mybatis 详解（八）------ 懒加载

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();
	}