How to create mock server response and connect to Android Retrofit

Published by Igor Khrupin on

Here I want to talk about mock responses for your Retrofit.

Sometimes we should continue mobile development even backend not ready. We can be never blocked on it if will use mock responses. Sure we need specify JSON response before starting. But we need only JSON format of the backend response to start.

Let’s see.

First we need create implementation of Interceptor and this class will contains all logic for server responses.

Feel free parse requests and create your own if/else cases

Here is sample

package com.hrupin.fakeinterceptorsample.retrofit;

import android.content.Context;
import android.util.Log;

import java.util.ArrayList;
import java.util.List;

import okhttp3.Interceptor;
import okhttp3.MediaType;
import okhttp3.Protocol;
import okhttp3.Request;
import okhttp3.Response;
import okhttp3.ResponseBody;
import okio.Buffer;

public class FakeInterceptor implements Interceptor {
    private static final String TAG = FakeInterceptor.class.getSimpleName();
    private static final String FILE_EXTENSION = ".json";
    private Context mContext;

    private String mContentType = "application/json";

    public FakeInterceptor(Context context) {
        mContext = context;

     * Set content type for header
     * @param contentType Content type
     * @return FakeInterceptor
    public FakeInterceptor setContentType(String contentType) {
        mContentType = contentType;
        return this;

    public Response intercept(Chain chain) throws IOException {
        List<String> listSuggestionFileName = new ArrayList<>();
        String method = chain.request().method().toLowerCase();

        Response response = null;
        // Get Request URI.
        final URI uri = chain.request().url().uri();
        Log.d(TAG, "--> Request url: [" + method.toUpperCase() + "]" + uri.toString());

        String defaultFileName = getFileName(chain);

        if (defaultFileName != null) {
            String fileName = getFilePath(uri, defaultFileName);
            Log.d(TAG, "Read data from file: " + fileName);
            try {
                InputStream is = mContext.getAssets().open(fileName);
                BufferedReader r = new BufferedReader(new InputStreamReader(is));
                StringBuilder responseStringBuilder = new StringBuilder();
                String line;
                while ((line = r.readLine()) != null) {
                Log.d(TAG, "Response: " + responseStringBuilder.toString());
                Response.Builder builder = new Response.Builder();
                builder.addHeader("content-type", mContentType);

                builder.body(ResponseBody.create(MediaType.parse(mContentType), responseStringBuilder.toString().getBytes()));

                response =;
            } catch (IOException e) {
                Log.e(TAG, e.getMessage(), e);
        } else {
            for (String file : listSuggestionFileName) {
                Log.e(TAG, "File not exist: " + getFilePath(uri, file));
            response = chain.proceed(chain.request());

        Log.d(TAG, "<-- END [" + method.toUpperCase() + "]" + uri.toString());
        return response;

    private String getFileName(Chain chain) {

        String method = chain.request().method();
        if (method.equals("POST")) {
            String fileName = null;
            String lastSegment = chain.request().url().pathSegments().get(chain.request().url().pathSegments().size() - 1);

            if (lastSegment.equals("login")) {
                fileName = "login";
            } else {
                fileName = lastSegment;

            return fileName + FILE_EXTENSION;
        return "none" + FILE_EXTENSION;

    private String getFilePath(URI uri, String fileName) {
        String path;
        if (uri.getPath().lastIndexOf('/') != uri.getPath().length() - 1) {
            path = uri.getPath().substring(0, uri.getPath().lastIndexOf('/') + 1);
        } else {
            path = uri.getPath();
        return uri.getHost() + path + fileName;

The key method is public Response intercept(Chain chain). You can play inside this method and prepare responses for your client side.

I’ve created sample login form with mock server requests

OkHttp log

D/OkHttp: --> POST http://mockapi/login http/1.1
D/OkHttp: Content-Type: application/json
D/OkHttp: Content-Length: 47
D/OkHttp: Accept: application/json
D/OkHttp: {"email":"","password":"password"}
D/OkHttp: --> END POST (47-byte body)
D/OkHttp: <-- 200 {
D/OkHttp:   "id":"uceruc65tfvifbvuyfd67t",
D/OkHttp:   "name":"John"
D/OkHttp: }
D/OkHttp:  http://mockapi/login (1ms)
D/OkHttp: content-type: application/json
D/OkHttp: {
D/OkHttp:   "id":"uceruc65tfvifbvuyfd67t",
D/OkHttp:   "name":"John"
D/OkHttp: }
D/OkHttp: <-- END HTTP (53-byte body)


Download it from github

Feel free download source code of the sample application from my github

Nice codding


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