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HIBERNATE - 符合Java习惯的关系数据库持久化
1
Hibernate参考文档
3.3.2.GA
由 Gavin King、Christian Bauer、Max Rydahl Andersen、Emmanuel Bernard和Steve Ebersole
and thanks to James Cobb (Graphic Design)、Cheyenne
Weaver (Graphic Design)和Cao Xiaogang
前言 ............................................................................ xi
1. Feedback .............................................................. xii
1. Tutorial ..................................................................... 1
1.1. 第一部分 - 第一个Hibernate应用程序 .................................... 1
1.1.1. Setup ........................................................... 1
1.1.2. 第一个class ..................................................... 3
1.1.3. 映射文件 ........................................................ 5
1.1.4. Hibernate配置 ................................................... 7
1.1.5. Building with Maven ........................................... 10
1.1.6. 启动和辅助类 ................................................... 10
1.1.7. 加载并存储对象 ................................................. 12
1.2. 第二部分 - 关联映射 ................................................. 15
1.2.1. 映射Person类 ................................................... 15
1.2.2. 单向Set-based的关联 ............................................ 16
1.2.3. 使关联工作 ..................................................... 18
1.2.4. 值类型的集合 ................................................... 20
1.2.5. 双向关联 ....................................................... 21
1.2.6. 使双向连起来 ................................................... 22
1.3. 第三部分 - EventManager web应用程序 .................................. 23
1.3.1. 编写基本的servlet .............................................. 23
1.3.2. 最后,当处理与渲染都结束的时候,这个工作单元就结束了。假若在处
理或渲染的时候有任何错误发生,会抛出一个异常,回滚数据库事务。这
样,session-per-request模式就完成了。为了避免在每个servlet中都编写事务边
界界定的代码,可以考虑写一个servlet 过滤器(filter)来更好地解决。关于
这一模式的更多信息,请参阅Hibernate网站和Wiki,这一模式叫做Open
Session in View-只要你考虑用JSP来渲染你的视图(view),而不是在servlet
中,你就会很快用到它。 ................................................ 25
1.3.3. 部署与测试 ..................................................... 27
1.4. 总结 ................................................................. 28
2. 体系结构(Architecture) ....................................................... 29
2.1. 概况(Overview) ........................................................ 29
2.2. 实例状态 .............................................................. 30
2.3. JMX整合 .............................................................. 31
2.4. 对JCA的支持 ........................................................... 31
2.5. Contextual sessions .................................................. 31
3. 配置 ........................................................................ 35
3.1. 可编程的配置方式 ...................................................... 35
3.2. 获得SessionFactory .................................................... 36
3.3. JDBC连接 .............................................................. 36
3.4. 可选的配置属性 ........................................................ 38
3.4.1. SQL方言 ........................................................ 45
3.4.2. 外连接抓取(Outer Join Fetching) ................................ 46
3.4.3. 二进制流 (Binary Streams) ...................................... 46
3.4.4. 二级缓存与查询缓存 ............................................. 46
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HIBERNATE - 符合Java习惯的关系数据库持久化
3.4.5. 查询语言中的替换 ...............................................
3.4.6. Hibernate的统计(statistics)机制 ................................
3.5. 日志 .................................................................
3.6. 实现NamingStrategy .....................................................
3.7. XML配置文件 ...........................................................
3.8. J2EE应用程序服务器的集成 ..............................................
3.8.1. 事务策略配置 ...................................................
3.8.2. JNDI绑定的SessionFactory ........................................
3.8.3. 在JTA环境下使用Current Session context (当前session上下文)管理 ..
3.8.4. JMX部署 ........................................................
4. 持久化类(Persistent Classes) ................................................
4.1. 一个简单的POJO例子 ....................................................
4.1.1. 实现一个默认的(即无参数的)构造方法(constructor) .............
4.1.2. 提供一个标识属性(identifier property)(可选) .................
4.1.3. 使用非final的类 (可选) .........................................
4.1.4. 为持久化字段声明访问器(accessors)和是否可变的标志(mutators)(可
选) ...................................................................
4.2. 实现继承(Inheritance) ...............................................
4.3. 实现equals()和hashCode() ................................................
4.4. 动态模型(Dynamic models) ..............................................
4.5. 元组片断映射(Tuplizers) ...............................................
4.6. EntityNameResolvers ...................................................
5. 对象/关系数据库映射基础(Basic O/R Mapping) ..................................
5.1. 映射定义(Mapping declaration) .......................................
5.1.1. Doctype ........................................................
5.1.2. Hibernate-mapping ..............................................
5.1.3. Class ..........................................................
5.1.4. id .............................................................
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 and dynamic-component ...............................
5.1.16. Properties ....................................................
5.1.17. Subclass ......................................................
5.1.18. Joined-subclass ...............................................
5.1.19. Union-subclass ................................................
5.1.20. Join ..........................................................
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5.1.24.
Key ..........................................................
Column and formula elements ..................................
Import .......................................................
Any ..........................................................
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5.2. Hibernate types .....................................................
5.2.1. 实体(Entities)和值(values) ....................................
5.2.2. 基本值类型 ....................................................
5.2.3. 自定义值类型 ..................................................
5.3. 多次映射同一个类 .....................................................
5.4. SQL中引号包围的标识符 ................................................
5.5. 其他元数据(Metadata) .................................................
5.5.1. 使用 XDoclet 标记 ............................................
5.5.2. 使用 JDK 5.0 的注解(Annotation) ...............................
5.6. Generated properties ................................................
5.7. Auxiliary database objects ..........................................
6. Collection mapping .........................................................
6.1. 持久化集合类(Persistent collections) .................................
6.2. 集合映射( Collection mappings ) ...................................
6.2.1. 集合外键(Collection foreign keys) .............................
6.2.2. 集合元素(Collection elements) ...............................
6.2.3. 索引集合类(Indexed collections) ...............................
6.2.4. 对于一个值集合, 我们使用<element>标签。 ........................
6.2.5. 一对多关联通过外键连接两个类对应的表,而没有中间集合表。 这个关系
模型失去了一些Java集合的语义: ........................................
6.3. 高级集合映射(Advanced collection mappings) .........................
6.3.1. 有序集合(Sorted collections) ................................
6.3.2. 双向关联(Bidirectional associations) ........................
6.3.3. 双向关联,涉及有序集合类 ......................................
6.3.4. 三重关联(Ternary associations) ..............................
6.3.5. 使用<idbag> ....................................................
6.4. 集合例子(Collection example) .......................................
7. 关联关系映射 ...............................................................
7.1. 介绍 ................................................................
7.2. 单向关联(Unidirectional associations) ..............................
7.2.1. Many-to-one ...................................................
7.2.2. One-to-one ....................................................
7.2.3. One-to-many ...................................................
7.3. 使用连接表的单向关联(Unidirectional associations with join tables) ..
7.3.1. One-to-many ...................................................
7.3.2. Many-to-one ...................................................
7.3.3. One-to-one ....................................................
7.3.4. Many-to-many ..................................................
7.4. 双向关联(Bidirectional associations) ...............................
7.4.1. one-to-many / many-to-one .....................................
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7.4.2. One-to-one ....................................................
7.5. 使用连接表的双向关联(Bidirectional associations with join tables) ...
7.5.1. one-to-many / many-to-one .....................................
7.5.2. 一对一(one to one) .........................................
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7.5.3. Many-to-many ..................................................
7.6. 更复杂的关联映射 .....................................................
8. 组件(Component)映射 .......................................................
8.1. 依赖对象(Dependent objects) ........................................
8.2. 在集合中出现的依赖对象 (Collections of dependent objects) ............
8.3. 组件作为Map的索引(Components as Map indices ) ......................
8.4. 组件作为联合标识符(Components as composite identifiers) ..............
8.5. 动态组件 (Dynamic components) ......................................
9. Inheritance mapping ........................................................
9.1. The three strategies ................................................
9.1.1. 每个类分层结构一张表(Table per class hierarchy) ...............
9.1.2. 每个子类一张表(Table per subclass) ............................
9.1.3. Table per subclass: using a discriminator .....................
9.1.4. 混合使用“每个类分层结构一张表”和“每个子类一张表” ...........
9.1.5. 每个具体类一张表(Table per concrete class) ....................
9.1.6. Table per concrete class using implicit polymorphism ..........
9.1.7. 隐式多态和其他继承映射混合使用 ................................
9.2. 限制 ................................................................
10. 与对象共事 ................................................................
10.1. Hibernate对象状态(object states) ....................................
10.2. 使对象持久化 ........................................................
10.3. 装载对象 ............................................................
10.4. 查询 ...............................................................
10.4.1. 执行查询 .....................................................
10.4.2. 过滤集合 .....................................................
10.4.3. 条件查询(Criteria queries) ...................................
10.4.4. 使用原生SQL的查询 ............................................
10.5. 修改持久对象 ........................................................
10.6. 修改脱管(Detached)对象 ..............................................
10.7. 自动状态检测 ........................................................
10.8. 删除持久对象 ........................................................
10.9. 在两个不同数据库间复制对象 ..........................................
10.10. Session刷出(flush) .................................................
10.11. 传播性持久化(transitive persistence) ...............................
10.12. 使用元数据 .........................................................
11. Transactions and Concurrency ..............................................
11.1. Session和事务范围(transaction scope) ................................
11.1.1. 操作单元(Unit of work) .......................................
11.1.2. 长对话 .......................................................
11.1.3. 关注对象标识(Considering object identity) ....................
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11.1.4. 常见问题 .....................................................
11.2. 数据库事务声明 ......................................................
11.2.1. 非托管环境 ...................................................
11.2.2. 使用JTA ......................................................
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11.2.3. 异常处理 ..................................................... 191
11.2.4. 事务超时 ..................................................... 192
11.3. 乐观并发控制(Optimistic concurrency control) ........................ 193
11.3.1. 应用程序级别的版本检查(Application version checking) .......... 193
11.3.2. 扩展周期的session和自动版本化 ................................ 194
11.3.3. 脱管对象(deatched object)和自动版本化 ........................ 195
11.3.4. 定制自动版本化行为 ........................................... 196
11.4. Pessimistic locking ................................................ 196
11.5. Connection release modes ........................................... 197
拦截器与事件(Interceptors and events) ..................................... 199
12.1. 拦截器(Interceptors) ................................................ 199
12.2. 事件系统(Event system) .............................................. 201
12.3. Hibernate的声明式安全机制 ........................................... 203
批鉉変牉)Batch processing) ......................................... 205
13.1.
批鉉扉å
¥ï¼‰Batch inserts) ...................................................... 206
13.2. 批鉉扴扰)Batch updates) ..................................... 206
13.3. StatelessSession (æ‰ ç‰¶æ‰‰session)扥剣 ........................... 207
13.4. DML(扰扮扉佉è¯è¨‰)é£‰æ ¼ç‰‰æ‰‰ä½‰(DML-style operations) ......... 208
HQL: Hibernate查询语言 .................................................... 211
14.1. 大小写敏感性问题 .................................................... 211
14.2. from子句 ............................................................ 211
14.3. 关联(Association)与连接(Join) ....................................... 212
14.4. join 语法的形式 .................................................... 214
14.5. Referring to identifier property ................................... 214
14.6. select子句 .......................................................... 214
14.7. 聚集函数 ............................................................ 216
14.8. 多态查询 ............................................................ 217
14.9. where子句 ........................................................... 217
14.10. 表达式 ............................................................. 219
14.11. order by子句 ...................................................... 224
14.12. group by子句 ...................................................... 224
14.13. 子查询 ............................................................. 225
14.14. HQL示例 ............................................................ 226
14.15. 批量的UPDATE和DELETE ............................................... 228
14.16. 小技巧 & 小窍门 ................................................... 228
14.17. translator-credits ................................................. 230
14.18. Row value constructor syntax ...................................... 231
条件查询(Criteria Queries) ................................................ 233
15.1. 创建一个Criteria 实例 ............................................... 233
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HIBERNATE - 符合Java习惯的关系数据库持久化
15.2.
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15.4.
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限制结果集内容 ......................................................
结果集排序 ..........................................................
关联 ...............................................................
动态关联抓取 ........................................................
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15.6. 查询示例 ............................................................
15.7. 投影(Projections)、聚合(aggregation)和分组(grouping) .............
15.8. 离线(detached)查询和子查询 ..........................................
15.9. 根据自然标识查询(Queries by natural identifier) .....................
Native SQL查询 ............................................................
16.1. 使用SQLQuery ........................................................
16.1.1. 标量查询(Scalar queries) ...................................
16.1.2. 实体查询(Entity queries) .....................................
16.1.3. 处理关联和集合类(Handling associations and collections) .......
16.1.4. 返回多个实体(Returning multiple entities) ....................
16.1.5. 返回非受管实体(Returning non-managed entities) ...............
16.1.6. 处理继承(Handling inheritance) .............................
16.1.7. 参数(Parameters) ...........................................
16.2. 命名SQL查询 .........................................................
16.2.1. 使用return-property来明确地指定字段/别名 ......................
16.2.2. 使用存储过程来查询 ...........................................
16.3. 定制SQL用来create,update和delete ...................................
16.4. 定制装载SQL .........................................................
过滤数据 ..................................................................
17.1. Hibernate 过滤器(filters) ...........................................
XML映射 ...................................................................
18.1. 用XML数据进行工作 ...................................................
18.1.1. 指定同时映射XML和类 ..........................................
18.1.2. 只定义XML映射 ................................................
18.2. XML映射元数据 .......................................................
18.3. 操作XML数据 .........................................................
提升性能 ..................................................................
19.1. 抓取策略(Fetching strategies) .......................................
19.1.1. 操作延迟加载的关联 ...........................................
19.1.2. 调整抓取策略(Tuning fetch strategies) ......................
19.1.3. 单端关联代理(Single-ended association proxies) ..............
19.1.4. 实例化集合和代理(Initializing collections and proxies) ......
19.1.5. 使用批量抓取(Using batch fetching) .........................
19.1.6. 使用子查询抓取(Using subselect fetching) ...................
19.1.7. 使用延迟属性抓取(Using lazy property fetching) .............
19.2. 二级缓存(The Second Level Cache) .................................
19.2.1. 缓存映射(Cache mappings) ...................................
19.2.2. 策略:只读缓存(Strategy: read only) ........................
19.2.3. 策略:读/写缓存(Strategy: read/write) .......................
19.2.4. 策略:非严格读/写缓存(Strategy: nonstrict read/write) ........
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19.2.5. 策略:事务缓存(transactional) ...............................
19.2.6. Cache-provider/concurrency-strategy compatibility .............
19.3. 管理缓存(Managing the caches) .....................................
19.4. 查询缓存(The Query Cache) ........................................
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19.5. 理解集合性能(Understanding Collection performance) ................
19.5.1. 分类(Taxonomy) .............................................
19.5.2. Lists, maps 和sets用于更新效率最高 ...........................
19.5.3. Bag和list是反向集合类中效率最高的 ............................
19.5.4. 一次性删除(One shot delete) ................................
19.6. 监测性能(Monitoring performance) ..................................
19.6.1. 监测SessionFactory ...........................................
19.6.2. 数据记录(Metrics) ..........................................
工具箱指南 ................................................................
20.1. Schema自动生成(Automatic schema generation) .......................
20.1.1. 对schema定制化(Customizing the schema) .......................
20.1.2. 运行该工具 ...................................................
20.1.3. 属性(Properties) .............................................
20.1.4. 使用Ant(Using Ant) ...........................................
20.1.5. 对schema的增量更新(Incremental schema updates) ...............
20.1.6. 用Ant来增量更新schema(Using Ant for incremental schema updates)
20.1.7. Schema 校验 ..................................................
20.1.8. 使用Ant进行schema校验 ........................................
示例:父子关系(Parent Child Relationships) .................................
21.1. 关于collections需要注意的一点 .......................................
21.2. 双向的一对多关系(Bidirectional one-to-many) .........................
21.3. 级联生命周期(Cascading life cycle) ................................
21.4. 级联与未保存值(Cascades and unsaved-value) ..........................
21.5. 结论 ...............................................................
示例:Weblog 应用程序 .....................................................
22.1. 持久化类 ............................................................
22.2. Hibernate 映射 .....................................................
22.3. Hibernate 代码 .....................................................
示例:复杂映射实例 ........................................................
23.1. Employer(雇主)/Employee(雇员) ......................................
23.2. Author(作家)/Work(作品) .............................................
23.3. Customer(客户)/Order(订单)/Product(产品) ............................
23.4. 杂例 ...............................................................
23.4.1. "Typed" one-to-one association ...............................
23.4.2. Composite key example ........................................
23.4.3. 共有组合键属性的多对多(Many-to-many with shared composite key
attribute) ...........................................................
23.4.4. Content based discrimination .................................
23.4.5. Associations on alternate keys ...............................
最佳实践(Best Practices) ..................................................
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HIBERNATE - 符合Java习惯的关系数据库持久化
25. Database Portability Considerations .......................................
25.1. Portability Basics .................................................
25.2. Dialect .............................................................
25.3. Dialect resolution .................................................
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25.6.
References
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Identifier generation ...............................................
Database functions .................................................
Type mappings ......................................................
.....................................................................
前言
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. It can also significantly reduce development time otherwise spent with
manual data handling in SQL and JDBC.
Hibernate's 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.
如果你对Hibernate和对象/关系数据库映射还是个新手,或者甚至对Java也不熟悉,请按照下面
的步骤来学习。
1. 阅读第 1 章 Tutorial,这是一篇包含详细的逐步指导的指南。本指南的源代码包含在发行包
中,你可以在doc/reference/tutorial/目录下找到。
2. 阅读第 2 章 体系结构(Architecture)来理解Hibernate可以使用的环境。
3. View 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 [JPwH] 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 from [JPwH].
5. 在Hibernate 的网站上可以找到经常提问的问题与解答(FAQ)。
6. Links to third party demos, examples, and tutorials are maintained on the Hibernate
website.
7. Hibernate网站的“社区(Community Area)”是讨论关于设计模式以及很多整合方案(Tomcat,
JBoss AS, Struts, EJB,等等)的好地方。
xi
前言
If you have questions, use the user forum linked on the Hibernate website. We also
provide a JIRA issue tracking 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.
商业开发、产品支持和Hibernate培训可以通过JBoss
Inc.获得。(请查阅:http://
www.hibernate.org/SupportTraining/)。
Hibernate是一个专业的开放源代码项目
(Professional Open Source project),也是JBoss Enterprise Middleware System(JEMS),JBoss
企业级中间件系统的一个核心组件。
1. Feedback
Use Hibernate JIRA [http://opensource.atlassian.com/projects/hibernate] to report
errors or request enhacements to this documentation.
xii
第 1
Tutorial
Intended for new users, this chapter provides an step-by-step introduction to Hibernate,
starting with a simple application using an in-memory database. The tutorial is based on
an earlier tutorial developed by Michael Gloegl. All code is contained in the tutorials/
web directory of the project source.
重要
This tutorial expects the user have knowledge of both Java and SQL. If
you have a limited knowledge of JAVA or SQL, it is advised that you
start with a good introduction to that technology prior to attempting
to learn Hibernate.
注意
The distribution contains another example application under the tutorial/
eg project source directory.
1.1. 第一部分 - 第一个Hibernate应用程序
For this example, we will set up a small database application that can store events we
want to attend and information about the host(s) of these events.
注意
Although you can use whatever database you feel comfortable using, we will
use HSQLDB [http://hsqldb.org/] (an in-memory, Java database) to avoid
describing installation/setup of any particular database servers.
1.1.1. Setup
The first thing we need to do is to set up the development environment.
We will be using the "standard layout" advocated by alot of build tools
such as Maven [http://maven.org]. Maven, in particular, has a good resource
describing this layout [http://maven.apache.org/guides/introduction/introduction-tothe-standard-directory-layout.html]. As this tutorial is to be a web application, we
will be creating and making use of src/main/java, src/main/resources and src/main/webapp
directories.
1
第 1 章 Tutorial
We will be using Maven in this tutorial, taking advantage of its transitive dependency
management capabilities as well as the ability of many IDEs to automatically set up
a project for us based on the maven descriptor.
<project xmlns="http://maven.apache.org/POM/4.0.0"
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">
<modelVersion>4.0.0</modelVersion>
<groupId>org.hibernate.tutorials</groupId>
<artifactId>hibernate-tutorial</artifactId>
<version>1.0.0-SNAPSHOT</version>
<name>First Hibernate Tutorial</name>
<build>
<!-- we dont want the version to be part of the generated war file name -->
<finalName>${artifactId}</finalName>
</build>
<dependencies>
<dependency>
<groupId>org.hibernate</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>
<!-- Hibernate uses slf4j for logging, for our purposes here use the simple backend -->
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-simple</artifactId>
</dependency>
<!-- Hibernate gives you a choice of bytecode providers between cglib and javassist -->
<dependency>
<groupId>javassist</groupId>
<artifactId>javassist</artifactId>
</dependency>
2
第一个class
</dependencies>
</project>
提示
It is not a requirement to use Maven. If you wish to use something
else to build this tutoial (such as Ant), the layout will remain the
same. The only change is that you will need to manually account for
all the needed dependencies. If you use something like Ivy [http://
ant.apache.org/ivy/] providing transitive dependency management you would
still use the dependencies mentioned below. Otherwise, you'd need to
grab all dependencies, both explicit and transitive, and add them to the
project's classpath. If working from the Hibernate distribution bundle,
this would mean hibernate3.jar, all artifacts in the lib/required directory
and all files from either the lib/bytecode/cglib or lib/bytecode/javassist
directory; additionally you will need both the servlet-api jar and one
of the slf4j logging backends.
Save this file as pom.xml in the project root directory.
1.1.2. 第一个class
Next, we create a class that represents the event we want to store in the database;
it 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;
public Event() {}
public Long getId() {
return id;
}
3
第 1 章 Tutorial
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;
}
}
This class uses standard JavaBean naming conventions for property getter and setter
methods, as well as private visibility for the fields. Although this is the recommended
design, it is not required. Hibernate can also access fields directly, the benefit of
accessor methods is robustness for refactoring.
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, especially web applications, need to distinguish objects by identifier,
so you should consider this a feature rather than a limitation. However, we usually do
not manipulate the identity of an object, hence the setter method should be private. Only
Hibernate will assign identifiers when an object is saved. Hibernate can access public,
private, and protected accessor methods, as well as public, private and 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 or public visibility is required for runtime proxy generation and
efficient data retrieval without bytecode instrumentation.
Save this file to the src/main/java/org/hibernate/tutorial/domain directory.
4
映射文件
1.1.3. 映射文件
Hibernate需要知道怎样去加载(load)和存储(store)持久化类的对象。这正是Hibernate映射
文件发挥作用的地方。映射文件告诉Hibernate它,应该访问数据库(database)里面的哪个表
(table)及应该使用表里面的哪些字段(column)。
一个映射文件的基本结构看起来像这样:
<?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="org.hibernate.tutorial.domain">
[...]
</hibernate-mapping>
Hibernate DTD is sophisticated. You can use it for auto-completion of XML mapping
elements and attributes in your editor or IDE. Opening up the DTD file in your text
editor is the easiest way to get an overview of all elements and attributes, and to
view the defaults, as well as some comments. 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 hibernate-core.jar (it is also included in the hibernate3.jar, if using
the distribution bundle).
重要
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, which are not first-class
entities) need a mapping to a table in the SQL database:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Event" table="EVENTS">
</class>
</hibernate-mapping>
5
第 1 章 Tutorial
So far we have told Hibernate how to persist and load object of class Event to the table
EVENTS. Each instance is now represented by a row in that table. Now we can continue
by mapping the unique identifier property to the tables primary key. As we do not want
to care about handling this identifier, we configure Hibernate's identifier generation
strategy for a surrogate primary key column:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="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. The name="id" mapping
attribute declares the name of the JavaBean property and tells Hibernate to use the
getId() and setId() methods to access the property. The column attribute tells Hibernate
which column of the EVENTS table holds the primary key value.
The nested generator element specifies the identifier generation strategy (aka how are
identifier values generated?). In this case we choose native, which offers a level of
portability depending on the configured database dialect. Hibernate supports database
generated, globally unique, as well as application assigned, identifiers. Identifier
value generation is also one of Hibernate's many extension points and you can plugin
in your own strategy.
提示
native is no longer consider the best strategy in terms of portability.
for further discussion, see 第 25.4 节 “Identifier generation”
Lastly, we need to tell Hibernate about the remaining entity class properties. By
default, no properties of the class are considered persistent:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Event" table="EVENTS">
<id name="id" column="EVENT_ID">
<generator class="native"/>
6
Hibernate配置
</id>
<property name="date" type="timestamp" column="EVENT_DATE"/>
<property name="title"/>
</class>
</hibernate-mapping>
Similar to the id element, the name attribute of the property element tells Hibernate
which getter and setter methods to use. In this case, Hibernate will search for getDate(),
setDate(), getTitle() and setTitle() methods.
注意
Why does the date property mapping include the column attribute, but the
title does not? Without the column attribute, Hibernate by default uses
the property name as the column name. This works for title, however, date
is a reserved keyword in most databases so you will need to map it to
a different name.
The title mapping also lacks a type attribute. The types declared and used in the mapping
files are not Java data types; they are not SQL database types either. These types are
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 cannot
know if the property, which is of java.util.Date, should map to a SQL date, timestamp,
or time column. Full date and time information is preserved by mapping the property
with a timestamp converter.
提示
Hibernate makes this mapping type determination using reflection when the
mapping files are processed. This can take time and resources, so if
startup performance is important you should consider explicitly defining
the type to use.
Save this mapping file as src/main/resources/org/hibernate/tutorial/domain/Event.hbm.xml.
1.1.4. Hibernate配置
At this point, you should have the persistent class and its mapping file in place. It
is now time to configure Hibernate. First let's set up HSQLDB to run in "server mode"
7
第 1 章 Tutorial
注意
We do this do that the data remains between runs.
We will utilize the Maven exec plugin to launch the HSQLDB server by running:
mvn exec:java -Dexec.mainClass="org.hsqldb.Server" -Dexec.args="-database.0 file:target/data/
tutorial" You will 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 HSQLDB, delete all files in the target/data directory, and start
HSQLDB again.
Hibernate will be connecting to the database on behalf of your application, so it needs
to know how to obtain connections. For this tutorial we will be using a standalone
connection pool (as opposed to a javax.sql.DataSource). Hibernate comes with support
for two third-party open source JDBC connection pools: c3p0 [https://sourceforge.net/
projects/c3p0] and proxool [http://proxool.sourceforge.net/]. However, we will be using
the Hibernate built-in connection pool for this tutorial.
小心
The built-in Hibernate connection pool is in no way intended for production
use. It lacks several features found on any decent connection pool.
For Hibernate's configuration, we can use a simple hibernate.properties file, a 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>
8
Hibernate配置
<!-- JDBC connection pool (use the built-in) -->
<property name="connection.pool_size">1</property>
<!-- SQL dialect -->
<property name="dialect">org.hibernate.dialect.HSQLDialect</property>
<!-- 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">update</property>
<mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/>
</session-factory>
</hibernate-configuration>
注意
Notice that this configuration file specifies a different DTD
You configure Hibernate's SessionFactory. SessionFactory is a global factory responsible
for a particular database. If you have several databases, for easier startup you should
use several <session-factory> configurations in several configuration files.
The first four property elements contain the necessary configuration for the JDBC
connection. The dialect property element specifies the particular SQL variant Hibernate
generates.
提示
In most cases, Hibernate is able to properly determine which dialect to
use. See 第 25.3 节 “Dialect resolution” for more information.
9
第 1 章 Tutorial
Hibernate's automatic session management for persistence contexts is particularly useful
in this context. The hbm2ddl.auto option turns on automatic generation of database schemas
directly into the database. This can also be turned off by removing the configuration
option, or redirected to a file with the help of the SchemaExport Ant task. Finally,
add the mapping file(s) for persistent classes to the configuration.
Save this file as hibernate.cfg.xml into the src/main/resources directory.
1.1.5. Building with Maven
We will now build the tutorial with Maven. You will need to have Maven installed; it is
available from the Maven download page [http://maven.apache.org/download.html]. Maven
will read the /pom.xml file we created earlier and know how to perform some basic project
tasks. First, lets run the compile goal to make sure we can compile everything so far:
[hibernateTutorial]$ mvn compile
[INFO] Scanning for projects...
[INFO] -----------------------------------------------------------------------[INFO] Building First Hibernate Tutorial
[INFO] task-segment: [compile]
[INFO] -----------------------------------------------------------------------[INFO] [resources:resources]
[INFO] Using default encoding to copy filtered resources.
[INFO] [compiler:compile]
[INFO] Compiling 1 source file to /home/steve/projects/sandbox/hibernateTutorial/target/classes
[INFO] -----------------------------------------------------------------------[INFO] BUILD SUCCESSFUL
[INFO] -----------------------------------------------------------------------[INFO] Total time: 2 seconds
[INFO] Finished at: Tue Jun 09 12:25:25 CDT 2009
[INFO] Final Memory: 5M/547M
[INFO] ------------------------------------------------------------------------
1.1.6. 启动和辅助类
It is time to load and store some Event objects, but first you have to complete the
setup with some infrastructure code. You have to startup Hibernate by building a
global org.hibernate.SessionFactory object and storing it somewhere for easy access in
application code. A org.hibernate.SessionFactory is used to obtain org.hibernate.Session
instances. A org.hibernate.Session represents a single-threaded unit of work. The
org.hibernate.SessionFactory is a thread-safe global object that is instantiated once.
We will create a HibernateUtil helper class that takes care of startup and makes accessing
the org.hibernate.SessionFactory more convenient.
10
启动和辅助类
package org.hibernate.tutorial.util;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
public class HibernateUtil {
private static final SessionFactory sessionFactory = buildSessionFactory();
private static SessionFactory buildSessionFactory() {
try {
// Create the SessionFactory from hibernate.cfg.xml
return 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;
}
}
Save this code as src/main/java/org/hibernate/tutorial/util/HibernateUtil.java
This class not only produces the global org.hibernate.SessionFactory reference in its
static initializer; it also hides the fact that it uses a static singleton. We might
just as well have looked up the org.hibernate.SessionFactory reference from JNDI in an
application server or any other location for that matter.
If you give the org.hibernate.SessionFactory a name in your configuration, Hibernate will
try to bind it to JNDI under that name after it has been built. Another, better option
is to use a JMX deployment and let the JMX-capable container instantiate and bind a
HibernateService to JNDI. Such advanced options are discussed later.
You now need to configure a logging system. Hibernate uses commons
provides two choices: Log4j and JDK 1.4 logging. Most developers prefer
log4j.properties from the Hibernate distribution in the etc/ directory
directory, next to hibernate.cfg.xml. If you prefer to have more verbose
logging and
Log4j: copy
to your src
output than
11
第 1 章 Tutorial
that provided in the example configuration, you can change the settings. By default,
only the Hibernate startup message is shown on stdout.
The tutorial infrastructure is complete and you are now ready to do some real work
with Hibernate.
1.1.7. 加载并存储对象
We are now ready to start doing some real worjk with Hibernate. Let's start by writing
an EventManager class with a main() method:
package org.hibernate.tutorial;
import org.hibernate.Session;
import java.util.*;
import org.hibernate.tutorial.domain.Event;
import org.hibernate.tutorial.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());
}
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();
}
12
加载并存储对象
}
In createAndStoreEvent() we created a new Event object and handed it over to Hibernate.
