mybatis中resultMap用法及懒加载

本章节主要介绍mybatis的resultMap的用法，包含了级联查询、关联查询、懒加载、鉴别器的相关讲解和代码用例

 

首先这是测试代码用到的实体类POJO

Employee.java：

package com.wcg.mybatis.entity;

import java.io.Serializable;

/**
 * @author wcg
 * @create 2019-05-06 10:04
 */
public class Employee implements Serializable {
    private static final long serialVersionUID = -6248806586248836314L;
    private Integer id;
    private String lastName;
    private String gender;
    private String email;
    private Department department;

    public Employee() {
    }
    // setting getting 省略
    
    @Override
    public String toString() {
        return "Employee{" +
                "id=" + id +
                ", lastName='" + lastName + '\'' +
                ", gender='" + gender + '\'' +
                ", email='" + email + '\'' +
                ", department=" + department +
                '}';
    }
}

Department.java

package com.wcg.mybatis.entity;

import java.io.Serializable;
import java.util.List;

/**
 * @author wcg
 * @create 2019-05-07 23:59
 */
public class Department implements Serializable {

    private static final long serialVersionUID = -6380361622841645277L;
    private Integer id;
    private String deptName;
    private List<Employee> employeeList;

    public Department() {
    }

    // setting getting 省略

    @Override
    public String toString() {
        return "Department{" +
                "id=" + id +
                ", deptName='" + deptName + '\'' +
                ", employeeList=" + employeeList +
                '}';
    }
}

数据库的建表语句DDL：

CREATE TABLE `tbl_employee` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `last_name` varchar(255) DEFAULT NULL,
  `gender` char(1) DEFAULT NULL,
  `email` varchar(255) DEFAULT NULL,
  `dept_id` int(10) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8;


CREATE TABLE `tbl_department` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `dept_name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

一、自定义结果集映射resultMap

EmployeeMapper.java 接口：

package com.wcg.mybatis.dao;

import com.wcg.mybatis.entity.Employee;
import org.apache.ibatis.annotations.MapKey;
import org.apache.ibatis.annotations.Param;

import java.util.List;
import java.util.Map;

/**
 * @author wcg
 * @create 2019-05-06 17:48
 */
public interface EmployeeMapperPlus {

    Employee getEmpById(Integer id);

}

EmployeeMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.EmployeeMapperPlus">
	<!--
		自定义某个javaBean的封装规则
			type：自定义规则的Java类型
			id:唯一id方便引用
	  -->
	<resultMap id="employee" type="com.wcg.mybatis.entity.Employee">
		<!--
			指定主键列：
				id定义主键会底层有优化；
				column：字段
				property：JavaBean属性
		-->
		<id column="id" property="id"></id>
		<!-- 定义普通列封装规则 -->
		<result column="last_name" property="lastName"></result>
		<!-- 其他不指定的列会自动封装：我们只要写resultMap就把全部的映射规则都写上。 -->
		<result column="gender" property="gender"></result>
		<result column="email" property="email"></result>
	</resultMap>

	<!-- resultMap:自定义结果集映射规则；  -->
	<!-- Employee getEmpById(Integer id); -->
	<select id="getEmpById" resultMap="employee">
		select id,last_name,email,gender from tbl_employee where id = #{id}
	</select>

</mapper>

测试代码：

package com.wcg.mybatis.test;

import com.wcg.mybatis.dao.EmployeeMapperPlus;
import com.wcg.mybatis.entity.Employee;
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.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

import java.io.InputStream;

/**
 * @author wcg
 * @create 2019-05-08 21:55
 */
public class ResultMapTest {
    public SqlSessionFactory sessionFactory = null;
    @BeforeEach
    public void test()throws Exception{
        // 根据全局配置文件（xml）创建一个SqlSessionFactory对象
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }

    @Test
    public void resultMapTest(){
        // 获取 SqlSession 的实例 。SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。通过 SqlSession 实例来直接执行已映射的 SQL 语句
        SqlSession sqlSession = null;
        try {
            sqlSession = sessionFactory.openSession();
            // 通过获取接口代理对象来执行sql语句
            EmployeeMapperPlus mapper = sqlSession.getMapper(EmployeeMapperPlus.class);
            Employee employee = mapper.getEmpById(2);
            System.out.println(employee);
        } finally {
            // 资源关闭，放在finally中确保一定会执行
            sqlSession.close();
        }
    }

