Hibernate 多对一关联映射

hibernate多对一关联映射是hibernate中比较常用的一种映射关系

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、编写实体类

package com.yx.zzg.pojo;

public class Group {
	private int id;

	private String name;

	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 com.yx.zzg.pojo;

public class User {

	private int id;

	private String name;

	private Group group;

	public Group getGroup() {
		return group;
	}

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

	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;
	}
}

二、编写实体对象映射文件

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.yx.zzg.pojo">
	<class name="Group" table="t_group">
		<id name="id">
			<!-- 指定主键生成策略 -->
			<generator class="native" />
		</id>
		<property name="name" />
	</class>
</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.yx.zzg.pojo">
	<class name="User" table="t_user">
		<id name="id">
			<!-- 指定主键生成策略 -->
			<generator class="native" />
		</id>
		<property name="name" />
		<many-to-one name="group" column="group_id" cascade="save-update" />
	</class>
</hibernate-mapping>

三、编写hibernate配置文件

<!DOCTYPE hibernate-configuration PUBLIC
	"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
	<session-factory>
		<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
		<property name="connection.url">jdbc:mysql://localhost/hibernate_many2one</property>
		<property name="connection.username">root</property>
		<property name="connection.password">123456</property>
		<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
		<!-- 是否显示执行的sql语句 -->
		<property name="show_sql">true</property>
		<!-- 显示执行的sql语句是否格式化 -->
		<property name="format_sql">true</property>
		<!-- 是否自动创建表，参数详见HIBERNATE_HOME/etc/hibernate.properties文件 -->
		<!--
			<property name="hibernate.hbm2ddl.auto">create</property>
		-->
		<mapping resource="com/yx/zzg/pojo/User.hbm.xml" />
		<mapping resource="com/yx/zzg/pojo/Group.hbm.xml" />
	</session-factory>
</hibernate-configuration>

四、测试类

package com.yx.zzg.test;

import junit.framework.TestCase;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.Transaction;

import com.yx.zzg.pojo.Group;
import com.yx.zzg.pojo.User;
import com.yx.zzg.util.HibernateUtil;


public class TestUser extends TestCase {

	public void testSave1() {
		Session session = null;
		Transaction tx = null;
		try {
			session = HibernateUtil.getSession();
			tx=session.beginTransaction();
			Group group=new Group();
			group.setName("开发部");
			//如果在<many-to-one>标签中没有使用cascade属性并且没有显示的保存group会抛出TransientObjectException异常
			//因为Group为Transient状态，group中的id没有分配值
			//persistent状态的对象是不能引用transient状态的对象的
			User user1=new User();
			user1.setName("zzg");
			user1.setGroup(group);

			User user2=new User();
			user2.setName("zlh");
			user2.setGroup(group);
			session.save(user1);
			session.save(user2);
			tx.commit();
		} catch (HibernateException e) {
			tx.rollback();
			throw e;
		} finally {
			if (session != null) {
				session.close();
			}
		}
	}

	public void testLoad1() {
		Session session = null;
		Transaction tx = null;
		try {
			session = HibernateUtil.getSession();
			tx=session.beginTransaction();
			User user =(User)session.load(User.class, 2);
			System.out.println("user.name="+user.getName()+";user.group.name="+user.getGroup().getName());
			tx.commit();
		} catch (HibernateException e) {
			throw e;
		} finally {
			if (session != null) {
				session.close();
			}
		}
	}

}