value. The second uses an Oracle-style sequence (where supported).
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id
<id name="id" type="long" column="cat_id">
<generator class="hilo">
<param name="table">hi_value</param>
<param name="column">next_value</param>
<param name="max_lo">100</param>
</generator>
</id>
<id name="id" type="long" column="cat_id">
<generator class="seqhilo">
<param name="sequence">hi_value</param>
<param name="max_lo">100</param>
</generator>
</id>
Unfortunately, you can't use hilo when supplying your own Connection
to Hibernate. When Hibernate is using an application server datasource
to obtain connections enlisted with JTA, you must properly configure the
hibernate.transaction.manager_lookup_class.
5.1.4.3. UUID algorithm
The UUID contains: IP address, startup time of the JVM (accurate to a
quarter second), system time and a counter value (unique within the JVM).
It's not possible to obtain a MAC address or memory address from Java
code, so this is the best we can do without using JNI.
5.1.4.4. Identity columns and sequences
For databases which support identity columns (DB2, MySQL, Sybase, MS
SQL), you may use identity key generation. For databases that support
sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you may
use sequence style key generation. Both these strategies require two SQL
queries to insert a new object.
<id name="id" type="long" column="person_id">
<generator class="sequence">
<param name="sequence">person_id_sequence</param>
</generator>
</id>
<id name="id" type="long" column="person_id" unsaved-value="0">
<generator class="identity"/>
</id>
For cross-platform development, the native strategy will choose from the
identity, sequence and hilo strategies, dependant upon the capabilities of
the underlying database.
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5.1.4.5. Assigned identifiers
If you want the application to assign identifiers (as opposed to having
Hibernate generate them), you may use the assigned generator. This special
generator will use the identifier value already assigned to the object's
identifier property. This generator is used when the primary key is a natural
key instead of a surrogate key. This is the default behavior if you do no
specify a <generator> element.
Choosing the assigned generator makes Hibernate use
unsaved-value="undefined", forcing Hibernate to go to the database to
determine if an instance is transient or detached, unless there is a version or
timestamp property, or you define Interceptor.isUnsaved().
5.1.4.6. Primary keys assigned by triggers
For legacy schemas only (Hibernate does not generate DDL with triggers).
<id name="id" type="long" column="person_id">
<generator class="select">
<param name="key">socialSecurityNumber</param>
</generator>
</id>
In the above example, there is a unique valued property named
socialSecurityNumber defined by the class, as a natural key, and a surrogate
key named person_id whose value is generated by a trigger.
5.1.5. Enhanced identifier generators
Starting with release 3.2.3, there are 2 new generators which represent a
re-thinking of 2 different aspects of identifier generation. The first aspect
is database portability; the second is optimization (not having to query
the database for every request for a new identifier value). These two new
generators are intended to take the place of some of the named generators
described above (starting in 3.3.x); however, they are included in the current
releases and can be referenced by FQN.
The first of these new generators is
which is intended
firstly as a replacement for the sequence generator and secondly as a
better portability generator than native (because native (generally)
chooses between identity and sequence which have largely different
semantics which can cause subtle isssues in applications eyeing portability).
org.hibernate.id.enhanced.SequenceStyleGenerator however achieves
portability in a different manner. It chooses between using a table or a
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sequence in the database to store its incrementing values depending on
the capabilities of the dialect being used. The difference between this and
native is that table-based and sequence-based storage have the same
exact semantic (in fact sequences are exactly what Hibernate tries to
emmulate with its table-based generators). This generator has a number of
configuration parameters:
• sequence_name (optional, defaults to hibernate_sequence): The name of the
sequence (or table) to be used.
• initial_value (optional, defaults to 1): The initial value to be retrieved from
the sequence/table. In sequence creation terms, this is analogous to the
clause typical named "STARTS WITH".
• increment_size (optional, defaults to 1): The value by which subsequent
calls to the sequence/table should differ. In sequence creation terms, this
is analogous to the clause typical named "INCREMENT BY".
• force_table_use (optional, defaults to false): Should we force the use of
a table as the backing structure even though the dialect might support
sequence?
• value_column (optional, defaults to next_val): Only relevant for table
structures! The name of the column on the table which is used to hold the
value.
• optimizer (optional, defaults to none): See Section 5.1.6, “Identifier
generator optimization”
The second of these new generators is
which is intended firstly as a
replacement for the table generator (although it actually functions much
more like org.hibernate.id.MultipleHiLoPerTableGenerator) and secondly
as a re-implementation of org.hibernate.id.MultipleHiLoPerTableGenerator
utilizing the notion of pluggable optimiziers. Essentially this generator
defines a table capable of holding a number of different increment values
simultaneously by using multiple distinctly keyed rows. This generator has a
number of configuration parameters:
org.hibernate.id.enhanced.TableGenerator
• table_name (optional, defaults to hibernate_sequences): The name of the
table to be used.
• value_column_name (optional, defaults to next_val): The name of the column
on the table which is used to hold the value.
• segment_column_name (optional, defaults to sequence_name): The name of the
column on the table which is used to hold the "segement key". This is the
value which distinctly identifies which increment value to use.
• segment_value (optional, defaults to default): The "segment key" value
for the segment from which we want to pull increment values for this
generator.
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• segment_value_length (optional, defaults to 255): Used for schema
generation; the column size to create this segment key column.
• initial_value (optional, defaults to 1): The initial value to be retrieved from
the table.
• increment_size (optional, defaults to 1): The value by which subsequent
calls to the table should differ.
• optimizer (optional, defaults to ): See Section 5.1.6, “Identifier generator
optimization”
5.1.6. Identifier generator optimization
For identifier generators which store values in the database, it is inefficient for
them to hit the database on each and every call to generate a new identifier
value. Instead, you'd ideally want to group a bunch of them in memory and
only hit the database when you have exhausted your in-memory value group.
This is the role of the pluggable optimizers. Currently only the two enhanced
generators (Section 5.1.5, “Enhanced identifier generators” support this
notion.
• none (generally this is the default if no optimizer was specified): This says
to not perform any optimizations, and hit the database each and every
request.
• hilo: applies a hi/lo algorithm around the database retrieved values. The
values from the database for this optimizer are expected to be sequential.
The values retrieved from the database structure for this optimizer
indicates the "group number"; the increment_size is multiplied by that value
in memory to define a group "hi value".
• pooled: like was discussed for hilo, this optimizers attempts to minimize
the number of hits to the database. Here, however, we simply store the
starting value for the "next group" into the database structure rather than
a sequential value in combination with an in-memory grouping algorithm.
increment_size here refers to the values coming from the database.
5.1.7. composite-id
<composite-id
name="propertyName"
class="ClassName"
mapped="true|false"
access="field|property|ClassName">
node="element-name|."
<key-property name="propertyName" type="typename"
column="column_name"/>
<key-many-to-one name="propertyName class="ClassName"
column="column_name"/>
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composite-id
......
</composite-id>
For a table with a composite key, you may map multiple properties of
the class as identifier properties. The <composite-id> element accepts
<key-property> property mappings and <key-many-to-one> mappings as child
elements.
<composite-id>
<key-property name="medicareNumber"/>
<key-property name="dependent"/>
</composite-id>
Your persistent class must override equals() and hashCode() to implement
composite identifier equality. It must also implements Serializable.
Unfortunately, this approach to composite identifiers means that a persistent
object is its own identifier. There is no convenient "handle" other than the
object itself. You must instantiate an instance of the persistent class itself
and populate its identifier properties before you can load() the persistent
state associated with a composite key. We call this approach an embedded
composite identifier, and discourage it for serious applications.
A second approach is what we call a mapped composite identifier, where the
identifier properties named inside the <composite-id> element are duplicated
on both the persistent class and a separate identifier class.
<composite-id class="MedicareId" mapped="true">
<key-property name="medicareNumber"/>
<key-property name="dependent"/>
</composite-id>
In this example, both the composite identifier class, MedicareId, and the
entity class itself have properties named medicareNumber and dependent.
The identifier class must override equals() and hashCode() and implement.
Serializable. The disadvantage of this approach is quite obviouscode
duplication.
The following attributes are used to specify a mapped composite identifier:
• mapped (optional, defaults to false): indicates that a mapped composite
identifier is used, and that the contained property mappings refer to both
the entity class and the composite identifier class.
• class (optional, but required for a mapped composite identifier): The class
used as a composite identifier.
We will describe a third, even more convenient approach where the
composite identifier is implemented as a component class in Section 8.4,
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“Components as composite identifiers”. The attributes described below apply
only to this alternative approach:
• name (optional, required for this approach): A property of component type
that holds the composite identifier (see chapter 9).
• access (optional - defaults to property): The strategy Hibernate should use
for accessing the property value.
• class (optional - defaults to the property type determined by reflection):
The component class used as a composite identifier (see next section).
This third approach, an identifier component is the one we recommend for
almost all applications.
5.1.8. discriminator
The <discriminator> element is required for polymorphic persistence using
the table-per-class-hierarchy mapping strategy and declares a discriminator
column of the table. The discriminator column contains marker values that
tell the persistence layer what subclass to instantiate for a particular row. A
restricted set of types may be used: string, character, integer, byte, short,
boolean, yes_no, true_false.
<discriminator
column="discriminator_column"
type="discriminator_type"
force="true|false"
insert="true|false"
formula="arbitrary sql expression"
/>
(1)
(2)
(3)
(4)
(5)
(1)
(2)
(3)
(4)
(5)
(optional - defaults to class) the name of the discriminator
column.
type (optional - defaults to string) a name that indicates the Hibernate
type
force (optional - defaults to false) "force" Hibernate to specify allowed
discriminator values even when retrieving all instances of the root class.
insert (optional - defaults to true) set this to false if your discriminator
column is also part of a mapped composite identifier. (Tells Hibernate to
not include the column in SQL INSERTs.)
formula (optional) an arbitrary SQL expression that is executed when a
type has to be evaluated. Allows content-based discrimination.
column
Actual values of the discriminator column are specified by the
discriminator-value attribute of the <class> and <subclass> elements.
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version (optional)
The force attribute is (only) useful if the table contains rows with "extra"
discriminator values that are not mapped to a persistent class. This will not
usually be the case.
Using the formula attribute you can declare an arbitrary SQL expression that
will be used to evaluate the type of a row:
<discriminator
formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1
end"
type="integer"/>
5.1.9. version (optional)
The <version> element is optional and indicates that the table contains
versioned data. This is particularly useful if you plan to use long transactions
(see below).
<version
column="version_column"
(1)
name="propertyName"
(2)
type="typename"
(3)
access="field|property|ClassName"
(4)
unsaved-value="null|negative|undefined"
(5)
generated="never|always"
(6)
insert="true|false"
(7)
node="element-name|@attribute-name|element/@attribute|."
/>
(1)
(2)
(3)
(4)
(5)
(optional - defaults to the property name): The name of the
column holding the version number.
name: The name of a property of the persistent class.
type (optional - defaults to integer): The type of the version number.
access (optional - defaults to property): The strategy Hibernate should
use for accessing the property value.
unsaved-value (optional - defaults to undefined): A version property
value that indicates that an instance is newly instantiated (unsaved),
distinguishing it from detached instances that were saved or loaded in a
previous session. (undefined specifies that the identifier property value
should be used.)
column
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(6)
(7)
(optional - defaults to never): Specifies that this version
property value is actually generated by the database. See the
discussion of generated properties.
insert (optional - defaults to true): Specifies whether the version
column should be included in SQL insert statements. May be set to
false if and only if the database column is defined with a default value of
0.
generated
Version numbers may be of Hibernate type long, integer, short, timestamp or
calendar.
A version or timestamp property should never be null for a detached
instance, so Hibernate will detect any instance with a null version or
timestamp as transient, no matter what other unsaved-value strategies are
specified. Declaring a nullable version or timestamp property is an easy way
to avoid any problems with transitive reattachment in Hibernate, especially
useful for people using assigned identifiers or composite keys!
5.1.10. timestamp (optional)
The optional <timestamp> element indicates that the table contains
timestamped data. This is intended as an alternative to versioning.
Timestamps are by nature a less safe implementation of optimistic locking.
However, sometimes the application might use the timestamps in other ways.
<timestamp
column="timestamp_column"
(1)
name="propertyName"
(2)
access="field|property|ClassName"
(3)
unsaved-value="null|undefined"
(4)
source="vm|db"
(5)
generated="never|always"
(6)
node="element-name|@attribute-name|element/@attribute|."
/>
(1)
(2)
(3)
80
(optional - defaults to the property name): The name of a column
holding the timestamp.
name: The name of a JavaBeans style property of Java type Date or
Timestamp of the persistent class.
access (optional - defaults to property): The strategy Hibernate should
use for accessing the property value.
column
Hibernate 3.3.1
property
(4)
(5)
(6)
(optional - defaults to null): A version property value that
indicates that an instance is newly instantiated (unsaved), distinguishing
it from detached instances that were saved or loaded in a previous
session. (undefined specifies that the identifier property value should be
used.)
source (optional - defaults to vm): From where should Hibernate retrieve
the timestamp value? From the database, or from the current JVM?
Database-based timestamps incur an overhead because Hibernate
must hit the database in order to determine the "next value", but will
be safer for use in clustered environments. Note also, that not all
Dialects are known to support retrieving of the database's current
timestamp, while others might be unsafe for usage in locking due to lack
of precision (Oracle 8 for example).
generated (optional - defaults to never): Specifies that this timestamp
property value is actually generated by the database. See the
discussion of generated properties.
unsaved-value
Note that <timestamp> is equivalent to <version type="timestamp">. And
<timestamp source="db"> is equivalent to <version type="dbtimestamp">
5.1.11. property
The <property> element declares a persistent, JavaBean style property of the
class.
<property
name="propertyName"
(1)
column="column_name"
(2)
type="typename"
(3)
update="true|false"
(4)
insert="true|false"
(4)
formula="arbitrary SQL expression"
(5)
access="field|property|ClassName"
(6)
lazy="true|false"
(7)
unique="true|false"
(8)
not-null="true|false"
(9)
optimistic-lock="true|false"
(10)
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generated="never|insert|always"
(11)
node="element-name|@attribute-name|element/@attribute|."
index="index_name"
unique_key="unique_key_id"
length="L"
precision="P"
scale="S"
/>
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
name:
the name of the property, with an initial lowercase letter.
column (optional - defaults to the property name): the name of the
mapped database table column. This may also be specified by nested
<column> element(s).
type (optional): a name that indicates the Hibernate type.
update, insert (optional - defaults to true) : specifies that the mapped
columns should be included in SQL UPDATE and/or INSERT statements.
Setting both to false allows a pure "derived" property whose value is
initialized from some other property that maps to the same colum(s) or
by a trigger or other application.
formula (optional): an SQL expression that defines the value for
a computed property. Computed properties do not have a column
mapping of their own.
access (optional - defaults to property): The strategy Hibernate should
use for accessing the property value.
lazy (optional - defaults to false): Specifies that this property should
be fetched lazily when the instance variable is first accessed (requires
build-time bytecode instrumentation).
unique (optional): Enable the DDL generation of a unique constraint for
the columns. Also, allow this to be the target of a property-ref.
not-null (optional): Enable the DDL generation of a nullability constraint
for the columns.
optimistic-lock (optional - defaults to true): Specifies that updates to
this property do or do not require acquisition of the optimistic lock. In
other words, determines if a version increment should occur when this
property is dirty.
generated (optional - defaults to never): Specifies that this property value
is actually generated by the database. See the discussion of generated
properties.
typename could be:
1. The name of a Hibernate basic type (eg. integer, string, character,
date, timestamp, float, binary, serializable, object, blob).
2. The name of a Java class with a default basic type (eg. int, float, char,
java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob).
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3. The name of a serializable Java class.
4. The class name of a custom type (eg. com.illflow.type.MyCustomType).
If you do not specify a type, Hibernate will use reflection upon the named
property to take a guess at the correct Hibernate type. Hibernate will try
to interpret the name of the return class of the property getter using rules
2, 3, 4 in that order. However, this is not always enough. In certain cases
you will still need the type attribute. (For example, to distinguish between
Hibernate.DATE and Hibernate.TIMESTAMP, or to specify a custom type.)
The access attribute lets you control how Hibernate will access the property
at runtime. By default, Hibernate will call the property get/set pair. If you
specify access="field", Hibernate will bypass the get/set pair and access
the field directly, using reflection. You may specify your own strategy
for property access by naming a class that implements the interface
org.hibernate.property.PropertyAccessor.
An especially powerful feature are derived properties. These properties are
by definition read-only, the property value is computed at load time. You
declare the computation as a SQL expression, this translates to a SELECT
clause subquery in the SQL query that loads an instance:
<property name="totalPrice"
formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li,
Product p
WHERE li.productId = p.productId
AND li.customerId = customerId
AND li.orderNumber = orderNumber )"/>
Note that you can reference the entities own table by not declaring an alias
on a particular column (customerId in the given example). Also note that you
can use the nested <formula> mapping element if you don't like to use the
attribute.
5.1.12. many-to-one
An ordinary association to another persistent class is declared using a
many-to-one element. The relational model is a many-to-one association: a
foreign key in one table is referencing the primary key column(s) of the target
table.
<many-to-one
name="propertyName"
(1)
column="column_name"
(2)
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class="ClassName"
(3)
cascade="cascade_style"
(4)
fetch="join|select"
(5)
update="true|false"
(6)
insert="true|false"
(6)
property-ref="propertyNameFromAssociatedClass"
(7)
access="field|property|ClassName"
(8)
unique="true|false"
(9)
not-null="true|false"
(10)
optimistic-lock="true|false"
(11)
lazy="proxy|no-proxy|false"
(12)
not-found="ignore|exception"
(13)
entity-name="EntityName"
(14)
formula="arbitrary SQL expression"
(15)
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
index="index_name"
unique_key="unique_key_id"
foreign-key="foreign_key_name"
/>
(1)
name:
(2)
column
(3)
(4)
(5)
(6)
84
The name of the property.
(optional): The name of the foreign key column. This may also be
specified by nested <column> element(s).
class (optional - defaults to the property type determined by reflection):
The name of the associated class.
cascade (optional): Specifies which operations should be cascaded from
the parent object to the associated object.
fetch (optional - defaults to select): Chooses between outer-join
fetching or sequential select fetching.
update, insert (optional - defaults to true) specifies that the mapped
columns should be included in SQL UPDATE and/or INSERT statements.
Setting both to false allows a pure "derived" association whose value is
initialized from some other property that maps to the same colum(s) or
by a trigger or other application.
Hibernate 3.3.1
many-to-one
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
property-ref:
(optional) The name of a property of the associated class
that is joined to this foreign key. If not specified, the primary key of the
associated class is used.
access (optional - defaults to property): The strategy Hibernate should
use for accessing the property value.
unique (optional): Enable the DDL generation of a unique constraint
for the foreign-key column. Also, allow this to be the target of a
property-ref. This makes the association multiplicity effectively one to
one.
not-null (optional): Enable the DDL generation of a nullability constraint
for the foreign key columns.
optimistic-lock (optional - defaults to true): Specifies that updates to
this property do or do not require acquisition of the optimistic lock. In
other words, dertermines if a version increment should occur when this
property is dirty.
lazy (optional - defaults to proxy): By default, single point associations
are proxied. lazy="no-proxy" specifies that the property should be
fetched lazily when the instance variable is first accessed (requires
build-time bytecode instrumentation). lazy="false" specifies that the
association will always be eagerly fetched.
not-found (optional - defaults to exception): Specifies how foreign keys
that reference missing rows will be handled: ignore will treat a missing
row as a null association.
entity-name (optional): The entity name of the associated class.
formula (optional): an SQL expression that defines the value for a
computed foreign key.
Setting a value of the cascade attribute to any meaningful value other
than none will propagate certain operations to the associated object.
The meaningful values are the names of Hibernate's basic operations,
persist, merge, delete, save-update, evict, replicate, lock,
refresh,
as well as the special values delete-orphan and all and
comma-separated combinations of operation names, for example,
cascade="persist,merge,evict" or cascade="all,delete-orphan". See
Section 10.11, “Transitive persistence” for a full explanation. Note that single
valued associations (many-to-one and one-to-one associations) do not
support orphan delete.
A typical many-to-one declaration looks as simple as this:
<many-to-one name="product" class="Product" column="PRODUCT_ID"/>
The property-ref attribute should only be used for mapping legacy data
where a foreign key refers to a unique key of the associated table other than
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the primary key. This is an ugly relational model. For example, suppose the
Product class had a unique serial number, that is not the primary key. (The
unique attribute controls Hibernate's DDL generation with the SchemaExport
tool.)
<property name="serialNumber" unique="true" type="string"
column="SERIAL_NUMBER"/>
Then the mapping for OrderItem might use:
<many-to-one name="product" property-ref="serialNumber"
column="PRODUCT_SERIAL_NUMBER"/>
This is certainly not encouraged, however.
If the referenced unique key comprises multiple properties of the associated
entity, you should map the referenced properties inside a named
<properties> element.
If the referenced unique key is the property of a component, you may specify
a property path:
<many-to-one name="owner" property-ref="identity.ssn"
column="OWNER_SSN"/>
5.1.13. one-to-one
A one-to-one association to another persistent class is declared using a
one-to-one element.
<one-to-one
name="propertyName"
(1)
class="ClassName"
(2)
cascade="cascade_style"
(3)
constrained="true|false"
(4)
fetch="join|select"
(5)
property-ref="propertyNameFromAssociatedClass"
(6)
access="field|property|ClassName"
(7)
formula="any SQL expression"
(8)
lazy="proxy|no-proxy|false"
(9)
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entity-name="EntityName"
(10)
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
foreign-key="foreign_key_name"
/>
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
name:
The name of the property.
class (optional - defaults to the property type determined by reflection):
The name of the associated class.
cascade (optional) specifies which operations should be cascaded from
the parent object to the associated object.
constrained (optional) specifies that a foreign key constraint on the
primary key of the mapped table references the table of the associated
class. This option affects the order in which save() and delete() are
cascaded, and determines whether the association may be proxied (it is
also used by the schema export tool).
fetch (optional - defaults to select): Chooses between outer-join
fetching or sequential select fetching.
property-ref: (optional) The name of a property of the associated class
that is joined to the primary key of this class. If not specified, the primary
key of the associated class is used.
access (optional - defaults to property): The strategy Hibernate should
use for accessing the property value.
formula (optional): Almost all one to one associations map
to the primary key of the owning entity. In the rare case that
this is not the case, you may specify a some other column,
columns or expression to join on using an SQL formula. (See
org.hibernate.test.onetooneformula for an example.)
lazy (optional - defaults to proxy): By default, single point associations
are proxied. lazy="no-proxy" specifies that the property should be
fetched lazily when the instance variable is first accessed (requires
build-time bytecode instrumentation). lazy="false" specifies
that the association will always be eagerly fetched. Note that if
constrained="false", proxying is impossible and Hibernate will eager
fetch the association!
entity-name (optional): The entity name of the associated class.
There are two varieties of one-to-one association:
• primary key associations
• unique foreign key associations
Primary key associations don't need an extra table column; if two rows
are related by the association then the two table rows share the same
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primary key value. So if you want two objects to be related by a primary key
association, you must make sure that they are assigned the same identifier
value!
For a primary key association, add the following mappings to Employee and
Person, respectively.
<one-to-one name="person" class="Person"/>
<one-to-one name="employee" class="Employee" constrained="true"/>
Now we must ensure that the primary keys of related rows in the PERSON
and EMPLOYEE tables are equal. We use a special Hibernate identifier
generation strategy called foreign:
<class name="person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="foreign">
<param name="property">employee</param>
</generator>
</id>
...
<one-to-one name="employee"
class="Employee"
constrained="true"/>
</class>
A newly saved instance of Person is then assigned the same primary key
value as the Employee instance refered with the employee property of that
Person.
Alternatively, a foreign key with a unique constraint, from Employee to Person,
may be expressed as:
<many-to-one name="person" class="Person" column="PERSON_ID"
unique="true"/>
And this association may be made bidirectional by adding the following to the
Person mapping:
<one-to-one name="employee" class="Employee" property-ref="person"/>
5.1.14. natural-id
<natural-id mutable="true|false"/>
<property ... />
<many-to-one ... />
......
</natural-id>
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component, dynamic-component
Even though we recommend the use of surrogate keys as primary keys,
you should still try to identify natural keys for all entities. A natural key is
a property or combination of properties that is unique and non-null. If it is
also immutable, even better. Map the properties of the natural key inside the
<natural-id> element. Hibernate will generate the necessary unique key and
nullability constraints, and your mapping will be more self-documenting.
We strongly recommend that you implement equals() and hashCode() to
compare the natural key properties of the entity.
This mapping is not intended for use with entities with natural primary keys.
• mutable (optional, defaults to false): By default, natural identifier properties
as assumed to be immutable (constant).
5.1.15. component, dynamic-component
The <component> element maps properties of a child object to columns of
the table of a parent class. Components may, in turn, declare their own
properties, components or collections. See "Components" below.
<component
name="propertyName"
class="className"
insert="true|false"
update="true|false"
access="field|property|ClassName"
lazy="true|false"
optimistic-lock="true|false"
unique="true|false"
node="element-name|."
>
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
<property ...../>
<many-to-one .... />
........
</component>
(1)
(2)
(3)
(4)
(5)
(6)
name:
The name of the property.
class (optional - defaults to the property type determined by reflection):
The name of the component (child) class.
insert: Do the mapped columns appear in SQL INSERTs?
update: Do the mapped columns appear in SQL UPDATEs?
access (optional - defaults to property): The strategy Hibernate should
use for accessing the property value.
lazy (optional - defaults to false): Specifies that this component should
be fetched lazily when the instance variable is first accessed (requires
build-time bytecode instrumentation).
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(optional - defaults to true): Specifies that updates to
this component do or do not require acquisition of the optimistic lock. In
other words, determines if a version increment should occur when this
property is dirty.
unique (optional - defaults to false): Specifies that a unique constraint
exists upon all mapped columns of the component.
optimistic-lock
The child <property> tags map properties of the child class to table columns.
The <component> element allows a <parent> subelement that maps a property
of the component class as a reference back to the containing entity.
The <dynamic-component> element allows a Map to be mapped as a
component, where the property names refer to keys of the map, see
Section 8.5, “Dynamic components”.
5.1.16. properties
The <properties> element allows the definition of a named, logical grouping
of properties of a class. The most important use of the construct is that it
allows a combination of properties to be the target of a property-ref. It is
also a convenient way to define a multi-column unique constraint.
<properties
name="logicalName"
insert="true|false"
update="true|false"
optimistic-lock="true|false"
unique="true|false"
>
(1)
(2)
(3)
(4)
(5)
<property ...../>
<many-to-one .... />
........
</properties>
(1)
(2)
(3)
(4)
(5)
name:
The logical name of the grouping - not an actual property name.
insert: Do the mapped columns appear in SQL INSERTs?
update: Do the mapped columns appear in SQL UPDATEs?
optimistic-lock (optional - defaults to true): Specifies that updates to
these properties do or do not require acquisition of the optimistic lock. In
other words, determines if a version increment should occur when these
properties are dirty.
unique (optional - defaults to false): Specifies that a unique constraint
exists upon all mapped columns of the component.
For example, if we have the following <properties> mapping:
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subclass
<class name="Person">
<id name="personNumber"/>
...
<properties name="name"
unique="true" update="false">
<property name="firstName"/>
<property name="initial"/>
<property name="lastName"/>
</properties>
</class>
Then we might have some legacy data association which refers to this
unique key of the Person table, instead of to the primary key:
<many-to-one name="person"
class="Person" property-ref="name">
<column name="firstName"/>
<column name="initial"/>
<column name="lastName"/>
</many-to-one>
We don't recommend the use of this kind of thing outside the context of
mapping legacy data.
5.1.17. subclass
Finally, polymorphic persistence requires the declaration of each subclass of
the root persistent class. For the table-per-class-hierarchy mapping strategy,
the <subclass> declaration is used.
<subclass
name="ClassName"
discriminator-value="discriminator_value"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
entity-name="EntityName"
node="element-name"
extends="SuperclassName">
(1)
(2)
(3)
(4)
<property .... />
.....
</subclass>
(1)
(2)
(3)
name:
The fully qualified class name of the subclass.
discriminator-value (optional - defaults to the class name): A value that
distiguishes individual subclasses.
proxy (optional): Specifies a class or interface to use for lazy initializing
proxies.
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(optional, defaults to true): Setting lazy="false" disables the use of
lazy fetching.
lazy
Each subclass should declare its own persistent properties and
subclasses. <version> and <id> properties are assumed to be inherited
from the root class. Each subclass in a heirarchy must define a unique
discriminator-value. If none is specified, the fully qualified Java class name
is used.
For information about inheritance mappings, see Chapter 9, Inheritance
Mapping.
5.1.18. joined-subclass
Alternatively, each subclass may be mapped to its own table
(table-per-subclass mapping strategy). Inherited state is retrieved by joining
with the table of the superclass. We use the <joined-subclass> element.
<joined-subclass
name="ClassName"
table="tablename"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
schema="schema"
catalog="catalog"
extends="SuperclassName"
persister="ClassName"
subselect="SQL expression"
entity-name="EntityName"
node="element-name">
(1)
(2)
(3)
(4)
<key .... >
<property .... />
.....
</joined-subclass>
(1)
(2)
(3)
(4)
name:
The fully qualified class name of the subclass.
table: The name of the subclass table.
proxy (optional): Specifies a class or interface to use for lazy initializing
proxies.
lazy (optional, defaults to true): Setting lazy="false" disables the use of
lazy fetching.
No discriminator column is required for this mapping strategy. Each subclass
must, however, declare a table column holding the object identifier using the
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<key>
element. The mapping at the start of the chapter would be re-written
as:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class name="Cat" table="CATS">
<id name="id" column="uid" type="long">
<generator class="hilo"/>
</id>
<property name="birthdate" type="date"/>
<property name="color" not-null="true"/>
<property name="sex" not-null="true"/>
<property name="weight"/>
<many-to-one name="mate"/>
<set name="kittens">
<key column="MOTHER"/>
<one-to-many class="Cat"/>
</set>
<joined-subclass name="DomesticCat"
table="DOMESTIC_CATS">
<key column="CAT"/>
<property name="name" type="string"/>
</joined-subclass>
</class>
<class name="eg.Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>
For information about inheritance mappings, see Chapter 9, Inheritance
Mapping.
5.1.19. union-subclass
A third option is to map only the concrete classes of an inheritance hierarchy
to tables, (the table-per-concrete-class strategy) where each table defines all
persistent state of the class, including inherited state. In Hibernate, it is not
absolutely necessary to explicitly map such inheritance hierarchies. You can
simply map each class with a separate <class> declaration. However, if you
wish use polymorphic associations (e.g. an association to the superclass of
your hierarchy), you need to use the <union-subclass> mapping.
<union-subclass
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name="ClassName"
table="tablename"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
schema="schema"
catalog="catalog"
extends="SuperclassName"
abstract="true|false"
persister="ClassName"
subselect="SQL expression"
entity-name="EntityName"
node="element-name">
(1)
(2)
(3)
(4)
<property .... />
.....
</union-subclass>
(1)
name:
(2)
table:
(3)
(4)
The fully qualified class name of the subclass.
The name of the subclass table.
proxy (optional): Specifies a class or interface to use for lazy initializing
proxies.
lazy (optional, defaults to true): Setting lazy="false" disables the use of
lazy fetching.
No discriminator column or key column is required for this mapping strategy.
For information about inheritance mappings, see Chapter 9, Inheritance
Mapping.
5.1.20. join
Using the <join> element, it is possible to map properties of one class to
several tables, when there's a 1-to-1 relationship between the tables.
<join
table="tablename"
schema="owner"
catalog="catalog"
fetch="join|select"
inverse="true|false"
optional="true|false">
(1)
(2)
(3)
(4)
(5)
(6)
<key ... />
<property ... />
...
</join>
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The name of the joined table.
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key
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(3)
(4)
(5)
(6)
(optional): Override the schema name specified by the root
<hibernate-mapping> element.
catalog (optional): Override the catalog name specified by the root
<hibernate-mapping> element.
fetch (optional - defaults to join): If set to join, the default, Hibernate
will use an inner join to retrieve a <join> defined by a class or its
superclasses and an outer join for a <join> defined by a subclass. If set
to select then Hibernate will use a sequential select for a <join> defined
on a subclass, which will be issued only if a row turns out to represent
an instance of the subclass. Inner joins will still be used to retrieve a
<join> defined by the class and its superclasses.
inverse (optional - defaults to false): If enabled, Hibernate will not try to
insert or update the properties defined by this join.
optional (optional - defaults to false): If enabled, Hibernate will insert a
row only if the properties defined by this join are non-null and will always
use an outer join to retrieve the properties.
schema
For example, the address information for a person can be mapped to a
separate table (while preserving value type semantics for all properties):
<class name="Person"
table="PERSON">
<id name="id" column="PERSON_ID">...</id>
<join table="ADDRESS">
<key column="ADDRESS_ID"/>
<property name="address"/>
<property name="zip"/>
<property name="country"/>
</join>
...
This feature is often only useful for legacy data models, we recommend
fewer tables than classes and a fine-grained domain model. However, it
is useful for switching between inheritance mapping strategies in a single
hierarchy, as explained later.
5.1.21. key
We've seen the <key> element crop up a few times now. It appears anywhere
the parent mapping element defines a join to a new table, and defines the
foreign key in the joined table, that references the primary key of the original
table.
<key
column="columnname"
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on-delete="noaction|cascade"
property-ref="propertyName"
not-null="true|false"
update="true|false"
unique="true|false"
(2)
(3)
(4)
(5)
(6)
/>
(1)
(2)
(3)
(4)
(5)
(6)
(optional): The name of the foreign key column. This may also be
specified by nested <column> element(s).
on-delete (optional, defaults to noaction): Specifies whether the foreign
key constraint has database-level cascade delete enabled.
property-ref (optional): Specifies that the foreign key refers to columns
that are not the primary key of the orginal table. (Provided for legacy
data.)
not-null (optional): Specifies that the foreign key columns are not
nullable (this is implied whenever the foreign key is also part of the
primary key).
update (optional): Specifies that the foreign key should never be updated
(this is implied whenever the foreign key is also part of the primary key).
unique (optional): Specifies that the foreign key should have a unique
constraint (this is implied whenever the foreign key is also the primary
key).
column
We recommend that for systems where delete performance is important,
all keys should be defined on-delete="cascade", and Hibernate will use a
database-level ON CASCADE DELETE constraint, instead of many individual
DELETE statements. Be aware that this feature bypasses Hibernate's usual
optimistic locking strategy for versioned data.
The not-null and update attributes are useful when mapping a unidirectional
one to many association. If you map a unidirectional one to many to a
non-nullable foreign key, you must declare the key column using <key
not-null="true">.
5.1.22. column and formula elements
Any mapping element which accepts a column attribute will alternatively
accept a <column> subelement. Likewise, <formula> is an alternative to the
formula attribute.
<column
name="column_name"
length="N"
precision="N"
scale="N"
not-null="true|false"
unique="true|false"
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unique-key="multicolumn_unique_key_name"
index="index_name"
sql-type="sql_type_name"
check="SQL expression"
default="SQL expression"/>
<formula>SQL expression</formula>
and formula attributes may even be combined within the same
property or association mapping to express, for example, exotic join
conditions.
column
<many-to-one name="homeAddress" class="Address"
insert="false" update="false">
<column name="person_id" not-null="true" length="10"/>
<formula>'MAILING'</formula>
</many-to-one>
5.1.23. import
Suppose your application has two persistent classes with the same name,
and you don't want to specify the fully qualified (package) name in Hibernate
queries. Classes may be "imported" explicitly, rather than relying upon
auto-import="true". You may even import classes and interfaces that are not
explicitly mapped.
<import class="java.lang.Object" rename="Universe"/>
<import
class="ClassName"
rename="ShortName"
(1)
(2)
/>
(1)
(2)
class:
The fully qualified class name of of any Java class.
rename (optional - defaults to the unqualified class name): A name that
may be used in the query language.
5.1.24. any
There is one further type of property mapping. The <any> mapping element
defines a polymorphic association to classes from multiple tables. This type
of mapping always requires more than one column. The first column holds
the type of the associated entity. The remaining columns hold the identifier. It
is impossible to specify a foreign key constraint for this kind of association, so
this is most certainly not meant as the usual way of mapping (polymorphic)
associations. You should use this only in very special cases (eg. audit logs,
user session data, etc).
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The meta-type attribute lets the application specify a custom type that maps
database column values to persistent classes which have identifier properties
of the type specified by id-type. You must specify the mapping from values
of the meta-type to class names.
<any name="being" id-type="long" meta-type="string">
<meta-value value="TBL_ANIMAL" class="Animal"/>
<meta-value value="TBL_HUMAN" class="Human"/>
<meta-value value="TBL_ALIEN" class="Alien"/>
<column name="table_name"/>
<column name="id"/>
</any>
<any
name="propertyName"
id-type="idtypename"
meta-type="metatypename"
cascade="cascade_style"
access="field|property|ClassName"
optimistic-lock="true|false"
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(2)
(3)
(4)
(5)
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>
<meta-value ... />
<meta-value ... />
.....