}

测试结果：

DEBUG 05-08 22:43:39,908 ==>  Preparing: select id,last_name,email,gender from tbl_employee where id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 22:43:39,989 ==> Parameters: 2(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 22:43:40,022 <==      Total: 1  (BaseJdbcLogger.java:145) 
Employee{id=2, lastName='BB', gender='0', email='bb@qq.com', department=null}

二、级联属性封装结果集

EmployeeMapper.java 接口方法：

Employee getEmpByIdWithDeptName(Integer id);

EmployeeMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.EmployeeMapperPlus">
	<!--
		联合查询：级联属性封装结果集
	  -->
	<resultMap id="employeeWithDeptName" type="com.wcg.mybatis.entity.Employee">
		<id column="eid" property="id"></id>
		<result column="last_name" property="lastName"></result>
		<result column="gender" property="gender"></result>
		<result column="email" property="email"></result>
		<result column="did" property="department.id"></result>
		<result column="dept_name" property="department.deptName"></result>
	</resultMap>

	<select id="getEmpByIdWithDeptName" resultMap="employeeWithDeptName">
		select
			e.id as eid, e.last_name, e.email, e.gender,
			d.id as did, d.dept_name
		from
			tbl_employee e
		LEFT JOIN
			tbl_department d on e.dept_id = d.id
		where e.id = #{id}
	</select>

</mapper>

测试代码：

@Test
    public void resultMapTest(){
        // 获取 SqlSession 的实例 。SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。通过 SqlSession 实例来直接执行已映射的 SQL 语句
        SqlSession sqlSession = null;
        try {
            sqlSession = sessionFactory.openSession();
            // 通过获取接口代理对象来执行sql语句
            EmployeeMapperPlus mapper = sqlSession.getMapper(EmployeeMapperPlus.class);
            Employee employee = mapper.getEmpById(2);
            System.out.println(employee);
        } finally {
            // 资源关闭，放在finally中确保一定会执行
            sqlSession.close();
        }
    }

测试结果：

DEBUG 05-08 22:51:26,592 ==>  Preparing: select e.id as eid, e.last_name, e.email, e.gender, d.id as did, d.dept_name from tbl_employee e LEFT JOIN tbl_department d on e.dept_id = d.id where e.id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 22:51:26,676 ==> Parameters: 2(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 22:51:26,700 <==      Total: 1  (BaseJdbcLogger.java:145) 
Employee{id=2, lastName='BB', gender='0', email='bb@qq.com', department=Department{id=1, deptName='开发部', employeeList=null}}

 

三、association定义关联对象封装结果集

（一）、普通查询

EmployeeMapper.java 接口方法：

Employee getByIdWithDept(Integer id);

EmployeeMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.EmployeeMapperPlus">
    <!--
		使用association定义关联的单个对象的封装规则；
	 -->
	<resultMap id="employeeWithDept" type="com.wcg.mybatis.entity.Employee">
		<id column="eid" property="id"></id>
		<result column="last_name" property="lastName"></result>
		<result column="gender" property="gender"></result>
		<result column="email" property="email"></result>
		<!--  
			association可以指定联合的javaBean对象
				property="dept"：指定哪个属性是联合的对象
				javaType:指定这个属性对象的类型[不能省略]
		-->
		<association property="department" javaType="com.wcg.mybatis.entity.Department">
			<id column="did" property="id"></id>
			<result column="dept_name" property="deptName"></result>
		</association>
	</resultMap>

	<select id="getByIdWithDept" resultMap="employeeWithDept">
		select
			e.id as eid, e.last_name, e.email, e.gender,
			d.id as did, d.dept_name
		from
			tbl_employee e
		LEFT JOIN
			tbl_department d on e.dept_id = d.id
		where e.id = #{id}
	</select>

</mapper>

测试代码：

@Test
public void associationTest(){
    // 获取 SqlSession 的实例 。SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。通过 SqlSession 实例来直接执行已映射的 SQL 语句
    SqlSession sqlSession = null;
    try {
        sqlSession = sessionFactory.openSession();
        // 通过获取接口代理对象来执行sql语句
        EmployeeMapperPlus mapper = sqlSession.getMapper(EmployeeMapperPlus.class);
        Employee employee = mapper.getByIdWithDept(2);
        System.out.println(employee);
    } finally {
        // 资源关闭，放在finally中确保一定会执行
        sqlSession.close();
    }
}

