ApiModule.java

## ApiModule.java
// Dagger 1 example
@Module(
        complete = false,
        library = true
)
public final class ApiModule {
    @Provides
    @Singleton
    Retrofit provideRetrofit(Gson gson, Application app) {
        return new Retrofit.Builder()
                .baseUrl(app.getString(R.string.base_url))
                .addConverterFactory(GsonConverterFactory.create(gson))
                .addCallAdapterFactory(new ErrorHandlingExecutorCallAdapterFactory(new ErrorHandlingExecutorCallAdapterFactory.MainThreadExecutor()))
                .build();
    }

    @Provides
    @Singleton
    LoginService provideLoginService(Retrofit retrofit) {
        return retrofit.create(LoginService.class);
    }
}

## ErrorHandlingExecutorCallAdapterFactory.java
// Modified version of https://github.com/square/retrofit/blob/master/retrofit/src/main/java/retrofit/ExecutorCallAdapterFactory.java
public class ErrorHandlingExecutorCallAdapterFactory implements CallAdapter.Factory {
    private final Executor callbackExecutor;

    ErrorHandlingExecutorCallAdapterFactory(Executor callbackExecutor) {
        this.callbackExecutor = callbackExecutor;
    }

    @Override
    public CallAdapter<Call<?>> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
        if ($Gson$Types.getRawType(returnType) != Call.class) {
            return null;
        }
        final Type responseType = getCallResponseType(returnType);
        return new CallAdapter<Call<?>>() {
            @Override
            public Type responseType() {
                return responseType;
            }

            @Override
            public <R> Call<R> adapt(Call<R> call) {
                return new ExecutorCallbackCall<>(callbackExecutor, call);
            }
        };
    }

    static final class ExecutorCallbackCall<T> implements Call<T> {
        private final Executor callbackExecutor;
        private final Call<T> delegate;

        ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
            this.callbackExecutor = callbackExecutor;
            this.delegate = delegate;
        }

        @Override
        public void enqueue(Callback<T> callback) {
            delegate.enqueue(new ExecutorCallback<>(callbackExecutor, callback));
        }

        @Override
        public Response<T> execute() throws IOException {
            return delegate.execute();
        }

        @Override
        public void cancel() {
            delegate.cancel();
        }

        @SuppressWarnings("CloneDoesntCallSuperClone") // Performing deep clone.
        @Override
        public Call<T> clone() {
            return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone());
        }
    }

    static final class ExecutorCallback<T> implements Callback<T> {
        private final Executor callbackExecutor;
        private final Callback<T> delegate;

        ExecutorCallback(Executor callbackExecutor, Callback<T> delegate) {
            this.callbackExecutor = callbackExecutor;
            this.delegate = delegate;
        }

        @Override
        public void onResponse(final Response<T> response, final Retrofit retrofit) {
            if (response.isSuccess()) {
                callbackExecutor.execute(new Runnable() {
                    @Override
                    public void run() {
                        delegate.onResponse(response, retrofit);
                    }
                });
            } else {
                callbackExecutor.execute(new Runnable() {
                    @Override
                    public void run() {
                        delegate.onFailure(RetrofitException.httpError(response.raw().request().urlString(), response, retrofit));
                    }
                });
            }
        }

        @SuppressWarnings("ThrowableResultOfMethodCallIgnored")
        @Override
        public void onFailure(final Throwable t) {
            RetrofitException exception;
            if (t instanceof IOException) {
                exception = RetrofitException.networkError((IOException) t);
            } else {
                exception = RetrofitException.unexpectedError(t);
            }
            final RetrofitException finalException = exception;
            callbackExecutor.execute(new Runnable() {
                @Override
                public void run() {
                    delegate.onFailure(finalException);
                }
            });
        }
    }

    public static class MainThreadExecutor implements Executor {
        private final Handler handler = new Handler(Looper.getMainLooper());

        @Override
        public void execute(@NonNull Runnable r) {
            handler.post(r);
        }
    }

    static Type getCallResponseType(Type returnType) {
        if (!(returnType instanceof ParameterizedType)) {
            throw new IllegalArgumentException(
                    "Call return type must be parameterized as Call<Foo> or Call<? extends Foo>");
        }
        final Type responseType = getSingleParameterUpperBound((ParameterizedType) returnType);

        // Ensure the Call response type is not Response, we automatically deliver the Response object.
        if ($Gson$Types.getRawType(responseType) == retrofit.Response.class) {
            throw new IllegalArgumentException(
                    "Call<T> cannot use Response as its generic parameter. "
                            + "Specify the response body type only (e.g., Call<TweetResponse>).");
        }
        return responseType;
    }

    public static Type getSingleParameterUpperBound(ParameterizedType type) {
        Type[] types = type.getActualTypeArguments();
        if (types.length != 1) {
            throw new IllegalArgumentException(
                    "Expected one type argument but got: " + Arrays.toString(types));
        }
        Type paramType = types[0];
        if (paramType instanceof WildcardType) {
            return ((WildcardType) paramType).getUpperBounds()[0];
        }
        return paramType;
    }
}

## example.java
Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("https://api.github.com")
    .addConverterFactory(GsonConverterFactory.create(gson))
    .addCallAdapterFactory(new ErrorHandlingExecutorCallAdapterFactory(new ErrorHandlingExecutorCallAdapterFactory.MainThreadExecutor()))
    .build();

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

## RetrofitException.java
// This is RetrofitError converted to Retrofit 2
public class RetrofitException extends RuntimeException {
    public static RetrofitException httpError(String url, Response response, Retrofit retrofit) {
        String message = response.code() + " " + response.message();
        return new RetrofitException(message, url, response, Kind.HTTP, null, retrofit);
    }

    public static RetrofitException networkError(IOException exception) {
        return new RetrofitException(exception.getMessage(), null, null, Kind.NETWORK, exception, null);
    }

    public static RetrofitException unexpectedError(Throwable exception) {
        return new RetrofitException(exception.getMessage(), null, null, Kind.UNEXPECTED, exception, null);
    }

    /** Identifies the event kind which triggered a {@link RetrofitException}. */
    public enum Kind {
        /** An {@link IOException} occurred while communicating to the server. */
        NETWORK,
        /** A non-200 HTTP status code was received from the server. */
        HTTP,
        /**
         * An internal error occurred while attempting to execute a request. It is best practice to
         * re-throw this exception so your application crashes.
         */
        UNEXPECTED
    }

    private final String url;
    private final Response response;
    private final Kind kind;
    private final Retrofit retrofit;

    RetrofitException(String message, String url, Response response, Kind kind, Throwable exception, Retrofit retrofit) {
        super(message, exception);
        this.url = url;
        this.response = response;
        this.kind = kind;
        this.retrofit = retrofit;
    }

    /** The request URL which produced the error. */
    public String getUrl() {
        return url;
    }

    /** Response object containing status code, headers, body, etc. */
    public Response getResponse() {
        return response;
    }

    /** The event kind which triggered this error. */
    public Kind getKind() {
        return kind;
    }

    /** The Retrofit this request was executed on */
    public Retrofit getRetrofit() {
        return retrofit;
    }

    /**
     * HTTP response body converted to specified {@code type}. {@code null} if there is no
     * response.
     *
     * @throws IOException if unable to convert the body to the specified {@code type}.
     */
    public <T> T getErrorBodyAs(Class<T> type) throws IOException {
        if (response == null || response.errorBody() == null) {
            return null;
        }
        Converter<ResponseBody, T> converter = retrofit.responseConverter(type, new Annotation[0]);
        return converter.convert(response.errorBody());
    }
}