FeignClient throws instead of returning ResponseEntity with error http status

Question

As I'm using ResponseEntity<T> as return value for my FeignClient method, I was expecting it to return a ResponseEntity with 400 status if it's what the server returns. But instead it throws a FeignException.

How can I get a proper ResponseEntity instead of an Exception from FeignClient ?

Here is my FeignClient:

@FeignClient(value = "uaa", configuration = OauthFeignClient.Conf.class)
public interface OauthFeignClient {

    @RequestMapping(
            value = "/oauth/token",
            method = RequestMethod.POST,
            consumes = MULTIPART_FORM_DATA_VALUE,
            produces = APPLICATION_JSON_VALUE)
    ResponseEntity<OauthTokenResponse> token(Map<String, ?> formParams);

    class Conf {

        @Value("${oauth.client.password}")
        String oauthClientPassword;

        @Bean
        public Encoder feignFormEncoder() {
            return new SpringFormEncoder();
        }

        @Bean
        public Contract feignContract() {
            return new SpringMvcContract();
        }

        @Bean
        public BasicAuthRequestInterceptor basicAuthRequestInterceptor() {
            return new BasicAuthRequestInterceptor("web-client", oauthClientPassword);
        }

    }
}

and here how I use it:

@PostMapping("/login")
public ResponseEntity<LoginTokenPair> getTokens(@RequestBody @Valid LoginRequest userCredentials) {
    Map<String, String> formData = new HashMap<>();

    ResponseEntity<OauthTokenResponse> response = oauthFeignClient.token(formData);

    //code never reached if contacted service returns a 400
    ...
}

Answer 1

So, looking at source code, it seams that only solution is actually using feign.Response as return type for FeignClient methods and hand decoding the body with something like new ObjectMapper().readValue(response.body().asReader(), clazz) (with a guard on 2xx status of course because for error statuses, it's very likely that body is an error description and not a valid payload ;).

This makes possible to extract and forward status, header, body, etc. even if status is not in 2xx range.

Edit: Here is a way to forward status, headers and mapped JSON body (if possible):

public static class JsonFeignResponseHelper {
    private final ObjectMapper json = new ObjectMapper();

    public <T> Optional<T> decode(Response response, Class<T> clazz) {
        if(response.status() >= 200 && response.status() < 300) {
            try {
                return Optional.of(json.readValue(response.body().asReader(), clazz));
            } catch(IOException e) {
                return Optional.empty();
            }
        } else {
            return Optional.empty();
        }
    }

    public <T, U> ResponseEntity<U> toResponseEntity(Response response, Class<T> clazz, Function<? super T, ? extends U> mapper) {
        Optional<U> payload = decode(response, clazz).map(mapper);

        return new ResponseEntity(
                payload.orElse(null),//didn't find a way to feed body with original content if payload is empty
                convertHeaders(response.headers()),
                HttpStatus.valueOf(response.status()));
    }

    public MultiValueMap<String, String>  convertHeaders(Map<String, Collection<String>> responseHeaders) {
        MultiValueMap<String, String> responseEntityHeaders = new LinkedMultiValueMap<>();
        responseHeaders.entrySet().stream().forEach(e -> 
                responseEntityHeaders.put(e.getKey(), new ArrayList<>(e.getValue())));
        return responseEntityHeaders;
    }
}

that can be used as follow:

@PostMapping("/login")
public ResponseEntity<LoginTokenPair> getTokens(@RequestBody @Valid LoginRequest userCredentials) throws IOException {
    Response response = oauthFeignClient.token();

    return feignHelper.toResponseEntity(
            response,
            OauthTokenResponse.class,
            oauthTokenResponse -> new LoginTokenPair(
                    new BearerToken(oauthTokenResponse.access_token, oauthTokenResponse.expires_in),
                    new BearerToken(oauthTokenResponse.refresh_token, refreshTokenValidity)));
}

This saves headers and status code, but error message is lost :/