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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