测试结果：

DEBUG 05-08 23:01:10,678 ==>  Preparing: select e.id as eid, e.last_name, e.email, e.gender, d.id as did, d.dept_name from tbl_employee e LEFT JOIN tbl_department d on e.dept_id = d.id where e.id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:01:10,746 ==> Parameters: 2(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:01:10,763 <==      Total: 1  (BaseJdbcLogger.java:145) 
Employee{id=2, lastName='BB', gender='0', email='bb@qq.com', department=Department{id=1, deptName='开发部', employeeList=null}}

（二）、分步查询

EmployeeMapper.java 接口方法：

Employee getByIdWithDeptStep(Integer id);

EmployeeMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.EmployeeMapperPlus">
    <!--
		使用association进行分步查询：
            1、先按照员工id查询员工信息
            2、根据查询员工信息中的d_id值去部门表查出部门信息
            3、部门设置到员工中；
	 -->
	<resultMap id="employeeWithDeptStep" type="com.wcg.mybatis.entity.Employee">
		<id column="id" property="id"></id>
		<result column="last_name" property="lastName"></result>
		<result column="gender" property="gender"></result>
		<result column="email" property="email"></result>
		<!-- association定义关联对象的封装规则
	 		select:表明当前属性是调用select指定的方法查出的结果
	 		column:指定将哪一列的值传给这个方法 注：如果多列情况使用{key=value...}的形式
	 		流程：使用select指定的方法（传入column指定的这列参数的值）查出对象，并封装给property指定的属性
	 	 -->
		<association property="department"
					 select="com.wcg.mybatis.dao.DepartmentMapper.getById"
					 column="dept_id">
		</association>
	</resultMap>

	<select id="getByIdWithDeptStep" resultMap="employeeWithDeptStep">
		select id,last_name,email,gender, dept_id from tbl_employee where id = #{id}
	</select>

</mapper>

DepartmentMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.DepartmentMapper">

	<resultMap id="department" type="com.wcg.mybatis.entity.Department">
		<id column="did" property="id"></id>
		<result column="dept_name" property="deptName"></result>
	</resultMap>

	<select id="getById" resultMap="department">
		SELECT id , dept_name FROM tbl_department WHERE id = #{id}
	</select>
</mapper>

测试代码：

@Test
public void associationStepTest(){
    // 获取 SqlSession 的实例 。SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。通过 SqlSession 实例来直接执行已映射的 SQL 语句
    SqlSession sqlSession = null;
    try {
        sqlSession = sessionFactory.openSession();
        // 通过获取接口代理对象来执行sql语句
        EmployeeMapperPlus mapper = sqlSession.getMapper(EmployeeMapperPlus.class);
        Employee employee = mapper.getByIdWithDeptStep(2);
        System.out.println(employee);
    } finally {
        // 资源关闭，放在finally中确保一定会执行
        sqlSession.close();
    }
}

测试结果：

DEBUG 05-08 23:15:31,272 ==>  Preparing: select id,last_name,email,gender, dept_id from tbl_employee where id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:15:31,339 ==> Parameters: 2(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:15:31,370 ====>  Preparing: SELECT id , dept_name FROM tbl_department WHERE id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:15:31,376 ====> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:15:31,386 <====      Total: 1  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:15:31,387 <==      Total: 1  (BaseJdbcLogger.java:145) 
Employee{id=2, lastName='BB', gender='0', email='bb@qq.com', department=Department{id=1, deptName='开发部', employeeList=null}}

四、collection定义关联对象封装结果集

（一）、普通查询

DepartmentMapper.java 接口方法：

Department getByIdWithEmp(Integer id);

DepartmentMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.DepartmentMapper">

	<!-- collection 结果集的方式：定义关联集合类型元素的封装规则 -->
	<resultMap id="departmentWithEmp" type="com.wcg.mybatis.entity.Department">
		<id column="did" property="id"></id>
		<result column="dept_name" property="deptName"></result>
		<!--
			collection定义关联集合类型的属性的封装规则
			ofType：指定集合里面元素的类型
		-->
		<collection property="employeeList" ofType="com.wcg.mybatis.entity.Employee">
			<!-- 定义这个集合中元素的封装规则 -->
			<id column="eid" property="id"></id>
			<result column="last_name" property="lastName"></result>
			<result column="gender" property="gender"></result>
			<result column="email" property="email"></result>
		</collection>
	</resultMap>
	<!-- collection 结果集的方式 -->
	<select id="getByIdWithEmp" resultMap="departmentWithEmp">
		SELECT
			d.id as did, d.dept_name,
			e.id as eid, e.last_name, e.email, e.gender
		FROM
			tbl_department d
		LEFT JOIN tbl_employee e ON e.dept_id = d.id
		WHERE d.id = #{id}
	</select>
</mapper>

