6.Hibernate的多对一关联映射

hibernate多对一关联映射

关联映射的本质：
    * 将关联关系映射到数据库，所谓的关联关系是对象模型在内存中的一个或多个引用

<many-to-one>会在多的一端加入一个外键，指向一的一端，这个外键是由<many-to-one>
中的column属性定义的，如果忽略了这个属性那么默认的外键与实体的属性一致

<many-to-one>标签的定义示例：
     * <many-to-one name="group" column="groupid"/>
     
理解级联的含义？
    * 是对象的连锁操作   

 

一：实体类

package com.tao.hibernate.many2one;

public class Group {
	private Integer id;
	private String name;

	public Group() {
		super();
	}

	public Integer getId() {
		return id;
	}

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

	public String getName() {
		return name;
	}

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

}

 

package com.tao.hibernate.many2one;

public class User {
	private Integer id;
	private String name;
	private Group group;

	public User() {
		super();
	}

	public Integer getId() {
		return id;
	}

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

	public String getName() {
		return name;
	}

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

	public Group getGroup() {
		return group;
	}

	public void setGroup(Group group) {
		this.group = group;
	}

}

 

二：映射文件

<?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 package="com.tao.hibernate.many2one">
	<class name="Group" table="t_group">
		<id name="id" column="id" type="integer">
			<generator class="native">
			</generator>
		</id>
		<property name="name" column="name" />
	</class>
</hibernate-mapping>

 

<?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 package="com.tao.hibernate.many2one">
	<class name="User" table="t_user">
		<id name="id" column="id" type="integer">
			<generator class="native">
			</generator>
		</id>
		<property name="name" column="name" />
		<many-to-one name="group" column="groupId"/>
	</class>
</hibernate-mapping>

 

三：测试类：

package com.tao.hibernate.many2one;

import org.hibernate.Session;
import org.hibernate.Transaction;
import org.junit.Test;

import com.tao.hibernate.HibernateUtils;

public class TestMain {
	Session session = HibernateUtils.getFactory().openSession();

	@Test
	public void testSave() {
		Group group = new Group();
		group.setName("second group");

		User user = new User();
		user.setName("Tao2");
		user.setGroup(group);

		Transaction ts = session.beginTransaction();
		ts.begin();
		session.save(group);
		session.save(user);
		ts.commit();
	}

	@Test
	public void testLoad() {
		Session session = HibernateUtils.getFactory().openSession();
		User user = (User) session.load(User.class, 1);
		System.out.println(user.getName());
		System.out.println(user.getGroup().getName());
	}

}