Why developers set OkHttp to Retrofit if Retrofit already uses OkHttp internally

Question

I know, that Retrofit uses OkHttp internally. But, I can see that some developers provided applying of approach as bellow

return new Retrofit.Builder()
        .baseUrl(BuildConfig.API_ENDPOINT)
        .client(getClient()) // we can add OkHttpClient client there

So, can someone clarify why this needed?

I have heard that this can be helpful for testing, when we can apply custom Interceptor during mocking of HTTP requests

Answer 1

Retrofit does get an instance of OkHttp by itself but does not perform any customization of the OkHttpClient. In a lot of cases, you might need to customize you client to take advantage of the flexibility of OkHttp. Take a look at what you can do in this JavaDoc. You can see that you can do a lot of things like set your own timeout, your own DNS, your own custom cache (could come in handy in graphics intensive apps), your own proxy, limit protocols based on user device (a country might block HTTPS so you want to be able to fall back to HTTP in this case as quickly as possible) and many more.

EDIT: In most usage cases this might not be needed such as in a simple API call to a REST API endpoint. But in some cases, such as video streaming, VPN or proxy services, or whatever case that requires you to customize your app to geographical regions or different network connections you could benefit from this. Other examples that come to mind are some messaging or social apps that are blocked by certain countries. Even an app like Spotify or YouTube. When the user is connected through WiFi you want to route them to your high speed server through a high speed protocol (say UDP). But if the user is connected via 3G you want to route them to a different server and using TCP protocol to ensure quality.

Answer 2

You should explicitly provide an OkHttp instance to Retrofit, otherwise Retrofit would implicitly perform .client(new OkHttpClient()), thus you'd not get a lot of smart things, e.g. disk caching, connection pooling.

See how Jake Wharton clarifies the case in "Making Retrofit Work For You" talk.

Edit

Above mentioned case makes sense if you have multiple retrofit instances.