【mybatis笔记】 resultType与resultMap的区别

序言：

昨天做一个项目，看到很多刚开始用mybatis的同事对于resultType和resultMap的理解与使用含糊不清，这里我试图用最好理解的说法写一写，欢迎大家勘误。

两者异同：

相同点：resultType和resultMap都是映射结果集到Javabean用的

不同点：

1. resultType属于自动映射到javabean，而resultMap是手动映射到Javabean的，其中简单的映射关系可以使用resultType，复杂映射关系的推荐使用resultMap。
2. 使用resultMap需要先在mapper.xml中定义resultMap。而resultType则无需定义。

下面我举两个正例、两个反例：

需要映射的JavaBean：User

package com.github.hellxz.entity;

/**
 * @Author : Hellxz
 * @Description: 被映射的Javabean，常见的User
 * @Date : 2018/3/9 8:25
 */
public class User {

    private Long userId;
    private String username;
    private String password;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

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

    public Long getUserId() {
        return userId;
    }

    public void setUserId(Long userId) {
        this.userId = userId;
    }
}

UserMapper接口定义：

package com.github.hellxz.dao;

/**
 * @Author : Hellxz
 * @Description: User的dao接口
 * @Date : 2018/3/9 8:44
 */
public interface UserMapper {
    
    User selectByUsername(String username);
}

正例：

【resultType正例】：resultType指向具体类型或别名 ✔

<?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.github.hellxz.dao.UserMapper" >
  <sql id="Base_Column_List" >
    user_id, user_name, password
  </sql>
  <select id="selectByUsername" resultType="com.github.hellxz.entity.User" parameterType="string" >
    select
    <include refid="Base_Column_List" />
    from user
    where user_name = #{username}
  </select>
  
</mapper>

【resultMap正例】：resultMap引用定义好的resultMap的id ✔

<?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.github.hellxz.dao.UserMapper" >
  <resultMap id="BaseResultMap" type="com.github.hellxz.entity.User" >
    <id column="user_id" property="userId" jdbcType="BIGINT" />
    <result column="user_name" property="username" jdbcType="VARCHAR" />
    <result column="password" property="password" jdbcType="VARCHAR" />
  </resultMap>
  <sql id="Base_Column_List" >
    user_id, user_name, password
  </sql>
  <select id="selectByUsername" resultMap="BaseResultMap" parameterType="string" >
    select 
    <include refid="Base_Column_List" />
    from user
    where user_name = #{username}
  </select>

</mapper>

反例：

【resultType反例】：使用resultType去引用定义的resultMap ✘

<?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.github.hellxz.dao.UserMapper" >
  <resultMap id="BaseResultMap" type="com.github.hellxz.entity.User" >
    <id column="user_id" property="userId" jdbcType="BIGINT" />
    <result column="user_name" property="username" jdbcType="VARCHAR" />
    <result column="password" property="password" jdbcType="VARCHAR" />
  </resultMap>
  <sql id="Base_Column_List" >
    user_id, user_name, password
  </sql>
  <select id="selectByUsername" resultType="BaseResultMap" parameterType="string" >
    select 
    <include refid="Base_Column_List" />
    from user
    where user_name = #{username}
  </select>

</mapper>

【resultMap反例】：使用resultType去引用定义的resultMap或者引用没有定义的resultMap ✘

<?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.github.hellxz.dao.UserMapper" >
  <sql id="Base_Column_List" >
    user_id, user_name, password
  </sql>
  <select id="selectByUsername" resultMap="com.github.hellxz.entity.User" parameterType="string" >
    select 
    <include refid="Base_Column_List" />
    from user
    where user_name = #{username}
  </select>

</mapper>