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。
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。