At that point, Hibernate takes care of the SQL and executes an INSERT on the database.
A org.hibernate.Session is designed to represent a single unit of work (a single
atmoic piece of work to be performed). For now we will keep things simple and assume
a one-to-one granularity between a Hibernate org.hibernate.Session and a database
transaction. To shield our code from the actual underlying transaction system we use
the Hibernate org.hibernate.Transaction API. In this particular case we are using JDBCbased transactional semantics, but it could also run with JTA.
What does sessionFactory.getCurrentSession() do? First, you can call it as many times
and anywhere you like once you get hold of your org.hibernate.SessionFactory. The
getCurrentSession() method always returns the "current" unit of work. Remember that
we switched the configuration option for this mechanism to "thread" in our src/main/
resources/hibernate.cfg.xml? Due to that setting, the context of a current unit of work
is bound to the current Java thread that executes the application.
重要
Hibernate offers three methods of current session tracking. The "thread"
based method is not intended for production use; it is merely useful for
prototyping and tutorials such as this one. Current session tracking is
discussed in more detail later on.
A org.hibernate.Session begins when the first call to getCurrentSession() is made
for the current thread. It is then bound by Hibernate to the current thread. When
the transaction ends, either through commit or rollback, Hibernate automatically
unbinds the org.hibernate.Session from the thread and closes it for you. If you call
getCurrentSession() again, you get a new org.hibernate.Session and can start a new unit
of work.
Related to the unit of work scope, should the Hibernate org.hibernate.Session be
used to execute one or several database operations? The above example uses one
org.hibernate.Session for one operation. However this is pure coincidence; the example
is just not complex enough to show any other approach. The scope of a Hibernate
org.hibernate.Session is flexible but you should never design your application to use
a new Hibernate org.hibernate.Session for every database operation. Even though it is
used in the following examples, consider session-per-operation an anti-pattern. A real
web application is shown later in the tutorial which will help illustrate this.
See 第 11 章 Transactions and Concurrency for more information about transaction handling
and demarcation. The previous example also skipped any error handling and rollback.
13
第 1 章 Tutorial
To run this, we will make use of the Maven exec plugin to call our class with the necessary
classpath setup: mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" Dexec.args="store"
注意
You may need to perform mvn compile first.
You should see Hibernate starting up and, depending on your configuration, lots of log
output. Towards the end, the following line will be displayed:
[java] Hibernate: insert into EVENTS (EVENT_DATE, title, EVENT_ID) values (?, ?, ?)
This is the INSERT executed by Hibernate.
To list stored events an option is added to the main method:
if (args[0].equals("store")) {
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()
);
}
}
A new listEvents() method is also added:
private List listEvents() {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
List result = session.createQuery("from Event").list();
session.getTransaction().commit();
return result;
}
14
第二部分 - 关联映射
Here, we are using a Hibernate Query Language (HQL) 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. See 第 14 章 HQL: Hibernate查询语言 for more information.
Now we can call our new functionality, again using the Maven exec plugin: mvn exec:java
-Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="list"
1.2. 第二部分 - 关联映射
So far we have mapped a single persistent entity class to a table in isolation. Let's
expand on that a bit and add some class associations. We will add people to the
application and store a list of events in which they participate.
1.2.1. 映射Person类
The first cut of the Person class looks like this:
package org.hibernate.tutorial.domain;
public class Person {
private Long id;
private int age;
private String firstname;
private String lastname;
public Person() {}
// Accessor methods for all properties, private setter for 'id'
}
Save this to a file named src/main/java/org/hibernate/tutorial/domain/Person.java
Next, create the new mapping file as src/main/resources/org/hibernate/tutorial/domain/
Person.hbm.xml
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="native"/>
</id>
15
第 1 章 Tutorial
<property name="age"/>
<property name="firstname"/>
<property name="lastname"/>
</class>
</hibernate-mapping>
最后,把新的映射加入到Hibernate的配置中:
<mapping resource="events/Event.hbm.xml"/>
<mapping resource="events/Person.hbm.xml"/>
Create an association between these two entities. Persons can participate in events, and
events have participants. The design questions you have to deal with are: directionality,
multiplicity, and collection behavior.
1.2.2. 单向Set-based的关联
By adding a collection of events to the Person class, you can easily navigate to
the events for a particular person, without executing an explicit query - by calling
Person‰getEvents. Multi-valued associations are represented in Hibernate by one of the
Java Collection Framework contracts; here we choose a java.util.Set because the collection
will not contain duplicate elements and the ordering is not relevant to our examples:
public class Person {
private Set events = new HashSet();
public Set getEvents() {
return events;
}
public void setEvents(Set events) {
this.events = events;
}
}
Before mapping this association, let's consider the other side. We could just keep this
unidirectional or create another collection on the Event, if we wanted to be able to
navigate it from both directions. This is not necessary, from a functional perspective.
You can 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
16
单向Set-based的关联
the multiplicity of the association: "many" valued on both sides is called a many-tomany association. Hence, we use Hibernate's many-to-many mapping:
<class name="Person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="native"/>
</id>
<property name="age"/>
<property name="firstname"/>
<property name="lastname"/>
<set name="events" table="PERSON_EVENT">
<key column="PERSON_ID"/>
<many-to-many column="EVENT_ID" class="Event"/>
</set>
</class>
Hibernate supports a broad range of collection mappings, a set being most common. For a
many-to-many association, or n:m entity relationship, an association table is required.
Each row in this table represents a link between a person and an event. The table name
is decalred using the table attribute of the set element. The identifier column name
in the association, for the person 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 (the class on the other
side of the collection of references).
因而这个映射的数据库schema是:
_____________
__________________
|
|
|
|
_____________
| EVENTS |
| PERSON_EVENT |
|
|
|_____________|
|__________________|
| PERSON |
|
|
|
|
|_____________|
| *EVENT_ID | <--> | *EVENT_ID
|
|
|
| EVENT_DATE |
| *PERSON_ID
| <--> | *PERSON_ID |
| TITLE
|
|__________________|
| AGE
|
|_____________|
| FIRSTNAME |
| LASTNAME |
|_____________|
17
第 1 章 Tutorial
1.2.3. 使关联工作
Now we will 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();
}
After loading a Person and an Event, simply modify the collection using the normal
collection methods. 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. 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 org.hibernate.Session, 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.
You can load person and event in different units of work. Or you can modify an
object outside of a org.hibernate.Session, when it is not in persistent state (if it was
persistent before, this state is called 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
18
使关联工作
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
session2.getTransaction().commit();
}
The call to update makes a detached object persistent again by binding 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.
This is not much use in our example, but it is an important concept you can
into your own application. Complete this exercise by adding a new action
method of the EventManager and call it from the command line. If you need the
of a person and an event - the save() method returns it (you might have to
of the previous methods to return that identifier):
incorporate
to the main
identifiers
modify some
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 is 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 java.lang.String.
We call these classes value types, and their instances depend on a particular entity.
Instances of these types do not have their own identity, nor are they shared between
entities. Two persons do not reference the same firstname object, even if they have
the same first name. Value types cannot only be found in the JDK , but you can also
write dependent classes yourself such as an Address or MonetaryAmount class. In fact, in
a Hibernate application all JDK classes are considered value types.
You can also design a collection of value types. This is conceptually different from
a collection of references to other entities, but looks almost the same in Java.
19
第 1 章 Tutorial
1.2.4. 值类型的集合
Let's add a collection of email addresses to the Person entity. This will be represented
as a java.util.Set of java.lang.String instances:
private Set emailAddresses = new HashSet();
public Set getEmailAddresses() {
return emailAddresses;
}
public void setEmailAddresses(Set emailAddresses) {
this.emailAddresses = emailAddresses;
}
The mapping of this Set is as follows:
<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 use of the element part which
tells Hibernate that the collection does not contain references to another entity,
but is rather a collection whose elements are values types, here specifically of type
string. The lowercase name tells you it is a Hibernate mapping type/converter. 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 email address values
will actually be stored.
Here is 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
|
20
|
双向关联
|_____________|
| FIRSTNAME |
| LASTNAME |
|___________________|
|_____________|
You can see that the primary key of the collection table is in fact a composite key that
uses both columns. This also implies that there cannot be duplicate email addresses
per person, which is exactly the semantics we need for a set in Java.
You can now try to add elements to this collection, just like we did before by linking
persons and events. It is 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);
// adding to the emailAddress collection might trigger a lazy load of the collection
aPerson.getEmailAddresses().add(emailAddress);
session.getTransaction().commit();
}
This time we did not use a fetch query to initialize the collection. Monitor the SQL
log and try to optimize this with an eager fetch.
1.2.5. 双向关联
Next you will map a bi-directional association. You will make the association between
person and event work from both sides in Java. The database schema does not change,
so you will still have many-to-many multiplicity.
注意
A relational database is more flexible than a network programming language,
in that it does not need a navigation direction; data can be viewed and
retrieved in any possible way.
First, add a collection of participants to the Event class:
private Set participants = new HashSet();
21
第 1 章 Tutorial
public Set getParticipants() {
return participants;
}
public void setParticipants(Set participants) {
this.participants = participants;
}
Now map this side of the association 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>
These are normal set mappings in both mapping documents. Notice that the column names
in key and many-to-many swap 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.
1.2.6. 使双向连起来
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? You add an
instance of Event to the collection of event references, of an instance of Person. If
you want to make this link bi-directional, you have to do the same on the other side
by adding a Person reference to the collection in an Event. This process of "setting
the link on both sides" is absolutely necessary with bi-directional links.
Many developers program defensively and create link management methods to correctly
set both sides (for example, in Person):
protected Set getEvents() {
return events;
}
protected void setEvents(Set events) {
this.events = events;
}
22
第三部分 - EventManager web应用程序
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);
}
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 altering the collections directly. Repeat the steps for 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 correctly. Hibernate, however,
does not have enough information to correctly arrange SQL INSERT and UPDATE statements
(to avoid constraint violations). Making one side of the association inverse tells
Hibernate to consider it a mirror of the other side. That is all that is necessary for
Hibernate to resolve any issues that arise when transforming a directional navigation
model to a SQL database schema. The rules are straightforward: all bi-directional
associations need one side as inverse. In a one-to-many association it has to be the
many-side, and in many-to-many association you can select either side.
1.3. 第三部分 - EventManager web应用程序
A Hibernate web application uses Session and Transaction almost like a standalone
application. However, some common patterns are useful. You can now write an
EventManagerServlet. This servlet can list all events stored in the database, and it
provides an HTML form to enter new events.
1.3.1. 编写基本的servlet
First we need create our basic processing servlet. Since our servlet only handles HTTP
GET requests, we will only implement the doGet() method:
package org.hibernate.tutorial.web;
// Imports
public class EventManagerServlet extends HttpServlet {
23
第 1 章 Tutorial
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...
// End unit of work
HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().commit();
}
catch (Exception ex) {
HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().rollback();
if ( ServletException.class.isInstance( ex ) ) {
throw ( ServletException ) ex;
}
else {
throw new ServletException( ex );
}
}
}
}
Save this servlet as src/main/java/org/hibernate/tutorial/web/EventManagerServlet.java
The pattern applied 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. A database transaction is then started. All data access occurs inside a
transaction irrespective of whether the data is read or written. Do not use the autocommit mode in applications.
我们称这里应用的模式为每次请求一个session(session-per-request)。当有请求到达这个
servlet的时候,通过对SessionFactory的第一次调用,打开一个新的Hibernate Session。然后启
动一个数据库事务-所有的数据访问都是在事务中进行,不管是读还是写(我们在应用程序中不使
用auto-commit模式)。
Next, the possible actions of the request are processed and the response HTML is
rendered. We will get to that part soon.
24
式叫做Open Session in View-只要你考虑用JSP来渲染你的视图(view),而不是在servlet中,你就会很快用到它。
Finally, the unit of work ends when processing and rendering are complete.
problems occurred during processing or rendering, an exception will be thrown
database transaction rolled back. This completes the session-per-request pattern.
of the transaction demarcation code in every servlet, you could also write a
If any
and the
Instead
servlet
filter. See the Hibernate website and Wiki for more information about this pattern
called Open Session in View. You will need it as soon as you consider rendering your
view in JSP, not in a servlet.
1.3.2. 最后,当处理与渲染都结束的时候,这个工作单元就结束
了。假若在处理或渲染的时候有任何错误发生,会抛出一个异常,回
滚数据库事务。这样,session-per-request模式就完成了。为了避免在每个
servlet中都编写事务边界界定的代码,可以考虑写一个servlet 过
滤器(filter)来更好地解决。关于这一模式的更多信息,请参阅
Hibernate网站和Wiki,这一模式叫做Open Session in View-只要你
考虑用JSP来渲染你的视图(view),而不是在servlet中,你就会很
快用到它。
Now you can implement the processing of the request and the 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")) ) {
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);
25
第 1 章 Tutorial
// Write HTML footer
out.println("</body></html>");
out.flush();
out.close();
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>");
}
listEvents()方法使用绑定到当前线程的Hibernate Session来执行查询:
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'>");
out.println("<tr>");
out.println("<th>Event title</th>");
out.println("<th>Event date</th>");
out.println("</tr>");
Iterator it = result.iterator();
while (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>");
}
26
部署与测试
out.println("</table>");
}
}
最后,store动作会被导向到createAndStoreEvent()方法,它也使用当前线程的Session:
protected void createAndStoreEvent(String title, Date theDate) {
Event theEvent = new Event();
theEvent.setTitle(title);
theEvent.setDate(theDate);
HibernateUtil.getSessionFactory()
.getCurrentSession().save(theEvent);
}
The servlet is now 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 would
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.
1.3.3. 部署与测试
To deploy this application for testing we must create a Web ARchive (WAR). First we
must define the WAR descriptor as src/main/webapp/WEB-INF/web.xml
<?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/webapp_2_4.xsd">
<servlet>
<servlet-name>Event Manager</servlet-name>
<servlet-class>org.hibernate.tutorial.web.EventManagerServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Event Manager</servlet-name>
<url-pattern>/eventmanager</url-pattern>
27
第 1 章 Tutorial
</servlet-mapping>
</web-app>
To build and deploy call mvn package in your project directory and copy the hibernatetutorial.war file into your Tomcat webapps directory.
注意
If you do not have Tomcat installed, download it from http://
tomcat.apache.org/ and follow the installation instructions. Our
application requires no changes to the standard Tomcat configuration.
在部署完,启动Tomcat之后,通过http://localhost:8080/hibernate-tutorial/eventmanager进行访
问你的应用,在第一次servlet 请求发生时,请在Tomcat log中确认你看到Hibernate被初始化了
(HibernateUtil的静态初始化器被调用),假若有任何异常抛出,也可以看到详细的输出。
1.4. 总结
This tutorial covered the basics of writing a simple standalone Hibernate application
and a small web application. More tutorials are available from the Hibernate website
[http://hibernate.org].
28
第 2
体系结构(Architecture)
2.1. 概况(Overview)
The diagram below provides a high-level view of the Hibernate architecture:
We do not have the scope in this document to provide a more detailed view of
all the runtime architectures available; Hibernate is flexible and supports several
different approaches. We will, however, show the two extremes: "minimal" architecture
and "comprehensive" architecture.
This next diagram illustrates how Hibernate utilizes database and configuration data
to provide persistence services, and persistent objects, to the application.
The "minimal" architecture has the application provide its own JDBC connections and
manage its own transactions. This approach uses a minimal subset of Hibernate's APIs:
The "comprehensive" architecture abstracts the application away from the underlying
JDBC/JTA APIs and allows Hibernate to manage the details.
Here are some definitions of the objects depicted in the diagrams:
SessionFactory (org.hibernate.SessionFactory)
A threadsafe, immutable cache of compiled mappings for a single database. A factory
for Session and a client of ConnectionProvider, SessionFactory can 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. It wraps a JDBC connection and is a factory
for Transaction. Session holds a mandatory first-level cache of persistent objects
that are used when navigating the object graph or looking up objects by identifier.
持久的对象及其集合
Short-lived, single threaded objects containing persistent state and business
function. These can be ordinary JavaBeans/POJOs. They are associated with exactly
one Session. Once the Session is closed, they will be detached and free to use
in any application layer (for example, directly as data transfer objects to and
from presentation).
29
第 2 章 体系结构(Architecture)
瞬态(transient)和脱管(detached)的对象及其集合
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.
事务Transaction (org.hibernate.Transaction)
(Optional) A single-threaded, short-lived object used by the application to specify
atomic units of work. It abstracts the application from the 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. It abstracts the application
from underlying Datasource or DriverManager. It is not exposed to application, but
it can be extended and/or implemented by the developer.
TransactionFactory (org.hibernate.TransactionFactory)
(Optional) A factory for Transaction instances. It is not exposed to the application,
but it can be extended and/or implemented by the developer.
扩展接口
Hibernate offers a range of optional extension interfaces you can implement to
customize the behavior of your persistence layer. See the API documentation for
details.
Given
a
"minimal" architecture, the application bypasses the Transaction/
TransactionFactory and/or ConnectionProvider APIs to communicate with JTA or JDBC directly.
2.2. 实例状态
An instance of a persistent class can be in one of three different states. These states
are defined in relation to a persistence context. The Hibernate Session object is the
persistence context. The three different states are as follows:
瞬态(transient)
The instance is not associated with any persistence context. It has no persistent
identity or primary key value.
持久化(persistent)
The instance is currently associated with a persistence context. It has a persistent
identity (primary key value) and can have a corresponding row in the database. For
a particular persistence context, Hibernate guarantees that persistent identity is
equivalent to Java identity in relation to the in-memory location of the object.
脱管(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
30
JMX整合
identity and can have a corresponding row in the database. For detached instances,
Hibernate does not guarantee the relationship between persistent identity and Java
identity.
2.3. JMX整合
JMX is the J2EE standard for the management of Java components. Hibernate can be managed
via a JMX standard service. AN MBean implementation is provided in the distribution:
org.hibernate.jmx.HibernateService.
For an example of how to deploy Hibernate as a JMX service on the JBoss Application
Server, please see the JBoss User Guide. JBoss AS also provides 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 that you no longer have to manually open
and close the Session; this becomes the job of a JBoss EJB interceptor. You also
do not have to worry about transaction demarcation in your code (if you would like
to write a portable persistence layer use the optional Hibernate Transaction API for
this). You call the HibernateContext to access a Session.
•HAR deployment: the Hibernate JMX service is deployed using a JBoss service deployment
descriptor in an EAR and/or SAR file, as it supports all the usual configuration
options of a Hibernate SessionFactory. However, you still need to name all your mapping
files in the deployment descriptor. If you use the optional HAR deployment, JBoss
will automatically detect all mapping files in your HAR file.
这些选项更多的描述,请参考JBoss 应用程序用户指南。
Another feature available as a JMX service is runtime Hibernate statistics. See
第 3.4.6 节 “Hibernate的统计(statistics)机制” for more information.
2.4. 对JCA的支持
Hibernate can also be configured as a JCA connector. Please see the website for more
information. Please note, however, that at this stage Hibernate JCA support is under
development.
2.5. Contextual sessions
Most applications using Hibernate need some form of "contextual" session, 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;
different contexts define different scopes to the notion of current. Applications
using Hibernate prior to version 3.0 tended to utilize either home-grown ThreadLocalbased contextual sessions, helper classes such as HibernateUtil, or utilized third-party
31
第 2 章 体系结构(Architecture)
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. Given the maturity of the numerous
stand-alone JTA TransactionManager implementations, 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 are all you 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.
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. 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 not open, flush, or close a Session.
The first two implementations provide a "one session - one database transaction"
programming model. This is also 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, you can utilize the JTA interfaces to demarcate transactions. If
you execute in an EJB container that supports CMT, transaction boundaries are defined
declaratively and you do not need any transaction or session demarcation operations
in your code. Refer to 第 11 章 Transactions and Concurrency for more information
and code examples.
The
configuration parameter defines which
implementation
should
be
used.
For
backwards compatibility, if this configuration parameter is not set but a
org.hibernate.transaction.TransactionManagerLookup is configured, Hibernate will use the
hibernate.current_session_context_class
org.hibernate.context.CurrentSessionContext
32
Contextual sessions
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".
33
34
第 3
配置
Hibernate is designed to operate in many different environments and, as such, there
is a broad range of configuration parameters. Fortunately, most have sensible default
values and Hibernate is distributed with an example hibernate.properties file in etc/
that displays the various options. Simply put the example file in your classpath and
customize it to suit your needs.
3.1. 可编程的配置方式
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 can 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(). For example:
Configuration cfg = new Configuration()
.addResource("Item.hbm.xml")
.addResource("Bid.hbm.xml");
An alternative way is to specify the mapped class and allow Hibernate to find the
mapping document for you:
Configuration cfg = new Configuration()
.addClass(org.hibernate.auction.Item.class)
.addClass(org.hibernate.auction.Bid.class);
Hibernate will then search 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.
For example:
Configuration cfg = new Configuration()
.addClass(org.hibernate.auction.Item.class)
.addClass(org.hibernate.auction.Bid.class)
.setProperty("hibernate.dialect", "org.hibernate.dialect.MySQLInnoDBDialect")
35
第 3 章 配置
.setProperty("hibernate.connection.datasource", "java:comp/env/jdbc/test")
.setProperty("hibernate.order_updates", "true");
This is not the only way to pass configuration properties to Hibernate. Some alternative
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. 通过java -Dproperty=value来设置系统 (System)属性.
4. Include <property> elements in hibernate.cfg.xml (this is discussed later).
If you want to get started quicklyhibernate.properties is the easiest approach.
The org.hibernate.cfg.Configuration is intended as a startup-time object that will be
discarded once a SessionFactory is created.
3.2. 获得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连接
It is advisable 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
Once you start a task that requires access to the database, a JDBC connection will
be obtained from the pool.
Before you can do this, you first need to pass some JDBC connection properties
to Hibernate. All Hibernate property names and semantics are defined on the
class org.hibernate.cfg.Environment. The most important settings for JDBC connection
configuration are outlined below.
Hibernate will obtain and pool connections using java.sql.DriverManager if you set the
following properties:
36
JDBC连接
表 3.1. Hibernate JDBC属性
属性名
用途
hibernate.connection.driver_class
jdbc驱动类
hibernate.connection.url
jdbc URL
hibernate.connection.username
数据库用户
hibernate.connection.password
数据库用户密码
hibernate.connection.pool_size
连接池容量上限数目
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 would like to use
Proxool, refer to the packaged hibernate.properties and the Hibernate web site for more
information.
The following 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 javax.sql.Datasource registered in JNDI.
You will need to set at least one of the following properties:
表 3.2. Hibernate数据源属性
属性名
用途
hibernate.connection.datasource
数据源JNDI名字
hibernate.jndi.url
URL of the JNDI provider (optional)
37
第 3 章 配置
属性名
用途
hibernate.jndi.class
class of the
(optional)
hibernate.connection.username
database user (optional)
hibernate.connection.password
database user password (optional)
JNDI
InitialContextFactory
Here is an example hibernate.properties file for an application server provided JNDI
datasource:
hibernate.connection.datasource = java:/comp/env/jdbc/test
hibernate.transaction.factory_class = \
org.hibernate.transaction.JTATransactionFactory
hibernate.transaction.manager_lookup_class = \
org.hibernate.transaction.JBossTransactionManagerLookup
hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect
从JNDI数据源获得的JDBC连接将自动参与到应用程序服务器中容器管理的事务(containermanaged transactions)中去.
Arbitrary connection properties can be given by prepending "hibernate.connection" to the
connection property name. For example, you can specify a charSet connection property
using hibernate.connection.charSet.
You can define your own plugin strategy for obtaining JDBC connections by implementing
the interface org.hibernate.connection.ConnectionProvider, and specifying your custom
implementation via the hibernate.connection.provider_class property.
3.4. 可选的配置属性
There are a number of other properties that control the behavior of Hibernate at
runtime. All are optional and have reasonable default values.
警告
Some of these properties are "system-level" only. System-level properties
can be set only via java -Dproperty=value or hibernate.properties. They cannot
be set by the other techniques described above.
表 3.3. Hibernate配置属性
属性名
用途
hibernate.dialect
The
38
classname
of
a
Hibernate
org.hibernate.dialect.Dialect which allows
可选的配置属性
属性名
用途
Hibernate to generate SQL optimized for a
particular relational database.
e.g. full.classname.of.Dialect
In most cases Hibernate will actually
be
able
to
choose
the
correct
org.hibernate.dialect.Dialect implementation
based on the JDBC metadata returned by the
JDBC driver.
hibernate.show_sql
Write all SQL statements to console. This
is an alternative to setting the log
category org.hibernate.SQL to debug.
e.g. true | false
hibernate.format_sql
Pretty print the SQL in the log and
console.
e.g. true | false
hibernate.default_schema
Qualify unqualified table names with the
given schema/tablespace in generated SQL.
e.g. SCHEMA_NAME
hibernate.default_catalog
Qualifies unqualified table names with the
given catalog in generated SQL.
e.g. CATALOG_NAME
hibernate.session_factory_name
The org.hibernate.SessionFactory will be
automatically bound to this name in JNDI
after it has been created.
e.g. jndi/composite/name
hibernate.max_fetch_depth
Sets 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.
e.g. recommended values between 0 and 3
hibernate.default_batch_fetch_size
Sets a default size for Hibernate batch
fetching of associations.
e.g. recommended values 4, 8, 16
39
第 3 章 配置
属性名
用途
hibernate.default_entity_mode
Sets
a
default
mode
for
entity
representation for all sessions opened
from this SessionFactory
取值dynamic-map, dom4j, pojo
hibernate.order_updates
Forces 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.
e.g. true | false
hibernate.generate_statistics
If enabled, Hibernate will collect
statistics useful for performance tuning.
e.g. true | false
hibernate.use_identifer_rollback
If
enabled,
generated
identifier
properties will be reset to default values
when objects are deleted.
e.g. true | false
hibernate.use_sql_comments
If turned on, Hibernate will generate
comments inside the SQL, for easier
debugging, defaults to false.
e.g. true | false
表 3.4. Hibernate JDBC和连接(connection)属性
属性名
用途
hibernate.jdbc.fetch_size
A non-zero value determines the JDBC fetch
size (calls Statement.setFetchSize()).
hibernate.jdbc.batch_size
A non-zero value enables use of JDBC2 batch
updates by Hibernate.
e.g. recommended values between 5 and 30
hibernate.jdbc.batch_versioned_data
Set this property to true if your JDBC
driver returns correct row counts from
executeBatch(). Iit is usually safe to turn
this option on. Hibernate will then use
batched DML for automatically versioned
data. Defaults to false.
e.g. true | false
40
可选的配置属性
属性名
用途
hibernate.jdbc.factory_class
Select a custom org.hibernate.jdbc.Batcher.
Most applications will not need this
configuration property.
e.g. classname.of.BatcherFactory
hibernate.jdbc.use_scrollable_resultset
Enables use of JDBC2 scrollable resultsets
by Hibernate. This property is only
necessary when using user-supplied JDBC
connections. Hibernate uses connection
metadata otherwise.
e.g. true | false
hibernate.jdbc.use_streams_for_binary
Use streams when writing/reading binary or
serializable types to/from JDBC. *systemlevel property*
e.g. true | false
hibernate.jdbc.use_get_generated_keys
Enables
use
of
PreparedStatement.getGeneratedKeys()
JDBC3
to
retrieve natively generated keys after
insert. Requires JDBC3+ driver and
JRE1.4+, set to false if your driver has
problems with the Hibernate identifier
generators. By default, it tries to
determine the driver capabilities using
connection metadata.
e.g. true|false
hibernate.connection.provider_class
The
classname
of
a
custom
org.hibernate.connection.ConnectionProvider
which provides
Hibernate.
JDBC
connections
to
e.g. classname.of.ConnectionProvider
hibernate.connection.isolation
Sets the JDBC transaction isolation level.
Check java.sql.Connection for meaningful
values, but note that most databases
do not support all isolation levels
and some define additional, non-standard
isolations.
e.g. 1, 2, 4, 8
41
第 3 章 配置
属性名
用途
hibernate.connection.autocommit
Enables autocommit for JDBC pooled
connections (it is not recommended).
e.g. true | false
hibernate.connection.release_mode
Specifies 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,
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.
e.g.
(default)
|
on_close
after_transaction | after_statement
auto
|
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
第 2.5 节 “Contextual sessions”
hibernate.connection.<propertyName>
Pass the JDBC property <propertyName> to
DriverManager.getConnection().
hibernate.jndi.<propertyName>
Pass the property <propertyName> to the
JNDI InitialContextFactory.
表 3.5. Hibernate缓存属性
属性名
用途
hibernate.cache.provider_class
The classname of a custom CacheProvider.
e.g. classname.of.CacheProvider
42
可选的配置属性
属性名
用途
hibernate.cache.use_minimal_puts
Optimizes 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.
e.g. true|false
hibernate.cache.use_query_cache
Enables the query cache. Individual
queries still have to be set cachable.
e.g. true|false
hibernate.cache.use_second_level_cache
Can be used to completely disable the
second level cache, which is enabled by
default for classes which specify a <cache>
mapping.
e.g. true|false
hibernate.cache.query_cache_factory
The classname of a custom QueryCache
interface, defaults to the built-in
StandardQueryCache.
e.g. classname.of.QueryCache
hibernate.cache.region_prefix
A prefix to use for second-level cache
region names.
e.g. prefix
hibernate.cache.use_structured_entries
Forces Hibernate to store data in the
second-level cache in a more humanfriendly format.
e.g. true|false
表 3.6. Hibernate事务属性
属性名
用途
hibernate.transaction.factory_class
The classname of a TransactionFactory to use
with Hibernate Transaction API (defaults to
JDBCTransactionFactory).
e.g. classname.of.TransactionFactory
jta.UserTransaction
A JNDI name used by JTATransactionFactory
to obtain the JTA UserTransaction from the
application server.
43
第 3 章 配置
属性名
用途
e.g. jndi/composite/name
hibernate.transaction.manager_lookup_class
The classname of a TransactionManagerLookup.
It is required when JVM-level caching is
enabled or when using hilo generator in a
JTA environment.
e.g. 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 第 2.5 节
“Contextual sessions”.
e.g. 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 automatic session context
management is preferred, see 第 2.5 节
“Contextual sessions”.
e.g. true | false
表 3.7. 其他属性
属性名
用途
hibernate.current_session_context_class
Supply a custom strategy for the
scoping of the "current" Session. See
第 2.5 节 “Contextual sessions” for more
information about the built-in strategies.
e.g. jta | thread | managed | custom.Class
hibernate.query.factory_class
Chooses the HQL parser implementation.
e.g.
org.hibernate.hql.ast.ASTQueryTranslatorFactory
or
org.hibernate.hql.classic.ClassicQueryTranslatorFactory
hibernate.query.substitutions
Is used to map from tokens in Hibernate
queries to SQL tokens (tokens might be
function or literal names, for example).