<column .... />
<column .... />
.....
</any>
(1)
(2)
(3)
(4)
(5)
(6)
name:
the property name.
id-type: the identifier type.
meta-type (optional - defaults to string): Any type that is allowed for a
discriminator mapping.
cascade (optional- defaults to none): the cascade style.
access (optional - defaults to property): The strategy Hibernate should
use for accessing the property value.
optimistic-lock (optional - defaults to true): Specifies that updates to
this property do or do not require acquisition of the optimistic lock. In
other words, define if a version increment should occur if this property is
dirty.
5.2. Hibernate Types
5.2.1. Entities and values
To understand the behaviour of various Java language-level objects with
respect to the persistence service, we need to classify them into two groups:
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An entity exists independently of any other objects holding references to
the entity. Contrast this with the usual Java model where an unreferenced
object is garbage collected. Entities must be explicitly saved and deleted
(except that saves and deletions may be cascaded from a parent entity to
its children). This is different from the ODMG model of object persistence
by reachablity - and corresponds more closely to how application objects
are usually used in large systems. Entities support circular and shared
references. They may also be versioned.
An entity's persistent state consists of references to other entities and
instances of value types. Values are primitives, collections (not what's inside
a collection), components and certain immutable objects. Unlike entities,
values (in particular collections and components) are persisted and deleted
by reachability. Since value objects (and primitives) are persisted and deleted
along with their containing entity they may not be independently versioned.
Values have no independent identity, so they cannot be shared by two
entities or collections.
Up until now, we've been using the term "persistent class" to refer to entities.
We will continue to do that. Strictly speaking, however, not all user-defined
classes with persistent state are entities. A component is a user defined
class with value semantics. A Java property of type java.lang.String
also has value semantics. Given this definition, we can say that all types
(classes) provided by the JDK have value type semantics in Java, while
user-defined types may be mapped with entity or value type semantics. This
decision is up to the application developer. A good hint for an entity class in a
domain model are shared references to a single instance of that class, while
composition or aggregation usually translates to a value type.
We'll revisit both concepts throughout the documentation.
The challenge is to map the Java type system (and the developers' definition
of entities and value types) to the SQL/database type system. The bridge
between both systems is provided by Hibernate: for entities we use <class>,
<subclass> and so on. For value types we use <property>, <component>,
etc, usually with a type attribute. The value of this attribute is the name of a
Hibernate mapping type. Hibernate provides many mappings (for standard
JDK value types) out of the box. You can write your own mapping types and
implement your custom conversion strategies as well, as you'll see later.
All built-in Hibernate types except collections support null semantics.
5.2.2. Basic value types
The built-in basic mapping types may be roughly categorized into
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integer, long, short, float, double, character, byte, boolean, yes_no,
true_false
Type mappings from Java primitives or wrapper classes to appropriate
(vendor-specific) SQL column types. boolean, yes_no and true_false are
all alternative encodings for a Java boolean or java.lang.Boolean.
string
A type mapping from java.lang.String to VARCHAR (or Oracle VARCHAR2).
date, time, timestamp
Type mappings from java.util.Date and its subclasses to SQL types
DATE, TIME and TIMESTAMP (or equivalent).
calendar, calendar_date
Type mappings from java.util.Calendar to SQL types TIMESTAMP and
DATE (or equivalent).
big_decimal, big_integer
Type mappings from java.math.BigDecimal and java.math.BigInteger to
NUMERIC (or Oracle NUMBER).
locale, timezone, currency
Type mappings from java.util.Locale, java.util.TimeZone and
java.util.Currency to VARCHAR (or Oracle VARCHAR2). Instances of Locale
and Currency are mapped to their ISO codes. Instances of TimeZone are
mapped to their ID.
class
A type mapping from java.lang.Class to VARCHAR (or Oracle VARCHAR2). A
Class is mapped to its fully qualified name.
binary
Maps byte arrays to an appropriate SQL binary type.
text
Maps long Java strings to a SQL CLOB or TEXT type.
serializable
Maps serializable Java types to an appropriate SQL binary type. You
may also indicate the Hibernate type serializable with the name of a
serializable Java class or interface that does not default to a basic type.
clob, blob
Type mappings for the JDBC classes java.sql.Clob and java.sql.Blob.
These types may be inconvenient for some applications, since the blob
or clob object may not be reused outside of a transaction. (Furthermore,
driver support is patchy and inconsistent.)
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imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date,
imm_serializable, imm_binary
Type mappings for what are usually considered mutable Java types,
where Hibernate makes certain optimizations appropriate only for
immutable Java types, and the application treats the object as immutable.
For example, you should not call Date.setTime() for an instance mapped
as imm_timestamp. To change the value of the property, and have that
change made persistent, the application must assign a new (nonidentical)
object to the property.
Unique identifiers of entities and collections may be of any basic type except
binary, blob and clob. (Composite identifiers are also allowed, see below.)
The basic value types have corresponding Type constants defined on
org.hibernate.Hibernate. For example, Hibernate.STRING represents the
string type.
5.2.3. Custom value types
It is relatively easy for developers to create their own value types. For
example, you might want to persist properties of type java.lang.BigInteger
to VARCHAR columns. Hibernate does not provide a built-in type for this. But
custom types are not limited to mapping a property (or collection element)
to a single table column. So, for example, you might have a Java property
getName()/setName() of type java.lang.String that is persisted to the columns
FIRST_NAME, INITIAL, SURNAME.
To implement a custom type, implement either org.hibernate.UserType
or org.hibernate.CompositeUserType and declare properties
using the fully qualified classname of the type. Check out
org.hibernate.test.DoubleStringType to see the kind of things that are
possible.
<property name="twoStrings"
type="org.hibernate.test.DoubleStringType">
<column name="first_string"/>
<column name="second_string"/>
</property>
Notice the use of <column> tags to map a property to multiple columns.
The CompositeUserType, EnhancedUserType, UserCollectionType, and
UserVersionType interfaces provide support for more specialized uses.
You may even supply parameters to a UserType in the
mapping file. To do this, your UserType must implement the
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interface. To supply parameters
to your custom type, you can use the <type> element in your mapping files.
org.hibernate.usertype.ParameterizedType
<property name="priority">
<type name="com.mycompany.usertypes.DefaultValueIntegerType">
<param name="default">0</param>
</type>
</property>
The UserType can now retrieve the value for the parameter named default
from the Properties object passed to it.
If you use a certain UserType very often, it may be useful to define a shorter
name for it. You can do this using the <typedef> element. Typedefs assign
a name to a custom type, and may also contain a list of default parameter
values if the type is parameterized.
<typedef class="com.mycompany.usertypes.DefaultValueIntegerType"
name="default_zero">
<param name="default">0</param>
</typedef>
<property name="priority" type="default_zero"/>
It is also possible to override the parameters supplied in a typedef on a
case-by-case basis by using type parameters on the property mapping.
Even though Hibernate's rich range of built-in types and support for
components means you will very rarely need to use a custom type, it is
nevertheless considered good form to use custom types for (non-entity)
classes that occur frequently in your application. For example, a
MonetaryAmount class is a good candidate for a CompositeUserType, even
though it could easily be mapped as a component. One motivation for
this is abstraction. With a custom type, your mapping documents would
be future-proofed against possible changes in your way of representing
monetary values.
5.3. Mapping a class more than once
It is possible to provide more than one mapping for a particular persistent
class. In this case you must specify an entity name do disambiguate between
instances of the two mapped entities. (By default, the entity name is the
same as the class name.) Hibernate lets you specify the entity name when
working with persistent objects, when writing queries, or when mapping
associations to the named entity.
<class name="Contract" table="Contracts"
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entity-name="CurrentContract">
...
<set name="history" inverse="true"
order-by="effectiveEndDate desc">
<key column="currentContractId"/>
<one-to-many entity-name="HistoricalContract"/>
</set>
</class>
<class name="Contract" table="ContractHistory"
entity-name="HistoricalContract">
...
<many-to-one name="currentContract"
column="currentContractId"
entity-name="CurrentContract"/>
</class>
Notice how associations are now specified using entity-name instead of
class.
5.4. SQL quoted identifiers
You may force Hibernate to quote an identifier in the generated SQL by
enclosing the table or column name in backticks in the mapping document.
Hibernate will use the correct quotation style for the SQL Dialect (usually
double quotes, but brackets for SQL Server and backticks for MySQL).
<class name="LineItem" table="`Line Item`">
<id name="id" column="`Item Id`"/><generator
class="assigned"/></id>
<property name="itemNumber" column="`Item #`"/>
...
</class>
5.5. Metadata alternatives
XML isn't for everyone, and so there are some alternative ways to define O/R
mapping metadata in Hibernate.
5.5.1. Using XDoclet markup
Many Hibernate users prefer to embed mapping information directly in
sourcecode using XDoclet @hibernate.tags. We will not cover this approach
in this document, since strictly it is considered part of XDoclet. However, we
include the following example of the Cat class with XDoclet mappings.
package eg;
import java.util.Set;
import java.util.Date;
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/**
* @hibernate.class
* table="CATS"
*/
public class Cat {
private Long id; // identifier
private Date birthdate;
private Cat mother;
private Set kittens
private Color color;
private char sex;
private float weight;
/*
* @hibernate.id
* generator-class="native"
* column="CAT_ID"
*/
public Long getId() {
return id;
}
private void setId(Long id) {
this.id=id;
}
/**
* @hibernate.many-to-one
* column="PARENT_ID"
*/
public Cat getMother() {
return mother;
}
void setMother(Cat mother) {
this.mother = mother;
}
/**
* @hibernate.property
* column="BIRTH_DATE"
*/
public Date getBirthdate() {
return birthdate;
}
void setBirthdate(Date date) {
birthdate = date;
}
/**
* @hibernate.property
* column="WEIGHT"
*/
public float getWeight() {
return weight;
}
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void setWeight(float weight) {
this.weight = weight;
}
/**
* @hibernate.property
* column="COLOR"
* not-null="true"
*/
public Color getColor() {
return color;
}
void setColor(Color color) {
this.color = color;
}
/**
* @hibernate.set
* inverse="true"
* order-by="BIRTH_DATE"
* @hibernate.collection-key
* column="PARENT_ID"
* @hibernate.collection-one-to-many
*/
public Set getKittens() {
return kittens;
}
void setKittens(Set kittens) {
this.kittens = kittens;
}
// addKitten not needed by Hibernate
public void addKitten(Cat kitten) {
kittens.add(kitten);
}
/**
* @hibernate.property
* column="SEX"
* not-null="true"
* update="false"
*/
public char getSex() {
return sex;
}
void setSex(char sex) {
this.sex=sex;
}
}
See the Hibernate web site for more examples of XDoclet and Hibernate.
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5.5.2. Using JDK 5.0 Annotations
JDK 5.0 introduced XDoclet-style annotations at the language level,
type-safe and checked at compile time. This mechnism is more powerful
than XDoclet annotations and better supported by tools and IDEs. IntelliJ
IDEA, for example, supports auto-completion and syntax highlighting of JDK
5.0 annotations. The new revision of the EJB specification (JSR-220) uses
JDK 5.0 annotations as the primary metadata mechanism for entity beans.
Hibernate3 implements the EntityManager of JSR-220 (the persistence API),
support for mapping metadata is available via the Hibernate Annotations
package, as a separate download. Both EJB3 (JSR-220) and Hibernate3
metadata is supported.
This is an example of a POJO class annotated as an EJB entity bean:
@Entity(access = AccessType.FIELD)
public class Customer implements Serializable {
@Id;
Long id;
String firstName;
String lastName;
Date birthday;
@Transient
Integer age;
@Embedded
private Address homeAddress;
@OneToMany(cascade=CascadeType.ALL)
@JoinColumn(name="CUSTOMER_ID")
Set<Order> orders;
// Getter/setter and business methods
}
Note that support for JDK 5.0 Annotations (and JSR-220) is still work in
progress and not completed. Please refer to the Hibernate Annotations
module for more details.
5.6. Generated Properties
Generated properties are properties which have their values generated by
the database. Typically, Hibernate applications needed to refresh objects
which contain any properties for which the database was generating values.
Marking properties as generated, however, lets the application delegate this
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responsibility to Hibernate. Essentially, whenever Hibernate issues an SQL
INSERT or UPDATE for an entity which has defined generated properties, it
immediately issues a select afterwards to retrieve the generated values.
Properties marked as generated must additionally be non-insertable and
non-updateable. Only versions, timestamps, and simple properties can be
marked as generated.
(the default) - means that the given property value is not generated
within the database.
never
- states that the given property value is generated on insert, but is not
regenerated on subsequent updates. Things like created-date would fall into
this category. Note that even thought version and timestamp properties can
be marked as generated, this option is not available there...
insert
- states that the property value is generated both on insert and on
update.
always
5.7. Auxiliary Database Objects
Allows CREATE and DROP of arbitrary database objects, in conjunction with
Hibernate's schema evolution tools, to provide the ability to fully define a user
schema within the Hibernate mapping files. Although designed specifically
for creating and dropping things like triggers or stored procedures, really any
SQL command that can be run via a java.sql.Statement.execute() method
is valid here (ALTERs, INSERTS, etc). There are essentially two modes for
defining auxiliary database objects...
The first mode is to explicitly list the CREATE and DROP commands out in
the mapping file:
<hibernate-mapping>
...
<database-object>
<create>CREATE TRIGGER my_trigger ...</create>
<drop>DROP TRIGGER my_trigger</drop>
</database-object>
</hibernate-mapping>
The second mode is to supply a custom class which knows how to construct
the CREATE and DROP commands. This custom class must implement the
org.hibernate.mapping.AuxiliaryDatabaseObject interface.
<hibernate-mapping>
...
<database-object>
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<definition class="MyTriggerDefinition"/>
</database-object>
</hibernate-mapping>
Additionally, these database objects can be optionally scoped such that they
only apply when certain dialects are used.
<hibernate-mapping>
...
<database-object>
<definition class="MyTriggerDefinition"/>
<dialect-scope name="org.hibernate.dialect.Oracle9Dialect"/>
<dialect-scope name="org.hibernate.dialect.OracleDialect"/>
</database-object>
</hibernate-mapping>
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6.1. Persistent collections
Hibernate requires that persistent collection-valued fields be declared as an
interface type, for example:
public class Product {
private String serialNumber;
private Set parts = new HashSet();
public Set getParts() { return parts; }
void setParts(Set parts) { this.parts = parts; }
public String getSerialNumber() { return serialNumber; }
void setSerialNumber(String sn) { serialNumber = sn; }
}
The actual interface might be java.util.Set, java.util.Collection,
java.util.List, java.util.Map, java.util.SortedSet, java.util.SortedMap or
... anything you like! (Where "anything you like" means you will have to write
an implementation of org.hibernate.usertype.UserCollectionType.)
Notice how we initialized the instance variable with an instance of HashSet.
This is the best way to initialize collection valued properties of newly
instantiated (non-persistent) instances. When you make the instance
persistent - by calling persist(), for example - Hibernate will actually replace
the HashSet with an instance of Hibernate's own implementation of Set. Watch
out for errors like this:
Cat cat = new DomesticCat();
Cat kitten = new DomesticCat();
....
Set kittens = new HashSet();
kittens.add(kitten);
cat.setKittens(kittens);
session.persist(cat);
kittens = cat.getKittens(); // Okay, kittens collection is a Set
(HashSet) cat.getKittens(); // Error!
The persistent collections injected by Hibernate behave like HashMap, HashSet,
TreeMap, TreeSet or ArrayList, depending upon the interface type.
Collections instances have the usual behavior of value types. They are
automatically persisted when referenced by a persistent object and
automatically deleted when unreferenced. If a collection is passed from one
persistent object to another, its elements might be moved from one table
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to another. Two entities may not share a reference to the same collection
instance. Due to the underlying relational model, collection-valued properties
do not support null value semantics; Hibernate does not distinguish between
a null collection reference and an empty collection.
You shouldn't have to worry much about any of this. Use persistent
collections the same way you use ordinary Java collections. Just make sure
you understand the semantics of bidirectional associations (discussed later).
6.2. Collection mappings
Tip
There are quite a range of mappings that can be generated for
collections, covering many common relational models. We suggest
you experiment with the schema generation tool to get a feeling for
how various mapping declarations translate to database tables.
The Hibernate mapping element used for mapping a collection depends upon
the type of the interface. For example, a <set> element is used for mapping
properties of type Set.
<class name="Product">
<id name="serialNumber" column="productSerialNumber"/>
<set name="parts">
<key column="productSerialNumber" not-null="true"/>
<one-to-many class="Part"/>
</set>
</class>
Apart from <set>, there is also <list>, <map>, <bag>, <array> and
<primitive-array> mapping elements. The <map> element is representative:
<map
name="propertyName"
table="table_name"
schema="schema_name"
lazy="true|extra|false"
inverse="true|false"
(1)
(2)
(3)
(4)
(5)
cascade="all|none|save-update|delete|all-delete-orphan|delet(6)eorphan"
sort="unsorted|natural|comparatorClass"
(7)
order-by="column_name asc|desc"
(8)
where="arbitrary sql where condition"
(9)
fetch="join|select|subselect"
(10)
batch-size="N"
(11)
access="field|property|ClassName"
(12)
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optimistic-lock="true|false"
mutable="true|false"
node="element-name|."
embed-xml="true|false"
(13)
(14)
>
<key .... />
<map-key .... />
<element .... />
</map>
the collection property name
(optional - defaults to property name) the name of the collection
table (not used for one-to-many associations)
schema (optional) the name of a table schema to override the schema
declared on the root element
lazy (optional - defaults to true) may be used to disable lazy fetching
and specify that the association is always eagerly fetched, or to
enable "extra-lazy" fetching where most operations do not initialize the
collection (suitable for very large collections)
inverse (optional - defaults to false) mark this collection as the "inverse"
end of a bidirectional association
cascade (optional - defaults to none) enable operations to cascade to
child entities
sort (optional) specify a sorted collection with natural sort order, or a
given comparator class
order-by (optional, JDK1.4 only) specify a table column (or columns)
that define the iteration order of the Map, Set or bag, together with an
optional asc or desc
where (optional) specify an arbitrary SQL WHERE condition to be used
when retrieving or removing the collection (useful if the collection should
contain only a subset of the available data)
fetch (optional, defaults to select) Choose between outer-join fetching,
fetching by sequential select, and fetching by sequential subselect.
batch-size (optional, defaults to 1) specify a "batch size" for lazily
fetching instances of this collection.
access (optional - defaults to property): The strategy Hibernate should
use for accessing the collection property value.
optimistic-lock (optional - defaults to true): Species that changes to
the state of the collection results in increment of the owning entity's
version. (For one to many associations, it is often reasonable to disable
this setting.)
mutable (optional - defaults to true): A value of false specifies that
the elements of the collection never change (a minor performance
optimization in some cases).
(1)
name
(2)
table
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
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6.2.1. Collection foreign keys
Collection instances are distinguished in the database by the foreign key
of the entity that owns the collection. This foreign key is referred to as the
collection key column (or columns) of the collection table. The collection key
column is mapped by the <key> element.
There may be a nullability constraint on the foreign key column. For most
collections, this is implied. For unidirectional one to many associations,
the foreign key column is nullable by default, so you might need to specify
not-null="true".
<key column="productSerialNumber" not-null="true"/>
The foreign key constraint may use ON DELETE CASCADE.
<key column="productSerialNumber" on-delete="cascade"/>
See the previous chapter for a full definition of the <key> element.
6.2.2. Collection elements
Collections may contain almost any other Hibernate type, including all basic
types, custom types, components, and of course, references to other entities.
This is an important distinction: an object in a collection might be handled
with "value" semantics (its life cycle fully depends on the collection owner) or
it might be a reference to another entity, with its own life cycle. In the latter
case, only the "link" between the two objects is considered to be state held by
the collection.
The contained type is referred to as the collection element type. Collection
elements are mapped by <element> or <composite-element>, or in the case
of entity references, with <one-to-many> or <many-to-many>. The first two
map elements with value semantics, the next two are used to map entity
associations.
6.2.3. Indexed collections
All collection mappings, except those with set and bag semantics, need
an index column in the collection table - a column that maps to an array
index, or List index, or Map key. The index of a Map may be of any basic
type, mapped with <map-key>, it may be an entity reference mapped with
<map-key-many-to-many>, or it may be a composite type, mapped with
<composite-map-key>. The index of an array or list is always of type integer
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associations
and is mapped using the <list-index> element. The mapped column
contains sequential integers (numbered from zero, by default).
<list-index
column="column_name"
base="0|1|..."/>
(1)
(1)
(1)
(required): The name of the column holding the collection
index values.
base (optional, defaults to 0): The value of the index column that
corresponds to the first element of the list or array.
column_name
<map-key
column="column_name"
formula="any SQL expression"
type="type_name"
node="@attribute-name"
length="N"/>
(1)
(2)
(3)
(optional): The name of the column holding the collection index
values.
formula (optional): A SQL formula used to evaluate the key of the map.
type (reguired): The type of the map keys.
column
<map-key-many-to-many
column="column_name"
formula="any SQL expression"
class="ClassName"
/>
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)(3)
(optional): The name of the foreign key column for the collection
index values.
formula (optional): A SQL formula used to evaluate the foreign key of
the map key.
class (required): The entity class used as the map key.
column
If your table doesn't have an index column, and you still wish to use List as
the property type, you should map the property as a Hibernate <bag>. A bag
does not retain its order when it is retrieved from the database, but it may be
optionally sorted or ordered.
6.2.4. Collections of values and many-to-many
associations
Any collection of values or many-to-many association requires a dedicated
collection table with a foreign key column or columns, collection element
column or columns and possibly an index column or columns.
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For a collection of values, we use the <element> tag.
<element
column="column_name"
formula="any SQL expression"
type="typename"
length="L"
precision="P"
scale="S"
not-null="true|false"
unique="true|false"
node="element-name"
(1)
(2)
(3)
/>
(1)
(2)
(3)
(optional): The name of the column holding the collection
element values.
formula (optional): An SQL formula used to evaluate the element.
type (required): The type of the collection element.
column
A many-to-many association is specified using the <many-to-many> element.
<many-to-many
column="column_name"
formula="any SQL expression"
class="ClassName"
fetch="select|join"
unique="true|false"
not-found="ignore|exception"
entity-name="EntityName"
property-ref="propertyNameFromAssociatedClass"
node="element-name"
embed-xml="true|false"
/>
column
(2)
formula
(4)
(5)
(6)
114
(optional): The name of the element foreign key column.
(optional): An SQL formula used to evaluate the element foreign
key value.
class (required): The name of the associated class.
fetch (optional - defaults to join): enables outer-join or sequential select
fetching for this association. This is a special case; for full eager fetching
(in a single SELECT) of an entity and its many-to-many relationships to
other entities, you would enable join fetching not only of the collection
itself, but also with this attribute on the <many-to-many> nested element.
unique (optional): Enable the DDL generation of a unique constraint
for the foreign-key column. This makes the association multiplicity
effectively one to many.
not-found (optional - defaults to exception): Specifies how foreign keys
that reference missing rows will be handled: ignore will treat a missing
row as a null association.
(1)
(3)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Hibernate 3.3.1
(7)
(8)
Collections of values and many-to-many
associations
entity-name (optional): The entity name of the associated class, as an
alternative to class.
property-ref: (optional) The name of a property of the associated class
that is joined to this foreign key. If not specified, the primary key of the
associated class is used.
Some examples, first, a set of strings:
<set name="names" table="person_names">
<key column="person_id"/>
<element column="person_name" type="string"/>
</set>
A bag containing integers (with an iteration order determined by the order-by
attribute):
<bag name="sizes"
table="item_sizes"
order-by="size asc">
<key column="item_id"/>
<element column="size" type="integer"/>
</bag>
An array of entities - in this case, a many to many association:
<array name="addresses"
table="PersonAddress"
cascade="persist">
<key column="personId"/>
<list-index column="sortOrder"/>
<many-to-many column="addressId" class="Address"/>
</array>
A map from string indices to dates:
<map name="holidays"
table="holidays"
schema="dbo"
order-by="hol_name asc">
<key column="id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>
A list of components (discussed in the next chapter):
<list name="carComponents"
table="CarComponents">
<key column="carId"/>
<list-index column="sortOrder"/>
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<composite-element class="CarComponent">
<property name="price"/>
<property name="type"/>
<property name="serialNumber" column="serialNum"/>
</composite-element>
</list>
6.2.5. One-to-many associations
A one to many association links the tables of two classes via a foreign key,
with no intervening collection table. This mapping loses certain semantics of
normal Java collections:
• An instance of the contained entity class may not belong to more than one
instance of the collection
• An instance of the contained entity class may not appear at more than one
value of the collection index
An association from Product to Part requires existence of a foreign key
column and possibly an index column to the Part table. A <one-to-many> tag
indicates that this is a one to many association.
<one-to-many
class="ClassName"
not-found="ignore|exception"
entity-name="EntityName"
node="element-name"
embed-xml="true|false"
/>
(1)
(2)
(3)
(1)
(2)
(3)
(required): The name of the associated class.
not-found (optional - defaults to exception): Specifies how cached
identifiers that reference missing rows will be handled: ignore will treat a
missing row as a null association.
entity-name (optional): The entity name of the associated class, as an
alternative to class.
class
Notice that the <one-to-many> element does not need to declare any columns.
Nor is it necessary to specify the table name anywhere.
Very important note: If the foreign key column of a <one-to-many> association
is declared NOT NULL, you must declare the <key> mapping not-null="true"
or use a bidirectional association with the collection mapping marked
inverse="true". See the discussion of bidirectional associations later in this
chapter.
This example shows a map of Part entities by name (where partName is a
persistent property of Part). Notice the use of a formula-based index.
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<map name="parts"
cascade="all">
<key column="productId" not-null="true"/>
<map-key formula="partName"/>
<one-to-many class="Part"/>
</map>
6.3. Advanced collection mappings
6.3.1. Sorted collections
Hibernate supports collections implementing java.util.SortedMap and
java.util.SortedSet. You must specify a comparator in the mapping file:
<set name="aliases"
table="person_aliases"
sort="natural">
<key column="person"/>
<element column="name" type="string"/>
</set>
<map name="holidays" sort="my.custom.HolidayComparator">
<key column="year_id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>
Allowed values of the sort attribute are unsorted, natural and the name of a
class implementing java.util.Comparator.
Sorted collections actually behave like java.util.TreeSet or
java.util.TreeMap.
If you want the database itself to order the collection elements use the
order-by attribute of set, bag or map mappings. This solution is only
available under JDK 1.4 or higher (it is implemented using LinkedHashSet or
LinkedHashMap). This performs the ordering in the SQL query, not in memory.
<set name="aliases" table="person_aliases" order-by="lower(name)
asc">
<key column="person"/>
<element column="name" type="string"/>
</set>
<map name="holidays" order-by="hol_date, hol_name">
<key column="year_id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date type="date"/>
</map>
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Note that the value of the order-by attribute is an SQL ordering, not a HQL
ordering!
Associations may even be sorted by some arbitrary criteria at runtime using a
collection filter().
sortedUsers = s.createFilter( group.getUsers(), "order by this.name"
).list();
6.3.2. Bidirectional associations
A bidirectional association allows navigation from both "ends" of the
association. Two kinds of bidirectional association are supported:
one-to-many
set or bag valued at one end, single-valued at the other
many-to-many
set or bag valued at both ends
You may specify a bidirectional many-to-many association simply by mapping
two many-to-many associations to the same database table and declaring
one end as inverse (which one is your choice, but it can not be an indexed
collection).
Here's an example of a bidirectional many-to-many association; each
category can have many items and each item can be in many categories:
<class name="Category">
<id name="id" column="CATEGORY_ID"/>
...
<bag name="items" table="CATEGORY_ITEM">
<key column="CATEGORY_ID"/>
<many-to-many class="Item" column="ITEM_ID"/>
</bag>
</class>
<class name="Item">
<id name="id" column="ITEM_ID"/>
...
<!-- inverse end -->
<bag name="categories" table="CATEGORY_ITEM" inverse="true">
<key column="ITEM_ID"/>
<many-to-many class="Category" column="CATEGORY_ID"/>
</bag>
</class>
Changes made only to the inverse end of the association are not persisted.
This means that Hibernate has two representations in memory for every
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collections
bidirectional association, one link from A to B and another link from B to A.
This is easier to understand if you think about the Java object model and how
we create a many-to-many relationship in Java:
category.getItems().add(item);
about the relationship
item.getCategories().add(category);
about the relationship
// The category now "knows"
session.persist(item);
be saved!
session.persist(category);
saved
// The item now "knows"
// The relationship won't
// The relationship will be
The non-inverse side is used to save the in-memory representation to the
database.
You may define a bidirectional one-to-many association by mapping a
one-to-many association to the same table column(s) as a many-to-one
association and declaring the many-valued end inverse="true".
<class name="Parent">
<id name="id" column="parent_id"/>
....
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<many-to-one name="parent"
class="Parent"
column="parent_id"
not-null="true"/>
</class>
Mapping one end of an association with inverse="true" doesn't affect the
operation of cascades, these are orthogonal concepts!
6.3.3. Bidirectional associations with indexed collections
A bidirectional association where one end is represented as a <list> or
<map> requires special consideration. If there is a property of the child
class which maps to the index column, no problem, we can continue using
inverse="true" on the collection mapping:
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<class name="Parent">
<id name="id" column="parent_id"/>
....
<map name="children" inverse="true">
<key column="parent_id"/>
<map-key column="name"
type="string"/>
<one-to-many class="Child"/>
</map>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<property name="name"
not-null="true"/>
<many-to-one name="parent"
class="Parent"
column="parent_id"
not-null="true"/>
</class>
But, if there is no such property on the child class, we can't think of the
association as truly bidirectional (there is information available at one end
of the association that is not available at the other end). In this case, we
can't map the collection inverse="true". Instead, we could use the following
mapping:
<class name="Parent">
<id name="id" column="parent_id"/>
....
<map name="children">
<key column="parent_id"
not-null="true"/>
<map-key column="name"
type="string"/>
<one-to-many class="Child"/>
</map>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<many-to-one name="parent"
class="Parent"
column="parent_id"
insert="false"
update="false"
not-null="true"/>
</class>
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Note that in this mapping, the collection-valued end of the association is
responsible for updates to the foreign key. TODO: Does this really result in
some unnecessary update statements?
6.3.4. Ternary associations
There are three possible approaches to mapping a ternary association. One
is to use a Map with an association as its index:
<map name="contracts">
<key column="employer_id" not-null="true"/>
<map-key-many-to-many column="employee_id" class="Employee"/>
<one-to-many class="Contract"/>
</map>
<map name="connections">
<key column="incoming_node_id"/>
<map-key-many-to-many column="outgoing_node_id" class="Node"/>
<many-to-many column="connection_id" class="Connection"/>
</map>
A second approach is to simply remodel the association as an entity class.
This is the approach we use most commonly.
A final alternative is to use composite elements, which we will discuss later.
6.3.5. Using
an <idbag>
If you've fully embraced our view that composite keys are a bad thing and
that entities should have synthetic identifiers (surrogate keys), then you
might find it a bit odd that the many to many associations and collections of
values that we've shown so far all map to tables with composite keys! Now,
this point is quite arguable; a pure association table doesn't seem to benefit
much from a surrogate key (though a collection of composite values might).
Nevertheless, Hibernate provides a feature that allows you to map many to
many associations and collections of values to a table with a surrogate key.
The <idbag> element lets you map a List (or Collection) with bag semantics.
<idbag name="lovers" table="LOVERS">
<collection-id column="ID" type="long">
<generator class="sequence"/>
</collection-id>
<key column="PERSON1"/>
<many-to-many column="PERSON2" class="Person" fetch="join"/>
</idbag>
As you can see, an <idbag> has a synthetic id generator, just like an entity
class! A different surrogate key is assigned to each collection row. Hibernate
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does not provide any mechanism to discover the surrogate key value of a
particular row, however.
Note that the update performance of an <idbag> is much better than a regular
<bag>! Hibernate can locate individual rows efficiently and update or delete
them individually, just like a list, map or set.
In the current implementation, the native identifier generation strategy is not
supported for <idbag> collection identifiers.
6.4. Collection examples
The previous sections are pretty confusing. So lets look at an example. This
class:
package eg;
import java.util.Set;
public class Parent {
private long id;
private Set children;
public long getId() { return id; }
private void setId(long id) { this.id=id; }
private Set getChildren() { return children; }
private void setChildren(Set children) { this.children=children;
}
....
....
}
has a collection of Child instances. If each child has at most one parent, the
most natural mapping is a one-to-many association:
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
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<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
This maps to the following table definitions:
create table parent ( id bigint not null primary key )
create table child ( id bigint not null primary key, name
varchar(255), parent_id bigint )
alter table child add constraint childfk0 (parent_id) references
parent
If the parent is required, use a bidirectional one-to-many association:
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
<many-to-one name="parent" class="Parent" column="parent_id"
not-null="true"/>
</class>
</hibernate-mapping>
Notice the NOT NULL constraint:
create table parent ( id bigint not null primary key )
create table child ( id bigint not null
primary key,
name varchar(255),
parent_id bigint not null )
alter table child add constraint childfk0 (parent_id) references
parent
Alternatively, if you absolutely insist that this association should be
unidirectional, you can declare the NOT NULL constraint on the <key> mapping:
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<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children">
<key column="parent_id" not-null="true"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
On the other hand, if a child might have multiple parents, a many-to-many
association is appropriate:
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" table="childset">
<key column="parent_id"/>
<many-to-many class="Child" column="child_id"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
Table definitions:
create table parent ( id bigint not null primary key )
create table child ( id bigint not null primary key, name
varchar(255) )
create table childset ( parent_id bigint not null,
child_id bigint not null,
primary key ( parent_id, child_id ) )
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alter table
references
alter table
references
childset add constraint childsetfk0 (parent_id)
parent
childset add constraint childsetfk1 (child_id)
child
For more examples and a complete walk-through a parent/child relationship
mapping, see Chapter 21, Example: Parent/Child.
Even more exotic association mappings are possible, we will catalog all
possibilities in the next chapter.
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Chapter 7. Association Mappings
7.1. Introduction
Association mappings are the often most difficult thing to get right. In this
section we'll go through the canonical cases one by one, starting with
unidirectional mappings, and then considering the bidirectional cases. We'll
use Person and Address in all the examples.
We'll classify associations by whether or not they map to an intervening join
table, and by multiplicity.
Nullable foreign keys are not considered good practice in traditional data
modelling, so all our examples use not null foreign keys. This is not a
requirement of Hibernate, and the mappings will all work if you drop the
nullability constraints.
7.2. Unidirectional associations
7.2.1. many to one
A unidirectional many-to-one association is the most common kind of
unidirectional association.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key,
addressId bigint not null )
create table Address ( addressId bigint not null primary key )
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7.2.2. one to one
A unidirectional one-to-one association on a foreign key is almost identical.
The only difference is the column unique constraint.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
unique="true"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key,
addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )
A unidirectional one-to-one association on a primary key usually uses
a special id generator. (Notice that we've reversed the direction of the
association in this example.)
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
</class>
<class name="Address">
<id name="id" column="personId">
<generator class="foreign">
<param name="property">person</param>
</generator>
</id>
<one-to-one name="person" constrained="true"/>
</class>
create table Person ( personId bigint not null primary key )
create table Address ( personId bigint not null primary key )
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7.2.3. one to many
A unidirectional one-to-many association on a foreign key is a very unusual
case, and is not really recommended.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses">
<key column="personId"
not-null="true"/>
<one-to-many class="Address"/>
</set>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table Address ( addressId bigint not null primary key,
personId bigint not null )
We think it's better to use a join table for this kind of association.
7.3. Unidirectional associations with join tables
7.3.1. one to many
A unidirectional one-to-many association on a join table is much preferred.
Notice that by specifying unique="true", we have changed the multiplicity
from many-to-many to one-to-many.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses" table="PersonAddress">
<key column="personId"/>
<many-to-many column="addressId"
unique="true"
class="Address"/>
</set>
</class>
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<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId not null, addressId bigint not
null primary key )
create table Address ( addressId bigint not null primary key )
7.3.2. many to one
A unidirectional many-to-one association on a join table is quite common
when the association is optional.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<join table="PersonAddress"
optional="true">
<key column="personId" unique="true"/>
<many-to-one name="address"
column="addressId"
not-null="true"/>
</join>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null primary key,
addressId bigint not null )
create table Address ( addressId bigint not null primary key )
7.3.3. one to one
A unidirectional one-to-one association on a join table is extremely unusual,
but possible.
<class name="Person">
<id name="id" column="personId">
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<generator class="native"/>
</id>
<join table="PersonAddress"
optional="true">
<key column="personId"
unique="true"/>
<many-to-one name="address"
column="addressId"
not-null="true"
unique="true"/>
</join>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null primary key,
addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )
7.3.4. many to many
Finally, we have a unidirectional many-to-many association.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses" table="PersonAddress">
<key column="personId"/>
<many-to-many column="addressId"
class="Address"/>
</set>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null, addressId
bigint not null, primary key (personId, addressId) )
create table Address ( addressId bigint not null primary key )
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7.4. Bidirectional associations
7.4.1. one to many / many to one
A bidirectional many-to-one association is the most common kind of
association. (This is the standard parent/child relationship.)
