Retrofit 2 synchronous call error handling for 4xx Errors

Question

I'm using a android-priority-jobqueue and I use retrofit to make synchronous calls to my rest api but i'm unsure how to handle errors like 401 Unauthorized errors which I send back json stating the error. Simple when doing async calls but I'm adapting my app for job manager. below is a simple try catch for IO exceptions, but 401's 422's etc? How to do this?

try {
    PostService postService = ServiceGenerator.createService(PostService.class);
    final Call<Post> call = postService.addPost(post);
    Post newPost = call.execute().body();

    // omitted code here

} catch (IOException e) {
    // handle error
}

EDIT

The use of the retrofit response object was the clincher for me, returning the retrofit response object allowed me to

Response<Post> response = call.execute();

if (response.isSuccessful()) {
    // request successful (status code 200, 201)
    Post result = response.body();

    // publish the post added event
    EventBus.getDefault().post(new PostAddedEvent(result));
} else {
    // request not successful (like 400,401,403 etc and 5xx)
    renderApiError(response);
}

Answer 1

Check response code and show the appropriate message.

Try this:

 PostService postService = ServiceGenerator.createService(PostService.class);
 final Call<Post> call = postService.addPost(post);

Response<Post> newPostResponse = call.execute();

// Here call newPostResponse.code() to get response code
int statusCode = newPostResponse.code();
if(statusCode == 200)
    Post newPost = newPostResponse.body();
else if(statusCode == 401)
    // Do some thing... 

Answer 2

Putting checks for 401 on every response is not a very good approach. Instead one can apply this check at the base level i.e. while creating an object for Retrofit, through the interceptors. Have a look:

public synchronized static Retrofit getClientWithRetry(final Context ctx) {
    if (clientWithRetry == null) {
        Interceptor responseCodeInterceptor = new Interceptor() {
            @Override
            public Response intercept(Chain chain) throws IOException {
                Request request = chain.request();
                Response response = chain.proceed(request);
                if (response.code() == 401) {
                    Log.d(LOG_TAG, "Intercepted Req: " + response.toString());
                    Response r = retryWithFreshToken(request, chain);
                    return r;
                }
                return response;
            }
        };

        int cacheSize = 10 * 1024 * 1024; // 10 MB
        Cache cache = new Cache(ctx.getCacheDir(), cacheSize);

        HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
        logging.setLevel(HttpLoggingInterceptor.Level.BODY);
        OkHttpClient client = new OkHttpClient.Builder()
                .addInterceptor(logging)
                .addInterceptor(responseCodeInterceptor)
                .cache(cache)
                .build();

        Retrofit.Builder builder = new Retrofit.Builder()
                .baseUrl(API_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .client(client);
        clientWithRetry = builder.build();
    }
    return clientWithRetry;
}

Here internally if a 401 is observed a new chained request can be made and token can be fetched. Post which the original request can be completed. Taken from this Retrofit retry tutorial.