Hibernate lazy加载FOR 单端关联

Hibernate单端关联懒加载策略：即在<one-to-one>/<many-to-one>标签上可以配置
懒加载策略。

可以取值为：false/proxy/no-proxy
false:取消懒加载策略，即在加载对象的同时，发出查询语句，加载其关联对象
proxy:这是hibernate对单端关联的默认懒加载策略，即只有在调用到其关联对象的方法的时候
      才真正发出查询语句查询其对象数据，其关联对象是代理类
no-proxy:这种懒加载特性需要对类进行增强，使用no-proxy，其关联对象不是代理类

注意：在class标签上配置的lazy属性不会影响到关联对象!!!

 

如下例以多对一为例：

package com.bjsxt.hibernate;

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.bjsxt.hibernate;

import java.util.Date;

public class User {
	
	private int id;
	
	private String name;
	
	private 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;
	}

	public Group getGroup() {
		return group;
	}

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

 

Group.hbm.xml

<?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>
	<class name="com.bjsxt.hibernate.Group" table="t_group">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
	</class>
</hibernate-mapping>	

 

User.hbm.xml

<?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>
	<class name="com.bjsxt.hibernate.User" table="t_user" >
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<!--
		<many-to-one name="group" column="relatedGroup" />
		 -->
		
		<many-to-one name="group" column="relatedGroup"/> 
	</class>
</hibernate-mapping>	

 

生成数据：

package com.bjsxt.hibernate;

import org.hibernate.Session;

public class InitData {

	public static void main(String[] args) {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			Group group = new Group();
			group.setName("java开发组");
			
			//将group转换为持久对对象 
			session.save(group);
			
			for (int i = 0; i < 10 ; i++) {
				User user = new User();
				user.setName("User_" + i);
				
				//建立对象之间的关系
				user.setGroup(group);
				
				//因为group为持久对象，所以可以成功保存
				session.save(user);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}

}

 

 

测试类：

package com.bjsxt.hibernate;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 所有lazy的属性默认
 * @author Administrator
 *
 */
public class SingleEndTest1 extends TestCase {

	public void testLoadUser() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			
			//不会发出查询sql
			User user = (User)session.load(User.class, 2);
			
			//会发出一条查询sql，转载数据到User
			System.out.println("user name=" + user.getName());
			
			//不会发出查询sql
			Group group = user.getGroup();
			
			//会发出一条查询sql，转载数据到Group
			System.out.println("related group=" + group.getName());
			
		}catch(Exception e) {
			e.printStackTrace();
		}finally {
			
		}
	}
}

 

 

将<class>标签中的lazy设置为false

package com.bjsxt.hibernate;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 将<class>标签中的lazy设置为false
 * @author Administrator
 *
 */
public class SingleEndTest2 extends TestCase {

	public void testLoadUser() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			
			//会发出一条查询sql，转载数据到User
			User user = (User)session.load(User.class, 2);
			
			//不会发出查询sql
			System.out.println("user name=" + user.getName());
			
			//不会发出查询sql
			Group group = user.getGroup();
			
			//会发出一条查询sql，转载数据到Group
			System.out.println("related group=" + group.getName());
			
		}catch(Exception e) {
			e.printStackTrace();
		}finally {
			
		}
	}
}

 

 保持<class>标签中的lazy默认设置， 将<many-to-one>标签中加入lazy=false

package com.bjsxt.hibernate;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 保持<class>标签中的lazy默认设置
 * 将<many-to-one>标签中加入lazy=false
 * @author Administrator
 *
 */
public class SingleEndTest3 extends TestCase {

	public void testLoadUser() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			
			//不会发出一条查询sql
			User user = (User)session.load(User.class, 2);
			
			//会发出查询sql，这里因为many-to-one设置了lazy=false所以这里会把group也加载过来，这里会发出两条SQL
			//Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_, user0_.relatedGroup as relatedG3_0_0_ from t_user user0_ where user0_.id=?
			//Hibernate: select group0_.id as id1_0_, group0_.name as name1_0_ from t_group group0_ where group0_.id=?

			System.out.println("user name=" + user.getName());
			
			//不会发出查询sql
			Group group = user.getGroup();
			
			//前面一次性查出了数据，不再发出查询SQL
			System.out.println("related group=" + group.getName());
			
		}catch(Exception e) {
			e.printStackTrace();
		}finally {
			
		}
	}
}