Hibernate关联关系映射-----单向多对一映射配置

   在这里举了一个不太恰当的例子：双亲和孩子。当然举这个例子也有一定的道理，一个孩子至少有两个parent，但是例子只是例子，重点不是例子而是配置方法。下面我们看一下配置的详细步骤：

实体：

package uni.many2one;public class Child {private int id;private String name;public Child(int id, String name) {super();this.id = id;this.name = name;}public Child() {}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}}package uni.many2one;public class Parent {private int id;private String name;private Child child;public Parent() {}public Parent(int id, String name, Child child) {super();this.id = id;this.name = name;this.child = child;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Child getChild() {return child;}public void setChild(Child child) {this.child = child;}}

hbm配置文件：

<hibernate-mapping><class name="uni.many2one.Child"><id name="id" column="did"><generator class="native" /></id><property name="name" type="string" column="name"></property></class></hibernate-mapping><hibernate-mapping><!-- <class name="org.hibernate.wk.Student" dynamic-insert="true" dynamic-update="true"> --><class name="uni.many2one.Parent"><id name="id" column="did"><generator class="native" /></id><property name="name" type="string" column="name"></property><!-- configure the many to one association --><many-to-one name="child" column="child"></many-to-one></class></hibernate-mapping>

测试文件：

public void testAdd() {SessionFactory sf = HibernateUtil.getSessionFactory();Session session = sf.getCurrentSession();session.beginTransaction();Child c1 = new Child();c1.setName("child1");Parent mother = new Parent();mother.setName("Mother");mother.setChild(c1);Parent father = new Parent();father.setName("Daddy");father.setChild(c1);session.save(c1);session.save(mother);session.save(father);session.beginTransaction().commit();}

测试结果：

Hibernate: insert into Child (name) values (?)Hibernate: insert into Parent (name, child) values (?, ?)Hibernate: insert into Parent (name, child) values (?, ?)

这里我没有贴出来表的结构，因为表hibernate会自动生成的，多以就没有浪费地方。

hibernate关联关系映射的配置中务必要搞清楚谁是主动配置方，谁是被配置的，如果搞不清楚很容易在实体操作的时候出现错误。尤其是在保存实体的时候，很容易出现引用为空的情况。