mybatis 详解（八）------ 懒加载编程语言

select * from orders o ,user u where o.user_id = u.id; 

//查询所有的订单信息，包括用户id 
select * from orders; 
//如果需要用户信息，我们在根据上一步查询的用户id查询用户信息 
select * from user where id=user_id 

package com.ys.lazyload; 
 
import java.util.List; 
 
public class User { 
	//用户ID 
	private int id; 
	//用户姓名 
	private String username; 
	//用户性别 
	private String sex; 
	//一个用户能创建多个订单，用户和订单构成一对多的关系 
	public List<Orders> orders; 
	 
	public List<Orders> getOrders() { 
		return orders; 
	} 
	public void setOrders(List<Orders> orders) { 
		this.orders = orders; 
	} 
	public int getId() { 
		return id; 
	} 
	public void setId(int id) { 
		this.id = id; 
	} 
	public String getUsername() { 
		return username; 
	} 
	public void setUsername(String username) { 
		this.username = username; 
	} 
	public String getSex() { 
		return sex; 
	} 
	public void setSex(String sex) { 
		this.sex = sex; 
	} 
	@Override 
	public String toString() { 
		return "User [id=" + id + ", username=" + username + ", sex=" + sex 
				+ "]"; 
	} 
} 

package com.ys.lazyload; 
 
public class Orders { 
	//订单ID 
	private int id; 
	//用户ID 
	private int userId; 
	//订单数量 
	private String number; 
	//和用户表构成一对一的关系，即一个订单只能由一个用户创建 
	private User user; 
	 
	public int getId() { 
		return id; 
	} 
 
	public void setId(int id) { 
		this.id = id; 
	} 
 
	public int getUserId() { 
		return userId; 
	} 
 
	public void setUserId(int userId) { 
		this.userId = userId; 
	} 
 
	public String getNumber() { 
		return number; 
	} 
 
	public void setNumber(String number) { 
		this.number = number; 
	} 
 
	public User getUser() { 
		return user; 
	} 
 
	public void setUser(User user) { 
		this.user = user; 
	} 
 
	@Override 
	public String toString() { 
		return "Orders [id=" + id + ", userId=" + userId + ", number=" + number 
				+ ", user=" + user + "]"; 
	} 
 
} 

package com.ys.lazyload; 
 
import java.util.List; 
 
import com.ys.lazyload.Orders; 
import com.ys.lazyload.User; 
 
public interface OrdersMapper { 
	/** 
	 * select * from order //得到user_id 
	 * select * from user WHERE id=1   //1 是上一个查询得到的user_id的值 
	 * @param userID 
	 * @return 
	 */ 
	//得到订单信息(包含user_id) 
	public List<Orders> getOrderByOrderId(); 
	//根据用户ID查询用户信息 
	public User getUserByUserId(int userID); 
 
} 

<?xml version="1.0" encoding="UTF-8" ?> 
<!DOCTYPE mapper 
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" 
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> 
<mapper namespace="com.ys.lazyload.OrdersMapper"> 
	<!-- 
         延迟加载： 
         select user_id from order WHERE id=1;//得到user_id 
         select * from user WHERE id=1   //1 是上一个查询得到的user_id的值 
         property:别名(属性名)    column：列名 --> 
    <select id="getOrderByOrderId" resultMap="getOrderMap"> 
    	select * from orders 
    </select> 
    <resultMap type="com.ys.lazyload.Orders" id="getOrderMap"> 
    	<id column="id" property="id"/> 
		<result column="number" property="number"/> 
		<!-- select:指定延迟加载需要执行的statement的id（根据user_id查询的statement） 
					如果不在本文件中，需要加上namespace 
			 property:resultMap中type指定类中的属性名 
			 column:和select查询关联的字段user_id 
		 --> 
		<association property="user" javaType="com.ys.lazyload.User"  column="user_id" select="getUserByUserId"> 
		 
		</association> 
    </resultMap> 
    <select id="getUserByUserId" resultType="com.ys.lazyload.User"> 
    	select * from user where id=#{id} 
    </select> 
	 
</mapper> 

<!-- 开启懒加载配置 --> 
<settings> 
	<!-- 全局性设置懒加载。如果设为‘false'，则所有相关联的都会被初始化加载。 --> 
	<setting name="lazyLoadingEnabled" value="true"/> 
	<!-- 当设置为‘true'的时候，懒加载的对象可能被任何懒属性全部加载。否则，每个属性都按需加载。 --> 
	<setting name="aggressiveLazyLoading" value="false"/> 
</settings> 

	@Test 
	public void testLazy(){ 
		String statement = "com.ys.lazyload.OrdersMapper.getOrderByOrderId"; 
		//创建OrdersMapper对象，mybatis自动生成mapepr代理对象 
		OrdersMapper orderMapper = session.getMapper(OrdersMapper.class); 
		List<Orders> orders = orderMapper.getOrderByOrderId();//第一步 
		for(Orders order : orders){ 
			System.out.println(order.getUser());//第二步 
		} 
		session.close(); 
	}