MyBatis处理对象映射（一对多，多对一，多对多）

首先创建两张表：

  1. CREATE TABLE `teacher` (
  2.   `id` varchar(20) NOT NULL DEFAULT '',
  3.   `name` varchar(20) NOT NULL DEFAULT '',
  4.   PRIMARY KEY (`id`) USING BTREE
  5. ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
  7. INSERT INTO `test`.`teacher`(`t_id`, `t_name`) VALUES ('01', '张三');
  8. INSERT INTO `test`.`teacher`(`t_id`, `t_name`) VALUES ('02', '李四');
  9. INSERT INTO `test`.`teacher`(`t_id`, `t_name`) VALUES ('03', '王五');
  10. INSERT INTO `test`.`teacher`(`t_id`, `t_name`) VALUES ('04', '李强');
  14. CREATE TABLE `student` (
  15.   `id` varchar(20) NOT NULL DEFAULT '',
  16.   `name` varchar(20) NOT NULL DEFAULT '',
  17.   `birth` varchar(20) NOT NULL DEFAULT '',
  18.   `sex` varchar(10) NOT NULL DEFAULT '',
  19.   `tid` varchar(20) NOT NULL,
  20.   PRIMARY KEY (`id`) USING BTREE,
  21.   KEY `t_id` (`tid`)
  22. ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
  24. INSERT INTO `test`.`student`(`s_id`, `s_name`, `s_birth`, `s_sex`, `t_id`) VALUES ('01', '赵雷', '1990-01-01', '男', '01');
  25. INSERT INTO `test`.`student`(`s_id`, `s_name`, `s_birth`, `s_sex`, `t_id`) VALUES ('02', '钱电', '1990-12-21', '男', '02');
  26. INSERT INTO `test`.`student`(`s_id`, `s_name`, `s_birth`, `s_sex`, `t_id`) VALUES ('03', '孙风', '1990-05-20', '男', '03');
  27. INSERT INTO `test`.`student`(`s_id`, `s_name`, `s_birth`, `s_sex`, `t_id`) VALUES ('04', '李云', '1990-08-06', '男', '04');
  28. INSERT INTO `test`.`student`(`s_id`, `s_name`, `s_birth`, `s_sex`, `t_id`) VALUES ('05', '周梅', '1991-12-01', '女', '02');
  29. INSERT INTO `test`.`student`(`s_id`, `s_name`, `s_birth`, `s_sex`, `t_id`) VALUES ('06', '吴兰', '1992-03-01', '女', '04');
  30. INSERT INTO `test`.`student`(`s_id`, `s_name`, `s_birth`, `s_sex`, `t_id`) VALUES ('07', '郑竹', '1989-07-01', '女', '03');
  31. INSERT INTO `test`.`student`(`s_id`, `s_name`, `s_birth`, `s_sex`, `t_id`) VALUES ('08', '王菊', '1990-01-20', '女', '01');

配置文件：

  1. spring.datasource.driver-class-name=com.mysql.jdbc.Driver
  2. spring.datasource.url=jdbc:mysql://localhost:3306/test
  3. spring.datasource.username=root
  4. spring.datasource.password=123456
  6. #mybatis
  7. mybatis.mapper-locations=classpath*:com/example/spring_mybatis/mapper/*.xml
  8. mybatis.type-aliases-package=com.example.spring_mybatis.pojo

多对一（多个学生拥有一个老师）：

Student实体类：

  1. package com.example.spring_mybatis.pojo;
  3. import java.io.Serializable;
  4. import lombok.Data;
  6. @Data
  7. public class Student implements Serializable {
  8.     private String id;
  10.     private String name;
  12.     private String birth;
  14.     private String sex;
  16.     private String tid;
  18.     private Teacher teacher;
  20.     private static final long serialVersionUID = 1L;
  21. }

Teacher实体类：

  1. package com.example.spring_mybatis.pojo;
  3. import java.io.Serializable;
  4. import lombok.Data;
  6. @Data
  7. public class Teacher implements Serializable {
  8.     private String id;
  10.     private String name;
  12.     private static final long serialVersionUID = 1L;
  13. }

