自定义映射resultMap代码
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MyBatis Code

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pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>top.qaqaq.mybatis</groupId>
  <artifactId>MyBatis_demo3</artifactId>
  <version>1.0-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>MyBatis_demo3</name>
  <url>http://maven.apache.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>

    <!-- Mybatis核心 -->
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.5.7</version>
    </dependency>
    <!-- junit测试 -->
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.12</version>
      <scope>test</scope>
    </dependency>
    <!-- MySQL驱动 -->
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>8.0.31</version>
    </dependency>
    <!-- log4j日志 -->
    <dependency>
      <groupId>log4j</groupId>
      <artifactId>log4j</artifactId>
      <version>1.2.17</version>
    </dependency>

  </dependencies>
</project>

resources

mybatis-config.xml

<?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 resource="jdbc.properties"/>

    <!--设置MyBatis的全局配置-->
    <settings>
        <!--将_自动映射为驼峰，emp_name:empName-->
        <setting name="mapUnderscoreToCamelCase" value="true"/>
        <!--开启延迟加载-->
        <setting name="lazyLoadingEnabled" value="true"/>
    </settings>

    <typeAliases>
        <package name="top.qaqaq.mybatis.pojo"/>
    </typeAliases>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <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>

    <mappers>
        <package name="top.qaqaq.mybatis.mapper"/>
    </mappers>
</configuration>

jdbc.properties

jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mybatis
jdbc.username=root
jdbc.password=123456

log4j.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
    <appender name="STDOUT" class="org.apache.log4j.ConsoleAppender">
        <param name="Encoding" value="UTF-8" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%-5p %d{MM-dd HH:mm:ss,SSS}%m (%F:%L) \n" />
        </layout>
    </appender>
    <logger name="java.sql">
        <level value="debug" />
    </logger>
    <logger name="org.apache.ibatis">
        <level value="info" />
    </logger>
    <root>
        <level value="debug" />
        <appender-ref ref="STDOUT" />
    </root>
</log4j:configuration>

resources/mapper

EmpMapper.xml

<?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="top.qaqaq.mybatis.mapper.EmpMapper">

    <!--
        resultMap：设置自定义映射关系
        id：唯一标识，不能重复
        type：设置映射关系中的类型
        子标签：
        id：设置主键的映射关系
        result：设置普通字段的映射关系
        属性：
        property：设置映射关系中的属性名，必须是type属性所设置的实体类类型中的属性名
        column：设置映射关系中的字段名，必须是sql语句查询出的字段名

    -->
    <resultMap id="empResultMap" type="Emp">
        <id property="eid" column="eid"></id>
        <result property="empName" column="emp_name"></result>
        <result property="age" column="age"></result>
        <result property="sex" column="sex"></result>
        <result property="email" column="email"></result>
    </resultMap>

    <!--List<Emp> getAllEmp();-->
    <select id="getAllEmp" resultMap="empResultMap">
        select * from t_emp
    </select>

    <!--处理多对一映射关系方式一：级联属性赋值-->
    <resultMap id="empAndDeptResultMapOne" type="Emp">
        <id property="eid" column="eid"></id>
        <result property="empName" column="emp_name"></result>
        <result property="age" column="age"></result>
        <result property="sex" column="sex"></result>
        <result property="email" column="email"></result>
        <result property="dept.did" column="did"></result>
        <result property="dept.deptName" column="dept_name"></result>
    </resultMap>

    <!--处理多对一映射关系方式二：级联属性赋值-->
    <resultMap id="empAndDeptResultMapTwo" type="Emp">
        <id property="eid" column="eid"></id>
        <result property="empName" column="emp_name"></result>
        <result property="age" column="age"></result>
        <result property="sex" column="sex"></result>
        <result property="email" column="email"></result>
        <!--
            association:处理多对一的映射关系
            property:需要处理多对一映射关系的属性名
            javaType:该属性的类型
        -->
        <association property="dept" javaType="Dept">
            <id property="did" column="did"></id>
            <result property="deptName" column="dept_name"></result>
        </association>
    </resultMap>

    <!--Emp getEmpAndDept(@Param("eid") Integer eid);-->
    <select id="getEmpAndDept" resultMap="empAndDeptResultMapTwo">
        select * from t_emp left join t_dept on t_emp.did = t_dept.did where t_emp.eid = #{eid}
    </select>