44
SQL方言
属性名
用途
e.g.
hqlLiteral=SQL_LITERAL,
hqlFunction=SQLFUNC
Automatically validates or exports schema
DDL to the database when the SessionFactory
hibernate.hbm2ddl.auto
is created. With create-drop, the database
schema will be dropped when the
SessionFactory is closed explicitly.
e.g. validate | update | create | create-drop
hibernate.cglib.use_reflection_optimizer
Enables the
of runtime
use of CGLIB instead
reflection (System-level
property). Reflection can sometimes be
useful when troubleshooting. Hibernate
always requires CGLIB even if you turn
off the optimizer. You cannot set this
property in hibernate.cfg.xml.
e.g. true | false
3.4.1. SQL方言
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. This means that you will not
have to specify them manually.
表 3.8. Hibernate SQL方言 (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
org.hibernate.dialect.Oracle9iDialect
Oracle 10g
org.hibernate.dialect.Oracle10gDialect
Sybase
org.hibernate.dialect.SybaseDialect
Sybase Anywhere
org.hibernate.dialect.SybaseAnywhereDialect
45
第 3 章 配置
RDBMS
Dialect
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. 外连接抓取(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 trips to and from
the database. This is, however, 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
can
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 that have been mapped with fetch="join".
参见第 19.1 节 “抓取策略(Fetching strategies)”获得更多信息.
3.4.3. 二进制流 (Binary Streams)
Oracle limits the size of byte arrays that can be passed to and/or 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. 二级缓存与查询缓存
The properties prefixed by hibernate.cache allow you to use a process or cluster scoped
second-level cache system with Hibernate. See the 第 19.2 节 “二级缓存(The Second
Level Cache)” for more information.
3.4.5. 查询语言中的替换
You can define new Hibernate query tokens using hibernate.query.substitutions. For example:
46
Hibernate的统计(statistics)机制
hibernate.query.substitutions true=1, false=0
This would cause the tokens true and false to be translated to integer literals in
the generated SQL.
hibernate.query.substitutions toLowercase=LOWER
This would allow you to rename the SQL LOWER function.
3.4.6. Hibernate的统计(statistics)机制
If you enable hibernate.generate_statistics, Hibernate exposes 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.
3.5. 日志
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 you will need slf4j-api.jar
in your classpath together with the jar file for your preferred binding - slf4jlog4j12.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.
It is recommended 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:
表 3.9. Hibernate日志类别
类别
功能
org.hibernate.SQL
在所有SQL DML语句被执行时为它们记录日志
org.hibernate.type
为所有JDBC参数记录日志
org.hibernate.tool.hbm2ddl 在所有SQL DDL语句执行时为它们记录日志
org.hibernate.pretty
在session清洗(flush)时,为所有与其关联的实体(最多20个)的状
态记录日志
47
第 3 章 配置
类别
功能
org.hibernate.cache
为所有二级缓存的活动记录日志
org.hibernate.transaction
为事务相关的活动记录日志
org.hibernate.jdbc
为所有JDBC资源的获取记录日志
org.hibernate.hql.AST
在解析查询的时候,记录HQL和SQL的AST分析日志
org.hibernate.secure
为JAAS认证请求做日志
org.hibernate
Log everything. This is a lot of information but it is useful
for troubleshooting
在使用Hibernate开发应用程序时, 你应当总是为org.hibernate.SQL 开启debug级别的日志记录,或
者开启hibernate.show_sql属性。
3.6. 实现NamingStrategy
org.hibernate.cfg.NamingStrategy接口允许你为数据库中的对象和schema
元素指定一个“命名标
准”.
You can 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 can 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();
org.hibernate.cfg.ImprovedNamingStrategy是一个内建的命名策略, 对 一些应用程序而言,可能是
非常有用的起点.
3.7. XML配置文件
另一个配置方法是在hibernate.cfg.xml文件中指定一套完整的配置.
成hibernate.properties的替代。 若两个文件同时存在,它将覆盖前者的属性.
这个文件可以当
The XML configuration file is by default expected to be in the root of your CLASSPATH.
Here is an example:
48
XML配置文件
<?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>
<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>
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. It 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();
49
第 3 章 配置
You can select a different XML configuration file using:
SessionFactory sf = new Configuration()
.configure("catdb.cfg.xml")
.buildSessionFactory();
3.8. J2EE应用程序服务器的集成
针对J2EE体系,Hibernate有如下几个集成的方面:
•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), especially distributed
transaction handling across several datasources. You can also demarcate transaction
boundaries programmatically (BMT), or you might want to use the optional Hibernate
Transaction API for this to keep your code portable.
•自动JNDI绑定: Hibernate可以在启动后将 SessionFactory绑定到JNDI.
•JTA Session binding: the Hibernate Session can be automatically bound to the scope
of JTA transactions. Simply lookup the SessionFactory from JNDI and get the current
Session. Let Hibernate manage 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
choose 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 also take care of service dependencies (datasource has to be
available before Hibernate starts, etc).
如果应用程序服务器抛出"connection containment"异常,
hibernate.connection.release_mode设为after_statement.
根据你的环境,也许该将配置属性
3.8.1. 事务策略配置
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
can 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.
为了让你的代码在两种(或其他)环境中可以移植,我们建议使用可选的Hibernate
Transaction
API,
它包装并隐藏了底层系统.
你必须通过设置Hibernate配置属
性hibernate.transaction.factory_class来指定 一个Transaction实例的工厂类.
50
JNDI绑定的SessionFactory
There are three standard, or built-in, choices:
org.hibernate.transaction.JDBCTransactionFactory
委托给数据库(JDBC)事务(默认)
org.hibernate.transaction.JTATransactionFactory
delegates to container-managed transactions if an existing transaction is underway
in this context (for example, EJB session bean method). Otherwise, a new transaction
is started and bean-managed transactions are used.
org.hibernate.transaction.CMTTransactionFactory
委托给容器管理的JTA事务
You can 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, since J2EE does not standardize a single mechanism, you have to
specify how Hibernate should obtain a reference to the TransactionManager:
表 3.10. JTA TransactionManagers
Transaction工厂类
org.hibernate.transaction.JBossTransactionManagerLookup
应用程序服务器
JBoss
org.hibernate.transaction.WeblogicTransactionManagerLookup
Weblogic
org.hibernate.transaction.WebSphereTransactionManagerLookup
WebSphere
org.hibernate.transaction.WebSphereExtendedJTATransactionLookup
WebSphere 6
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
Borland ES
3.8.2. JNDI绑定的SessionFactory
A JNDI-bound Hibernate SessionFactory can simplify the lookup function of the factory
and create new Sessions. This is not, however, 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 (e.g.
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
51
第 3 章 配置
useful in environments with a read-only JNDI default implementation (in Tomcat, for
example).
在将SessionFactory绑定至JNDI时,
Hibernate将使用hibernate.jndi.url,
和hibernate.jndi.class的值来实例化初始环境(initial context). 如果它们没有被指定, 将使用
默认的InitialContext.
Hibernate
will
automatically place the SessionFactory in JNDI after you call
cfg.buildSessionFactory(). This means you will have this call in some startup code, or
utility class in your application, unless you use JMX deployment with the HibernateService
(this is discussed later in greater detail).
If you use a JNDI SessionFactory, an EJB or any other class, you can obtain the
SessionFactory using a JNDI lookup.
It is recommended 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. 在JTA环境下使用Current Session context (当前session上
下文)管理
The easiest way to handle Sessions and transactions is Hibernate's automatic "current"
Session management. For a discussion of contextual sessions see 第 2.5 节 “Contextual
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 the"jta" context are 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部署
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 can 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:
52
JMX部署
<?xml version="1.0"?>
<server>
<mbean code="org.hibernate.jmx.HibernateService"
name="jboss.jca:service=HibernateFactory,name=HibernateFactory">
<!-- Required services -->
<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>
53
第 3 章 配置
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) can be kept in their
own JAR file, but you can 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.
54
第 4
持久化类(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. For
example, an instance can 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 can express a domain model in other ways (using trees of Map
instances, for example).
4.1. 一个简单的POJO例子
Most Java applications require a persistent class representing felines. For example:
package eg;
import java.util.Set;
import java.util.Date;
public class Cat {
private Long id; // identifier
private Date birthdate;
private Color color;
private char sex;
private float weight;
private 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;
}
55
第 4 章 持久化类(Persistent Classes)
public Date getBirthdate() {
return birthdate;
}
void setWeight(float weight) {
this.weight = weight;
}
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;
}
56
实现一个默认的(即无参数的)构造方法(constructor)
// addKitten not needed by Hibernate
public void addKitten(Cat kitten) {
kitten.setMother(this);
kitten.setLitterId( kittens.size() );
kittens.add(kitten);
}
}
The four main rules of persistent classes are explored in more detail in the following
sections.
4.1.1. 实现一个默认的(即无参数的)构造方法(constructor)
Cat has a no-argument constructor. All persistent classes must have a default
constructor (which can be non-public) so that Hibernate can instantiate them using
Constructor.newInstance(). It is recommended that you have a default constructor with at
least package visibility for runtime proxy generation in Hibernate.
4.1.2. 提供一个标识属性(identifier property)(可选)
Cat 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 use a user-defined class
with properties of these types (see the section on composite identifiers later in
the chapter.)
标识符属性是可选的。可以不用管它,让Hibernate内部来追踪对象的识别。
这样做。
但是我们并不推荐
In fact, some functionality is available only to classes that declare an identifier
property:
•Transitive reattachment for detached objects (cascade update or cascade merge) - see
第 10.11 节 “传播性持久化(transitive persistence)”
•Session.saveOrUpdate()
•Session.merge()
We recommend that you declare consistently-named identifier properties on persistent
classes and that you use a nullable (i.e., non-primitive) type.
4.1.3. 使用非final的类 (可选)
代理(proxies)是Hibernate的一个重要的功能,它依赖的条件是,持久
的,或者是实现了一个所有方法都声明为public的接口。
化类或者是非final
57
第 4 章 持久化类(Persistent Classes)
You can persist final classes that do not implement an interface with Hibernate. You
will not, however, be able to use proxies for lazy association fetching which will
ultimately limit your options for performance tuning.
你也应该避免在非final类中声明 public final的方法。如果你想使用一 个有public final方法的
类,你必须通过设置lazy="false" 来明确地禁用代理。
4.1.4. 为持久化字段声明访问器(accessors)和是否可变的标志
(mutators)(可选)
Cat declares accessor methods for all its persistent fields. Many other ORM tools
directly persist instance variables. It is better to provide 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. If required, you can switch to direct field access for particular properties.
属性不需要要声明为public的。Hibernate可以持久化一个有 default、protected或private的get/
set方法对 的属性进行持久化。
4.2. 实现继承(Inheritance)
A subclass must also observe the first and second rules. It inherits its identifier
property from the superclass, Cat. For example:
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. 实现equals()和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
58
实现equals()和hashCode()
•想重用脱管实例
Hibernate guarantees equivalence of persistent identity (database row) and Java identity
only inside a particular session scope. When you mix instances retrieved in different
sessions, you must implement equals() and hashCode() if you 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,
because they are equal. If both are added to a Set, you will only have one element
in the Set). Unfortunately, you cannot 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() 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. This is not a Hibernate issue, but normal Java semantics of object
identity and equality.
It is recommended that you implement equals() and hashCode() using Business key equality.
Business key equality means that the equals() method compares only the properties that
form the business key. It is 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;
}
59
第 4 章 持久化类(Persistent Classes)
}
A business key does not have to be as solid as a database primary key candidate
(see 第 11.1.3 节 “关注对象标识(Considering object identity)”). Immutable or unique
properties are usually good candidates for a business key.
4.4. 动态模型(Dynamic models)
Note
The following features are currently considered experimental and may
change in the near future.
Persistent entities do not 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
do not write persistent classes, only mapping files.
By default, Hibernate works in normal POJO mode. You can set a default entity
representation mode for a particular SessionFactory using the default_entity_mode
configuration option (see 表 3.3 “Hibernate配置属性”).
The following examples demonstrate 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"/>
60
动态模型(Dynamic models)
<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>
Even though associations are declared using target class names, the target type of
associations can also be a dynamic entity instead of a POJO.
After setting the default entity mode to dynamic-map for the SessionFactory, you can,
at runtime, work with Maps of Maps:
Session s = openSession();
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();
61
第 4 章 持久化类(Persistent Classes)
s.close();
One of the main advantages of dynamic mapping is quick turnaround time for prototyping,
without the need for entity class implementation. However, you lose compile-time type
checking and will likely deal with many exceptions at runtime. As a result of the
Hibernate mapping, the database schema can easily be normalized and sound, allowing to
add a proper domain model implementation on top later on.
实体表示模式也能在每个Session的基础上设置:
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 do not have to call flush()
and close() on the secondary Session, and also leave the transaction and connection
handling to the primary unit of work.
关于XML表示能力的更多信息可以在第 18 章 XML映射中找到。
4.5. 元组片断映射(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 that 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 corresponding tuplizer knows how create the POJO through
its constructor. It also knows how to access the POJO properties using the defined
property accessors.
There
types
of
Tuplizers,
represented
by
the
org.hibernate.tuple.entity.EntityTuplizer and org.hibernate.tuple.component.ComponentTuplizer
62
are
two
high-level
元组片断映射(Tuplizers)
interfaces. EntityTuplizers are responsible for managing the above mentioned contracts
in regards to entities, while ComponentTuplizers do the same for components.
Users can 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 entitymode. 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.
Tuplizer 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 {
// 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();
}
}
63
第 4 章 持久化类(Persistent Classes)
}
4.6. EntityNameResolvers
The org.hibernate.EntityNameResolver interface is a contract for resolving the entity name
of a given entity instance. The interface defines a single method resolveEntityName which
is passed the entity instance and is expected to return the appropriate entity name (null
is allowed and would indicate that the resolver does not know how to resolve the entity
name of the given entity instance). Generally speaking, an org.hibernate.EntityNameResolver
is going to be most useful in the case of dynamic models. One example might be using
proxied interfaces as your domain model. The hibernate test suite has an example of
this exact style of usage under the org.hibernate.test.dynamicentity.tuplizer2. Here
is some of the code from that package for illustration.
/**
* A very trivial JDK Proxy InvocationHandler implementation where we proxy an interface as
* the domain model and simply store persistent state in an internal Map. This is an extremely
* trivial example meant only for illustration.
*/
public final class DataProxyHandler implements InvocationHandler {
private String entityName;
private HashMap data = new HashMap();
public DataProxyHandler(String entityName, Serializable id) {
this.entityName = entityName;
data.put( "Id", id );
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
String methodName = method.getName();
if ( methodName.startsWith( "set" ) ) {
String propertyName = methodName.substring( 3 );
data.put( propertyName, args[0] );
}
else if ( methodName.startsWith( "get" ) ) {
String propertyName = methodName.substring( 3 );
return data.get( propertyName );
}
else if ( "toString".equals( methodName ) ) {
return entityName + "#" + data.get( "Id" );
}
else if ( "hashCode".equals( methodName ) ) {
return new Integer( this.hashCode() );
64
EntityNameResolvers
}
return null;
}
public String getEntityName() {
return entityName;
}
public HashMap getData() {
return data;
}
}
/**
*
*/
public class ProxyHelper {
public static String extractEntityName(Object object) {
// Our custom java.lang.reflect.Proxy instances actually bundle
// their appropriate entity name, so we simply extract it from there
// if this represents one of our proxies; otherwise, we return null
if ( Proxy.isProxyClass( object.getClass() ) ) {
InvocationHandler handler = Proxy.getInvocationHandler( object );
if ( DataProxyHandler.class.isAssignableFrom( handler.getClass() ) ) {
DataProxyHandler myHandler = ( DataProxyHandler ) handler;
return myHandler.getEntityName();
}
}
return null;
}
// various other utility methods ....
}
/**
* The EntityNameResolver implementation.
* IMPL NOTE : An EntityNameResolver really defines a strategy for how entity names should be
* resolved. Since this particular impl can handle resolution for all of our entities we want to
* take advantage of the fact that SessionFactoryImpl keeps these in a Set so that we only ever
* have one instance registered. Why? Well, when it comes time to resolve an entity name,
* Hibernate must iterate over all the registered resolvers. So keeping that number down
* helps that process be as speedy as possible. Hence the equals and hashCode impls
*/
65
第 4 章 持久化类(Persistent Classes)
public class MyEntityNameResolver implements EntityNameResolver {
public static final MyEntityNameResolver INSTANCE = new MyEntityNameResolver();
public String resolveEntityName(Object entity) {
return ProxyHelper.extractEntityName( entity );
}
public boolean equals(Object obj) {
return getClass().equals( obj.getClass() );
}
public int hashCode() {
return getClass().hashCode();
}
}
public class MyEntityTuplizer extends PojoEntityTuplizer {
public MyEntityTuplizer(EntityMetamodel entityMetamodel, PersistentClass mappedEntity) {
super( entityMetamodel, mappedEntity );
}
public EntityNameResolver[] getEntityNameResolvers() {
return new EntityNameResolver[] { MyEntityNameResolver.INSTANCE };
}
public String determineConcreteSubclassEntityName(Object entityInstance,
SessionFactoryImplementor factory) {
String entityName = ProxyHelper.extractEntityName( entityInstance );
if ( entityName == null ) {
entityName = super.determineConcreteSubclassEntityName( entityInstance, factory );
}
return entityName;
}
...
}
In order to register an org.hibernate.EntityNameResolver users must either:
1. Implement a custom Tuplizer, implementing the getEntityNameResolvers method.
2. Register it with the org.hibernate.impl.SessionFactoryImpl (which is the implementation
class for org.hibernate.SessionFactory) using the registerEntityNameResolver method.
66
第 5
对象/关系数据库映射基础(Basic O/R
Mapping)
5.1. 映射定义(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 and not
table declarations.
Please note that even though many Hibernate users choose to write the XML by hand,
a number of tools exist to generate the mapping document. These include XDoclet,
Middlegen and AndroMDA.
Here is 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"
67
第 5 章 对象/关系数据库映射基础(Basic O/R ...
type="eg.types.ColorUserType"
not-null="true"
update="false"/>
<property name="sex"
not-null="true"
update="false"/>
<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,
however, 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 notnull attribute).
68
Doctype
5.1.1. Doctype
All XML mappings should declare the doctype shown. The actual DTD can 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 the DTD declaration against
the contents of your classpath.
5.1.1.1. EntityResolver
Hibernate will first attempt to resolve DTDs in its classpath. It does this is by
registering a custom org.xml.sax.EntityResolver implementation with the SAXReader it uses
to read in the xml files. This custom EntityResolver recognizes two different systemId
namespaces:
•a hibernate namespace is recognized whenever the resolver encounters a systemId starting
with http://hibernate.sourceforge.net/. The resolver attempts to resolve these entities
via the classloader which loaded the Hibernate classes.
•a user namespace is recognized whenever the resolver encounters 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.
The following is 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.
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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 they are specified, tablenames will be qualified by the given schema and catalog
names. If they are missing, tablenames will be unqualified. The default-cascade attribute
specifies what cascade style should be assumed for properties and collections that do
not specify a cascade attribute. By default, the auto-import attribute allows you to use
unqualified class names in the query language.
<hibernate-mapping
schema="schemaName"
catalog="catalogName"
default-cascade="cascade_style"
default-access="field|property|ClassName"
default-lazy="true|false"
auto-import="true|false"
package="package.name"
/>
schema (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. It can be a custom implementation of PropertyAccessor.
default-lazy (optional - defaults to true): the default value for unspecified 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 use for unqualified class names
in the mapping document.
If you have two persistent classes with the same unqualified name, you should set
auto-import="false". An exception will result if you attempt to assign two classes to
the same "imported" name.
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
70
Class
it after the persistent superclass. For example, Cat.hbm.xml, Dog.hbm.xml, or if using
inheritance, Animal.hbm.xml.
5.1.3. Class
You can declare a persistent class using the class element. For example:
<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"
where="arbitrary sql where condition"
persister="PersisterClass"
batch-size="N"
optimistic-lock="none|version|dirty|all"
lazy="true|false"
(16)
entity-name="EntityName"
(17)
check="arbitrary sql check condition"
(18)
rowid="rowid"
(19)
subselect="SQL expression"
(20)
abstract="true|false"
(21)
node="element-name"
/>
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name (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.
(optional - defaults to the class name): a value that
distinguishes individual subclasses that is used for polymorphic behavior.
Acceptable values include null and not null.
mutable (optional - defaults to true): specifies that instances of the class are
(not) mutable.
schema (optional): overrides the schema name specified by the root <hibernatemapping> element.
catalog (optional): overrides the catalog name specified by the root <hibernatemapping> element.
proxy (optional): specifies an interface to use for lazy initializing proxies. You
can specify the name of the class itself.
dynamic-update (optional - defaults to false): specifies that UPDATE SQL should be
generated at runtime and can 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.
Only when a transient object has been associated with a new session using update(),
will Hibernate 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): specifies an arbitrary SQL WHERE condition to be used when
retrieving objects of this class.
persister (optional): specifies a custom ClassPersister.
discriminator-value
batch-size (optional - defaults to 1): specifies a "batch size" for fetching
instances of this class by identifier.
optimistic-lock (optional - defaults to version): determines the optimistic locking
strategy.
16
lazy (optional): lazy fetching can be disabled by setting lazy="false".
17
entity-name (optional - defaults to the class name): Hibernate3 allows a class to
18
19
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be mapped multiple times, potentially to different tables. It also 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 第 4.4 节 “动
态模型(Dynamic models)” and 第 18 章 XML映射 for more information.
check (optional): an SQL expression used to generate a multi-row check constraint
for automatic schema generation.
rowid (optional): Hibernate can use ROWIDs on databases. On Oracle, for example,
Hibernate can use the rowid extra column for fast updates once this option has
Class
20
21
been set 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.
This is useful if you want to have a view instead of a base table. See below
for more information.
abstract (optional): is used to mark abstract superclasses in <union-subclass>
hierarchies.
It is acceptable for the named persistent class to be an interface. You can declare
implementing classes of that interface using the <subclass> element. You can persist
any static inner class. Specify the class name using the standard form i.e. e.g.Foo$Bar.
Immutable classes, mutable="false", cannot 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 that implement the named
interface. The persistent object will load 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 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. Queries that name the class will return only instances of
subclasses mapped 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
for
attribute lets you customize the persistence strategy used
class. You can, for example, specify your own subclass of
org.hibernate.persister.EntityPersister, or you can even provide a completely new
implementation of the interface org.hibernate.persister.ClassPersister that implements,
for example, persistence via stored procedure calls, serialization to flat files or
LDAP. See org.hibernate.test.CustomPersister for a simple example of "persistence" to a
Hashtable.
persister
the
The dynamic-update and dynamic-insert settings are not inherited by subclasses, so they
can also be specified on the <subclass> or <joined-subclass> elements. Although these
settings can increase performance in some cases, they can actually decrease performance
in others.
Use of select-before-update will usually decrease performance. It is useful to prevent a
database update trigger being called unnecessarily if you reattach a graph of detached
instances to a Session.
如果你打开了dynamic-update,你可以选择几种乐观锁定的策略:
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•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
It is strongly recommended that you use version/timestamp columns for optimistic locking
with Hibernate. This strategy optimizes performance and correctly handles modifications
made to detached instances (i.e. when Session.merge() is used).
There is no difference between a view and a base table for a Hibernate mapping. This is
transparent at the database level, although some DBMS do not support views properly,
especially with updates. Sometimes you want to use a view, but you cannot create one
in the database (i.e. with a legacy schema). In this case, you can map an immutable
and read-only entity to a given SQL subselect expression:
<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 both as an attribute and a nested mapping element.
5.1.4. id
被映射的类必须定义对应数据库表主键字段。大多数类有一个JavaBeans风格的属性,
实例包含唯一的标识。<id> 元素定义了该属性到数据库表主键字段的映射。
<id
name="propertyName"
type="typename"
74
为每一个
id
column="column_name"
unsaved-value="null|any|none|undefined|id_value"
access="field|property|ClassName">
node="element-name|@attribute-name|element/@attribute|."
<generator class="generatorClass"/>
</id>
name (optional): the name of the identifier property.
type (可选): 一个Hibernate类型的名字。
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 an instance is newly instantiated (unsaved), distinguishing
it from detached instances that were saved or loaded in a previous session.
access (optional - defaults to property): the strategy Hibernate should use for
accessing the property value.
如果 name属性不存在,会认为这个类没有标识属性。
unsaved-value 属性在Hibernate3中几乎不再需要。
There is an alternative <composite-id> declaration that allows access to legacy data
with composite keys. Its use is strongly discouraged for anything else.
5.1.4.1. Generator
可选的<generator>子元素是一个Java类的名字, 用来为该持久化类的实例生成唯一的标识。如果
这个生成器实例需要某些配置值或者初始化参数, 用<param>元素来传递。
<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 can choose to provide their own specialized
implementations, however, Hibernate provides a range of built-in implementations. The
shortcut names for the built-in generators are as follows:
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increment
用于为long, short或者int类型生成 唯一标识。只有在没有其他进程往同一张表中插入数据时
才能使用。 在集群下不要使用。
identity
对DB2,MySQL, MS SQL Server, Sybase和HypersonicSQL的内置标识字段提供支持。 返回的标
识符是long, short 或者int类型的。
sequence
在DB2,PostgreSQL, Oracle, SAP DB, McKoi中使用序列(sequence), 而在Interbase中使用
生成器(generator)。返回的标识符是long, short或者 int类型的。
hilo
使用一个高/低位算法高效的生成long, short 或者 int类型的标识符。给定一个表和字段(默
认分别是 hibernate_unique_key 和next_hi)作为高位值的来源。 高/低位算法生成的标识符
只在一个特定的数据库中是唯一的。
seqhilo
使用一个高/低位算法来高效的生成long, short 或者 int类型的标识符,给定一个数据库序列
(sequence)的名字。
uuid
uses a 128-bit UUID algorithm to generate identifiers of type string that are unique
within a network (the IP address is used). The UUID is encoded as a string of 32
hexadecimal digits in length.
guid
在MS SQL Server 和 MySQL 中使用数据库生成的GUID字符串。
native
selects identity, sequence or hilo depending upon the capabilities of the underlying
database.
assigned
lets the application 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. It is usually used in conjunction
with a <one-to-one> primary key association.
sequence-identity
a specialized sequence generation strategy that utilizes a database sequence for the
actual value generation, but combines this with JDBC3 getGeneratedKeys to return
the generated identifier value as part of the insert statement execution. This
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id
strategy is only supported on Oracle 10g drivers targeted for JDK 1.4. Comments on
these insert statements are disabled due to a bug in the Oracle drivers.
5.1.4.2. 高/低位算法(Hi/Lo Algorithm)
The hilo and seqhilo generators provide two alternate implementations of the hi/lo
algorithm. The first implementation requires a "special" database table to hold the
next available "hi" value. Where supported, the second uses an Oracle-style sequence.
<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 cannot use hilo when supplying your own Connection to Hibernate. When
Hibernate uses an application server datasource to obtain connections enlisted with
JTA, you must configure the hibernate.transaction.manager_lookup_class.
5.1.4.3. UUID算法(UUID Algorithm )
The UUID contains: IP address, startup time of the JVM that is accurate to a quarter
second, system time and a counter value that is unique within the JVM. It is not
possible to obtain a MAC address or memory address from Java code, so this is the
best option without using JNI.
5.1.4.4. 标识字段和序列(Identity columns and Sequences)
For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you can use
identity key generation. For databases that support sequences (DB2, Oracle, PostgreSQL,
Interbase, McKoi, SAP DB) you can use sequence style key generation. Both of these
strategies require two SQL queries to insert a new object. For example:
<id name="id" type="long" column="person_id">
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<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, depending on the capabilities
of the underlying database, choose from the identity, sequence and hilo strategies.
5.1.4.5. 程序分配的标识符(Assigned Identifiers)
If you want the application to assign identifiers, as opposed to having Hibernate
generate them, you can use the assigned generator. This special generator uses the
identifier value already assigned to the object's identifier property. The generator
is used when the primary key is a natural key instead of a surrogate key. This is the
default behavior if you do not specify a <generator> element.
The assigned generator makes Hibernate use unsaved-value="undefined". This forces 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. 触发器实现的主键生成器(Primary keys assigned by
triggers)
Hibernate does not generate DDL with triggers. It is for legacy schemas only.
<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. It
is 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;
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Enhanced identifier generators
the second is optimization Optimization means that you do not have 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
which is
secondly,
generally
semantics
of these new generators is org.hibernate.id.enhanced.SequenceStyleGenerator
intended, firstly, as a replacement for the sequence generator and,
as a better portability generator than native. This is because native
chooses between identity and sequence which have largely different
that can cause subtle issues in applications eyeing portability.
org.hibernate.id.enhanced.SequenceStyleGenerator, however, achieves portability in a
different manner. It chooses between a table or a 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
emulate 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 typically
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 typically 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, it
is the name of the column on the table which is used to hold the value.
•optimizer (optional - defaults to none): See 第 5.1.6 节 “Identifier generator
optimization”
The second of these new generators is org.hibernate.id.enhanced.TableGenerator, which is
intended, firstly, as a replacement for the table generator, even though it actually
functions much more like org.hibernate.id.MultipleHiLoPerTableGenerator, and secondly, as
a re-implementation of org.hibernate.id.MultipleHiLoPerTableGenerator that utilizes the
notion of pluggable optimizers. 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:
•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 that is used to hold the value.
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•segment_column_name (optional - defaults to sequence_name): the name of the column on
the table that is used to hold the "segment key". This is the value which 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.
•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 第 5.1.6 节 “Identifier generator
optimization”
5.1.6. Identifier generator optimization
For identifier generators that 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 can 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 (第 5.1.5 节 “Enhanced identifier generators” support
this operation.
•none (generally this is the default if no optimizer was specified): this will not
perform any optimizations and hit the database for 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: as with the case of hilo, this optimizer 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. Here, increment_size 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|."
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composite-id
<key-property name="propertyName" type="typename" column="column_name"/>
<key-many-to-one name="propertyName class="ClassName" column="column_name"/>
......
</composite-id>
A table with a composite key can be mapped with 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>
The persistent class must override equals() and hashCode() to implement composite
identifier equality. It must also implement Serializable.
Unfortunately, this approach 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.
第二种方法我们称为mapped(映射式)组合标识符 (mapped composite identifier),<compositeid>元素中列出的标识属性不但在持久化类出现,还形成一个独立的标识符类。
<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 main disadvantage of
this approach is code duplication.
下面列出的属性是用来指定一个映射式组合标识符的:
•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.
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•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 第 8.4 节 “组件作为联合标识符(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. Please see chapter 9 for more information.
•access (optional - defaults to property): the strategy Hibernate uses for accessing
the property value.
•class (optional - defaults to the property type determined by reflection): the
component class used as a composite identifier. Please see the next section for
more information.
The third approach, an identifier component, is recommended for almost all applications.
5.1.8. Discriminator
The <discriminator> element is required for polymorphic persistence using the tableper-class-hierarchy mapping strategy. It 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 can 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"
/>
column (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): "forces" Hibernate to specify the 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. It tells Hibernate not to include
the column in SQL INSERTs.
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Version (optional)
formula (optional): an arbitrary SQL expression that is executed when a type has
to be evaluated. It allows content-based discrimination.
鉴别器字段的实际值是根据<class>和<subclass>元素中 的discriminator-value属性得来的。
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.
