BlogJava-chou's blog-随笔分类-about sshhttp://www.blogjava.net/chou/category/32794.htmlrecords of my java learning process zh-cnSun, 07 Sep 2008 13:57:15 GMTSun, 07 Sep 2008 13:57:15 GMT60Hibernate一对多双向关联及inverse的作用(转)http://www.blogjava.net/chou/archive/2008/09/06/227398.htmlchouchouSat, 06 Sep 2008 06:41:00 GMThttp://www.blogjava.net/chou/archive/2008/09/06/227398.htmlhttp://www.blogjava.net/chou/comments/227398.htmlhttp://www.blogjava.net/chou/archive/2008/09/06/227398.html#Feedback0http://www.blogjava.net/chou/comments/commentRss/227398.htmlhttp://www.blogjava.net/chou/services/trackbacks/227398.html在测试Hibernate的一对多双向关联映射时，碰到很有趣的问题，跟inverse属性直接相关。

1、People.hbm.xml

＜hibernate-mapping default-lazy="false"＞ 
＜class name="com.persistent.People" table="people"＞ 
＜id name="id" column="peopleId" unsaved-value="0"＞ 
＜generator class="increment"＞ 
＜/generator＞ 
＜/id＞ 
＜property name="name" column="name"＞＜/property＞ 
＜set name="addresses" cascade="save-update"＞
＜key column="peopleId" not-null="true" /＞
＜one-to-many class="com.persistent.Address"/＞
＜/set＞ 
＜/class＞
＜/hibernate-mapping＞

2、Address.hbm.xml

＜hibernate-mapping＞
＜class name="com.persistent.Address" table="address"＞ 
＜id name="id" column="addressId" unsaved-value="0"＞
＜generator class="increment"＞
＜/generator＞
＜/id＞ 
＜many-to-one name="people" column="peopleId" insert="false" update="false"＞＜/many-to-one＞ 
＜property name="addressName" column="addressName"＞＜/property＞ 
＜property name="codeNumber" column="codeNumber"＞＜/property＞ 
＜/class＞ 
＜/hibernate-mapping＞
3、People.java和Address.java

public class People ...{ 
private long id; 
private String name; 
private Set addresses = new HashSet(); 
...
}

public class Address ...{ 
private long id; 
private People people; 
private String addressName; 
private String codeNumber; 
...
} 

4、数据库结构

people表：{peopleId,name}

address表：{addressId,peopleId,addressName,codeNumber}

5、测试代码

People people = new People(); 
people.setName("linda"); 
Address address = new Address(); 
address.setAddressName("yunnan"); 
address.setCodeNumber("564123"); 
address.setPeople(people); 
people.getAddresses().add(address); 
Session session = HibernateSessionFactory.getSession(); 
session.beginTransaction(); 
session.save(people); 
session.getTransaction().commit(); 

6、运行结果

　　上面测试代码运行起来正确：

Hibernate: select max(peopleId) from people
Hibernate: select max(addressId) from address
Hibernate: insert into people (name, peopleId) values (?, ?)
Hibernate: insert into address (addressName, codeNumber, peopleId, addressId) values (?, ?, ?, ?)
Hibernate: update address set peopleId=? where addressId=?

　　如果将People.hbm.xml映射改写一下：

＜set name="addresses" cascade="save-update" inverse="true"＞
＜key column="peopleId" not-null="true" /＞
＜one-to-many class="com.persistent.Address"/＞
＜/set＞

　　不同之处在于添加了inverse="true"，结果：

Hibernate: select max(peopleId) from people
Hibernate: select max(addressId) from address
Hibernate: insert into people (name, peopleId) values (?, ?)
Hibernate: insert into address (addressName, codeNumber, addressId) values (?, ?, ?)

　　可以看到，peopleId并没有写入到关联的address当中，数据库address表中相应记录的peopleId字段为空。

7、分析

　　在Hibernate中，术语inverse是反转的意思，在关联关系中，inverse="false"为主控方，由主控方负责维护对象的关联关系。所以上面的映射文件改动之后，address为主控方，people为被控方，但是测试代码只进行了一个保存操作session.save(people)，这是针对people的，因此无法正确级联保存address。而原来的映射文件中（虽然没有明确指明，Hibernate默认inverse="false"），people为主控方，因此保存people时它会保证关联的address的正确保存。

　　也就是说，Hibernate仅仅按照主控方对象的状态的变化来同步更新数据库。按照原来的映射文件，people.getAddresses().add(address)，即主控方对象的状态发生了改变，因此数据库会跟着对象状态的变化来同步更新数据库；而address.setPeople(people)，即被控方对象的状态发生了改变，它是不能触发对象和数据库的同步更新的。

chou 2008-09-06 14:41 发表评论
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