hibernate inverse屬性的作用

hibernate配置文件中有這么一個屬性inverse，它是用來指定關聯的控制方的。
inverse屬性默認是false，若為false，則關聯由自己控制，若為true，則關聯
由對方控制。
見例子：

一個Parent有多個Child,一個Child只能有一個Parent。

package com.xian;

public class Child {
?private Long id;
?private String name;
?private Parent parent;

?public Long getId() {
??return id;
?}

?public void setId(Long id) {
??this.id = id;
?}

?public String getName() {
??return name;
?}

?public void setName(String name) {
??this.name = name;
?}

?public Parent getParent() {
??return parent;
?}

?public void setParent(Parent parent) {
??this.parent = parent;
?}

}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >
<hibernate-mapping>
?<class name="com.xian.Child" table="child">
??<id name="id" column="childId">
???<generator class="native" />
??</id>
??<property name="name" type="string"></property>
??<many-to-one name="parent" column="parentId" not-null="true">
??</many-to-one>
?</class>
</hibernate-mapping>

package com.xian;

import java.util.HashSet;
import java.util.Set;

public class Parent {
?private Long id;
?private String name;
?private Set children = new HashSet();

?public Long getId() {
??return id;
?}

?private void setId(Long id) {
??this.id = id;
?}

?public String getName() {
??return name;
?}

?public void setName(String name) {
??this.name = name;
?}

?public Set getChildren() {
??return children;
?}

?public void setChildren(Set children) {
??this.children = children;
?}
?
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >
<hibernate-mapping>
?<class name="com.xian.Parent" table="parent">
??<id name="id" column="parentId">
???<generator class="native"></generator>
??</id>
??<property name="name" type="string"></property>
??<set name="children" inverse="true">
??<!-- 此處inverse設置為true，則關聯的控制方交給了child，一對多關聯一般都將控制方交給多端 -->
???<key column="parentId" not-null="true"></key>
???<one-to-many class="com.xian.Child"/>
??</set>
?</class>
</hibernate-mapping>

Parent p = new Parent();
p.setName("parent");
Child c = (Child) session.load(Child.class, 4l);? //原child表中已存在id=4的child
p.getChildren().add(p);??// 我希望用parent端來修改關聯，即將id=4的child的外鍵變為現在這個parent的主鍵值
session.save(p);?//這里保存parent，同時希望關聯也得到修改

但是我們發現parent確實保存了，而child的外鍵還是原值，沒有關聯到我們現在的外鍵
原因就是我們將關聯的控制方交給了child端，這樣一來parent就不能夠控制關聯了。

Parent p = new Parent();
p.setName("parent");
Child c = (Child) session.load(Child.class, 4l);
c.setParent(p);?? //這樣由child端修改關聯就ok了?
session.save(p);