    <resultMap id="empAndDeptByStepResultMap" type="Emp">
        <id property="eid" column="eid"></id>
        <result property="empName" column="emp_name"></result>
        <result property="age" column="age"></result>
        <result property="sex" column="sex"></result>
        <result property="email" column="email"></result>
        <!--
            select:设置分步查询的sql的唯一标识（namespace.SQLId或mapper接口的全类名，方法名）
            column:设置分步查询的条件
            fetchType:当开启了全局的延迟加载以后，可通过此属性手动控制延迟加载的效果
            fetchType="lazy|eager":lazy表示延迟加载，eager表示立即加载
        -->
        <association property="dept"
                     select="top.qaqaq.mybatis.mapper.DeptMapper.getEmpAndDeptByStepTwo"
                     column="did"
                     fetchType="eager"></association>
    </resultMap>
    <!--Emp getEmpAndDeptByStepOne(@Param("eid") Integer eid);-->
    <select id="getEmpAndDeptByStepOne" resultMap="empAndDeptByStepResultMap">
        select * from t_emp where eid = #{eid}
    </select>

    <select id="getAllEmpOld" resultType="Emp">
        <!--select eid, emp_name empName, age, sex, email from t_emp-->
        select * from t_emp
    </select>

    <!--List<Emp> getDeptAndEmpByStepTwo(@Param("did") Integer did);-->
    <select id="getDeptAndEmpByStepTwo" resultType="Emp">
        select * from t_emp where did = #{did}
    </select>

</mapper>

DeptMapper.xml

<?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="top.qaqaq.mybatis.mapper.DeptMapper">

    <!--Dept getEmpAndDeptByStepTwo(@Param("did") Integer did);-->
    <select id="getEmpAndDeptByStepTwo" resultType="Dept">
        select * from t_dept where did = #{did}
    </select>

    <resultMap id="deptAndEmpResultMap" type="Dept">
        <id property="did" column="did"></id>
        <result property="deptName" column="dept_name"></result>
        <!--
            collection：处理一对多的映射关系
            ofType：表示该属性所对应的集合中存储数据的类型
        -->
        <collection property="emps" ofType="Emp">
            <id property="eid" column="eid"></id>
            <result property="empName" column="emp_name"></result>
            <result property="age" column="age"></result>
            <result property="sex" column="sex"></result>
            <result property="email" column="email"></result>
        </collection>
    </resultMap>
    <!--Dept getDeptAndEmp(@Param("did") Integer did);-->
    <select id="getDeptAndEmp" resultMap="deptAndEmpResultMap">
        select * from t_dept left join t_emp on t_dept.did = t_emp.did where t_dept.did = #{did}
    </select>

    <resultMap id="deptAndEmpByStepResultMap" type="Dept">
        <id property="did" column="did"></id>
        <result property="deptName" column="dept_name"></result>
        <!--
            select:设置分步查询的sql的唯一标识（namespace.SQLId或mapper接口的全类名，方法名）
            column:设置分步查询的条件
            fetchType:当开启了全局的延迟加载以后，可通过此属性手动控制延迟加载的效果
            fetchType="lazy|eager":lazy表示延迟加载，eager表示立即加载
        -->
        <collection property="emps"
                    select="top.qaqaq.mybatis.mapper.EmpMapper.getDeptAndEmpByStepTwo"
                    column="did"
                    fetchType="eager"></collection>
    </resultMap>
    <!--Dept getDeptAndEmpByStepOne(@Param("did") Integer did);-->
    <select id="getDeptAndEmpByStepOne" resultMap="deptAndEmpByStepResultMap">
        select * from t_dept where did = #{did}
    </select>

</mapper>

mapper

EmpMapper .java

package top.qaqaq.mybatis.mapper;

import org.apache.ibatis.annotations.Param;
import top.qaqaq.mybatis.pojo.Emp;

import java.util.List;

public interface EmpMapper {

    /**
     * 查询所有的员工信息
     */
    List<Emp> getAllEmp();

    /**
     * 查询员工以及员工所对应的部门信息
     */
    Emp getEmpAndDept(@Param("eid") Integer eid);

    /**
     * 通过分步查询查询员工以及员工所对应的部门信息
     * 分步查询第一步：查询员工信息
     */
    Emp getEmpAndDeptByStepOne(@Param("eid") Integer eid);

    /**
     * 通过分步查询查询部门以及部门中所有的员工信息
     * 分步查询第二步：根据did查询员工信息
     */
    List<Emp> getDeptAndEmpByStepTwo(@Param("did") Integer did);
}

DeptMapper .java

package top.qaqaq.mybatis.mapper;

import org.apache.ibatis.annotations.Param;
import top.qaqaq.mybatis.pojo.Dept;

public interface DeptMapper {

    /**
     * 通过分步查询查询员工以及员工所对应的部门信息
     * 分步查询第二步：通过did查询员工所对应的部门
     */
    Dept getEmpAndDeptByStepTwo(@Param("did") Integer did);

    /**
     * 获取部门以及部门中所有的员工信息
     */
    Dept getDeptAndEmp(@Param("did") Integer did);

