MyBatis resultmap结果映射

创建数据库和实体类

首先创建数据库student和teacher，并且每个学生对应一个老师，一个老师可以对应多个学生，数据库如下图：

 

 创建实体类Student

package com.loubin.pojo;

public class Student {
    private int id;
    private String name;

    Teacher teacher;

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", teacher=" + teacher +
                '}';
    }

    public Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

 

创建实体类Teacher

package com.loubin.pojo;

import java.util.List;

public class Teacher {
    private int id;
    private String name;

    List<Student> students;

    public List<Student> getStudents() {
        return students;
    }

    @Override
    public String toString() {
        return "Teacher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", students=" + students +
                '}';
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Teacher(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

 

一对多查询

Student类中除了基本类型id和name之外，还有一个自定义类teacher，所以在进行查询的时候，需要对查询到的结果进行映射处理，将查到的字段映射为对象teacher

在接口StudenMapper中定义getStudentById的方法，如下所示

package com.loubin.dao;import com.loubin.pojo.Student;
import org.apache.ibatis.annotations.Param;public interface StudentMapper {Student getStudentById(int id);
}

对应的映射文件StudentMapper.xm如下所示，其中使用了resultMap标签对结果进行映射，由于student对象中的属性teacher是类Teacher的对象，所以需要使用association标签进行映射

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.loubin.dao.StudentMapper"><resultMap id="studentMap" type="com.loubin.pojo.Student"><result column="sid" property="id"/><result column="sname" property="name"/><association property="teacher" javaType="teacher"><result property="id" column="tid"/><result property="name" column="tname"/></association></resultMap><select id="getStudentById"  resultMap="studentMap">select s.id  sid, s.name  sname, t.id tid, t.name tname from student as s, teacher as t WHERE s.tid = t.id and s.id = #{id};</select>
</mapper>

 

查询得到的结果如下

 

多对一查询

在类Teacher中有属性students，该属性是类Student的集合，所以需要使用resultMap中的collection标签进行映射，如下所示，创建接口TeacherMapper

package com.loubin.dao;import com.loubin.pojo.Teacher;
import org.apache.ibatis.annotations.Param;public interface TeacherMapper {Teacher getTeacherById(@Param("id") int id);
}

对应的映射文件TeacherMapper.xml如下所示

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.loubin.dao.TeacherMapper"><resultMap id="teacherMap" type="teacher"><result column="tid" property="id"/><result column="tname" property="name"/><collection property="students" ofType="student"><result column="sid" property="id"/><result column="sname" property="name"/></collection></resultMap><select id="getTeacherById"  resultMap="teacherMap">select s.id  sid, s.name  sname,  t.id tid, t.name tname from student as s, teacher as t WHERE s.tid = t.id and t.id = #{id};</select>
</mapper>

查询得到的结果如下