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。