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
<set name="people" inverse="true">
<key column="addressId"/>
<one-to-many class="Person"/>
</set>
</class>
create table Person ( personId bigint not null primary key,
addressId bigint not null )
create table Address ( addressId bigint not null primary key )
If you use a List (or other indexed collection) you need to set the key column
of the foreign key to not null, and let Hibernate manage the association from
the collections side to maintain the index of each element (making the other
side virtually inverse by setting update="false" and insert="false"):
<class name="Person">
<id name="id"/>
...
<many-to-one name="address"
column="addressId"
not-null="true"
insert="false"
update="false"/>
</class>
<class name="Address">
<id name="id"/>
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...
<list name="people">
<key column="addressId" not-null="true"/>
<list-index column="peopleIdx"/>
<one-to-many class="Person"/>
</list>
</class>
It is important that you define not-null="true" on the <key> element of the
collection mapping if the underlying foreign key column is NOT NULL. Don't
only declare not-null="true" on a possible nested <column> element, but on
the <key> element.
7.4.2. one to one
A bidirectional one-to-one association on a foreign key is quite common.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address"
column="addressId"
unique="true"
not-null="true"/>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
<one-to-one name="person"
property-ref="address"/>
</class>
create table Person ( personId bigint not null primary key,
addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )
A bidirectional one-to-one association on a primary key uses the special id
generator.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<one-to-one name="address"/>
</class>
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<class name="Address">
<id name="id" column="personId">
<generator class="foreign">
<param name="property">person</param>
</generator>
</id>
<one-to-one name="person"
constrained="true"/>
</class>
create table Person ( personId bigint not null primary key )
create table Address ( personId bigint not null primary key )
7.5. Bidirectional associations with join tables
7.5.1. one to many / many to one
A bidirectional one-to-many association on a join table. Note that the
inverse="true" can go on either end of the association, on the collection, or
on the join.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses"
table="PersonAddress">
<key column="personId"/>
<many-to-many column="addressId"
unique="true"
class="Address"/>
</set>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
<join table="PersonAddress"
inverse="true"
optional="true">
<key column="addressId"/>
<many-to-one name="person"
column="personId"
not-null="true"/>
</join>
</class>
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create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null, addressId
bigint not null primary key )
create table Address ( addressId bigint not null primary key )
7.5.2. one to one
A bidirectional one-to-one association on a join table is extremely unusual,
but possible.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<join table="PersonAddress"
optional="true">
<key column="personId"
unique="true"/>
<many-to-one name="address"
column="addressId"
not-null="true"
unique="true"/>
</join>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
<join table="PersonAddress"
optional="true"
inverse="true">
<key column="addressId"
unique="true"/>
<many-to-one name="person"
column="personId"
not-null="true"
unique="true"/>
</join>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null primary key,
addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )
7.5.3. many to many
Finally, we have a bidirectional many-to-many association.
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<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<set name="addresses" table="PersonAddress">
<key column="personId"/>
<many-to-many column="addressId"
class="Address"/>
</set>
</class>
<class name="Address">
<id name="id" column="addressId">
<generator class="native"/>
</id>
<set name="people" inverse="true" table="PersonAddress">
<key column="addressId"/>
<many-to-many column="personId"
class="Person"/>
</set>
</class>
create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null, addressId
bigint not null, primary key (personId, addressId) )
create table Address ( addressId bigint not null primary key )
7.6. More complex association mappings
More complex association joins are extremely rare. Hibernate makes
it possible to handle more complex situations using SQL fragments
embedded in the mapping document. For example, if a table with historical
account information data defines accountNumber, effectiveEndDate and
effectiveStartDatecolumns, mapped as follows:
<properties name="currentAccountKey">
<property name="accountNumber" type="string" not-null="true"/>
<property name="currentAccount" type="boolean">
<formula>case when effectiveEndDate is null then 1 else 0
end</formula>
</property>
</properties>
<property name="effectiveEndDate" type="date"/>
<property name="effectiveStateDate" type="date" not-null="true"/>
Then we can map an association to the current instance (the one with null
effectiveEndDate) using:
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<many-to-one name="currentAccountInfo"
property-ref="currentAccountKey"
class="AccountInfo">
<column name="accountNumber"/>
<formula>'1'</formula>
</many-to-one>
In a more complex example, imagine that the association between Employee
and Organization is maintained in an Employment table full of historical
employment data. Then an association to the employee's most recent
employer (the one with the most recent startDate) might be mapped this
way:
<join>
<key column="employeeId"/>
<subselect>
select employeeId, orgId
from Employments
group by orgId
having startDate = max(startDate)
</subselect>
<many-to-one name="mostRecentEmployer"
class="Organization"
column="orgId"/>
</join>
You can get quite creative with this functionality, but it is usually more
practical to handle these kinds of cases using HQL or a criteria query.
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Chapter 8. Component Mapping
The notion of a component is re-used in several different contexts, for
different purposes, throughout Hibernate.
8.1. Dependent objects
A component is a contained object that is persisted as a value type, not an
entity reference. The term "component" refers to the object-oriented notion of
composition (not to architecture-level components). For example, you might
model a person like this:
public class Person {
private java.util.Date birthday;
private Name name;
private String key;
public String getKey() {
return key;
}
private void setKey(String key) {
this.key=key;
}
public java.util.Date getBirthday() {
return birthday;
}
public void setBirthday(java.util.Date birthday) {
this.birthday = birthday;
}
public Name getName() {
return name;
}
public void setName(Name name) {
this.name = name;
}
......
......
}
public class Name {
char initial;
String first;
String last;
public String getFirst() {
return first;
}
void setFirst(String first) {
this.first = first;
}
public String getLast() {
return last;
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}
void setLast(String last) {
this.last = last;
}
public char getInitial() {
return initial;
}
void setInitial(char initial) {
this.initial = initial;
}
}
Now Name may be persisted as a component of Person. Notice that Name
defines getter and setter methods for its persistent properties, but doesn't
need to declare any interfaces or identifier properties.
Our Hibernate mapping would look like:
<class name="eg.Person" table="person">
<id name="Key" column="pid" type="string">
<generator class="uuid"/>
</id>
<property name="birthday" type="date"/>
<component name="Name" class="eg.Name"> <!-- class attribute
optional -->
<property name="initial"/>
<property name="first"/>
<property name="last"/>
</component>
</class>
The person table would have the columns pid, birthday, initial, first and
last.
Like all value types, components do not support shared references. In
other words, two persons could have the same name, but the two person
objects would contain two independent name ojects, only "the same" by
value. The null value semantics of a component are ad hoc. When reloading
the containing object, Hibernate will assume that if all component columns
are null, then the entire component is null. This should be okay for most
purposes.
The properties of a component may be of any Hibernate type (collections,
many-to-one associations, other components, etc). Nested components
should not be considered an exotic usage. Hibernate is intended to support a
very fine-grained object model.
The <component> element allows a <parent> subelement that maps a property
of the component class as a reference back to the containing entity.
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<class name="eg.Person" table="person">
<id name="Key" column="pid" type="string">
<generator class="uuid"/>
</id>
<property name="birthday" type="date"/>
<component name="Name" class="eg.Name" unique="true">
<parent name="namedPerson"/> <!-- reference back to the
Person -->
<property name="initial"/>
<property name="first"/>
<property name="last"/>
</component>
</class>
8.2. Collections of dependent objects
Collections of components are supported (eg. an array of type Name).
Declare your component collection by replacing the <element> tag with a
<composite-element> tag.
<set name="someNames" table="some_names" lazy="true">
<key column="id"/>
<composite-element class="eg.Name"> <!-- class attribute
required -->
<property name="initial"/>
<property name="first"/>
<property name="last"/>
</composite-element>
</set>
Note: if you define a Set of composite elements, it is very important to
implement equals() and hashCode() correctly.
Composite elements may contain components but not collections.
If your composite element itself contains components, use the
<nested-composite-element> tag. This is a pretty exotic case - a collection of
components which themselves have components. By this stage you should
be asking yourself if a one-to-many association is more appropriate. Try
remodelling the composite element as an entity - but note that even though
the Java model is the same, the relational model and persistence semantics
are still slightly different.
Please note that a composite element mapping doesn't support null-able
properties if you're using a <set>. Hibernate has to use each columns
value to identify a record when deleting objects (there is no separate
primary key column in the composite element table), which is not possible
with null values. You have to either use only not-null properties in a
composite-element or choose a <list>, <map>, <bag> or <idbag>.
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A special case of a composite element is a composite element with a
nested <many-to-one> element. A mapping like this allows you to map extra
columns of a many-to-many association table to the composite element
class. The following is a many-to-many association from Order to Item where
purchaseDate, price and quantity are properties of the association:
<class name="eg.Order" .... >
....
<set name="purchasedItems" table="purchase_items" lazy="true">
<key column="order_id">
<composite-element class="eg.Purchase">
<property name="purchaseDate"/>
<property name="price"/>
<property name="quantity"/>
<many-to-one name="item" class="eg.Item"/> <!-- class
attribute is optional -->
</composite-element>
</set>
</class>
Of course, there can't be a reference to the purchae on the other side, for
bidirectional association navigation. Remember that components are value
types and don't allow shared references. A single Purchase can be in the set
of an Order, but it can't be referenced by the Item at the same time.
Even ternary (or quaternary, etc) associations are possible:
<class name="eg.Order" .... >
....
<set name="purchasedItems" table="purchase_items" lazy="true">
<key column="order_id">
<composite-element class="eg.OrderLine">
<many-to-one name="purchaseDetails class="eg.Purchase"/>
<many-to-one name="item" class="eg.Item"/>
</composite-element>
</set>
</class>
Composite elements may appear in queries using the same syntax as
associations to other entities.
8.3. Components as Map indices
The <composite-map-key> element lets you map a component class as the
key of a Map. Make sure you override hashCode() and equals() correctly on
the component class.
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8.4. Components as composite identifiers
You may use a component as an identifier of an entity class. Your
component class must satisfy certain requirements:
• It must implement java.io.Serializable.
• It must re-implement equals() and hashCode(), consistently with the
database's notion of composite key equality.
Note: in Hibernate3, the second requirement is not an absolutely hard
requirement of Hibernate. But do it anyway.
You can't use an IdentifierGenerator to generate composite keys. Instead
the application must assign its own identifiers.
Use the <composite-id> tag (with nested <key-property> elements) in place
of the usual <id> declaration. For example, the OrderLine class has a primary
key that depends upon the (composite) primary key of Order.
<class name="OrderLine">
<composite-id name="id" class="OrderLineId">
<key-property name="lineId"/>
<key-property name="orderId"/>
<key-property name="customerId"/>
</composite-id>
<property name="name"/>
<many-to-one name="order" class="Order"
insert="false" update="false">
<column name="orderId"/>
<column name="customerId"/>
</many-to-one>
....
</class>
Now, any foreign keys referencing the OrderLine table are also composite.
You must declare this in your mappings for other classes. An association to
OrderLine would be mapped like this:
<many-to-one name="orderLine" class="OrderLine">
<!-- the "class" attribute is optional, as usual -->
<column name="lineId"/>
<column name="orderId"/>
<column name="customerId"/>
</many-to-one>
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(Note that the <column> tag is an alternative to the column attribute
everywhere.)
A many-to-many association to OrderLine also uses the composite foreign key:
<set name="undeliveredOrderLines">
<key column name="warehouseId"/>
<many-to-many class="OrderLine">
<column name="lineId"/>
<column name="orderId"/>
<column name="customerId"/>
</many-to-many>
</set>
The collection of OrderLines in Order would use:
<set name="orderLines" inverse="true">
<key>
<column name="orderId"/>
<column name="customerId"/>
</key>
<one-to-many class="OrderLine"/>
</set>
(The <one-to-many> element, as usual, declares no columns.)
If OrderLine itself owns a collection, it also has a composite foreign key.
<class name="OrderLine">
....
....
<list name="deliveryAttempts">
<key>
<!-- a collection inherits the composite key type
-->
<column name="lineId"/>
<column name="orderId"/>
<column name="customerId"/>
</key>
<list-index column="attemptId" base="1"/>
<composite-element class="DeliveryAttempt">
...
</composite-element>
</set>
</class>
8.5. Dynamic components
You may even map a property of type Map:
<dynamic-component name="userAttributes">
<property name="foo" column="FOO" type="string"/>
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<property name="bar" column="BAR" type="integer"/>
<many-to-one name="baz" class="Baz" column="BAZ_ID"/>
</dynamic-component>
The semantics of a <dynamic-component> mapping are identical to
<component>. The advantage of this kind of mapping is the ability to determine
the actual properties of the bean at deployment time, just by editing the
mapping document. Runtime manipulation of the mapping document is also
possible, using a DOM parser. Even better, you can access (and change)
Hibernate's configuration-time metamodel via the Configuration object.
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Chapter 9. Inheritance Mapping
9.1. The Three Strategies
Hibernate supports the three basic inheritance mapping strategies:
• table per class hierarchy
• table per subclass
• table per concrete class
In addition, Hibernate supports a fourth, slightly different kind of
polymorphism:
• implicit polymorphism
It is possible to use different mapping strategies for different branches of the
same inheritance hierarchy, and then make use of implicit polymorphism to
achieve polymorphism across the whole hierarchy. However, Hibernate does
not support mixing <subclass>, and <joined-subclass> and <union-subclass>
mappings under the same root <class> element. It is possible to mix together
the table per hierarchy and table per subclass strategies, under the the same
<class> element, by combining the <subclass> and <join> elements (see
below).
It is possible to define subclass, union-subclass, and joined-subclass
mappings in separate mapping documents, directly beneath
hibernate-mapping. This allows you to extend a class hierachy just by adding
a new mapping file. You must specify an extends attribute in the subclass
mapping, naming a previously mapped superclass. Note: Previously this
feature made the ordering of the mapping documents important. Since
Hibernate3, the ordering of mapping files does not matter when using the
extends keyword. The ordering inside a single mapping file still needs to be
defined as superclasses before subclasses.
<hibernate-mapping>
<subclass name="DomesticCat" extends="Cat"
discriminator-value="D">
<property name="name" type="string"/>
</subclass>
</hibernate-mapping>
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9.1.1. Table per class hierarchy
Suppose we have an interface Payment, with implementors
CreditCardPayment, CashPayment, ChequePayment. The table per hierarchy
mapping would look like:
<class name="Payment" table="PAYMENT">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="native"/>
</id>
<discriminator column="PAYMENT_TYPE" type="string"/>
<property name="amount" column="AMOUNT"/>
...
<subclass name="CreditCardPayment" discriminator-value="CREDIT">
<property name="creditCardType" column="CCTYPE"/>
...
</subclass>
<subclass name="CashPayment" discriminator-value="CASH">
...
</subclass>
<subclass name="ChequePayment" discriminator-value="CHEQUE">
...
</subclass>
</class>
Exactly one table is required. There is one big limitation of this mapping
strategy: columns declared by the subclasses, such as CCTYPE, may not have
NOT NULL constraints.
9.1.2. Table per subclass
A table per subclass mapping would look like:
<class name="Payment" table="PAYMENT">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="native"/>
</id>
<property name="amount" column="AMOUNT"/>
...
<joined-subclass name="CreditCardPayment"
table="CREDIT_PAYMENT">
<key column="PAYMENT_ID"/>
<property name="creditCardType" column="CCTYPE"/>
...
</joined-subclass>
<joined-subclass name="CashPayment" table="CASH_PAYMENT">
<key column="PAYMENT_ID"/>
...
</joined-subclass>
<joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
<key column="PAYMENT_ID"/>
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...
</joined-subclass>
</class>
Four tables are required. The three subclass tables have primary key
associations to the superclass table (so the relational model is actually a
one-to-one association).
9.1.3. Table per subclass, using a discriminator
Note that Hibernate's implementation of table per subclass requires no
discriminator column. Other object/relational mappers use a different
implementation of table per subclass which requires a type discriminator
column in the superclass table. The approach taken by Hibernate is much
more difficult to implement but arguably more correct from a relational point
of view. If you would like to use a discriminator column with the table per
subclass strategy, you may combine the use of <subclass> and <join>, as
follow:
<class name="Payment" table="PAYMENT">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="native"/>
</id>
<discriminator column="PAYMENT_TYPE" type="string"/>
<property name="amount" column="AMOUNT"/>
...
<subclass name="CreditCardPayment" discriminator-value="CREDIT">
<join table="CREDIT_PAYMENT">
<key column="PAYMENT_ID"/>
<property name="creditCardType" column="CCTYPE"/>
...
</join>
</subclass>
<subclass name="CashPayment" discriminator-value="CASH">
<join table="CASH_PAYMENT">
<key column="PAYMENT_ID"/>
...
</join>
</subclass>
<subclass name="ChequePayment" discriminator-value="CHEQUE">
<join table="CHEQUE_PAYMENT" fetch="select">
<key column="PAYMENT_ID"/>
...
</join>
</subclass>
</class>
The optional fetch="select" declaration tells Hibernate not to fetch
the ChequePayment subclass data using an outer join when querying the
superclass.
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9.1.4. Mixing table per class hierarchy with table per
subclass
You may even mix the table per hierarchy and table per subclass strategies
using this approach:
<class name="Payment" table="PAYMENT">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="native"/>
</id>
<discriminator column="PAYMENT_TYPE" type="string"/>
<property name="amount" column="AMOUNT"/>
...
<subclass name="CreditCardPayment" discriminator-value="CREDIT">
<join table="CREDIT_PAYMENT">
<property name="creditCardType" column="CCTYPE"/>
...
</join>
</subclass>
<subclass name="CashPayment" discriminator-value="CASH">
...
</subclass>
<subclass name="ChequePayment" discriminator-value="CHEQUE">
...
</subclass>
</class>
For any of these mapping strategies, a polymorphic association to the root
Payment class is mapped using <many-to-one>.
<many-to-one name="payment" column="PAYMENT_ID" class="Payment"/>
9.1.5. Table per concrete class
There are two ways we could go about mapping the table per concrete class
strategy. The first is to use <union-subclass>.
<class name="Payment">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="sequence"/>
</id>
<property name="amount" column="AMOUNT"/>
...
<union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
<property name="creditCardType" column="CCTYPE"/>
...
</union-subclass>
<union-subclass name="CashPayment" table="CASH_PAYMENT">
...
</union-subclass>
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<union-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
...
</union-subclass>
</class>
Three tables are involved for the subclasses. Each table defines columns for
all properties of the class, including inherited properties.
The limitation of this approach is that if a property is mapped on the
superclass, the column name must be the same on all subclass tables. (We
might relax this in a future release of Hibernate.) The identity generator
strategy is not allowed in union subclass inheritance, indeed the primary key
seed has to be shared accross all unioned subclasses of a hierarchy.
If your superclass is abstract, map it with abstract="true". Of course, if it is
not abstract, an additional table (defaults to PAYMENT in the example above) is
needed to hold instances of the superclass.
9.1.6. Table per concrete class, using implicit
polymorphism
An alternative approach is to make use of implicit polymorphism:
<class name="CreditCardPayment" table="CREDIT_PAYMENT">
<id name="id" type="long" column="CREDIT_PAYMENT_ID">
<generator class="native"/>
</id>
<property name="amount" column="CREDIT_AMOUNT"/>
...
</class>
<class name="CashPayment" table="CASH_PAYMENT">
<id name="id" type="long" column="CASH_PAYMENT_ID">
<generator class="native"/>
</id>
<property name="amount" column="CASH_AMOUNT"/>
...
</class>
<class name="ChequePayment" table="CHEQUE_PAYMENT">
<id name="id" type="long" column="CHEQUE_PAYMENT_ID">
<generator class="native"/>
</id>
<property name="amount" column="CHEQUE_AMOUNT"/>
...
</class>
Notice that nowhere do we mention the Payment interface explicitly. Also
notice that properties of Payment are mapped in each of the subclasses. If
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you want to avoid duplication, consider using XML entities (e.g. [ <!ENTITY
allproperties SYSTEM "allproperties.xml"> ] in the DOCTYPE declartion and
&allproperties; in the mapping).
The disadvantage of this approach is that Hibernate does not generate SQL
UNIONs when performing polymorphic queries.
For this mapping strategy, a polymorphic association to Payment is usually
mapped using <any>.
<any name="payment" meta-type="string" id-type="long">
<meta-value value="CREDIT" class="CreditCardPayment"/>
<meta-value value="CASH" class="CashPayment"/>
<meta-value value="CHEQUE" class="ChequePayment"/>
<column name="PAYMENT_CLASS"/>
<column name="PAYMENT_ID"/>
</any>
9.1.7. Mixing implicit polymorphism with other inheritance
mappings
There is one further thing to notice about this mapping. Since the subclasses
are each mapped in their own <class> element (and since Payment is just an
interface), each of the subclasses could easily be part of another inheritance
hierarchy! (And you can still use polymorphic queries against the Payment
interface.)
<class name="CreditCardPayment" table="CREDIT_PAYMENT">
<id name="id" type="long" column="CREDIT_PAYMENT_ID">
<generator class="native"/>
</id>
<discriminator column="CREDIT_CARD" type="string"/>
<property name="amount" column="CREDIT_AMOUNT"/>
...
<subclass name="MasterCardPayment" discriminator-value="MDC"/>
<subclass name="VisaPayment" discriminator-value="VISA"/>
</class>
<class name="NonelectronicTransaction" table="NONELECTRONIC_TXN">
<id name="id" type="long" column="TXN_ID">
<generator class="native"/>
</id>
...
<joined-subclass name="CashPayment" table="CASH_PAYMENT">
<key column="PAYMENT_ID"/>
<property name="amount" column="CASH_AMOUNT"/>
...
</joined-subclass>
<joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
<key column="PAYMENT_ID"/>
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<property name="amount" column="CHEQUE_AMOUNT"/>
...
</joined-subclass>
</class>
Once again, we don't mention Payment explicitly. If we execute a query
against the Payment interface - for example, from Payment - Hibernate
automatically returns instances of CreditCardPayment (and its subclasses,
since they also implement Payment), CashPayment and ChequePayment but not
instances of NonelectronicTransaction.
9.2. Limitations
There are certain limitations to the "implicit polymorphism" approach to
the table per concrete-class mapping strategy. There are somewhat less
restrictive limitations to <union-subclass> mappings.
The following table shows the limitations of table per concrete-class
mappings, and of implicit polymorphism, in Hibernate.
Table 9.1. Features of inheritance mappings
Inheritance
Polymorphic
Polymorphic
Polymorphic
Polymorphic
Polymorphic
Polymorphic
Polymorphic
Outer
strategymany- one-to- one-to- many- load()/ queries joins join fetching
to-one one
many toget()
many
table
<many- <one<oneper
to-one> to-one> toclassmany>
hierarchy
<many-
s.get(Payment.class,
from
from
to-
id)
table
<many- <one<oneper subclass
to-one> to-one> to-
<many-
s.get(Payment.class,
from
from
to-
id)
many>
many>
supported
Payment Order
p
o join o.payment
p
many>
supported
Payment Order
p
o join o.payment
p
table
<many- <oneper concreteto-one> to-one>
class
(unionsubclass)
<one-
<many-
s.get(Payment.class,
from
from
to-
to-
id)
many>
many>
(for inverse="true"
only)
supported
Payment Order
p
o join o.payment
p
table
<any>
not supported
not supported
<many- s.createCriteria(Payment.class).add(
from
not supported
not supported
per concrete
to-any> Restrictions.idEq(id)
Payment
class
).uniqueResult()
p
(implicit polymorphism)
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Hibernate is a full object/relational mapping solution that not only shields
the developer from the details of the underlying database management
system, but also offers state management of objects. This is, contrary to the
management of SQL statements in common JDBC/SQL persistence layers, a
very natural object-oriented view of persistence in Java applications.
In other words, Hibernate application developers should always think about
the state of their objects, and not necessarily about the execution of SQL
statements. This part is taken care of by Hibernate and is only relevant for
the application developer when tuning the performance of the system.
10.1. Hibernate object states
Hibernate defines and supports the following object states:
• Transient - an object is transient if it has just been instantiated using the
new operator, and it is not associated with a Hibernate Session. It has no
persistent representation in the database and no identifier value has been
assigned. Transient instances will be destroyed by the garbage collector
if the application doesn't hold a reference anymore. Use the Hibernate
Session to make an object persistent (and let Hibernate take care of the
SQL statements that need to be executed for this transition).
• Persistent - a persistent instance has a representation in the database and
an identifier value. It might just have been saved or loaded, however, it is
by definition in the scope of a Session. Hibernate will detect any changes
made to an object in persistent state and synchronize the state with the
database when the unit of work completes. Developers don't execute
manual UPDATE statements, or DELETE statements when an object should be
made transient.
• Detached - a detached instance is an object that has been persistent, but
its Session has been closed. The reference to the object is still valid, of
course, and the detached instance might even be modified in this state.
A detached instance can be reattached to a new Session at a later point
in time, making it (and all the modifications) persistent again. This feature
enables a programming model for long running units of work that require
user think-time. We call them application transactions, i.e. a unit of work
from the point of view of the user.
We'll now discuss the states and state transitions (and the Hibernate
methods that trigger a transition) in more detail.
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10.2. Making objects persistent
Newly instantiated instances of a a persistent class are considered transient
by Hibernate. We can make a transient instance persistent by associating it
with a session:
DomesticCat fritz = new DomesticCat();
fritz.setColor(Color.GINGER);
fritz.setSex('M');
fritz.setName("Fritz");
Long generatedId = (Long) sess.save(fritz);
If Cat has a generated identifier, the identifier is generated and assigned to
the cat when save() is called. If Cat has an assigned identifier, or a composite
key, the identifier should be assigned to the cat instance before calling
save(). You may also use persist() instead of save(), with the semantics
defined in the EJB3 early draft.
• persist() makes a transient instance persistent. However, it doesn't
guarantee that the identifier value will be assigned to the persistent
instance immediately, the assignment might happen at flush time.
persist() also guarantees that it will not execute an INSERT statement if it
is called outside of transaction boundaries. This is useful in long-running
conversations with an extended Session/persistence context.
• save() does guarantee to return an identifier. If an INSERT has to be
executed to get the identifier ( e.g. "identity" generator, not "sequence"),
this INSERT happens immediately, no matter if you are inside or outside
of a transaction. This is problematic in a long-running conversation with an
extended Session/persistence context.
Alternatively, you may assign the identifier using an overloaded version of
save().
DomesticCat pk = new DomesticCat();
pk.setColor(Color.TABBY);
pk.setSex('F');
pk.setName("PK");
pk.setKittens( new HashSet() );
pk.addKitten(fritz);
sess.save( pk, new Long(1234) );
If the object you make persistent has associated objects (e.g. the kittens
collection in the previous example), these objects may be made persistent in
any order you like unless you have a NOT NULL constraint upon a foreign key
column. There is never a risk of violating foreign key constraints. However,
you might violate a NOT NULL constraint if you save() the objects in the wrong
order.
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Usually you don't bother with this detail, as you'll very likely use Hibernate's
transitive persistence feature to save the associated objects automatically.
Then, even NOT NULL constraint violations don't occur - Hibernate will take
care of everything. Transitive persistence is discussed later in this chapter.
10.3. Loading an object
The load() methods of Session gives you a way to retrieve a persistent
instance if you already know its identifier. load() takes a class object and will
load the state into a newly instantiated instance of that class, in persistent
state.
Cat fritz = (Cat) sess.load(Cat.class, generatedId);
// you need to wrap primitive identifiers
long id = 1234;
DomesticCat pk = (DomesticCat) sess.load( DomesticCat.class, new
Long(id) );
Alternatively, you can load state into a given instance:
Cat cat = new DomesticCat();
// load pk's state into cat
sess.load( cat, new Long(pkId) );
Set kittens = cat.getKittens();
Note that load() will throw an unrecoverable exception if there is no matching
database row. If the class is mapped with a proxy, load() just returns an
uninitialized proxy and does not actually hit the database until you invoke a
method of the proxy. This behaviour is very useful if you wish to create an
association to an object without actually loading it from the database. It also
allows multiple instances to be loaded as a batch if batch-size is defined for
the class mapping.
If you are not certain that a matching row exists, you should use the get()
method, which hits the database immediately and returns null if there is no
matching row.
Cat cat = (Cat) sess.get(Cat.class, id);
if (cat==null) {
cat = new Cat();
sess.save(cat, id);
}
return cat;
You may even load an object using an SQL SELECT ... FOR UPDATE, using a
LockMode. See the API documentation for more information.
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Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);
Note that any associated instances or contained collections are not selected
FOR UPDATE, unless you decide to specify lock or all as a cascade style for
the association.
It is possible to re-load an object and all its collections at any time, using the
refresh() method. This is useful when database triggers are used to initialize
some of the properties of the object.
sess.save(cat);
sess.flush(); //force the SQL INSERT
sess.refresh(cat); //re-read the state (after the trigger executes)
An important question usually appears at this point: How much does
Hibernate load from the database and how many SQL SELECTs will it use?
This depends on the fetching strategy and is explained in Section 19.1,
“Fetching strategies”.
10.4. Querying
If you don't know the identifiers of the objects you are looking for, you need a
query. Hibernate supports an easy-to-use but powerful object oriented query
language (HQL). For programmatic query creation, Hibernate supports a
sophisticated Criteria and Example query feature (QBC and QBE). You may
also express your query in the native SQL of your database, with optional
support from Hibernate for result set conversion into objects.
10.4.1. Executing queries
HQL and native SQL queries are represented with an instance of
org.hibernate.Query. This interface offers methods for parameter binding,
result set handling, and for the execution of the actual query. You always
obtain a Query using the current Session:
List cats = session.createQuery(
"from Cat as cat where cat.birthdate < ?")
.setDate(0, date)
.list();
List mothers = session.createQuery(
"select mother from Cat as cat join cat.mother as mother where
cat.name = ?")
.setString(0, name)
.list();
List kittens = session.createQuery(
"from Cat as cat where cat.mother = ?")
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.setEntity(0, pk)
.list();
Cat mother = (Cat) session.createQuery(
"select cat.mother from Cat as cat where cat = ?")
.setEntity(0, izi)
.uniqueResult();]]
Query mothersWithKittens = (Cat) session.createQuery(
"select mother from Cat as mother left join fetch
mother.kittens");
Set uniqueMothers = new HashSet(mothersWithKittens.list());
A query is usually executed by invoking list(), the result of the query will be
loaded completely into a collection in memory. Entity instances retrieved by
a query are in persistent state. The uniqueResult() method offers a shortcut
if you know your query will only return a single object. Note that queries that
make use of eager fetching of collections usually return duplicates of the root
objects (but with their collections initialized). You can filter these duplicates
simply through a Set.
10.4.1.1. Iterating results
Occasionally, you might be able to achieve better performance by executing
the query using the iterate() method. This will only usually be the case if
you expect that the actual entity instances returned by the query will already
be in the session or second-level cache. If they are not already cached,
iterate() will be slower than list() and might require many database hits
for a simple query, usually 1 for the initial select which only returns identifiers,
and n additional selects to initialize the actual instances.
// fetch ids
Iterator iter = sess.createQuery("from eg.Qux q order by
q.likeliness").iterate();
while ( iter.hasNext() ) {
Qux qux = (Qux) iter.next(); // fetch the object
// something we couldnt express in the query
if ( qux.calculateComplicatedAlgorithm() ) {
// delete the current instance
iter.remove();
// dont need to process the rest
break;
}
}
10.4.1.2. Queries that return tuples
Hibernate queries sometimes return tuples of objects, in which case each
tuple is returned as an array:
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Iterator kittensAndMothers = sess.createQuery(
"select kitten, mother from Cat kitten join
kitten.mother mother")
.list()
.iterator();
while ( kittensAndMothers.hasNext() ) {
Object[] tuple = (Object[]) kittensAndMothers.next();
Cat kitten = (Cat) tuple[0];
Cat mother = (Cat) tuple[1];
....
}
10.4.1.3. Scalar results
Queries may specify a property of a class in the select clause. They may
even call SQL aggregate functions. Properties or aggregates are considered
"scalar" results (and not entities in persistent state).
Iterator results = sess.createQuery(
"select cat.color, min(cat.birthdate), count(cat) from Cat
cat " +
"group by cat.color")
.list()
.iterator();
while ( results.hasNext() ) {
Object[] row = (Object[]) results.next();
Color type = (Color) row[0];
Date oldest = (Date) row[1];
Integer count = (Integer) row[2];
.....
}
10.4.1.4. Bind parameters
Methods on Query are provided for binding values to named parameters or
JDBC-style ? parameters. Contrary to JDBC, Hibernate numbers parameters
from zero. Named parameters are identifiers of the form :name in the query
string. The advantages of named parameters are:
• named parameters are insensitive to the order they occur in the query
string
• they may occur multiple times in the same query
• they are self-documenting
//named parameter (preferred)
Query q = sess.createQuery("from DomesticCat cat where cat.name =
:name");
q.setString("name", "Fritz");
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Iterator cats = q.iterate();
//positional parameter
Query q = sess.createQuery("from DomesticCat cat where cat.name =
?");
q.setString(0, "Izi");
Iterator cats = q.iterate();
//named parameter list
List names = new ArrayList();
names.add("Izi");
names.add("Fritz");
Query q = sess.createQuery("from DomesticCat cat where cat.name in
(:namesList)");
q.setParameterList("namesList", names);
List cats = q.list();
10.4.1.5. Pagination
If you need to specify bounds upon your result set (the maximum number
of rows you want to retrieve and / or the first row you want to retrieve) you
should use methods of the Query interface:
Query q = sess.createQuery("from DomesticCat cat");
q.setFirstResult(20);
q.setMaxResults(10);
List cats = q.list();
Hibernate knows how to translate this limit query into the native SQL of your
DBMS.
10.4.1.6. Scrollable iteration
If your JDBC driver supports scrollable ResultSets, the Query interface may
be used to obtain a ScrollableResults object, which allows flexible navigation
of the query results.
Query q = sess.createQuery("select cat.name, cat from DomesticCat
cat " +
"order by cat.name");
ScrollableResults cats = q.scroll();
if ( cats.first() ) {
// find the first name on each page of an alphabetical list of
cats by name
firstNamesOfPages = new ArrayList();
do {
String name = cats.getString(0);
firstNamesOfPages.add(name);
}
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while ( cats.scroll(PAGE_SIZE) );
// Now get the first page of cats
pageOfCats = new ArrayList();
cats.beforeFirst();
int i=0;
while( ( PAGE_SIZE > i++ ) && cats.next() ) pageOfCats.add(
cats.get(1) );
}
cats.close()
Note that an open database connection (and cursor) is required for this
functionality, use setMaxResult()/setFirstResult() if you need offline
pagination functionality.
10.4.1.7. Externalizing named queries
You may also define named queries in the mapping document. (Remember
to use a CDATA section if your query contains characters that could be
interpreted as markup.)
<query name="ByNameAndMaximumWeight"><![CDATA[
from eg.DomesticCat as cat
where cat.name = ?
and cat.weight > ?
] ]></query>
Parameter binding and executing is done programatically:
Query q = sess.getNamedQuery("ByNameAndMaximumWeight");
q.setString(0, name);
q.setInt(1, minWeight);
List cats = q.list();
Note that the actual program code is independent of the query language that
is used, you may also define native SQL queries in metadata, or migrate
existing queries to Hibernate by placing them in mapping files.
Also note that a query declaration inside a <hibernate-mapping> element
requires a global unique name for the query, while a query declaration inside
a <class> element is made unique automatically by prepending the fully
qualified name of the class, for example eg.Cat.ByNameAndMaximumWeight.
10.4.2. Filtering collections
A collection filter is a special type of query that may be applied to a persistent
collection or array. The query string may refer to this, meaning the current
collection element.
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Collection blackKittens = session.createFilter(
pk.getKittens(),
"where this.color = ?")
.setParameter( Color.BLACK,
Hibernate.custom(ColorUserType.class) )
.list()
);
The returned collection is considered a bag, and it's a copy of the given
collection. The original collection is not modified (this is contrary to the
implication of the name "filter", but consistent with expected behavior).
Observe that filters do not require a from clause (though they may have
one if required). Filters are not limited to returning the collection elements
themselves.
Collection blackKittenMates = session.createFilter(
pk.getKittens(),
"select this.mate where this.color = eg.Color.BLACK.intValue")
.list();
Even an empty filter query is useful, e.g. to load a subset of elements in a
huge collection:
Collection tenKittens = session.createFilter(
mother.getKittens(), "")
.setFirstResult(0).setMaxResults(10)
.list();
10.4.3. Criteria queries
HQL is extremely powerful but some developers prefer to build queries
dynamically, using an object-oriented API, rather than building query strings.
Hibernate provides an intuitive Criteria query API for these cases:
Criteria crit = session.createCriteria(Cat.class);
crit.add( Restrictions.eq( "color", eg.Color.BLACK ) );
crit.setMaxResults(10);
List cats = crit.list();
The Criteria and the associated Example API are discussed in more detail in
Chapter 15, Criteria Queries.
10.4.4. Queries in native SQL
You may express a query in SQL, using createSQLQuery() and let Hibernate
take care of the mapping from result sets to objects. Note that you may at
any time call session.connection() and use the JDBC Connection directly.