The formula attribute allows you to declare an arbitrary SQL expression that will be
used to evaluate the type of a row. For example:
<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 for more
information:
<version
column="version_column"
name="propertyName"
type="typename"
access="field|property|ClassName"
unsaved-value="null|negative|undefined"
generated="never|always"
insert="true|false"
node="element-name|@attribute-name|element/@attribute|."
/>
column (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 uses to access
the property value.
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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.
generated (optional - defaults to never): specifies that this version property value
is generated by the database. See the discussion of generated properties for more
information.
insert (optional - defaults to true): specifies whether the version column should
be included in SQL insert statements. It can be set to false if the database column
is defined with a default value of 0.
Version numbers can be of Hibernate type long, integer, short, timestamp or calendar.
A version or timestamp property should never be null for a detached instance. Hibernate
will detect any instance with a null version or timestamp as transient, irrespective
of what other unsaved-value strategies are specified. Declaring a nullable version or
timestamp property is an easy way to avoid problems with transitive reattachment in
Hibernate. It is 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 provides an alternative to versioning. Timestamps are a less safe implementation
of optimistic locking. However, sometimes the application might use the timestamps
in other ways.
<timestamp
column="timestamp_column"
name="propertyName"
access="field|property|ClassName"
unsaved-value="null|undefined"
source="vm|db"
generated="never|always"
node="element-name|@attribute-name|element/@attribute|."
/>
column (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.
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Property
access (optional - defaults to property): the strategy Hibernate uses for accessing
the property value.
unsaved-value (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): Where should Hibernate retrieve the timestamp
value from? 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". It is safer to use in clustered environments. Not all Dialects are
known to support the retrieval of the database's current timestamp. Others may also
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 for more information.
Note
<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"
column="column_name"
type="typename"
update="true|false"
insert="true|false"
formula="arbitrary SQL expression"
access="field|property|ClassName"
lazy="true|false"
unique="true|false"
not-null="true|false"
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optimistic-lock="true|false"
generated="never|insert|always"
node="element-name|@attribute-name|element/@attribute|."
index="index_name"
unique_key="unique_key_id"
length="L"
precision="P"
scale="S"
/>
name: 属性的名字,以小写字母开头。
column (optional - defaults to the property name): the name of the mapped database
table column. This can also be specified by nested <column> element(s).
type (可选): 一个Hibernate类型的名字。
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 column(s), or by a trigger or other application.
formula (可选): 一个SQL表达式,定义了这个计算 (computed) 属性的值。计算属性没有
和它对应的数据库字段。
access (optional - defaults to property): the strategy Hibernate uses 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. It requires build-time bytecode
instrumentation.
unique (optional): enables the DDL generation of a unique constraint for the columns.
Also, allow this to be the target of a property-ref.
not-null (optional): enables 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, it
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
for more information.
typename可以是如下几种:
1. The name of a Hibernate basic type: integer, string, character, date, timestamp, float,
binary, serializable, object, blob etc.
2. The name of a Java class with a default basic type: int, float, char, java.lang.String,
java.util.Date, java.lang.Integer, java.sql.Clob etc.
3. 一个可以序列化的Java类的名字。
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Many-to-one
4. The class name of a custom type: com.illflow.type.MyCustomType etc.
If you do not
and guess the
of the return
certain cases
specify a type, Hibernate will use reflection upon the named property
correct Hibernate type. Hibernate will attempt to interpret the name
class of the property getter using, in order, rules 2, 3, and 4. In
you will need the type attribute. For example, to distinguish between
Hibernate.DATE and Hibernate.TIMESTAMP, or to specify a custom type.
The access attribute allows you to control how Hibernate accesses 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 can specify your own strategy for property access by naming a
class that implements the interface org.hibernate.property.PropertyAccessor.
A powerful feature is derived properties. These properties are by definition readonly. The property value is computed at load time. You declare the computation as
an SQL expression. This then 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 )"/>
You can reference the entity table by not declaring an alias on a particular column.
This would be customerId in the given example. You can also use the nested <formula>
mapping element if you do not want 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"
column="column_name"
class="ClassName"
cascade="cascade_style"
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fetch="join|select"
update="true|false"
insert="true|false"
property-ref="propertyNameFromAssociatedClass"
access="field|property|ClassName"
unique="true|false"
not-null="true|false"
optimistic-lock="true|false"
lazy="proxy|no-proxy|false"
not-found="ignore|exception"
entity-name="EntityName"
formula="arbitrary SQL expression"
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
index="index_name"
unique_key="unique_key_id"
foreign-key="foreign_key_name"
/>
name: the name of the property.
column (optional): the name of the foreign key column. This can 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 another property that
maps to the same column(s), or by a trigger or other application.
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.
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Many-to-one
access (optional - defaults to property): the strategy Hibernate uses for accessing
the property value.
unique (optional): enables the DDL generation of a unique constraint for the foreignkey column. By allowing this to be the target of a property-ref, you can make the
association multiplicity one-to-one.
not-null (optional): enables 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, it
determines 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. This 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 (可选): SQL表达式,用于定义computed(计算出的)外键值。
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 divided
into three categories. First, basic operations, which include: persist, merge, delete,
save-update, evict, replicate, lock and refresh; second, special values: delete-orphan; and
third,all comma-separated combinations of operation names: cascade="persist,merge,evict"
or cascade="all,delete-orphan". See 第 10.11 节 “传播性持久化(transitive persistence)”
for a full explanation. Note that single valued, many-to-one and one-to-one,
associations do not support orphan delete.
Here is an example of a typical many-to-one declaration:
<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 the primary key. This
is a complicated and confusing relational model. For example, if 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"/>
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那么关于OrderItem 的映射可能是:
<many-to-one
name="product"
property-ref="serialNumber"
column="PRODUCT_SERIAL_NUMBER"/>
This is not encouraged, however.
如果被引用的唯一主键由关联实体的多个属性组成,你应该在名称为<properties>的元素
射所有关联的属性。
里面映
If the referenced unique key is the property of a component, you can specify a property
path:
<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>
5.1.13. One-to-one
持久化对象之间一对一的关联关系是通过one-to-one元素定义的。
<one-to-one
name="propertyName"
class="ClassName"
cascade="cascade_style"
constrained="true|false"
fetch="join|select"
property-ref="propertyNameFromAssociatedClass"
access="field|property|ClassName"
formula="any SQL expression"
lazy="proxy|no-proxy|false"
entity-name="EntityName"
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
foreign-key="foreign_key_name"
/>
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One-to-one
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 and references the table of the associated class. This option
affects the order in which save() and delete() are cascaded, and determines whether
the association can 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 uses for accessing
the property value.
formula (optional): almost all one-to-one associations map to the primary key of
the owning entity. If this is not the case, you can specify another 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. It 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
eagerly fetch the association.
entity-name (optional): the entity name of the associated class.
There are two varieties of one-to-one associations:
•主键关联
•惟一外键关联
Primary key associations do not need an extra table column. If two rows are related
by the association, then the two table rows share the same primary key value. To
relate two objects by a primary key association, ensure 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"/>
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<one-to-one name="employee" class="Employee" constrained="true"/>
Ensure that the primary keys of the related rows in the PERSON and EMPLOYEE tables are
equal. You 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 assigned the same primary key value as the Employee
instance referred with the employee property of that Person.
Alternatively, a foreign key with a unique constraint, from Employee to Person, can
be expressed as:
<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>
This association can 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 and dynamic-component
Although we recommend the use of surrogate keys as primary keys, you should try to
identify natural keys for all entities. A natural key is a property or combination of
properties that is unique and non-null. It is also immutable. Map the properties of
the natural key inside the <natural-id> element. Hibernate will generate the necessary
unique key and nullability constraints and, as a result, your mapping will be more
self-documenting.
It is recommended that you implement equals() and hashCode() to compare the natural key
properties of the entity.
This mapping is not intended for use with entities that have natural primary keys.
•mutable (optional - defaults to false): by default, natural identifier properties are
assumed to be immutable (constant).
5.1.15. Component and dynamic-component
The <component> element maps properties of a child object to columns of the table of
a parent class. Components can, in turn, declare their own properties, components or
collections. See the "Component" examples 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|."
>
<property ...../>
<many-to-one .... />
........
</component>
name: the name of the property.
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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 uses 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. It requires build-time bytecode
instrumentation.
optimistic-lock (optional - defaults to true): specifies that updates to this
component either do or do not require acquisition of the optimistic lock. It
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.
其<property>子标签为子类的一些属性与表字段之间建立映射。
<component>元素允许加入一个<parent>子元素,在组件类内部就可以有一个指向其容器的实体的
反向引用。
The <dynamic-component> element allows a Map to be mapped as a component, where
the property names refer to keys of the map. See 第 8.5 节 “动态组件 (Dynamic
components)” for more information.
5.1.16. Properties
The <properties> element allows the definition of a named, logical grouping of the
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. For example:
<properties
name="logicalName"
insert="true|false"
update="true|false"
optimistic-lock="true|false"
unique="true|false"
>
<property ...../>
<many-to-one .... />
........
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Subclass
</properties>
name: the logical name of the grouping. It is 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 either do or do not require acquisition of the optimistic lock. It
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.
例如,如果我们有如下的<properties>映射:
<class name="Person">
<id name="personNumber"/>
...
<properties name="name"
unique="true" update="false">
<property name="firstName"/>
<property name="initial"/>
<property name="lastName"/>
</properties>
</class>
You might have some legacy data association that 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>
The use of this outside the context of mapping legacy data is not recommended.
5.1.17. Subclass
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. For example:
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<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">
<property .... />
.....
</subclass>
name: the fully qualified class name of the subclass.
(optional - defaults to the class name): a value that
distinguishes individual subclasses.
proxy (optional): specifies a class or interface used for lazy initializing proxies.
discriminator-value
lazy (optional - defaults to true): setting lazy="false" disables the use of lazy
fetching.
Each subclass declares its own persistent properties and subclasses. <version> and <id>
properties are assumed to be inherited from the root class. Each subclass in a hierarchy
must define a unique discriminator-value. If this is not specified, the fully qualified
Java class name is used.
For information about inheritance mappings see 第 9 章 Inheritance mapping.
5.1.18. Joined-subclass
Each subclass can also be mapped to its own table. This is called the table-per-subclass
mapping strategy. An inherited state is retrieved by joining with the table of the
superclass. To do this you use the <joined-subclass> element. For example:
<joined-subclass
name="ClassName"
table="tablename"
proxy="ProxyInterface"
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Joined-subclass
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">
<key .... >
<property .... />
.....
</joined-subclass>
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.
A discriminator column is not required for this mapping strategy. Each subclass must,
however, declare a table column holding the object identifier using the <key> element.
The mapping at the start of the chapter would then 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"/>
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<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 第 9 章 Inheritance mapping.
5.1.19. Union-subclass
A third option is to map only the concrete classes of an inheritance hierarchy to
tables. This is called the table-per-concrete-class strategy. Each table defines all
persistent states of the class, including the inherited state. In Hibernate, it is
not necessary to explicitly map such inheritance hierarchies. You can 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 <unionsubclass> mapping. For example:
<union-subclass
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"
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Join
subselect="SQL expression"
entity-name="EntityName"
node="element-name">
<property .... />
.....
</union-subclass>
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.
这种映射策略不需要指定辨别标志(discriminator)字段。
For information about inheritance mappings see 第 9 章 Inheritance mapping.
5.1.20. Join
Using the <join> element, it is possible to map properties of one class to several
tables that have a one-to-one relationship. For example:
<join
table="tablename"
schema="owner"
catalog="catalog"
fetch="join|select"
inverse="true|false"
optional="true|false">
<key ... />
<property ... />
...
</join>
table: the name of the joined table.
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schema (optional): overrides the schema name specified by the root <hibernatemapping> element.
catalog (optional): overrides the catalog name specified by the root <hibernatemapping> 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. It
will use 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. This
will be issued only if a row represents 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 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. It will always use an
outer join to retrieve the properties.
For example, 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
The <key> element has featured a few times within this guide. It appears anywhere the
parent mapping element defines a join to a new table that references the primary key
of the original table. It also defines the foreign key in the joined table:
<key
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Column and formula elements
column="columnname"
on-delete="noaction|cascade"
property-ref="propertyName"
not-null="true|false"
update="true|false"
unique="true|false"
/>
column (optional): the name of the foreign key column. This can 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 original table. It is 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.
For systems where delete performance is important, we recommend that all keys should be
defined on-delete="cascade". Hibernate uses 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 association to a non-nullable
foreign key, you must declare the key column using <key not-null="true">.
5.1.22. Column and formula elements
Mapping elements which accept a column attribute will alternatively accept a <column>
subelement. Likewise, <formula> is an alternative to the formula attribute. For example:
<column
name="column_name"
length="N"
precision="N"
scale="N"
not-null="true|false"
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unique="true|false"
unique-key="multicolumn_unique_key_name"
index="index_name"
sql-type="sql_type_name"
check="SQL expression"
default="SQL expression"/>
<formula>SQL expression</formula>
column and formula attributes can even be combined within the same property or association
mapping to express, for example, exotic join conditions.
<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
If your application has two persistent classes with the same name, and you do not
want to specify the fully qualified package name in Hibernate queries, classes can be
"imported" explicitly, rather than relying upon auto-import="true". You can also import
classes and interfaces that are not explicitly mapped:
<import class="java.lang.Object" rename="Universe"/>
<import
class="ClassName"
rename="ShortName"
/>
class: the fully qualified class name of any Java class.
rename (optional - defaults to the unqualified class name): a name that can be
used in the query language.
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Any
5.1.24. Any
There is one more type of property mapping. The <any> mapping element defines a
polymorphic association to classes from multiple tables. This type of mapping requires
more than one column. The first column contains the type of the associated entity.
The remaining columns contain the identifier. It is impossible to specify a foreign
key constraint for this kind of association. This is not the usual way of mapping
polymorphic associations and you should use this only in special cases. For example,
for audit logs, user session data, etc.
The meta-type attribute allows the application to specify a custom type that maps
database column values to persistent classes that 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"
>
<meta-value ... />
<meta-value ... />
.....
<column .... />
<column .... />
.....
</any>
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name: 属性名
id-type: 标识符类型
meta-type (optional - defaults to string): any type that is allowed for a
discriminator mapping.
cascade (可选 -默认是none): 级联的类型
access (optional - defaults to property): the strategy Hibernate uses for accessing
the property value.
optimistic-lock (optional - defaults to true): specifies that updates to this property
either do or do not require acquisition of the optimistic lock. It defines whether
a version increment should occur if this property is dirty.
5.2. Hibernate types
5.2.1. 实体(Entities)和值(values)
In relation to the persistence service, Java language-level objects are classified
into two groups:
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. Saves and deletions, however,
can be cascaded from a parent entity to its children. This is different from the
ODMG model of object persistence by reachability and corresponds more closely to how
application objects are usually used in large systems. Entities support circular and
shared references. They can 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 is 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 cannot be independently versioned. Values have no independent identity, so they
cannot be shared by two entities or collections.
Until now, we have been using the term "persistent class" to refer to entities. We will
continue to do that. Not all user-defined classes with a persistent state, however,
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, all types
(classes) provided by the JDK have value type semantics in Java, while user-defined
types can be mapped with entity or value type semantics. This decision is up to the
application developer. An entity class in a domain model will normally have shared
references to a single instance of that class, while composition or aggregation usually
translates to a value type.
We will revisit both concepts throughout this reference guide.
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基本值类型
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, <class>, <subclass> and so on are used. For value
types we use <property>, <component>etc., that usually have a type attribute. The value
of this attribute is the name of a Hibernate mapping type. Hibernate provides a range
of mappings for standard JDK value types out of the box. You can write your own mapping
types and implement your own custom conversion strategies.
With the exception of collections, all built-in Hibernate types support null semantics.
5.2.2. 基本值类型
The built-in basic mapping types can be roughly categorized into the following:
integer, long, short, float, double, character, byte, boolean, yes_no, true_false
这些类型都对应Java的原始类型或者其封装类,来符合(特定厂商的)SQL 字段类型。boolean,
yes_no 和 true_false都是Java 中boolean 或者java.lang.Boolean的另外说法。
string
从java.lang.String 到 VARCHAR (或者 Oracle的 VARCHAR2)的映射。
date, time, timestamp
从java.util.Date和其子类到SQL类型DATE, TIME 和TIMESTAMP (或等价类型)的映射。
calendar, calendar_date
从java.util.Calendar 到SQL 类型TIMESTAMP和 DATE(或等价类型)的映射。
big_decimal, big_integer
从java.math.BigDecimal和java.math.BigInteger到NUMERIC (或者 Oracle 的NUMBER类型)的映
射。
locale, timezone, currency
从java.util.Locale, java.util.TimeZone 和java.util.Currency 到VARCHAR (或者 Oracle
的VARCHAR2类型)的映射. Locale和 Currency 的实例被映射为它们的ISO代码。TimeZone的实例
被影射为它的ID。
class
从java.lang.Class 到 VARCHAR (或者 Oracle 的VARCHAR2类型)的映射。Class被映射为它的全
限定名。
binary
把字节数组(byte arrays)映射为对应的 SQL二进制类型。
text
把长Java字符串映射为SQL的CLOB或者TEXT类型。
serializable
Maps serializable Java types to an appropriate SQL binary type. You can also
indicate the Hibernate type serializable with the name of a serializable Java class
or interface that does not default to a basic type.
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clob, blob
Type mappings for the JDBC classes java.sql.Clob and java.sql.Blob. These types can be
inconvenient for some applications, since the blob or clob object cannot be reused
outside of a transaction. Driver support is patchy and inconsistent.
imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date, imm_serializable, imm_binary
Type mappings for what are considered mutable Java types. This is 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 can be of any basic type except binary,
blob and clob. Composite identifiers are also allowed. See below for more information.
在org.hibernate.Hibernate中,定义了基础类型对应的Type常量。比如,Hibernate.STRING代
表string 类型。
5.2.3. 自定义值类型
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. 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. View 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>
注意使用<column>标签来把一个属性映射到多个字段的做法。
CompositeUserType, EnhancedUserType, UserCollectionType, 和 UserVersionType 接口为更特殊的使
用方式提供支持。
You can even supply parameters to a UserType in the mapping file. To do this, your
UserType must implement the org.hibernate.usertype.ParameterizedType interface. To supply
parameters to your custom type, you can use the <type> element in your mapping files.
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多次映射同一个类
<property name="priority">
<type name="com.mycompany.usertypes.DefaultValueIntegerType">
<param name="default">0</param>
</type>
</property>
现在,UserType 可以从传入的Properties对象中得到default 参数的值。
If you regularly use a certain UserType, it is 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 can 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"/>
也可以根据具体案例通过属性映射中的类型参数覆盖在typedef中提供的参数。
Even though Hibernate's rich range of built-in types and support for components means
you will rarely need to use a custom type, it is considered good practice 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 be mapped as a component. One reason for this is abstraction. With a custom
type, your mapping documents would be protected against changes to the way monetary
values are represented.
5.3. 多次映射同一个类
It is possible to provide more than one mapping for a particular persistent class. In
this case, you must specify an entity name to 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"
entity-name="CurrentContract">
...
<set name="history" inverse="true"
order-by="effectiveEndDate desc">
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<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>
Associations are now specified using entity-name instead of class.
5.4. SQL中引号包围的标识符
You can 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. This is usually double quotes, but the SQL
Server uses brackets and MySQL uses backticks.
<class name="LineItem" table="`Line Item`">
<id name="id" column="`Item Id`"/><generator class="assigned"/></id>
<property name="itemNumber" column="`Item #`"/>
...
</class>
5.5. 其他元数据(Metadata)
XML does not suit all users so there are some alternative ways to define O/R mapping
metadata in Hibernate.
5.5.1. 使用 XDoclet 标记
Many Hibernate users prefer to embed mapping information directly in sourcecode using
XDoclet @hibernate.tags. We do not cover this approach in this reference guide since
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;
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使用 XDoclet 标记
import java.util.Date;
/**
* @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;
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}
void setBirthdate(Date date) {
birthdate = date;
}
/**
* @hibernate.property
* column="WEIGHT"
*/
public float getWeight() {
return weight;
}
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);
}
110
使用 JDK 5.0 的注解(Annotation)
/**
* @hibernate.property
* column="SEX"
* not-null="true"
* update="false"
*/
public char getSex() {
return sex;
}
void setSex(char sex) {
this.sex=sex;
}
}
See the Hibernate website for more examples of XDoclet and Hibernate.
5.5.2. 使用 JDK 5.0 的注解(Annotation)
JDK 5.0 introduced XDoclet-style annotations at the language level that are type-safe
and checked at compile time. This mechanism is more powerful than XDoclet annotations
and better supported by tools and IDEs. IntelliJ IDEA, for example, supports autocompletion 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.
这是一个被注解为EJB entity bean 的POJO类的例子
@Entity(access = AccessType.FIELD)
public class Customer implements Serializable {
@Id;
Long id;
String firstName;
String lastName;
Date birthday;
@Transient
Integer age;
@Embedded
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第 5 章 对象/关系数据库映射基础(Basic O/R ...
private Address homeAddress;
@OneToMany(cascade=CascadeType.ALL)
@JoinColumn(name="CUSTOMER_ID")
Set<Order> orders;
// Getter/setter and business methods
}
Note
Support for JDK 5.0 Annotations (and JSR-220) is currently under
development. Please refer to the Hibernate Annotations module for more
details.
5.6. Generated properties
Generated properties are properties that have their values generated by the database.
Typically, Hibernate applications needed to refresh objects that contain any properties
for which the database was generating values. Marking properties as generated, however,
lets the application delegate this responsibility to Hibernate. When Hibernate issues an
SQL INSERT or UPDATE for an entity that 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.
never (the default): the given property value is not generated within the database.
insert: the given property value is generated on insert, but is not regenerated on
subsequent updates. Properties like created-date fall into this category. Even though
version and timestamp properties can be marked as generated, this option is not
available.
always: the property value is generated both on insert and on update.
5.7. Auxiliary database objects
Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects.
In conjunction with Hibernate's schema evolution tools, they have 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, any SQL command
that can be run via a java.sql.Statement.execute() method is valid (for example, ALTERs,
INSERTS, etc.). There are essentially two modes for defining auxiliary database objects:
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Auxiliary database objects
The first mode is to explicitly list the CREATE and DROP commands 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 that constructs the CREATE and DROP commands.
This custom class must implement the org.hibernate.mapping.AuxiliaryDatabaseObject
interface.
<hibernate-mapping>
...
<database-object>
<definition class="MyTriggerDefinition"/>
</database-object>
</hibernate-mapping>
Additionally, these database objects can be optionally scoped so that they only apply
when certain dialects are used.
<hibernate-mapping>
...
<database-object>
<definition class="MyTriggerDefinition"/>
<dialect-scope name="org.hibernate.dialect.Oracle9iDialect"/>
<dialect-scope name="org.hibernate.dialect.Oracle10gDialect"/>
</database-object>
</hibernate-mapping>
113
114
第 6
Collection mapping
6.1. 持久化集合类(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
("anything you like" means you will have to write an implementation of
org.hibernate.usertype.UserCollectionType.)
Notice how the instance variable was initialized with an instance of HashSet. This is
the best way to initialize collection valued properties of newly instantiated (nonpersistent) 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. Be aware of the following errors:
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 on the interface type.
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第 6 章 Collection mapping
Collections instances have the usual behavior of value types. They are automatically
persisted when referenced by a persistent object and are automatically deleted when
unreferenced. If a collection is passed from one persistent object to another, its
elements might be moved from one table to another. Two entities cannot share a reference
to the same collection instance. Due to the underlying relational model, collectionvalued properties do not support null value semantics. Hibernate does not distinguish
between a null collection reference and an empty collection.
Use persistent collections the same way you use ordinary Java collections. However,
please ensure you understand the semantics of bidirectional associations (these are
discussed later).
6.2. 集合映射( Collection mappings )
提示
There are quite a range of mappings that can be generated for collections
that cover many common relational models. We suggest you experiment with
the schema generation tool so that you understand how various mapping
declarations translate to database tables.
The Hibernate mapping element used for mapping a collection depends upon the type of
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>
除了<set>,还有<list>, <map>, <bag>, <array> 和 <primitive-array> 映射元素。<map>具有代表
性:
<map
name="propertyName"
table="table_name"
schema="schema_name"
116
集合映射( Collection mappings )
lazy="true|extra|false"
inverse="true|false"
cascade="all|none|save-update|delete|all-delete-orphan|delet
e-orphan"
sort="unsorted|natural|comparatorClass"
order-by="column_name asc|desc"
where="arbitrary sql where condition"
fetch="join|select|subselect"
batch-size="N"
access="field|property|ClassName"
optimistic-lock="true|false"
mutable="true|false"
node="element-name|."
embed-xml="true|false"
>
<key .... />
<map-key .... />
<element .... />
</map>
name: the collection property name
table (optional - defaults to property name): the name of the collection table.
It is 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): disables lazy fetching and specifies that the
association is always eagerly fetched. It can also be used to enable "extra-lazy"
fetching where most operations do not initialize the collection. This is suitable
for large collections.
inverse (optional - defaults to false): marks this collection as the "inverse" end
of a bidirectional association.
cascade (optional - defaults to none): enables operations to cascade to child
entities.
sort (optional): specifies a sorted collection with natural sort order or a given
comparator class.
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第 6 章 Collection mapping
order-by (optional, JDK1.4 only): specifies 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): specifies an arbitrary SQL WHERE condition that is used when
retrieving or removing the collection. This is useful if the collection needs to
contain only a subset of the available data.
fetch (optional, defaults to select): chooses between outer-join fetching, fetching
by sequential select, and fetching by sequential subselect.
batch-size (optional, defaults to 1): specifies a "batch size" for lazily fetching
instances of this collection.
access (optional - defaults to property): the strategy Hibernate uses for accessing
the collection property value.
optimistic-lock (optional - defaults to true): specifies that changes to the state
of the collection results in increments of the owning entity's version. For oneto-many associations you may want to disable this setting.
mutable (optional - defaults to true): a value of false specifies that the elements
of the collection never change. This allows for minor performance optimization
in some cases.
6.2.1. 集合外键(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 can 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 may need to specify not-null="true".
<key column="productSerialNumber" not-null="true"/>
The foreign key constraint can use ON DELETE CASCADE.
<key column="productSerialNumber" on-delete="cascade"/>
对<key> 元素的完整定义,请参阅前面的章节。
6.2.2. 集合元素(Collection elements)
Collections can contain almost any other Hibernate type, including: basic types, custom
types, components and 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
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索引集合类(Indexed collections)
its own life cycle. In the latter case, only the "link" between the two objects is
considered to be a state held by the collection.
被包容的类型被称为集合元素类型(collection
element
type)。集合元素通
过<element>或<composite-element>映射,或在其是实体引用的时候,通过<one-to-many>
to-many>映射。前两种用于使用值语义映射元素,后两种用于映射实体关联。
或<many-
6.2.3. 索引集合类(Indexed collections)
All collection mappings, except those with set and bag semantics, need an index column
in the collection table. An index column is 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 <mapkey>. It can be an entity reference mapped with <map-key-many-to-many>, or it can be a
composite type mapped with <composite-map-key>. The index of an array or list is always
of type integer and is mapped using the <list-index> element. The mapped column contains
sequential integers that are numbered from zero by default.
<list-index
column="column_name"
base="0|1|..."/>
column_name (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.
<map-key
column="column_name"
formula="any SQL expression"
type="type_name"
node="@attribute-name"
length="N"/>
column (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 (required): the type of the map keys.
<map-key-many-to-many
column="column_name"
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第 6 章 Collection mapping
formula="any SQL expression"
class="ClassName"
/>
column (optional): the name of the foreign key column for the collection index
values.
formula (optional): a SQ formula used to evaluate the foreign key of the map key.
class (required): the entity class used as the map key.
If your table does not have an index column, and you still wish to use List as the
property type, you can map the property as a Hibernate <bag>. A bag does not retain its
order when it is retrieved from the database, but it can be optionally sorted or ordered.
6.2.4. 对于一个值集合, 我们使用<element>标签。
Any collection of values or many-to-many associations requires a dedicated collection
table with a foreign key column or columns, collection element column or columns, and
possibly an index column or columns.
For a collection of values use the <element> tag. For example:
<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"
/>
column (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.
A many-to-many association is specified using the <many-to-many> element.
<many-to-many
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对于一个值集合, 我们使用<element>标签。
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 (optional): the name of the element foreign key column.
formula (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): enables the DDL generation of a unique constraint for the foreignkey 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.
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.
Here are some examples.
A set of strings:
<set name="names" table="person_names">
<key column="person_id"/>
<element column="person_name" type="string"/>
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</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>
一个组件的列表:(下一章讨论)
<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 (this is discussed in the next chapter):
<list name="carComponents"
table="CarComponents">
<key column="carId"/>
<list-index column="sortOrder"/>
<composite-element class="CarComponent">
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一对多关联通过外键连接两个类对应的表,而没有中间集合表。 这个关系模型失去了一些Java集合的语义:
<property name="price"/>
<property name="type"/>
<property name="serialNumber" column="serialNum"/>
</composite-element>
</list>
6.2.5. 一对多关联通过外键连接两个类对应的表,而没有中间集合
表。 这个关系模型失去了一些Java集合的语义:
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 cannot belong to more than one instance
of the collection.
•An instance of the contained entity class cannot appear at more than one value of
the collection index.
An association from Product to Part requires the 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"
/>
class (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.
The <one-to-many> element does not need to declare any columns. Nor is it necessary to
specify the table name anywhere.
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第 6 章 Collection mapping
警告
If the foreign key column of a <one-to-many> association is declared
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 for more information.
NOT
The following 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:
<map name="parts"
cascade="all">
<key column="productId" not-null="true"/>
<map-key formula="partName"/>
<one-to-many class="Part"/>
</map>
6.3. 高级集合映射(Advanced collection mappings)
6.3.1. 有序集合(Sorted collections)
Hibernate支持实现java.util.SortedMap和java.util.SortedSet的集合。 你必须在映射文件中指定
一个比较器:
<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>
sort属性中允许的值包括unsorted,natural和某个实现了java.util.Comparator的类的名称。
分类集合的行为事实上象java.util.TreeSet或者java.util.TreeMap。
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双向关联(Bidirectional associations)
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 and is implemented using LinkedHashSet or LinkedHashMap. This performs the
ordering in the SQL query and not in the 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>
Note
The value of the order-by attribute is an SQL ordering, not an HQL ordering.
Associations can even be sorted by arbitrary criteria at runtime using a collection
filter():
sortedUsers = s.createFilter( group.getUsers(), "order by this.name" ).list();
6.3.2. 双向关联(Bidirectional associations)
A bidirectional association allows navigation from both "ends" of the association. Two
kinds of bidirectional association are supported:
一对多(one-to-many)
set or bag valued at one end and single-valued at the other
多对多(many-to-many)
两端都是set或bag值
You can specify a bidirectional many-to-many association by mapping two many-to-many
associations to the same database table and declaring one end as inverse. You cannot
select an indexed collection.
Here is an example of a bidirectional many-to-many association that illustrates how
each category can have many items and each item can be in many categories:
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第 6 章 Collection mapping
<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 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 a many-to-many relationship in Javais created:
category.getItems().add(item);
// The category now "knows" about the relationship
item.getCategories().add(category); // The item now "knows" about the relationship
session.persist(item);
session.persist(category);
// The relationship won't be saved!
