Unable to parse error in Retrofit 2

Question

I am using Retrofit 2 and I am getting a null pointer exception at this line:

 RetrofitClient.APIError error = RetrofitClient.ErrorUtils.parseError(response, retrofit);

The error is null. More details:

This is the format that the API returns the error in:

{
  "error": {
    "message": "Incorrect credentials",
    "statusCode": 401
  }
}

Here is my login Callback code:

new Callback<LoginResponse>() {

    @Override
    public void onResponse(Response<LoginResponse> response, Retrofit retrofit) {
       if (listener != null) {
           if (response.isSuccess() && response.body() != null) {

               User user = RetrofitUserToUserMapper.fromRetrofitUser(response.body().getLoginUser());

           } else {

               RetrofitClient.APIError error = RetrofitClient.ErrorUtils.parseError(response, retrofit);
               listener.onUserLoginFailure(error.getErrorMessage()); // NPE - error is null
           }
        }
    }

    @Override
    public void onFailure(Throwable t) {

        if (listener != null) {
            listener.onUserLoginFailure("");
        }
    }
}

This is my Retrofit 2 class:

public class RetrofitClient {

    public static final String API_ROOT = "http://example.com/api/v1/";
    private static final String HEADER_OS_VERSION = "X-OS-Type";
    private static final String HEADER_APP_VERSION = "X-App-Version";
    private static final String HEADER_OS_VERSION_VALUE_ANDROID = "android";


    private RetrofitClient() {
    }

    private static Retrofit INSTANCE;

    public static Retrofit getInstance() {
        if (INSTANCE == null) {
            setupRestClient();
        }
        return INSTANCE;
    }

    public static void setupRestClient() {

        OkHttpClient httpClient = new OkHttpClient();
        addHeadersRequiredForAllRequests(httpClient, BuildConfig.VERSION_NAME);

        INSTANCE = new Retrofit.Builder()
                .baseUrl(API_ROOT)
                .addConverterFactory(GsonConverterFactory.create())
                .client(httpClient)
                .build();
    }

    private static void addHeadersRequiredForAllRequests(OkHttpClient httpClient, final String appVersion) {

        class RequestInterceptor implements Interceptor {

            @Override
            public com.squareup.okhttp.Response intercept(Chain chain) throws IOException {
                Request request = chain.request().newBuilder()
                        .addHeader(HEADER_OS_VERSION, HEADER_OS_VERSION_VALUE_ANDROID)
                        .addHeader(HEADER_APP_VERSION, appVersion)
                        .build();
                return chain.proceed(request);
            }
        }

        httpClient.networkInterceptors().add(new RequestInterceptor());
    }

    public static class ErrorUtils {

        public static APIError parseError(Response<?> response, Retrofit retrofit) {
            Converter<ResponseBody, APIError> converter =
                    retrofit.responseConverter(APIError.class, new Annotation[0]);

            APIError error;

            try {
                error = converter.convert(response.errorBody());
            } catch (IOException e) {
                e.printStackTrace();
                return new APIError();
            } catch (Exception e) {
                e.printStackTrace();
                return new APIError();
            }

            return error;
        }
    }

    public static class APIError {
        @SerializedName("error")
        public
        ErrorResponse loginError;

        public ErrorResponse getLoginError() {
            return loginError;
        }

        public String getErrorMessage() {
            return loginError.message;
        }

        private class ErrorResponse {
            @SerializedName("message")
            private String message;
            @SerializedName("statusCode")
            public int statusCode;

            public String getMessage() {
                return message;
            }

            public int getStatusCode() {
                return statusCode;
            }

            @Override
            public String toString() {
                return "LoginErrorResponseBody{" +
                        "message='" + getMessage() + '\'' +
                        ", statusCode=" + statusCode +
                        '}';
            }

            public void setMessage(String message) {
                this.message = message;
            }
        }
    }


}

I got the error utils class from this tutorial, but changed it slightly because the error formatting in their example is different:

EDIT: This is the Converter class:

/**
 * Convert objects to and from their representation as HTTP bodies. Register a converter with
 * Retrofit using {@link Retrofit.Builder#addConverterFactory(Factory)}.
 */
public interface Converter<F, T> {
  T convert(F value) throws IOException;

  abstract class Factory {
    /**
     * Create a {@link Converter} for converting an HTTP response body to {@code type} or null if it
     * cannot be handled by this factory.
     */
    public Converter<ResponseBody, ?> fromResponseBody(Type type, Annotation[] annotations) {
      return null;
    }

