Hibernate一对多双向关联及inverse的作用

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 peopleHibernate: select max(addressId) from addressHibernate: 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 peopleHibernate: select max(addressId) from addressHibernate: 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。