在測試Hibernate的一對多雙向關(guān)聯(lián)映射時，碰到很有趣的問題，跟inverse屬性直接相關(guān)。

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" /＞
＜o(jì)ne-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(shù)據(jù)庫結(jié)構(gòu)

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(jié)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" /＞
＜o(jì)ne-to-many class="com.persistent.Address"/＞
＜/set＞

　　不同之處在于添加了inverse="true"，結(jié)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(guān)聯(lián)的address當(dāng)中，數(shù)據(jù)庫address表中相應(yīng)記錄的peopleId字段為空。

7、分析

　　在Hibernate中，術(shù)語inverse是反轉(zhuǎn)的意思，在關(guān)聯(lián)關(guān)系中，inverse="false"為主控方，由主控方負(fù)責(zé)維護(hù)對象的關(guān)聯(lián)關(guān)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(jìn)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(lián)保存address。而原來的映射文件中（雖然沒有明確指明，Hibernate默認(rèn)inverse="false"），people為主控方，因此保存people時它會保證關(guān)聯(lián)的address的正確保存。

　　也就是說，Hibernate僅僅按照主控方對象的狀態(tài)的變化來同步更新數(shù)據(jù)庫。按照原來的映射文件，people.getAddresses().add(address)，即主控方對象的狀態(tài)發(fā)生了改變，因此數(shù)據(jù)庫會跟著對象狀態(tài)的變化來同步更新數(shù)據(jù)庫；而address.setPeople(people)，即被控方對象的狀態(tài)發(fā)生了改變，它是不能觸發(fā)對象和數(shù)據(jù)庫的同步更新的。