第8讲_MyBatis_实体关系映射

MyBatis既然是一个ORM框架，则它也有像Hibernate那样的一对多，多对多，多对一的实体关系映射功能。下面我们就来介绍一下如何使用MyBatis的实体关系映射
1.MyBatis实体关系映射，对于我个人来讲常用的有下面两种

• 多对一：在子表的映射文件中添加association
• 一对多：在父表的映射文件中添加collection
  2.MyBatis中多对一的案例
• 先创建两张表

CREATE TABLE `student` (
  `sid` int(11) default NULL,
  `sname` varchar(10) default NULL,
  `t_id` int(11) default NULL
) ;

CREATE TABLE `teacher` (
  `t_id` int(11) NOT NULL,
  `t_name` varchar(20) default NULL,
  PRIMARY KEY  (`t_id`)
) ;

• 创建实体类

package com.gxa.pojo;

public class Teacher {
    private intt_id;
    private String t_name;
    public intgetT_id() {
        return t_id;
    }
    public void setT_id(intt_id) {
        this.t_id = t_id;
    }
    public String getT_name() {
        return t_name;
    }
    public void setT_name(String t_name) {
        this.t_name = t_name;
    }
}

package com.gxa.pojo;

public class Student {
    private intsid;
    private String sname;
    private Teacher teacher;
    public intgetSid() {
        return sid;
    }
    public void setSid(intsid) {
        this.sid = sid;
    }
    public String getSname() {
        return sname;
    }
    public void setSname(String sname) {
        this.sname = sname;
    }
    public Teacher getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
}

• 创建StudentMapper.xml文件，在此XML文件中配置多对一的关系

<?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.gxa.mapper.StudentMapper">
    <resultMap type="com.gxa.pojo.Student" id="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname"/>
        <association property="teacher" javaType="com.gxa.pojo.Teacher">
            <id property="t_id" column="t_id"/>
            <result property="t_name" column="t_name"/>
        </association>
    </resultMap>
    
    <select id="getStudent" resultMap="Student">
        select * from student a, teacher b where a.t_id = b.t_id and sid = 123
    </select>
</mapper>

• 完成多对一关系的代码测试

package com.gxa.test;

import java.io.IOException;
import java.io.Reader;

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.Test;

import com.gxa.pojo.Student;

public class Test03 {
    private static SqlSessionFactorysqlSessionFactory;
    private static Reader reader;
    
    static {
        try {
            reader = Resources.getResourceAsReader("config.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    
    @Test
    public void m01() {
        SqlSessionsqlSession = sqlSessionFactory.openSession();
        String sql = "com.gxa.mapper.StudentMapper.getStudent";
        Student student = sqlSession.selectOne(sql);
        System.out.println(student.getSname() + " ===  " + student.getTeacher().getT_name());
        sqlSession.close();
    }
}

3.MyBatis一对多的案例

• 修改Student和Teacher这两个实体类

package com.gxa.pojo;

public class Student {
    private intsid;
    private String sname;
    private Teacher teacher;
    public intgetSid() {
        return sid;
    }
    public void setSid(intsid) {
        this.sid = sid;
    }
    public String getSname() {
        return sname;
    }
    public void setSname(String sname) {
        this.sname = sname;
    }
    public Teacher getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
}

package com.gxa.pojo;

import java.util.List;

public class Teacher {
    private intt_id;
    private String t_name;
    private List<Student> student;
    public List<Student>getStudent() {
        return student;
    }
    public void setStudent(List<Student> student) {
        this.student = student;
    }
    public intgetT_id() {
        return t_id;
    }
    public void setT_id(intt_id) {
        this.t_id = t_id;
    }
    public String getT_name() {
        return t_name;
    }
    public void setT_name(String t_name) {
        this.t_name = t_name;
    }
}

• 创建TeacherMapper的映射文件，在此文件的<resultMap>标签中加入<collection>

<?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.gxa.mapper.TeacherMapper">
    <resultMap type="com.gxa.pojo.Teacher" id="Teacher">
        <id property="t_id" column="t_id"/>
        <result property="t_name" column="t_name"/>
        <collection property="student" ofType="com.gxa.pojo.Student">
            <id property="sid" column="sid"/>
            <result property="sname" column="sname"/>
        </collection>
    </resultMap>
    
    <select id="getTeacher" resultMap="Teacher">
        select * from teacher a,student b where a.t_id = b.t_id and a.t_id = 1
    </select>
</mapper>

• 测试

package com.gxa.test;

import java.io.IOException;
import java.io.Reader;
import java.util.List;

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.Test;

import com.gxa.pojo.Student;
import com.gxa.pojo.Teacher;

public class Test03 {
    private static SqlSessionFactorysqlSessionFactory;
    private static Reader reader;
    
    static {
        try {
            reader = Resources.getResourceAsReader("config.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    
    @Test
    public void m01() {
        SqlSessionsqlSession = sqlSessionFactory.openSession();
        String sql = "com.gxa.mapper.StudentMapper.getStudent";
        Student student = sqlSession.selectOne(sql);
        System.out.println(student.getSname() + " ===  " + student.getTeacher().getT_name());
        sqlSession.close();
    }
    
    @Test
    public void m02() {
        SqlSessionsqlSession = sqlSessionFactory.openSession();
        String sql = "com.gxa.mapper.TeacherMapper.getTeacher";
        Teacher teacher = sqlSession.selectOne(sql);
        List<Student> student = teacher.getStudent();
        for (Student s : student) {
            System.out.println(teacher.getT_name() + "====" + s.getSname());
        }
        sqlSession.close();
    }
}

转载于:https://www.jianshu.com/p/94db8864c2fc