// The relationship will be saved
非反向端用于把内存中的表示保存到数据库中。
You can 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"/>
....
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双向关联,涉及有序集合类
<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" does not affect the operation of
cascades as these are orthogonal concepts.
6.3.3. 双向关联,涉及有序集合类
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 that maps to the index
column you can use inverse="true" on the collection mapping:
<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"/>
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第 6 章 Collection mapping
</class>
If there is no such property on the child class, the association cannot be considered
truly bidirectional. That is, there is information available at one end of the
association that is not available at the other end. In this case, you cannot map the
collection inverse="true". Instead, you 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>
Note that in this mapping, the collection-valued end of the association is responsible
for updates to the foreign key.
6.3.4. 三重关联(Ternary associations)
There are three possible approaches to mapping a ternary association. One approach 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"/>
128
使用<idbag>
</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 remodel the association as an entity class. This is the most
common approach.
A final alternative is to use composite elements, which will be discussed later.
6.3.5.
使用<idbag>
The majority of the many-to-many associations and collections of values shown previously
all map to tables with composite keys, even though it has been have suggested that
entities should have synthetic identifiers (surrogate keys). A pure association table
does not seem to benefit much from a surrogate key, although a collection of composite
values might. It is for this reason that 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. For example:
<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>
An <idbag> has a synthetic id generator, just like an entity class. A different surrogate
key is assigned to each collection row. Hibernate does not, however, provide any
mechanism for discovering the surrogate key value of a particular row.
The update performance of an <idbag> supersedes a regular <bag>. Hibernate can locate
individual rows efficiently and update or delete them individually, similar to a list,
map or set.
在目前的实现中,还不支持使用identity标识符生成器策略来生成<idbag>集合的标识符。
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第 6 章 Collection mapping
6.4. 集合例子(Collection example)
This section covers collection examples.
The following class has a collection of Child instances:
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; }
....
....
}
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">
<generator class="sequence"/>
</id>
<property name="name"/>
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集合例子(Collection example)
</class>
</hibernate-mapping>
在以下的表定义中反应了这个映射关系:
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
如果父亲是必须的, 那么就可以使用双向one-to-many的关联了:
<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>
请注意NOT NULL的约束:
create table parent ( id bigint not null primary key )
create table child ( id bigint not null
primary key,
name varchar(255),
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第 6 章 Collection mapping
parent_id bigint not null )
alter table child add constraint childfk0 (parent_id) references parent
Alternatively, if this association must be unidirectional you can declare the NOT NULL
constraint on the <key> mapping:
<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 has 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>
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集合例子(Collection example)
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
表定义:
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 ) )
alter table childset add constraint childsetfk0 (parent_id) references parent
alter table childset add constraint childsetfk1 (child_id) references child
For more examples and a complete explanation of a parent/child relationship mapping,
see 第 21 章 示例:父子关系(Parent Child Relationships) for more information.
Even more complex association mappings are covered in the next chapter.
133
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第 7
关联关系映射
7.1. 介绍
Association mappings are often the most difficult thing to implement correctly. In
this section we examine some canonical cases one by one, starting with unidirectional
mappings and then bidirectional cases. We will use Person and Address in all the examples.
Associations will be classified by multiplicity and whether or not they map to an
intervening join table.
Nullable foreign keys are not considered to be good practice in traditional data
modelling, so our examples do not use nullable foreign keys. This is not a requirement
of Hibernate, and the mappings will work if you drop the nullability constraints.
7.2. 单向关联(Unidirectional associations)
7.2.1. Many-to-one
单向many-to-one关联是最常见的单向关联关系。
<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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第 7 章 关联关系映射
7.2.2. One-to-one
基于外键关联的单向一对一关联和单向多对一关联几乎是一样的。唯一的不同就是单向一对一关
联中的外键字段具有唯一性约束。
<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 In this example, however, we have reversed the direction of the association:
<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>
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One-to-many
<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 )
7.2.3. One-to-many
A unidirectional one-to-many association on a foreign key is an unusual case, and is
not 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 )
You should instead use a join table for this kind of association.
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第 7 章 关联关系映射
7.3. 使用连接表的单向关联(Unidirectional
associations with join tables)
7.3.1. One-to-many
A unidirectional one-to-many association on a join table is the preferred option.
Specifying unique="true", changes 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>
<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 common when the association
is optional. For example:
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
138
One-to-one
</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 possible, but extremely
unusual.
<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>
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第 7 章 关联关系映射
<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, here is an example of 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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双向关联(Bidirectional associations)
7.4. 双向关联(Bidirectional associations)
7.4.1. one-to-many / many-to-one
A bidirectional many-to-one association is the most common kind of association. The
following example illustrates 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, set the key column of the foreign
key to not null. Hibernate will 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"
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第 7 章 关联关系映射
not-null="true"
insert="false"
update="false"/>
</class>
<class name="Address">
<id name="id"/>
...
<list name="people">
<key column="addressId" not-null="true"/>
<list-index column="peopleIdx"/>
<one-to-many class="Person"/>
</list>
</class>
If the underlying foreign key column is NOT NULL, it is important that you define notnull="true" on the <key> element of the collection mapping. Do not only declare notnull="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 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>
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使用连接表的双向关联(Bidirectional associations with join tables)
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>
<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. 使用连接表的双向关联(Bidirectional associations
with join tables)
7.5.1. one-to-many / many-to-one
The following is an example of a bidirectional one-to-many association on a join table.
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">
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第 7 章 关联关系映射
<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>
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. 一对一(one to one)
A bidirectional one-to-one association on a join table is possible, but extremely
unusual.
<class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<join table="PersonAddress"
optional="true">
144
Many-to-many
<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
Here is an example of a bidirectional 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"
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第 7 章 关联关系映射
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. 更复杂的关联映射
More complex association joins are extremely rare. Hibernate handles more complex
situations by using SQL fragments embedded in the mapping document. For example, if
a table with historical account information data defines accountNumber, effectiveEndDate
and effectiveStartDatecolumns, it would be 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"/>
You can then map an association to the current instance, the one with null
effectiveEndDate, by using:
146
更复杂的关联映射
<many-to-one name="currentAccountInfo"
property-ref="currentAccountKey"
class="AccountInfo">
<column name="accountNumber"/>
<formula>'1'</formula>
</many-to-one>
In
is
to
be
a more complex example, imagine that the association between Employee and Organization
maintained in an Employment table full of historical employment data. An association
the employee's most recent employer, the one with the most recent startDate, could
mapped in the following 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>
This functionality allows a degree of creativity and flexibility, but it is more
practical to handle these kinds of cases using HQL or a criteria query.
147
148
第 8
组件(Component)映射
The notion of a component is re-used in several different contexts and purposes
throughout Hibernate.
8.1. 依赖对象(Dependent objects)
A component is a contained object that is persisted as a value type and not an entity
reference. The term "component" refers to the object-oriented notion of composition and
not to architecture-level components. For example, you can 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() {
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第 8 章 组件(Component)映射
return first;
}
void setFirst(String first) {
this.first = first;
}
public String getLast() {
return last;
}
void setLast(String last) {
this.last = last;
}
public char getInitial() {
return initial;
}
void setInitial(char initial) {
this.initial = initial;
}
}
Now Name can be persisted as a component of Person. Name defines getter and setter
methods for its persistent properties, but it does not need to declare any interfaces
or identifier properties.
Our Hibernate mapping would look like this:
<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>
人员(Person)表中将包括pid, birthday, initial, first和 last等字段。
Like 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 objects that were only "the same" by value. The null value semantics
of a component are ad hoc. When reloading the containing object, Hibernate will assume
150
在集合中出现的依赖对象 (Collections of dependent objects)
that if all component columns are null, then the entire component is null. This is
suitable for most purposes.
The properties of a component can 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 fine-grained object model.
<component> 元素还允许有 <parent>子元素,用来表明component类中的一个属性是指向包含它的
实体的引用。
<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. 在集合中出现的依赖对象 (Collections of dependent
objects)
Collections of components are supported (e.g. 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>
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第 8 章 组件(Component)映射
重要
If you define a Set of composite elements, it is important to implement
equals() and hashCode() correctly.
Composite elements can contain components but not collections. If your composite element
contains components, use the <nested-composite-element> tag. This case is a collection
of components which themselves have components. You may want to consider if a oneto-many association is more appropriate. Remodel the composite element as an entity,
but be aware that even though the Java model is the same, the relational model and
persistence semantics are still slightly different.
A composite element mapping does not support null-able properties if you are using
a <set>. There is no separate primary key column in the composite element table.
Hibernate uses each column's value to identify a record when deleting objects, 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>.
A special case of a composite element is a composite element with a nested <many-to-one>
element. This mapping 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>
There cannot be a reference to the purchase on the other side for bidirectional
association navigation. Components are value types and do not allow shared references.
A single Purchase can be in the set of an Order, but it cannot be referenced by the
Item at the same time.
其实组合元素的这个用法可以扩展到三重或多重关联:
152
组件作为Map的索引(Components as Map indices )
<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 can appear in queries using the same syntax as associations to
other entities.
8.3. 组件作为Map的索引(Components as Map indices )
The <composite-map-key> element allows you to map a component class as the key of a Map.
Ensure that you override hashCode() and equals() correctly on the component class.
8.4. 组件作为联合标识符(Components as composite
identifiers)
You can use a component as an identifier of an entity class. Your component class must
satisfy certain requirements:
•它必须实现java.io.Serializable接口
•It must re-implement equals() and hashCode() consistently with the database's notion
of composite key equality.
Note
In Hibernate3, although the second requirement is not an absolutely hard
requirement of Hibernate, it is recommended.
You cannot 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">
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第 8 章 组件(Component)映射
<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>
Any foreign keys referencing the OrderLine table are now composite. Declare this in your
mappings for other classes. An association to OrderLine is 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>
提示
The column element is an alternative to the column attribute everywhere.
Using the column element just gives more declaration options, which are
mostly useful when utilizing hbm2ddl
指向OrderLine的多对多关联也使用联合外键:
<set name="undeliveredOrderLines">
<key column name="warehouseId"/>
<many-to-many class="OrderLine">
<column name="lineId"/>
<column name="orderId"/>
154
动态组件 (Dynamic components)
<column name="customerId"/>
</many-to-many>
</set>
在Order中,OrderLine的集合则是这样:
<set name="orderLines" inverse="true">
<key>
<column name="orderId"/>
<column name="customerId"/>
</key>
<one-to-many class="OrderLine"/>
</set>
The <one-to-many> element declares no columns.
假若OrderLine本身拥有一个集合,它也具有组合外键。
<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. 动态组件 (Dynamic components)
You can also map a property of type Map:
<dynamic-component name="userAttributes">
<property name="foo" column="FOO" type="string"/>
<property name="bar" column="BAR" type="integer"/>
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第 8 章 组件(Component)映射
<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. You can also access, and change,
Hibernate's configuration-time metamodel via the Configuration object.
156
第 9
Inheritance mapping
9.1. The three strategies
Hibernate支持三种基本的继承映射策略:
•每个类分层结构一张表(table per class hierarchy)
•table per subclass
•每个具体类一张表(table per concrete class)
此外,Hibernate还支持第四种稍有不同的多态映射策略:
•隐式多态(implicit polymorphism)
It is possible to use different mapping strategies for different branches of the
same inheritance hierarchy. You can then make use of implicit polymorphism to achieve
polymorphism across the whole hierarchy. However, Hibernate does not support mixing
<subclass>, <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 for an example).
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
hierarchy by adding a new mapping file. You must specify an extends attribute in the
subclass mapping, naming a previously mapped superclass. Previously this feature made
the ordering of the mapping documents important. Since Hibernate3, the ordering of
mapping files is irrelevant 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>
9.1.1. 每个类分层结构一张表(Table per class hierarchy)
Suppose we have an interface Payment with the implementors CreditCardPayment, CashPayment,
and ChequePayment. The table per hierarchy mapping would display in the following way:
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第 9 章 Inheritance mapping
<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 a limitation of this mapping strategy: columns
declared by the subclasses, such as CCTYPE, cannot have NOT NULL constraints.
9.1.2. 每个子类一张表(Table per subclass)
A table per subclass mapping looks like this:
<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"/>
158
Table per subclass: using a discriminator
...
</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
Hibernate's implementation of table per subclass does not require a discriminator
column. Other object/relational mappers use a different implementation of table per
subclass that 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 want to use a discriminator column with the table
per subclass strategy, you can combine the use of <subclass> and <join>, as follows:
<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>
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第 9 章 Inheritance mapping
可选的声明fetch="select",是用来告诉Hibernate,在查询超类时,
join)来抓取子类ChequePayment的数据。
不要使用外部连接(outer
9.1.4. 混合使用“每个类分层结构一张表”和“每个子类一张表”
You can even mix the table per hierarchy and table per subclass strategies using the
following 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>
对上述任何一种映射策略而言,指向根类Payment的 关联是使用<many-to-one>进行映射的。
<many-to-one name="payment" column="PAYMENT_ID" class="Payment"/>
9.1.5. 每个具体类一张表(Table per concrete class)
There are two ways we can map the table per concrete class strategy. First, you can
use <union-subclass>.
<class name="Payment">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="sequence"/>
</id>
160
Table per concrete class using implicit polymorphism
<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>
<union-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
...
</union-subclass>
</class>
这里涉及三张与子类相关的表。每张表为对应类的所有属性(包括从超类继承的属性)定义相应
字段。
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. The identity generator strategy
is not allowed in union subclass inheritance. The primary key seed has to be shared
across all unioned subclasses of a hierarchy.
If your superclass is abstract, map it with abstract="true". If it is not abstract,
an additional table (it 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
另一种可供选择的方法是采用隐式多态:
<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"/>
...
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第 9 章 Inheritance mapping
</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 the Payment interface is not mentioned explicitly. Also notice that
properties of Payment are mapped in each of the subclasses. If you want to avoid
duplication, consider using XML entities (for example, [ <!ENTITY allproperties SYSTEM
"allproperties.xml"> ] in the DOCTYPE declaration and &allproperties; in the mapping).
这种方法的缺陷在于,在Hibernate执行多态查询时(polymorphic queries)无法生成带 UNION的
SQL语句。
对于这种映射策略而言,通常用<any>来实现到 Payment的多态关联映射。
<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. 隐式多态和其他继承映射混合使用
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. 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"/>
162
限制
</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"/>
<property name="amount" column="CHEQUE_AMOUNT"/>
...
</joined-subclass>
</class>
Once again, Payment is not mentioned 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. 限制
There are limitations to the "implicit polymorphism" approach to the table per concreteclass mapping strategy. There are somewhat less restrictive limitations to <unionsubclass> mappings.
下面表格中列出了在Hibernte中“每个具体类一张表”的策略和隐式多态的限制。
表 9.1. 继承映射特性(Features of inheritance mappings)
继承
多态多
策略
对一
(Inheritance
strategy)
多态一
对一
每个类
分层结
<many-
<one-to-
<one-to-
<many-
s.get(Payment.class,
from
from
to-one>
one>
many>
to-many>
id)
构一张
表
多态一
对多
多态多
对多
Polymorphic
多态查
load()/
询
get()
多态
连接
(join)
Payment
Order
p
o join
外连接
(Outer
join)读
取
支持
o.payment
p
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第 9 章 Inheritance mapping
继承
多态多
策略
对一
(Inheritance
strategy)
多态一
对一
多态一
对多
多态多
对多
Polymorphic
多态查
load()/
询
table
per
<many-
<one-to-
<one-to-
<many-
s.get(Payment.class,
from
from
to-one>
one>
many>
to-many>
id)
get()
subclass
多态
连接
(join)
Payment
Order
p
o join
外连接
(Outer
join)读
取
支持
o.payment
p
每个具
体类
<many-
<one-to-
<one-to-
<many-
s.get(Payment.class,
from
from
to-one>
one>
many>
to-many>
id)
一张表
(unionsubclass)
每个具
体类一
张表(隐
式多态)
164
(for
<any>
不支持
Payment
Order
p
o join
inverse="true"
o.payment
only)
p
不支持
<manyto-any>
支持
s.createCriteria(Payment.class).add(
from
不支持
不支持
Restrictions.idEq(id) )
Payment
p
第 10
与对象共事
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 natural object-oriented view of persistence in
Java applications.
换句话说,使用Hibernate的开发者应该总是关注对象的状态(state),不必考虑SQL语句的执行。
这部分细节已经由Hibernate掌管妥当,只有开发者在进行系统性能调优的时候才需要进行了解。
10.1. Hibernate对象状态(object states)
Hibernate定义并支持下列对象状态(state):
•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 does not
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 do not 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 will now discuss the states and state transitions (and the Hibernate methods that
trigger a transition) in more detail.
10.2. 使对象持久化
Hibernate认为持久化类(persistent class)新实例化的对象是瞬时(Transient)的。 我们可通过
将瞬时(Transient)对象与session关联而把它变为持久(Persistent)的。
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第 10 章 与对象共事
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 can also
use persist() instead of save(), with the semantics defined in the EJB3 early draft.
•persist() makes a transient instance persistent. However, it does not 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 can 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 can 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.
Usually you do not bother with this detail, as you will normally use Hibernate's
transitive persistence feature to save the associated objects automatically. Then, even
NOT NULL constraint violations do not occur - Hibernate will take care of everything.
Transitive persistence is discussed later in this chapter.
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装载对象
10.3. 装载对象
The load() methods of Session provide a way of retrieving a persistent instance if you
know its identifier. load() takes a class object and loads the state into a newly
instantiated instance of that class in a 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) );
此外, 你可以把数据(state)加载到指定的对象实例上(覆盖掉该实例原来的数据)。
Cat cat = new DomesticCat();
// load pk's state into cat
sess.load( cat, new Long(pkId) );
Set kittens = cat.getKittens();
Be aware 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 is 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 can even load an object using an SQL SELECT ... FOR UPDATE, using a LockMode. See
the API documentation for more information.
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第 10 章 与对象共事
Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);
Any associated instances or contained collections will not be selected FOR UPDATE,
unless you decide to specify lock or all as a cascade style for the association.
任何时候都可以使用refresh()方法强迫装载对象和它的集合。如果你使用数据库触发器功能来处
理对象的某些属性,这个方法就很有用了。
sess.save(cat);
sess.flush(); //force the SQL INSERT
sess.refresh(cat); //re-read the state (after the trigger executes)
How much does Hibernate load from the database and how many SQL SELECTs will it use?
This depends on the fetching strategy. This is explained in 第 19.1 节 “抓取策略
(Fetching strategies)”.
10.4. 查询
If you do not 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 can 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. 执行查询
HQL和原生SQL(native SQL)查询要通过为org.hibernate.Query的实例来表达。 这个接口提供了
参数绑定、结果集处理以及运行实际查询的方法。 你总是可以通过当前Session获取一个Query对
象:
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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执行查询
.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 a
persistent state. The uniqueResult() method offers a shortcut if you know your query will
only return a single object. 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 through a Set.
10.4.1.1. 迭代式获取结果(Iterating results)
Occasionally, you might be able to achieve better performance by executing the query
using the iterate() method. This will 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;
}
}
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第 10 章 与对象共事
10.4.1.2. 返回元组(tuples)的查询
Hibernate queries sometimes return tuples of objects. Each tuple is returned as an array:
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. 标量(Scalar)结果
Queries can specify a property of a class in the select clause. They can 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. 绑定参数
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 as follows:
170
执行查询
•命名参数(named parameters)与其在查询串中出现的顺序无关
•they can occur multiple times in the same query
•它们本身是自我说明的
//named parameter (preferred)
Query q = sess.createQuery("from DomesticCat cat where cat.name = :name");
q.setString("name", "Fritz");
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. 分页
If you need to specify bounds upon your result set, that is, the maximum number of rows
you want to retrieve and/or the first row you want to retrieve, you can use methods
of the Query interface:
Query q = sess.createQuery("from DomesticCat cat");
q.setFirstResult(20);
q.setMaxResults(10);
List cats = q.list();
Hibernate 知道如何将这个有限定条件的查询转换成你的数据库的原生SQL(native SQL)。
10.4.1.6. 可滚动遍历(Scrollable iteration)
If your JDBC driver supports scrollable ResultSets, the Query interface can be used to
obtain a ScrollableResults object that allows flexible navigation of the query results.
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第 10 章 与对象共事
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);
}
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. 外置命名查询(Externalizing named queries)
You can 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>
参数绑定及执行以编程方式(programatically)完成:
Query q = sess.getNamedQuery("ByNameAndMaximumWeight");
q.setString(0, name);
q.setInt(1, minWeight);
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过滤集合
List cats = q.list();
The actual program code is independent of the query language that is used. You can
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. 过滤集合
A collection filter is a special type of query that can be applied to a persistent
collection or array. The query string can refer to this, meaning the current collection
element.
Collection blackKittens = session.createFilter(
pk.getKittens(),
"where this.color = ?")
.setParameter( Color.BLACK, Hibernate.custom(ColorUserType.class) )
.list()
);
The returned collection is considered a bag that is 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, although they can 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 large
collection:
Collection tenKittens = session.createFilter(
mother.getKittens(), "")
.setFirstResult(0).setMaxResults(10)
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第 10 章 与对象共事
.list();
10.4.3. 条件查询(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();
Criteria以及相关的样例(Example)API将会再第 15 章 条件查询(Criteria Queries)中详细讨论。
10.4.4. 使用原生SQL的查询
You can
mapping
use the
enclose
express a query in SQL, using createSQLQuery() and let Hibernate manage the
from result sets to objects. You can at any time call session.connection() and
JDBC Connection directly. If you choose to use the Hibernate API, you must
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 can contain named and positional parameters, just like Hibernate queries.
More information about native SQL queries in Hibernate can be found in 第 16 章 Native
SQL查询.
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修改持久对象
10.5. 修改持久对象
Transactional persistent instances (i.e. objects loaded, saved, created or queried
by the Session) can be manipulated by the application, and any changes to persistent
state will be persisted when the Session is flushed. This is 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. 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, as it requires in the same session
both an SQL SELECT to load an object and an SQL UPDATE to persist its updated state.
Hibernate offers an alternate approach by using detached instances.
重要
Hibernate does not offer its own API for direct execution of UPDATE or
DELETE statements. Hibernate is a state management service, you do not
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
applications. Future versions of Hibernate can, however, provide special
mass operation functions. See 第 13 章 批鉉変牉)Batch processing)
for some possible batch operation tricks.
10.6. 修改脱管(Detached)对象
很多程序需要在某个事务中获取对象,然后将对象发送到界面层去操作,最后在一个新的事务保
存所做的修改。
在高并发访问的环境中使用这种方式,通常使用附带版本信息的数据来保证这
些“长“工作单元之间的隔离。
Hibernate通过提供Session.update()或Session.merge()
型。
重新关联脱管实例的办法来支持这种模
// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catId);
Cat potentialMate = new Cat();
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第 10 章 与对象共事
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
如果具有catId持久化标识的Cat之前已经被另一Session(secondSession)装载了,
关联操作(reattach)的时候会抛出一个异常。
应用程序进行重
Use update() if you are certain that the session does not contain an already persistent
instance with the same identifier. Use 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 the
reattachment of your detached instances is the first operation that is executed.
The application should individually update() detached instances that are reachable from
the given detached instance only if it wants their state to be updated. This can
be automated using transitive persistence. See 第 10.11 节 “传播性持久化(transitive
persistence)” for more information.
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);
//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().
其他用于长时间工作单元的模型会在第
control)”中讨论。
11.3
节
“乐观并发控制(Optimistic
concurrency
10.7. 自动状态检测
Hibernate的用户曾要求一个既可自动分配新持久化标识(identifier)保存瞬时(transient)对
象,又可更新/重新关联脱管(detached)实例的通用方法。 saveOrUpdate()方法实现了这个功能。
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自动状态检测
// 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); // update existing state (cat has a non-null id)
secondSession.saveOrUpdate(mate); // save the new instance (mate has a null id)
saveOrUpdate()用途和语义可能会使新用户感到迷惑。 首先,只要你没有尝试在某个session中使
用来自另一session的实例,你就应该不需要使用update(), saveOrUpdate(),或merge()。有些程
序从来不用这些方法。
通常下面的场景会使用update()或saveOrUpdate():
•程序在第一个session中加载对象
•该对象被传递到表现层
•对象发生了一些改动
•该对象被返回到业务逻辑层
•程序调用第二个session的update()方法持久这些改动
saveOrUpdate()做下面的事:
•如果对象已经在本session中持久化了,不做任何事
•如果另一个与本session关联的对象拥有相同的持久化标识(identifier),抛出一个异常
•如果对象没有持久化标识(identifier)属性,对其调用save()
•如果对象的持久标识(identifier)表明其是一个新实例化的对象,对其调用save()
•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
•否则update() 这个对象
merge()可非常不同:
•如果session中存在相同持久化标识(identifier)的实例,用用户给出的对象的状态覆盖旧有的
持久实例
•如果session没有相应的持久实例,则尝试从数据库中加载,或创建新的持久化实例
•最后返回该持久实例
•用户给出的这个对象没有被关联到session上,它依旧是脱管的
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第 10 章 与对象共事
10.8. 删除持久对象
Session.delete() will remove an object's state from the database. Your application,
however, can still hold a reference to a deleted object. It is best to think of delete()
as making a persistent instance, transient.
sess.delete(cat);
You can delete objects in any order, 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. 在两个不同数据库间复制对象
It is sometimes 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();
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: ignores the object when there is an existing database row with
the same identifier
•ReplicationMode.OVERWRITE: overwrites any existing database row with the same identifier
•ReplicationMode.EXCEPTION: throws an exception if there is an existing database row
with the same identifier
•ReplicationMode.LATEST_VERSION: overwrites the row if its version number is earlier than
the version number of the object, or ignore the object otherwise
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Session刷出(flush)
这个功能的用途包括使录入的数据在不同数据库中一致,产品升级时升级系统配置信息,回滚
non-ACID事务中的修改等等。
(译注,non-ACID,非
ACID;ACID,Atomic,Consistent,Isolated and Durable的缩写)
10.10. Session刷出(flush)
Sometimes 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, called
flush, occurs by default at the following points:
•在某些查询执行之前
•在调用org.hibernate.Transaction.commit()的时候
•在调用Session.flush()的时候
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. 所有对实体进行更新的语句
3. 所有进行集合删除的语句
4. 所有对集合元素进行删除,更新或者插入的语句
5. 所有进行集合插入的语句
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 explicitly 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 or incorrect
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 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 第 11.3.2 节 “扩展周期的session和自动版本化”).
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);
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// 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();
刷出(flush)期间,可能会抛出异常。(例如一个DML操作违反了约束)
异常处理涉及到对
Hibernate事务性行为的理解,因此我们将在第 11 章 Transactions and Concurrency中讨论。
10.11. 传播性持久化(transitive persistence)
对每一个对象都要执行保存,删除或重关联操作让人感觉有点麻烦,尤其是在处理许多彼此关联
的对象的时候。 一个常见的例子是父子关系。考虑下面的例子:
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 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 and when the parent is deleted, the
children will be deleted, etc. This works for operations such as the removal of a
child from the collection. Since value-typed objects cannot have shared references,
Hibernate will detect this and delete the child from the database.
Now consider the same scenario with parent and child objects being entities, not valuetypes (e.g. categories and items, or parent and child cats). Entities have their own
life cycle and support shared references. 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.
每个Hibernate session的基本操作 - 包括 persist(), merge(), saveOrUpdate(), delete(), lock(),
refresh(), evict(), replicate() - 都有对应的级联风格(cascade style)。 这些级联风格(cascade
style)风格分别命名为 create, merge, save-update, delete, lock, refresh, evict, replicate。 如
果你希望一个操作被顺着关联关系级联传播,你必须在映射文件中指出这一点。例如:
<one-to-one name="person" cascade="persist"/>
级联风格(cascade style)是可组合的:
<one-to-one name="person" cascade="persist,delete,lock"/>
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You can 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.
注意有一个特殊的级联风格(cascade
style)
delete-orphan,只应用于one-to-many关联,表
明delete()操作 应该被应用于所有从关联中删除的对象。
建议:
•It does not 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.
•如果子对象的寿命限定在父亲对象的寿命之内,可通过指定cascade="all,delete-orphan"将其变
为自动生命周期管理的对象(life cycle object)。
•其他情况,你可根本不需要级联(cascade)。但是如果你认为你会经常在某个事务中同时用到父
对象与子对象,并且你希望少打点儿字,可以考虑使用cascade="persist,merge,save-update"。
可以使用cascade="all"将一个关联关系(无论是对值对象的关联,或者对一个集合的关联)标记
为父/子关系的关联。
这样对父对象进行save/update/delete操作就会导致子对象也进行save/
update/delete操作。
Furthermore, 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-tomany> association mapped with cascade="delete-orphan". The precise semantics of cascading
operations for a parent/child relationship are as follows:
•如果父对象被persist(),那么所有子对象也会被persist()
•如果父对象被merge(),那么所有子对象也会被merge()
•如果父对象被save(),update()或 saveOrUpdate(),那么所有子对象则会被saveOrUpdate()
•如果某个持久的父对象引用了瞬时(transient)或者脱管(detached)的子对象,那么子对象将会
被saveOrUpdate()
•如果父对象被删除,那么所有子对象也会被delete()
•除非被标记为cascade="delete-orphan"(删除“孤儿”模式,此时不被任何一个父对象引用的子
对象会被删除), 否则子对象失掉父对象对其的引用时,什么事也不会发生。 如果有特殊需
要,应用程序可通过显式调用delete()删除子对象。
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-update and delete-orphan are
transitive for all associated entities reachable during flush of the Session.
10.12. 使用元数据
Hibernate requires a rich meta-level model of all entity and value types. This model can
be 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
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should be copied (eg. mutable value types) and which objects that should not (e.g.
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 can be obtained from the
SessionFactory.
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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Transactions and Concurrency
The most important point about Hibernate and concurrency control is that it is easy to
understand. Hibernate directly uses JDBC connections and JTA resources without adding
any additional locking behavior. It is recommended that 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. Through Session, which
is also a transaction-scoped cache, Hibernate provides repeatable reads for lookup by
identifier and entity queries and not reporting queries that return scalar values.
In addition to versioning for automatic optimistic concurrency control, Hibernate also
offers, using the SELECT FOR UPDATE syntax, a (minor) API for pessimistic locking of
rows. Optimistic concurrency control and this API are discussed later in this chapter.
The discussion of concurrency control in Hibernate begins with the granularity of
Configuration, SessionFactory, and Session, as well as database transactions and long
conversations.
11.1. Session和事务范围(transaction scope)
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 and then
discarded for: a single request, a conversation or a single unit of work. A Session
will not obtain a JDBC Connection, or a Datasource, unless it is needed. It will not
consume any resources until used.
In order to reduce lock contention in the database, a database transaction has to be
as short as possible. Long database transactions will prevent your application from
scaling to a highly concurrent load. It is not recommended that you 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? When should you open and
close a Session and how do you demarcate the database transaction boundaries? These
questions are addressed in the following sections.
11.1.1. 操作单元(Unit of work)
First, let's define a unit of work. A unit of work is a design pattern described by
Martin Fowler as “ [maintaining] a list of objects affected by a business transaction
and coordinates the writing out of changes and the resolution of concurrency problems.