StudentMapper.xml

  1. <?xml version="1.0" encoding="UTF-8"?>
  2. <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
  3. <mapper namespace="com.example.spring_mybatis.mapper.StudentMapper">
  4.   <!--子查询-->
  5. <!--  <select id="getStudent" resultMap="StudentTeacher">
  6.     select id,name,tid from student
  7.   </select>
  9.   <resultMap id="StudentTeacher" type="com.example.spring_mybatis.pojo.Student">
  10.     <result property="id" column="id"/>
  11.     <result property="name" column="name"/>
  12.     <result property="tid" column="tid"/>
  13.     &lt;!&ndash;复杂的属性单独处理
  14.     &ndash;&gt;
  15.     <association property="teacher" column="tid" javaType="com.example.spring_mybatis.pojo.Teacher" select="getTeacher"/>
  16.   </resultMap>
  18.   <select id="getTeacher" resultType="com.example.spring_mybatis.pojo.Teacher">
  19.     select name from teacher where id = #{tid}
  20.   </select>-->
  22.   <!--按照结果嵌套处理，连表查询，推荐使用这种-->
  23.   <select id="getStudent" resultMap="StudentTeacher">
  24.     select s.id sid,s.name sname,t.name tname
  25.     from student s,teacher t
  26.     where s.tid = t.id;
  27.   </select>
  29.   <resultMap id="StudentTeacher" type="Student">
  30.     <result property="id" column="sid"/>
  31.     <result property="name" column="sname"/>
  32.     <association property="teacher" javaType="Teacher">
  33.       <result property="name" column="tname"/>
  34.     </association>
  35.   </resultMap>
  36. </mapper>

测试代码:

  1. package com.example.spring_mybatis;
  3. import com.example.spring_mybatis.mapper.StudentMapper;
  4. import com.example.spring_mybatis.mapper.TeacherMapper;
  5. import com.example.spring_mybatis.pojo.Student;
  6. import org.junit.jupiter.api.Test;
  8. import org.springframework.beans.factory.annotation.Autowired;
  9. import org.springframework.boot.test.context.SpringBootTest;
  11. import java.util.List;
  13. @SpringBootTest
  14. class SpringMybatisApplicationTests {
  15.     @Autowired
  16.     private StudentMapper studentMapper;
  17.     @Autowired
  18.     private TeacherMapper teacherMapper;
  20.     @Test
  21.     void contextLoads() {
  22.         List<Student> studentList = studentMapper.getStudent();
  23.         for (Student student : studentList) {
  24.             System.out.println(student);
  25.         }
  26.     }
  28. }

查询结果：
1.png

一对多（一个老师拥有多个学生）：

首先给老师的实体类修改:

  1. package com.example.spring_mybatis.pojo;
  3. import java.io.Serializable;
  4. import java.util.List;
  6. import lombok.Data;
  8. @Data
  9. public class Teacher implements Serializable {
  10.     private String id;
  12.     private String name;
  14.     private List<Student> students;
  16.     private static final long serialVersionUID = 1L;
  17. }

TeacherMapper.xml

  1. <?xml version="1.0" encoding="UTF-8"?>
  2. <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
  3. <mapper namespace="com.example.spring_mybatis.mapper.TeacherMapper">
  4.   <!--连表查询-->
  5. <!--  <select id="getTeacher" resultMap="TeacherStudent" parameterType="String">
  6.     select s.id sid,s.name sname,t.id tid,t.name tname
  7.     from teacher t,student s
  8.     where t.id = s.tid
  9.     and t.id = #{id};
  10.   </select>
  11.   &lt;!&ndash;按结果嵌套查询&ndash;&gt;
  12.   <resultMap id="TeacherStudent" type="Teacher">
  13.     <result property="id" column="tid"/>
  14.     <result property="name" column="tname"/>
  15.     &lt;!&ndash;此处要用集合
  16.       javaType,指定属性的类型
  17.       ofType,获取集合中的泛型
  18.     &ndash;&gt;
  19.     <collection property="students" ofType="Student">
  20.       <result property="id" column="sid"/>
  21.       <result property="name" column="sname"/>
  22.     </collection>
  23.   </resultMap>--> 
  24.   <!--子查询-->
  25.   <select id="getTeacher" resultMap="TeacherStudent">
  26.     select * from teacher where id = #{id};
  27.   </select>
  29.   <resultMap id="TeacherStudent" type="Teacher">
  30.     <collection property="students" javaType="ArrayList" ofType="Student" select="getStudent" column="id"/>
  31.   </resultMap>
  33.   <select id="getStudent" resultType="Student">
  34.     select * from student where tid = #{tid};
  35.   </select>
  37. </mapper>

测试类：

  1. package com.example.spring_mybatis;
  3. import com.example.spring_mybatis.mapper.StudentMapper;
  4. import com.example.spring_mybatis.mapper.TeacherMapper;
  5. import com.example.spring_mybatis.pojo.Student;
  6. import com.example.spring_mybatis.pojo.Teacher;
  7. import org.junit.jupiter.api.Test;
  9. import org.springframework.beans.factory.annotation.Autowired;
  10. import org.springframework.boot.test.context.SpringBootTest;
  12. import java.util.List;
  14. @SpringBootTest
  15. class SpringMybatisApplicationTests {
  16.     @Autowired
  17.     private TeacherMapper teacherMapper;
  19.     @Test
  20.     void contextLoads() {
  21.         Teacher teacher = teacherMapper.getTeacher("01");
  22.         System.out.println(teacher);
  23.     }
  25. }

输出结果：
2.png

注意点：

1.JavaType和ofType都是用来指定对象类型的，但是JavaType是用来指定pojo中属性的类型，而ofType指定的是映射到list集合属性中pojo的类型。