为什么在 Spring Reactor 中使用 WebClient 时 SwitchIfEmpty 不起作用

问：

控制器：

@PostMapping("/login")
public Mono<ResponseEntity<String>> logIn(@RequestBody @Valid Mono<LoginAndPasswordData> user) {
    return authenticationUserPort.logIn(user).flatMap(convertObjectToJsonResponse::convert);
}

服务：

public Mono<Result<IsCorrectCredentials>> logIn(Mono<LoginAndPasswordData> userLoginDataMono) {
    return userLoginDataMono.flatMap(userLoginData -> 
        userRepositoryPort.findUserWithEmail(userLoginData.login())
            .flatMap(userFromDb -> {
                if (hashPasswordPort.checkPassword(userLoginData.password(),
                    userFromDb.password())) {
                        return Mono.just(Result.<IsCorrectCredentials>success(new IsCorrectCredentials(true)));
                    } else {
                        return Mono.just(Result.<IsCorrectCredentials>success(new IsCorrectCredentials(false)));
                    }
                }).switchIfEmpty(
                    Mono.just(Result.IsCorrectCredentials>error(ErrorMessage.USER_NOT_FOUND
                        .getMessage()))))
                .onErrorResume(ex -> Mono.just(
                    Result.IsCorrectCredentials>error(ErrorMessage.RESPONSE_NOT_AVAILABLE
                       .getMessage())));
    }

当我将 Postman 与不存在的用户一起使用时，一切正常。例如;

数据：

{
  "login":"noExistsUser",
  "password":"example"
}

结果：

{
  "ErrorMessage": " User not found "
}

但是当我使用 WebClient 请求时：

@Override
public Mono<ResponseEntity<String>> isCorrectCredentials(Mono<LoginAndPasswordData> loginAndPasswordData) {

    Mono<ResponseEntity<String>> result = WebClient.create().post()
        .uri(uri)
        .contentType(MediaType.APPLICATION_JSON)
        .body(BodyInserters.fromPublisher(loginAndPasswordData, LoginAndPasswordData.class))
        .retrieve()
        .toEntity(String.class)
        .map(responseEntity -> new ResponseEntity<>(responseEntity.getBody(), responseEntity.getHeaders(), responseEntity.getStatusCode()));
    return result;
}

当用户存在时，响应是OK，但是当数据库中不存在用户时，我从webclient得到以下响应：

{
  "timestamp": "2023-10-30T20:56:09.878+00:00",
  "path": "/login",
  "status": 500,
  "error": "Internal Server Error",
  "requestId": "7273059f-30"
}

和内部例外：

400 Bad Request from POST http://localhost:8082/authentication/login

这看起来像是逻辑错误。switchIfEmpty