测试代码：

@Test
public void collectionTest(){
    // 获取 SqlSession 的实例 。SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。通过 SqlSession 实例来直接执行已映射的 SQL 语句
    SqlSession sqlSession = null;
    try {
        sqlSession = sessionFactory.openSession();
        // 通过获取接口代理对象来执行sql语句
        DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);
        Department department = mapper.getByIdWithEmp(1);
        System.out.println(department);
    } finally {
        // 资源关闭，放在finally中确保一定会执行
        sqlSession.close();
    }
}

测试结果：

DEBUG 05-08 23:40:25,183 ==>  Preparing: SELECT d.id as did, d.dept_name, e.id as eid, e.last_name, e.email, e.gender FROM tbl_department d LEFT JOIN tbl_employee e ON e.dept_id = d.id WHERE d.id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:40:25,291 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:40:25,333 <==      Total: 2  (BaseJdbcLogger.java:145) 
Department{id=1, deptName='开发部', employeeList=[Employee{id=2, lastName='BB', gender='0', email='bb@qq.com', department=null}, Employee{id=7, lastName='FF', gender='0', email='ff@qq.com', department=null}]}

（二）、分步查询

DepartmentMapper.java 接口方法：

​Employee getByIdWithDeptStep(Integer id);

​

DepartmentMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.DepartmentMapper">

	<resultMap id="departmentWithEmpStep" type="com.wcg.mybatis.entity.Department">
		<id column="id" property="id"></id>
		<result column="dept_name" property="deptName"></result>
		<collection property="employeeList"
					ofType="com.wcg.mybatis.entity.Employee"
					select="com.wcg.mybatis.dao.EmployeeMapperPlus.getEmpByDeptId"
					column="id">
		</collection>
	</resultMap>

	<!-- collection 结果集的方式 -->
	<select id="getByIdWithEmpStep" resultMap="departmentWithEmpStep">
		SELECT
			d.id, d.dept_name
		FROM
			tbl_department d
		WHERE d.id = #{id}
	</select>
</mapper>

EmployeeMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.EmployeeMapperPlus">
	<!--
		自定义某个javaBean的封装规则
			type：自定义规则的Java类型
			id:唯一id方便引用
	  -->
	<resultMap id="employee" type="com.wcg.mybatis.entity.Employee">
		<!--
			指定主键列：
				id定义主键会底层有优化；
				column：字段
				property：JavaBean属性
		-->
		<id column="id" property="id"></id>
		<!-- 定义普通列封装规则 -->
		<result column="last_name" property="lastName"></result>
		<!-- 其他不指定的列会自动封装：我们只要写resultMap就把全部的映射规则都写上。 -->
		<result column="gender" property="gender"></result>
		<result column="email" property="email"></result>
	</resultMap>
	<select id="getEmpByDeptId" resultMap="employee">
		select id,last_name,email,gender from tbl_employee where dept_id = #{id}
	</select>
</mapper>

测试代码：

@Test
public void collectionStepTest(){
    // 获取 SqlSession 的实例 。SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。通过 SqlSession 实例来直接执行已映射的 SQL 语句
    SqlSession sqlSession = null;
    try {
        sqlSession = sessionFactory.openSession();
        // 通过获取接口代理对象来执行sql语句
        DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);
        Department department = mapper.getByIdWithEmpStep(1);
        System.out.println(department);
    } finally {
        // 资源关闭，放在finally中确保一定会执行
        sqlSession.close();
    }
}

测试结果：

DEBUG 05-08 23:43:04,330 ==>  Preparing: SELECT d.id, d.dept_name FROM tbl_department d WHERE d.id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,428 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,507 <==      Total: 1  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,509 ==>  Preparing: select id,last_name,email,gender from tbl_employee where dept_id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,509 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,511 <==      Total: 2  (BaseJdbcLogger.java:145) 
Department{id=1, deptName='开发部', employeeList=[Employee{id=2, lastName='BB', gender='0', email='bb@qq.com', department=null}, Employee{id=7, lastName='FF', gender='0', email='ff@qq.com', department=null}]}