    /**
     * 通过分步查询查询部门以及部门中所有的员工信息
     * 分步查询第一步：查询部门信息
     */
    Dept getDeptAndEmpByStepOne(@Param("did") Integer did);

}

pojo

Emp .java

package top.qaqaq.mybatis.pojo;

public class Emp {

    private Integer eid;
    private String empName;
    private Integer age;
    private String sex;
    private String email;
    private Dept dept;

    public Emp() {
    }

    public Emp(Integer eid, String empName, Integer age, String sex, String email) {
        this.eid = eid;
        this.empName = empName;
        this.age = age;
        this.sex = sex;
        this.email = email;
    }

    public Integer getEid() {
        return eid;
    }

    public void setEid(Integer eid) {
        this.eid = eid;
    }

    public String getEmpName() {
        return empName;
    }

    public void setEmpName(String empName) {
        this.empName = empName;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public Dept getDept() {
        return dept;
    }

    public void setDept(Dept dept) {
        this.dept = dept;
    }

    @Override
    public String toString() {
        return "Emp{" +
                "eid=" + eid +
                ", empName='" + empName + '\'' +
                ", age=" + age +
                ", sex='" + sex + '\'' +
                ", email='" + email + '\'' +
                ", dept=" + dept +
                '}';
    }

}

Dept .java

package top.qaqaq.mybatis.pojo;

import java.util.List;

public class Dept {

    private Integer did;
    private String deptName;
    private List<Emp> emps;

    public Dept() {
    }

    public Dept(Integer did, String deptName) {
        this.did = did;
        this.deptName = deptName;
    }

    public Integer getDid() {
        return did;
    }

    public void setDid(Integer did) {
        this.did = did;
    }

    public String getDeptName() {
        return deptName;
    }

    public void setDeptName(String deptName) {
        this.deptName = deptName;
    }

    public List<Emp> getEmps() {
        return emps;
    }

    public void setEmps(List<Emp> emps) {
        this.emps = emps;
    }

    @Override
    public String toString() {
        return "Dept{" +
                "did=" + did +
                ", deptName='" + deptName + '\'' +
                ", emps=" + emps +
                '}';
    }
}

utils

SqlSessionUtils .java

package top.qaqaq.mybatis.utils;

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 java.io.IOException;
import java.io.InputStream;

public class SqlSessionUtils {

    public static SqlSession getSqlSession() {
        SqlSession sqlSession = null;
        try {
            InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
            sqlSession = sqlSessionFactory.openSession(true);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        return sqlSession;
    }
}

test

ResultMapTest .java

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import top.qaqaq.mybatis.mapper.DeptMapper;
import top.qaqaq.mybatis.mapper.EmpMapper;
import top.qaqaq.mybatis.pojo.Dept;
import top.qaqaq.mybatis.pojo.Emp;
import top.qaqaq.mybatis.utils.SqlSessionUtils;

import java.util.List;

public class ResultMapTest {

    /**
     * 解决字段名和属性名不一致的情况：
     * a>为字段起别名，保持和属性名的一致
     * b>设置全局配置，将_自动映射为驼峰
     * <setting name="mapUnderscoreToCamelCase" value="true"/>
     * c>通过resultMap设置自定义的映射关系
     *<resultMap id="empResultMap" type="Emp">
     *    <id property="eid" column="eid"></id>
     *    <result property="empName" column="emp_name"></result>
     *    <result property="age" column="age"></result>
     *    <result property="sex" column="sex"></result>
     *    <result property="email" column="email"></result>
     *</resultMap>
     *
     * 处理多对一的映射关系：
     * a>级联属性赋值
     * b>association
     * c>分步查询
     *
     * 处理一对多的映射关系
     * a>collection
     * b>分步查询
     */

    @Test
    public void testGetEmpAndDeptByStep() {
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        Emp emp = mapper.getEmpAndDeptByStepOne(3);
        System.out.println(emp.getEmpName());
        System.out.println("++++++++++++++++++++++++++++++++++");
        System.out.println(emp.getDept());
    }

    @Test
    public void testGetEmpAndDept() {
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        Emp emp = mapper.getEmpAndDept(3);
        System.out.println(emp);
    }

    @Test
    public void testGetAllEmp() {
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
        List<Emp> list = mapper.getAllEmp();
        list.forEach(emp -> System.out.println(emp));
    }

    @Test
    public void testGetDeptAndEmp() {
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);
        Dept dept = mapper.getDeptAndEmp(1);
        System.out.println(dept);
    }

    @Test
    public void testGetDeptAndEmpByStep() {
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);
        Dept dept = mapper.getDeptAndEmpByStepOne(1);
        System.out.println(dept.getDeptName());
    }
}
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