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”[PoEAA] In other words, its a series of operations we wish to carry out against
the database together. Basically, it is a transaction, though fulfilling a unit of
work will often span multiple physical database transactions (see 第 11.1.2 节 “长
对话”). So really we are talking about a more abstract notion of a transaction. The
term "business transaction" is also sometimes used in lieu of unit of work.
Do not use the session-per-operation antipattern: do not open and close a Session
for every simple database call in a single thread. 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. This also means that auto-commit after every
single SQL statement is useless in an application as this mode is intended for ad-hoc SQL
console work. Hibernate disables, or expects the application server to disable, autocommit mode immediately. Database transactions are never optional. All communication
with a database has to occur inside a transaction. 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 more maintainable and extensible.
The most common pattern in a multi-user client/server application is session-perrequest. 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. On completion of the work, and once the
response for the client has been prepared, the session is flushed and closed. 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. Start a transaction when a server request
has to be processed, and end the transaction before the response is sent to the client.
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 use programmatic transaction demarcation, for ease of
use and code portability use the Hibernate Transaction API shown later in this chapter.
Your application code can access a "current session" to process the request by calling
sessionFactory.getCurrentSession(). You will always get a Session scoped to the current
database transaction. This has to be configured for either resource-local or JTA
environments, see 第 2.5 节 “Contextual sessions”.
You can 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 achieved by implementing your own interceptor. However, this
will be difficult if you rely on EJBs with container-managed transactions. A transaction
will be completed when an EJB method returns, before rendering of any view can start.
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See the Hibernate website and forum for tips and examples relating to this Open Session
in View pattern.
11.1.2. 长对话
The session-per-request pattern is not the only way of designing units of work. Many
business processes require a whole series of interactions with the user that are
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 their modifications to be made
persistent. The user also expects that they were the only person editing this
information and that no conflicting modification has occurred.
From the point of view of the user, we call this unit of work a long-running conversation
or application transaction. There are many ways to 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 an anti-pattern, since lock contention
would not allow the application to scale with the number of concurrent users.
You 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 (for example, in a wizard-style dialog
spanning several request/response cycles). This is easier to implement than it might
sound, especially if you utilize some of Hibernate's features:
•Automatic Versioning: Hibernate can perform automatic optimistic concurrency control
for you. It can automatically detect if a concurrent modification occurred during
user think time. Check for this at the end of the conversation.
•Detached Objects: if you decide to use the session-per-request pattern, all loaded
instances will be in the 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 can 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-
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conversation and makes even reattachment unnecessary. Automatic versioning is used
to isolate concurrent modifications and the Session will not be allowed to be flushed
automatically, but explicitly.
Both session-per-request-with-detached-objects and session-per-conversation have
advantages and disadvantages. These disadvantages are discussed later in this chapter
in the context of optimistic concurrency control.
11.1.3. 关注对象标识(Considering object identity)
An application can concurrently access the same persistent state in two different
Sessions. However, an instance of a persistent class is never shared between two Session
instances. It is for this reason that there are two different notions of identity:
数据库标识
foo.getId().equals( bar.getId() )
JVM 标识
foo==bar
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. 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 an optimistic approach and
automatic versioning at flush/commit time.
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 means that it does not need expensive locking or other means of synchronization.
The application does not need to synchronize on any business object, as long as it
maintains a single thread per Session. Within a Session the application can safely use
== to compare objects.
However, an application that uses == outside of a Session might produce 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). JVM identity, however, is by definition not
guaranteed for instances in a detached state. The developer has to override the equals()
and hashCode() methods in persistent classes and implement their own notion of object
equality. There is one caveat: never use the database identifier to implement equality.
Use a business key that is 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 do not have
to be as stable as database primary keys; you only have to guarantee stability as
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常见问题
long as the objects are in the same Set. See the Hibernate website for a more thorough
discussion of this issue. Please note that this is not a Hibernate issue, but simply
how Java object identity and equality has to be implemented.
11.1.4. 常见问题
Do not use the anti-patterns session-per-user-session or session-per-application (there
are, however, rare exceptions to this rule). Some of the following issues might also
arise within the recommended patterns, so ensure that you understand the implications
before making a design decision:
•A Session is not thread-safe. Things that work concurrently, like HTTP requests,
session beans, or Swing workers, will cause race conditions if a Session instance
is shared. If you keep your Hibernate Session in your HttpSession (this is discussed
later in the chapter), you should consider synchronizing access to your Http session.
Otherwise, a user that clicks reload fast enough can 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 (this is discussed in more detail later in the
chapter). If your Session is bound to the application, you have to stop the application.
Rolling back the database transaction does not put your business objects back into the
state they were at the start of the transaction. This means that the database state
and the business objects will be out of sync. Usually this is not a problem, because
exceptions are not recoverable and you will have to start over after rollback anyway.
•The Session caches every object that is in a persistent state (watched and checked
for dirty state by Hibernate). If you keep it open for a long time or simply load too
much data, it will grow endlessly until you get an OutOfMemoryException. One solution
is to call clear() and evict() to manage the Session cache, but you should consider
a Stored Procedure if you need mass data operations. Some solutions are shown in
第 13 章 批鉉変牉)Batch processing). Keeping a Session open for the duration
of a user session also means a higher probability of stale data.
11.2. 数据库事务声明
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
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is usually responsible for its own database connection pool. The application developer
has to manually set transaction boundaries (begin, commit, or rollback database
transactions) themselves. 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 nonmanaged resource-local environments, and systems that can rely on JTA but use BMT
instead of CMT. In both cases 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.
Ending a Session usually involves four distinct phases:
•同步session(flush,刷出到磁盘)
•提交事务
•关闭session
•处理异常
We discussed Flushing the session earlier, so we will now have a closer look
at transaction demarcation and exception handling in both managed and non-managed
environments.
11.2.1. 非托管环境
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) {
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使用JTA
if (tx != null) tx.rollback();
throw e; // or display error message
}
finally {
sess.close();
}
You do not have to flush() the Session explicitly: the call to commit() automatically
triggers the synchronization depending on 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.
As outlined earlier, a much more flexible solution is Hibernate's built-in "current
session" context management:
// Non-managed environment idiom with getCurrentSession()
try {
factory.getCurrentSession().beginTransaction();
// do some work
...
factory.getCurrentSession().getTransaction().commit();
}
catch (RuntimeException e) {
factory.getCurrentSession().getTransaction().rollback();
throw e; // or display error message
}
You will not 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 by accessing a SessionFactory. Exception
handling is discussed later in this chapter.
You should select org.hibernate.transaction.JDBCTransactionFactory, which is the default,
and for the second example select "thread" as your hibernate.current_session_context_class.
11.2.2. 使用JTA
If your persistence layer runs in an application server (for example, behind EJB session
beans), every datasource connection obtained by Hibernate will automatically be part
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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. 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();
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, 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) {
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tx.rollback();
throw e; // or display error message
}
With CMT, transaction demarcation is completed in session bean deployment descriptors,
not programmatically. The code is reduced to:
// CMT idiom
Session sess = factory.getCurrentSession();
// do some work
...
In a CMT/EJB, even rollback happens automatically. An unhandled RuntimeException thrown
by a session bean method tells the container to set the global transaction to rollback.
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.
When
configuring
Hibernate's
transaction
factory,
choose
org.hibernate.transaction.JTATransactionFactory
if you use JTA directly (BMT),
and
org.hibernate.transaction.CMTTransactionFactory
in
a
CMT
session
bean.
Remember to also set hibernate.transaction.manager_lookup_class. Ensure that your
hibernate.current_session_context_class is either unset (backwards compatibility), or is
set to "jta".
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 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. Most
applications can easily avoid using scroll() or iterate() from the JTA or CMT code.)
11.2.3. 异常处理
If the Session throws an exception, including any SQLException, 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 was not in older versions of Hibernate.
In our opinion, we should not force the application developer to catch an unrecoverable
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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 either
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 that are not
a HibernateException. These are 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.
•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. 事务超时
An 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
...
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乐观并发控制(Optimistic concurrency control)
sess.getTransaction().commit()
}
catch (RuntimeException e) {
sess.getTransaction().rollback();
throw e; // or display error message
}
finally {
sess.close();
}
setTimeout() cannot be called in a CMT bean, where transaction timeouts must be defined
declaratively.
11.3. 乐观并发控制(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 prevent lost updates. Hibernate
provides three possible approaches to writing application code that uses optimistic
concurrency. The use cases we discuss 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. 应用程序级别的版本检查(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. The application is forced 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();
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第 11 章 Transactions and Concu...
version 属性使用 <version>来映射,如果对象 是脏数据,在同步的时候,Hibernate会自动增加
版本号。
If you are operating in a low-data-concurrency environment, and do not require version
checking, you can use this approach and skip the version check. In this case, last
commit wins is the default strategy for long conversations. Be aware 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.
Manual version checking is only feasible in 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. 扩展周期的session和自动版本化
A single Session instance and its persistent instances that are used for the whole
conversation are known as session-per-conversation. Hibernate checks instance versions
at flush time, throwing an exception if concurrent modification is detected. It is 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 does not version check or reattach 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(); // Only for last transaction in conversation
t.commit();
// Also return JDBC connection
session.close();
// Only for last transaction in conversation
The foo object 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 are
not updating, you can call Session.lock() with LockMode.READ on any objects that might
have been updated by another transaction. You do not 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
194
脱管对象(deatched object)和自动版本化
made in this conversation. Only this last database transaction will include the flush()
operation, and then close() the session to end the conversation.
This pattern is problematic if the Session is too big to be stored during user think
time (for example, an HttpSession should be kept as small as possible). As the Session
is also the first-level cache and contains all loaded objects, we can probably use
this strategy only for a few request/response cycles. Use a Session only for a single
conversation as it will soon have stale data.
Note
Earlier versions of Hibernate required explicit disconnection and
reconnection of a Session. These methods are deprecated, as beginning and
ending a transaction has the same effect.
Keep the disconnected Session close to the persistence layer. Use an EJB stateful session
bean to hold the Session in a three-tier environment. Do not 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. 脱管对象(deatched object)和自动版本化
这种方式下,与持久化存储的每次交互都发生在一个新的Session中。 然而,同一持久化对象实例
可以在多次与数据库的交互中重用。应用程序操纵脱管对象实例 的状态,这个脱管对象实例最初
是在另一个Session 中载入的,然后 调用 Session.update(),Session.saveOrUpdate(), 或者
Session.merge() 来重新关联该对象实例。
// 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 can also call lock() instead of update(), and use LockMode.READ (performing a version
check and bypassing all caches) if you are sure that the object has not been modified.
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第 11 章 Transactions and Concu...
11.3.4. 定制自动版本化行为
You can 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 cannot be modified. Or, other applications
might access the same database and will not know how to handle version numbers or
even timestamps. In both cases, versioning cannot rely on a particular column in a
table. To force a version check with a comparison of the state of all fields in a
row but without a version or timestamp property mapping, turn on optimistic-lock="all"
in the <class> mapping. This conceptually only works if Hibernate can compare the old
and the new state (i.e., if you use a single long Session and not session-per-requestwith-detached-objects).
Concurrent modification can be permitted in instances where the changes that have
been made do not 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 a 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 may 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 occur
before updating the row.
11.4. Pessimistic locking
It is not intended that users spend much time worrying about locking strategies. It is
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 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; it never lock objects
in memory.
The LockMode class defines the different lock levels that can be acquired by Hibernate.
A lock is obtained by the following mechanisms:
•当Hibernate更新或者插入一行记录的时候,锁定级别自动设置为LockMode.WRITE。
•LockMode.UPGRADE can be acquired upon explicit user request using SELECT ... FOR UPDATE
on databases which support that syntax.
•LockMode.UPGRADE_NOWAIT can be acquired upon explicit user request using a SELECT ...
FOR UPDATE NOWAIT under Oracle.
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Connection release modes
•LockMode.READ is acquired automatically when Hibernate reads data under Repeatable
Read or Serializable isolation level. It can be re-acquired by explicit user request.
•LockMode.NONE 代表无需锁定。在Transaction结束时, 所有的对象都切换到该模式上来。与
session相关联的对象通过调用update() 或者saveOrUpdate()脱离该模式。
"显式的用户指定"可以通过以下几种方式之一来表示:
•调用 Session.load()的时候指定锁定模式(LockMode)。
•调用Session.lock()。
•调用Query.setLockMode()。
如果在UPGRADE或者UPGRADE_NOWAIT锁定模式下调
用Session.load(),并且要读取的对象尚未被
session载入过,那么对象 通过SELECT ... FOR UPDATE这样的SQL语句被载入。如果为一个对象调
用 load()方法时,该对象已经在另一个较少限制的锁定模式下被载入了,那 么Hibernate就对该
对象调用lock() 方法。
Session.lock() 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.
If the requested lock mode is not supported by the database, Hibernate uses an
appropriate alternate mode instead of throwing an exception. This ensures that
applications are portable.
11.5. Connection release modes
One of the legacies of Hibernate 2.x JDBC connection management meant that a
Session would obtain a connection when it was first required and then maintain
that connection until the session was closed. Hibernate 3.x introduced the notion
of connection release modes that would instruct a session how to handle its JDBC
connections. 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 the legacy behavior described above. The Hibernate session obtains
a connection when it first needs to perform some JDBC access and maintains that
connection until the session is closed.
•AFTER_TRANSACTION: releases connections after a org.hibernate.Transaction has been
completed.
•AFTER_STATEMENT (also referred to as aggressive release): releases connections after
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 are as follows:
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第 11 章 Transactions and Concu...
•auto (the default): this choice delegates to the release mode returned
by the org.hibernate.transaction.TransactionFactory.getDefaultReleaseMode() method. For
JTATransactionFactory, this returns ConnectionReleaseMode.AFTER_STATEMENT; for
JDBCTransactionFactory, this returns ConnectionReleaseMode.AFTER_TRANSACTION. Do not
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: uses ConnectionReleaseMode.ON_CLOSE. This setting is left for backwards
compatibility, but its use is discouraged.
•after_transaction: uses 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 autocommit mode, connections will be released as if the release mode were AFTER_STATEMENT.
•after_statement: uses 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 you make
a call into ConnectionProvider.getConnection() or in auto-commit environments where it
does not matter if we re-establish the same connection.
198
第 12
拦截器与事件(Interceptors and
events)
It is useful for the application to react to certain events that occur inside Hibernate.
This allows for the implementation of generic functionality and the extension of
Hibernate functionality.
12.1. 拦截器(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 can either implement Interceptor directly or 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
}
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第 12 章 拦截器与事件(Interceptors an...
public boolean onFlushDirty(Object entity,
Serializable id,
Object[] currentState,
Object[] previousState,
String[] propertyNames,
Type[] types) {
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;
}
}
200
事件系统(Event system)
}
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;
}
}
There are two kinds of inteceptors: Session-scoped and SessionFactory-scoped.
当使用某个重载的SessionFactory.openSession()使用Interceptor作为参数调用打开一个session
的时候,就指定了Session范围内的拦截器。
Session session = sf.openSession( new AuditInterceptor() );
A SessionFactory-scoped interceptor is registered with the Configuration object prior
to building the SessionFactory. Unless a session is opened explicitly specifying the
interceptor to use, the supplied interceptor will be applied to all sessions opened
from that SessionFactory. SessionFactory-scoped interceptors must be thread safe. Ensure
that you do not store session-specific states, since multiple sessions will use this
interceptor potentially concurrently.
new Configuration().setInterceptor( new AuditInterceptor() );
12.2. 事件系统(Event system)
If you have to react to particular events in your persistence layer, you can also use
the Hibernate3 event architecture. The event system can be used in addition, or as
a replacement, for interceptors.
All 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
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第 12 章 拦截器与事件(Interceptors an...
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 implementation 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 singletons. This means they are shared between
requests, and should not save any state as instance variables.
A custom listener implements the appropriate interface for the event it wants to process
and/or extend one of the convenience base classes (or 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 is 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");
}
}
}
你还需要修改一处配置,来告诉Hibernate,除了默认的监听器,还要附加选定的监听器。
<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 can register it programmatically:
Configuration cfg = new Configuration();
202
Hibernate的声明式安全机制
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 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? A listener
implementation could implement multiple event listener interfaces. Having the type
additionally defined during registration makes it easier to turn custom listeners on
or off during configuration.
12.3. Hibernate的声明式安全机制
Usually, declarative security in Hibernate applications is managed in a session facade
layer. Hibernate3 allows certain actions to be permissioned via JACC, and authorized via
JAAS. This is an optional functionality that is built on top of the event architecture.
首先,你必须要配置适当的事件监听器(event listener),来激活使用JAAS管理授权的功能。
<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 shorthand for <event type="..."><listener
class="..."/></event> when there is exactly one listener for a particular event type.
Next, while 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="*"/>
这些角色的名字就是你的JACC provider所定义的角色的名字。
203
204
第 13
批鉉変牉)Batch processing)
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
is because Hibernate caches all the newly inserted Customer instances in the sessionlevel cache. In this chapter we will show you how to avoid this problem.
If you are undertaking batch processing you will need to enable the use of JDBC batching.
This is absolutely essential if you want to achieve optimal performance. Set the JDBC
batch size to a reasonable number (10-50, for example):
hibernate.jdbc.batch_size 20
Hibernate disables insert batching at the JDBC level transparently if you use an identity
identifier generator.
You can also do this kind of work in a process where interaction with the secondlevel cache is completely disabled:
hibernate.cache.use_second_level_cache false
佉扯)迉三扯绉对å¿
é¡»ç‰‰ï¼‰å‰ ä¸ºæ‰‰ä»¬å‰¯ä»¥æ‰¾å¼‰è®¾ç½®CacheMode扥å
³é‰ä¸‰äº‰çº§ç¼‰å­‰ç‰‰äº¤äº‰ã‰‰
205
第 13 章 批鉉変牉)Batch p...
13.1. 批鉉扉å
¥ï¼‰Batch inserts)
When making new objects persistent flush() and then clear() the session regularly in
order 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);
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. 批鉉扴扰)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();
206
StatelessSession (æ‰ ç‰¶æ‰‰session)扥剣
session.clear();
}
}
tx.commit();
session.close();
13.3. StatelessSession (æ‰ ç‰¶æ‰‰session)扥剣
Alternatively, Hibernate provides a command-oriented API that can 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 firstlevel 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 never cascade to associated instances. Collections are ignored by a
stateless session. Operations performed via a stateless session bypass Hibernate's event
model and interceptors. Due to the lack of a first-level cache, Stateless sessions are
vulnerable to data aliasing effects. A stateless session is a lower-level abstraction
that is 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();
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. They result in the immediate
execution of a SQL INSERT, UPDATE or DELETE respectively. They have different semantics
to the save(), saveOrUpdate() and delete() operations defined by the Session interface.
207
第 13 章 批鉉変牉)Batch p...
13.4. DML(扰扮扉佉è¯è¨‰)é£‰æ ¼ç‰‰æ‰‰ä½‰(DML-style
operations)
As already discussed, automatic and transparent object/relational mapping is concerned
with the management of the object state. The object state is available in memory. This
means that manipulating data directly in the database (using the SQL Data Manipulation
Language (DML) the statements: INSERT, UPDATE, DELETE) will not affect in-memory state.
However, Hibernate provides methods for bulk SQL-style DML statement execution that is
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:
•å‰¨FROM孉剥)from-clause)ä¸ï¼‰FROMå
³é‰®å­‰æ‰¯å‰¯é‰‰ç‰‰
•There can only be a single entity named in the from-clause. It can, however, 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. Subqueries can be used in the where-clause, where the subqueries themselves may contain
joins.
•æ‰´ä¸ªWHERE孉剥扯副鉉牉㉉
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();
In keeping with the EJB3 specification, HQL UPDATE statements, by default, do not
effect the version or the timestamp property values for the affected entities. However,
you can force Hibernate to reset the version or timestamp property values through the
208
DML(扰扮扉佉语訉)é£‰æ ¼ç‰‰æ‰‰ä½‰(DML-style operations)
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();
Custom version types, org.hibernate.usertype.UserVersionType,
conjunction with a update versioned statement.
are
not
allowed
in
执衉三个HQL DELETEï¼‰å‰‰æ ·ä½¿ç‰¨ Query.executeUpdate() 批泉:
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();
session.close();
The int value returned by the Query.executeUpdate() method indicates the number of entities
effected by the operation. 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 inheritance hierarchy.
INSERTè¯å‰¥ç‰‰ä¼ªç
‰æ‰¯:
INSERT
INTO
EntityName
properties_list
select_statement.
覉注扉牉扯:
•å‰ªæ‰¯æ‰‰INSERT INTO ... SELECT ...形弉,三扯扉INSERT INTO ... VALUES ...形弉.
209
第 13 章 批鉉変牉)Batch p...
The properties_list is analogous to the column specification in the SQL INSERT statement.
For entities involved in mapped inheritance, 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. This
might, however, 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 latter 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. For the purposes of this discussion, in-database generators are considered
to be org.hibernate.id.SequenceGenerator (and its subclasses) and any implementers
of org.hibernate.id.PostInsertIdentifierGenerator. The most notable exception here is
org.hibernate.id.TableHiLoGenerator, 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.
The following is an example of an 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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第 14
HQL: Hibernate查询语言
Hibernate uses a powerful query language (HQL) that is similar in appearance to SQL.
Compared with SQL, however, HQL is fully object-oriented and understands notions like
inheritance, polymorphism and association.
14.1. 大小写敏感性问题
With the exception of names of Java classes and properties, queries are case-insensitive.
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 this convention is unsuitable for queries embedded in Java code.
14.2. from子句
Hibernate中最简单的查询语句的形式如下:
from eg.Cat
This returns all instances of the class eg.Cat. You do not usually need to qualify the
class name, since auto-import is the default. For example:
from Cat
In order to refer to the Cat in other parts of the query, you will need to assign
an alias. For example:
from Cat as cat
This query assigns the alias cat to Cat instances, so you can use that alias later in
the query. The as keyword is optional. You could also write:
from Cat cat
Multiple classes can appear, resulting in a cartesian product or "cross" join.
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第 14 章 HQL: Hibernate查询语言
from Formula, Parameter
from Formula as form, Parameter as param
It is good practice to name query aliases using an initial lowercase as this is
consistent with Java naming standards for local variables (e.g. domesticCat).
14.3. 关联(Association)与连接(Join)
You can also assign aliases to associated entities or to elements of a collection of
values using a join. For example:
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(内连接)
•left outer join(左外连接)
•right outer join(右外连接)
•full join (全连接,并不常用)
语句inner join, left outer join 以及 right outer join 可以简写。
from Cat as cat
join cat.mate as mate
left join cat.kittens as kitten
通过HQL的with关键字,你可以提供额外的join条件。
from Cat as cat
212
关联(Association)与连接(Join)
left join cat.kittens as kitten
with kitten.bodyWeight > 10.0
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 第 19.1 节 “抓取策略(Fetching
strategies)” for more information.
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). The associated objects
are also not returned directly in the query results. Instead, they may be accessed via
the parent object. The only reason you might need an alias is if you 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
The fetch construct cannot be used in queries called using iterate() (though scroll()
can be used). Fetch should 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 would expect.
Fetch should also not be used together with impromptu 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 can produce unexpected
results for bag mappings, so user discretion is advised when formulating 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 in the first query immediately
using fetch all properties.
from Document fetch all properties order by name
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第 14 章 HQL: Hibernate查询语言
from Document doc fetch all properties where lower(doc.name) like '%cats%'
14.4. join 语法的形式
HQL支持两种关联join的形式:implicit(隐式) 与explicit(显式)。
The queries shown in the previous section all use the explicit form, that is, where the
join keyword is explicitly used in the from clause. This is the recommended form.
implicit(隐式)形式不使用join关键字。关联使用"点号"来进行“引用”。implicit join可以在
任何HQL子句中出现.implicit join在最终的SQL语句中以inner join的方式出现。
from Cat as cat where cat.mate.name like '%s%'
14.5. Referring to identifier property
There are 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 the entity does not define a non-identifier property named id.
•If the entity defines a named identifier property, you can 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 reference the identifier property.
重要
Please note that, starting in version 3.2.2, this has changed
significantly. In previous versions, id always referred to the identifier
property regardless of its actual name. A ramification of that decision
was that non-identifier properties named id could never be referenced in
Hibernate queries.
14.6. select子句
The select clause picks which objects and properties to return in the query result
set. Consider the following:
select mate
from Cat as cat
214
select子句
inner join cat.mate as mate
The query will select mates of other Cats. You can express this query more compactly as:
select cat.mate from Cat cat
Queries can return properties of any value type including properties of component type:
select cat.name from DomesticCat cat
where cat.name like 'fri%'
select cust.name.firstName from Customer as cust
Queries can 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 - assuming that the class Family has an appropriate constructor - 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
You can assign aliases to selected expressions using as:
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第 14 章 HQL: Hibernate查询语言
select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
from Cat cat
这种做法在与子句select new map一起使用时最有用:
select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n )
from Cat cat
该查询返回了一个Map的对象,内容是别名与被选择的值组成的名-值映射。
14.7. 聚集函数
HQL queries can 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:
•avg(...), sum(...), min(...), max(...)
•count(*)
•count(...), count(distinct ...), count(all...)
You can 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 can be used and have the same semantics as in SQL.
select distinct cat.name from Cat cat
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多态查询
select count(distinct cat.name), count(cat) from Cat cat
14.8. 多态查询
一个如下的查询语句:
from Cat as cat
returns instances not only of Cat, but also of subclasses like DomesticCat. Hibernate
queries can 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
接口Named 可能被各种各样的持久化类声明:
from Named n, Named m where n.name = m.name
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 cannot call
these queries using Query.scroll().
14.9. where子句
The where clause allows you to refine the list of instances returned. If no alias
exists, you can refer to properties by name:
from Cat where name='Fritz'
如果指派了别名,需要使用完整的属性名:
from Cat as cat where cat.name='Fritz'
This returns instances of Cat named 'Fritz'.
The following query:
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第 14 章 HQL: Hibernate查询语言
select foo
from Foo foo, Bar bar
where foo.startDate = bar.date
returns all instances of Foo with 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 the following:
from Cat cat where cat.mate.name is not null
This query translates to an SQL query with a table (inner) join. For example:
from Foo foo
where foo.bar.baz.customer.address.city is not null
would result in a query that would require four table joins in SQL.
The = operator can 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 can be used to reference the unique identifier of
an object. See 第 14.5 节 “Referring 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 and does not require a table join.
Properties of composite identifiers can also be used. Consider the following example
where Person has composite identifiers consisting of country and medicareNumber:
218
表达式
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 does not require a table join.
See 第 14.5 节 “Referring to identifier property” for more information regarding
referencing identifier properties)
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 can also use components or composite user types, or properties of said component
types. See 第 14.17 节 “translator-credits” for more information.
An "any" type has the special properties id and class that allows you 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
The log.item.class and payment.class would refer to the values of completely different
database columns in the above query.
14.10. 表达式
Expressions used in the where clause include the following:
•mathematical operators: +, -, *, /
•binary comparison operators: =, >=, <=, <>, !=, like
•逻辑运算符and, or, not
•Parentheses ( ) that indicates grouping
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第 14 章 HQL: Hibernate查询语言
•in, not in, between, is null, is not null, is empty, is not empty, member of and not member of
•"简单的" case, case ... when ... then ... else ... end,和 "搜索" case, case when ...
then ... else ... end
•字符串连接符...||... or concat(...,...)
•current_date(), current_time(), and current_timestamp()
•second(...), minute(...), hour(...), day(...), month(...), and year(...)
•EJB-QL 3.0定义的任何函数或操作:substring(), trim(), lower(), upper(), length(), locate(),
abs(), sqrt(), bit_length(), mod()
•coalesce() 和 nullif()
•str() 把数字或者时间值转换为可读的字符串
•cast(... as ...), 其第二个参数是某Hibernate类型的名字,以及extract(... from ...),只要
ANSI cast() 和 extract() 被底层数据库支持
•HQL index() 函数,作用于join的有序集合的别名。
•HQL functions that take collection-valued path expressions: size(), minelement(),
maxelement(), minindex(), maxindex(), along with the special elements() and indices
functions that can be quantified using some, all, exists, any, in.
•Any database-supported SQL scalar function like sign(), trunc(), rtrim(), and sin()
•JDBC风格的参数传入 ?
•named parameters :name, :start_date, and :x1
•SQL 直接常量 'foo', 69, 6.66E+2, '1970-01-01 10:00:01.0'
•Java public static final 类型的常量 eg.Color.TABBY
in and between can be used as follows:
from DomesticCat cat where cat.name between 'A' and 'B'
from DomesticCat cat where cat.name in ( 'Foo', 'Bar', 'Baz' )
The negated forms can be written as follows:
from DomesticCat cat where cat.name not between 'A' and 'B'
from DomesticCat cat where cat.name not in ( 'Foo', 'Bar', 'Baz' )
Similarly, is null and is not null can be used to test for null values.
Booleans can be easily used in expressions by declaring HQL query substitutions in
Hibernate configuration:
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表达式
<property name="hibernate.query.substitutions">true 1, false 0</property>
系统将该HQL转换为SQL语句时,该设置表明将用字符 1 和 0 来 取代关键字true 和 false:
from Cat cat where cat.alive = true
You can 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 can refer to the minimum and maximum indices using minindex
and maxindex functions. Similarly, you can refer to the minimum and maximum elements of
a collection of basic type using the minelement and maxelement functions. For example:
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
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第 14 章 HQL: Hibernate查询语言
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 - can only be used in the where clause in Hibernate3.
Elements of indexed collections (arrays, lists, and maps) can 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
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 [] can even be an arithmetic expression:
select item from Item item, Order order
where order.items[ size(order.items) - 1 ] = item
222
表达式
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 can be used:
from DomesticCat cat where upper(cat.name) like 'FRI%'
Consider 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)
提示: 会像如下的语句
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
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
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第 14 章 HQL: Hibernate查询语言
)
14.11. order by子句
The list returned by a query can be ordered by any property of a returned class or
components:
from DomesticCat cat
order by cat.name asc, cat.weight desc, cat.birthdate
可选的asc或desc关键字指明了按照升序或降序进行排序.
14.12. group by子句
A query that returns aggregate values can 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
having子句在这里也允许使用.
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
they are supported by the underlying database (i.e., not in MySQL).
select cat
from Cat cat
join cat.kittens kitten
224
子查询
group by cat.id, cat.name, cat.other, cat.properties
having avg(kitten.weight) > 100
order by count(kitten) asc, sum(kitten.weight) desc
Neither the group by clause nor the order by clause can contain arithmetic expressions.
Hibernate also does not currently expand a grouped entity, so you cannot write group
by cat if all properties of cat are non-aggregated. You have to list all non-aggregated
properties explicitly.
14.13. 子查询
对于支持子查询的数据库,Hibernate支持在查询中使用子查询。一个子查询必须被圆括号包围起
来(经常是SQL聚集函数的圆括号)。 甚至相互关联的子查询(引用到外部查询中的别名的子查
询)也是允许的。
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)
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第 14 章 HQL: Hibernate查询语言
from Cat as cat
Note that HQL subqueries can occur only in the select or where clauses.
Note that subqueries can also utilize row value constructor syntax. See 第 14.18 节 “Row
value constructor syntax” for more information.