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If you chose to use the Hibernate API, you must enclose SQL aliases in
braces:
List cats = session.createSQLQuery("SELECT {cat.*} FROM CAT {cat}
WHERE ROWNUM<10")
.addEntity("cat", Cat.class)
.list();
List cats = session.createSQLQuery(
"SELECT {cat}.ID AS {cat.id}, {cat}.SEX AS {cat.sex}, " +
"{cat}.MATE AS {cat.mate}, {cat}.SUBCLASS AS {cat.class},
... " +
"FROM CAT {cat} WHERE ROWNUM<10")
.addEntity("cat", Cat.class)
.list()
SQL queries may contain named and positional parameters, just like
Hibernate queries. More information about native SQL queries in Hibernate
can be found in Chapter 16, Native SQL.
10.5. Modifying persistent objects
Transactional persistent instances (ie. objects loaded, saved, created or
queried by the Session) may be manipulated by the application and any
changes to persistent state will be persisted when the Session is flushed
(discussed later in this chapter). There is no need to call a particular method
(like update(), which has a different purpose) to make your modifications
persistent. So the most straightforward way to update the state of an object is
to load() it, and then manipulate it directly, while the Session is open:
DomesticCat cat = (DomesticCat) sess.load( Cat.class, new Long(69)
);
cat.setName("PK");
sess.flush(); // changes to cat are automatically detected and
persisted
Sometimes this programming model is inefficient since it would require both
an SQL SELECT (to load an object) and an SQL UPDATE (to persist its updated
state) in the same session. Therefore Hibernate offers an alternate approach,
using detached instances.
Note that Hibernate does not offer its own API for direct execution of UPDATE
or DELETE statements. Hibernate is a state management service, you don't
have to think in statements to use it. JDBC is a perfect API for executing
SQL statements, you can get a JDBC Connection at any time by calling
session.connection(). Furthermore, the notion of mass operations conflicts
with object/relational mapping for online transaction processing-oriented
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applications. Future versions of Hibernate may however provide special mass
operation functions. See Chapter 13, Batch processing for some possible
batch operation tricks.
10.6. Modifying detached objects
Many applications need to retrieve an object in one transaction, send
it to the UI layer for manipulation, then save the changes in a new
transaction. Applications that use this kind of approach in a high-concurrency
environment usually use versioned data to ensure isolation for the "long" unit
of work.
Hibernate supports this model by providing for reattachment of detached
instances using the Session.update() or Session.merge() methods:
// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catId);
Cat potentialMate = new Cat();
firstSession.save(potentialMate);
// in a higher layer of the application
cat.setMate(potentialMate);
// later, in a new session
secondSession.update(cat); // update cat
secondSession.update(mate); // update mate
If the Cat with identifier catId had already been loaded by secondSession
when the application tried to reattach it, an exception would have been
thrown.
Use update() if you are sure that the session does not contain an already
persistent instance with the same identifier, and merge() if you want to merge
your modifications at any time without consideration of the state of the
session. In other words, update() is usually the first method you would call in
a fresh session, ensuring that reattachment of your detached instances is the
first operation that is executed.
The application should individually update() detached instances reachable
from the given detached instance if and only if it wants their state also
updated. This can be automated of course, using transitive persistence, see
Section 10.11, “Transitive persistence”.
The lock() method also allows an application to reassociate an object with a
new session. However, the detached instance has to be unmodified!
//just reassociate:
sess.lock(fritz, LockMode.NONE);
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//do a version check, then reassociate:
sess.lock(izi, LockMode.READ);
//do a version check, using SELECT ... FOR UPDATE, then reassociate:
sess.lock(pk, LockMode.UPGRADE);
Note that lock() can be used with various LockModes, see the API
documentation and the chapter on transaction handling for more information.
Reattachment is not the only usecase for lock().
Other models for long units of work are discussed in Section 11.3, “Optimistic
concurrency control”.
10.7. Automatic state detection
Hibernate users have requested a general purpose method that either saves
a transient instance by generating a new identifier or updates/reattaches the
detached instances associated with its current identifier. The saveOrUpdate()
method implements this functionality.
// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catID);
// in a higher tier of the application
Cat mate = new Cat();
cat.setMate(mate);
// later, in a new session
secondSession.saveOrUpdate(cat);
a non-null id)
secondSession.saveOrUpdate(mate);
has a null id)
// update existing state (cat has
// save the new instance (mate
The usage and semantics of saveOrUpdate() seems to be confusing for
new users. Firstly, so long as you are not trying to use instances from one
session in another new session, you should not need to use update(),
saveOrUpdate(), or merge(). Some whole applications will never use either of
these methods.
Usually update() or saveOrUpdate() are used in the following scenario:
•
•
•
•
•
the application loads an object in the first session
the object is passed up to the UI tier
some modifications are made to the object
the object is passed back down to the business logic tier
the application persists these modifications by calling update() in a second
session
saveOrUpdate()
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• if the object is already persistent in this session, do nothing
• if another object associated with the session has the same identifier, throw
an exception
• if the object has no identifier property, save() it
• if the object's identifier has the value assigned to a newly instantiated
object, save() it
• if the object is versioned (by a <version> or <timestamp>), and the version
property value is the same value assigned to a newly instantiated object,
save() it
• otherwise update() the object
and merge() is very different:
• if there is a persistent instance with the same identifier currently associated
with the session, copy the state of the given object onto the persistent
instance
• if there is no persistent instance currently associated with the session, try
to load it from the database, or create a new persistent instance
• the persistent instance is returned
• the given instance does not become associated with the session, it
remains detached
10.8. Deleting persistent objects
will remove an object's state from the database. Of course,
your application might still hold a reference to a deleted object. It's best to
think of delete() as making a persistent instance transient.
Session.delete()
sess.delete(cat);
You may delete objects in any order you like, without risk of foreign key
constraint violations. It is still possible to violate a NOT NULL constraint on a
foreign key column by deleting objects in the wrong order, e.g. if you delete
the parent, but forget to delete the children.
10.9. Replicating object between two different
datastores
It is occasionally useful to be able to take a graph of persistent instances and
make them persistent in a different datastore, without regenerating identifier
values.
//retrieve a cat from one database
Session session1 = factory1.openSession();
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Transaction tx1 = session1.beginTransaction();
Cat cat = session1.get(Cat.class, catId);
tx1.commit();
session1.close();
//reconcile with a second database
Session session2 = factory2.openSession();
Transaction tx2 = session2.beginTransaction();
session2.replicate(cat, ReplicationMode.LATEST_VERSION);
tx2.commit();
session2.close();
The ReplicationMode determines how replicate() will deal with conflicts with
existing rows in the database.
• ReplicationMode.IGNORE - ignore the object when there is an existing
database row with the same identifier
• ReplicationMode.OVERWRITE - overwrite any existing database row with the
same identifier
• ReplicationMode.EXCEPTION - throw an exception if there is an existing
database row with the same identifier
• ReplicationMode.LATEST_VERSION - overwrite the row if its version number is
earlier than the version number of the object, or ignore the object otherwise
Usecases for this feature include reconciling data entered into different
database instances, upgrading system configuration information during
product upgrades, rolling back changes made during non-ACID transactions
and more.
10.10. Flushing the Session
From time to time the Session will execute the SQL statements needed to
synchronize the JDBC connection's state with the state of objects held in
memory. This process, flush, occurs by default at the following points
• before some query executions
• from org.hibernate.Transaction.commit()
• from Session.flush()
The SQL statements are issued in the following order
1. all entity insertions, in the same order the corresponding objects were
saved using Session.save()
2. all entity updates
3. all collection deletions
4. all collection element deletions, updates and insertions
5. all collection insertions
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6. all entity deletions, in the same order the corresponding objects were
deleted using Session.delete()
(An exception is that objects using native ID generation are inserted when
they are saved.)
Except when you explicity flush(), there are absolutely no guarantees about
when the Session executes the JDBC calls, only the order in which they are
executed. However, Hibernate does guarantee that the Query.list(..) will
never return stale data; nor will they return the wrong data.
It is possible to change the default behavior so that flush occurs less
frequently. The FlushMode class defines three different modes: only flush at
commit time (and only when the Hibernate Transaction API is used), flush
automatically using the explained routine, or never flush unless flush() is
called explicitly. The last mode is useful for long running units of work, where
a Session is kept open and disconnected for a long time (see Section 11.3.2,
“Extended session and automatic versioning”).
sess = sf.openSession();
Transaction tx = sess.beginTransaction();
sess.setFlushMode(FlushMode.COMMIT); // allow queries to return
stale state
Cat izi = (Cat) sess.load(Cat.class, id);
izi.setName(iznizi);
// might return stale data
sess.find("from Cat as cat left outer join cat.kittens kitten");
// change to izi is not flushed!
...
tx.commit(); // flush occurs
sess.close();
During flush, an exception might occur (e.g. if a DML operation violates
a constraint). Since handling exceptions involves some understanding of
Hibernate's transactional behavior, we discuss it in Chapter 11, Transactions
And Concurrency.
10.11. Transitive persistence
It is quite cumbersome to save, delete, or reattach individual objects,
especially if you deal with a graph of associated objects. A common case is a
parent/child relationship. Consider the following example:
If the children in a parent/child relationship would be value typed (e.g. a
collection of addresses or strings), their life cycle would depend on the parent
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and no further action would be required for convenient "cascading" of state
changes. When the parent is saved, the value-typed child objects are saved
as well, when the parent is deleted, the children will be deleted, etc. This
even works for operations such as the removal of a child from the collection;
Hibernate will detect this and, since value-typed objects can't have shared
references, delete the child from the database.
Now consider the same scenario with parent and child objects being entities,
not value-types (e.g. categories and items, or parent and child cats). Entities
have their own life cycle, support shared references (so removing an entity
from the collection does not mean it can be deleted), and there is by default
no cascading of state from one entity to any other associated entities.
Hibernate does not implement persistence by reachability by default.
For each basic operation of the Hibernate session - including persist(),
merge(), saveOrUpdate(), delete(), lock(), refresh(), evict(),
- there is a corresponding cascade style. Respectively, the
cascade styles are named create, merge, save-update, delete, lock,
refresh, evict, replicate. If you want an operation to be cascaded
along an association, you must indicate that in the mapping document. For
example:
replicate()
<one-to-one name="person" cascade="persist"/>
Cascade styles my be combined:
<one-to-one name="person" cascade="persist,delete,lock"/>
You may even use cascade="all" to specify that all operations should be
cascaded along the association. The default cascade="none" specifies that no
operations are to be cascaded.
A special cascade style, delete-orphan, applies only to one-to-many
associations, and indicates that the delete() operation should be applied to
any child object that is removed from the association.
Recommendations:
• It doesn't usually make sense to enable cascade on a <many-to-one> or
<many-to-many> association. Cascade is often useful for <one-to-one> and
<one-to-many> associations.
• If the child object's lifespan is bounded by the lifespan of the parent object,
make it a life cycle object by specifying cascade="all,delete-orphan".
• Otherwise, you might not need cascade at all. But if you think that you
will often be working with the parent and children together in the same
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transaction, and you want to save yourself some typing, consider using
cascade="persist,merge,save-update".
Mapping an association (either a single valued association, or a collection)
with cascade="all" marks the association as a parent/child style relationship
where save/update/delete of the parent results in save/update/delete of the
child or children.
Futhermore, a mere reference to a child from a persistent parent will
result in save/update of the child. This metaphor is incomplete, however.
A child which becomes unreferenced by its parent is not automatically
deleted, except in the case of a <one-to-many> association mapped with
cascade="delete-orphan". The precise semantics of cascading operations for
a parent/child relationship are as follows:
• If a parent is passed to persist(), all children are passed to persist()
• If a parent is passed to merge(), all children are passed to merge()
• If a parent is passed to save(), update() or saveOrUpdate(), all children are
passed to saveOrUpdate()
• If a transient or detached child becomes referenced by a persistent parent,
it is passed to saveOrUpdate()
• If a parent is deleted, all children are passed to delete()
• If a child is dereferenced by a persistent parent, nothing special happens
- the application should explicitly delete the child if necessary - unless
cascade="delete-orphan", in which case the "orphaned" child is deleted.
Finally, note that cascading of operations can be applied to an object graph
at call time or at flush time. All operations, if enabled, are cascaded to
associated entities reachable when the operation is executed. However,
save-upate and delete-orphan are transitive for all associated entities
reachable during flush of the Session.
10.12. Using metadata
Hibernate requires a very rich meta-level model of all entity and value types.
From time to time, this model is very useful to the application itself. For
example, the application might use Hibernate's metadata to implement a
"smart" deep-copy algorithm that understands which objects should be
copied (eg. mutable value types) and which should not (eg. immutable value
types and, possibly, associated entities).
Hibernate exposes metadata via the ClassMetadata and CollectionMetadata
interfaces and the Type hierarchy. Instances of the metadata interfaces may
be obtained from the SessionFactory.
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Cat fritz = ......;
ClassMetadata catMeta = sessionfactory.getClassMetadata(Cat.class);
Object[] propertyValues = catMeta.getPropertyValues(fritz);
String[] propertyNames = catMeta.getPropertyNames();
Type[] propertyTypes = catMeta.getPropertyTypes();
// get a Map of all properties which are not collections or
associations
Map namedValues = new HashMap();
for ( int i=0; i<propertyNames.length; i++ ) {
if ( !propertyTypes[i].isEntityType() &&
!propertyTypes[i].isCollectionType() ) {
namedValues.put( propertyNames[i], propertyValues[i] );
}
}
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Concurrency
The most important point about Hibernate and concurrency control is that it
is very easy to understand. Hibernate directly uses JDBC connections and
JTA resources without adding any additional locking behavior. We highly
recommend you spend some time with the JDBC, ANSI, and transaction
isolation specification of your database management system.
Hibernate does not lock objects in memory. Your application can expect
the behavior as defined by the isolation level of your database transactions.
Note that thanks to the Session, which is also a transaction-scoped cache,
Hibernate provides repeatable reads for lookup by identifier and entity
queries (not reporting queries that return scalar values).
In addition to versioning for automatic optimistic concurrency control,
Hibernate also offers a (minor) API for pessimistic locking of rows, using the
SELECT FOR UPDATE syntax. Optimistic concurrency control and this API are
discussed later in this chapter.
We start the discussion of concurrency control in Hibernate with the
granularity of Configuration, SessionFactory, and Session, as well as
database transactions and long conversations.
11.1. Session and transaction scopes
A SessionFactory is an expensive-to-create, threadsafe object intended to be
shared by all application threads. It is created once, usually on application
startup, from a Configuration instance.
A Session is an inexpensive, non-threadsafe object that should be used once,
for a single request, a conversation, single unit of work, and then discarded.
A Session will not obtain a JDBC Connection (or a Datasource) unless it is
needed, hence consume no resources until used.
To complete this picture you also have to think about database transactions.
A database transaction has to be as short as possible, to reduce lock
contention in the database. Long database transactions will prevent your
application from scaling to highly concurrent load. Hence, it is almost never
good design to hold a database transaction open during user think time, until
the unit of work is complete.
What is the scope of a unit of work? Can a single Hibernate Session span
several database transactions or is this a one-to-one relationship of scopes?
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When should you open and close a Session and how do you demarcate the
database transaction boundaries?
11.1.1. Unit of work
First, don't use the session-per-operation antipattern, that is, don't open and
close a Session for every simple database call in a single thread! Of course,
the same is true for database transactions. Database calls in an application
are made using a planned sequence, they are grouped into atomic units of
work. (Note that this also means that auto-commit after every single SQL
statement is useless in an application, this mode is intended for ad-hoc
SQL console work. Hibernate disables, or expects the application server to
do so, auto-commit mode immediately.) Database transactions are never
optional, all communication with a database has to occur inside a transaction,
no matter if you read or write data. As explained, auto-commit behavior for
reading data should be avoided, as many small transactions are unlikely to
perform better than one clearly defined unit of work. The latter is also much
more maintainable and extensible.
The most common pattern in a multi-user client/server application is
session-per-request. In this model, a request from the client is sent to the
server (where the Hibernate persistence layer runs), a new Hibernate Session
is opened, and all database operations are executed in this unit of work.
Once the work has been completed (and the response for the client has been
prepared), the session is flushed and closed. You would also use a single
database transaction to serve the clients request, starting and committing it
when you open and close the Session. The relationship between the two is
one-to-one and this model is a perfect fit for many applications.
The challenge lies in the implementation. Hibernate provides built-in
management of the "current session" to simplify this pattern. All you have
to do is start a transaction when a server request has to be processed, and
end the transaction before the response is sent to the client. You can do this
in any way you like, common solutions are ServletFilter, AOP interceptor
with a pointcut on the service methods, or a proxy/interception container.
An EJB container is a standardized way to implement cross-cutting aspects
such as transaction demarcation on EJB session beans, declaratively with
CMT. If you decide to use programmatic transaction demarcation, prefer the
Hibernate Transaction API shown later in this chapter, for ease of use and
code portability.
Your application code can access a "current session" to process the request
by simply calling sessionFactory.getCurrentSession() anywhere and as often
as needed. You will always get a Session scoped to the current database
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transaction. This has to be configured for either resource-local or JTA
environments, see Section 2.5, “Contextual Sessions”.
Sometimes it is convenient to extend the scope of a Session and database
transaction until the "view has been rendered". This is especially useful in
servlet applications that utilize a separate rendering phase after the request
has been processed. Extending the database transaction until view rendering
is complete is easy to do if you implement your own interceptor. However, it
is not easily doable if you rely on EJBs with container-managed transactions,
as a transaction will be completed when an EJB method returns, before
rendering of any view can start. See the Hibernate website and forum for tips
and examples around this Open Session in View pattern.
11.1.2. Long conversations
The session-per-request pattern is not the only useful concept you can use
to design units of work. Many business processes require a whole series of
interactions with the user interleaved with database accesses. In web and
enterprise applications it is not acceptable for a database transaction to span
a user interaction. Consider the following example:
• The first screen of a dialog opens, the data seen by the user has been
loaded in a particular Session and database transaction. The user is free to
modify the objects.
• The user clicks "Save" after 5 minutes and expects his modifications to be
made persistent; he also expects that he was the only person editing this
information and that no conflicting modification can occur.
We call this unit of work, from the point of view of the user, a long running
conversation (or application transaction). There are many ways how you can
implement this in your application.
A first naive implementation might keep the Session and database transaction
open during user think time, with locks held in the database to prevent
concurrent modification, and to guarantee isolation and atomicity. This is of
course an anti-pattern, since lock contention would not allow the application
to scale with the number of concurrent users.
Clearly, we have to use several database transactions to implement the
conversation. In this case, maintaining isolation of business processes
becomes the partial responsibility of the application tier. A single
conversation usually spans several database transactions. It will be atomic
if only one of these database transactions (the last one) stores the updated
data, all others simply read data (e.g. in a wizard-style dialog spanning
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several request/response cycles). This is easier to implement than it might
sound, especially if you use Hibernate's features:
• Automatic Versioning - Hibernate can do automatic optimistic concurrency
control for you, it can automatically detect if a concurrent modification
occurred during user think time. Usually we only check at the end of the
conversation.
• Detached Objects - If you decide to use the already discussed
session-per-request pattern, all loaded instances will be in detached
state during user think time. Hibernate allows you to reattach
the objects and persist the modifications, the pattern is called
session-per-request-with-detached-objects. Automatic versioning is used to
isolate concurrent modifications.
• Extended (or Long) Session - The Hibernate Session may be disconnected
from the underlying JDBC connection after the database transaction has
been committed, and reconnected when a new client request occurs.
This pattern is known as session-per-conversation and makes even
reattachment unnecessary. Automatic versioning is used to isolate
concurrent modifications and the Session is usually not allowed to be
flushed automatically, but explicitly.
Both session-per-request-with-detached-objects and
session-per-conversation have advantages and disadvantages, we discuss
them later in this chapter in the context of optimistic concurrency control.
11.1.3. Considering object identity
An application may concurrently access the same persistent state in two
different Sessions. However, an instance of a persistent class is never shared
between two Session instances. Hence there are two different notions of
identity:
Database Identity
foo.getId().equals( bar.getId() )
JVM Identity
foo==bar
Then for objects attached to a particular Session (i.e. in the scope of a
Session) the two notions are equivalent, and JVM identity for database
identity is guaranteed by Hibernate. However, while the application might
concurrently access the "same" (persistent identity) business object in two
different sessions, the two instances will actually be "different" (JVM identity).
Conflicts are resolved using (automatic versioning) at flush/commit time,
using an optimistic approach.
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This approach leaves Hibernate and the database to worry about
concurrency; it also provides the best scalability, since guaranteeing identity
in single-threaded units of work only doesn't need expensive locking or other
means of synchronization. The application never needs to synchronize on
any business object, as long as it sticks to a single thread per Session. Within
a Session the application may safely use == to compare objects.
However, an application that uses == outside of a Session, might see
unexpected results. This might occur even in some unexpected places, for
example, if you put two detached instances into the same Set. Both might
have the same database identity (i.e. they represent the same row), but
JVM identity is by definition not guaranteed for instances in detached state.
The developer has to override the equals() and hashCode() methods in
persistent classes and implement his own notion of object equality. There is
one caveat: Never use the database identifier to implement equality, use a
business key, a combination of unique, usually immutable, attributes. The
database identifier will change if a transient object is made persistent. If the
transient instance (usually together with detached instances) is held in a Set,
changing the hashcode breaks the contract of the Set. Attributes for business
keys don't have to be as stable as database primary keys, you only have
to guarantee stability as long as the objects are in the same Set. See the
Hibernate website for a more thorough discussion of this issue. Also note that
this is not a Hibernate issue, but simply how Java object identity and equality
has to be implemented.
11.1.4. Common issues
Never use the anti-patterns session-per-user-session or
session-per-application (of course, there are rare exceptions to this
rule). Note that some of the following issues might also appear with the
recommended patterns, make sure you understand the implications before
making a design decision:
• A Session is not thread-safe. Things which are supposed to work
concurrently, like HTTP requests, session beans, or Swing workers, will
cause race conditions if a Session instance would be shared. If you keep
your Hibernate Session in your HttpSession (discussed later), you should
consider synchronizing access to your Http session. Otherwise, a user that
clicks reload fast enough may use the same Session in two concurrently
running threads.
• An exception thrown by Hibernate means you have to rollback your
database transaction and close the Session immediately (discussed later
in more detail). If your Session is bound to the application, you have to
stop the application. Rolling back the database transaction doesn't put
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your business objects back into the state they were at the start of the
transaction. This means the database state and the business objects do
get out of sync. Usually this is not a problem, because exceptions are not
recoverable and you have to start over after rollback anyway.
• The Session caches every object that is in persistent state (watched and
checked for dirty state by Hibernate). This means it grows endlessly until
you get an OutOfMemoryException, if you keep it open for a long time
or simply load too much data. One solution for this is to call clear() and
evict() to manage the Session cache, but you most likely should consider
a Stored Procedure if you need mass data operations. Some solutions are
shown in Chapter 13, Batch processing. Keeping a Session open for the
duration of a user session also means a high probability of stale data.
11.2. Database transaction demarcation
Database (or system) transaction boundaries are always necessary.
No communication with the database can occur outside of a database
transaction (this seems to confuse many developers who are used to the
auto-commit mode). Always use clear transaction boundaries, even for
read-only operations. Depending on your isolation level and database
capabilities this might not be required but there is no downside if you always
demarcate transactions explicitly. Certainly, a single database transaction is
going to perform better than many small transactions, even for reading data.
A Hibernate application can run in non-managed (i.e. standalone, simple
Web- or Swing applications) and managed J2EE environments. In a
non-managed environment, Hibernate is usually responsible for its own
database connection pool. The application developer has to manually
set transaction boundaries, in other words, begin, commit, or rollback
database transactions himself. A managed environment usually provides
container-managed transactions (CMT), with the transaction assembly
defined declaratively in deployment descriptors of EJB session beans,
for example. Programmatic transaction demarcation is then no longer
necessary.
However, it is often desirable to keep your persistence layer portable
between non-managed resource-local environments, and systems that
can rely on JTA but use BMT instead of CMT. In both cases you'd use
programmatic transaction demarcation. Hibernate offers a wrapper API
called Transaction that translates into the native transaction system of your
deployment environment. This API is actually optional, but we strongly
encourage its use unless you are in a CMT session bean.
Usually, ending a Session involves four distinct phases:
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•
•
•
•
flush the session
commit the transaction
close the session
handle exceptions
Flushing the session has been discussed earlier, we'll now have a closer look
at transaction demarcation and exception handling in both managed- and
non-managed environments.
11.2.1. Non-managed environment
If a Hibernate persistence layer runs in a non-managed environment,
database connections are usually handled by simple (i.e. non-DataSource)
connection pools from which Hibernate obtains connections as needed. The
session/transaction handling idiom looks like this:
// Non-managed environment idiom
Session sess = factory.openSession();
Transaction tx = null;
try {
tx = sess.beginTransaction();
// do some work
...
tx.commit();
}
catch (RuntimeException e) {
if (tx != null) tx.rollback();
throw e; // or display error message
}
finally {
sess.close();
}
You don't have to flush() the Session explicitly - the call to commit()
automatically triggers the synchronization (depending upon the FlushMode
for the session. A call to close() marks the end of a session. The main
implication of close() is that the JDBC connection will be relinquished by the
session. This Java code is portable and runs in both non-managed and JTA
environments.
A much more flexible solution is Hibernate's built-in "current session" context
management, as described earlier:
// Non-managed environment idiom with getCurrentSession()
try {
factory.getCurrentSession().beginTransaction();
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// do some work
...
factory.getCurrentSession().getTransaction().commit();
}
catch (RuntimeException e) {
factory.getCurrentSession().getTransaction().rollback();
throw e; // or display error message
}
You will very likely never see these code snippets in a regular application;
fatal (system) exceptions should always be caught at the "top". In other
words, the code that executes Hibernate calls (in the persistence layer)
and the code that handles RuntimeException (and usually can only clean
up and exit) are in different layers. The current context management by
Hibernate can significantly simplify this design, as all you need is access to a
SessionFactory. Exception handling is discussed later in this chapter.
Note that you should select
(which
is the default), and for the second example "thread" as your
hibernate.current_session_context_class.
org.hibernate.transaction.JDBCTransactionFactory
11.2.2. Using JTA
If your persistence layer runs in an application server (e.g. behind EJB
session beans), every datasource connection obtained by Hibernate will
automatically be part of the global JTA transaction. You can also install a
standalone JTA implementation and use it without EJB. Hibernate offers two
strategies for JTA integration.
If you use bean-managed transactions (BMT) Hibernate will tell the
application server to start and end a BMT transaction if you use the
Transaction API. So, the transaction management code is identical to the
non-managed environment.
// BMT idiom
Session sess = factory.openSession();
Transaction tx = null;
try {
tx = sess.beginTransaction();
// do some work
...
tx.commit();
}
catch (RuntimeException e) {
if (tx != null) tx.rollback();
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throw e; // or display error message
}
finally {
sess.close();
}
If you want to use a transaction-bound Session, that is, the
getCurrentSession() functionality for easy context propagation, you will have
to use the JTA UserTransaction API directly:
// BMT idiom with getCurrentSession()
try {
UserTransaction tx = (UserTransaction)new InitialContext()
.lookup("java:comp/UserTransaction");
tx.begin();
// Do some work on Session bound to transaction
factory.getCurrentSession().load(...);
factory.getCurrentSession().persist(...);
tx.commit();
}
catch (RuntimeException e) {
tx.rollback();
throw e; // or display error message
}
With CMT, transaction demarcation is done in session bean deployment
descriptors, not programmatically, hence, the code is reduced to:
// CMT idiom
Session sess = factory.getCurrentSession();
// do some work
...
In a CMT/EJB even rollback happens automatically, since an unhandled
RuntimeException thrown by a session bean method tells the container to set
the global transaction to rollback. This means you do not need to use the
Hibernate Transaction API at all with BMT or CMT, and you get automatic
propagation of the "current" Session bound to the transaction.
Note that you should choose
if you use JTA directly
(BMT), and org.hibernate.transaction.CMTTransactionFactory in a
CMT session bean, when you configure Hibernate's transaction factory.
Remember to also set hibernate.transaction.manager_lookup_class.
Furthermore, make sure that your hibernate.current_session_context_class
is either unset (backwards compatibility), or set to "jta".
org.hibernate.transaction.JTATransactionFactory
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The getCurrentSession() operation has one downside in a JTA environment.
There is one caveat to the use of after_statement connection release
mode, which is then used by default. Due to a silly limitation of the
JTA spec, it is not possible for Hibernate to automatically clean up any
unclosed ScrollableResults or Iterator instances returned by scroll()
or iterate(). You must release the underlying database cursor by calling
ScrollableResults.close() or Hibernate.close(Iterator) explicitly from a
finally block. (Of course, most applications can easily avoid using scroll()
or iterate() at all from the JTA or CMT code.)
11.2.3. Exception handling
If the Session throws an exception (including any SQLException), you should
immediately rollback the database transaction, call Session.close() and
discard the Session instance. Certain methods of Session will not leave the
session in a consistent state. No exception thrown by Hibernate can be
treated as recoverable. Ensure that the Session will be closed by calling
close() in a finally block.
The HibernateException, which wraps most of the errors that can occur in
a Hibernate persistence layer, is an unchecked exception (it wasn't in older
versions of Hibernate). In our opinion, we shouldn't force the application
developer to catch an unrecoverable exception at a low layer. In most
systems, unchecked and fatal exceptions are handled in one of the first
frames of the method call stack (i.e. in higher layers) and an error message is
presented to the application user (or some other appropriate action is taken).
Note that Hibernate might also throw other unchecked exceptions which are
not a HibernateException. These are, again, not recoverable and appropriate
action should be taken.
Hibernate wraps SQLExceptions thrown while interacting with the database in
a JDBCException. In fact, Hibernate will attempt to convert the exception into
a more meaningful subclass of JDBCException. The underlying SQLException
is always available via JDBCException.getCause(). Hibernate converts
the SQLException into an appropriate JDBCException subclass using the
SQLExceptionConverter attached to the SessionFactory. By default, the
SQLExceptionConverter is defined by the configured dialect; however, it is
also possible to plug in a custom implementation (see the javadocs for the
SQLExceptionConverterFactory class for details). The standard JDBCException
subtypes are:
• JDBCConnectionException - indicates an error with the underlying JDBC
communication.
• SQLGrammarException - indicates a grammar or syntax problem with the
issued SQL.
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• ConstraintViolationException - indicates some form of integrity constraint
violation.
• LockAcquisitionException - indicates an error acquiring a lock level
necessary to perform the requested operation.
• GenericJDBCException - a generic exception which did not fall into any of
the other categories.
11.2.4. Transaction timeout
One extremely important feature provided by a managed environment like
EJB that is never provided for non-managed code is transaction timeout.
Transaction timeouts ensure that no misbehaving transaction can indefinitely
tie up resources while returning no response to the user. Outside a managed
(JTA) environment, Hibernate cannot fully provide this functionality. However,
Hibernate can at least control data access operations, ensuring that
database level deadlocks and queries with huge result sets are limited by
a defined timeout. In a managed environment, Hibernate can delegate
transaction timeout to JTA. This functionality is abstracted by the Hibernate
Transaction object.
Session sess = factory.openSession();
try {
//set transaction timeout to 3 seconds
sess.getTransaction().setTimeout(3);
sess.getTransaction().begin();
// do some work
...
sess.getTransaction().commit()
}
catch (RuntimeException e) {
sess.getTransaction().rollback();
throw e; // or display error message
}
finally {
sess.close();
}
Note that setTimeout() may not be called in a CMT bean, where transaction
timeouts must be defined declaratively.
11.3. Optimistic concurrency control
The only approach that is consistent with high concurrency and high
scalability is optimistic concurrency control with versioning. Version checking
uses version numbers, or timestamps, to detect conflicting updates (and to
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prevent lost updates). Hibernate provides for three possible approaches to
writing application code that uses optimistic concurrency. The use cases we
show are in the context of long conversations, but version checking also has
the benefit of preventing lost updates in single database transactions.
11.3.1. Application version checking
In an implementation without much help from Hibernate, each interaction
with the database occurs in a new Session and the developer is responsible
for reloading all persistent instances from the database before manipulating
them. This approach forces the application to carry out its own version
checking to ensure conversation transaction isolation. This approach is the
least efficient in terms of database access. It is the approach most similar to
entity EJBs.
// foo is an instance loaded by a previous Session
session = factory.openSession();
Transaction t = session.beginTransaction();
int oldVersion = foo.getVersion();
session.load( foo, foo.getKey() ); // load the current state
if ( oldVersion != foo.getVersion() ) throw new
StaleObjectStateException();
foo.setProperty("bar");
t.commit();
session.close();
The version property is mapped using <version>, and Hibernate will
automatically increment it during flush if the entity is dirty.
Of course, if you are operating in a low-data-concurrency environment
and don't require version checking, you may use this approach and just
skip the version check. In that case, last commit wins will be the default
strategy for your long conversations. Keep in mind that this might confuse the
users of the application, as they might experience lost updates without error
messages or a chance to merge conflicting changes.
Clearly, manual version checking is only feasible in very trivial circumstances
and not practical for most applications. Often not only single instances, but
complete graphs of modified objects have to be checked. Hibernate offers
automatic version checking with either an extended Session or detached
instances as the design paradigm.
11.3.2. Extended session and automatic versioning
A single Session instance and its persistent instances are used for the whole
conversation, known as session-per-conversation. Hibernate checks instance
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versions at flush time, throwing an exception if concurrent modification
is detected. It's up to the developer to catch and handle this exception
(common options are the opportunity for the user to merge changes or to
restart the business conversation with non-stale data).
The Session is disconnected from any underlying JDBC connection when
waiting for user interaction. This approach is the most efficient in terms
of database access. The application need not concern itself with version
checking or with reattaching detached instances, nor does it have to reload
instances in every database transaction.
// foo is an instance loaded earlier by the old session
Transaction t = session.beginTransaction(); // Obtain a new JDBC
connection, start transaction
foo.setProperty("bar");
session.flush();
t.commit();
session.close();
// Only for last transaction in conversation
// Also return JDBC connection
// Only for last transaction in conversation
The foo object still knows which Session it was loaded in. Beginning a new
database transaction on an old session obtains a new connection and
resumes the session. Committing a database transaction disconnects a
session from the JDBC connection and returns the connection to the pool.
After reconnection, to force a version check on data you aren't updating, you
may call Session.lock() with LockMode.READ on any objects that might have
been updated by another transaction. You don't need to lock any data that
you are updating. Usually you would set FlushMode.MANUAL on an extended
Session, so that only the last database transaction cycle is allowed to actually
persist all modifications made in this conversation. Hence, only this last
database transaction would include the flush() operation, and then also
close() the session to end the conversation.
This pattern is problematic if the Session is too big to be stored during user
think time, e.g. an HttpSession should be kept as small as possible. As the
Session is also the (mandatory) first-level cache and contains all loaded
objects, we can probably use this strategy only for a few request/response
cycles. You should use a Session only for a single conversation, as it will
soon also have stale data.
(Note that earlier Hibernate versions required explicit disconnection and
reconnection of a Session. These methods are deprecated, as beginning and
ending a transaction has the same effect.)
Also note that you should keep the disconnected Session close to the
persistence layer. In other words, use an EJB stateful session bean to hold
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the Session in a three-tier environment, and don't transfer it to the web layer
(or even serialize it to a separate tier) to store it in the HttpSession.
The extended session pattern, or session-per-conversation, is more difficult
to implement with automatic current session context management. You need
to supply your own implementation of the CurrentSessionContext for this, see
the Hibernate Wiki for examples.
11.3.3. Detached objects and automatic versioning
Each interaction with the persistent store occurs in a new Session. However,
the same persistent instances are reused for each interaction with the
database. The application manipulates the state of detached instances
originally loaded in another Session and then reattaches them using
Session.update(), Session.saveOrUpdate(), or Session.merge().
// foo is an instance loaded by a previous Session
foo.setProperty("bar");
session = factory.openSession();
Transaction t = session.beginTransaction();
session.saveOrUpdate(foo); // Use merge() if "foo" might have been
loaded already
t.commit();
session.close();
Again, Hibernate will check instance versions during flush, throwing an
exception if conflicting updates occurred.
You may also call lock() instead of update() and use LockMode.READ
(performing a version check, bypassing all caches) if you are sure that the
object has not been modified.
11.3.4. Customizing automatic versioning
You may disable Hibernate's automatic version increment for particular
properties and collections by setting the optimistic-lock mapping attribute to
false. Hibernate will then no longer increment versions if the property is dirty.
Legacy database schemas are often static and can't be modified. Or, other
applications might also access the same database and don't know how
to handle version numbers or even timestamps. In both cases, versioning
can't rely on a particular column in a table. To force a version check
without a version or timestamp property mapping, with a comparison
of the state of all fields in a row, turn on optimistic-lock="all" in the
<class> mapping. Note that this conceptually only works if Hibernate can
compare the old and new state, i.e. if you use a single long Session and not
session-per-request-with-detached-objects.
