延迟加载-association来实现

resultMap可以实现高级映射（使用association、collection实现一对一及一对多映射），association、collection具备延迟加载功能。

需求：

如果查询订单并且关联查询用户信息。如果先查询订单信息即可满足要求，当我们需要查询用户信息时再查询用户信息。把对用户信息的按需去查询就是延迟加载。

延迟加载：先从单表查询、需要时再从关联表去关联查询，大大提高 数据库性能，因为查询单表要比关联查询多张表速度要快。

接下来我们用assocation来实现延迟加载：

先给出需求：

查询订单并且关联查询用户信息

老规矩：1.sql语句，2.pojo类，3.mapper.xml和mapper.java接口

第一步：

1。我们要实现延时加载,相当于先查订单（orders）的信息，等需要用到user的信息时再去查询User表。

也就是把

  <select id="findOrdersUserLazyLoading" resultMap="OrdersUserLazyLoadingResultMap">
          Select * from orders;
      </select>

  <select id="finduserByid" parameterType="int" resultType="cn.itcast.mybatis.po.User">
         SELECT * FROM USER WHERE ID=#{VALUE}
     </select>
     

上面两条select语句用延迟加载的方式实现。（用resultMap来连接这两条select语句）

第二步：

2.pojo类：

package cn.itcast.mybatis.po;

import java.util.Date;
import java.util.List;

public class Orders {
private Integer id;
private Integer user_id;
private String number;
private Date createtime;
private String note;
//用户信息，新增了一个User属性，为了保存查询得到的关联的User表的信息(一对一)
private User user;

public List<Orderdetail> getOrderdetail() {
    return orderdetails;
}
public void setOrderdetail(List<Orderdetail> orderdetail) {
    this.orderdetails = orderdetail;
}
public User getUser() {
    return user;
}
public void setUser(User user) {
    this.user = user;
}
public Integer getId() {
    return id;
}
public void setId(Integer id) {
    this.id = id;
}
public Integer getUser_id() {
    return user_id;
}
public void setUser_id(Integer user_id) {
    this.user_id = user_id;
}
public String getNumber() {
    return number;
}
public void setNumber(String number) {
    this.number = number;
}
public Date getCreatetime() {
    return createtime;
}
public void setCreatetime(Date createtime) {
    this.createtime = createtime;
}
public String getNote() {
    return note;
}
public void setNote(String note) {
    this.note = note;
}

}

第三步：

3.编写OrderMapperCustom.xml代码：使用association中的select指定延迟加载去执行的statement的id。

 <resultMap type="cn.itcast.mybatis.po.Orders" id="OrdersUserLazyLoadingResultMap">这句代码的作用就是用来连接上面那两条select语句来实现延迟加载。

<?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">
<!-- nanmespace:命名空间。 作用就是对sql进行分类话管理，理解Sal分离

注意：使用mapper代理方式，namespace有特殊重要的作用
-->

<mapper namespace="cn.itcast.mybatis.mapper.OrdersMapperCustom">

     <!-- 延迟加载的resultMap -->
    <resultMap type="cn.itcast.mybatis.po.Orders" id="OrdersUserLazyLoadingResultMap">
            <!--对订单信息进行映射配置  -->

     <resultMap type="cn.itcast.mybatis.po.Orders" id="OrdersUserLazyLoadingResultMap">
         <id column="id" property="id"/>
         <result column="user_id" property="user_id"/>
         <result column="number" property="number"/>
         <result column="createtime" property="createtime"/>
         <result column="note" property="note"/>

         <!-- 实现对用户信息进行延迟加载
            select：指定延迟加载需要执行的statement的id（是根据user_id查询用户信息的statement）
            要使用userMapper.xml中findUserById完成根据用户id(user_id)用户信息的查询，如果findUserById不在本mapper中需要前边加namespace
            column：订单信息中关联用户信息查询的列，是user_id
            关联查询的sql理解为：
            SELECT orders.*,
    (SELECT username FROM USER WHERE orders.user_id = user.id)username,
    (SELECT sex FROM USER WHERE orders.user_id = user.id)sex
     FROM orders
             -->

         <association property="user" javaType="cn.itcast.mybatis.po.User"
             select="finduserByid" column="user_id"
             >
         </association>
     </resultMap>

     <select id="finduserByid" parameterType="int" resultType="cn.itcast.mybatis.po.User">
         SELECT * FROM USER WHERE ID=#{VALUE}
     </select>

      <select id="findOrdersUserLazyLoading" resultMap="OrdersUserLazyLoadingResultMap">
          Select * from orders;
      </select>

