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

By the way, solution I gave before works, but my initial intention is bad idea: an error is an error and should not be handled on nominal flow. Throwing an exception, like Feign does, and handling it with an @ExceptionHandler is a better way to go in Spring MVC world.

So two solutions:

• add an @ExceptionHandler for FeignException
• configure the FeignClient with an ErrorDecoder to translate the error in an Exception your business layer knows about (and already provide @ExceptionHandler for)

I prefer second solution because received error message structure is likely to change from a client to an other, so you can extract finer grained data from those error with a per-client error decoding.

FeignClient with conf (sorry for the noise introduced by feign-form)

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

    @RequestMapping(
            value = "/oauth/token",
            method = RequestMethod.POST,
            consumes = MULTIPART_FORM_DATA_VALUE,
            produces = APPLICATION_JSON_VALUE)
    DefaultOAuth2AccessToken token(Map<String, ?> formParams);

    @Configuration
    class Config {

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

        @Autowired
        private ObjectFactory<HttpMessageConverters> messageConverters;

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

        @Bean
        public Decoder springDecoder() {
            return new ResponseEntityDecoder(new SpringDecoder(messageConverters));
        }

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

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

        @Bean
        public ErrorDecoder uaaErrorDecoder(Decoder decoder) {
            return (methodKey, response) -> {
                try {
                    OAuth2Exception uaaException = (OAuth2Exception) decoder.decode(response, OAuth2Exception.class);
                    return new SroException(
                            uaaException.getHttpErrorCode(),
                            uaaException.getOAuth2ErrorCode(),
                            Arrays.asList(uaaException.getSummary()));

                } catch (Exception e) {
                    return new SroException(
                            response.status(),
                            "Authorization server responded with " + response.status() + " but failed to parse error payload",
                            Arrays.asList(e.getMessage()));
                }
            };
        }
    }
}

Common business exception

public class SroException extends RuntimeException implements Serializable {
    public final int status;

    public final List<String> errors;

    public SroException(final int status, final String message, final Collection<String> errors) {
        super(message);
        this.status = status;
        this.errors = Collections.unmodifiableList(new ArrayList<>(errors));
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof SroException)) return false;
        SroException sroException = (SroException) o;
        return status == sroException.status &&
                Objects.equals(super.getMessage(), sroException.getMessage()) &&
                Objects.equals(errors, sroException.errors);
    }

    @Override
    public int hashCode() {
        return Objects.hash(status, super.getMessage(), errors);
    }
}

Error handler (extracted from a ResponseEntityExceptionHandler extension)

@ExceptionHandler({SroException.class})
public ResponseEntity<Object> handleSroException(SroException ex) {
    return new SroError(ex).toResponse();
}

Error response DTO

@XmlRootElement
public class SroError implements Serializable {
    public final int status;

    public final String message;

    public final List<String> errors;

    public SroError(final int status, final String message, final Collection<String> errors) {
        this.status = status;
        this.message = message;
        this.errors = Collections.unmodifiableList(new ArrayList<>(errors));
    }

    public SroError(final SroException e) {
        this.status = e.status;
        this.message = e.getMessage();
        this.errors = Collections.unmodifiableList(e.errors);
    }

    protected SroError() {
        this.status = -1;
        this.message = null;
        this.errors = null;
    }

    public ResponseEntity<Object> toResponse() {
        return new ResponseEntity(this, HttpStatus.valueOf(this.status));
    }

    public ResponseEntity<Object> toResponse(HttpHeaders headers) {
        return new ResponseEntity(this, headers, HttpStatus.valueOf(this.status));
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof SroError)) return false;
        SroError sroException = (SroError) o;
        return status == sroException.status &&
                Objects.equals(message, sroException.message) &&
                Objects.equals(errors, sroException.errors);
    }

    @Override
    public int hashCode() {

        return Objects.hash(status, message, errors);
    }
}

Feign client usage notice how errors are transparently handled (no try / catch) thanks to @ControllerAdvice & @ExceptionHandler({SroException.class})

@RestController
@RequestMapping("/uaa")
public class AuthenticationController {
    private static final BearerToken REVOCATION_TOKEN = new BearerToken("", 0L);

    private final OauthFeignClient oauthFeignClient;

    private final int refreshTokenValidity;

    @Autowired
    public AuthenticationController(
            OauthFeignClient oauthFeignClient,
            @Value("${oauth.ttl.refresh-token}") int refreshTokenValidity) {
        this.oauthFeignClient = oauthFeignClient;
        this.refreshTokenValidity = refreshTokenValidity;
    }

    @PostMapping("/login")
    public ResponseEntity<LoginTokenPair> getTokens(@RequestBody @Valid LoginRequest userCredentials) {
        Map<String, String> formData = new HashMap<>();
        formData.put("grant_type", "password");
        formData.put("client_id", "web-client");
        formData.put("username", userCredentials.username);
        formData.put("password", userCredentials.password);
        formData.put("scope", "openid");

        DefaultOAuth2AccessToken response = oauthFeignClient.token(formData);
        return ResponseEntity.ok(new LoginTokenPair(
                new BearerToken(response.getValue(), response.getExpiresIn()),
                new BearerToken(response.getRefreshToken().getValue(), refreshTokenValidity)));
    }

    @PostMapping("/logout")
    public ResponseEntity<LoginTokenPair> revokeTokens() {
        return ResponseEntity
                .ok(new LoginTokenPair(REVOCATION_TOKEN, REVOCATION_TOKEN));
    }

    @PostMapping("/refresh")
    public ResponseEntity<BearerToken> refreshToken(@RequestHeader("refresh_token") String refresh_token) {
        Map<String, String> formData = new HashMap<>();
        formData.put("grant_type", "refresh_token");
        formData.put("client_id", "web-client");
        formData.put("refresh_token", refresh_token);
        formData.put("scope", "openid");

        DefaultOAuth2AccessToken response = oauthFeignClient.token(formData);
        return ResponseEntity.ok(new BearerToken(response.getValue(), response.getExpiresIn()));
    }
}

Answer 2

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 :/