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Sometimes concurrent modification can be permitted as long as the changes
that have been made don't overlap. If you set optimistic-lock="dirty" when
mapping the <class>, Hibernate will only compare dirty fields during flush.
In both cases, with dedicated version/timestamp columns or with full/dirty
field comparison, Hibernate uses a single UPDATE statement (with an
appropriate WHERE clause) per entity to execute the version check and update
the information. If you use transitive persistence to cascade reattachment
to associated entities, Hibernate might execute unnecessary updates. This
is usually not a problem, but on update triggers in the database might be
executed even when no changes have been made to detached instances.
You can customize this behavior by setting select-before-update="true" in
the <class> mapping, forcing Hibernate to SELECT the instance to ensure that
changes did actually occur, before updating the row.
11.4. Pessimistic Locking
It is not intended that users spend much time worrying about locking
strategies. It's usually enough to specify an isolation level for the JDBC
connections and then simply let the database do all the work. However,
advanced users may sometimes wish to obtain exclusive pessimistic locks, or
re-obtain locks at the start of a new transaction.
Hibernate will always use the locking mechanism of the database, never lock
objects in memory!
The LockMode class defines the different lock levels that may be acquired by
Hibernate. A lock is obtained by the following mechanisms:
• LockMode.WRITE is acquired automatically when Hibernate updates or
inserts a row.
• LockMode.UPGRADE may be acquired upon explicit user request using SELECT
... FOR UPDATE on databases which support that syntax.
• LockMode.UPGRADE_NOWAIT may be acquired upon explicit user request using
a SELECT ... FOR UPDATE NOWAIT under Oracle.
• LockMode.READ is acquired automatically when Hibernate reads data under
Repeatable Read or Serializable isolation level. May be re-acquired by
explicit user request.
• LockMode.NONE represents the absence of a lock. All objects switch to this
lock mode at the end of a Transaction. Objects associated with the session
via a call to update() or saveOrUpdate() also start out in this lock mode.
The "explicit user request" is expressed in one of the following ways:
• A call to Session.load(), specifying a LockMode.
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• A call to Session.lock().
• A call to Query.setLockMode().
If Session.load() is called with UPGRADE or UPGRADE_NOWAIT, and the requested
object was not yet loaded by the session, the object is loaded using SELECT
... FOR UPDATE. If load() is called for an object that is already loaded with a
less restrictive lock than the one requested, Hibernate calls lock() for that
object.
performs a version number check if the specified lock mode
is READ, UPGRADE or UPGRADE_NOWAIT. (In the case of UPGRADE or UPGRADE_NOWAIT,
SELECT ... FOR UPDATE is used.)
Session.lock()
If the database does not support the requested lock mode, Hibernate will
use an appropriate alternate mode (instead of throwing an exception). This
ensures that applications will be portable.
11.5. Connection Release Modes
The legacy (2.x) behavior of Hibernate in regards to JDBC connection
management was that a Session would obtain a connection when it was first
needed and then hold unto that connection until the session was closed.
Hibernate 3.x introduced the notion of connection release modes to tell
a session how to handle its JDBC connections. Note that the following
discussion is pertinent only to connections provided through a configured
ConnectionProvider; user-supplied connections are outside the breadth of
this discussion. The different release modes are identified by the enumerated
values of org.hibernate.ConnectionReleaseMode:
• ON_CLOSE - is essentially the legacy behavior described above. The
Hibernate session obtains a connection when it first needs to perform
some JDBC access and holds unto that connection until the session is
closed.
• AFTER_TRANSACTION - says to release connections after a
org.hibernate.Transaction has completed.
• AFTER_STATEMENT (also referred to as aggressive release) - says to release
connections after each and every statement execution. This aggressive
releasing is skipped if that statement leaves open resources associated
with the given session; currently the only situation where this occurs is
through the use of org.hibernate.ScrollableResults.
The configuration parameter hibernate.connection.release_mode is used to
specify which release mode to use. The possible values:
• auto (the default) - this choice delegates
to the release mode returned by the
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org.hibernate.transaction.TransactionFactory.getDefaultReleaseMode()
method. For JTATransactionFactory, this returns
ConnectionReleaseMode.AFTER_STATEMENT;
for JDBCTransactionFactory, this returns
ConnectionReleaseMode.AFTER_TRANSACTION. It is rarely a good idea
to change this default behavior as failures due to the value of this setting
tend to indicate bugs and/or invalid assumptions in user code.
• on_close - says to use ConnectionReleaseMode.ON_CLOSE. This setting
is left for backwards compatibility, but its use is highly discouraged.
• after_transaction - says to use
ConnectionReleaseMode.AFTER_TRANSACTION. This setting
should not be used in JTA environments. Also note that with
ConnectionReleaseMode.AFTER_TRANSACTION, if a session is
considered to be in auto-commit mode connections will be released as if
the release mode were AFTER_STATEMENT.
• after_statement - says to use
ConnectionReleaseMode.AFTER_STATEMENT. Additionally, the
configured ConnectionProvider is consulted to see if it supports this
setting (supportsAggressiveRelease()). If not, the release mode is
reset to ConnectionReleaseMode.AFTER_TRANSACTION. This
setting is only safe in environments where we can either re-acquire
the same underlying JDBC connection each time we make a call into
ConnectionProvider.getConnection() or in auto-commit environments
where it does not matter whether we get back the same connection.
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It is often useful for the application to react to certain events that occur
inside Hibernate. This allows implementation of certain kinds of generic
functionality, and extension of Hibernate functionality.
12.1. Interceptors
The Interceptor interface provides callbacks from the session to the
application allowing the application to inspect and/or manipulate properties
of a persistent object before it is saved, updated, deleted or loaded. One
possible use for this is to track auditing information. For example, the
following Interceptor automatically sets the createTimestamp when an
Auditable is created and updates the lastUpdateTimestamp property when an
Auditable is updated.
You may either implement Interceptor directly or (better) extend
EmptyInterceptor.
package org.hibernate.test;
import java.io.Serializable;
import java.util.Date;
import java.util.Iterator;
import org.hibernate.EmptyInterceptor;
import org.hibernate.Transaction;
import org.hibernate.type.Type;
public class AuditInterceptor extends EmptyInterceptor {
private int updates;
private int creates;
private int loads;
public void onDelete(Object entity,
Serializable id,
Object[] state,
String[] propertyNames,
Type[] types) {
// do nothing
}
public boolean onFlushDirty(Object entity,
Serializable id,
Object[] currentState,
Object[] previousState,
String[] propertyNames,
Type[] types) {
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if ( entity instanceof Auditable ) {
updates++;
for ( int i=0; i < propertyNames.length; i++ ) {
if ( "lastUpdateTimestamp".equals( propertyNames[i]
) ) {
currentState[i] = new Date();
return true;
}
}
}
return false;
}
public boolean onLoad(Object entity,
Serializable id,
Object[] state,
String[] propertyNames,
Type[] types) {
if ( entity instanceof Auditable ) {
loads++;
}
return false;
}
public boolean onSave(Object entity,
Serializable id,
Object[] state,
String[] propertyNames,
Type[] types) {
if ( entity instanceof Auditable ) {
creates++;
for ( int i=0; i<propertyNames.length; i++ ) {
if ( "createTimestamp".equals( propertyNames[i] ) )
{
state[i] = new Date();
return true;
}
}
}
return false;
}
public void afterTransactionCompletion(Transaction tx) {
if ( tx.wasCommitted() ) {
System.out.println("Creations: " + creates + ", Updates:
" + updates, "Loads: " + loads);
}
updates=0;
creates=0;
loads=0;
}
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}
Interceptors come in two flavors: Session-scoped and
SessionFactory-scoped.
A Session-scoped interceptor is specified when a session is opened using
one of the overloaded SessionFactory.openSession() methods accepting an
Interceptor.
Session session = sf.openSession( new AuditInterceptor() );
A SessionFactory-scoped interceptor is registered with the Configuration
object prior to building the SessionFactory. In this case, the supplied
interceptor will be applied to all sessions opened from that SessionFactory;
this is true unless a session is opened explicitly specifying the interceptor
to use. SessionFactory-scoped interceptors must be thread safe, taking
care to not store session-specific state since multiple sessions will use this
interceptor (potentially) concurrently.
new Configuration().setInterceptor( new AuditInterceptor() );
12.2. Event system
If you have to react to particular events in your persistence layer, you may
also use the Hibernate3 event architecture. The event system can be used in
addition or as a replacement for interceptors.
Essentially all of the methods of the Session interface correlate to an event.
You have a LoadEvent, a FlushEvent, etc (consult the XML configuration-file
DTD or the org.hibernate.event package for the full list of defined event
types). When a request is made of one of these methods, the Hibernate
Session generates an appropriate event and passes it to the configured event
listeners for that type. Out-of-the-box, these listeners implement the same
processing in which those methods always resulted. However, you are free to
implement a customization of one of the listener interfaces (i.e., the LoadEvent
is processed by the registered implemenation of the LoadEventListener
interface), in which case their implementation would be responsible for
processing any load() requests made of the Session.
The listeners should be considered effectively singletons; meaning, they are
shared between requests, and thus should not save any state as instance
variables.
A custom listener should implement the appropriate interface for the event
it wants to process and/or extend one of the convenience base classes (or
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even the default event listeners used by Hibernate out-of-the-box as these
are declared non-final for this purpose). Custom listeners can either be
registered programmatically through the Configuration object, or specified
in the Hibernate configuration XML (declarative configuration through the
properties file is not supported). Here's an example of a custom load event
listener:
public class MyLoadListener implements LoadEventListener {
// this is the single method defined by the LoadEventListener
interface
public void onLoad(LoadEvent event, LoadEventListener.LoadType
loadType)
throws HibernateException {
if ( !MySecurity.isAuthorized( event.getEntityClassName(),
event.getEntityId() ) ) {
throw MySecurityException("Unauthorized access");
}
}
}
You also need a configuration entry telling Hibernate to use the listener in
addition to the default listener:
<hibernate-configuration>
<session-factory>
...
<event type="load">
<listener class="com.eg.MyLoadListener"/>
<listener
class="org.hibernate.event.def.DefaultLoadEventListener"/>
</event>
</session-factory>
</hibernate-configuration>
Instead, you may register it programmatically:
Configuration cfg = new Configuration();
LoadEventListener[] stack = { new MyLoadListener(), new
DefaultLoadEventListener() };
cfg.EventListeners().setLoadEventListeners(stack);
Listeners registered declaratively cannot share instances. If the same class
name is used in multiple <listener/> elements, each reference will result in
a separate instance of that class. If you need the capability to share listener
instances between listener types you must use the programmatic registration
approach.
Why implement an interface and define the specific type during
configuration? Well, a listener implementation could implement multiple event
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listener interfaces. Having the type additionally defined during registration
makes it easier to turn custom listeners on or off during configuration.
12.3. Hibernate declarative security
Usually, declarative security in Hibernate applications is managed in
a session facade layer. Now, Hibernate3 allows certain actions to be
permissioned via JACC, and authorized via JAAS. This is optional
functionality built on top of the event architecture.
First, you must configure the appropriate event listeners, to enable the use of
JAAS authorization.
<listener type="pre-delete"
class="org.hibernate.secure.JACCPreDeleteEventListener"/>
<listener type="pre-update"
class="org.hibernate.secure.JACCPreUpdateEventListener"/>
<listener type="pre-insert"
class="org.hibernate.secure.JACCPreInsertEventListener"/>
<listener type="pre-load"
class="org.hibernate.secure.JACCPreLoadEventListener"/>
Note that <listener type="..." class="..."/> is just a shorthand for <event
type="..."><listener class="..."/></event> when there is exactly one
listener for a particular event type.
Next, still in hibernate.cfg.xml, bind the permissions to roles:
<grant role="admin" entity-name="User"
actions="insert,update,read"/>
<grant role="su" entity-name="User" actions="*"/>
The role names are the roles understood by your JACC provider.
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A naive approach to inserting 100 000 rows in the database using Hibernate
might look like this:
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
Customer customer = new Customer(.....);
session.save(customer);
}
tx.commit();
session.close();
This would fall over with an OutOfMemoryException somewhere around the 50
000th row. That's because Hibernate caches all the newly inserted Customer
instances in the session-level cache.
In this chapter we'll show you how to avoid this problem. First, however, if
you are doing batch processing, it is absolutely critical that you enable the
use of JDBC batching, if you intend to achieve reasonable performance. Set
the JDBC batch size to a reasonable number (say, 10-50):
hibernate.jdbc.batch_size 20
Note that Hibernate disables insert batching at the JDBC level transparently if
you use an identiy identifier generator.
You also might like to do this kind of work in a process where interaction with
the second-level cache is completely disabled:
hibernate.cache.use_second_level_cache false
However, this is not absolutely necessary, since we can explicitly set the
CacheMode to disable interaction with the second-level cache.
13.1. Batch inserts
When making new objects persistent, you must flush() and then clear() the
session regularly, to control the size of the first-level cache.
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
Customer customer = new Customer(.....);
session.save(customer);
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if ( i % 20 == 0 ) { //20, same as the JDBC batch size
//flush a batch of inserts and release memory:
session.flush();
session.clear();
}
}
tx.commit();
session.close();
13.2. Batch updates
For retrieving and updating data the same ideas apply. In addition, you need
to use scroll() to take advantage of server-side cursors for queries that
return many rows of data.
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
ScrollableResults customers = session.getNamedQuery("GetCustomers")
.setCacheMode(CacheMode.IGNORE)
.scroll(ScrollMode.FORWARD_ONLY);
int count=0;
while ( customers.next() ) {
Customer customer = (Customer) customers.get(0);
customer.updateStuff(...);
if ( ++count % 20 == 0 ) {
//flush a batch of updates and release memory:
session.flush();
session.clear();
}
}
tx.commit();
session.close();
13.3. The StatelessSession interface
Alternatively, Hibernate provides a command-oriented API that may be used
for streaming data to and from the database in the form of detached objects.
A StatelessSession has no persistence context associated with it and does
not provide many of the higher-level life cycle semantics. In particular,
a stateless session does not implement a first-level cache nor interact
with any second-level or query cache. It does not implement transactional
write-behind or automatic dirty checking. Operations performed using a
stateless session do not ever cascade to associated instances. Collections
are ignored by a stateless session. Operations performed via a stateless
session bypass Hibernate's event model and interceptors. Stateless sessions
are vulnerable to data aliasing effects, due to the lack of a first-level cache. A
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stateless session is a lower-level abstraction, much closer to the underlying
JDBC.
StatelessSession session = sessionFactory.openStatelessSession();
Transaction tx = session.beginTransaction();
ScrollableResults customers = session.getNamedQuery("GetCustomers")
.scroll(ScrollMode.FORWARD_ONLY);
while ( customers.next() ) {
Customer customer = (Customer) customers.get(0);
customer.updateStuff(...);
session.update(customer);
}
tx.commit();
session.close();
Note that in this code example, the Customer instances returned by the query
are immediately detached. They are never associated with any persistence
context.
The insert(), update() and delete() operations defined by the
StatelessSession interface are considered to be direct database row-level
operations, which result in immediate execution of a SQL INSERT, UPDATE or
DELETE respectively. Thus, they have very different semantics to the save(),
saveOrUpdate() and delete() operations defined by the Session interface.
13.4. DML-style operations
As already discussed, automatic and transparent object/relational mapping
is concerned with the management of object state. This implies that the
object state is available in memory, hence manipulating (using the SQL
Data Manipulation Language (DML) statements: INSERT, UPDATE, DELETE) data
directly in the database will not affect in-memory state. However, Hibernate
provides methods for bulk SQL-style DML statement execution which are
performed through the Hibernate Query Language (HQL).
The pseudo-syntax for UPDATE and DELETE statements is: ( UPDATE | DELETE )
FROM? EntityName (WHERE where_conditions)?. Some points to note:
• In the from-clause, the FROM keyword is optional
• There can only be a single entity named in the from-clause; it can
optionally be aliased. If the entity name is aliased, then any property
references must be qualified using that alias; if the entity name is not
aliased, then it is illegal for any property references to be qualified.
• No joins (either implicit or explicit) can be specified in a bulk HQL
query. Sub-queries may be used in the where-clause; the subqueries,
themselves, may contain joins.
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• The where-clause is also optional.
As an example, to execute an HQL UPDATE, use the Query.executeUpdate()
method (the method is named for those familiar with JDBC's
PreparedStatement.executeUpdate()):
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlUpdate = "update Customer c set c.name = :newName where
c.name = :oldName";
// or String hqlUpdate = "update Customer set name = :newName where
name = :oldName";
int updatedEntities = s.createQuery( hqlUpdate )
.setString( "newName", newName )
.setString( "oldName", oldName )
.executeUpdate();
tx.commit();
session.close();
HQL UPDATE statements, by default do not effect the version or the timestamp
property values for the affected entities; this is in keeping with the EJB3
specification. However, you can force Hibernate to properly reset the version
or timestamp property values through the use of a versioned update. This is
achieved by adding the VERSIONED keyword after the UPDATE keyword.
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlVersionedUpdate = "update versioned Customer set name =
:newName where name = :oldName";
int updatedEntities = s.createQuery( hqlUpdate )
.setString( "newName", newName )
.setString( "oldName", oldName )
.executeUpdate();
tx.commit();
session.close();
Note that custom version types (org.hibernate.usertype.UserVersionType)
are not allowed in conjunction with a update versioned statement.
To execute an HQL DELETE, use the same Query.executeUpdate() method:
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlDelete = "delete Customer c where c.name = :oldName";
// or String hqlDelete = "delete Customer where name = :oldName";
int deletedEntities = s.createQuery( hqlDelete )
.setString( "oldName", oldName )
.executeUpdate();
tx.commit();
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session.close();
The int value returned by the Query.executeUpdate() method indicate the
number of entities effected by the operation. Consider this may or may
not correlate to the number of rows effected in the database. An HQL bulk
operation might result in multiple actual SQL statements being executed, for
joined-subclass, for example. The returned number indicates the number
of actual entities affected by the statement. Going back to the example of
joined-subclass, a delete against one of the subclasses may actually result
in deletes against not just the table to which that subclass is mapped, but
also the "root" table and potentially joined-subclass tables further down the
inheritence hierarchy.
The pseudo-syntax for INSERT statements is: INSERT INTO EntityName
properties_list select_statement. Some points to note:
• Only the INSERT INTO ... SELECT ... form is supported; not the INSERT
INTO ... VALUES ... form.
The properties_list is analogous to the column speficiation in the SQL
INSERT statement. For entities involved in mapped inheritence, only
properties directly defined on that given class-level can be used in the
properties_list. Superclass properties are not allowed; and subclass
properties do not make sense. In other words, INSERT statements are
inherently non-polymorphic.
• select_statement can be any valid HQL select query, with the caveat
that the return types must match the types expected by the insert.
Currently, this is checked during query compilation rather than allowing
the check to relegate to the database. Note however that this might cause
problems between Hibernate Types which are equivalent as opposed
to equal. This might cause issues with mismatches between a property
defined as a org.hibernate.type.DateType and a property defined as a
org.hibernate.type.TimestampType, even though the database might not
make a distinction or might be able to handle the conversion.
• For the id property, the insert statement gives you two options. You can
either explicitly specify the id property in the properties_list (in which case
its value is taken from the corresponding select expression) or omit it
from the properties_list (in which case a generated value is used). This
later option is only available when using id generators that operate in
the database; attempting to use this option with any "in memory" type
generators will cause an exception during parsing. Note that for the
purposes of this discussion, in-database generators are considered to
be org.hibernate.id.SequenceGenerator (and its subclasses) and any
implementors of org.hibernate.id.PostInsertIdentifierGenerator. The
most notable exception here is org.hibernate.id.TableHiLoGenerator,
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which cannot be used because it does not expose a selectable way to get
its values.
• For properties mapped as either version or timestamp, the insert
statement gives you two options. You can either specify the property in
the properties_list (in which case its value is taken from the corresponding
select expressions) or omit it from the properties_list (in which case the
seed value defined by the org.hibernate.type.VersionType is used).
An example HQL INSERT statement execution:
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlInsert = "insert into DelinquentAccount (id, name) select
c.id, c.name from Customer c where ...";
int createdEntities = s.createQuery( hqlInsert )
.executeUpdate();
tx.commit();
session.close();
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Chapter 14. HQL: The Hibernate
Query Language
Hibernate is equipped with an extremely powerful query language that
(quite intentionally) looks very much like SQL. But don't be fooled by the
syntax; HQL is fully object-oriented, understanding notions like inheritence,
polymorphism and association.
14.1. Case Sensitivity
Queries are case-insensitive, except for names of Java classes and
properties. So SeLeCT is the same as sELEct is the same as SELECT but
org.hibernate.eg.FOO is not org.hibernate.eg.Foo and foo.barSet is not
foo.BARSET.
This manual uses lowercase HQL keywords. Some users find queries with
uppercase keywords more readable, but we find this convention ugly when
embedded in Java code.
14.2. The from clause
The simplest possible Hibernate query is of the form:
from eg.Cat
which simply returns all instances of the class eg.Cat. We don't usually need
to qualify the class name, since auto-import is the default. So we almost
always just write:
from Cat
Most of the time, you will need to assign an alias, since you will want to refer
to the Cat in other parts of the query.
from Cat as cat
This query assigns the alias cat to Cat instances, so we could use that alias
later in the query. The as keyword is optional; we could also write:
from Cat cat
Multiple classes may appear, resulting in a cartesian product or "cross" join.
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from Formula, Parameter
from Formula as form, Parameter as param
It is considered good practice to name query aliases using an initial
lowercase, consistent with Java naming standards for local variables (eg.
domesticCat).
14.3. Associations and joins
We may also assign aliases to associated entities, or even to elements of a
collection of values, using a join.
from Cat as cat
inner join cat.mate as mate
left outer join cat.kittens as kitten
from Cat as cat left join cat.mate.kittens as kittens
from Formula form full join form.parameter param
The supported join types are borrowed from ANSI SQL
•
•
•
•
inner join
left outer join
right outer join
full join
(not usually useful)
The inner join, left outer join and right outer join constructs may be
abbreviated.
from Cat as cat
join cat.mate as mate
left join cat.kittens as kitten
You may supply extra join conditions using the HQL with keyword.
from Cat as cat
left join cat.kittens as kitten
with kitten.bodyWeight > 10.0
In addition, a "fetch" join allows associations or collections of values to
be initialized along with their parent objects, using a single select. This is
particularly useful in the case of a collection. It effectively overrides the outer
join and lazy declarations of the mapping file for associations and collections.
See Section 19.1, “Fetching strategies” for more information.
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from Cat as cat
inner join fetch cat.mate
left join fetch cat.kittens
A fetch join does not usually need to assign an alias, because the associated
objects should not be used in the where clause (or any other clause). Also,
the associated objects are not returned directly in the query results. Instead,
they may be accessed via the parent object. The only reason we might need
an alias is if we are recursively join fetching a further collection:
from Cat as cat
inner join fetch cat.mate
left join fetch cat.kittens child
left join fetch child.kittens
Note that the fetch construct may not be used in queries called using
iterate() (though scroll() can be used). Nor should fetch be used together
with setMaxResults() or setFirstResult() as these operations are based on
the result rows, which usually contain duplicates for eager collection fetching,
hence, the number of rows is not what you'd expect. Nor may fetch be used
together with an ad hoc with condition. It is possible to create a cartesian
product by join fetching more than one collection in a query, so take care
in this case. Join fetching multiple collection roles also sometimes gives
unexpected results for bag mappings, so be careful about how you formulate
your queries in this case. Finally, note that full join fetch and right join
fetch are not meaningful.
If you are using property-level lazy fetching (with bytecode instrumentation), it
is possible to force Hibernate to fetch the lazy properties immediately (in the
first query) using fetch all properties.
from Document fetch all properties order by name
from Document doc fetch all properties where lower(doc.name) like
'%cats%'
14.4. Forms of join syntax
HQL supports two forms of association joining: implicit and explicit.
The queries shown in the previous section all use the explicit form
where the join keyword is explicitly used in the from clause. This is the
recommended form.
The implicit form does not use the join keyword. Instead, the associations
are "dereferenced" using dot-notation. implicit joins can appear in any
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of the HQL clauses. implicit join result in inner joins in the resulting SQL
statement.
from Cat as cat where cat.mate.name like '%s%'
14.5. Refering to identifier property
There are, generally speaking, 2 ways to refer to an entity's identifier
property:
• The special property (lowercase) id may be used to reference the identifier
property of an entity provided that entity does not define a non-identifier
property named id.
• If the entity defines a named identifier property, you may use that property
name.
References to composite identifier properties follow the same naming rules.
If the entity has a non-identifier property named id, the composite identifier
property can only be referenced by its defined named; otherwise, the special
id property can be used to rerference the identifier property.
Note: this has changed significantly starting in version 3.2.2. In previous
versions, id always referred to the identifier property no matter what its actual
name. A ramification of that decision was that non-identifier properties named
id could never be referenced in Hibernate queries.
14.6. The select clause
The select clause picks which objects and properties to return in the query
result set. Consider:
select mate
from Cat as cat
inner join cat.mate as mate
The query will select mates of other Cats. Actually, you may express this
query more compactly as:
select cat.mate from Cat cat
Queries may return properties of any value type including properties of
component type:
select cat.name from DomesticCat cat
where cat.name like 'fri%'
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select cust.name.firstName from Customer as cust
Queries may return multiple objects and/or properties as an array of type
Object[],
select mother, offspr, mate.name
from DomesticCat as mother
inner join mother.mate as mate
left outer join mother.kittens as offspr
or as a List,
select new list(mother, offspr, mate.name)
from DomesticCat as mother
inner join mother.mate as mate
left outer join mother.kittens as offspr
or as an actual typesafe Java object,
select new Family(mother, mate, offspr)
from DomesticCat as mother
join mother.mate as mate
left join mother.kittens as offspr
assuming that the class Family has an appropriate constructor.
You may assign aliases to selected expressions using as:
select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
from Cat cat
This is most useful when used together with select new map:
select new map( max(bodyWeight) as max, min(bodyWeight) as min,
count(*) as n )
from Cat cat
This query returns a Map from aliases to selected values.
14.7. Aggregate functions
HQL queries may even return the results of aggregate functions on
properties:
select avg(cat.weight), sum(cat.weight), max(cat.weight), count(cat)
from Cat cat
The supported aggregate functions are
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• avg(...), sum(...), min(...), max(...)
• count(*)
• count(...), count(distinct ...), count(all...)
You may use arithmetic operators, concatenation, and recognized SQL
functions in the select clause:
select cat.weight + sum(kitten.weight)
from Cat cat
join cat.kittens kitten
group by cat.id, cat.weight
select firstName||' '||initial||' '||upper(lastName) from Person
The distinct and all keywords may be used and have the same semantics
as in SQL.
select distinct cat.name from Cat cat
select count(distinct cat.name), count(cat) from Cat cat
14.8. Polymorphic queries
A query like:
from Cat as cat
returns instances not only of Cat, but also of subclasses like DomesticCat.
Hibernate queries may name any Java class or interface in the from clause.
The query will return instances of all persistent classes that extend that class
or implement the interface. The following query would return all persistent
objects:
from java.lang.Object o
The interface Named might be implemented by various persistent classes:
from Named n, Named m where n.name = m.name
Note that these last two queries will require more than one SQL SELECT. This
means that the order by clause does not correctly order the whole result set.
(It also means you can't call these queries using Query.scroll().)
14.9. The where clause
The where clause allows you to narrow the list of instances returned. If no
alias exists, you may refer to properties by name:
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from Cat where name='Fritz'
If there is an alias, use a qualified property name:
from Cat as cat where cat.name='Fritz'
returns instances of Cat named 'Fritz'.
select foo
from Foo foo, Bar bar
where foo.startDate = bar.date
will return all instances of Foo for which there exists an instance of bar with
a date property equal to the startDate property of the Foo. Compound path
expressions make the where clause extremely powerful. Consider:
from Cat cat where cat.mate.name is not null
This query translates to an SQL query with a table (inner) join. If you were to
write something like
from Foo foo
where foo.bar.baz.customer.address.city is not null
you would end up with a query that would require four table joins in SQL.
The = operator may be used to compare not only properties, but also
instances:
from Cat cat, Cat rival where cat.mate = rival.mate
select cat, mate
from Cat cat, Cat mate
where cat.mate = mate
The special property (lowercase) id may be used to reference the unique
identifier of an object. See Section 14.5, “Refering to identifier property” for
more information.
from Cat as cat where cat.id = 123
from Cat as cat where cat.mate.id = 69
The second query is efficient. No table join is required!
Properties of composite identifiers may also be used. Suppose Person has
a composite identifier consisting of country and medicareNumber. Again, see
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Section 14.5, “Refering to identifier property” for more information regarding
referencing identifier properties.
from bank.Person person
where person.id.country = 'AU'
and person.id.medicareNumber = 123456
from bank.Account account
where account.owner.id.country = 'AU'
and account.owner.id.medicareNumber = 123456
Once again, the second query requires no table join.
Likewise, the special property class accesses the discriminator value of
an instance in the case of polymorphic persistence. A Java class name
embedded in the where clause will be translated to its discriminator value.
from Cat cat where cat.class = DomesticCat
You may also use components or composite user types, or properties of said
component types. See Section 14.17, “Components” for more details.
An "any" type has the special properties id and class, allowing us to express
a join in the following way (where AuditLog.item is a property mapped with
<any>).
from AuditLog log, Payment payment
where log.item.class = 'Payment' and log.item.id = payment.id
Notice that log.item.class and payment.class would refer to the values of
completely different database columns in the above query.
14.10. Expressions
Expressions allowed in the where clause include most of the kind of things
you could write in SQL:
•
•
•
•
•
mathematical operators +, -, *, /
binary comparison operators =, >=, <=, <>, !=, like
logical operations and, or, not
Parentheses ( ), indicating grouping
in, not in, between, is null, is not null, is empty, is not empty, member
of and not member of
• "Simple" case, case ... when ... then ... else ... end, and "searched"
case, case when ... then ... else ... end
• string concatenation ...||... or concat(...,...)
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• current_date(), current_time(), current_timestamp()
• second(...), minute(...), hour(...), day(...), month(...), year(...),
• Any function or operator defined by EJB-QL 3.0: substring(), trim(),
lower(), upper(), length(), locate(), abs(), sqrt(), bit_length(),
mod()
• coalesce() and nullif()
• str() for converting numeric or temporal values to a readable string
• cast(... as ...), where the second argument is the name of a Hibernate
type, and extract(... from ...) if ANSI cast() and extract() is supported
by the underlying database
• the HQL index() function, that applies to aliases of a joined indexed
collection
• HQL functions that take collection-valued path expressions: size(),
minelement(), maxelement(), minindex(), maxindex(), along with the
special elements() and indices functions which may be quantified using
some, all, exists, any, in.
• Any database-supported SQL scalar function like sign(), trunc(), rtrim(),
sin()
•
•
•
•
JDBC-style positional parameters ?
named parameters :name, :start_date, :x1
SQL literals 'foo', 69, 6.66E+2, '1970-01-01 10:00:01.0'
Java public static final constants eg.Color.TABBY
in
and between may be used as follows:
from DomesticCat cat where cat.name between 'A' and 'B'
from DomesticCat cat where cat.name in ( 'Foo', 'Bar', 'Baz' )
and the negated forms may be written
from DomesticCat cat where cat.name not between 'A' and 'B'
from DomesticCat cat where cat.name not in ( 'Foo', 'Bar', 'Baz' )
Likewise, is null and is not null may be used to test for null values.
Booleans may be easily used in expressions by declaring HQL query
substitutions in Hibernate configuration:
<property name="hibernate.query.substitutions">true 1, false
0</property>
This will replace the keywords true and false with the literals 1 and 0 in the
translated SQL from this HQL:
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from Cat cat where cat.alive = true
You may test the size of a collection with the special property size, or the
special size() function.
from Cat cat where cat.kittens.size > 0
from Cat cat where size(cat.kittens) > 0
For indexed collections, you may refer to the minimum and maximum
indices using minindex and maxindex functions. Similarly, you may refer to
the minimum and maximum elements of a collection of basic type using the
minelement and maxelement functions.
from Calendar cal where maxelement(cal.holidays) > current_date
from Order order where maxindex(order.items) > 100
from Order order where minelement(order.items) > 10000
The SQL functions any, some, all, exists, in are supported when passed
the element or index set of a collection (elements and indices functions) or
the result of a subquery (see below).
select mother from Cat as mother, Cat as kit
where kit in elements(foo.kittens)
select p from NameList list, Person p
where p.name = some elements(list.names)
from Cat cat where exists elements(cat.kittens)
from Player p where 3 > all elements(p.scores)
from Show show where 'fizard' in indices(show.acts)
Note that these constructs - size, elements, indices, minindex, maxindex,
minelement, maxelement - may only be used in the where clause in Hibernate3.
Elements of indexed collections (arrays, lists, maps) may be referred to by
index (in a where clause only):
from Order order where order.items[0].id = 1234
select person from Person person, Calendar calendar
where calendar.holidays['national day'] = person.birthDay
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and person.nationality.calendar = calendar
select item from Item item, Order order
where order.items[ order.deliveredItemIndices[0] ] = item and
order.id = 11
select item from Item item, Order order
where order.items[ maxindex(order.items) ] = item and order.id = 11
The expression inside [] may even be an arithmetic expression.
select item from Item item, Order order
where order.items[ size(order.items) - 1 ] = item
HQL also provides the built-in index() function, for elements of a
one-to-many association or collection of values.
select item, index(item) from Order order
join order.items item
where index(item) < 5
Scalar SQL functions supported by the underlying database may be used
from DomesticCat cat where upper(cat.name) like 'FRI%'
If you are not yet convinced by all this, think how much longer and less
readable the following query would be in SQL:
select cust
from Product prod,
Store store
inner join store.customers cust
where prod.name = 'widget'
and store.location.name in ( 'Melbourne', 'Sydney' )
and prod = all elements(cust.currentOrder.lineItems)
Hint: something like
SELECT cust.name, cust.address, cust.phone, cust.id,
cust.current_order
FROM customers cust,
stores store,
locations loc,
store_customers sc,
product prod
WHERE prod.name = 'widget'
AND store.loc_id = loc.id
AND loc.name IN ( 'Melbourne', 'Sydney' )
AND sc.store_id = store.id
AND sc.cust_id = cust.id
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AND prod.id = ALL(
SELECT item.prod_id
FROM line_items item, orders o
WHERE item.order_id = o.id
AND cust.current_order = o.id
)
14.11. The order by clause
The list returned by a query may be ordered by any property of a returned
class or components:
from DomesticCat cat
order by cat.name asc, cat.weight desc, cat.birthdate
The optional asc or desc indicate ascending or descending order respectively.
14.12. The group by clause
A query that returns aggregate values may be grouped by any property of a
returned class or components:
select cat.color, sum(cat.weight), count(cat)
from Cat cat
group by cat.color
select foo.id, avg(name), max(name)
from Foo foo join foo.names name
group by foo.id
A having clause is also allowed.
select cat.color, sum(cat.weight), count(cat)
from Cat cat
group by cat.color
having cat.color in (eg.Color.TABBY, eg.Color.BLACK)
SQL functions and aggregate functions are allowed in the having and order
by clauses, if supported by the underlying database (eg. not in MySQL).
select cat
from Cat cat
join cat.kittens kitten
group by cat.id, cat.name, cat.other, cat.properties
having avg(kitten.weight) > 100
order by count(kitten) asc, sum(kitten.weight) desc
Note that neither the group by clause nor the order by clause may contain
arithmetic expressions. Also note that Hibernate currently does not expand
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a grouped entity, so you can't write group by cat if all properties of cat are
non-aggregated. You have to list all non-aggregated properties explicitly.
14.13. Subqueries
For databases that support subselects, Hibernate supports subqueries within
queries. A subquery must be surrounded by parentheses (often by an SQL
aggregate function call). Even correlated subqueries (subqueries that refer to
an alias in the outer query) are allowed.
from Cat as fatcat
where fatcat.weight > (
select avg(cat.weight) from DomesticCat cat
)
from DomesticCat as cat
where cat.name = some (
select name.nickName from Name as name
)
from Cat as cat
where not exists (
from Cat as mate where mate.mate = cat
)
from DomesticCat as cat
where cat.name not in (
select name.nickName from Name as name
)
select cat.id, (select max(kit.weight) from cat.kitten kit)
from Cat as cat
Note that HQL subqueries may occur only in the select or where clauses.
Note that subqueries can also utilize row value constructor syntax. See
Section 14.18, “Row value constructor syntax” for more details.
14.14. HQL examples
Hibernate queries can be quite powerful and complex. In fact, the power of
the query language is one of Hibernate's main selling points. Here are some
example queries very similar to queries that I used on a recent project. Note
that most queries you will write are much simpler than these!
