作用

• MyBatis 中最重要最強大的元素

• 解決欄位名和屬性名不一致的問題

• ResultMap 的設計思想是，對簡單的語句做到零配置，對於複雜一點的語句，只需要描述語句之間的關係

ResultMap配置詳解

<!--
    id為這個resultMap的標識
    type為對映的型別，一個Java類的全限定名，或一個類型別名
-->
<resultMap id="" type="">
  <!--
    id和result都將一個列的值對映到一個簡單資料型別的屬性或欄位
         column：對應資料庫中的列
         property：對應實體類中的屬性
 
         javaType：一個Java類的全限定名，或一個類型別名，結果集對映返回的結果
         jdbcType：所支援的JDBC型別
         typeHandler：型別處理器
  -->
  <id column="" property="" javaType="" jdbcType="" typeHandler=""></id>
  <result column="" property="" javaType="" jdbcType="" typeHandler=""></result>
  <!--
    association處理“有一個”型別的關係
 
    collection處理“有多個”型別的關係
      column：對應資料庫中的列
      property：對應實體類中的屬性（物件）
      javaType：指定實體類中屬性的型別
      ofType：指定對映到集合中的實體類型別，泛型中的約束型別
      select：用於載入複雜型別屬性的對映語句的id
          會從column屬性指定的列中檢索資料，作為引數傳遞給目標select語句
  -->
  <association column="" property="" javaType="" select=""></association>
 
  <collection column="" property="" javaType="" select=""></collection>
</resultMap>

1、解決資料庫中的欄位和實體類中屬性不一致

• 實體類

  @Data
  @NoArgsConstructor
  @AllArgsConstructor
  public class User {
  ​
    private Integer id;
    private String name;
    private String password;
  }

• 介面類

  User selectById(int id);

• 介面類的實現

  <resultMap id="UserMap" type="user">
    <!--
          id,name屬性和資料庫中的欄位一樣，所有不需要顯式寫出，可省略
         <result column="id" property="id"/>
         <result column="name" property="name"/>
      -->
    <result column="pwd" property="password"/>
  </resultMap>
  ​
  <!--resultMap裡的值為結果集對映<resultMap></resultMap>的id值-->
  <select id="selectById" resultMap="UserMap">
     select * from mybatistest where id = #{id};
  </select>

2、多對一以及一對多問題

• Teacher表

  id：老師id
  name：老師姓名

• Student表

  id：學生id
  name：學生姓名
  tid：學生關聯的老師id

• 多對一：多個學生對應一個老師，查詢學生的資訊以及學生的老師的資訊

  • 1、老師類：Teacher

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

  • 2、老師的介面類：TeacherMapper

    public interface TeacherMapper {
    }

  • 3、老師介面類的實現：TeacherMapper.xml

    <?xml version="1.0" encoding="UTF8" ?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    ​
    <mapper namespace="com.hmx.mapper.TeacherMapper">
    </mapper>

  • 4、學生類：Student

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

  • 5、學生的介面類：StudentMapper

    public interface StudentMapper {
      List<Student> getStudent();
    }

  • 6、學生介面類的實現：StudentMapper.xml

    • 方法一：根據查詢巢狀處理，子查詢

      <?xml version="1.0" encoding="UTF8" ?>
      <!DOCTYPE mapper
          PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
          "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
      ​
      <mapper namespace="com.hmx.mapper.StudentMapper">
        
        <!--查詢所有的學生資訊-->
        <select id="getStudent" resultMap="StudentTeacher">
           select * from student
        </select>
      ​
        <!--
              根據查詢出來的學生的tid屬性，查詢老師的資訊
              tid的值是從resultMap中傳遞過來的
          -->
        <select id="getTeacher" resultType="Teacher">
           select * from teacher where id = #{tid}
        </select>
        
        <resultMap id="StudentTeacher" type="Student">
          <result property="id" column="id"/>
          <result property="name" column="name"/>
          
          <!--從tid列中檢索資料，作為引數傳遞給id為getTeacher的select語句-->
          <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
        </resultMap>
      </mapper>

    • 方法二：根據結果巢狀處理，關聯查詢

      <?xml version="1.0" encoding="UTF8" ?>
      <!DOCTYPE mapper
          PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
          "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
      ​
      <mapper namespace="com.hmx.mapper.StudentMapper">
        
        <select id="getStudent" resultMap="StudentTeacher">
           select s.id as sid,s.`name` as sname,t.`name` tname
           from student s,teacher t
           where s.tid = t.id
        </select>
      ​
        <resultMap id="StudentTeacher" type="Student">
          <result property="id" column="sid"/>
          <result property="name" column="sname"/>
          <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
          </association>
        </resultMap>
      ​
      </mapper>

  • 7、測試

    public static void selectAll() {
      SqlSession sqlSession = MybatisUtils.getSqlSession();
    ​
      StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
      List<Student> studentList = studentMapper.getStudent();
      for (Student student : studentList) {
        System.out.println(student);
       }
    ​
      sqlSession.close();
    }

• 一對多：一個老師有多個學生，查詢指定老師下的學生的資訊，以及該老師的資訊

  • 1、學生類：Student

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

  • 2、學生的介面類：StudentMapper

    public interface StudentMapper {
    }

  • 3、學生介面類的實現類：StudentMapper.xml

    <?xml version="1.0" encoding="UTF8" ?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    ​
    <mapper namespace="com.hmx.mapper.StudentMapper">
    </mapper>

  • 4、老師類：Teacher

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

  • 5、老師的介面類：TeacherMapper

    public Teacher getTeacher(int id);

  • 6、老師介面類的實現類：TeacherMapper.xml

    • 方法一：根據查詢巢狀處理，子查詢

      <?xml version="1.0" encoding="UTF8" ?>
      <!DOCTYPE mapper
          PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
          "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
      <mapper namespace="com.hmx.mapper.TeacherMapper">
      ​
        <!--查詢老師的所有資訊-->
        <select id="getTeacher" resultMap="TeacherStudent">
           select * from teacher where id = #{id}
        </select>
      ​
        <!--根據老師的id查詢學生的所有資訊-->
        <select id="getStudentByTeacherId" resultType="Student">
           select * from student where tid = #{tid}
        </select>
      ​
        <!--把老師裡的id傳給select查詢塊-->
        <resultMap id="TeacherStudent" type="Teacher">
          <collection property="studentList" column="id" javaType="ArrayList" ofType="student" select="getStudentByTeacherId">
          </collection>
        </resultMap>
      ​
      </mapper>

    • 方法二：根據結果巢狀處理，關聯查詢

      <?xml version="1.0" encoding="UTF8" ?>
      <!DOCTYPE mapper
          PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
          "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
      <mapper namespace="com.hmx.mapper.TeacherMapper">
      ​
        <select id="getTeacher" resultMap="TeacherStudent">
           SELECT
               s.id AS sid,
               s.`name` AS sname,
             t.id AS tid,
               t.`name` tname
           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="studentList" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
          </collection>
        </resultMap>
      ​
      </mapper>

  • 7、測試

    public static void testTeacher() {
      SqlSession sqlSession = MybatisUtils.getSqlSession();
    ​
      TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);
      Teacher teacher = teacherMapper.getTeacher(1);
      System.out.println(teacher);
    ​
      sqlSession.close();
    }