來源：thinkGhoster的專欄 - CSDNBlog
在測試Hibernate的一對多雙向關聯映射時，碰到很有趣的問題，跟inverse屬性直接相關。
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、數據庫結構
people表：{peopleId,name}
address表：{addressId,peopleId,addressName,codeNumber}
5、測試代碼
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、運行結果
上面測試代碼運行起來正確：
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=?
如果將People.hbm.xml映射改寫一下：
＜set name="addresses" cascade="save-update" inverse="true"＞
＜key column="peopleId" not-null="true" /＞
＜one-to-many class="com.persistent.Address"/＞
＜/set＞
不同之處在于添加了inverse="true"，結果：
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并沒有寫入到關聯的address當中，數據庫address表中相應記錄的peopleId字段為空。
7、分析
在Hibernate中，術語inverse是反轉的意思，在關聯關系中，inverse="false"為主控方，由主控方負責維護對象的關聯關系。所以上面的映射文件改動之后，address為主控方，people為被控方，但是測試代碼只進行了一個保存操作session.save(people)，這是針對people的，因此無法正確級聯保存address。而原來的映射文件中（雖然沒有明確指明，Hibernate默認inverse="false"），people為主控方，因此保存people時它會保證關聯的address的正確保存。
也就是說，Hibernate僅僅按照主控方對象的狀態的變化來同步更新數據庫。按照原來的映射文件，people.getAddresses().add(address)，即主控方對象的狀態發生了改變，因此數據庫會跟著對象狀態的變化來同步更新數據庫；而address.setPeople(people)，即被控方對象的狀態發生了改變，它是不能觸發對象和數據庫的同步更新的。