The following query returns the order id, number of items and total value of
the order for all unpaid orders for a particular customer and given minimum
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total value, ordering the results by total value. In determining the prices,
it uses the current catalog. The resulting SQL query, against the ORDER,
ORDER_LINE, PRODUCT, CATALOG and PRICE tables has four inner joins and an
(uncorrelated) subselect.
select order.id, sum(price.amount), count(item)
from Order as order
join order.lineItems as item
join item.product as product,
Catalog as catalog
join catalog.prices as price
where order.paid = false
and order.customer = :customer
and price.product = product
and catalog.effectiveDate < sysdate
and catalog.effectiveDate >= all (
select cat.effectiveDate
from Catalog as cat
where cat.effectiveDate < sysdate
)
group by order
having sum(price.amount) > :minAmount
order by sum(price.amount) desc
What a monster! Actually, in real life, I'm not very keen on subqueries, so my
query was really more like this:
select order.id, sum(price.amount), count(item)
from Order as order
join order.lineItems as item
join item.product as product,
Catalog as catalog
join catalog.prices as price
where order.paid = false
and order.customer = :customer
and price.product = product
and catalog = :currentCatalog
group by order
having sum(price.amount) > :minAmount
order by sum(price.amount) desc
The next query counts the number of payments in each status, excluding
all payments in the AWAITING_APPROVAL status where the most recent status
change was made by the current user. It translates to an SQL query with two
inner joins and a correlated subselect against the PAYMENT, PAYMENT_STATUS
and PAYMENT_STATUS_CHANGE tables.
select count(payment), status.name
from Payment as payment
join payment.currentStatus as status
join payment.statusChanges as statusChange
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where payment.status.name <> PaymentStatus.AWAITING_APPROVAL
or (
statusChange.timeStamp = (
select max(change.timeStamp)
from PaymentStatusChange change
where change.payment = payment
)
and statusChange.user <> :currentUser
)
group by status.name, status.sortOrder
order by status.sortOrder
If I would have mapped the statusChanges collection as a list, instead of a set,
the query would have been much simpler to write.
select count(payment), status.name
from Payment as payment
join payment.currentStatus as status
where payment.status.name <> PaymentStatus.AWAITING_APPROVAL
or payment.statusChanges[ maxIndex(payment.statusChanges) ].user
<> :currentUser
group by status.name, status.sortOrder
order by status.sortOrder
The next query uses the MS SQL Server isNull() function to return all the
accounts and unpaid payments for the organization to which the current user
belongs. It translates to an SQL query with three inner joins, an outer join
and a subselect against the ACCOUNT, PAYMENT, PAYMENT_STATUS, ACCOUNT_TYPE,
ORGANIZATION and ORG_USER tables.
select account, payment
from Account as account
left outer join account.payments as payment
where :currentUser in elements(account.holder.users)
and PaymentStatus.UNPAID = isNull(payment.currentStatus.name,
PaymentStatus.UNPAID)
order by account.type.sortOrder, account.accountNumber,
payment.dueDate
For some databases, we would need to do away with the (correlated)
subselect.
select account, payment
from Account as account
join account.holder.users as user
left outer join account.payments as payment
where :currentUser = user
and PaymentStatus.UNPAID = isNull(payment.currentStatus.name,
PaymentStatus.UNPAID)
order by account.type.sortOrder, account.accountNumber,
payment.dueDate
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14.15. Bulk update and delete
HQL now supports update, delete and insert ... select ... statements.
See Section 13.4, “DML-style operations” for details.
14.16. Tips & Tricks
You can count the number of query results without actually returning them:
( (Integer) session.createQuery("select count(*) from
....").iterate().next() ).intValue()
To order a result by the size of a collection, use the following query:
select usr.id, usr.name
from User as usr
left join usr.messages as msg
group by usr.id, usr.name
order by count(msg)
If your database supports subselects, you can place a condition upon
selection size in the where clause of your query:
from User usr where size(usr.messages) >= 1
If your database doesn't support subselects, use the following query:
select usr.id, usr.name
from User usr.name
join usr.messages msg
group by usr.id, usr.name
having count(msg) >= 1
As this solution can't return a User with zero messages because of the inner
join, the following form is also useful:
select usr.id, usr.name
from User as usr
left join usr.messages as msg
group by usr.id, usr.name
having count(msg) = 0
Properties of a JavaBean can be bound to named query parameters:
Query q = s.createQuery("from foo Foo as foo where foo.name=:name
and foo.size=:size");
q.setProperties(fooBean); // fooBean has getName() and getSize()
List foos = q.list();
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Collections are pageable by using the Query interface with a filter:
Query q = s.createFilter( collection, "" ); // the trivial filter
q.setMaxResults(PAGE_SIZE);
q.setFirstResult(PAGE_SIZE * pageNumber);
List page = q.list();
Collection elements may be ordered or grouped using a query filter:
Collection orderedCollection = s.filter( collection, "order by
this.amount" );
Collection counts = s.filter( collection, "select this.type,
count(this) group by this.type" );
You can find the size of a collection without initializing it:
( (Integer) session.createQuery("select count(*) from
....").iterate().next() ).intValue();
14.17. Components
Components might be used in just about every way that simple value types
can be used in HQL queries. They can appear in the select clause:
select p.name from Person p
select p.name.first from Person p
where the Person's name property is a component. Components can also be
used in the where clause:
from Person p where p.name = :name
from Person p where p.name.first = :firstName
Components can also be used in the order by clause:
from Person p order by p.name
from Person p order by p.name.first
Another common use of components is in row value constructors.
14.18. Row value constructor syntax
HQL supports the use of ANSI SQL row value constructor syntax
(sometimes called tuple syntax), even though the underlying database
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may not support that notion. Here we are generally referring to multi-valued
comparisons, typically associated with components. Consider an entity
Person which defines a name component:
from Person p where p.name.first='John' and
p.name.last='Jingleheimer-Schmidt'
That's valid syntax, although a little verbose. It be nice to make this a bit
more concise and use row value constructor syntax:
from Person p where p.name=('John', 'Jingleheimer-Schmidt')
It can also be useful to specify this in the select clause:
select p.name from Person p
Another time using row value constructor syntax can be beneficial is when
using subqueries needing to compare against multiple values:
from Cat as cat
where not ( cat.name, cat.color ) in (
select cat.name, cat.color from DomesticCat cat
)
One thing to consider when deciding if you want to use this syntax is that the
query will be dependent upon the ordering of the component sub-properties
in the metadata.
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Chapter 15. Criteria Queries
Hibernate features an intuitive, extensible criteria query API.
15.1. Creating a Criteria instance
The interface org.hibernate.Criteria represents a query against a particular
persistent class. The Session is a factory for Criteria instances.
Criteria crit = sess.createCriteria(Cat.class);
crit.setMaxResults(50);
List cats = crit.list();
15.2. Narrowing the result set
An individual query criterion is an instance of the
interface org.hibernate.criterion.Criterion. The class
org.hibernate.criterion.Restrictions defines factory methods for obtaining
certain built-in Criterion types.
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "Fritz%") )
.add( Restrictions.between("weight", minWeight, maxWeight) )
.list();
Restrictions may be grouped logically.
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "Fritz%") )
.add( Restrictions.or(
Restrictions.eq( "age", new Integer(0) ),
Restrictions.isNull("age")
) )
.list();
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.in( "name", new String[] { "Fritz", "Izi",
"Pk" } ) )
.add( Restrictions.disjunction()
.add( Restrictions.isNull("age") )
.add( Restrictions.eq("age", new Integer(0) ) )
.add( Restrictions.eq("age", new Integer(1) ) )
.add( Restrictions.eq("age", new Integer(2) ) )
) )
.list();
There are quite a range of built-in criterion types (Restrictions subclasses),
but one that is especially useful lets you specify SQL directly.
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List cats = sess.createCriteria(Cat.class)
.add( Restrictions.sqlRestriction("lower({alias}.name) like
lower(?)", "Fritz%", Hibernate.STRING) )
.list();
The {alias} placeholder with be replaced by the row alias of the queried
entity.
An alternative approach to obtaining a criterion is to get it from a Property
instance. You can create a Property by calling Property.forName().
Property age = Property.forName("age");
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.disjunction()
.add( age.isNull() )
.add( age.eq( new Integer(0) ) )
.add( age.eq( new Integer(1) ) )
.add( age.eq( new Integer(2) ) )
) )
.add( Property.forName("name").in( new String[] { "Fritz",
"Izi", "Pk" } ) )
.list();
15.3. Ordering the results
You may order the results using org.hibernate.criterion.Order.
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "F%")
.addOrder( Order.asc("name") )
.addOrder( Order.desc("age") )
.setMaxResults(50)
.list();
List cats = sess.createCriteria(Cat.class)
.add( Property.forName("name").like("F%") )
.addOrder( Property.forName("name").asc() )
.addOrder( Property.forName("age").desc() )
.setMaxResults(50)
.list();
15.4. Associations
You may easily specify constraints upon related entities by navigating
associations using createCriteria().
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "F%") )
.createCriteria("kittens")
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.add( Restrictions.like("name", "F%") )
.list();
note that the second createCriteria() returns a new instance of Criteria,
which refers to the elements of the kittens collection.
The following, alternate form is useful in certain circumstances.
List cats = sess.createCriteria(Cat.class)
.createAlias("kittens", "kt")
.createAlias("mate", "mt")
.add( Restrictions.eqProperty("kt.name", "mt.name") )
.list();
(createAlias() does not create a new instance of Criteria.)
Note that the kittens collections held by the Cat instances returned by the
previous two queries are not pre-filtered by the criteria! If you wish to retrieve
just the kittens that match the criteria, you must use a ResultTransformer.
List cats = sess.createCriteria(Cat.class)
.createCriteria("kittens", "kt")
.add( Restrictions.eq("name", "F%") )
.setResultTransformer(Criteria.ALIAS_TO_ENTITY_MAP)
.list();
Iterator iter = cats.iterator();
while ( iter.hasNext() ) {
Map map = (Map) iter.next();
Cat cat = (Cat) map.get(Criteria.ROOT_ALIAS);
Cat kitten = (Cat) map.get("kt");
}
15.5. Dynamic association fetching
You may specify association fetching semantics at runtime using
setFetchMode().
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "Fritz%") )
.setFetchMode("mate", FetchMode.EAGER)
.setFetchMode("kittens", FetchMode.EAGER)
.list();
This query will fetch both mate and kittens by outer join. See Section 19.1,
“Fetching strategies” for more information.
15.6. Example queries
The class org.hibernate.criterion.Example allows you to construct a query
criterion from a given instance.
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Cat cat = new Cat();
cat.setSex('F');
cat.setColor(Color.BLACK);
List results = session.createCriteria(Cat.class)
.add( Example.create(cat) )
.list();
Version properties, identifiers and associations are ignored. By default, null
valued properties are excluded.
You can adjust how the Example is applied.
Example example = Example.create(cat)
.excludeZeroes()
//exclude zero valued properties
.excludeProperty("color") //exclude the property named "color"
.ignoreCase()
//perform case insensitive string
comparisons
.enableLike();
//use like for string comparisons
List results = session.createCriteria(Cat.class)
.add(example)
.list();
You can even use examples to place criteria upon associated objects.
List results = session.createCriteria(Cat.class)
.add( Example.create(cat) )
.createCriteria("mate")
.add( Example.create( cat.getMate() ) )
.list();
15.7. Projections, aggregation and grouping
The class org.hibernate.criterion.Projections is a factory for Projection
instances. We apply a projection to a query by calling setProjection().
List results = session.createCriteria(Cat.class)
.setProjection( Projections.rowCount() )
.add( Restrictions.eq("color", Color.BLACK) )
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Projections.projectionList()
.add( Projections.rowCount() )
.add( Projections.avg("weight") )
.add( Projections.max("weight") )
.add( Projections.groupProperty("color") )
)
.list();
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There is no explicit "group by" necessary in a criteria query. Certain
projection types are defined to be grouping projections, which also appear in
the SQL group by clause.
An alias may optionally be assigned to a projection, so that the projected
value may be referred to in restrictions or orderings. Here are two different
ways to do this:
List results = session.createCriteria(Cat.class)
.setProjection( Projections.alias(
Projections.groupProperty("color"), "colr" ) )
.addOrder( Order.asc("colr") )
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Projections.groupProperty("color").as("colr") )
.addOrder( Order.asc("colr") )
.list();
The alias() and as() methods simply wrap a projection instance in another,
aliased, instance of Projection. As a shortcut, you can assign an alias when
you add the projection to a projection list:
List results = session.createCriteria(Cat.class)
.setProjection( Projections.projectionList()
.add( Projections.rowCount(), "catCountByColor" )
.add( Projections.avg("weight"), "avgWeight" )
.add( Projections.max("weight"), "maxWeight" )
.add( Projections.groupProperty("color"), "color" )
)
.addOrder( Order.desc("catCountByColor") )
.addOrder( Order.desc("avgWeight") )
.list();
List results = session.createCriteria(Domestic.class, "cat")
.createAlias("kittens", "kit")
.setProjection( Projections.projectionList()
.add( Projections.property("cat.name"), "catName" )
.add( Projections.property("kit.name"), "kitName" )
)
.addOrder( Order.asc("catName") )
.addOrder( Order.asc("kitName") )
.list();
You can also use Property.forName() to express projections:
List results = session.createCriteria(Cat.class)
.setProjection( Property.forName("name") )
.add( Property.forName("color").eq(Color.BLACK) )
.list();
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List results = session.createCriteria(Cat.class)
.setProjection( Projections.projectionList()
.add( Projections.rowCount().as("catCountByColor") )
.add( Property.forName("weight").avg().as("avgWeight") )
.add( Property.forName("weight").max().as("maxWeight") )
.add( Property.forName("color").group().as("color" )
)
.addOrder( Order.desc("catCountByColor") )
.addOrder( Order.desc("avgWeight") )
.list();
15.8. Detached queries and subqueries
The DetachedCriteria class lets you create a query outside the scope of a
session, and then later execute it using some arbitrary Session.
DetachedCriteria query = DetachedCriteria.forClass(Cat.class)
.add( Property.forName("sex").eq('F') );
Session session = ....;
Transaction txn = session.beginTransaction();
List results =
query.getExecutableCriteria(session).setMaxResults(100).list();
txn.commit();
session.close();
A DetachedCriteria may also be used to express a subquery. Criterion
instances involving subqueries may be obtained via Subqueries or Property.
DetachedCriteria avgWeight = DetachedCriteria.forClass(Cat.class)
.setProjection( Property.forName("weight").avg() );
session.createCriteria(Cat.class)
.add( Property.forName("weight").gt(avgWeight) )
.list();
DetachedCriteria weights = DetachedCriteria.forClass(Cat.class)
.setProjection( Property.forName("weight") );
session.createCriteria(Cat.class)
.add( Subqueries.geAll("weight", weights) )
.list();
Even correlated subqueries are possible:
DetachedCriteria avgWeightForSex =
DetachedCriteria.forClass(Cat.class, "cat2")
.setProjection( Property.forName("weight").avg() )
.add( Property.forName("cat2.sex").eqProperty("cat.sex") );
session.createCriteria(Cat.class, "cat")
.add( Property.forName("weight").gt(avgWeightForSex) )
.list();
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15.9. Queries by natural identifier
For most queries, including criteria queries, the query cache is not
very efficient, because query cache invalidation occurs too frequently.
However, there is one special kind of query where we can optimize the
cache invalidation algorithm: lookups by a constant natural key. In some
applications, this kind of query occurs frequently. The criteria API provides
special provision for this use case.
First, you should map the natural key of your entity using <natural-id>, and
enable use of the second-level cache.
<class name="User">
<cache usage="read-write"/>
<id name="id">
<generator class="increment"/>
</id>
<natural-id>
<property name="name"/>
<property name="org"/>
</natural-id>
<property name="password"/>
</class>
Note that this functionality is not intended for use with entities with mutable
natural keys.
Next, enable the Hibernate query cache.
Now, Restrictions.naturalId() allows us to make use of the more efficient
cache algorithm.
session.createCriteria(User.class)
.add( Restrictions.naturalId()
.set("name", "gavin")
.set("org", "hb")
).setCacheable(true)
.uniqueResult();
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Chapter 16. Native SQL
You may also express queries in the native SQL dialect of your database.
This is useful if you want to utilize database specific features such as query
hints or the CONNECT keyword in Oracle. It also provides a clean migration path
from a direct SQL/JDBC based application to Hibernate.
Hibernate3 allows you to specify handwritten SQL (including stored
procedures) for all create, update, delete, and load operations.
16.1. Using a SQLQuery
Execution of native SQL queries is controlled via the SQLQuery interface,
which is obtained by calling Session.createSQLQuery(). The following
describes how to use this API for querying.
16.1.1. Scalar queries
The most basic SQL query is to get a list of scalars (values).
sess.createSQLQuery("SELECT * FROM CATS").list();
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();
These will both return a List of Object arrays (Object[]) with scalar values for
each column in the CATS table. Hibernate will use ResultSetMetadata to
deduce the actual order and types of the returned scalar values.
To avoid the overhead of using ResultSetMetadata or simply to be more
explicit in what is returned one can use addScalar().
sess.createSQLQuery("SELECT * FROM CATS")
.addScalar("ID", Hibernate.LONG)
.addScalar("NAME", Hibernate.STRING)
.addScalar("BIRTHDATE", Hibernate.DATE)
This query specified:
• the SQL query string
• the columns and types to return
This will still return Object arrays, but now it will not use ResultSetMetadata
but will instead explicitly get the ID, NAME and BIRTHDATE column as
respectively a Long, String and a Short from the underlying resultset. This
also means that only these three columns will be returned, even though the
query is using * and could return more than the three listed columns.
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It is possible to leave out the type information for all or some of the scalars.
sess.createSQLQuery("SELECT * FROM CATS")
.addScalar("ID", Hibernate.LONG)
.addScalar("NAME")
.addScalar("BIRTHDATE")
This is essentially the same query as before, but now ResultSetMetaData is
used to decide the type of NAME and BIRTHDATE where as the type of ID is
explicitly specified.
How the java.sql.Types returned from ResultSetMetaData is mapped to
Hibernate types is controlled by the Dialect. If a specific type is not mapped
or does not result in the expected type it is possible to customize it via calls
to registerHibernateType in the Dialect.
16.1.2. Entity queries
The above queries were all about returning scalar values, basically returning
the "raw" values from the resultset. The following shows how to get entity
objects from a native sql query via addEntity().
sess.createSQLQuery("SELECT * FROM CATS").addEntity(Cat.class);
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM
CATS").addEntity(Cat.class);
This query specified:
• the SQL query string
• the entity returned by the query
Assuming that Cat is mapped as a class with the columns ID, NAME and
BIRTHDATE the above queries will both return a List where each element is
a Cat entity.
If the entity is mapped with a many-to-one to another entity it is required to
also return this when performing the native query, otherwise a database
specific "column not found" error will occur. The additional columns will
automatically be returned when using the * notation, but we prefer to be
explicit as in the following example for a many-to-one to a Dog:
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, DOG_ID FROM
CATS").addEntity(Cat.class);
This will allow cat.getDog() to function properly.
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16.1.3. Handling associations and collections
It is possible to eagerly join in the Dog to avoid the possible extra roundtrip for
initializing the proxy. This is done via the addJoin() method, which allows you
to join in an association or collection.
sess.createSQLQuery("SELECT c.ID, NAME, BIRTHDATE, DOG_ID, D_ID,
D_NAME FROM CATS c, DOGS d WHERE c.DOG_ID = d.D_ID")
.addEntity("cat", Cat.class)
.addJoin("cat.dog");
In this example the returned Cat's will have their dog property fully initialized
without any extra roundtrip to the database. Notice that we added a alias
name ("cat") to be able to specify the target property path of the join. It is
possible to do the same eager joining for collections, e.g. if the Cat had a
one-to-many to Dog instead.
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, D_ID, D_NAME,
CAT_ID FROM CATS c, DOGS d WHERE c.ID = d.CAT_ID")
.addEntity("cat", Cat.class)
.addJoin("cat.dogs");
At this stage we are reaching the limits of what is possible with native queries
without starting to enhance the sql queries to make them usable in Hibernate;
the problems starts to arise when returning multiple entities of the same type
or when the default alias/column names are not enough.
16.1.4. Returning multiple entities
Until now the result set column names are assumed to be the same as the
column names specified in the mapping document. This can be problematic
for SQL queries which join multiple tables, since the same column names
may appear in more than one table.
Column alias injection is needed in the following query (which most likely will
fail):
sess.createSQLQuery("SELECT c.*, m.*
c.MOTHER_ID = c.ID")
.addEntity("cat", Cat.class)
.addEntity("mother", Cat.class)
FROM CATS c, CATS m WHERE
The intention for this query is to return two Cat instances per row, a cat
and its mother. This will fail since there is a conflict of names since they are
mapped to the same column names and on some databases the returned
column aliases will most likely be on the form "c.ID", "c.NAME", etc. which
are not equal to the columns specified in the mappings ("ID" and "NAME").
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The following form is not vulnerable to column name duplication:
sess.createSQLQuery("SELECT {cat.*}, {mother.*}
WHERE c.MOTHER_ID = c.ID")
.addEntity("cat", Cat.class)
.addEntity("mother", Cat.class)
FROM CATS c, CATS m
This query specified:
• the SQL query string, with placeholders for Hibernate to inject column
aliases
• the entities returned by the query
The {cat.*} and {mother.*} notation used above is a shorthand for "all
properties". Alternatively, you may list the columns explicitly, but even in this
case we let Hibernate inject the SQL column aliases for each property. The
placeholder for a column alias is just the property name qualified by the table
alias. In the following example, we retrieve Cats and their mothers from a
different table (cat_log) to the one declared in the mapping metadata. Notice
that we may even use the property aliases in the where clause if we like.
String sql = "SELECT ID as {c.id}, NAME as {c.name}, " +
"BIRTHDATE as {c.birthDate}, MOTHER_ID as {c.mother},
{mother.*} " +
"FROM CAT_LOG c, CAT_LOG m WHERE {c.mother} = c.ID";
List loggedCats = sess.createSQLQuery(sql)
.addEntity("cat", Cat.class)
.addEntity("mother", Cat.class).list()
16.1.4.1. Alias and property references
For most cases the above alias injection is needed, but for queries
relating to more complex mappings like composite properties, inheritance
discriminators, collections etc. there are some specific aliases to use to allow
Hibernate to inject the proper aliases.
The following table shows the different possibilities of using the alias
injection. Note: the alias names in the result are examples, each alias will
have a unique and probably different name when used.
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Table 16.1. Alias injection names
Description
Syntax
Example
A simple
property
{[aliasname].[propertyname]
A_NAME as {item.name}
A composite
property
{[aliasname].[componentname].[propertyname]}
CURRENCY as {item.amount.currency},
VALUE as {item.amount.value}
Discriminator of {[aliasname].class}
DISC as {item.class}
an entity
All properties of {[aliasname].*} {item.*}
an entity
A collection key {[aliasname].key}
ORGID as {coll.key}
The id of an
collection
{[aliasname].id}EMPID as {coll.id}
The element of {[aliasname].element}
XID as {coll.element}
an collection
roperty of the
element in the
collection
{[aliasname].element.[propertyname]}
NAME as {coll.element.name}
All properties of {[aliasname].element.*}
{coll.element.*}
the element in
the collection
All properties
of the the
collection
{[aliasname].*} {coll.*}
16.1.5. Returning non-managed entities
It is possible to apply a ResultTransformer to native sql queries. Allowing it to
e.g. return non-managed entities.
sess.createSQLQuery("SELECT NAME, BIRTHDATE FROM CATS")
.setResultTransformer(Transformers.aliasToBean(CatDTO.class))
This query specified:
• the SQL query string
• a result transformer
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The above query will return a list of CatDTO which has been instantiated
and injected the values of NAME and BIRTHNAME into its corresponding
properties or fields.
16.1.6. Handling inheritance
Native sql queries which query for entities that is mapped as part of an
inheritance must include all properties for the baseclass and all it subclasses.
16.1.7. Parameters
Native sql queries support positional as well as named parameters:
Query query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME
like ?").addEntity(Cat.class);
List pusList = query.setString(0, "Pus%").list();
query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like
:name").addEntity(Cat.class);
List pusList = query.setString("name", "Pus%").list();
16.2. Named SQL queries
Named SQL queries may be defined in the mapping document and called in
exactly the same way as a named HQL query. In this case, we do not need
to call addEntity().
<sql-query name="persons">
<return alias="person" class="eg.Person"/>
SELECT person.NAME AS {person.name},
person.AGE AS {person.age},
person.SEX AS {person.sex}
FROM PERSON person
WHERE person.NAME LIKE :namePattern
</sql-query>
List people = sess.getNamedQuery("persons")
.setString("namePattern", namePattern)
.setMaxResults(50)
.list();
The <return-join> and <load-collection> elements are used to join
associations and define queries which initialize collections, respectively.
<sql-query name="personsWith">
<return alias="person" class="eg.Person"/>
<return-join alias="address" property="person.mailingAddress"/>
SELECT person.NAME AS {person.name},
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person.AGE AS {person.age},
person.SEX AS {person.sex},
address.STREET AS {address.street},
address.CITY AS {address.city},
address.STATE AS {address.state},
address.ZIP AS {address.zip}
FROM PERSON person
JOIN ADDRESS address
ON person.ID = address.PERSON_ID AND address.TYPE='MAILING'
WHERE person.NAME LIKE :namePattern
</sql-query>
A named SQL query may return a scalar value. You must declare the column
alias and Hibernate type using the <return-scalar> element:
<sql-query name="mySqlQuery">
<return-scalar column="name" type="string"/>
<return-scalar column="age" type="long"/>
SELECT p.NAME AS name,
p.AGE AS age,
FROM PERSON p WHERE p.NAME LIKE 'Hiber%'
</sql-query>
You can externalize the resultset mapping informations in a <resultset>
element to either reuse them across several named queries or through the
setResultSetMapping() API.
<resultset name="personAddress">
<return alias="person" class="eg.Person"/>
<return-join alias="address" property="person.mailingAddress"/>
</resultset>
<sql-query name="personsWith" resultset-ref="personAddress">
SELECT person.NAME AS {person.name},
person.AGE AS {person.age},
person.SEX AS {person.sex},
address.STREET AS {address.street},
address.CITY AS {address.city},
address.STATE AS {address.state},
address.ZIP AS {address.zip}
FROM PERSON person
JOIN ADDRESS address
ON person.ID = address.PERSON_ID AND address.TYPE='MAILING'
WHERE person.NAME LIKE :namePattern
</sql-query>
You can alternatively use the resultset mapping information in your hbm files
directly in java code.
List cats = sess.createSQLQuery(
"select {cat.*}, {kitten.*} from cats cat, cats kitten where
kitten.mother = cat.id"
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)
.setResultSetMapping("catAndKitten")
.list();
16.2.1. Using return-property to explicitly specify
column/alias names
With <return-property> you can explicitly tell Hibernate what column aliases
to use, instead of using the {}-syntax to let Hibernate inject its own aliases.
<sql-query name="mySqlQuery">
<return alias="person" class="eg.Person">
<return-property name="name" column="myName"/>
<return-property name="age" column="myAge"/>
<return-property name="sex" column="mySex"/>
</return>
SELECT person.NAME AS myName,
person.AGE AS myAge,
person.SEX AS mySex,
FROM PERSON person WHERE person.NAME LIKE :name
</sql-query>
also works with multiple columns. This solves a limitation
with the {}-syntax which can not allow fine grained control of multi-column
properties.
<return-property>
<sql-query name="organizationCurrentEmployments">
<return alias="emp" class="Employment">
<return-property name="salary">
<return-column name="VALUE"/>
<return-column name="CURRENCY"/>
</return-property>
<return-property name="endDate" column="myEndDate"/>
</return>
SELECT EMPLOYEE AS {emp.employee}, EMPLOYER AS
{emp.employer},
STARTDATE AS {emp.startDate}, ENDDATE AS {emp.endDate},
REGIONCODE as {emp.regionCode}, EID AS {emp.id}, VALUE,
CURRENCY
FROM EMPLOYMENT
WHERE EMPLOYER = :id AND ENDDATE IS NULL
ORDER BY STARTDATE ASC
</sql-query>
Notice that in this example we used <return-property> in combination with
the {}-syntax for injection. Allowing users to choose how they want to refer
column and properties.
If your mapping has a discriminator you must use <return-discriminator> to
specify the discriminator column.
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Using stored procedures for querying
16.2.2. Using stored procedures for querying
Hibernate 3 introduces support for queries via stored procedures and
functions. Most of the following documentation is equivalent for both. The
stored procedure/function must return a resultset as the first out-parameter
to be able to work with Hibernate. An example of such a stored function in
Oracle 9 and higher is as follows:
CREATE OR REPLACE FUNCTION selectAllEmployments
RETURN SYS_REFCURSOR
AS
st_cursor SYS_REFCURSOR;
BEGIN
OPEN st_cursor FOR
SELECT EMPLOYEE, EMPLOYER,
STARTDATE, ENDDATE,
REGIONCODE, EID, VALUE, CURRENCY
FROM EMPLOYMENT;
RETURN st_cursor;
END;
To use this query in Hibernate you need to map it via a named query.
<sql-query name="selectAllEmployees_SP" callable="true">
<return alias="emp" class="Employment">
<return-property name="employee" column="EMPLOYEE"/>
<return-property name="employer" column="EMPLOYER"/>
<return-property name="startDate" column="STARTDATE"/>
<return-property name="endDate" column="ENDDATE"/>
<return-property name="regionCode" column="REGIONCODE"/>
<return-property name="id" column="EID"/>
<return-property name="salary">
<return-column name="VALUE"/>
<return-column name="CURRENCY"/>
</return-property>
</return>
{ ? = call selectAllEmployments() }
</sql-query>
Notice stored procedures currently only return scalars and entities.
<return-join> and <load-collection> are not supported.
16.2.2.1. Rules/limitations for using stored procedures
To use stored procedures with Hibernate the procedures/functions have to
follow some rules. If they do not follow those rules they are not usable with
Hibernate. If you still want to use these procedures you have to execute them
via session.connection(). The rules are different for each database, since
database vendors have different stored procedure semantics/syntax.
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Stored procedure queries can't be paged with
setFirstResult()/setMaxResults().
Recommended call form is standard SQL92: { ? = call
functionName(<parameters>) } or { ? = call procedureName(<parameters>}.
Native call syntax is not supported.
For Oracle the following rules apply:
• A function must return a result set. The first parameter of a procedure must
be an OUT that returns a result set. This is done by using a SYS_REFCURSOR
type in Oracle 9 or 10. In Oracle you need to define a REF CURSOR type, see
Oracle literature.
For Sybase or MS SQL server the following rules apply:
• The procedure must return a result set. Note that since these servers
can/will return multiple result sets and update counts, Hibernate will iterate
the results and take the first result that is a result set as its return value.
Everything else will be discarded.
• If you can enable SET NOCOUNT ON in your procedure it will probably be more
efficient, but this is not a requirement.
16.3. Custom SQL for create, update and delete
Hibernate3 can use custom SQL statements for create, update, and delete
operations. The class and collection persisters in Hibernate already contain
a set of configuration time generated strings (insertsql, deletesql, updatesql
etc.). The mapping tags <sql-insert>, <sql-delete>, and <sql-update>
override these strings:
<class name="Person">
<id name="id">
<generator class="increment"/>
</id>
<property name="name" not-null="true"/>
<sql-insert>INSERT INTO PERSON (NAME, ID) VALUES ( UPPER(?), ?
)</sql-insert>
<sql-update>UPDATE PERSON SET NAME=UPPER(?) WHERE
ID=?</sql-update>
<sql-delete>DELETE FROM PERSON WHERE ID=?</sql-delete>
</class>
The SQL is directly executed in your database, so you are free to use any
dialect you like. This will of course reduce the portability of your mapping if
you use database specific SQL.
Stored procedures are supported if the callable attribute is set:
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<class name="Person">
<id name="id">
<generator class="increment"/>
</id>
<property name="name" not-null="true"/>
<sql-insert callable="true">{call createPerson (?,
?)}</sql-insert>
<sql-delete callable="true">{? = call deletePerson
(?)}</sql-delete>
<sql-update callable="true">{? = call updatePerson (?,
?)}</sql-update>
</class>
The order of the positional parameters are currently vital, as they must be in
the same sequence as Hibernate expects them.
You can see the expected order by enabling debug logging for the
org.hibernate.persister.entity level. With this level enabled Hibernate will
print out the static SQL that is used to create, update, delete etc. entities. (To
see the expected sequence, remember to not include your custom SQL in
the mapping files as that will override the Hibernate generated static sql.)
The stored procedures are in most cases (read: better do it than not) required
to return the number of rows inserted/updated/deleted, as Hibernate has
some runtime checks for the success of the statement. Hibernate always
registers the first statement parameter as a numeric output parameter for the
CUD operations:
CREATE OR REPLACE FUNCTION updatePerson (uid IN NUMBER, uname IN
VARCHAR2)
RETURN NUMBER IS
BEGIN
update PERSON
set
NAME = uname,
where
ID = uid;
return SQL%ROWCOUNT;
END updatePerson;
16.4. Custom SQL for loading
You may also declare your own SQL (or HQL) queries for entity loading:
<sql-query name="person">
<return alias="pers" class="Person" lock-mode="upgrade"/>
SELECT NAME AS {pers.name}, ID AS {pers.id}
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FROM PERSON
WHERE ID=?
FOR UPDATE
</sql-query>
This is just a named query declaration, as discussed earlier. You may
reference this named query in a class mapping:
<class name="Person">
<id name="id">
<generator class="increment"/>
</id>
<property name="name" not-null="true"/>
<loader query-ref="person"/>
</class>
This even works with stored procedures.
You may even define a query for collection loading:
<set name="employments" inverse="true">
<key/>
<one-to-many class="Employment"/>
<loader query-ref="employments"/>
</set>
<sql-query name="employments">
<load-collection alias="emp" role="Person.employments"/>
SELECT {emp.*}
FROM EMPLOYMENT emp
WHERE EMPLOYER = :id
ORDER BY STARTDATE ASC, EMPLOYEE ASC
</sql-query>
You could even define an entity loader that loads a collection by join fetching:
<sql-query name="person">
<return alias="pers" class="Person"/>
<return-join alias="emp" property="pers.employments"/>
SELECT NAME AS {pers.*}, {emp.*}
FROM PERSON pers
LEFT OUTER JOIN EMPLOYMENT emp
ON pers.ID = emp.PERSON_ID
WHERE ID=?
</sql-query>
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Chapter 17. Filtering data
Hibernate3 provides an innovative new approach to handling data with
"visibility" rules. A Hibernate filter is a global, named, parameterized filter that
may be enabled or disabled for a particular Hibernate session.
17.1. Hibernate filters
Hibernate3 adds the ability to pre-define filter criteria and attach those filters
at both a class and a collection level. A filter criteria is the ability to define a
restriction clause very similiar to the existing "where" attribute available on
the class and various collection elements. Except these filter conditions can
be parameterized. The application can then make the decision at runtime
whether given filters should be enabled and what their parameter values
should be. Filters can be used like database views, but parameterized inside
the application.
In order to use filters, they must first be defined and then attached to the
appropriate mapping elements. To define a filter, use the <filter-def/>
element within a <hibernate-mapping/> element:
<filter-def name="myFilter">
<filter-param name="myFilterParam" type="string"/>
</filter-def>
Then, this filter can be attached to a class:
<class name="myClass" ...>
...
<filter name="myFilter" condition=":myFilterParam =
MY_FILTERED_COLUMN"/>
</class>
or, to a collection:
<set ...>
<filter name="myFilter" condition=":myFilterParam =
MY_FILTERED_COLUMN"/>
</set>
or, even to both (or multiples of each) at the same time.
The methods on Session are: enableFilter(String filterName),
getEnabledFilter(String filterName), and disableFilter(String
filterName). By default, filters are not enabled for a given session; they must
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be explcitly enabled through use of the Session.enableFilter() method,
which returns an instance of the Filter interface. Using the simple filter
defined above, this would look like:
session.enableFilter("myFilter").setParameter("myFilterParam",
"some-value");
Note that methods on the org.hibernate.Filter interface do allow the
method-chaining common to much of Hibernate.
A full example, using temporal data with an effective record date pattern:
<filter-def name="effectiveDate">
<filter-param name="asOfDate" type="date"/>
</filter-def>
<class name="Employee" ...>
...
<many-to-one name="department" column="dept_id"
class="Department"/>
<property name="effectiveStartDate" type="date"
column="eff_start_dt"/>
<property name="effectiveEndDate" type="date"
column="eff_end_dt"/>
...
<!-Note that this assumes non-terminal records have an
eff_end_dt set to
a max db date for simplicity-sake
-->
<filter name="effectiveDate"
condition=":asOfDate BETWEEN eff_start_dt and
eff_end_dt"/>
</class>
<class name="Department" ...>
...
<set name="employees" lazy="true">
<key column="dept_id"/>
<one-to-many class="Employee"/>
<filter name="effectiveDate"
condition=":asOfDate BETWEEN eff_start_dt and
eff_end_dt"/>
</set>
</class>
Then, in order to ensure that you always get back currently effective records,
simply enable the filter on the session prior to retrieving employee data:
Session session = ...;
session.enableFilter("effectiveDate").setParameter("asOfDate", new
Date());
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List results = session.createQuery("from Employee as e where
e.salary > :targetSalary")
.setLong("targetSalary", new Long(1000000))
.list();
In the HQL above, even though we only explicitly mentioned a salary
constraint on the results, because of the enabled filter the query will return
only currently active employees who have a salary greater than a million
dollars.
