一对多的理解：

• 一个老师拥有多个学生
• 如果对于老师这边，就是一个一对多的现象，即从一个老师下面拥有一群学生（集合）！

1、实体类编写

package com.study.pojo;

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

package com.study.pojo;

import java.util.List;

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

2、按结果嵌套处理

1. TeacherMapper接口编写方法

   //获取指定老师，及老师下的所有学生
   public Teacher getTeacher(int id);
   

2. 编写接口对应的Mapper配置文件

   <?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.study.mapper.TeacherMapper">
       <select id="getTeacherById" resultMap="TeacherStudent">
           select s.tid tid,t.name tname,s.id sid,s.name sname
           from student s,teacher t
           where tid = #{id}
       </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>
   </mapper>
   

3. 将Mapper文件注册到MyBatis-config文件中

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

4. 测试

   @Test
   public void getTeacherById(){
       SqlSession sqlSession = mybatisUtils.getSqlSession();
       TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);
       Teacher teacher = teacherMapper.getTeacherById(1);
       System.out.println(teacher);
       sqlSession.close();
   }
   

3、按查询嵌套处理

1. TeacherMapper接口编写方法

   public Teacher getTeacher2(int id);
   

2. 编写接口对应的Mapper配置文件

   <?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.study.mapper.TeacherMapper">
       <select id="getTeacherById2" resultMap="TeacherStudent2">
           select id,name from teacher where id = #{id}
       </select>
       <resultMap id="TeacherStudent2" type="Teacher">
           <result property="id" column="id"/>
           <result property="name" column="name"/>
           <collection property="students" column="id" javaType="ArrayList" ofType="Student" select="getStudentListByTeacherId"/>
       </resultMap>
       <select id="getStudentListByTeacherId" resultType="Student">
           select * from student where tid = #{id}
       </select>
   </mapper>
   

3. 将Mapper文件注册到MyBatis-config文件中

4. 测试

   @Test
   public void getTeacherById2(){
       SqlSession sqlSession = mybatisUtils.getSqlSession();
       TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);
       Teacher teacher = teacherMapper.getTeacherById2(1);
       System.out.println(teacher);
       sqlSession.close();
   }
   

9.4、小结

1. 关联-association
2. 集合-collection
3. 所以association是用于一对一和多对一，而collection是用于一对多的关系
4. JavaType和ofType都是用来指定对象类型的
   • JavaType是用来指定pojo中属性的类型
   • ofType指定的是映射到list集合属性中pojo的类型。

注意说明：

1. 保证SQL的可读性，尽量通俗易懂
2. 根据实际要求，尽量编写性能更高的SQL语句
3. 注意属性名和字段不一致的问题
4. 注意一对多和多对一 中：字段和属性对应的问题
5. 尽量使用Log4j，通过日志来查看自己的错误

【狂神说Java】Mybatis最新完整教程IDEA版通俗易懂