Hibernate的一对一关联实例
Hibernate中的表的关联有一对一,一对多和多对多三种关联方式,在这篇笔记和接下来的笔记中,我将用我自己的实际例子来说明如何具体实施。
我使用的Hibernate版本是2.1.8,在Hibernate的网站2.1.6版本的中文文档中有关一对一的关联有下面一段表述:
5.1.11. 一对一
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" (1)
class ="ClassName" (2)
cascade ="all|none|save-update|delete" (3)
constrained ="true|false" (4)
outer-join ="true|false|auto" (5)
property-ref ="propertyNameFromAssociatedClass" (6)
access ="field|property|ClassName" (7)
/>
(1) name: 属性的名字。
(2) class (可选 - 默认是通过反射得到的属性类型):被关联的类的名字。
(3) cascade(级联) (可选) 表明操作是否从父对象级联到被关联的对象。
(4) constrained(约束) (可选) 表明该类对应的表对应的数据库表,和被关联的对象所对应的数据库表之间,通过一个外键引用对主键进行约束。这个选项影响save()和delete()在级联执行时的先后顺序(也在schema export tool中被使用)。
(5) outer-join(外连接) (可选 - 默认为 自动): 当设置hibernate.use_outer_join的时候,对这个关联允许外连接抓取。
(6) property-ref: (可选) 指定关联类的一个属性,这个属性将会和本外键相对应。如果没有指定,会使用对方关联类的主键。
(7) access (可选 - 默认是 property): Hibernate用来访问属性的策略。
有两种不同的一对一关联:
主键关联
惟一外键关联
主键关联不需要额外的表字段;两行是通过这种一对一关系相关联的,那么这两行就共享同样的主关键字值。所以如果你希望两个对象通过主键一对一关联,你必须确认它们被赋予同样的标识值!
比如说,对下面的Employee和Person进行主键一对一关联:
< one-to-one name ="person" class ="Person" />
< one-to-one name ="employee" class ="Employee" constrained ="true" />
现在我们必须确保PERSON和EMPLOYEE中相关的字段是相等的。我们使用一个特别的称为foreign的Hibernate标识符生成器策略:
< 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 >
一个刚刚保存的Person实例被赋予和该Person的employee属性所指向的Employee实例同样的关键字值。
另一种方式是一个外键和一个惟一关键字对应,上面的Employee和Person的例子,如果使这种关联方式,应该表达成:
< many-to-one name ="person" class ="Person" column ="PERSON_ID" unique ="true" />
如果在Person的映射加入下面几句,这种关联就是双向的:
< one-to-one name"employee" class ="Employee" property-ref ="person" />
下面是我的一个一对一主键关联的例子,使用的数据库是MySQL 4.1.11:
我有两个表:UserBasic和UserInfo,UserBasic记录的是用户的基本注册信息,UserInfo表记录的是用户的详细信息。表的结构如下:
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CREATE TABLE IF NOT EXISTS UserBasic
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(
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Guid INT NOT NULL AUTO_INCREMENT,
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Account VARCHAR ( 64 ) NOT NULL ,
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Password VARCHAR ( 16 ) NOT NULL ,
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Email VARCHAR ( 128 ) NOT NULL ,
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PRIMARY KEY (Guid)
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) TYPE = InnoDB;
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CREATE TABLE IF NOT EXISTS UserInfo
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(
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Guid INT NOT NULL ,
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Username VARCHAR ( 128 ),
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Gender CHAR ( 1 ),
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Birthday DATETIME ,
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PRIMARY KEY (Guid)
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) TYPE = InnoDB;
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ALTER TABLE UserInfo ADD CONSTRAINT UserInfoRFUserBasic FOREIGN KEY (Guid)
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REFERENCES UserBasic (Guid) ON DELETE CASCADE ON UPDATE RESTRICT ;
UserInfo的主键值和UserBasic的主键值是一样的,两个表是单向的一对一关系。UserBasic为主控方,UserInfo是被动方。
用Middlegen生成的UserBasic.hbm.xml文件,修改后的内容如下:
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<? xml version="1.0" ?>
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<! DOCTYPE hibernate-mapping PUBLIC
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"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
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"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd" >
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< hibernate-mapping >
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<!--
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Created by the Middlegen Hibernate plugin 2.1
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http://boss.bekk.no/boss/middlegen/
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http://www.hibernate.org/
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-->
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< class
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name ="com.xxx.hibernate.UserBasic"
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table ="UserBasic"
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dynamic-update ="true"
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dynamic-insert ="true"
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>
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< meta attribute ="class-description" inherit ="false" >
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@hibernate.class
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table="UserBasic"
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dynamic-update="true"
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dynamic-insert="true"
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</ meta >
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< id
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name ="guid"
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type ="int"
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column ="Guid"
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>
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< meta attribute ="field-description" >
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@hibernate.id
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generator-class="native"
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type="int"
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column="Guid"
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</ meta >
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< generator class ="native" />
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</ id >
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< property
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name ="account"
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type ="java.lang.String"
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column ="Account"
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not-null ="true"
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length ="64"
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>
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< meta attribute ="field-description" >
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@hibernate.property
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column="Account"
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length="64"
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not-null="true"
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</ meta >
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</ property >
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< property
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name ="password"
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type ="java.lang.String"
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column ="Password"
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not-null ="true"
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length ="16"
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>
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< meta attribute ="field-description" >
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@hibernate.property
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column="Password"
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length="16"
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not-null="true"
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</ meta >
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</ property >
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< property
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name ="email"
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type ="java.lang.String"
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column ="Email"
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not-null ="true"
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length ="128"
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>
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< meta attribute ="field-description" >
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@hibernate.property
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column="Email"
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length="128"
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not-null="true"
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</ meta >
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</ property >
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<!-- Associations -->
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<!-- bi-directional one-to-one association to UserInfo -->
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< one-to-one
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name ="userInfo"
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class ="com.xxx.hibernate.UserInfo"
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cascade ="save-update"
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>
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< meta attribute ="field-description" >
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@hibernate.one-to-one
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cascade="save-update"
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</ meta >
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</ one-to-one >
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</ class >
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</ hibernate-mapping >
由于在建立外键的时候就声明了ON DELETE CASCADE,所以在xml的配置文件中第97行声明为save-update。如果声明为all,那么在删除UserBasic表的数据时,会无谓的多出一条删除UserInfo的delete语句出来。
UserInfo.hbm.xml文件的内容如下:
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<? xml version = " 1.0 " ?>
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<! DOCTYPE hibernate - mapping PUBLIC