Note: if you plan on using filters with outer joining (either through HQL or load
fetching) be careful of the direction of the condition expression. Its safest to
set this up for left outer joining; in general, place the parameter first followed
by the column name(s) after the operator.
After being defined a filter might be attached to multiple entities and/or
collections each with its own condition. That can be tedious when the
conditions are the same each time. Thus <filter-def/> allows defining a
default condition, either as an attribute or CDATA:
<filter-def name="myFilter" condition="abc > xyz">...</filter-def>
<filter-def name="myOtherFilter">abc=xyz</filter-def>
This default condition will then be used whenever the filter is attached to
something without specifying a condition. Note that this means you can give
a specific condition as part of the attachment of the filter which overrides the
default condition in that particular case.
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Chapter 18. XML Mapping
Note that this is an experimental feature in Hibernate 3.0 and is under
extremely active development.
18.1. Working with XML data
Hibernate lets you work with persistent XML data in much the same way you
work with persistent POJOs. A parsed XML tree can be thought of as just
another way to represent the relational data at the object level, instead of
POJOs.
Hibernate supports dom4j as API for manipulating XML trees. You can
write queries that retrieve dom4j trees from the database and have any
modification you make to the tree automatically synchronized to the
database. You can even take an XML document, parse it using dom4j, and
write it to the database with any of Hibernate's basic operations: persist(),
saveOrUpdate(), merge(), delete(), replicate() (merging is not yet
supported).
This feature has many applications including data import/export,
externalization of entity data via JMS or SOAP and XSLT-based reporting.
A single mapping may be used to simultaneously map properties of a class
and nodes of an XML document to the database, or, if there is no class to
map, it may be used to map just the XML.
18.1.1. Specifying XML and class mapping together
Here is an example of mapping a POJO and XML simultaneously:
<class name="Account"
table="ACCOUNTS"
node="account">
<id name="accountId"
column="ACCOUNT_ID"
node="@id"/>
<many-to-one name="customer"
column="CUSTOMER_ID"
node="customer/@id"
embed-xml="false"/>
<property name="balance"
column="BALANCE"
node="balance"/>
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...
</class>
18.1.2. Specifying only an XML mapping
Here is an example where there is no POJO class:
<class entity-name="Account"
table="ACCOUNTS"
node="account">
<id name="id"
column="ACCOUNT_ID"
node="@id"
type="string"/>
<many-to-one name="customerId"
column="CUSTOMER_ID"
node="customer/@id"
embed-xml="false"
entity-name="Customer"/>
<property name="balance"
column="BALANCE"
node="balance"
type="big_decimal"/>
...
</class>
This mapping allows you to access the data as a dom4j tree, or as a graph of
property name/value pairs (java Maps). The property names are purely logical
constructs that may be referred to in HQL queries.
18.2. XML mapping metadata
Many Hibernate mapping elements accept the node attribute. This let's you
specify the name of an XML attribute or element that holds the property or
entity data. The format of the node attribute must be one of the following:
•
•
•
•
- map to the named XML element
"@attribute-name" - map to the named XML attribute
"." - map to the parent element
"element-name/@attribute-name" - map to the named attribute of the
named element
"element-name"
For collections and single valued associations, there is an additional
embed-xml attribute. If embed-xml="true", the default, the XML tree for the
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associated entity (or collection of value type) will be embedded directly
in the XML tree for the entity that owns the association. Otherwise, if
embed-xml="false", then only the referenced identifier value will appear in the
XML for single point associations and collections will simply not appear at all.
You should be careful not to leave embed-xml="true" for too many
associations, since XML does not deal well with circularity!
<class name="Customer"
table="CUSTOMER"
node="customer">
<id name="id"
column="CUST_ID"
node="@id"/>
<map name="accounts"
node="."
embed-xml="true">
<key column="CUSTOMER_ID"
not-null="true"/>
<map-key column="SHORT_DESC"
node="@short-desc"
type="string"/>
<one-to-many entity-name="Account"
embed-xml="false"
node="account"/>
</map>
<component name="name"
node="name">
<property name="firstName"
node="first-name"/>
<property name="initial"
node="initial"/>
<property name="lastName"
node="last-name"/>
</component>
...
</class>
in this case, we have decided to embed the collection of account ids, but not
the actual account data. The following HQL query:
from Customer c left join fetch c.accounts where c.lastName like
:lastName
Would return datasets such as this:
<customer id="123456789">
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<account short-desc="Savings">987632567</account>
<account short-desc="Credit Card">985612323</account>
<name>
<first-name>Gavin</first-name>
<initial>A</initial>
<last-name>King</last-name>
</name>
...
</customer>
If you set embed-xml="true" on the <one-to-many> mapping, the data might
look more like this:
<customer id="123456789">
<account id="987632567" short-desc="Savings">
<customer id="123456789"/>
<balance>100.29</balance>
</account>
<account id="985612323" short-desc="Credit Card">
<customer id="123456789"/>
<balance>-2370.34</balance>
</account>
<name>
<first-name>Gavin</first-name>
<initial>A</initial>
<last-name>King</last-name>
</name>
...
</customer>
18.3. Manipulating XML data
Let's rearead and update XML documents in the application. We do this by
obtaining a dom4j session:
Document doc = ....;
Session session = factory.openSession();
Session dom4jSession = session.getSession(EntityMode.DOM4J);
Transaction tx = session.beginTransaction();
List results = dom4jSession
.createQuery("from Customer c left join fetch c.accounts where
c.lastName like :lastName")
.list();
for ( int i=0; i<results.size(); i++ ) {
//add the customer data to the XML document
Element customer = (Element) results.get(i);
doc.add(customer);
}
tx.commit();
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session.close();
Session session = factory.openSession();
Session dom4jSession = session.getSession(EntityMode.DOM4J);
Transaction tx = session.beginTransaction();
Element cust = (Element) dom4jSession.get("Customer", customerId);
for ( int i=0; i<results.size(); i++ ) {
Element customer = (Element) results.get(i);
//change the customer name in the XML and database
Element name = customer.element("name");
name.element("first-name").setText(firstName);
name.element("initial").setText(initial);
name.element("last-name").setText(lastName);
}
tx.commit();
session.close();
It is extremely useful to combine this feature with Hibernate's replicate()
operation to implement XML-based data import/export.
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Chapter 19. Improving performance
19.1. Fetching strategies
A fetching strategy is the strategy Hibernate will use for retrieving associated
objects if the application needs to navigate the association. Fetch strategies
may be declared in the O/R mapping metadata, or over-ridden by a particular
HQL or Criteria query.
Hibernate3 defines the following fetching strategies:
• Join fetching - Hibernate retrieves the associated instance or collection in
the same SELECT, using an OUTER JOIN.
• Select fetching - a second SELECT is used to retrieve the associated entity
or collection. Unless you explicitly disable lazy fetching by specifying
lazy="false", this second select will only be executed when you actually
access the association.
• Subselect fetching - a second SELECT is used to retrieve the associated
collections for all entities retrieved in a previous query or fetch. Unless
you explicitly disable lazy fetching by specifying lazy="false", this second
select will only be executed when you actually access the association.
• Batch fetching - an optimization strategy for select fetching - Hibernate
retrieves a batch of entity instances or collections in a single SELECT, by
specifying a list of primary keys or foreign keys.
Hibernate also distinguishes between:
• Immediate fetching - an association, collection or attribute is fetched
immediately, when the owner is loaded.
• Lazy collection fetching - a collection is fetched when the application
invokes an operation upon that collection. (This is the default for
collections.)
• "Extra-lazy" collection fetching - individual elements of the collection are
accessed from the database as needed. Hibernate tries not to fetch the
whole collection into memory unless absolutely needed (suitable for very
large collections)
• Proxy fetching - a single-valued association is fetched when a method
other than the identifier getter is invoked upon the associated object.
• "No-proxy" fetching - a single-valued association is fetched when the
instance variable is accessed. Compared to proxy fetching, this approach
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is less lazy (the association is fetched even when only the identifier
is accessed) but more transparent, since no proxy is visible to the
application. This approach requires buildtime bytecode instrumentation and
is rarely necessary.
• Lazy attribute fetching - an attribute or single valued association is fetched
when the instance variable is accessed. This approach requires buildtime
bytecode instrumentation and is rarely necessary.
We have two orthogonal notions here: when is the association fetched, and
how is it fetched (what SQL is used). Don't confuse them! We use fetch to
tune performance. We may use lazy to define a contract for what data is
always available in any detached instance of a particular class.
19.1.1. Working with lazy associations
By default, Hibernate3 uses lazy select fetching for collections and lazy
proxy fetching for single-valued associations. These defaults make sense for
almost all associations in almost all applications.
Note: if you set hibernate.default_batch_fetch_size, Hibernate will use
the batch fetch optimization for lazy fetching (this optimization may also be
enabled at a more granular level).
However, lazy fetching poses one problem that you must be aware of.
Access to a lazy association outside of the context of an open Hibernate
session will result in an exception. For example:
s = sessions.openSession();
Transaction tx = s.beginTransaction();
User u = (User) s.createQuery("from User u where u.name=:userName")
.setString("userName", userName).uniqueResult();
Map permissions = u.getPermissions();
tx.commit();
s.close();
Integer accessLevel = (Integer) permissions.get("accounts");
Error!
//
Since the permissions collection was not initialized when the Session was
closed, the collection will not be able to load its state. Hibernate does not
support lazy initialization for detached objects. The fix is to move the code
that reads from the collection to just before the transaction is committed.
Alternatively, we could use a non-lazy collection or association, by specifying
lazy="false" for the association mapping. However, it is intended that lazy
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initialization be used for almost all collections and associations. If you define
too many non-lazy associations in your object model, Hibernate will end up
needing to fetch the entire database into memory in every transaction!
On the other hand, we often want to choose join fetching (which is non-lazy
by nature) instead of select fetching in a particular transaction. We'll now see
how to customize the fetching strategy. In Hibernate3, the mechanisms for
choosing a fetch strategy are identical for single-valued associations and
collections.
19.1.2. Tuning fetch strategies
Select fetching (the default) is extremely vulnerable to N+1 selects problems,
so we might want to enable join fetching in the mapping document:
<set name="permissions"
fetch="join">
<key column="userId"/>
<one-to-many class="Permission"/>
</set
<many-to-one name="mother" class="Cat" fetch="join"/>
The fetch strategy defined in the mapping document affects:
• retrieval via get() or load()
• retrieval that happens implicitly when an association is navigated
• Criteria queries
• HQL queries if subselect fetching is used
No matter what fetching strategy you use, the defined non-lazy graph is
guaranteed to be loaded into memory. Note that this might result in several
immediate selects being used to execute a particular HQL query.
Usually, we don't use the mapping document to customize fetching. Instead,
we keep the default behavior, and override it for a particular transaction,
using left join fetch in HQL. This tells Hibernate to fetch the association
eagerly in the first select, using an outer join. In the Criteria query API, you
would use setFetchMode(FetchMode.JOIN).
If you ever feel like you wish you could change the fetching strategy used by
get() or load(), simply use a Criteria query, for example:
User user = (User) session.createCriteria(User.class)
.setFetchMode("permissions", FetchMode.JOIN)
.add( Restrictions.idEq(userId) )
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.uniqueResult();
(This is Hibernate's equivalent of what some ORM solutions call a "fetch
plan".)
A completely different way to avoid problems with N+1 selects is to use the
second-level cache.
19.1.3. Single-ended association proxies
Lazy fetching for collections is implemented using Hibernate's own
implementation of persistent collections. However, a different mechanism
is needed for lazy behavior in single-ended associations. The target entity
of the association must be proxied. Hibernate implements lazy initializing
proxies for persistent objects using runtime bytecode enhancement (via the
excellent CGLIB library).
By default, Hibernate3 generates proxies (at startup) for all persistent
classes and uses them to enable lazy fetching of many-to-one and one-to-one
associations.
The mapping file may declare an interface to use as the proxy interface for
that class, with the proxy attribute. By default, Hibernate uses a subclass of
the class. Note that the proxied class must implement a default constructor
with at least package visibility. We recommend this constructor for all
persistent classes!
There are some gotchas to be aware of when extending this approach to
polymorphic classes, eg.
<class name="Cat" proxy="Cat">
......
<subclass name="DomesticCat">
.....
</subclass>
</class>
Firstly, instances of Cat will never be castable to DomesticCat, even if the
underlying instance is an instance of DomesticCat:
Cat cat = (Cat) session.load(Cat.class, id);
(does not hit the db)
if ( cat.isDomesticCat() ) {
initialize the proxy
DomesticCat dc = (DomesticCat) cat;
....
}
// instantiate a proxy
// hit the db to
// Error!
Secondly, it is possible to break proxy ==.
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Cat cat = (Cat) session.load(Cat.class, id);
//
instantiate a Cat proxy
DomesticCat dc =
(DomesticCat) session.load(DomesticCat.class, id); //
acquire new DomesticCat proxy!
System.out.println(cat==dc);
// false
However, the situation is not quite as bad as it looks. Even though we now
have two references to different proxy objects, the underlying instance will
still be the same object:
cat.setWeight(11.0); // hit the db to initialize the proxy
System.out.println( dc.getWeight() ); // 11.0
Third, you may not use a CGLIB proxy for a final class or a class with any
final methods.
Finally, if your persistent object acquires any resources upon instantiation
(eg. in initializers or default constructor), then those resources will also be
acquired by the proxy. The proxy class is an actual subclass of the persistent
class.
These problems are all due to fundamental limitations in Java's single
inheritance model. If you wish to avoid these problems your persistent
classes must each implement an interface that declares its business
methods. You should specify these interfaces in the mapping file. eg.
<class name="CatImpl" proxy="Cat">
......
<subclass name="DomesticCatImpl" proxy="DomesticCat">
.....
</subclass>
</class>
where CatImpl implements the interface Cat and DomesticCatImpl implements
the interface DomesticCat. Then proxies for instances of Cat and DomesticCat
may be returned by load() or iterate(). (Note that list() does not usually
return proxies.)
Cat cat = (Cat) session.load(CatImpl.class, catid);
Iterator iter = session.createQuery("from CatImpl as cat where
cat.name='fritz'").iterate();
Cat fritz = (Cat) iter.next();
Relationships are also lazily initialized. This means you must declare any
properties to be of type Cat, not CatImpl.
Certain operations do not require proxy initialization
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• equals(), if the persistent class does not override equals()
• hashCode(), if the persistent class does not override hashCode()
• The identifier getter method
Hibernate will detect persistent classes that override equals() or hashCode().
By choosing lazy="no-proxy" instead of the default lazy="proxy", we can
avoid the problems associated with typecasting. However, we will require
buildtime bytecode instrumentation, and all operations will result in immediate
proxy initialization.
19.1.4. Initializing collections and proxies
A LazyInitializationException will be thrown by Hibernate if an uninitialized
collection or proxy is accessed outside of the scope of the Session, ie. when
the entity owning the collection or having the reference to the proxy is in the
detached state.
Sometimes we need to ensure that a proxy or collection is initialized before
closing the Session. Of course, we can alway force initialization by calling
cat.getSex() or cat.getKittens().size(), for example. But that is confusing
to readers of the code and is not convenient for generic code.
The static methods Hibernate.initialize() and Hibernate.isInitialized()
provide the application with a convenient way of working with lazily initialized
collections or proxies. Hibernate.initialize(cat) will force the initialization
of a proxy, cat, as long as its Session is still open. Hibernate.initialize(
cat.getKittens() ) has a similar effect for the collection of kittens.
Another option is to keep the Session open until all needed collections and
proxies have been loaded. In some application architectures, particularly
where the code that accesses data using Hibernate, and the code that uses
it are in different application layers or different physical processes, it can be
a problem to ensure that the Session is open when a collection is initialized.
There are two basic ways to deal with this issue:
• In a web-based application, a servlet filter can be used to close the Session
only at the very end of a user request, once the rendering of the view is
complete (the Open Session in View pattern). Of course, this places heavy
demands on the correctness of the exception handling of your application
infrastructure. It is vitally important that the Session is closed and the
transaction ended before returning to the user, even when an exception
occurs during rendering of the view. See the Hibernate Wiki for examples
of this "Open Session in View" pattern.
• In an application with a separate business tier, the business logic must
"prepare" all collections that will be needed by the web tier before
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returning. This means that the business tier should load all the data
and return all the data already initialized to the presentation/web tier
that is required for a particular use case. Usually, the application calls
Hibernate.initialize() for each collection that will be needed in the
web tier (this call must occur before the session is closed) or retrieves
the collection eagerly using a Hibernate query with a FETCH clause or
a FetchMode.JOIN in Criteria. This is usually easier if you adopt the
Command pattern instead of a Session Facade.
• You may also attach a previously loaded object to a new Session with
merge() or lock() before accessing uninitialized collections (or other
proxies). No, Hibernate does not, and certainly should not do this
automatically, since it would introduce ad hoc transaction semantics!
Sometimes you don't want to initialize a large collection, but still need some
information about it (like its size) or a subset of the data.
You can use a collection filter to get the size of a collection without initializing
it:
( (Integer) s.createFilter( collection, "select count(*)"
).list().get(0) ).intValue()
The createFilter() method is also used to efficiently retrieve subsets of a
collection without needing to initialize the whole collection:
s.createFilter( lazyCollection,
"").setFirstResult(0).setMaxResults(10).list();
19.1.5. Using batch fetching
Hibernate can make efficient use of batch fetching, that is, Hibernate can
load several uninitialized proxies if one proxy is accessed (or collections.
Batch fetching is an optimization of the lazy select fetching strategy. There
are two ways you can tune batch fetching: on the class and the collection
level.
Batch fetching for classes/entities is easier to understand. Imagine you have
the following situation at runtime: You have 25 Cat instances loaded in a
Session, each Cat has a reference to its owner, a Person. The Person class is
mapped with a proxy, lazy="true". If you now iterate through all cats and call
getOwner() on each, Hibernate will by default execute 25 SELECT statements,
to retrieve the proxied owners. You can tune this behavior by specifying a
batch-size in the mapping of Person:
<class name="Person" batch-size="10">...</class>
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Hibernate will now execute only three queries, the pattern is 10, 10, 5.
You may also enable batch fetching of collections. For example, if each
Person has a lazy collection of Cats, and 10 persons are currently loaded in
the Session, iterating through all persons will generate 10 SELECTs, one for
every call to getCats(). If you enable batch fetching for the cats collection in
the mapping of Person, Hibernate can pre-fetch collections:
<class name="Person">
<set name="cats" batch-size="3">
...
</set>
</class>
With a batch-size of 3, Hibernate will load 3, 3, 3, 1 collections in four
SELECTs. Again, the value of the attribute depends on the expected number of
uninitialized collections in a particular Session.
Batch fetching of collections is particularly useful if you have a nested tree
of items, ie. the typical bill-of-materials pattern. (Although a nested set or a
materialized path might be a better option for read-mostly trees.)
19.1.6. Using subselect fetching
If one lazy collection or single-valued proxy has to be fetched, Hibernate
loads all of them, re-running the original query in a subselect. This works in
the same way as batch-fetching, without the piecemeal loading.
19.1.7. Using lazy property fetching
Hibernate3 supports the lazy fetching of individual properties. This
optimization technique is also known as fetch groups. Please note that this is
mostly a marketing feature, as in practice, optimizing row reads is much more
important than optimization of column reads. However, only loading some
properties of a class might be useful in extreme cases, when legacy tables
have hundreds of columns and the data model can not be improved.
To enable lazy property loading, set the lazy attribute on your particular
property mappings:
<class name="Document">
<id name="id">
<generator class="native"/>
</id>
<property name="name" not-null="true" length="50"/>
<property name="summary" not-null="true" length="200"
lazy="true"/>
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<property name="text" not-null="true" length="2000"
lazy="true"/>
</class>
Lazy property loading requires buildtime bytecode instrumentation! If your
persistent classes are not enhanced, Hibernate will silently ignore lazy
property settings and fall back to immediate fetching.
For bytecode instrumentation, use the following Ant task:
<target name="instrument" depends="compile">
<taskdef name="instrument"
classname="org.hibernate.tool.instrument.InstrumentTask">
<classpath path="${jar.path}"/>
<classpath path="${classes.dir}"/>
<classpath refid="lib.class.path"/>
</taskdef>
<instrument verbose="true">
<fileset
dir="${testclasses.dir}/org/hibernate/auction/model">
<include name="*.class"/>
</fileset>
</instrument>
</target>
A different (better?) way to avoid unnecessary column reads, at least for
read-only transactions is to use the projection features of HQL or Criteria
queries. This avoids the need for buildtime bytecode processing and is
certainly a preferred solution.
You may force the usual eager fetching of properties using fetch all
properties in HQL.
19.2. The Second Level Cache
A Hibernate Session is a transaction-level cache of persistent data. It is
possible to configure a cluster or JVM-level (SessionFactory-level) cache on
a class-by-class and collection-by-collection basis. You may even plug in a
clustered cache. Be careful. Caches are never aware of changes made to
the persistent store by another application (though they may be configured to
regularly expire cached data).
You have the option to tell Hibernate which caching implementation
to use by specifying the name of a class that implements
org.hibernate.cache.CacheProvider using the property
hibernate.cache.provider_class. Hibernate comes bundled with a number
of built-in integrations with open-source cache providers (listed below);
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additionally, you could implement your own and plug it in as outlined above.
Note that versions prior to 3.2 defaulted to use EhCache as the default cache
provider; that is no longer the case as of 3.2.
Table 19.1. Cache Providers
Cache
Provider class
Type
Cluster
Safe
Query
Cache Supported
Hashtable org.hibernate.cache.HashtableCacheProvider
memory
(not
intended
for production
use)
yes
EHCache org.hibernate.cache.EhCacheProvider
memory,
disk
yes
OSCache org.hibernate.cache.OSCacheProvider
memory,
disk
yes
SwarmCache
org.hibernate.cache.SwarmCacheProvider
clustered yes
(ip
(clustered invalidation)
multicast)
JBoss
Cache
1.x
org.hibernate.cache.TreeCacheProvider
clustered
yes (replication)
yes (clock
(ip multicast),
sync req.)
transactional
JBoss
Cache 2
org.hibernate.cache.jbc2.JBossCacheRegionFactory
clustered yes (replication
yes (clock
(ip multicast),
or invalidation)
sync req.)
transactional
19.2.1. Cache mappings
The <cache> element of a class or collection mapping has the following form:
<cache
usage="transactional|read-write|nonstrict-read-write|read-only"
(1)
region="RegionName"
(2)
include="all|non-lazy"
(3)
/>
(1)
(2)
260
(required) specifies the caching strategy: transactional,
read-write, nonstrict-read-write or read-only
region (optional, defaults to the class or collection role name) specifies
the name of the second level cache region
usage
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(3)
(optional, defaults to all) non-lazy specifies that properties
of the entity mapped with lazy="true" may not be cached when
attribute-level lazy fetching is enabled
include
Alternatively (preferably?), you may specify <class-cache> and
<collection-cache> elements in hibernate.cfg.xml.
The usage attribute specifies a cache concurrency strategy.
19.2.2. Strategy: read only
If your application needs to read but never modify instances of a persistent
class, a read-only cache may be used. This is the simplest and best
performing strategy. It's even perfectly safe for use in a cluster.
<class name="eg.Immutable" mutable="false">
<cache usage="read-only"/>
....
</class>
19.2.3. Strategy: read/write
If the application needs to update data, a read-write cache
might be appropriate. This cache strategy should never be used
if serializable transaction isolation level is required. If the cache
is used in a JTA environment, you must specify the property
hibernate.transaction.manager_lookup_class, naming a strategy for
obtaining the JTA TransactionManager. In other environments, you
should ensure that the transaction is completed when Session.close() or
Session.disconnect() is called. If you wish to use this strategy in a cluster,
you should ensure that the underlying cache implementation supports
locking. The built-in cache providers do not.
<class name="eg.Cat" .... >
<cache usage="read-write"/>
....
<set name="kittens" ... >
<cache usage="read-write"/>
....
</set>
</class>
19.2.4. Strategy: nonstrict read/write
If the application only occasionally needs to update data (ie. if it
is extremely unlikely that two transactions would try to update the
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same item simultaneously) and strict transaction isolation is not
required, a nonstrict-read-write cache might be appropriate.
If the cache is used in a JTA environment, you must specify
hibernate.transaction.manager_lookup_class. In other environments, you
should ensure that the transaction is completed when Session.close() or
Session.disconnect() is called.
19.2.5. Strategy: transactional
The transactional cache strategy provides support for fully
transactional cache providers such as JBoss TreeCache. Such a
cache may only be used in a JTA environment and you must specify
hibernate.transaction.manager_lookup_class.
19.2.6. Cache-provider/concurrency-strategy
compatibility
Important
None of the cache providers support all of the cache concurrency
strategies.
The following table shows which providers are compatible with which
concurrency strategies.
Table 19.2. Cache Concurrency Strategy Support
Cache
262
read-only
nonstrictread-write
read-write
Hashtable
yes
(not intended
for production
use)
yes
yes
EHCache
yes
yes
yes
OSCache
yes
yes
yes
SwarmCache yes
yes
transactional
JBoss Cache yes
1.x
yes
JBoss Cache yes
2
yes
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19.3. Managing the caches
Whenever you pass an object to save(), update() or saveOrUpdate() and
whenever you retrieve an object using load(), get(), list(), iterate() or
scroll(), that object is added to the internal cache of the Session.
When flush() is subsequently called, the state of that object will be
synchronized with the database. If you do not want this synchronization to
occur or if you are processing a huge number of objects and need to manage
memory efficiently, the evict() method may be used to remove the object
and its collections from the first-level cache.
ScrollableResult cats = sess.createQuery("from Cat as
cat").scroll(); //a huge result set
while ( cats.next() ) {
Cat cat = (Cat) cats.get(0);
doSomethingWithACat(cat);
sess.evict(cat);
}
The Session also provides a contains() method to determine if an instance
belongs to the session cache.
To completely evict all objects from the session cache, call Session.clear()
For the second-level cache, there are methods defined on SessionFactory for
evicting the cached state of an instance, entire class, collection instance or
entire collection role.
sessionFactory.evict(Cat.class, catId); //evict a particular Cat
sessionFactory.evict(Cat.class); //evict all Cats
sessionFactory.evictCollection("Cat.kittens", catId); //evict a
particular collection of kittens
sessionFactory.evictCollection("Cat.kittens"); //evict all kitten
collections
The CacheMode controls how a particular session interacts with the
second-level cache.
• CacheMode.NORMAL - read items from and write items to the second-level
cache
• CacheMode.GET - read items from the second-level cache, but don't write to
the second-level cache except when updating data
• CacheMode.PUT - write items to the second-level cache, but don't read from
the second-level cache
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• CacheMode.REFRESH - write items to the second-level cache, but
don't read from the second-level cache, bypass the effect of
hibernate.cache.use_minimal_puts, forcing a refresh of the second-level
cache for all items read from the database
To browse the contents of a second-level or query cache region, use the
Statistics API:
Map cacheEntries = sessionFactory.getStatistics()
.getSecondLevelCacheStatistics(regionName)
.getEntries();
You'll need to enable statistics, and, optionally, force Hibernate to keep the
cache entries in a more human-understandable format:
hibernate.generate_statistics true
hibernate.cache.use_structured_entries true
19.4. The Query Cache
Query result sets may also be cached. This is only useful for queries that are
run frequently with the same parameters. To use the query cache you must
first enable it:
hibernate.cache.use_query_cache true
This setting causes the creation of two new cache regions - one holding
cached query result sets (org.hibernate.cache.StandardQueryCache), the
other holding timestamps of the most recent updates to queryable tables
(org.hibernate.cache.UpdateTimestampsCache). Note that the query cache
does not cache the state of the actual entities in the result set; it caches only
identifier values and results of value type. So the query cache should always
be used in conjunction with the second-level cache.
Most queries do not benefit from caching, so by default queries are not
cached. To enable caching, call Query.setCacheable(true). This call allows
the query to look for existing cache results or add its results to the cache
when it is executed.
If you require fine-grained control over query cache expiration policies,
you may specify a named cache region for a particular query by calling
Query.setCacheRegion().
List blogs = sess.createQuery("from Blog blog where blog.blogger =
:blogger")
.setEntity("blogger", blogger)
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.setMaxResults(15)
.setCacheable(true)
.setCacheRegion("frontpages")
.list();
If the query should force a refresh of its query cache region, you should call
Query.setCacheMode(CacheMode.REFRESH). This is particularly useful in cases
where underlying data may have been updated via a separate process (i.e.,
not modified through Hibernate) and allows the application to selectively
refresh particular query result sets. This is a more efficient alternative to
eviction of a query cache region via SessionFactory.evictQueries().
19.5. Understanding Collection performance
We've already spent quite some time talking about collections. In this section
we will highlight a couple more issues about how collections behave at
runtime.
19.5.1. Taxonomy
Hibernate defines three basic kinds of collections:
• collections of values
• one to many associations
• many to many associations
This classification distinguishes the various table and foreign key
relationships but does not tell us quite everything we need to know about the
relational model. To fully understand the relational structure and performance
characteristics, we must also consider the structure of the primary key that
is used by Hibernate to update or delete collection rows. This suggests the
following classification:
• indexed collections
• sets
• bags
All indexed collections (maps, lists, arrays) have a primary key consisting
of the <key> and <index> columns. In this case collection updates are
usually extremely efficient - the primary key may be efficiently indexed and
a particular row may be efficiently located when Hibernate tries to update or
delete it.
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Sets have a primary key consisting of <key> and element columns. This may
be less efficient for some types of collection element, particularly composite
elements or large text or binary fields; the database may not be able to index
a complex primary key as efficiently. On the other hand, for one to many or
many to many associations, particularly in the case of synthetic identifiers, it
is likely to be just as efficient. (Side-note: if you want SchemaExport to actually
create the primary key of a <set> for you, you must declare all columns as
not-null="true".)
mappings define a surrogate key, so they are always very efficient to
update. In fact, they are the best case.
<idbag>
Bags are the worst case. Since a bag permits duplicate element values and
has no index column, no primary key may be defined. Hibernate has no way
of distinguishing between duplicate rows. Hibernate resolves this problem
by completely removing (in a single DELETE) and recreating the collection
whenever it changes. This might be very inefficient.
Note that for a one-to-many association, the "primary key" may not be the
physical primary key of the database table - but even in this case, the above
classification is still useful. (It still reflects how Hibernate "locates" individual
rows of the collection.)
19.5.2. Lists, maps, idbags and sets are the most efficient
collections to update
From the discussion above, it should be clear that indexed collections and
(usually) sets allow the most efficient operation in terms of adding, removing
and updating elements.
There is, arguably, one more advantage that indexed collections have over
sets for many to many associations or collections of values. Because of the
structure of a Set, Hibernate doesn't ever UPDATE a row when an element
is "changed". Changes to a Set always work via INSERT and DELETE (of
individual rows). Once again, this consideration does not apply to one to
many associations.
After observing that arrays cannot be lazy, we would conclude that lists,
maps and idbags are the most performant (non-inverse) collection types,
with sets not far behind. Sets are expected to be the most common kind of
collection in Hibernate applications. This is because the "set" semantics are
most natural in the relational model.
However, in well-designed Hibernate domain models, we usually see that
most collections are in fact one-to-many associations with inverse="true".
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Bags and lists are the most efficient inverse
collections
For these associations, the update is handled by the many-to-one end of the
association, and so considerations of collection update performance simply
do not apply.
19.5.3. Bags and lists are the most efficient inverse
collections
Just before you ditch bags forever, there is a particular case in which
bags (and also lists) are much more performant than sets. For a collection
with inverse="true" (the standard bidirectional one-to-many relationship
idiom, for example) we can add elements to a bag or list without needing
to initialize (fetch) the bag elements! This is because Collection.add() or
Collection.addAll() must always return true for a bag or List (unlike a Set).
This can make the following common code much faster.
Parent p = (Parent) sess.load(Parent.class, id);
Child c = new Child();
c.setParent(p);
p.getChildren().add(c); //no need to fetch the collection!
sess.flush();
19.5.4. One shot delete
Occasionally, deleting collection elements one by one can be extremely
inefficient. Hibernate isn't completely stupid, so it knows not to do that in the
case of an newly-empty collection (if you called list.clear(), for example).
In this case, Hibernate will issue a single DELETE and we are done!
Suppose we add a single element to a collection of size twenty and then
remove two elements. Hibernate will issue one INSERT statement and two
DELETE statements (unless the collection is a bag). This is certainly desirable.
However, suppose that we remove eighteen elements, leaving two and then
add thee new elements. There are two possible ways to proceed
• delete eighteen rows one by one and then insert three rows
• remove the whole collection (in one SQL DELETE) and insert all five current
elements (one by one)
Hibernate isn't smart enough to know that the second option is probably
quicker in this case. (And it would probably be undesirable for Hibernate to
be that smart; such behaviour might confuse database triggers, etc.)
Fortunately, you can force this behaviour (ie. the second strategy) at any time
by discarding (ie. dereferencing) the original collection and returning a newly
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instantiated collection with all the current elements. This can be very useful
and powerful from time to time.
Of course, one-shot-delete does not apply to collections mapped
inverse="true".
19.6. Monitoring performance
Optimization is not much use without monitoring and access to performance
numbers. Hibernate provides a full range of figures about its internal
operations. Statistics in Hibernate are available per SessionFactory.
19.6.1. Monitoring a SessionFactory
You can access SessionFactory metrics in two ways. Your first option is to
call sessionFactory.getStatistics() and read or display the Statistics
yourself.
Hibernate can also use JMX to publish metrics if you enable the
StatisticsService MBean. You may enable a single MBean for all your
SessionFactory or one per factory. See the following code for minimalistic
configuration examples:
// MBean service registration for a specific SessionFactory
Hashtable tb = new Hashtable();
tb.put("type", "statistics");
tb.put("sessionFactory", "myFinancialApp");
ObjectName on = new ObjectName("hibernate", tb); // MBean object
name
StatisticsService stats = new StatisticsService(); // MBean
implementation
stats.setSessionFactory(sessionFactory); // Bind the stats to a
SessionFactory
server.registerMBean(stats, on); // Register the Mbean on the server
// MBean service registration for all SessionFactory's
Hashtable tb = new Hashtable();
tb.put("type", "statistics");
tb.put("sessionFactory", "all");
ObjectName on = new ObjectName("hibernate", tb); // MBean object
name
StatisticsService stats = new StatisticsService(); // MBean
implementation
server.registerMBean(stats, on); // Register the MBean on the server
TODO: This doesn't make sense: In the first case, we retrieve
and use the MBean directly. In the second one, we must give the
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JNDI name in which the session factory is held before using it. Use
hibernateStatsBean.setSessionFactoryJNDIName("my/JNDI/Name")
You can (de)activate the monitoring for a SessionFactory
• at configuration time, set hibernate.generate_statistics to false
• at runtime: sf.getStatistics().setStatisticsEnabled(true) or
hibernateStatsBean.setStatisticsEnabled(true)
Statistics can be reset programmatically using the clear() method. A
summary can be sent to a logger (info level) using the logSummary() method.
19.6.2. Metrics
Hibernate provides a number of metrics, from very basic to the specialized
information only relevant in certain scenarios. All available counters are
described in the Statistics interface API, in three categories:
• Metrics related to the general Session usage, such as number of open
sessions, retrieved JDBC connections, etc.
• Metrics related to he entities, collections, queries, and caches as a whole
(aka global metrics),
• Detailed metrics related to a particular entity, collection, query or cache
region.
For example, you can check the cache hit, miss, and put ratio of entities,
collections and queries, and the average time a query needs. Beware that
the number of milliseconds is subject to approximation in Java. Hibernate
is tied to the JVM precision, on some platforms this might even only be
accurate to 10 seconds.
Simple getters are used to access the global metrics (i.e. not tied to a
particular entity, collection, cache region, etc.). You can access the metrics of
a particular entity, collection or cache region through its name, and through
its HQL or SQL representation for queries. Please refer to the Statistics,
EntityStatistics, CollectionStatistics, SecondLevelCacheStatistics, and
QueryStatistics API Javadoc for more information. The following code
shows a simple example:
Statistics stats = HibernateUtil.sessionFactory.getStatistics();
double queryCacheHitCount = stats.getQueryCacheHitCount();
double queryCacheMissCount = stats.getQueryCacheMissCount();
double queryCacheHitRatio =
queryCacheHitCount / (queryCacheHitCount + queryCacheMissCount);
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log.info("Query Hit ratio:" + queryCacheHitRatio);
EntityStatistics entityStats =
stats.getEntityStatistics( Cat.class.getName() );
long changes =
entityStats.getInsertCount()
+ entityStats.getUpdateCount()
+ entityStats.getDeleteCount();
log.info(Cat.class.getName() + " changed " + changes + "times"
);
To work on all entities, collections, queries and region caches, you can
retrieve the list of names of entities, collections, queries and region
caches with the following methods: getQueries(), getEntityNames(),
getCollectionRoleNames(), and getSecondLevelCacheRegionNames().
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Chapter 20. Toolset Guide
Roundtrip engineering with Hibernate is possible using a set of Eclipse
plugins, commandline tools, as well as Ant tasks.
