如内容所示的Java注解什么意思？

Question

我在 spring-security-jwt-guide 看到一个 看不懂的写法，其中AuthController有如下代码：

/**
 * @author shuang.kou
 * @description 认证授权
 **/
@RestController
@RequestMapping("/auth")
@RequiredArgsConstructor(onConstructor = @__(@Autowired)) //此处@__没看懂
@Api(tags = "认证")
public class AuthController {

    private final AuthService authService;

    @PostMapping("/login")
    @ApiOperation("登录")
    public ResponseEntity<Void> login(@RequestBody LoginRequest loginRequest) {
        String token = authService.createToken(loginRequest);
        HttpHeaders httpHeaders = new HttpHeaders();
        httpHeaders.set(SecurityConstants.TOKEN_HEADER, token);
        return new ResponseEntity<>(httpHeaders, HttpStatus.OK);
    }

    @PostMapping("/logout")
    @ApiOperation("退出")
    public ResponseEntity<Void> logout() {
        authService.removeToken();
        return new ResponseEntity<>(HttpStatus.OK);
    }
}

请问：

@RequiredArgsConstructor(onConstructor = @__(@Autowired))

中的 @__ 是什么意思？

Answer 1

lombok doc

The reason of the weird syntax is to make this feature work in javac 7 compilers; the @__ type is an annotation reference to the annotation type __ (double underscore) which doesn't actually exist; this makes javac 7 delay aborting the compilation process due to an error because it is possible an annotation processor will later create the __ type. Instead, lombok applies the annotations and removes the references so that the error will never actually occur. The point is: The __ type must not exist, otherwise the feature does not work. In the rare case that the __ type does exist (and is imported or in the package), you can simply add more underscores. Technically any non-existent type would work, but to maintain consistency and readability and catch erroneous use, lombok considers it an error if the 'wrapper' annotation is anything but a series of underscores.

In javac8, the above feature should work but due to a bug in javac8 it does not. However, starting in javac8, if the parameter name does not exist in the annotation type, compilation proceeds to a phase where lombok can fix it.

To reiterate: This feature can disappear at any time; if you use this feature, be prepared to adjust your code when we find a nicer way of implementing this feature, or, if a future version of javac forces us to remove this feature entirely with no alternative.

The onX parameter is not legal on any type-wide variant. For example, a @Getter annotation on a class does not support onMethod.

Answer 2

1. 含义
   @RequiredArgsConstructor会将类的每一个final字段或者non-null字段生成一个构造方法
2. 用法
   @RequiredArgsConstructor(onConstructor =@_(@Autowired)) 写在类上可以代替@AutoWired注解
   需要注意的是在注入时需要用final定义，或者使用@notnull注解
   如： private final ImgService imgService;