mybatis高级映射——(延迟加载)

一.什么是延迟加载

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。

• resultMap可以实现高级映射(使用association和collection实现一对一以及一对多映射)所以assocation. collection具备延迟加载功能.

使用association：中的select指定延迟加载去执行的statement的id

查询sql语句：

select students.*,
(select a_username from admin where students.s_studentid = admin.a_nameid) a_username,
(select a_trank from admin where students.s_studentid = admin.a_nameid ) a_trank
from students;

1.只查询订单信息
select * from students
在查询订单的statement中使用association去延迟加载（执行）下边的statement（关联查询）
2.关联查询用户信息：
通过上边查询到的订单信息中的id去关联用户ID去关联查询管理身份信息
使用mapper.xml中的findUserById
上面先去执行findStudentLoading 当需要去查询管理信息时 再去执行findUserById通过resultMap的定义将延迟加载执行配置起来

---

1.pojo代码：
AdminBean.java

package mybatis.ma.entity;

/**
 * Created by 马欢欢 on 2017/6/6.
 */
public class AdminBean {
    private int a_id;
    private String  a_nameid;
    private String  a_username;
    private String  a_password;
    private String  a_trank;

//省略getter and setter 方法

package mybatis.ma.entity;


/**
 * Created by 马欢欢 on 2017/6/6.
 */
public class StudentBean {
    private int s_id;
    private String  s_studentid;
    private String s_username;
    private String s_password;
    private String s_sex;
    private String s_class;
    private String s_rank;
    private  AdminBean adminBean;
//省略getter and setter 方法

2.接口：

package mybatis.ma.mapper;

import mybatis.ma.entity.StudentBean;

import java.util.List;

/**
 * Created by 马欢欢  on 17-7-1.
 */
public interface StudentEvaluateMapper {

    List<StudentBean> findStudentLoading() throws Exception;
}

3.配置文件：

<?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="mybatis.ma.mapper.StudentEvaluateMapper">
    <resultMap id="StudentLoading" type="mybatis.ma.entity.StudentBean">
        <id column="s_id" property="s_id"/>
        <result column="s_sex" property="s_sex"/>
        <result column="s_username" property="s_username"/>
        <result column="s_password" property="s_password"/>
        <result column="s_class" property="s_class"/>
        <!--实现对用户信息惊醒延迟加载
        select：制定延迟加载需要执行的statement的id
        column:用户信息中关联查询的列-->
        <association property="adminBean" javaType="mybatis.ma.entity.AdminBean" select="mybatis.ma.mapper.StudentEvaluateMapper.findUserById" column="s_id">

        </association>
    </resultMap>


    <select id="findStudentLoading" resultMap="StudentLoading">
        select *from students
    </select>


    <select id="findUserById" parameterType="int" resultType="mybatis.ma.entity.AdminBean">
        SELECT * FROM admin WHERE a_id =#{a_id}
    </select>
</mapper>

4.打开延迟加载开关,在SqlMapConfig.xml配置如下:

    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

测试：

package mybatis.ma.mybatisTest;



import mybatis.ma.entity.AdminBean;
import mybatis.ma.entity.StudentBean;
import mybatis.ma.mapper.StudentEvaluateMapper;
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 java.io.IOException;
import java.io.InputStream;
import java.util.List;

/**
 * Created by 马欢欢  on 17-7-1.
 */
public class StudentEvaluateMapperText {
    private SqlSessionFactory sqlSessionFactory;
    @Before
    //测试前执行
    public void setUp() throws IOException {
        //配置文件
        String resource = "mybatis-config.xml";
        //的到配置文件流
        InputStream inputStream =  Resources.getResourceAsStream(resource);
        //创建会话工厂
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }
    @Test
    public void findStudentLoading() throws Exception{
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //StudentCustomMapper，mybatis自动生成mapper代理对象
        StudentEvaluateMapper studentEvaluateMapper = sqlSession.getMapper(StudentEvaluateMapper.class);
        List<StudentBean> studentBeans = studentEvaluateMapper.findStudentLoading();
        //遍历上边循环列表
        for (StudentBean studentBean:studentBeans){
            AdminBean adminBean =studentBean.getAdminBean();
            System.out.println(adminBean);
        }

        sqlSession.close();

    }
}

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