【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>