三種方式主動，一種被動

//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>

一：三種方式主動

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;
  }

可以得出在@Results注解的Id屬性預設ResultMap的id是與 @ConstructorArgs一致的，是com.mybatis.demo.mapper.UserMapper.selectAll2-void，如果存在@Results注解的Id屬性就是type.getName() + "." + results.id();本例的ResultMap的id就會是com.mybatis.demo.mapper.UserMapper.selectAll2.${id}

注：${id}為@Results注解的Id屬性設定的值

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>

二：一種被動

4 . @AutomapConstructor和無任何ResultMap指定

在建構式中@AutomapConstructor

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

將會在代碼沒有ResultMap時，回傳的型別為建構式類或者建構式類集合的時候被動生成ResultMap
無任何ResultMap指定：不寫ResultMap系統也會自動生成一個該方法的ResultMap，id與@Results上文預設一樣，

5. 驗證

5. 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

5. 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

5. 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

5. 4驗證 @AutomapConstructor無任何ResultMap指定

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

單元測驗 隨便打出有啥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);
    }
}

預設selectALL1方法預設ResultMap情況下

系統自動為他生成了ResultMap

無任何ResultMap指定（ 無@AtomapConstructor注解同樣也會生成）

去掉 @AutomapConstructor

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

同樣系統生成默認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為xml檔案中生成的ResultMap
結果就不生產默認的ResultMap

可以見得這是一種被動的ResultMap生成方式