 

五、分步查询延迟加载（懒加载）

（1）什么是mybatis的懒加载

        通俗的讲就是按需加载，我们需要什么的时候再去进行什么操作。而且先从单表查询，需要时再从关联表去关联查询，能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。

        在mybatis中，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。

（二）、如何开启懒加载

在mybatis中开启延迟加载只要在全局配置文件中添加以下配置：

<settings>
    <!-- 打开延迟加载的开关 -->
    <setting name="lazyLoadingEnabled" value="true" />
    <!-- 将积极加载改为消息加载即按需加载 -->
    <setting name="aggressiveLazyLoading" value="false" /> 
    <setting name="lazyLoadTriggerMethods" value=""/>
</settings>

mybatis全局配置：https://blog.youkuaiyun.com/qq_37776015/article/details/89892897

或者可以设置association或者collection中的fetchType属性来覆盖全局变量中的设置

• lazy：懒加载
• eager：急加载

 

根据上面的第四章：collection定义关联对象封装结果集的第二节：分步查询的测试代码如果没有开启懒加载的时候的测试结果为：

DEBUG 05-08 23:43:04,330 ==>  Preparing: SELECT d.id, d.dept_name FROM tbl_department d WHERE d.id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,428 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,507 <==      Total: 1  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,509 ==>  Preparing: select id,last_name,email,gender from tbl_employee where dept_id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,509 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:43:04,511 <==      Total: 2  (BaseJdbcLogger.java:145) 
Department{id=1, deptName='开发部', employeeList=[Employee{id=2, lastName='BB', gender='0', email='bb@qq.com', department=null}, Employee{id=7, lastName='FF', gender='0', email='ff@qq.com', department=null}]}

如果开启了懒加载：

DEBUG 05-08 23:53:01,207 ==>  Preparing: SELECT d.id, d.dept_name FROM tbl_department d WHERE d.id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:53:01,278 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-08 23:53:01,362 <==      Total: 1  (BaseJdbcLogger.java:145) 
Department{id=1, deptName='开发部', employeeList=null}

我们会发现没有开启懒加载的时候执行了两条sql语句，而开启了懒加载的只执行了一条sql语句

把测试代码进行修改：

@Test
public void collectionStepTest(){
    // 获取 SqlSession 的实例 。SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。通过 SqlSession 实例来直接执行已映射的 SQL 语句
    SqlSession sqlSession = null;
    try {
        sqlSession = sessionFactory.openSession();
        // 通过获取接口代理对象来执行sql语句
        DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);
        Department department = mapper.getByIdWithEmpStep(1);
        System.out.println(department);
        System.out.println(department.getEmployeeList());
    } finally {
        // 资源关闭，放在finally中确保一定会执行
        sqlSession.close();
    }
}

测试结果：

DEBUG 05-09 00:03:48,057 ==>  Preparing: SELECT d.id, d.dept_name FROM tbl_department d WHERE d.id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:03:48,164 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:03:48,222 <==      Total: 1  (BaseJdbcLogger.java:145) 
Department{id=1, deptName='开发部', employeeList=null}
DEBUG 05-09 00:03:48,225 ==>  Preparing: select id,last_name,email,gender from tbl_employee where dept_id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:03:48,226 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:03:48,227 <==      Total: 2  (BaseJdbcLogger.java:145) 
[Employee{id=2, lastName='BB', gender='0', email='bb@qq.com', department=null}, Employee{id=7, lastName='FF', gender='0', email='ff@qq.com', department=null}]

通过查看测试结果可以发现，先从单表查询，需要时再从关联表去关联查询

 

注意：如果没有在全局配置中设置lazyLoadTriggerMethods属性的话，可能在调用toString方法时，使懒加载失效

 

六、鉴别器

概述：有时一个数据库查询语句会返回很多不同数据类型的结果集。鉴别器用于处理这种情况，还包括类的继承层次结构，其表现相当于Java中的switch语句。

下面情况具体测试代码：

EmployeeMapper.java 接口方法：

Employee getByIdWithDeptDiscriminator(Integer id);

EmployeeMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.EmployeeMapperPlus">
    <!-- =======================鉴别器============================ -->
	<!-- <discriminator javaType=""></discriminator>
		鉴别器：mybatis可以使用discriminator判断某列的值，然后根据某列的值改变封装行为
		封装Employee：
			如果查出的是女生：就把部门信息查询出来，否则不查询；
			如果是男生，把last_name这一列的值赋值给email;
	 -->
	<resultMap type="com.wcg.mybatis.entity.Employee" id="discriminatorEmployee">
		<id column="id" property="id"/>
		<result column="last_name" property="lastName"/>
		<result column="email" property="email"/>
		<result column="gender" property="gender"/>
		<!--
            column：指定判定的列名
            javaType：列值对应的java类型  -->
		<discriminator javaType="string" column="gender">
			<!--女生  resultType:指定封装的结果类型；不能缺少。/resultMap-->
			<case value="0" resultType="com.wcg.mybatis.entity.Employee">
				<association property="dept"
							 select="com.wcg.mybatis.dao.DepartmentMapper.getById"
							 column="dept_id">
				</association>
			</case>
			<!--男生 ;如果是男生，把last_name这一列的值赋值给email; -->
			<case value="1" resultType="com.wcg.mybatis.entity.Employee">
				<id column="id" property="id"/>
				<result column="last_name" property="lastName"/>
				<result column="last_name" property="email"/>
				<result column="gender" property="gender"/>
			</case>
		</discriminator>
	</resultMap>

	<select id="getByIdWithDeptDiscriminator" resultMap="discriminatorEmployee">
		select id,last_name,email,gender, dept_id from tbl_employee where id = #{id}
	</select>

</mapper>

DepartmentMapper.xml sql映射文件：

<?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="com.wcg.mybatis.dao.DepartmentMapper">

	<resultMap id="department" type="com.wcg.mybatis.entity.Department">
		<id column="did" property="id"></id>
		<result column="dept_name" property="deptName"></result>
	</resultMap>

	<select id="getById" resultMap="department">
		SELECT id , dept_name FROM tbl_department WHERE id = #{id}
	</select>
</mapper>

测试代码1：

数据： 2    BB    0    bb@qq.com    1

@Test
public void discriminatorTest(){
    // 获取 SqlSession 的实例 。SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。通过 SqlSession 实例来直接执行已映射的 SQL 语句
    SqlSession sqlSession = null;
    try {
        sqlSession = sessionFactory.openSession();
        // 通过获取接口代理对象来执行sql语句
        EmployeeMapperPlus mapper = sqlSession.getMapper(EmployeeMapperPlus.class);
        Employee employee = mapper.getByIdWithDeptDiscriminator(2);
        System.out.println(employee);
    } finally {
        // 资源关闭，放在finally中确保一定会执行
        sqlSession.close();
    }
}

测试结果1：

DEBUG 05-09 00:28:44,411 ==>  Preparing: select id,last_name,email,gender, dept_id from tbl_employee where id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:28:44,499 ==> Parameters: 2(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:28:44,563 ====>  Preparing: SELECT id , dept_name FROM tbl_department WHERE id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:28:44,565 ====> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:28:44,568 <====      Total: 1  (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:28:44,568 <==      Total: 1  (BaseJdbcLogger.java:145) 
Employee{id=2, lastName='BB', gender='0', email='bb@qq.com', department=Department{id=1, deptName='开发部', employeeList=null}}

 得出结论：该gender为0，走了分支一查询出了部门信息

 

测试代码2：

数据：3    CC    1    cc@qq.com    2

@Test
public void discriminatorTest(){
    // 获取 SqlSession 的实例 。SqlSession 完全包含了面向数据库执行 SQL 命令所需的所有方法。通过 SqlSession 实例来直接执行已映射的 SQL 语句
    SqlSession sqlSession = null;
    try {
        sqlSession = sessionFactory.openSession();
        // 通过获取接口代理对象来执行sql语句
        EmployeeMapperPlus mapper = sqlSession.getMapper(EmployeeMapperPlus.class);
        Employee employee = mapper.getByIdWithDeptDiscriminator(3);
        System.out.println(employee);
    } finally {
        // 资源关闭，放在finally中确保一定会执行
        sqlSession.close();
    }
}

测试结果2：

DEBUG 05-09 00:32:43,805 ==>  Preparing: select id,last_name,email,gender, dept_id from tbl_employee where id = ?   (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:32:43,868 ==> Parameters: 3(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 05-09 00:32:43,890 <==      Total: 1  (BaseJdbcLogger.java:145) 
Employee{id=3, lastName='CC', gender='1', email='CC', department=null}

 得出结论：该gender为1，走了分支二把last_name的值赋给了email