14.14. HQL示例
Hibernate queries can be quite powerful and complex. In fact, the power of the query
language is one of Hibernate's main strengths. The following example queries are similar
to queries that have been used on recent projects. Please note that most queries you
will write will be much simpler than the following examples.
The following query returns the order id, number of items, the given minimum total
value and the total value of the order for all unpaid orders for a particular customer.
The results are ordered 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
这简直是一个怪物!实际上,在现实生活中,我并不热衷于子查询,所以我的查询语句看起来更
像这个:
select order.id, sum(price.amount), count(item)
226
HQL示例
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
下面一个查询计算每一种状态下的支付的数目,除去所有处于AWAITING_APPROVAL状态的支付,因
为在该状态下 当前的用户作出了状态的最新改变。该查询被转换成含有两个内连接以及一个相关
联的子选择的SQL查询,该查询使用了表 PAYMENT, PAYMENT_STATUS 以及 PAYMENT_STATUS_CHANGE。
select count(payment), status.name
from Payment as payment
join payment.currentStatus as status
join payment.statusChanges as statusChange
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 the statusChanges collection was mapped 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
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第 14 章 HQL: Hibernate查询语言
order by status.sortOrder
下面一个查询使用了MS SQL Server的 isNull()函数用以返回当前用户所属组织的组织帐号及组织
未支付的账。 它被转换成一个对表ACCOUNT, PAYMENT, PAYMENT_STATUS, ACCOUNT_TYPE, ORGANIZATION
以及 ORG_USER进行的三个内连接, 一个外连接和一个子选择的SQL查询。
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
对于一些数据库,我们需要弃用(相关的)子选择。
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
14.15. 批量的UPDATE和DELETE
HQL now supports update, delete and insert ... select ... statements. See 第 13.4 节
“DML(扰扮扉佉è¯è¨‰)é£‰æ ¼ç‰‰æ‰‰ä½‰(DML-style operations)” for more information.
14.16. 小技巧 & 小窍门
You can count the number of query results without returning them:
( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue()
若想根据一个集合的大小来进行排序,可以使用如下的语句:
select usr.id, usr.name
from User as usr
left join usr.messages as msg
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小技巧 & 小窍门
group by usr.id, usr.name
order by count(msg)
如果你的数据库支持子选择,你可以在你的查询的where子句中为选择的大小(selection size)
指定一个条件:
from User usr where size(usr.messages) >= 1
If your database does not 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 cannot 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
JavaBean的属性可以被绑定到一个命名查询(named query)的参数上:
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();
通过将接口Query与一个过滤器(filter)一起使用,集合(Collections)是可以分页的:
Query q = s.createFilter( collection, "" ); // the trivial filter
q.setMaxResults(PAGE_SIZE);
q.setFirstResult(PAGE_SIZE * pageNumber);
List page = q.list();
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第 14 章 HQL: Hibernate查询语言
Collection elements can 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" );
不用通过初始化,你就可以知道一个集合(Collection)的大小:
( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue();
14.17. translator-credits
Components can be used similarly to the simple value types that are used in HQL queries.
They can appear in the select clause as follows:
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.
230
Row value constructor syntax
14.18. Row value constructor syntax
HQL supports the use of ANSI SQL row value constructor syntax, sometimes referred to
AS tuple syntax, even though the underlying database 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 is valid syntax although it is a little verbose. You can make this more concise
by using 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
Using row value constructor syntax can also be beneficial when using subqueries that
need 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.
231
232
第 15
条件查询(Criteria Queries)
具有一个直观的、可扩展的条件查询API是Hibernate的特色。
15.1. 创建一个Criteria 实例
org.hibernate.Criteria接口表示特定持久类的一个查询。Session是 Criteria实例的工厂。
Criteria crit = sess.createCriteria(Cat.class);
crit.setMaxResults(50);
List cats = crit.list();
15.2. 限制结果集内容
一个单独的查询条件是org.hibernate.criterion.Criterion
接口的一个实
例。org.hibernate.criterion.Restrictions类 定义了获得某些内置Criterion类型的工厂方法。
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "Fritz%") )
.add( Restrictions.between("weight", minWeight, maxWeight) )
.list();
Restrictions can 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) ) )
233
第 15 章 条件查询(Criteria Queries)
))
.list();
There are a range of built-in criterion types (Restrictions subclasses). One of the most
useful allows you to specify SQL directly.
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.sqlRestriction("lower({alias}.name)
Hibernate.STRING) )
.list();
like
lower(?)",
"Fritz%",
{alias}占位符应当被替换为被查询实体的列别名。
You can also obtain a criterion 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. 结果集排序
You can 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();
234
关联
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. 关联
By navigating associations using createCriteria() you can specify constraints upon
related entities:
List cats = sess.createCriteria(Cat.class)
.add( Restrictions.like("name", "F%") )
.createCriteria("kittens")
.add( Restrictions.like("name", "F%") )
.list();
The second createCriteria() returns a new instance of Criteria that refers to the elements
of the kittens collection.
There is also an alternate form that 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()并不创建一个新的 Criteria实例。)
The kittens collections held by the Cat instances returned by the previous two queries
are not pre-filtered by the criteria. If you want 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();
235
第 15 章 条件查询(Criteria Queries)
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. 动态关联抓取
You can 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();
这个查询可以通过外连接抓取mate和kittens。
strategies)”可以获得更多信息。
查看第
19.1
节
“抓取策略(Fetching
15.6. 查询示例
org.hibernate.criterion.Example类允许你通过一个给定实例 构建一个条件查询。
Cat cat = new Cat();
cat.setSex('F');
cat.setColor(Color.BLACK);
List results = session.createCriteria(Cat.class)
.add( Example.create(cat) )
.list();
版本属性、标识符和关联被忽略。默认情况下值为null的属性将被排除。
你可以自行调整Example使之更实用。
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)
236
投影(Projections)、聚合(aggregation)和分组(grouping)
.add(example)
.list();
你甚至可以使用examples在关联对象上放置条件。
List results = session.createCriteria(Cat.class)
.add( Example.create(cat) )
.createCriteria("mate")
.add( Example.create( cat.getMate() ) )
.list();
15.7. 投影(Projections)、聚合(aggregation)和分组
(grouping)
The class org.hibernate.criterion.Projections is a factory for Projection instances. You
can 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();
在一个条件查询中没有必要显式的使用 "group by" 。某些投影类型就是被定义为 分组投影,他
们也出现在SQL的group by子句中。
An alias can be assigned to a projection so that the projected value can 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" ) )
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第 15 章 条件查询(Criteria Queries)
.addOrder( Order.asc("colr") )
.list();
List results = session.createCriteria(Cat.class)
.setProjection( Projections.groupProperty("color").as("colr") )
.addOrder( Order.asc("colr") )
.list();
alias()和as()方法简便的将一个投影实例包装到另外一个
别名的Projection实例中。简而言之,
当你添加一个投影到一个投影列表中时 你可以为它指定一个别名:
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();
你也可以使用Property.forName()来表示投影:
List results = session.createCriteria(Cat.class)
.setProjection( Property.forName("name") )
.add( Property.forName("color").eq(Color.BLACK) )
.list();
238
离线(detached)查询和子查询
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. 离线(detached)查询和子查询
The DetachedCriteria class allows you to create a query outside the scope of a session
and then execute it using an 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 can also be used to express a subquery. Criterion instances involving
subqueries can 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) )
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第 15 章 条件查询(Criteria Queries)
.list();
Correlated subqueries are also 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();
15.9. 根据自然标识查询(Queries by natural identifier)
For most queries, including criteria queries, the query cache is not efficient because
query cache invalidation occurs too frequently. However, there is a special kind of
query where you 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, 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>
This functionality is not intended for use with entities with mutable natural keys.
Once you have enabled the Hibernate query cache, the Restrictions.naturalId() allows you
to make use of the more efficient cache algorithm.
session.createCriteria(User.class)
.add( Restrictions.naturalId()
240
根据自然标识查询(Queries by natural identifier)
.set("name", "gavin")
.set("org", "hb")
).setCacheable(true)
.uniqueResult();
241
242
第 16
Native SQL查询
You can 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. 使用SQLQuery
Execution of native SQL queries is controlled via the SQLQuery interface, which is
obtained by calling Session.createSQLQuery(). The following sections describe how to use
this API for querying.
16.1.1. 标量查询(Scalar queries)
最基本的SQL查询就是获得一个标量(数值)的列表。
sess.createSQLQuery("SELECT * FROM CATS").list();
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();
These will 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:
•SQL查询字符串
•要返回的字段和类型
This will 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
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第 16 章 Native SQL查询
will be returned, even though the query is using * and could return more than the
three listed columns.
对全部或者部分的标量值不设置类型信息也是可以的。
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
determine 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. 实体查询(Entity queries)
上面的查询都是返回标量值的,也就是从resultset中返回的“裸”数据。下面展示如何通
过addEntity()让原生查询返回实体对象。
sess.createSQLQuery("SELECT * FROM CATS").addEntity(Cat.class);
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").addEntity(Cat.class);
This query specified:
•SQL查询字符串
•要返回的实体
假设Cat被映射为拥有ID,NAME和BIRTHDATE三个字段的类,以上的两个查询都返回一个List,每个
元素都是一个Cat实体。
假若实体在映射时有一个many-to-one的关联指向另外一个实体,在查询时必须也返回那个实体,
否则会导致发生一个"column not found"的数据库错误。这些附加的字段可以使用*标注来自动返
回,但我们希望还是明确指明,看下面这个具有指向Dog的many-to-one的例子:
sess.createSQLQuery("SELECT
CATS").addEntity(Cat.class);
这样cat.getDog()就能正常运作。
244
ID,
NAME,
BIRTHDATE,
DOG_ID
FROM
处理关联和集合类(Handling associations and collections)
16.1.3. 处理关联和集合类(Handling associations and
collections)
通过提前抓取将Dog连接获得,而避免初始化proxy带来的额外开销也是可能的。这是通
过addJoin()方法进行的,这个方法可以让你将关联或集合连接进来。
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 you added an 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 you are reaching the limits of what is possible with native queries,
without starting to enhance the sql queries to make them usable in Hibernate. Problems
can arise when returning multiple entities of the same type or when the default alias/
column names are not enough.
16.1.4. 返回多个实体(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 that join
multiple tables, since the same column names can appear in more than one table.
下面的查询中需要使用字段别名注射(这个例子本身会失败):
sess.createSQLQuery("SELECT c.*, m.* FROM CATS c, CATS m WHERE c.MOTHER_ID =
c.ID")
.addEntity("cat", Cat.class)
.addEntity("mother", Cat.class)
The query was intended to return two Cat instances per row: a cat and its mother.
The query will, however, fail because there is a conflict of names; the instances are
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第 16 章 Native SQL查询
mapped to the same column names. Also, 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").
下面的形式可以解决字段名重复:
sess.createSQLQuery("SELECT {cat.*}, {mother.*}
c.MOTHER_ID = c.ID")
.addEntity("cat", Cat.class)
.addEntity("mother", Cat.class)
FROM CATS c, CATS m WHERE
This query specified:
•SQL查询语句,其中包含占位附来让Hibernate注射字段别名
•查询返回的实体
The {cat.*} and {mother.*} notation used above is a shorthand for "all properties".
Alternatively, you can list the columns explicitly, but even in this case Hibernate
injects 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, you
retrieve Cats and their mothers from a different table (cat_log) to the one declared
in the mapping metadata. You can even use the property aliases in the where clause.
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. 别名和属性引用(Alias and property references)
In most cases the above alias injection is needed. For queries relating to more complex
mappings, like composite properties, inheritance discriminators, collections etc., you
can use specific aliases that allow Hibernate to inject the proper aliases.
The following table shows the different ways you can use the alias injection. Please
note that the alias names in the result are simply examples; each alias will have a
unique and probably different name when used.
246
返回非受管实体(Returning non-managed entities)
表 16.1. 别名注射(alias injection names)
描述
语法
示例
简单属性
{[aliasname].
A_NAME as {item.name}
[propertyname]
复合属性
{[aliasname].
CURRENCY
as
{item.amount.currency},
[componentname].
{item.amount.value}
VALUE
as
[propertyname]}
实体辨别器
{[aliasname].class} DISC as {item.class}
(Discriminator of
an entity)
实体的所有属性
{[aliasname].*}
{item.*}
集合键(collection {[aliasname].key}
key)
ORGID as {coll.key}
集合id
{[aliasname].id}
EMPID as {coll.id}
集合元素
{[aliasname].element}XID as {coll.element}
property of
element in
the {[aliasname].element.NAME as {coll.element.name}
the [propertyname]}
collection
集合元素的所有属
性
{[aliasname].element.*}
{coll.element.*}
集合的所有属性
{[aliasname].*}
{coll.*}
16.1.5. 返回非受管实体(Returning non-managed entities)
It is possible to apply a ResultTransformer to native SQL queries, allowing it to
return non-managed entities.
sess.createSQLQuery("SELECT NAME, BIRTHDATE FROM CATS")
.setResultTransformer(Transformers.aliasToBean(CatDTO.class))
This query specified:
•SQL查询字符串
•结果转换器(result transformer)
上面的查询将会返回CatDTO的列表,它将被实例化并且将NAME和BIRTHDAY的值注射入对应的属性或
者字段。
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第 16 章 Native SQL查询
16.1.6. 处理继承(Handling inheritance)
Native SQL queries which query for entities that are mapped as part of an inheritance
must include all properties for the baseclass and all its subclasses.
16.1.7. 参数(Parameters)
Native SQL queries support positional as well as named parameters:
Query query = sess.createSQLQuery("SELECT
like ?").addEntity(Cat.class);
List pusList = query.setString(0, "Pus%").list();
*
FROM
query
=
sess.createSQLQuery("SELECT
*
like :name").addEntity(Cat.class);
List pusList = query.setString("name", "Pus%").list();
FROM
CATS
CATS
WHERE
WHERE
NAME
NAME
16.2. 命名SQL查询
Named SQL queries can be defined in the mapping document and called in exactly the same
way as a named HQL query. In this case, you 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> element is use to join associations and the <load-collection> element
is used to define queries which initialize collections,
<sql-query name="personsWith">
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命名SQL查询
<return alias="person" class="eg.Person"/>
<return-join alias="address" property="person.mailingAddress"/>
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>
一个命名查询可能会返回一个标量值.你必须使用<return-scalar>元素来指定字段的别名和
Hibernate类型
<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 information in a <resultset> element which
will allow you 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},
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第 16 章 Native SQL查询
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"
)
.setResultSetMapping("catAndKitten")
.list();
16.2.1. 使用return-property来明确地指定字段/别名
You can explicitly tell Hibernate what column aliases to use with <return-property>,
instead of using the {}-syntax to let Hibernate inject its own aliases.For example:
<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>
<return-property> also works with multiple columns. This solves a limitation with the
{}-syntax which cannot allow fine grained control of multi-column properties.
<sql-query name="organizationCurrentEmployments">
<return alias="emp" class="Employment">
<return-property name="salary">
<return-column name="VALUE"/>
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使用存储过程来查询
<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>
In this example <return-property> was used in combination with the {}-syntax for
injection. This allows users to choose how they want to refer column and properties.
如果你映射一个识别器(discriminator),你必须使用<return-discriminator> 来指定识别器字段
16.2.2. 使用存储过程来查询
Hibernate3 provides 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;
在Hibernate里要要使用这个查询,你需要通过命名查询来映射它.
<sql-query name="selectAllEmployees_SP" callable="true">
<return alias="emp" class="Employment">
<return-property name="employee" column="EMPLOYEE"/>
<return-property name="employer" column="EMPLOYER"/>
251
第 16 章 Native SQL查询
<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>
Stored procedures currently only return scalars and entities. <return-join> and <loadcollection> are not supported.
16.2.2.1. 使用存储过程的规则和限制
You cannot use stored procedures with Hibernate unless you follow some procedure/
function 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.
Stored procedure queries cannot be paged with setFirstResult()/setMaxResults().
The recommended call form is standard SQL92: { ? = call functionName(<parameters>) } or
{ ? = call procedureName(<parameters>}. Native call syntax is not supported.
对于Oracle有如下规则:
•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
further information.
对于Sybase或者MS SQL server有如下规则:
•The procedure must return a result set. Note that since these servers can 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.
•如果你能够在存储过程里设定SET NOCOUNT ON,这可能会效率更高,但这不是必需的。
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定制SQL用来create,update和delete
16.3. 定制SQL用来create,update和delete
Hibernate3能够使用定制的SQL语句来执行create,update和delete操作。在Hibernate中,持久化
的类和集合已经 包含了一套配置期产生的语句(insertsql, deletesql, updatesql等等),这些
映射标记 <sql-insert>, <sql-delete>, and <sql-update>重载了 这些语句。
<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 can use any dialect you like.
This will reduce the portability of your mapping if you use database specific SQL.
如果设定callable,则能够支持存储过程了。
<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 is vital, as they must be in the same sequence
as Hibernate expects them.
You
can
view
the
expected order by enabling debug logging
org.hibernate.persister.entity level. With this level enabled, Hibernate will
the static SQL that is used to create, update, delete etc. entities. To
expected sequence, do not include your custom SQL in the mapping files, as
override the Hibernate generated static SQL.
for the
print out
view the
this will
The stored procedures are in most cases required to return the number of rows inserted,
updated and deleted, as Hibernate has some runtime checks for the success of the
253
第 16 章 Native SQL查询
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. 定制装载SQL
You can 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}
FROM PERSON
WHERE ID=?
FOR UPDATE
</sql-query>
This is just a named query declaration, as discussed earlier. You can 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>
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定制装载SQL
这也可以用于存储过程
You can 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 can also 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>
255
256
第 17
过滤数据
Hibernate3 provides an innovative new approach to handling data with "visibility"
rules. A Hibernate filter is a global, named, parameterized filter that can be enabled
or disabled for a particular Hibernate session.
17.1. Hibernate 过滤器(filters)
Hibernate3 has the ability to pre-define filter criteria and attach those filters at
both a class level and a collection level. A filter criteria allows you to define a
restriction clause similar to the existing "where" attribute available on the class and
various collection elements. These filter conditions, however, can be parameterized.
The application can then decide at runtime whether certain filters should be enabled
and what their parameter values should be. Filters can be used like database views,
but they are parameterized inside the application.
要使用过滤器,必须首先在相应的映射节点中定义。而定义一个过滤器,要用到位于<hibernatemapping/> 节点之内的<filter-def/>节点:
<filter-def name="myFilter">
<filter-param name="myFilterParam" type="string"/>
</filter-def>
This filter can then 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, 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
257
第 17 章 过滤数据
a given session. Filters must be enabled through use of the Session.enableFilter() method,
which returns an instance of the Filter interface. If you used the simple filter defined
above, it would look like this:
session.enableFilter("myFilter").setParameter("myFilterParam", "some-value");
Methods on the org.hibernate.Filter interface do allow the method-chaining common to
much of Hibernate.
The following is 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>
In order to ensure that you are provided with currently effective records, enable the
filter on the session prior to retrieving employee data:
258
Hibernate 过滤器(filters)
Session session = ...;
session.enableFilter("effectiveDate").setParameter("asOfDate", new Date());
List results = session.createQuery("from Employee as e where e.salary > :targetSalary")
.setLong("targetSalary", new Long(1000000))
.list();
Even though a salary constraint was mentioned explicitly on the results in the above HQL,
because of the enabled filter, the query will return only currently active employees
who have a salary greater than one million dollars.
If you want to use filters with outer joining, either through HQL or load fetching,
be careful of the direction of the condition expression. It is safest to set this
up for left outer joining. 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. This can be problematic when the conditions are the same
each time. Using <filter-def/> allows you to definine 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 be used whenever the filter is attached to something without
specifying a condition. This means you can give a specific condition as part of the
attachment of the filter that overrides the default condition in that particular case.
259
260
第 18
XML映射
XML Mapping is an experimental feature in Hibernate 3.0 and is currently under active
development.
18.1. 用XML数据进行工作
Hibernate allows you to work with persistent XML data in much the same way you work with
persistent POJOs. A parsed XML tree can be thought of as another way of representing
the relational data at the object level, instead of POJOs.
Hibernate支持采用dom4j作为操作XML树的API。你可以写一些查询从数据库中检索出 dom4j树,
随后你对这颗树做的任何修改都将自动同步回数据库。你甚至可以用dom4j解析
一篇XML文档,
然后使用Hibernate的任一基本操作将它写入数据库: persist(), saveOrUpdate(), merge(),
delete(), replicate() (合并操作merge()目前还不支持)。
这一特性可以应用在很多场合,包括数据导入导出,通过JMS或SOAP具体化实体数据以及
基于
XSLT的报表。
A single mapping can 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 can be used
to map just the XML.
18.1.1. 指定同时映射XML和类
这是一个同时映射POJO和XML的例子:
<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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第 18 章 XML映射
</class>
18.1.2. 只定义XML映射
这是一个不映射POJO的例子:
<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 or java Maps. The property names are purely logical constructs that
can be referred to in HQL queries.
18.2. XML映射元数据
A range of Hibernate mapping elements accept the node attribute. This lets 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:
•"element-name": map to the named XML element
•"@attribute-name": map to the named XML attribute
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XML映射元数据
•".": map to the parent element
•"element-name/@attribute-name": map to the named attribute of the named element
For collections and single valued associations, there is an additional embed-xml
attribute. If embed-xml="true", the default, the XML tree for the 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 not appear at all.
Do not 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>
...
263
第 18 章 XML映射
</class>
In this case, the collection of account ids is embedded, 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">
<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>
如果你把一对多映射<one-to-many>的embed-xml属性置为真(embed-xml="true"), 则数据看上去就
像这样:
<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>
...
264
操作XML数据
</customer>
18.3. 操作XML数据
You can also re-read and update XML documents in the application. You can 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();
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();
265
第 18 章 XML映射
session.close();
When implementing XML-based data import/export, it is useful to combine this feature
with Hibernate's replicate() operation.
266
第 19
提升性能
19.1. 抓取策略(Fetching strategies)
Hibernate uses a fetching strategy to retrieve associated objects if the application
needs to navigate the association. Fetch strategies can be declared in the O/R mapping
metadata, or over-ridden by a particular HQL or Criteria query.
Hibernate3 定义了如下几种抓取策略:
•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 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 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 or foreign keys.
Hibernate会区分下列各种情况:
•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. It is suitable for 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 is less lazy; the association
is fetched even when only the identifier is accessed. It is also more transparent,
267
第 19 章 提升性能
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. It is important that you do not confuse them. We use fetch to tune performance.
We can use lazy to define a contract for what data is always available in any detached
instance of a particular class.
19.1.1. 操作延迟加载的关联
By default, Hibernate3 uses lazy select fetching for collections and lazy proxy fetching
for single-valued associations. These defaults make sense for most associations in the
majority of applications.
If you set hibernate.default_batch_fetch_size, Hibernate will use the batch fetch
optimization for lazy fetching. This optimization can also be enabled at a more granular
level.
Please be aware that 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. This can be fixed by moving the code that reads
from the collection to just before the transaction is committed.
Alternatively, you can use a non-lazy collection or association, by specifying
lazy="false" for the association mapping. However, it is intended that lazy
initialization be used for almost all collections and associations. If you define
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调整抓取策略(Tuning fetch strategies)
too many non-lazy associations in your object model, Hibernate will fetch the entire
database into memory in every transaction.
On the other hand, you can use join fetching, which is non-lazy by nature, instead
of select fetching in a particular transaction. We will now explain 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. 调整抓取策略(Tuning fetch strategies)
查询抓取(默认的)在N+1查询的情况下是极其脆弱的,因此我们可能会要求在映射文档中定义使
用连接抓取:
<set name="permissions"
fetch="join">
<key column="userId"/>
<one-to-many class="Permission"/>
</set
<many-to-one name="mother" class="Cat" fetch="join"/>
在映射文档中定义的抓取策略将会对以下列表条目产生影响:
•通过get()或load()方法取得数据。
•只有在关联之间进行导航时,才会隐式的取得数据。
•条件查询
•使用了subselect抓取的HQL查询
Irrespective of the fetching strategy you use, the defined non-lazy graph is guaranteed
to be loaded into memory. This might, however, result in several immediate selects
being used to execute a particular HQL query.
Usually, the mapping document is not used 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 want to change the fetching strategy used by get() or load(), you can use a
Criteria query. For example:
User user = (User) session.createCriteria(User.class)
.setFetchMode("permissions", FetchMode.JOIN)
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.add( Restrictions.idEq(userId) )
.uniqueResult();
This is Hibernate's equivalent of what some ORM solutions call a "fetch plan".
A completely different approach to problems with N+1 selects is to use the secondlevel cache.
19.1.3. 单端关联代理(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 which is accessed via the CGLIB library.
At startup, Hibernate3 generates proxies by default 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. The
proxied class must implement a default constructor with at least package visibility.
This constructor is recommended for all persistent classes.
There are potential problems to note when extending this approach to polymorphic
classes.For example:
<class name="Cat" proxy="Cat">
......
<subclass name="DomesticCat">
.....
</subclass>
</class>
首先,Cat实例永远不可以被强制转换为DomesticCat, 即使它本身就是DomesticCat实例。
Cat cat = (Cat) session.load(Cat.class, id); // instantiate a proxy (does not hit the db)
if ( cat.isDomesticCat() ) {
// hit the db to initialize the proxy
DomesticCat dc = (DomesticCat) cat;
// Error!
....
}
Secondly, it is possible to break proxy ==:
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单端关联代理(Single-ended association proxies)
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
虽然如此,但实际情况并没有看上去那么糟糕。虽然我们现在有两个不同的引用,分别指向这两
个不同的代理对象, 但实际上,其底层应该是同一个实例对象:
cat.setWeight(11.0); // hit the db to initialize the proxy
System.out.println( dc.getWeight() ); // 11.0
Third, you cannot 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 (e.g. 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.
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
where CatImpl implements the interface Cat and DomesticCatImpl implements the interface
DomesticCat. For example:
<class name="CatImpl" proxy="Cat">
......
<subclass name="DomesticCatImpl" proxy="DomesticCat">
.....
</subclass>
</class>
Then proxies for instances of Cat and DomesticCat can be returned by load() or iterate().
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();
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Note
list() does not usually return proxies.
这里,对象之间的关系也将被延迟载入。这就意味着,你应该将属性声明为Cat,而不是CatImpl。
Certain operations do not require proxy initialization:
•equals(): if the persistent class does not override equals()
•hashCode(): if the persistent class does not override hashCode()
•标志符的getter方法。
Hibernate将会识别出那些重载了equals()、或hashCode()方法的持久化类。
By choosing lazy="no-proxy" instead of the default lazy="proxy", you can avoid problems
associated with typecasting. However, buildtime bytecode instrumentation is required,
and all operations will result in immediate proxy initialization.
19.1.4. 实例化集合和代理(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, i.e., when the entity owning
the collection or having the reference to the proxy is in the detached state.
Sometimes a proxy or collection needs to be initialized before closing the Session. You
can force initialization by calling cat.getSex() or cat.getKittens().size(), for example.
However, this can be confusing to readers of the code and it 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 required 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 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
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使用批量抓取(Using batch fetching)
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 the web tier needs before 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 can also attach a previously loaded object to a new Session with merge() or lock()
before accessing uninitialized collections or other proxies. Hibernate does not,
and certainly should not, do this automatically since it would introduce impromptu
transaction semantics.
Sometimes you do not want to initialize a large collection, but still need some
information about it, like its size, for example, or a subset of the data.
你可以使用集合过滤器得到其集合的大小,而不必实例化整个集合:
( (Integer) s.createFilter( collection, "select count(*)" ).list().get(0) ).intValue()
这里的createFilter()方法也可以被用来有效的抓取集合的部分内容,而无需实例化整个集合:
s.createFilter( lazyCollection, "").setFirstResult(0).setMaxResults(10).list();
19.1.5. 使用批量抓取(Using batch fetching)
Using batch fetching, Hibernate can load several uninitialized proxies if one proxy is
accessed. Batch fetching is an optimization of the lazy select fetching strategy. There
are two ways you can configure batch fetching: on the class level and the collection
level.
Batch fetching for classes/entities is easier to understand. Consider the following
example: at runtime you have 25 Cat instances loaded in a Session, and 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:
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<class name="Person" batch-size="10">...</class>
Hibernate will now execute only three queries: the pattern is 10, 10, 5.
You can 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>
如果整个的batch-size是3(笔误?),那么Hibernate将会分四次执行SELECT查询,
按照
3、3、3、1的大小分别载入数据。这里的每次载入的数据量还具体依赖于当前Session中未实例化
集合的个数。
Batch fetching of collections is particularly useful if you have a nested tree of items,
i.e. the typical bill-of-materials pattern. However, a nested set or a materialized
path might be a better option for read-mostly trees.
19.1.6. 使用子查询抓取(Using subselect fetching)
If one lazy collection or single-valued proxy has to be fetched, Hibernate will load
all of them, re-running the original query in a subselect. This works in the same way
as batch-fetching but without the piecemeal loading.
19.1.7. 使用延迟属性抓取(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; optimizing row reads is much more important than optimization of column reads.
However, only loading some properties of a class could be useful in extreme cases.
For example, when legacy tables have hundreds of columns and the data model cannot
be improved.
可以在映射文件中对特定的属性设置lazy,定义该属性为延迟载入。
<class name="Document">
<id name="id">
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二级缓存(The Second Level Cache)
<generator class="native"/>
</id>
<property name="name" not-null="true" length="50"/>
<property name="summary" not-null="true" length="200" lazy="true"/>
<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 ignore lazy property settings and return to
immediate fetching.
你可以在Ant的Task中,进行如下定义,对持久类代码加入“二进制指令。”
<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 way of avoiding 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 can force the usual eager fetching of properties using fetch all properties in HQL.
19.2. 二级缓存(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 can even plug in a clustered cache. Be aware that
caches are not aware of changes made to the persistent store by another application.
They can, however, 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 is bundled with a number of built-in integrations
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with the open-source cache providers that are listed below. You can also implement
your own and plug it in as outlined above. Note that versions prior to 3.2 use EhCache
as the default cache provider.
表 19.1. 缓存策略提供商(Cache Providers)
Cache
Provider class
Type
Hashtable
(not
org.hibernate.cache.HashtableCacheProvider
memory
Cluster
Safe
Query
Cache
Supported
yes
intended
for
production
use)
EHCache
org.hibernate.cache.EhCacheProvider
memory,
disk
yes
OSCache
org.hibernate.cache.OSCacheProvider
memory,
disk
yes
SwarmCache
org.hibernate.cache.SwarmCacheProvider
clustered
(ip
yes
(clustered
multicast)
invalidation)
clustered
(ip
yes
yes (clock
(replication)sync req.)
JBoss
Cache 1.x
org.hibernate.cache.TreeCacheProvider
multicast),
transactional
JBoss
Cache 2
org.hibernate.cache.jbc2.JBossCacheRegionFactory
clustered
(ip
yes
yes (clock
(replication sync req.)
multicast), or
transactionalinvalidation)
19.2.1. 缓存映射(Cache mappings)
类或者集合映射的“<cache>元素”可以有下列形式:
<cache
usage="transactional|read-write|nonstrict-read-write|read-only"
region="RegionName"
include="all|non-lazy"
/>
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策略:只读缓存(Strategy: read only)
usage(必须)说明了缓存的策略: transactional、 read-write、 nonstrict-read-write或 readonly。
region (optional: defaults to the class or collection role name): specifies the
name of the second level cache region
include (optional: defaults to all) non-lazy: specifies that properties of the entity
mapped with lazy="true" cannot be cached when attribute-level lazy fetching is
enabled
Alternatively,
you
can
specify
<class-cache>
and
<collection-cache>
elements
in
hibernate.cfg.xml.
