Retrofit – Mock service with an interceptor


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Many times, when starting a new project, we find that we still do not have a backend prepared to cover the needs of the frontend. In this case, how do we avoid slowing down development? Well, here we share you a possible solution to this scenario.

The ideal way to proceed in this case is to make a contract walidated by both the backend engineer and the frontend.

Now we are ready to start!

A real scenario

Usually we could imagine the json structure to create ours classes and write it down into a text file. But we can do it more real calling retrofit to emulate the complete scenario. To make it we might use an Interceptor implementation. 

Make a class called MockInterceptor and implement the interface Interceptor. Then you should override the intercept method. Inside this method and before doing anything, make sure you are only doing it for debug, to avoid using a mock in production

var response: Response? = null
if (BuildConfig.DEBUG) {

} else { // if not DEBUG proceed normally
    response = chain.proceed(chain.request())
return response


Well, to get up the json file we add this to the assets folder. For example, we will create an example folder and inside it another folder call target. Finally inside the target folder, create a service json file. The full path would be look like this: «assets/example/target/service.json». This means that our service it’s « Remind» it’s our default service url.

So the json files will be taken from the end of the base url as a folder structure.

Reading the json file

Once the file corresponding to the service to be mocked has been created, you need to read it from the assets and provide it to your response.

private fun getJsonDataFromAsset(context: Context, fileName: String): String? {
    val jsonString: String
    try {
        // Append the json extension
        jsonString ="$fileName.json").bufferedReader().use { it.readText() }
    } catch (ioException: IOException) {
        return null
    return jsonString


And use it in the intercept method, inside the debug condition, to send it as a response.file

override fun intercept(chain: Interceptor.Chain): Response {
    var response: Response? = null
    if (BuildConfig.DEBUG) {
        // Get Request URI.
        val uri: URI = chain.request().url.toUri()
        // Get path String.
        var path: String = uri.path
        // Remove the starter slide '/'
        path = if (path.startsWith('/')) path.substring(1) else path
        // Get the json file text
        val responseString: String = getJsonDataFromAsset(context, path).orEmpty()

        // Create the response
        response = Response.Builder()
            .body(ResponseBody.create("application/json".toMediaType(), responseString.toByteArray()))
            .addHeader("content-type", "application/json")
    } else {
        response = chain.proceed(chain.request())
    return response


Modify your «ServiceFactory» class to accept the mock interceptor option:

class ServiceFactory(private val context: Context, private val shouldUseMock: Boolean) {


and the okHttpClient method:

private fun okHttpClient(): OkHttpClient {
    var client = OkHttpClient.Builder()
    client = addMockInterceptor(client, context, shouldUseMock)


Finally add the method to pass the interceptor to the okhttp client:

private fun addMockInterceptor(builder: OkHttpClient.Builder, context: Context, shouldUseMock: Boolean): OkHttpClient.Builder {
    if (BuildConfig.DEBUG && shouldUseMock) {
        val mockInterceptor = MockInterceptor(context)
    return builder


That’s all! Now we have an interceptor to mock a service. Use it passing the variable «shouldUseMock» in true, like this:

ServiceFactory(this, true).createInstance(

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