resultMap如何處理復(fù)雜映射問題

更新時(shí)間：2025年04月11日 10:17:55 作者：長不大的大灰狼

這篇文章主要介紹了resultMap如何處理復(fù)雜映射問題,具有很好的參考價(jià)值,希望對大家有所幫助,如有錯(cuò)誤或未考慮完全的地方,望不吝賜教

resultMap復(fù)雜映射問題

association:關(guān)聯(lián)（多對一的情況）
collection: 集合（一對多的情況）
javaType: 用來指定實(shí)體類中屬性的類型。
ofType: 用來指定映射到List或集合中POJO的類型，泛型的約束類型。

Ⅰ 多對一查詢：學(xué)生——老師

數(shù)據(jù)庫表：

CREATE TABLE `teacher` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO teacher(`id`, `name`) VALUES (1, '王老師');

CREATE TABLE `student` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
`tid` INT(10) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fktid` (`tid`),
CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;


INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '小明', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小紅', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小張', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小李', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小王', '1');

(1) 創(chuàng)建實(shí)體類POJO

@Data
public class Student {
    private int id;
    private String name;
    private Teacher teacher;
}

@Data
public class Teacher {
    private int id;
    private String name;
}

(2) 創(chuàng)建學(xué)生實(shí)體類對應(yīng)的接口

public interface StudentMapper {

    //查詢所有學(xué)生的信息
    List<Student> getStudent();
    List<Student> getStudent2();
}

(3) 編寫學(xué)生接口對應(yīng)的Mapper.xml

為了達(dá)到和接口在同一個(gè)包中的效果，在resource文件夾下新建包結(jié)構(gòu)com.glp.dao:

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

<mapper namespace="com.glp.dao.StudentMapper">

<!--按照結(jié)果查詢——聯(lián)表查詢-->
    <select id="getStudent2" resultMap="StudentMap2">
         select s.id sid,s.name sname,t.name tname from student s, teacher t where s.tid=t.id;
    </select>

    <resultMap id="StudentMap2" type="Student">
         <result property="id" column="sid"/>
         <result property="name" column="sname"/>

        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>


    <!--按照查詢嵌套處理——子查詢-->
        <select id="getStudent" resultMap="StudentMap" >
           select * from student;
        </select>

    <resultMap id="StudentMap" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--復(fù)雜屬性：對象association, 集合collection-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
        
        <select id="getTeacher" resultType="Teacher">
            select * from teacher where id = #{id};
        </select>

</mapper>

在多對一查詢中，需要用到teacher這個(gè)表，每個(gè)學(xué)生都對應(yīng)著一個(gè)老師。而property只能處理單個(gè)屬性，像teacher這種復(fù)雜屬性（內(nèi)含多個(gè)屬性）需要進(jìn)行處理。處理復(fù)雜對象要用association 。

方式一：

聯(lián)表查詢（直接查出所有信息，再對結(jié)果進(jìn)行處理）

   <resultMap id="StudentMap2" type="Student">
         <result property="id" column="sid"/>
         <result property="name" column="sname"/>

        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

直接查詢出學(xué)生和老師，然后用association去取老師里面的屬性property。

方式二：

子查詢（先查出學(xué)生信息，再拿著學(xué)生中的tid，去查詢老師的信息）

  <resultMap id="StudentMap" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--復(fù)雜屬性：對象association-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
        
        <select id="getTeacher" resultType="Teacher">
            select * from teacher where id = #{id};
        </select>

在resultMap中引入屬性association，通過javaType指定property="teacher"的類型，javaType="Teacher"。通過select引入子查詢（嵌套查詢）。

這里是拿到學(xué)生中的tid，去查找對應(yīng)的老師。

（4）在核心配置類中引入Mapper

db.properties:數(shù)據(jù)庫連接參數(shù)配置文件

driver = com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?useSSL=true&useUnicode=true&chracterEncoding=utf8
username =root
password =mysql

mybatis.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="db.properties">
        <property name="username" value="root"/>
        <property name="password" value="mysql"/>
    </properties>

    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>

    <typeAliases>
        <typeAlias type="com.glp.POJO.Student" alias="Student"/>
        <typeAlias type="com.glp.POJO.Teacher" alias="Teacher"/>
    </typeAliases>


    <environments default="development">

        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>

    </environments>

    <mappers>
        <mapper class="com.glp.dao.StudentMapper"/>
        <mapper class="com.glp.dao.TeacherMapper"/>
    </mappers>

</configuration>

注意：

要保證接口和Mapper.xml都在同一個(gè)包中。

(5) 測試

public class UserDaoTest {
    @Test
    public void getStudent(){
        SqlSession sqlSession = MyUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> list = mapper.getStudent();

        for (Student stu:list ) {
            System.out.println(stu);
        }
        sqlSession.close();
    }

    @Test
    public void getStudent2(){
        SqlSession sqlSession = MyUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

        List<Student> list = mapper.getStudent2();

        for (Student stu:list ) {
            System.out.println(stu);
        }
        sqlSession.close();
    }
}

Ⅱ 一對多查詢：老師——學(xué)生

（1）實(shí)體類

@Data
public class Student {
    private int id;
    private String name;
    private int tid;
}

@Data
public class Teacher {
    private int id;
    private String name;
    private List<Student> students;
}

（2）接口

package com.glp.dao;

public interface TeacherMapper {

    Teacher getTeacher(@Param("tid") int id);

    Teacher getTeacher2(@Param("tid") int id);
}

（3）接口對應(yīng)的Mapper.xml

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


<mapper namespace="com.glp.dao.TeacherMapper">

  <!--方式一          =======================                  -->
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid, s.name sname, t.name tname, t.id tid from
        student s ,teacher t where s.tid = t.id and t.id = #{tid};
    </select>

    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>


    <!--方式二          =======================                  -->

    <select id="getTeacher2" resultMap="TeacherStudent2">
        select * from teacher where id = #{tid};
         <!--這里的tid和接口中指定的屬性名相同-->
    </select>

    <resultMap id="TeacherStudent2" type="Teacher">
	    <result property="id" column="id"/>
        <result property="name" column="name"/>
           <!--上面的兩個(gè)可以省略-->
        <collection property="students"  column="id" javaType="ArrayList" ofType="Student"  select="getStuById"/>
    </resultMap>

    <select id="getStuById" resultType="Student">
        select * from student where tid=#{tid};
           <!--查詢老師對應(yīng)的學(xué)生，#{tid}-->
    </select>
</mapper>

方式一：

聯(lián)表查詢，需要寫復(fù)雜SQL
collection 用來處理集合，ofType用來指定集合中的約束類型
聯(lián)合查詢時(shí)，查詢出所以結(jié)果，然后再解析結(jié)果中的屬性，將屬性property賦予到collection中。

方式二：

子查詢，需要寫復(fù)雜映射關(guān)系

查詢學(xué)生時(shí)，需要拿著老師的id去查找，column用來給出老師的id。

（4）測試：

package com.glp.dao;
public class UserDaoTest {

    @Test
    public void getTeacher(){
        SqlSession sqlSession = MyUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);

        Teacher teacher = mapper.getTeacher(1);

        System.out.println(teacher);

        sqlSession.close();
    }


    @Test
    public void getTeacher2(){
        SqlSession sqlSession = MyUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);

        Teacher teacher = mapper.getTeacher2(1);

        System.out.println(teacher);

        sqlSession.close();
    }
}