ResultMap

ResultMap:解决属性名和字段不一致的问题

sql

在原来的user表用加入age字段

-- 在user表中添加age新字段
alter table user add column age int(3) null;
-- 更新id为1的数据年龄为18
update user set test.user.age = 18 where id = 1;

User.java

@Data
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class User {
    private int id;  //id
    private String name;   //姓名
    private String pwd;   //密码

    private int userAge;//和数据库字段不同
}

UserTest.java

@Test
public void selectUserByIdTest(){
    SqlSession session = MyBatisUtil.getSession();
    UserMapper mapper = session.getMapper(UserMapper.class);
    System.out.println(mapper.selectUserById(1));
    session.close();
}

查询userAge为0

• select * from user where id = #{id} 可以看做 select id,name,pwd from user where id = #{id}
• mybatis会根据这些查询的列名(会将列名转化为小写,数据库不区分大小写) , 去对应的实体类中查找相应列名的set方法设值 , 由于找不到setUserAge() , 所以userAge返回0l ; 【自动映射】

解决方案

方案一：为列名指定别名 , 别名和java实体类的属性名一致(自动映射)

UserMapper.xml

<select id="selectUserById" resultType="com.sin.pojo.User" parameterType="int">
    select id,name,pwd,age as'userAge' from user where id = #{id}
</select>

UserTest.java

@Test
public void selectUserByIdTest(){
    SqlSession session = MyBatisUtil.getSession();
    UserMapper mapper = session.getMapper(UserMapper.class);
    System.out.println(mapper.selectUserById(1));
    session.close();
}

测试结果

方案二：使用结果集映射->ResultMap 【推荐】（手动映射）

UserMapper.xml

<resultMap id="UserMap" type="com.sin.pojo.User">
        <!-- id主键 -->
        <id column="id" property="id"/>
        <!-- property是实体类的属性名，column是数据库表的列民 -->
        <result property="name" column="name"/>
        <result property="pwd" column="pwd"/>
        <result property="userAge" column="age"/>
    </resultMap>
<select id="selectUserById" resultType="com.sin.pojo.User" parameterType="int" resultMap="UserMap">
    select id,name,pwd,age  from user where id = #{id}
</select>

UserTest.java

@Test
public void selectUserByIdTest(){
    SqlSession session = MyBatisUtil.getSession();
    UserMapper mapper = session.getMapper(UserMapper.class);
    System.out.println(mapper.selectUserById(1));
    session.close();
}

测试结果