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" -//Hibernate/Hibernate Mapping DTD 2.0//EN "
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" http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd " >
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< hibernate - mapping >
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<!--
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Created by the Middlegen Hibernate plugin 2.1
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http: // boss.bekk.no/boss/middlegen/
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http: // www.hibernate.org/
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-->
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< class
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name = " com.xxx.hibernate.UserInfo "
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table = " UserInfo "
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dynamic - update = " true "
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dynamic - insert = " true "
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>
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< meta attribute = " class-description " inherit = " false " >
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@hibernate. class
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table = " UserInfo "
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dynamic - update = " true "
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dynamic - insert = " true "
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</ meta >
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< id
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name = " guid "
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type = " int "
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column = " Guid "
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>
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< meta attribute = " field-description " >
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@hibernate.id
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generator - class = " foreign "
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type = " int "
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column = " Guid "
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</ meta >
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< generator class = " foreign " >
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< param name = " property " > userBasic </ param >
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</ generator >
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</ id >
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< property
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name = " username "
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type = " java.lang.String "
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column = " Username "
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length = " 128 "
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>
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< meta attribute = " field-description " >
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@hibernate.property
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column = " Username "
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length = " 128 "
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</ meta >
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</ property >
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< property
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name = " gender "
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type = " java.lang.String "
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column = " Gender "
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length = " 1 "
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>
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< meta attribute = " field-description " >
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@hibernate.property
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column = " Gender "
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length = " 1 "
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</ meta >
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</ property >
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< property
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name = " birthday "
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type = " java.sql.Date "
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column = " Birthday "
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length = " 19 "
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>
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< meta attribute = " field-description " >
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@hibernate.property
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column = " Birthday "
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length = " 19 "
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</ meta >
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</ property >
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<!-- Associations -->
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<!-- bi - directional one - to - one association to UserBasic -->
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< one - to - one
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name = " userBasic "
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class = " com.xxx.hibernate.UserBasic "
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constrained = " true "
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>
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< meta attribute = " field-description " >
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@hibernate.one - to - one
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class = " com.xxx.hibernate.UserBasic "
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constrained = " true "
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</ meta >
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</ one - to - one >
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</ class >
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</ hibernate - mapping >
用hbm2java生成对应的对应的Java类:hbm2java *.xml --output=xxx。
Hibernate的配置文件hibernate.cfg.xml内容如下:
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<? xml version="1.0" encoding="utf-8" ?>
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<! DOCTYPE hibernate-configuration
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PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN"
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"http://hibernate.sourceforge.net/hibernate-configuration-2.0.dtd" >
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< hibernate-configuration >
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< session-factory >
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<!-- local connection properties -->
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< property name ="hibernate.connection.url" > jdbc:mysql://127.0.0.1/xxx?useUnicode=true & characterEncoding=UTF-8 & autoReconnect=true </ property >
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< property name ="hibernate.connection.driver_class" > com.mysql.jdbc.Driver </ property >
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< property name ="hibernate.connection.username" > root </ property >
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< property name ="hibernate.connection.password" > 123456 </ property >
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<!-- property name="hibernate.connection.pool_size"></property -->
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<!-- dialect for MySQL -->
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< property name ="dialect" > net.sf.hibernate.dialect.MySQLDialect </ property >
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< property name ="hibernate.show_sql" > true </ property >
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< property name ="hibernate.use_outer_join" > true </ property >
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< property name ="hibernate.transaction.factory_class" > net.sf.hibernate.transaction.JDBCTransactionFactory </ property >
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< mapping resource ="com/xxx/hibernate/UserBasic.hbm.xml" />
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< mapping resource ="com/xxx/hibernate/UserInfo.hbm.xml" />
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</ session-factory >
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</ hibernate-configuration >
JUnit的测试用例程序片断如下:
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public void testInsertUser() throws Exception
{
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UserBasic user = new UserBasic();
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user.setAccount("test");
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user.setPassword("123456");
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user.setEmail("[email protected]");
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UserInfo info = new UserInfo();
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info.setUsername("George Hill");
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info.setGender("M");
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info.setBirthday(new Date());
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user.setUserInfo(info);
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info.setUserBasic(user);
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Transaction tx = session.beginTransaction();
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session.save(user);
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tx.commit();
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}
运行测试程序,可以看到输出了两条insert语句。