这里的usage 属性指明了缓存并发策略(cache concurrency strategy)。
19.2.2. 策略:只读缓存(Strategy: read only)
If your application needs to read, but not modify, instances of a persistent class,
a read-only cache can be used. This is the simplest and optimal performing strategy.
It is even safe for use in a cluster.
<class name="eg.Immutable" mutable="false">
<cache usage="read-only"/>
....
</class>
19.2.3. 策略:读/写缓存(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 and 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 want to use this
strategy in a cluster, you should ensure that the underlying cache implementation
supports locking. The built-in cache providers do not support locking.
<class name="eg.Cat" .... >
<cache usage="read-write"/>
....
<set name="kittens" ... >
<cache usage="read-write"/>
....
</set>
</class>
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19.2.4. 策略:非严格读/写缓存(Strategy: nonstrict read/
write)
If the application only occasionally needs to update data (i.e. if it is extremely
unlikely that two transactions would try to update the 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. 策略:事务缓存(transactional)
The transactional cache strategy provides support for fully transactional cache providers
such as JBoss TreeCache. Such a cache can only be used in a JTA environment and you
must specify hibernate.transaction.manager_lookup_class.
19.2.6. Cache-provider/concurrency-strategy compatibility
重要
None of the cache providers support all of the cache concurrency
strategies.
The following table shows which providers are compatible with which concurrency
strategies.
表 19.2. 各种缓存提供商对缓存并发策略的支持情况(Cache
Strategy Support)
Concurrency
Cache
read-only
nonstrict-readwrite
read-write
transactional
Hashtable (not
intended for
yes
yes
yes
EHCache
yes
yes
yes
OSCache
yes
yes
yes
SwarmCache
yes
yes
JBoss Cache 1.x
yes
yes
JBoss Cache 2
yes
yes
production use)
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管理缓存(Managing the caches)
19.3. 管理缓存(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 can
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);
}
Session还提供了一个contains()方法,用来判断某个实例是否处于当前session的缓存中。
To evict all objects from the session cache, call Session.clear()
对于二级缓存来说,在SessionFactory中定义了许多方法,
或者整个集合。
清除缓存中实例、整个类、集合实例
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: will read items from and write items to the second-level cache
•CacheMode.GET: will read items from the second-level cache. Do not write to the secondlevel cache except when updating data
•CacheMode.PUT: will write items to the second-level cache. Do not read from the secondlevel cache
•CacheMode.REFRESH: will write items to the second-level cache. Do not 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
如若需要查看二级缓存或查询缓存区域的内容,你可以使用统计(Statistics) API。
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Map cacheEntries = sessionFactory.getStatistics()
.getSecondLevelCacheStatistics(regionName)
.getEntries();
You will need to enable statistics and, optionally, force Hibernate to keep the cache
entries in a more readable format:
hibernate.generate_statistics true
hibernate.cache.use_structured_entries true
19.4. 查询缓存(The Query Cache)
Query result sets can also be cached. This is only useful for queries that are run
frequently with the same parameters. You will first need to enable the query cache:
hibernate.cache.use_query_cache true
This setting creates 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. 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 can
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)
.setMaxResults(15)
.setCacheable(true)
.setCacheRegion("frontpages")
.list();
如果查询需要强行刷新其查询缓存区域,那么你应该调
用Query.setCacheMode(CacheMode.REFRESH)方法。
这对在其他进程中修改底层数据(例如,不
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理解集合性能(Understanding Collection performance)
通过Hibernate修改数据),或对那些需要选择性更新特定查询结果集的情况特别有用。
对SessionFactory.evictQueries()的更为有效的替代方案,同样可以清除查询缓存区域。
这是
19.5. 理解集合性能(Understanding Collection
performance)
In the previous sections we have covered collections and their applications. In this
section we explore some more issues in relation to collections at runtime.
19.5.1. 分类(Taxonomy)
Hibernate定义了三种基本类型的集合:
•值数据集合
•one-to-many associations
•many-to-many associations
这个分类是区分了不同的表和外键关系类型,但是它没有告诉我们关系模型的所有内容。 要完全
理解他们的关系结构和性能特点,我们必须同时考虑“用于Hibernate更新或删除集合行数据的主
键的结构”。 因此得到了如下的分类:
•有序集合类
•集合(sets)
•包(bags)
All indexed collections (maps, lists, and arrays) have a primary key consisting of the
<key> and <index> columns. In this case, collection updates are extremely efficient. The
primary key can be efficiently indexed and a particular row can be efficiently located
when Hibernate tries to update or delete it.
Sets have a primary key consisting of <key> and element columns. This can be less
efficient for some types of collection element, particularly composite elements or
large text or binary fields, as the database may not be able to index a complex primary
key as efficiently. However, for one-to-many or many-to-many associations, particularly
in the case of synthetic identifiers, it is likely to be just as efficient. If you
want SchemaExport to actually create the primary key of a <set>, you must declare all
columns as not-null="true".
<idbag> mappings define a surrogate key, so they are efficient to update. In fact,
they are the best case.
Bags are the worst case since they permit duplicate element values and, as they have
no index column, no primary key can 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 can be inefficient.
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For a one-to-many association, the "primary key" may not be the physical primary key
of the database table. Even in this case, the above classification is still useful. It
reflects how Hibernate "locates" individual rows of the collection.
19.5.2. Lists, maps 和sets用于更新效率最高
From the discussion above, it should be clear that indexed collections and 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 does not 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, you can conclude that lists, maps and idbags
are the most performant (non-inverse) collection types, with sets not far behind. You
can expect sets 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
to-many associations with
by the many-to-one end of
performance simply do not
Hibernate domain models, most collections are in fact oneinverse="true". For these associations, the update is handled
the association, and so considerations of collection update
apply.
19.5.3. Bag和list是反向集合类中效率最高的
There is a particular case, however, 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, unlike a set,
Collection.add() or Collection.addAll() must always return true for a bag or List. 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. 一次性删除(One shot delete)
Deleting collection elements one by one can sometimes be extremely inefficient.
Hibernate 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.
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Suppose you added 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.
但是,假设我们删除了18个数据,只剩下2个,然后新增3个。则有两种处理方式:
•逐一的删除这18个数据,再新增三个;
•remove the whole collection in one SQL DELETE and insert all five current elements
one by one
Hibernate cannot know that the second option is probably quicker. It would probably be
undesirable for Hibernate to be that intuitive as such behavior might confuse database
triggers, etc.
Fortunately, you can force this behavior (i.e. the second strategy) at any time
by discarding (i.e. dereferencing) the original collection and returning a newly
instantiated collection with all the current elements.
One-shot-delete does not apply to collections mapped inverse="true".
19.6. 监测性能(Monitoring performance)
没有监测和性能参数而进行优化是毫无意义的。Hibernate为其内部操作提供了一系列的示意图,
因此可以从 每个SessionFactory抓取其统计数据。
19.6.1. 监测SessionFactory
你可以有两种方式访问SessionFactory的数据记录,第一种就是自己直接调用
sessionFactory.getStatistics()方法读取、显示统计数据。
Hibernate can also use JMX to publish metrics if you enable the StatisticsService MBean.
You can 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
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// 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
You can activate and deactivate the monitoring for a SessionFactory:
•在配置期间,将hibernate.generate_statistics设置为true或false;
•在运行期间,则可以可以通过sf.getStatistics().setStatisticsEnabled(true)
或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. 数据记录(Metrics)
Hibernate provides a number of metrics, from basic information to more specialized
information that is only relevant in certain scenarios. All available counters are
described in the Statistics interface API, in three categories:
•使用Session的普通数据记录,例如打开的Session的个数、取得的JDBC的连接数等;
•Metrics related to the entities, collections, queries, and caches as a whole (aka
global metrics).
•和具体实体、集合、查询、缓存相关的详细数据记录
For example, you can check the cache hit, miss, and put ratio of entities, collections
and queries, and the average time a query needs. Be aware that the number of milliseconds
is subject to approximation in Java. Hibernate is tied to the JVM precision and on some
platforms this might 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 is a simple example:
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数据记录(Metrics)
Statistics stats = HibernateUtil.sessionFactory.getStatistics();
double queryCacheHitCount = stats.getQueryCacheHitCount();
double queryCacheMissCount = stats.getQueryCacheMissCount();
double queryCacheHitRatio =
queryCacheHitCount / (queryCacheHitCount + queryCacheMissCount);
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" );
You can work on all entities, collections, queries and region caches, by retrieving
the list of names of entities, collections, queries and region caches using
the following methods: getQueries(), getEntityNames(), getCollectionRoleNames(), and
getSecondLevelCacheRegionNames().
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工具箱指南
Roundtrip engineering with Hibernate is possible using a set of Eclipse plugins,
commandline tools, and Ant tasks.
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 that supports autocompletion and syntax highlighting. It also supports semantic auto-completion for
class names and property/field names, making it 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 are also provided with 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
to reverse engineer an existing database schema into POJO source files and Hibernate
mapping files. The reverse engineering wizard supports customizable templates.
•
Please refer to the Hibernate Tools package documentation for more information.
However, the Hibernate main package comes bundled with an integrated tool : SchemaExport
aka hbm2ddl.It can even be used from "inside" Hibernate.
20.1. Schema自动生成(Automatic schema generation)
DDL can 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.
First, you must customize your mapping files to improve the generated schema. The next
section covers schema customization.
20.1.1. 对schema定制化(Customizing the schema)
Many Hibernate mapping elements define optional attributes named length, precision and
scale. You can set the length, precision and scale of a column with this attribute.
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第 20 章 工具箱指南
<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 can 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. It is only used 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 can be grouped into the same index by simply
specifying the same index name.
<property name="lastName" index="CustName"/>
<property name="firstName" index="CustName"/>
A foreign-key attribute can be used to override the name of any generated foreign key
constraint.
<many-to-one name="bar" column="barId" foreign-key="FKFooBar"/>
很多映射元素还接受<column>子元素。这在定义跨越多字段的类型时特别有用。
<property name="name" type="my.customtypes.Name"/>
<column name="last" not-null="true" index="bar_idx" length="30"/>
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对schema定制化(Customizing the schema)
<column name="first" not-null="true" index="bar_idx" length="20"/>
<column name="initial"/>
</property>
The default attribute allows you to 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>
sql-type属性允许用户覆盖默认的Hibernate类型到SQL数据类型的映射。
<property name="balance" type="float">
<column name="balance" sql-type="decimal(13,3)"/>
</property>
check属性允许用户指定一个约束检查。
<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>
The following table summarizes these optional attributes.
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第 20 章 工具箱指南
表 20.1. Summary
属性(Attribute)
值(Values)
解释(Interpretation)
length
数字
字段长度
precision
数字
精度(decimal precision)
scale
数字
小数点位数(decimal scale)
not-null
true|false
specifies that the column should be nonnullable
unique
true|false
指明是否该字段具有惟一约束
index
index_name
指明一个(多字段)的索引(index)的名字
unique-key
unique_key_name
指明多字段惟一约束的名字(参见上面的说明)
foreign-key
foreign_key_name
specifies the name of the foreign key
constraint generated for an association, for
a <one-to-one>, <many-to-one>, <key>, or <many-tomany> mapping element. Note that inverse="true"
sides will not be considered by SchemaExport.
sql-type
SQL 字段类型
overrides the default column type (attribute
of <column> element only)
default
SQL expression
为字段指定默认值
check
SQL expression
对字段或表加入SQL约束检查
<comment>元素可以让你在生成的schema中加入注释。
<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.
20.1.2. 运行该工具
SchemaExport工具把DDL脚本写到标准输出,同时/或者执行DDL语句。
290
属性(Properties)
The following table displays the SchemaExport command line options
java -cp hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaExport options mapping_files
表 20.2.
SchemaExport命令行选项
选项
Description
--quiet
do not output the script to stdout
--drop
只进行drop tables的步骤
--create
只创建表
--text
do not export to the database
--output=my_schema.ddl
把输出的ddl脚本输出到一个文件
--naming=eg.MyNamingStrategy
select a NamingStrategy
--config=hibernate.cfg.xml
从XML文件读入Hibernate配置
--properties=hibernate.properties
read database properties from a file
--format
把脚本中的SQL语句对齐和美化
--delimiter=;
为脚本设置行结束符
You can even embed SchemaExport in your application:
Configuration cfg = ....;
new SchemaExport(cfg).create(false, true);
20.1.3. 属性(Properties)
Database properties can be specified:
•通过-D<property>系统参数
•在hibernate.properties文件中
•位于一个其它名字的properties文件中,然后用 --properties参数指定
所需的参数包括:
表 20.3. SchemaExport 连接属性
属性名
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
方言(dialect)
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第 20 章 工具箱指南
20.1.4. 使用Ant(Using Ant)
你可以在你的Ant build脚本中调用SchemaExport:
<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. 对schema的增量更新(Incremental schema updates)
The SchemaUpdate tool will update an existing schema with "incremental" changes. The
SchemaUpdate depends upon the JDBC metadata API and, as such, will not work with all
JDBC drivers.
java -cp hibernate_classpaths org.hibernate.tool.hbm2ddl.SchemaUpdate options mapping_files
表 20.4.
SchemaUpdate命令行选项
选项
Description
--quiet
do not output the script to stdout
--text
do not export the script to the database
--naming=eg.MyNamingStrategy
select a NamingStrategy
--properties=hibernate.properties
read database properties from a file
--config=hibernate.cfg.xml
specify a .cfg.xml file
You can embed SchemaUpdate in your application:
Configuration cfg = ....;
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用Ant来增量更新schema(Using Ant for incremental schema updates)
new SchemaUpdate(cfg).execute(false);
20.1.6. 用Ant来增量更新schema(Using Ant for incremental
schema updates)
你可以在Ant脚本中调用SchemaUpdate:
<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 校验
The SchemaValidator tool will validate that the existing database schema "matches" your
mapping documents. The SchemaValidator depends heavily upon the JDBC metadata API and,
as such, will not work with all JDBC drivers. This tool is extremely useful for testing.
-cp
hibernate_classpaths
mapping_files
java
表 20.5.
org.hibernate.tool.hbm2ddl.SchemaValidator
options
SchemaValidator命令行参数
选项
Description
--naming=eg.MyNamingStrategy
select a NamingStrategy
--properties=hibernate.properties
read database properties from a file
--config=hibernate.cfg.xml
specify a .cfg.xml file
You can embed SchemaValidator in your application:
Configuration cfg = ....;
new SchemaValidator(cfg).validate();
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第 20 章 工具箱指南
20.1.8. 使用Ant进行schema校验
你可以在Ant脚本中调用SchemaValidator:
<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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第 21
示例:父子关系(Parent Child
Relationships)
One of the first things that new users want to do with Hibernate is to model a
parent/child type relationship. There are two different approaches to this. 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>. The 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.
21.1. 关于collections需要注意的一点
Hibernate collections are considered to be a logical part of their owning entity and
not of the contained entities. Be aware that this is a critical distinction that has
the following consequences:
•When you 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 (e.g.
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.
•Conversely, if an entity is removed from a collection (a one-to-many or many-tomany association), it will not be deleted by default. This behavior 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.
Adding an entity to a collection, by default, merely creates a link between the two
entities. Removing the entity will remove the link. This is appropriate for all sorts
of cases. However, it is not appropriate in the case of a parent/child relationship.
In this case, the life of the child is bound to the life cycle of the parent.
21.2. 双向的一对多关系(Bidirectional one-to-many)
假设我们要实现一个简单的从Parent到Child的<one-to-many>关联。
<set name="children">
<key column="parent_id"/>
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第 21 章 示例:父子关系(Parent Child R...
<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会产生两条SQL语句:
•一条INSERT语句,为c创建一条记录
•一条UPDATE语句,创建从p到c的连接
This is not only inefficient, but also violates any NOT NULL constraint on the parent_id
column. You 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>
然而,这并非是推荐的解决方法。
The underlying cause of this behavior 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. The solution is to make the link part of the Child mapping.
<many-to-one name="parent" column="parent_id" not-null="true"/>
You 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 to do this:
<set name="children" inverse="true">
<key column="parent_id"/>
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级联生命周期(Cascading life cycle)
<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();
Only one SQL INSERT would now be issued.
You could also create an addChild() method of Parent.
public void addChild(Child c) {
c.setParent(this);
children.add(c);
}
The code to add a Child looks like this:
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.save(c);
session.flush();
21.3. 级联生命周期(Cascading life cycle)
You can address the frustrations of the explicit call to save() 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:
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第 21 章 示例:父子关系(Parent Child R...
Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.flush();
Similarly, we do not 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);
session.delete(p);
session.flush();
However, the following 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. In this case, it will only remove the link to p
and cause a NOT NULL constraint violation. 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();
In our case, a Child cannot exist without its parent. So if we remove a Child from
the collection, we do want it to be deleted. To do this, we must use cascade="alldelete-orphan".
<set name="children" inverse="true" cascade="all-delete-orphan">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
298
级联与未保存值(Cascades and unsaved-value)
Even though the collection mapping specifies inverse="true", cascades are still processed
by iterating the collection elements. If you need 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. 级联与未保存值(Cascades and
)
unsaved-value
Suppose we loaded up a Parent in one Session, made some changes in a UI action and wanted
to persist these changes in a new session by calling update(). The Parent will contain
a collection of children and, since the cascading update is enabled, Hibernate needs
to know which children are newly instantiated and which represent existing rows in
the database. We will also assume that both Parent and Child have generated identifier
properties of type Long. Hibernate will use the identifier and version/timestamp property
value to determine which of the children are new. (See 第 10.7 节 “自动状态检测”.)
In Hibernate3, it is no longer necessary to specify an unsaved-value explicitly.
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();
This may be suitable for the case of a generated identifier, but what about assigned
identifiers and composite identifiers? This is more difficult, since Hibernate cannot
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. 结论
The sections we have just covered can be a bit confusing. However, in practice, it
all works out 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 with composite element
classes: composite elements cannot own collections and they should not be the child
of any entity other than the unique parent.
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第 22
示例:Weblog 应用程序
22.1. 持久化类
The persistent classes here 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;
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第 22 章 示例:Weblog 应用程序
public class BlogItem {
private Long _id;
private Calendar _datetime;
private String _text;
private String _title;
private Blog _blog;
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 映射
The XML mappings are now straightforward. For example:
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Hibernate 映射
<?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"/>
</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
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第 22 章 示例:Weblog 应用程序
"-//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"/>
<property
name="datetime"
column="DATE_TIME"
not-null="true"/>
<many-to-one
name="blog"
column="BLOG_ID"
not-null="true"/>
</class>
</hibernate-mapping>
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Hibernate 代码
22.3. Hibernate 代码
The following class demonstrates some of the kinds of things we can do with these
classes using Hibernate:
package eg;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Iterator;
import java.util.List;
import org.hibernate.HibernateException;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import 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 {
Blog blog = new Blog();
blog.setName(name);
blog.setItems( new ArrayList() );
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第 22 章 示例:Weblog 应用程序
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;
}
306
Hibernate 代码
public BlogItem createBlogItem(Long blogid, String title, String text)
throws HibernateException {
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;
}
307
第 22 章 示例:Weblog 应用程序
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);
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();
}
308
Hibernate 代码
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"
);
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();
309
第 22 章 示例:Weblog 应用程序
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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第 23
示例:复杂映射实例
This chapters explores some more complex association mappings.
23.1. Employer(雇主)/Employee(雇员)
The following model of the relationship between Employer and Employee uses an entity
class (Employment) to represent the association. You can do this when there might be
more than one period of employment for the same two parties. Components are used to
model monetary values and employee names.
Here is 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"/>
<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"/>
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第 23 章 示例:复杂映射实例
<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>
Here is 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,
firstName VARCHAR(255),
initial CHAR(1),
312
Author(作家)/Work(作品)
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(作家)/Work(作品)
Consider the following model of the relationships between Work, Author and Person. In
the example, the relationship between Work and Author is represented as a many-to-many
association and the relationship between Author and Person is represented as one-to-one
association. Another possibility would be to have Author extend Person.
下面的映射文件正确的描述了这些关系:
<hibernate-mapping>
<class name="Work" table="works" discriminator-value="W">
<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">
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第 23 章 示例:复杂映射实例
<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 table linking authors to works.
Here is 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,
314
Customer(客户)/Order(订单)/Product(产品)
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(客户)/Order(订单)/Product(产品)
In this section we consider a model of the relationships between Customer, Order, Line
Item and Product. There is a one-to-many association between Customer and Order, but how
can you represent Order / LineItem / Product? In the example, LineItem is mapped as an
association class representing the many-to-many association between Order and Product.
In Hibernate this is called a composite element.
The mapping document will look like this:
<hibernate-mapping>
<class name="Customer" table="customers">
<id name="id">
315
第 23 章 示例:复杂映射实例
<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>
customers, orders, line_items 和 products 分别保存着customer, order, order line item 和
product的数据。 line_items也作为连接orders 和 products的关联表。
create table customers (
id BIGINT not null generated by default as identity,
name VARCHAR(255),
primary key (id)
)
316
杂例
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. 杂例
These examples are available from the Hibernate test suite. You will find many
other useful example mappings there by searching in the test folder of the Hibernate
distribution.
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>
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第 23 章 示例:复杂映射实例
<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">
318
Composite key example
<synchronize table="LineItem"/>
<synchronize table="Product"/>
<composite-id name="id"
class="Order$Id">
<key-property name="customerId" length="10"/>
<key-property name="orderNumber"/>
</composite-id>
<property name="orderDate"
type="calendar_date"
not-null="true"/>
<property name="total">
<formula>
( select sum(li.quantity*p.price)
from LineItem li, Product p
where li.productId = p.productId
and li.customerId = customerId
and li.orderNumber = orderNumber )
</formula>
</property>
<many-to-one name="customer"
column="customerId"
insert="false"
update="false"
not-null="true"/>
<bag name="lineItems"
fetch="join"
inverse="true"
cascade="save-update">
<key>
<column name="customerId"/>
<column name="orderNumber"/>
</key>
<one-to-many class="LineItem"/>
</bag>
</class>
<class name="LineItem">
319
第 23 章 示例:复杂映射实例
<composite-id name="id"
class="LineItem$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"
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 )
320
共有组合键属性的多对多(Many-to-many with shared composite key attribute)
</formula>
</property>
</class>
23.4.3. 共有组合键属性的多对多(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"/>
<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>
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第 23 章 示例:复杂映射实例
</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"/>
<component name="address">
<property name="address"/>
<property name="zip"/>
<property name="country"/>
</component>
<subclass name="Employee"
discriminator-value="E">
<property name="title"
length="20"/>
322
Associations on alternate keys
<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"/>
</id>
323
第 23 章 示例:复杂映射实例
<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>
324
第 24
最佳实践(Best Practices)
Write fine-grained classes and map them using <component>:
使用一个Address持久类来封装 street, suburb, state, postcode. 这将有利于代码重用和简化
代码重构(refactoring)的工作。
Declare identifier properties on persistent classes:
Hibernate makes identifier properties optional. There are a range of reasons why you
should use them. We recommend that identifiers be 'synthetic', that is, generated
with no business meaning.
Identify natural keys:
对所有的实体都标识出自然键,用<natural-id>进行映射。实现equals()和hashCode(),在其中
用组成自然键的属性进行比较。
Place each class mapping in its own file:
Do not use a single monolithic mapping document. Map com.eg.Foo in the file com/eg/
Foo.hbm.xml. This makes sense, particularly in a team environment.
Load mappings as resources:
把映射文件和他们的映射类放在一起进行部署。
Consider externalizing query strings:
This is recommended if your queries call non-ANSI-standard SQL functions.
Externalizing the query strings to mapping files will make the application more
portable.
使用绑定变量
As in JDBC, always replace non-constant values by "?". Do not use string manipulation
to bind a non-constant value in a query. You should also consider using named
parameters in queries.
Do not manage your own JDBC connections:
Hibernate allows the application to manage JDBC connections, but his
approach should be considered a last-resort. If you cannot use the builtin connection providers, consider providing your own implementation of
org.hibernate.connection.ConnectionProvider.
Consider using a custom type:
Suppose you have a Java type from a library that needs to be persisted but does
not provide the accessors needed to map it as a component. You should consider
implementing org.hibernate.UserType. 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. Do not assume, however, that JDBC is necessarily faster. Please
325
第 24 章 最佳实践(Best Practices)
wait until you know something is a bottleneck. If you need to use direct JDBC, you
can open a Hibernate Session and usingfile:///usr/share/doc/HTML/en-US/index.html
that JDBC connection. This way you can still use the same transaction strategy and
underlying connection provider.
Understand Session flushing:
Sometimes the Session synchronizes its persistent state with the database.
Performance will be affected if this process occurs too often. You can 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 can 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 necessary 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 appropriate alternative in a two tiered
architecture, is to maintain a single open persistence contact session for the
whole life cycle of the application transaction. Then 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.
Do not 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 do not do this,
Hibernate cannot guarantee that in-memory state accurately represents the persistent
state. For example, do not use Session.load() to determine if an instance with the
given identifier exists on the database; use Session.get() or a query instead.
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 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
326
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. Unless you are prepared to hold the persistence context (the session)
open across the view rendering process, 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. 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, however, intended for
"sufficiently large" applications. It is not appropriate for an application with
five tables.
Do not use exotic association mappings:
Practical test cases for 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, most associations are one-to-many and many-to-one. For this reason, you
should proceed cautiously when using any other association style.
Prefer bidirectional associations:
单向关联更加难于查询。在大型应用中,几乎所有的关联必须在查询中可以双向导航。
327
328
第 25
Database Portability Considerations
25.1. Portability Basics
One of the selling points of Hibernate (and really Object/Relational Mapping as a whole)
is the notion of database portability. This could mean an internal IT user migrating from
one database vendor to another, or it could mean a framework or deployable application
consuming Hibernate to simultaneously target multiple database products by their users.
Regardless of the exact scenario, the basic idea is that you want Hibernate to help you
run against any number of databases without changes to your code, and ideally without
any changes to the mapping metadata.
25.2. Dialect
The first line of portability for Hibernate is the dialect, which is a specialization of
the org.hibernate.dialect.Dialect contract. A dialect encapsulates all the differences in
how Hibernate must communicate with a particular database to accomplish some task like
getting a sequence value or structuring a SELECT query. Hibernate bundles a wide range
of dialects for many of the most popular databases. If you find that your particular
database is not among them, it is not terribly difficult to write your own.
25.3. Dialect resolution
Originally, Hibernate would always require that users specify which dialect to use. In
the case of users looking to simultaneously target multiple databases with their build
that was problematic. Generally this required their users to configure the Hibernate
dialect or defining their own method of setting that value.
Starting with version 3.2, Hibernate introduced the notion of automatically detecting
the dialect to use based on the java.sql.DatabaseMetaData obtained from a java.sql.Connection
to that database. This was much better, expect that this resolution was limited
to databases Hibernate know about ahead of time and was in no way configurable or
overrideable.
Starting with version 3.3, Hibernate has a fare more powerful way to automatically
determine which dialect to should be used by relying on a series of delegates which
implement the org.hibernate.dialect.resolver.DialectResolver which defines only a single
method:
public Dialect resolveDialect(DatabaseMetaData metaData) throws JDBCConnectionException
. The basic contract here is that if the resolver 'understands' the given
database metadata then it returns the corresponding Dialect; if not it returns
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第 25 章 Database Portability C...
null and the process continues to the next resolver. The signature also
identifies org.hibernate.exception.JDBCConnectionException as possibly being thrown. A
JDBCConnectionException here is interpreted to imply a "non transient" (aka nonrecoverable) connection problem and is used to indicate an immediate stop to resolution
attempts. All other exceptions result in a warning and continuing on to the next
resolver.
The cool part about these resolvers is that users can also register their own custom
resolvers which will be processed ahead of the built-in Hibernate ones. This might
be useful in a number of different situations: it allows easy integration for autodetection of dialects beyond those shipped with HIbernate itself; it allows you to
specify to use a custom dialect when a particular database is recognized; etc. To
register one or more resolvers, simply specify them (seperated by commas, tabs or spaces)
using the 'hibernate.dialect_resolvers' configuration setting (see the DIALECT_RESOLVERS
constant on org.hibernate.cfg.Environment).
25.4. Identifier generation
When considering portability between databases, another important decision is selecting
the identifier generation stratagy you want to use. Originally Hibernate provided the
native generator for this purpose, which was intended to select between a sequence,
identity, or table strategy depending on the capability of the underlying database.
However, an insidious implication of this approach comes about when targtetting some
databases which support identity generation and some which do not. identity generation
relies on the SQL definition of an IDENTITY (or auto-increment) column to manage the
identifier value; it is what is known as a post-insert generation strategy becauase the
insert must actually happen before we can know the identifier value. Because Hibernate
relies on this identifier value to uniquely reference entities within a persistence
context it must then issue the insert immediately when the users requests the entitiy be
associated with the session (like via save() e.g.) regardless of current transactional
semantics.
注意
Hibernate was changed slightly once the implication of this was better
understood so that the insert is delayed in cases where that is feasible.
The underlying issue is that the actual semanctics of the application itself changes
in these cases.
Starting with version 3.2.3, Hibernate comes with a set of enhanced [http://
in.relation.to/2082.lace] identifier generators targetting portability in a much
different way.
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Database functions
注意
There are specifically 2 bundled enhancedgenerators:
•org.hibernate.id.enhanced.SequenceStyleGenerator
•org.hibernate.id.enhanced.TableGenerator
The idea
identifer
behind these generators is to port the actual semantics of the
value generation to the different databases. For example, the
org.hibernate.id.enhanced.SequenceStyleGenerator mimics the behavior of a sequence on
databases which do not support sequences by using a table.
25.5. Database functions
警告
This is an area in Hibernate in need of improvement. In terms of portability
concerns, this function handling currently works pretty well from HQL;
however, it is quite lacking in all other aspects.
SQL functions can be referenced in many ways by users. However, not all databases
support the same set of functions. Hibernate, provides a means of mapping a logical
function name to a a delegate which knows how to render that particular function,
perhaps even using a totally different physical function call.
重要
Technically
this
function
registration
is handled through the
org.hibernate.dialect.function.SQLFunctionRegistry class which is intended to
allow users to provide custom function definitions without having to
provide a custom dialect. This specific behavior is not fully completed
as of yet.
It is sort of implemented such that users can programatically register
functions with the org.hibernate.cfg.Configuration and those functions will
be recognized for HQL.
25.6. Type mappings
This section scheduled for completion at a later date...
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332
References
[PoEAA] Patterns of Enterprise Application Architecture. 0-321-12742-0. 由 Martin
Fowler. 版权 © 2003 Pearson Education, Inc.. Addison-Wesley Publishing Company.
[JPwH] Java Persistence with Hibernate. Second Edition of Hibernate in Action.
1-932394-88-5. http://www.manning.com/bauer2 . 由 Christian Bauer和Gavin King.
版权 © 2007 Manning Publications Co.. Manning Publications Co..
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