The Hibernate Tools currently include plugins for the Eclipse IDE as well as
Ant tasks for reverse engineering of existing databases:
• Mapping Editor: An editor for Hibernate XML mapping files, supporting
auto-completion and syntax highlighting. It also supports semantic
auto-completion for class names and property/field names, making it much
more versatile than a normal XML editor.
• Console: The console is a new view in Eclipse. In addition to a tree
overview of your console configurations, you also get an interactive view of
your persistent classes and their relationships. The console allows you to
execute HQL queries against your database and browse the result directly
in Eclipse.
• Development Wizards: Several wizards are provided with the Hibernate
Eclipse tools; you can use a wizard to quickly generate Hibernate
configuration (cfg.xml) files, or you may even completely reverse engineer
an existing database schema into POJO source files and Hibernate
mapping files. The reverse engineering wizard supports customizable
templates.
• Ant Tasks:
Please refer to the Hibernate Tools package and it's documentation for more
information.
However, the Hibernate main package comes bundled with an integrated tool
(it can even be used from "inside" Hibernate on-the-fly): SchemaExport aka
hbm2ddl.
20.1. Automatic schema generation
DDL may be generated from your mapping files by a Hibernate utility. The
generated schema includes referential integrity constraints (primary and
foreign keys) for entity and collection tables. Tables and sequences are also
created for mapped identifier generators.
You must specify a SQL Dialect via the hibernate.dialect property when
using this tool, as DDL is highly vendor specific.
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First, customize your mapping files to improve the generated schema.
20.1.1. Customizing the schema
Many Hibernate mapping elements define optional attributes named length,
precision and scale. You may set the length, precision and scale of a column
with this attribute.
<property name="zip" length="5"/>
<property name="balance" precision="12" scale="2"/>
Some tags also accept a not-null attribute (for generating a NOT NULL
constraint on table columns) and a unique attribute (for generating UNIQUE
constraint on table columns).
<many-to-one name="bar" column="barId" not-null="true"/>
<element column="serialNumber" type="long" not-null="true"
unique="true"/>
A unique-key attribute may be used to group columns in a single unique key
constraint. Currently, the specified value of the unique-key attribute is not
used to name the constraint in the generated DDL, only to group the columns
in the mapping file.
<many-to-one name="org" column="orgId" unique-key="OrgEmployeeId"/>
<property name="employeeId" unique-key="OrgEmployee"/>
An index attribute specifies the name of an index that will be created using
the mapped column or columns. Multiple columns may be grouped into the
same index, simply by specifying the same index name.
<property name="lastName" index="CustName"/>
<property name="firstName" index="CustName"/>
A foreign-key attribute may be used to override the name of any generated
foreign key constraint.
<many-to-one name="bar" column="barId" foreign-key="FKFooBar"/>
Many mapping elements also accept a child <column> element. This is
particularly useful for mapping multi-column types:
<property name="name" type="my.customtypes.Name"/>
<column name="last" not-null="true" index="bar_idx"
length="30"/>
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<column name="first" not-null="true" index="bar_idx"
length="20"/>
<column name="initial"/>
</property>
The default attribute lets you specify a default value for a column (you
should assign the same value to the mapped property before saving a new
instance of the mapped class).
<property name="credits" type="integer" insert="false">
<column name="credits" default="10"/>
</property>
<version name="version" type="integer" insert="false">
<column name="version" default="0"/>
</property>
The sql-type attribute allows the user to override the default mapping of a
Hibernate type to SQL datatype.
<property name="balance" type="float">
<column name="balance" sql-type="decimal(13,3)"/>
</property>
The check attribute allows you to specify a check constraint.
<property name="foo" type="integer">
<column name="foo" check="foo > 10"/>
</property>
<class name="Foo" table="foos" check="bar < 100.0">
...
<property name="bar" type="float"/>
</class>
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Table 20.1. Summary
Attribute
Values
Interpretation
length
number
column length
precision
number
column decimal precision
scale
number
column decimal scale
not-null
true|false
specfies that the column should be
non-nullable
unique
true|false
specifies that the column should have a
unique constraint
index
index_name
specifies the name of a (multi-column)
index
unique-key
unique_key_name
specifies the name of a multi-column
unique constraint
foreign-key
foreign_key_namespecifies
sql-type
SQL column type
default
SQL expression specify a default value for the column
check
SQL expression create an SQL check constraint on either
column or table
the name of the foreign key
constraint generated for an association,
for a <one-to-one>, <many-to-one>, <key>,
or <many-to-many> mapping element.
Note that inverse="true" sides will not be
considered by SchemaExport.
overrides the default column type
(attribute of <column> element only)
The <comment> element allows you to specify comments for the generated
schema.
<class name="Customer" table="CurCust">
<comment>Current customers only</comment>
...
</class>
<property name="balance">
<column name="bal">
<comment>Balance in USD</comment>
</column>
</property>
This results in a comment on table or comment on column statement in the
generated DDL (where supported).
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20.1.2. Running the tool
The SchemaExport tool writes a DDL script to standard out and/or executes
the DDL statements.
hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaExport
options mapping_files
java -cp
Table 20.2. SchemaExport Command Line Options
Option
Description
--quiet
don't output the script to stdout
--drop
only drop the tables
--create
only create the tables
--text
don't export to the database
--output=my_schema.ddl
output the ddl script to a file
--naming=eg.MyNamingStrategy
select a NamingStrategy
--config=hibernate.cfg.xml
read Hibernate configuration from an XML
file
--
read database properties from a file
properties=hibernate.properties
--format
format the generated SQL nicely in the
script
--delimiter=;
set an end of line delimiter for the script
You may even embed SchemaExport in your application:
Configuration cfg = ....;
new SchemaExport(cfg).create(false, true);
20.1.3. Properties
Database properties may be specified
• as system properties with -D<property>
• in hibernate.properties
• in a named properties file with --properties
The needed properties are:
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Table 20.3. SchemaExport Connection Properties
Property Name
Description
hibernate.connection.driver_class
jdbc
driver class
hibernate.connection.url
jdbc url
hibernate.connection.username
database user
hibernate.connection.password
user password
hibernate.dialect
dialect
20.1.4. Using Ant
You can call SchemaExport from your Ant build script:
<target name="schemaexport">
<taskdef name="schemaexport"
classname="org.hibernate.tool.hbm2ddl.SchemaExportTask"
classpathref="class.path"/>
<schemaexport
properties="hibernate.properties"
quiet="no"
text="no"
drop="no"
delimiter=";"
output="schema-export.sql">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemaexport>
</target>
20.1.5. Incremental schema updates
The SchemaUpdate tool will update an existing schema with "incremental"
changes. Note that SchemaUpdate depends heavily upon the JDBC metadata
API, so it will not work with all JDBC drivers.
hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaUpdate
options mapping_files
java -cp
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Table 20.4. SchemaUpdate Command Line Options
Option
Description
--quiet
don't output the script to stdout
--text
don't export the script to the database
--naming=eg.MyNamingStrategy
select a NamingStrategy
--
read database properties from a file
properties=hibernate.properties
--config=hibernate.cfg.xml
specify a .cfg.xml file
You may embed SchemaUpdate in your application:
Configuration cfg = ....;
new SchemaUpdate(cfg).execute(false);
20.1.6. Using Ant for incremental schema updates
You can call SchemaUpdate from the Ant script:
<target name="schemaupdate">
<taskdef name="schemaupdate"
classname="org.hibernate.tool.hbm2ddl.SchemaUpdateTask"
classpathref="class.path"/>
<schemaupdate
properties="hibernate.properties"
quiet="no">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemaupdate>
</target>
20.1.7. Schema validation
The SchemaValidator tool will validate that the existing database schema
"matches" your mapping documents. Note that SchemaValidator depends
heavily upon the JDBC metadata API, so it will not work with all JDBC
drivers. This tool is extremely useful for testing.
hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaValidator
options mapping_files
java -cp
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Table 20.5. SchemaValidator Command Line Options
Option
Description
--naming=eg.MyNamingStrategy
select a NamingStrategy
--
read database properties from a file
properties=hibernate.properties
--config=hibernate.cfg.xml
specify a .cfg.xml file
You may embed SchemaValidator in your application:
Configuration cfg = ....;
new SchemaValidator(cfg).validate();
20.1.8. Using Ant for schema validation
You can call SchemaValidator from the Ant script:
<target name="schemavalidate">
<taskdef name="schemavalidator"
classname="org.hibernate.tool.hbm2ddl.SchemaValidatorTask"
classpathref="class.path"/>
<schemavalidator
properties="hibernate.properties">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemavalidator>
</target>
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Chapter 21. Example: Parent/Child
One of the very first things that new users try to do with Hibernate is to model
a parent / child type relationship. There are two different approaches to this.
For various reasons the most convenient approach, especially for new users,
is to model both Parent and Child as entity classes with a <one-to-many>
association from Parent to Child. (The alternative approach is to declare the
Child as a <composite-element>.) Now, it turns out that default semantics of
a one to many association (in Hibernate) are much less close to the usual
semantics of a parent / child relationship than those of a composite element
mapping. We will explain how to use a bidirectional one to many association
with cascades to model a parent / child relationship efficiently and elegantly.
It's not at all difficult!
21.1. A note about collections
Hibernate collections are considered to be a logical part of their owning
entity; never of the contained entities. This is a crucial distinction! It has the
following consequences:
• When we remove / add an object from / to a collection, the version number
of the collection owner is incremented.
• If an object that was removed from a collection is an instance of a value
type (eg, a composite element), that object will cease to be persistent and
its state will be completely removed from the database. Likewise, adding a
value type instance to the collection will cause its state to be immediately
persistent.
• On the other hand, if an entity is removed from a collection (a one-to-many
or many-to-many association), it will not be deleted, by default. This
behaviour is completely consistent - a change to the internal state of
another entity should not cause the associated entity to vanish! Likewise,
adding an entity to a collection does not cause that entity to become
persistent, by default.
Instead, the default behaviour is that adding an entity to a collection merely
creates a link between the two entities, while removing it removes the link.
This is very appropriate for all sorts of cases. Where it is not appropriate at all
is the case of a parent / child relationship, where the life of the child is bound
to the life cycle of the parent.
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21.2. Bidirectional one-to-many
Suppose we start with a simple <one-to-many> association from Parent to
Child.
<set name="children">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
If we were to execute the following code
Parent p = .....;
Child c = new Child();
p.getChildren().add(c);
session.save(c);
session.flush();
Hibernate would issue two SQL statements:
• an INSERT to create the record for c
• an UPDATE to create the link from p to c
This is not only inefficient, but also violates any NOT NULL constraint on the
parent_id column. We can fix the nullability constraint violation by specifying
not-null="true" in the collection mapping:
<set name="children">
<key column="parent_id" not-null="true"/>
<one-to-many class="Child"/>
</set>
However, this is not the recommended solution.
The underlying cause of this behaviour is that the link (the foreign key
parent_id) from p to c is not considered part of the state of the Child object
and is therefore not created in the INSERT. So the solution is to make the link
part of the Child mapping.
<many-to-one name="parent" column="parent_id" not-null="true"/>
(We also need to add the parent property to the Child class.)
Now that the Child entity is managing the state of the link, we tell the
collection not to update the link. We use the inverse attribute.
<set name="children" inverse="true">
<key column="parent_id"/>
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<one-to-many class="Child"/>
</set>
The following code would be used to add a new Child
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
c.setParent(p);
p.getChildren().add(c);
session.save(c);
session.flush();
And now, only one SQL INSERT would be issued!
To tighten things up a bit, we could create an addChild() method of Parent.
public void addChild(Child c) {
c.setParent(this);
children.add(c);
}
Now, the code to add a Child looks like
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.save(c);
session.flush();
21.3. Cascading life cycle
The explicit call to save() is still annoying. We will address this by using
cascades.
<set name="children" inverse="true" cascade="all">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
This simplifies the code above to
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.flush();
Similarly, we don't need to iterate over the children when saving or deleting a
Parent. The following removes p and all its children from the database.
Parent p = (Parent) session.load(Parent.class, pid);
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session.delete(p);
session.flush();
However, this code
Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
c.setParent(null);
session.flush();
will not remove c from the database; it will ony remove the link to p (and
cause a NOT NULL constraint violation, in this case). You need to explicitly
delete() the Child.
Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
session.delete(c);
session.flush();
Now, in our case, a Child can't really exist without its parent. So if we remove
a Child from the collection, we really do want it to be deleted. For this, we
must use cascade="all-delete-orphan".
<set name="children" inverse="true" cascade="all-delete-orphan">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
Note: even though the collection mapping specifies inverse="true",
cascades are still processed by iterating the collection elements. So if you
require that an object be saved, deleted or updated by cascade, you must
add it to the collection. It is not enough to simply call setParent().
21.4. Cascades and unsaved-value
Suppose we loaded up a Parent in one Session, made some changes in
a UI action and wish to persist these changes in a new session by calling
update(). The Parent will contain a collection of childen and, since cascading
update is enabled, Hibernate needs to know which children are newly
instantiated and which represent existing rows in the database. Lets assume
that both Parent and Child have genenerated identifier properties of type
Long. Hibernate will use the identifier and version/timestamp property value
to determine which of the children are new. (See Section 10.7, “Automatic
state detection”.) In Hibernate3, it is no longer necessary to specify an
unsaved-value explicitly.
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The following code will update parent and child and insert newChild.
//parent and child were both loaded in a previous session
parent.addChild(child);
Child newChild = new Child();
parent.addChild(newChild);
session.update(parent);
session.flush();
Well, that's all very well for the case of a generated identifier, but what about
assigned identifiers and composite identifiers? This is more difficult, since
Hibernate can't use the identifier property to distinguish between a newly
instantiated object (with an identifier assigned by the user) and an object
loaded in a previous session. In this case, Hibernate will either use the
timestamp or version property, or will actually query the second-level cache
or, worst case, the database, to see if the row exists.
21.5. Conclusion
There is quite a bit to digest here and it might look confusing first time
around. However, in practice, it all works out very nicely. Most Hibernate
applications use the parent / child pattern in many places.
We mentioned an alternative in the first paragraph. None of the above issues
exist in the case of <composite-element> mappings, which have exactly the
semantics of a parent / child relationship. Unfortunately, there are two big
limitations to composite element classes: composite elements may not own
collections, and they should not be the child of any entity other than the
unique parent.
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Chapter 22. Example: Weblog
Application
22.1. Persistent Classes
The persistent classes represent a weblog, and an item posted in a weblog.
They are to be modelled as a standard parent/child relationship, but we will
use an ordered bag, instead of a set.
package eg;
import java.util.List;
public class Blog {
private Long _id;
private String _name;
private List _items;
public Long getId() {
return _id;
}
public List getItems() {
return _items;
}
public String getName() {
return _name;
}
public void setId(Long long1) {
_id = long1;
}
public void setItems(List list) {
_items = list;
}
public void setName(String string) {
_name = string;
}
}
package eg;
import java.text.DateFormat;
import java.util.Calendar;
public class BlogItem {
private Long _id;
private Calendar _datetime;
private String _text;
private String _title;
private Blog _blog;
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public Blog getBlog() {
return _blog;
}
public Calendar getDatetime() {
return _datetime;
}
public Long getId() {
return _id;
}
public String getText() {
return _text;
}
public String getTitle() {
return _title;
}
public void setBlog(Blog blog) {
_blog = blog;
}
public void setDatetime(Calendar calendar) {
_datetime = calendar;
}
public void setId(Long long1) {
_id = long1;
}
public void setText(String string) {
_text = string;
}
public void setTitle(String string) {
_title = string;
}
}
22.2. Hibernate Mappings
The XML mappings should now be quite straightforward.
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class
name="Blog"
table="BLOGS">
<id
name="id"
column="BLOG_ID">
<generator class="native"/>
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</id>
<property
name="name"
column="NAME"
not-null="true"
unique="true"/>
<bag
name="items"
inverse="true"
order-by="DATE_TIME"
cascade="all">
<key column="BLOG_ID"/>
<one-to-many class="BlogItem"/>
</bag>
</class>
</hibernate-mapping>
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class
name="BlogItem"
table="BLOG_ITEMS"
dynamic-update="true">
<id
name="id"
column="BLOG_ITEM_ID">
<generator class="native"/>
</id>
<property
name="title"
column="TITLE"
not-null="true"/>
<property
name="text"
column="TEXT"
not-null="true"/>
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<property
name="datetime"
column="DATE_TIME"
not-null="true"/>
<many-to-one
name="blog"
column="BLOG_ID"
not-null="true"/>
</class>
</hibernate-mapping>
22.3. Hibernate Code
The following class demonstrates some of the kinds of things we can do with
these classes, using Hibernate.
package eg;
import
import
import
import
java.util.ArrayList;
java.util.Calendar;
java.util.Iterator;
java.util.List;
import
import
import
import
import
import
import
org.hibernate.HibernateException;
org.hibernate.Query;
org.hibernate.Session;
org.hibernate.SessionFactory;
org.hibernate.Transaction;
org.hibernate.cfg.Configuration;
org.hibernate.tool.hbm2ddl.SchemaExport;
public class BlogMain {
private SessionFactory _sessions;
public void configure() throws HibernateException {
_sessions = new Configuration()
.addClass(Blog.class)
.addClass(BlogItem.class)
.buildSessionFactory();
}
public void exportTables() throws HibernateException {
Configuration cfg = new Configuration()
.addClass(Blog.class)
.addClass(BlogItem.class);
new SchemaExport(cfg).create(true, true);
}
public Blog createBlog(String name) throws HibernateException {
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Blog blog = new Blog();
blog.setName(name);
blog.setItems( new ArrayList() );
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
session.persist(blog);
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return blog;
}
public BlogItem createBlogItem(Blog blog, String title, String
text)
throws HibernateException {
BlogItem item = new BlogItem();
item.setTitle(title);
item.setText(text);
item.setBlog(blog);
item.setDatetime( Calendar.getInstance() );
blog.getItems().add(item);
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
session.update(blog);
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return item;
}
public BlogItem createBlogItem(Long blogid, String title, String
text)
throws HibernateException {
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BlogItem item = new BlogItem();
item.setTitle(title);
item.setText(text);
item.setDatetime( Calendar.getInstance() );
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
Blog blog = (Blog) session.load(Blog.class, blogid);
item.setBlog(blog);
blog.getItems().add(item);
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return item;
}
public void updateBlogItem(BlogItem item, String text)
throws HibernateException {
item.setText(text);
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
session.update(item);
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
}
public void updateBlogItem(Long itemid, String text)
throws HibernateException {
Session session = _sessions.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();
BlogItem item = (BlogItem) session.load(BlogItem.class,
itemid);
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item.setText(text);
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
}
public List listAllBlogNamesAndItemCounts(int max)
throws HibernateException {
Session session = _sessions.openSession();
Transaction tx = null;
List result = null;
try {
tx = session.beginTransaction();
Query q = session.createQuery(
"select blog.id, blog.name, count(blogItem) " +
"from Blog as blog " +
"left outer join blog.items as blogItem " +
"group by blog.name, blog.id " +
"order by max(blogItem.datetime)"
);
q.setMaxResults(max);
result = q.list();
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return result;
}
public Blog getBlogAndAllItems(Long blogid)
throws HibernateException {
Session session = _sessions.openSession();
Transaction tx = null;
Blog blog = null;
try {
tx = session.beginTransaction();
Query q = session.createQuery(
"from Blog as blog " +
"left outer join fetch blog.items " +
"where blog.id = :blogid"
);
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q.setParameter("blogid", blogid);
blog = (Blog) q.uniqueResult();
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return blog;
}
public List listBlogsAndRecentItems() throws HibernateException
{
Session session = _sessions.openSession();
Transaction tx = null;
List result = null;
try {
tx = session.beginTransaction();
Query q = session.createQuery(
"from Blog as blog " +
"inner join blog.items as blogItem " +
"where blogItem.datetime > :minDate"
);
Calendar cal = Calendar.getInstance();
cal.roll(Calendar.MONTH, false);
q.setCalendar("minDate", cal);
result = q.list();
tx.commit();
}
catch (HibernateException he) {
if (tx!=null) tx.rollback();
throw he;
}
finally {
session.close();
}
return result;
}
}
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Chapter 23. Example: Various
Mappings
This chapters shows off some more complex association mappings.
23.1. Employer/Employee
The following model of the relationship between Employer and Employee uses
an actual entity class (Employment) to represent the association. This is done
because there might be more than one period of employment for the same
two parties. Components are used to model monetary values and employee
names.
Heres a possible mapping document:
<hibernate-mapping>
<class name="Employer" table="employers">
<id name="id">
<generator class="sequence">
<param name="sequence">employer_id_seq</param>
</generator>
</id>
<property name="name"/>
</class>
<class name="Employment" table="employment_periods">
<id name="id">
<generator class="sequence">
<param name="sequence">employment_id_seq</param>
</generator>
</id>
<property name="startDate" column="start_date"/>
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<property name="endDate" column="end_date"/>
<component name="hourlyRate" class="MonetaryAmount">
<property name="amount">
<column name="hourly_rate" sql-type="NUMERIC(12,
2)"/>
</property>
<property name="currency" length="12"/>
</component>
<many-to-one name="employer" column="employer_id"
not-null="true"/>
<many-to-one name="employee" column="employee_id"
not-null="true"/>
</class>
<class name="Employee" table="employees">
<id name="id">
<generator class="sequence">
<param name="sequence">employee_id_seq</param>
</generator>
</id>
<property name="taxfileNumber"/>
<component name="name" class="Name">
<property name="firstName"/>
<property name="initial"/>
<property name="lastName"/>
</component>
</class>
</hibernate-mapping>
And heres the table schema generated by SchemaExport.
create table employers (
id BIGINT not null,
name VARCHAR(255),
primary key (id)
)
create table employment_periods (
id BIGINT not null,
hourly_rate NUMERIC(12, 2),
currency VARCHAR(12),
employee_id BIGINT not null,
employer_id BIGINT not null,
end_date TIMESTAMP,
start_date TIMESTAMP,
primary key (id)
)
create table employees (
id BIGINT not null,
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Author/Work
firstName VARCHAR(255),
initial CHAR(1),
lastName VARCHAR(255),
taxfileNumber VARCHAR(255),
primary key (id)
)
alter table employment_periods
add constraint employment_periodsFK0 foreign key (employer_id)
references employers
alter table employment_periods
add constraint employment_periodsFK1 foreign key (employee_id)
references employees
create sequence employee_id_seq
create sequence employment_id_seq
create sequence employer_id_seq
23.2. Author/Work
Consider the following model of the relationships between Work, Author
and Person. We represent the relationship between Work and Author as a
many-to-many association. We choose to represent the relationship between
Author and Person as one-to-one association. Another possibility would be to
have Author extend Person.
The following mapping document correctly represents these relationships:
<hibernate-mapping>
<class name="Work" table="works" discriminator-value="W">
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<id name="id" column="id">
<generator class="native"/>
</id>
<discriminator column="type" type="character"/>
<property name="title"/>
<set name="authors" table="author_work">
<key column name="work_id"/>
<many-to-many class="Author" column name="author_id"/>
</set>
<subclass name="Book" discriminator-value="B">
<property name="text"/>
</subclass>
<subclass name="Song" discriminator-value="S">
<property name="tempo"/>
<property name="genre"/>
</subclass>
</class>
<class name="Author" table="authors">
<id name="id" column="id">
<!-- The Author must have the same identifier as the
Person -->
<generator class="assigned"/>
</id>
<property name="alias"/>
<one-to-one name="person" constrained="true"/>
<set name="works" table="author_work" inverse="true">
<key column="author_id"/>
<many-to-many class="Work" column="work_id"/>
</set>
</class>
<class name="Person" table="persons">
<id name="id" column="id">
<generator class="native"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
There are four tables in this mapping. works, authors and persons hold
work, author and person data respectively. author_work is an association
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Customer/Order/Product
table linking authors to works. Heres the table schema, as generated by
SchemaExport.
create table works (
id BIGINT not null generated by default as identity,
tempo FLOAT,
genre VARCHAR(255),
text INTEGER,
title VARCHAR(255),
type CHAR(1) not null,
primary key (id)
)
create table author_work (
author_id BIGINT not null,
work_id BIGINT not null,
primary key (work_id, author_id)
)
create table authors (
id BIGINT not null generated by default as identity,
alias VARCHAR(255),
primary key (id)
)
create table persons (
id BIGINT not null generated by default as identity,
name VARCHAR(255),
primary key (id)
)
alter table authors
add constraint authorsFK0 foreign key (id) references persons
alter table author_work
add constraint author_workFK0 foreign key (author_id) references
authors
alter table author_work
add constraint author_workFK1 foreign key (work_id) references
works
23.3. Customer/Order/Product
Now consider a model of the relationships between Customer, Order and
LineItem and Product. There is a one-to-many association between Customer
and Order, but how should we represent Order / LineItem / Product?
I've chosen to map LineItem as an association class representing the
many-to-many association between Order and Product. In Hibernate, this is
called a composite element.
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The mapping document:
<hibernate-mapping>
<class name="Customer" table="customers">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="orders" inverse="true">
<key column="customer_id"/>
<one-to-many class="Order"/>
</set>
</class>
<class name="Order" table="orders">
<id name="id">
<generator class="native"/>
</id>
<property name="date"/>
<many-to-one name="customer" column="customer_id"/>
<list name="lineItems" table="line_items">
<key column="order_id"/>
<list-index column="line_number"/>
<composite-element class="LineItem">
<property name="quantity"/>
<many-to-one name="product" column="product_id"/>
</composite-element>
</list>
</class>
<class name="Product" table="products">
<id name="id">
<generator class="native"/>
</id>
<property name="serialNumber"/>
</class>
</hibernate-mapping>
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customers, orders, line_items
and products hold customer, order, order line
item and product data respectively. line_items also acts as an association
table linking orders with products.
create table customers (
id BIGINT not null generated by default as identity,
name VARCHAR(255),
primary key (id)
)
create table orders (
id BIGINT not null generated by default as identity,
customer_id BIGINT,
date TIMESTAMP,
primary key (id)
)
create table line_items (
line_number INTEGER not null,
order_id BIGINT not null,
product_id BIGINT,
quantity INTEGER,
primary key (order_id, line_number)
)
create table products (
id BIGINT not null generated by default as identity,
serialNumber VARCHAR(255),
primary key (id)
)
alter table orders
add constraint ordersFK0 foreign key (customer_id) references
customers
alter table line_items
add constraint line_itemsFK0 foreign key (product_id) references
products
alter table line_items
add constraint line_itemsFK1 foreign key (order_id) references
orders
23.4. Miscellaneous example mappings
These examples are all taken from the Hibernate test suite. You will find
many other useful example mappings there. Look in the test folder of the
Hibernate distribution.
TODO: put words around this stuff
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23.4.1. "Typed" one-to-one association
<class name="Person">
<id name="name"/>
<one-to-one name="address"
cascade="all">
<formula>name</formula>
<formula>'HOME'</formula>
</one-to-one>
<one-to-one name="mailingAddress"
cascade="all">
<formula>name</formula>
<formula>'MAILING'</formula>
</one-to-one>
</class>
<class name="Address" batch-size="2"
check="addressType in ('MAILING', 'HOME', 'BUSINESS')">
<composite-id>
<key-many-to-one name="person"
column="personName"/>
<key-property name="type"
column="addressType"/>
</composite-id>
<property name="street" type="text"/>
<property name="state"/>
<property name="zip"/>
</class>
23.4.2. Composite key example
<class name="Customer">
<id name="customerId"
length="10">
<generator class="assigned"/>
</id>
<property name="name" not-null="true" length="100"/>
<property name="address" not-null="true" length="200"/>
<list name="orders"
inverse="true"
cascade="save-update">
<key column="customerId"/>
<index column="orderNumber"/>
<one-to-many class="Order"/>
</list>
</class>
<class name="Order" table="CustomerOrder" lazy="true">
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<synchronize table="LineItem"/>
<synchronize table="Product"/>
<composite-id name="id"
class="Order$Id">
<key-property name="customerId" length="10"/>
<key-property name="orderNumber"/>
</composite-id>
<property name="orderDate"
type="calendar_date"
not-null="true"/>
<property name="total">
<formula>
( select sum(li.quantity*p.price)
from LineItem li, Product p
where li.productId = p.productId
and li.customerId = customerId
and li.orderNumber = orderNumber )
</formula>
</property>
<many-to-one name="customer"
column="customerId"
insert="false"
update="false"
not-null="true"/>
<bag name="lineItems"
fetch="join"
inverse="true"
cascade="save-update">
<key>
<column name="customerId"/>
<column name="orderNumber"/>
</key>
<one-to-many class="LineItem"/>
</bag>
</class>
<class name="LineItem">
<composite-id name="id"
class="LineItem$Id">
<key-property name="customerId" length="10"/>
<key-property name="orderNumber"/>
<key-property name="productId" length="10"/>
</composite-id>
<property name="quantity"/>
<many-to-one name="order"
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insert="false"
update="false"
not-null="true">
<column name="customerId"/>
<column name="orderNumber"/>
</many-to-one>
<many-to-one name="product"
insert="false"
update="false"
not-null="true"
column="productId"/>
</class>
<class name="Product">
<synchronize table="LineItem"/>
<id name="productId"
length="10">
<generator class="assigned"/>
</id>
<property name="description"
not-null="true"
length="200"/>
<property name="price" length="3"/>
<property name="numberAvailable"/>
<property name="numberOrdered">
<formula>
( select sum(li.quantity)
from LineItem li
where li.productId = productId )
</formula>
</property>
</class>
23.4.3. Many-to-many with shared composite key attribute
<class name="User" table="`User`">
<composite-id>
<key-property name="name"/>
<key-property name="org"/>
</composite-id>
<set name="groups" table="UserGroup">
<key>
<column name="userName"/>
<column name="org"/>
</key>
<many-to-many class="Group">
<column name="groupName"/>
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Content based discrimination
<formula>org</formula>
</many-to-many>
</set>
</class>
<class name="Group" table="`Group`">
<composite-id>
<key-property name="name"/>
<key-property name="org"/>
</composite-id>
<property name="description"/>
<set name="users" table="UserGroup" inverse="true">
<key>
<column name="groupName"/>
<column name="org"/>
</key>
<many-to-many class="User">
<column name="userName"/>
<formula>org</formula>
</many-to-many>
</set>
</class>
23.4.4. Content based discrimination
<class name="Person"
discriminator-value="P">
<id name="id"
column="person_id"
unsaved-value="0">
<generator class="native"/>
</id>
<discriminator
type="character">
<formula>
case
when title is not null then 'E'
when salesperson is not null then 'C'
else 'P'
end
</formula>
</discriminator>
<property name="name"
not-null="true"
length="80"/>
<property name="sex"
not-null="true"
update="false"/>
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<component name="address">
<property name="address"/>
<property name="zip"/>
<property name="country"/>
</component>
<subclass name="Employee"
discriminator-value="E">
<property name="title"
length="20"/>
<property name="salary"/>
<many-to-one name="manager"/>
</subclass>
<subclass name="Customer"
discriminator-value="C">
<property name="comments"/>
<many-to-one name="salesperson"/>
</subclass>
</class>
23.4.5. Associations on alternate keys
<class name="Person">
<id name="id">
<generator class="hilo"/>
</id>
<property name="name" length="100"/>
<one-to-one name="address"
property-ref="person"
cascade="all"
fetch="join"/>
<set name="accounts"
inverse="true">
<key column="userId"
property-ref="userId"/>
<one-to-many class="Account"/>
</set>
<property name="userId" length="8"/>
</class>
<class name="Address">
<id name="id">
<generator class="hilo"/>
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</id>
<property name="address" length="300"/>
<property name="zip" length="5"/>
<property name="country" length="25"/>
<many-to-one name="person" unique="true" not-null="true"/>
</class>
<class name="Account">
<id name="accountId" length="32">
<generator class="uuid"/>
</id>
<many-to-one name="user"
column="userId"
property-ref="userId"/>
<property name="type" not-null="true"/>
</class>
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Chapter 24. Best Practices
Write fine-grained classes and map them using <component>.
Use an Address class to encapsulate street, suburb, state, postcode. This
encourages code reuse and simplifies refactoring.
Declare identifier properties on persistent classes.
Hibernate makes identifier properties optional. There are all sorts of
reasons why you should use them. We recommend that identifiers be
'synthetic' (generated, with no business meaning).
Identify natural keys.
Identify natural keys for all entities, and map them using <natural-id>.
Implement equals() and hashCode() to compare the properties that make
up the natural key.
Place each class mapping in its own file.
Don't use a single monolithic mapping document. Map com.eg.Foo in the
file com/eg/Foo.hbm.xml. This makes particularly good sense in a team
environment.
Load mappings as resources.
Deploy the mappings along with the classes they map.
Consider externalising query strings.
This is a good practice if your queries call non-ANSI-standard SQL
functions. Externalising the query strings to mapping files will make the
application more portable.
Use bind variables.
As in JDBC, always replace non-constant values by "?". Never use
string manipulation to bind a non-constant value in a query! Even better,
consider using named parameters in queries.
Don't manage your own JDBC connections.
Hibernate lets the application manage JDBC connections. This
approach should be considered a last-resort. If you can't use the built-in
connections providers, consider providing your own implementation of
org.hibernate.connection.ConnectionProvider.
Consider using a custom type.
Suppose you have a Java type, say from some library, that needs to
be persisted but doesn't provide the accessors needed to map it as a
component. You should consider implementing org.hibernate.UserType.
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This approach frees the application code from implementing
transformations to / from a Hibernate type.
Use hand-coded JDBC in bottlenecks.
In performance-critical areas of the system, some kinds of operations
might benefit from direct JDBC. But please, wait until you know
something is a bottleneck. And don't assume that direct JDBC is
necessarily faster. If you need to use direct JDBC, it might be worth
opening a Hibernate Session and using that JDBC connection. That way
you can still use the same transaction strategy and underlying connection
provider.
Understand Session flushing.
From time to time the Session synchronizes its persistent state with
the database. Performance will be affected if this process occurs too
often. You may sometimes minimize unnecessary flushing by disabling
automatic flushing or even by changing the order of queries and other
operations within a particular transaction.
In a three tiered architecture, consider using detached objects.
When using a servlet / session bean architecture, you could pass
persistent objects loaded in the session bean to and from the
servlet / JSP layer. Use a new session to service each request. Use
Session.merge() or Session.saveOrUpdate() to synchronize objects with
the database.
In a two tiered architecture, consider using long persistence contexts.
Database Transactions have to be as short as possible for best
scalability. However, it is often neccessary to implement long running
application transactions, a single unit-of-work from the point of
view of a user. An application transaction might span several client
request/response cycles. It is common to use detached objects to
implement application transactions. An alternative, extremely appropriate
in two tiered architecture, is to maintain a single open persistence contact
(session) for the whole life cycle of the application transaction and simply
disconnect from the JDBC connection at the end of each request and
reconnect at the beginning of the subsequent request. Never share a
single session across more than one application transaction, or you will
be working with stale data.
Don't treat exceptions as recoverable.
This is more of a necessary practice than a "best" practice. When an
exception occurs, roll back the Transaction and close the Session. If
you don't, Hibernate can't guarantee that in-memory state accurately
represents persistent state. As a special case of this, do not use
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to determine if an instance with the given identifier exists
on the database; use Session.get() or a query instead.
Session.load()
Prefer lazy fetching for associations.
Use eager fetching sparingly. Use proxies and lazy collections for most
associations to classes that are not likely to be completely held in the
second-level cache. For associations to cached classes, where there
is an a extremely high probability of a cache hit, explicitly disable eager
fetching using lazy="false". When an join fetching is appropriate to a
particular use case, use a query with a left join fetch.
Use the open session in view pattern, or a disciplined assembly phase to
avoid problems with unfetched data.
Hibernate frees the developer from writing tedious Data Transfer Objects
(DTO). In a traditional EJB architecture, DTOs serve dual purposes:
first, they work around the problem that entity beans are not serializable;
second, they implicitly define an assembly phase where all data to
be used by the view is fetched and marshalled into the DTOs before
returning control to the presentation tier. Hibernate eliminates the first
purpose. However, you will still need an assembly phase (think of your
business methods as having a strict contract with the presentation tier
about what data is available in the detached objects) unless you are
prepared to hold the persistence context (the session) open across
the view rendering process. This is not a limitation of Hibernate! It is a
fundamental requirement of safe transactional data access.
Consider abstracting your business logic from Hibernate.
Hide (Hibernate) data-access code behind an interface. Combine the
DAO and Thread Local Session patterns. You can even have some
classes persisted by handcoded JDBC, associated to Hibernate via a
UserType. (This advice is intended for "sufficiently large" applications; it is
not appropriate for an application with five tables!)
Don't use exotic association mappings.
Good usecases for a real many-to-many associations are rare. Most
of the time you need additional information stored in the "link table".
In this case, it is much better to use two one-to-many associations to
an intermediate link class. In fact, we think that most associations are
one-to-many and many-to-one, you should be careful when using any
other association style and ask yourself if it is really neccessary.
Prefer bidirectional associations.
Unidirectional associations are more difficult to query. In a large
application, almost all associations must be navigable in both directions in
queries.
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Hibernate 3.3.1
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