Is there a nice way to implement "blocking" request interceptor?

The main idea is that all requests should be intercepted and added additional header - token.

If token does not exist yet it should be retrieved first, then added to that request and cached for future used. token is retrieved via API call.

I've tried to do synchronous request, but, that produces android.os.NetworkOnMainThreadException. And implementing with in_progress flags it doesn't look nice.

You can already do the 'intercept' part of this using RequestInterceptor. Just use RestAdapter.Builder.setRequestInterceptor().

It's a better idea to retrieve the token from the API outside the RequestInterceptor though, as it's not meant to do that. After that first call, you can just add the token anywhere you want in your requests inside RequestInterceptor.intercept().

Something like this:

Builder builder = new RestAdapter.Builder()
//Set Endpoint URL, Retrofit class... etc
.setRequestInterceptor(new RequestInterceptor() {
       @Override
       public void intercept(RequestFacade request) {
           String authToken = getAuthToken(); //Not included here, retrieve the token.
           request.addHeader("Authorization", "Bearer " + authToken);
       }
);