三種方式

//第一種
  @ConstructorArgs({
            @Arg(column = "id", javaType = Integer.class, id = true),
            @Arg(column = "userName", javaType = String.class, id = false),
            @Arg(column = "password", javaType = String.class, id = false)}
    )
    @Select("select id, userName,password from user")
    List<User> selectAll1();
//第二種
    @Results({
            @Result(property = "id", column = "id", id = true),
            @Result(property = "userName", column = "userName", id = false),
            @Result(property = "password", column = "id", id = false),
    })
    @Select("select id, userName,password from user")
    List<User> selectAll2();

//第三種
<mapper namespace="com.mybatis.demo.mapper.UserMapper">
    <resultMap id="commonResultMap" type="com.mybatis.demo.entity.User">
        <result column="id" property="id"></result>
        <result column="userName" property="userName"></result>
        <result column="password" property="password"></result>
    </resultMap>
</mapper>

1. @ConstructorArgs生成的ResultMap

參見mybatis原始碼

 private String parseResultMap(Method method) {
    //獲取方法的放回型別
    Class<?> returnType = getReturnType(method);
    //獲取方法上的@Arg注解陣列 參見@ConstructorArgs注解
    Arg[] args = method.getAnnotationsByType(Arg.class);
    //獲取方法上的@Result注解陣列 參見@Results
    Result[] results = method.getAnnotationsByType(Result.class);
    //獲取方法上的@TypeDiscriminator注解 參見@TypeDiscriminator
    TypeDiscriminator typeDiscriminator = method.getAnnotation(TypeDiscriminator.class);
    //生成resultMap的唯一標識
    String resultMapId = generateResultMapName(method);
    //生成ResultMap
    applyResultMap(resultMapId, returnType, args, results, typeDiscriminator);
    return resultMapId;
  }

generateResultMapName方法中生成默認的ResultMap

 private String generateResultMapName(Method method) {
    Results results = method.getAnnotation(Results.class);
    if (results != null && !results.id().isEmpty()) {
      return type.getName() + "." + results.id();
    }
    StringBuilder suffix = new StringBuilder();
    for (Class<?> c : method.getParameterTypes()) {
      suffix.append("-");
      suffix.append(c.getSimpleName());
    }
    if (suffix.length() < 1) {
      suffix.append("-void");
    }
    return type.getName() + "." + method.getName() + suffix;
  }

可以得知ResultMap名稱是 type.getName() + "." + method.getName() + suffix，其實就是方法的全限定名加上引數型別的getSimpleName()簡單類名加-隔開
這里上文 @ConstructorArgs生成的ResultMap的id就是com.mybatis.demo.mapper.UserMapper.selectAll1-void

2. @Results生成的ResultMap

呼叫的也是generateResultMapName方法中生成默認的ResultMap

 private String generateResultMapName(Method method) {
    Results results = method.getAnnotation(Results.class);
    if (results != null && !results.id().isEmpty()) {
      return type.getName() + "." + results.id();
    }
    StringBuilder suffix = new StringBuilder();
    for (Class<?> c : method.getParameterTypes()) {
      suffix.append("-");
      suffix.append(c.getSimpleName());
    }
    if (suffix.length() < 1) {
      suffix.append("-void");
    }
    return type.getName() + "." + method.getName() + suffix;
  }

所以ResultMap的id是com.mybatis.demo.mapper.UserMapper.selectAll2-void

3. <ResultMap>標簽生成的ResultMap

第三種比較常見 ResultMap的id就是標簽設定的

<mapper namespace="com.mybatis.demo.mapper.UserMapper">
    <resultMap id="commonResultMap" type="com.mybatis.demo.entity.User">
        <result column="id" property="id"></result>
        <result column="userName" property="userName"></result>
        <result column="password" property="password"></result>
    </resultMap>
</mapper>

4. 驗證

4. 1驗證 @ConstructorArgs

	@ResultMap("com.mybatis.demo.mapper.UserMapper.selectAll1-void")
    @Select("select id, userName,password from user")
    List<User> selectAll4();

單元測驗

結果

通過

4. 2驗證 @Results

	@ResultMap("com.mybatis.demo.mapper.UserMapper.selectAll2-void")
    @Select("select id, userName,password from user")
    List<User> selectAll4();

單元測驗

結果

通過

4. 3驗證 <ResultMap> 標簽

	@ResultMap("com.mybatis.demo.mapper.UserMapper.commonResultMap")
    @Select("select id, userName,password from user")
    List<User> selectAll4();

單元測驗

結果

通過