</mapper>

编写OrdersMapperCustom.java接口:

package cn.itcast.mybatis.mapper;

import java.util.List;

import cn.itcast.mybatis.po.Orders;
import cn.itcast.mybatis.po.OrdersCustom;
import cn.itcast.mybatis.po.User;

public interface OrdersMapperCustom {
    //函数的名字OrdersMapperCustom.xml中select中的id名一样

     public List<Orders> findOrdersUserLazyLoading();
}

第四步：

不要忘记修改总的配置文件，setting 加上延迟加载的功能

延迟加载配置

mybatis默认没有开启延迟加载，需要在SqlMapConfig.xml中setting配置。

在mybatis核心配置文件中配置：

lazyLoadingEnabled、aggressiveLazyLoading

设置项	描述	允许值	默认值
lazyLoadingEnabled	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。	true | false	false
aggressiveLazyLoading	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。	true | false	true

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" alt="" />

在SqlMapConfig.xml中代码为：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>

    <!-- properties的配置必须写在typeAliases的上面 -->
    <properties resource="db.properties"></properties>

<!-- 配置懒加载设置 -->
<settings>
    <setting name="lazyLoadingEnabled" value="true"/>
     <setting name="aggressiveLazyLoading" value="false"/>
</settings>

 <!-- 和spring整合后 environments配置将废除-->
    <environments default="development">
        <environment id="development">
        <!-- 使用jdbc事务管理-->
            <transactionManager type="JDBC" />
        <!-- 数据库连接池-->
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}" />
                <property name="url" value="${jdbc.url}" />
                <property name="username" value="${jdbc.username}" />
                <property name="password" value="${jdbc.password}" />
            </dataSource>

        </environment>

    </environments>

    <!-- 把映射文件(sqlmap/user.xml)加载进sqlMapConfig.xml-->
        <mappers>

           <!--  把mapper.xml加载进sqlMapConfig.xml-->
           <!--
           遵循一些规范：需要将mapper接口类名和mapper.xml映射文件名称保持一致
           且在一个目录中。
           当着这个规范的前提是：使用mapper代理方法。

            -->
            <mapper class="cn.itcast.mybatis.mapper.OrdersMapperCustom"/>

         </mappers>

</configuration>

第五步：

编写测试代码：

测试思路：

1、执行上边mapper方法（findOrdersUserLazyLoading），内部去调用cn.itcast.mybatis.mapper.OrdersMapperCustom中的findOrdersUserLazyLoading只查询orders信息（单表）。

2、在程序中去遍历上一步骤查询出的List<Orders>，当我们调用Orders中的getUser方法时，开始进行延迟加载。

3、延迟加载，去调用UserMapper.xml中findUserbyId这个方法获取用户信息。

测试代码Mybatis_mappertest.java如下：

package cn.itcast.mybatis.test;

import java.io.IOException;
import java.io.InputStream;
import java.util.Date;
import java.util.List;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;

import cn.itcast.mybatis.mapper.OrdersMapperCustom;
import cn.itcast.mybatis.mapper.userMapper;
import cn.itcast.mybatis.po.Orders;
import cn.itcast.mybatis.po.User;
import cn.itcast.mybatis.po.UserCustom;
import cn.itcast.mybatis.po.UserQueryVo;

public class Mybatis_mappertest {

    private SqlSessionFactory sqlSessionFactory;
    @Before
    public void setup() throws IOException
    {   String resource="SqlMapConfig.xml";
        InputStream inputStream= Resources.getResourceAsStream(resource);
        //主要是生成SqlsessionFactory。
        this.sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
    }
    @Test
    public void testMaper()
    {
        SqlSession sqlSession=null;

        sqlSession=sqlSessionFactory.openSession();
        //生成代理类
        OrdersMapperCustom orderMapper=sqlSession.getMapper(OrdersMapperCustom.class);
    //创建包装对象，设置查询条件
        //orderMapper.findOrdersUser();

       @SuppressWarnings("unused")

    List<Orders> list=orderMapper.findOrdersUserLazyLoading();
       for(Orders orders:list)

       {//只有执行了这一步才会执行延迟加载。
           User user=orders.getUser();
           System.out.println(user);

       }

    }

}

运行结果如下：

DEBUG [main] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@30647e13]
DEBUG [main] - ==>  Preparing: Select * from orders; 
DEBUG [main] - ==> Parameters: 
DEBUG [main] - <==      Total: 3
DEBUG [main] - ==>  Preparing: SELECT * FROM USER WHERE ID=? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 1
cn.itcast.mybatis.po.User@1ff90029

我们看一下上面的结果：第一条sql语句：Select * from orders;

然后才会有：SELECT * FROM USER WHERE ID=?

实现了延迟加载

不使用mybatis提供的association及collection中的延迟加载功能，如何实现延迟加载？？

实现方法如下：

定义两个mapper方法：

1、查询订单列表

2、根据用户id查询用户信息

实现思路：

先去查询第一个mapper方法，获取订单信息列表

在程序中（service），按需去调用第二个mapper方法去查询用户信息。

总之：

使用延迟加载方法，先去查询简单的sql（最好单表，也可以关联查询），再去按需要加载关联查询的其它信息。