解决属性名和字段名不一致的问题

1. 问题

在数据库中,密码字段为pwd,而在实体类中为password

package com.wang.pojo;

public class User {
    private int id;
    private String name;
    private String password;

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

    public User() {
    }

    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 String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ",name='" + name + ''' +
                ",password='" + password + ''' +
                '}';
    }
}

测试出现问题

package com.wang.dao;

import com.wang.pojo.User;
import com.wang.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class UserMapperTest {

    //
    @Test
    public void testGetUserById() {
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        User user = mapper.getUserById(1);
        System.out.println(user);

        sqlSession.close();
    }

}

User{id=1,name='张三',password='null'}

select * from mybatis.user where id = #{id};
等同于
select id,name,pwd from mybatis.user where id = #{id};

MyBatis 在设置预处理语句（PreparedStatement）中的参数或从结果集中取出一个值时， 都会用类型处理器将获取到的值以合适的方式转换成 Java 类型。

经过类型处理器处理后,此时,在实体类中找不到pwd的字段

.解决方法:

• 起别名: pwd as password

<?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">
<!--namespace=绑定一个对应的Dao/Mapper接口-->
<mapper namespace="com.wang.dao.UserMapper">

    <select id="getUserById" parameterType="int" resultType="User">
        select id,pwd as password from mybatis.user where id = #{id};
    </select>

</mapper>

2. ResultMap

结果集映射

数据库 id name pwd

实体类 id name password

<?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">
<!--namespace=绑定一个对应的Dao/Mapper接口-->
<mapper namespace="com.wang.dao.UserMapper">
  
	<!--使用resultMap时,此处不要指定resultType,在下面写resultMap时再指定其type属性-->
    <select id="getUserById" parameterType="int" resultMap="UserMap">
        select * from mybatis.user where id = #{id};
    </select>

    <!--结果集映射 resultMap id = 找到要映射的标签(在上面的resultMap中设定的标签)-->
    <resultMap id="UserMap" type="User">
    <!--column 数据库中的字段; property 实体类中的属性-->
        <result column="pwd" property="password"/>
    </resultMap>

</mapper>

（编辑：李大同）

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