三种方式主动，一种被动

//第一种
  @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();
//第二种
    @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();

//第三种
<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>

一：三种方式主动

1. @ConstructorArgs生成的ResultMap

参见mybatis源码

 private String parseResultMap(Method method) {
    //获取方法的放回类型
    Class<?> returnType = getReturnType(method);
    //获取方法上的@Arg注解数组 参见@ConstructorArgs注解
    Arg[] args = method.getAnnotationsByType(Arg.class);
    //获取方法上的@Result注解数组 参见@Results
    Result[] results = method.getAnnotationsByType(Result.class);
    //获取方法上的@TypeDiscriminator注解 参见@TypeDiscriminator
    TypeDiscriminator typeDiscriminator = method.getAnnotation(TypeDiscriminator.class);
    //生成resultMap的唯一标识
    String resultMapId = generateResultMapName(method);
    //生成ResultMap
    applyResultMap(resultMapId, returnType, args, results, typeDiscriminator);
    return resultMapId;
  }

generateResultMapName方法中生成默认的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名称是 type.getName() + "." + method.getName() + suffix，其实就是方法的全限定名加上参数类型的getSimpleName()简单类名加-隔开
这里上文 @ConstructorArgs生成的ResultMap的id就是com.mybatis.demo.mapper.UserMapper.selectAll1-void

2. @Results生成的ResultMap

调用的也是generateResultMapName方法中生成默认的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;
  }

可以得出在@Results注解的Id属性缺省ResultMap的id是与 @ConstructorArgs一致的，是com.mybatis.demo.mapper.UserMapper.selectAll2-void，如果存在@Results注解的Id属性就是type.getName() + "." + results.id();本例的ResultMap的id就会是com.mybatis.demo.mapper.UserMapper.selectAll2.${id}

注：${id}为@Results注解的Id属性设置的值

3. <ResultMap>标签生成的ResultMap

第三种比较常见 ResultMap的id就是标签设置的

<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>

二：一种被动

4 . @AutomapConstructor和无任何ResultMap指定

在构造函数中@AutomapConstructor

     @AutomapConstructor
    public User(Integer id, String userName, String password) {
        this.id = id;
        this.userName = userName;
        this.password = password;
    }

将会在代码没有ResultMap时，返回的类型为构造函数类或者构造函数类集合的时候被动生成ResultMap
无任何ResultMap指定：不写ResultMap系统也会自动生成一个该方法的ResultMap，id与@Results上文缺省一样。

5. 验证

5. 1验证 @ConstructorArgs

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

单元测试

结果

通过

5. 2验证 @Results

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

单元测试

结果

通过

5. 3验证 <ResultMap> 标签

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

单元测试

结果

通过

5. 4验证 @AutomapConstructor无任何ResultMap指定

  @Select("select id, userName,password from user")
    List<User> selectAll1();

单元测试 随便打出有啥ResultMap

@RunWith(SpringRunner.class)
@SpringBootTest(classes = ResultMapApplication.class)
public  class UserMapperTest {
    @Autowired
    private UserMapper userMapper;
    @Autowired
    private SqlSession sqlSession;
    @org.junit.Test
    public void selectAll1() {
        System.out.println(userMapper.selectAll1());
        sqlSession.getConfiguration().getResultMapNames().forEach(System.out::println);
    }
}

缺省selectALL1方法缺省ResultMap情况下

系统自动为他生成了ResultMap

无任何ResultMap指定（ 无@AtomapConstructor注解同样也会生成）

去掉 @AutomapConstructor

    public User(Integer id, String userName, String password) {
        this.id = id;
        this.userName = userName;
        this.password = password;
    }

同样系统生成默认ResultMap

方法加上ResultMapMap指定

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

注:com.mybatis.demo.mapper.UserMapper.commonResultMap为xml文件中生成的ResultMap
结果就不生产默认的ResultMap

可以见得这是一种被动的ResultMap生成方式