Returning error from OKHttp interceptor (using retrofit)

Just had the same scenario and this post helped me implementing the solution. Thanks to @mastov to point to the right direction.

Working with a back-end api that always returns HTTP 200 even if there was an error. This was my response sample of an error

{"status":403,"message":"Bad User credentials","time":1495597740061,"version":"1.0"}

Here is a simple implementation to complement this answer.

public Response intercept(Chain chain) throws IOException {        Request request   = chain.request();        Response response = chain.proceed(request);        ResponseBody body = response.body();        // Only intercept JSON type responses and ignore the rest.        if (body != null && body.contentType() != null && body.contentType().subtype() != null && body.contentType().subtype().toLowerCase().equals("json")) {            String errorMessage = "";            int errorCode       = 200; // Assume default OK            try {                BufferedSource source = body.source();                source.request(Long.MAX_VALUE); // Buffer the entire body.                Buffer buffer   = source.buffer();                Charset charset = body.contentType().charset(Charset.forName("UTF-8"));                // Clone the existing buffer is they can only read once so we still want to pass the original one to the chain.                String json     = buffer.clone().readString(charset);                JsonElement obj = new JsonParser().parse(json);                // Capture error code an message.                if (obj instanceof JsonObject && ((JsonObject) obj).has("status")) {                    errorCode   = ((JsonObject) obj).get("status").getAsInt();                }                if (obj instanceof JsonObject && ((JsonObject) obj).has("message")) {                    errorMessage= ((JsonObject) obj).get("message").getAsString();                }            } catch (Exception e) {                Log.e(TAG, "Error: " + e.getMessage());            }            // Check if status has an error code then throw and exception so retrofit can trigger the onFailure callback method.            // Anything above 400 is treated as a server error.            if(errorCode > 399){                throw new Exception("Server error code: " + errorCode + " with error message: " + errorMessage);            }        }        return response;    }

My solution taken from okhttp3.logging.HttpLoggingInterceptor

class ErrorResponseInterceptor : Interceptor {    override fun intercept(chain: Interceptor.Chain): Response {        val response = chain.proceed(chain.request())        val code = response.code()        if (code in 400..500) {            responseBody(response)?.also { errorString ->                // error string here is a body of server error            }        }        return response    }    private fun responseBody(response: Response): String? {        val responseBody = response.body() ?: return null        val contentLength = responseBody.contentLength()        if (contentLength == 0L) {            return null        }        val source = responseBody.source()        source.request(Long.MAX_VALUE) // Buffer the entire body.        var buffer = source.buffer()        val headers = response.headers()        if ("gzip".equals(headers.get("Content-Encoding"), ignoreCase = true)) {            var gzippedResponseBody: GzipSource? = null            try {                gzippedResponseBody = GzipSource(buffer.clone())                buffer = okio.Buffer()                buffer.writeAll(gzippedResponseBody)            } finally {                gzippedResponseBody?.close()            }        }        val charset: Charset = responseBody.contentType()?.charset(UTF8) ?: UTF8        return buffer.clone().readString(charset)    }    private companion object {        val UTF8: Charset = Charset.forName("UTF-8")    }}

You should throw IOException. In this case retrofit2 will use onFailure path.