    /**
     * Create a {@link Converter} for converting {@code type} to an HTTP request body or null if it
     * cannot be handled by this factory.
     */
    public Converter<?, RequestBody> toRequestBody(Type type, Annotation[] annotations) {
      return null;
    }
  }
}

Answer 1

UPDATE:

If you want to use a custom class instead of JSONObject below, you can refer to the following:

Custom class:

public class ResponseError {

    Error error;

    class Error {
        int statusCode;
        String message;
    }
}

Add the following to WebAPIService interface:

@GET("/api/geterror")
Call<ResponseError> getError2();

Then, inside MainActivity.java:

Call<ResponseError> responseErrorCall = service.getError2();
responseErrorCall.enqueue(new Callback<ResponseError>() {
    @Override
    public void onResponse(Response<ResponseError> response, Retrofit retrofit) {
        if (response.isSuccess() && response.body() != null){
            Log.i(LOG_TAG, response.body().toString());
        } else {
            if (response.errorBody() != null){
                RetrofitClient.APIError error = RetrofitClient.ErrorUtils.parseError(response, retrofit);
                Log.e(LOG_TAG, error.getErrorMessage());
            }
        }
    }

    @Override
    public void onFailure(Throwable t) {
        Log.e(LOG_TAG, t.toString());
    }
});

---

I have just tested your RetrofitClient class with my web service. I made a small update to your APIError class as the following (add 2 constructors, actually they are not called):

public APIError(){
    this.loginError = new ErrorResponse();
}

public APIError(int statusCode, String message) {
    this.loginError = new ErrorResponse();
    this.loginError.statusCode = statusCode;
    this.loginError.message = message;
}

Interface:

public interface WebAPIService {
    @GET("/api/geterror")
    Call<JSONObject> getError();
}

MainActivity:

// Retrofit 2.0-beta2
Retrofit retrofit = new Retrofit.Builder()
        .baseUrl(API_URL_BASE)
        .addConverterFactory(GsonConverterFactory.create())
        .build();

WebAPIService service = retrofit.create(WebAPIService.class);

Call<JSONObject> jsonObjectCall = service.getError();
jsonObjectCall.enqueue(new Callback<JSONObject>() {
    @Override
    public void onResponse(Response<JSONObject> response, Retrofit retrofit) {
        if (response.isSuccess() && response.body() != null){
            Log.i(LOG_TAG, response.body().toString());
        } else {
            if (response.errorBody() != null){
                RetrofitClient.APIError error = RetrofitClient.ErrorUtils.parseError(response, retrofit);
                Log.e(LOG_TAG, error.getErrorMessage());
            }
        }
    }

    @Override
    public void onFailure(Throwable t) {
        Log.e(LOG_TAG, t.toString());
    }
});

My web service (Asp.Net Web API):

According to your JSON response data, I have used the following code:

[Route("api/geterror")]
public HttpResponseMessage GetError()
{
    var detailError = new
    {
        message = "Incorrect credentials",
        statusCode = 401
    };

    var myError = new
    {
        error = detailError
    };            

    return Request.CreateResponse(HttpStatusCode.Unauthorized, myError);
}

It's working! Hope it helps!

Answer 2

The problem was really strange and I cannot explain how was this happening, and my solution is really ugly.

Retrofit actually had two converter factories and the converter factory that it returned when I asked it to convert the response, was null.

I found out about this while I was debugging retrofit's method that returns converter factories. The second factory that it returned actually did the conversion successfully and I was finally able to parse my response. I still don't know what I did wrong.

Here is what ended up being my ugly solution:

String errorMessage = "";
for (Converter.Factory factory : retrofit.converterFactories()) {
    try {
       LoginError loginError = (LoginError) factory.fromResponseBody(LoginError.class, new Annotation[0]).convert(errorBody);
       if (loginError != null) {
           errorMessage = loginError.error.message;
       }
    } catch (IOException e) {
       e.printStackTrace();
    } catch (Exception e) {
       e.printStackTrace();
    }
 }
 if (!TextUtils.isEmpty(errorMessage)) {
     listener.onUserLoginFailure(errorMessage);
 }

The first time around in the loop I got a NPE. The second time I got the error message

Here is the error class I ended up using:

private class LoginError {
    Error error;

    class Error {
        String message;
        int statusCode;
    }
}

EDIT: I suppose it might be the WildCard's fault for confusing retrofit what converter to return, the one that I was passing here:

public static APIError parseError(Response<?> response, Retrofit retrofit) {