Refreshing OAuth token using Retrofit without modifying all calls

Please do not use Interceptors to deal with authentication.

Currently, the best approach to handle authentication is to use the new Authenticator API, designed specifically for this purpose.

OkHttp will automatically ask the Authenticator for credentials when a response is 401 Not Authorised retrying last failed request with them.

public class TokenAuthenticator implements Authenticator {
    @Override
    public Request authenticate(Proxy proxy, Response response) throws IOException {
        // Refresh your access_token using a synchronous api request
        newAccessToken = service.refreshToken();

        // Add new header to rejected request and retry it
        return response.request().newBuilder()
                .header(AUTHORIZATION, newAccessToken)
                .build();
    }

    @Override
    public Request authenticateProxy(Proxy proxy, Response response) throws IOException {
        // Null indicates no attempt to authenticate.
        return null;
    }

Attach an Authenticator to an OkHttpClient the same way you do with Interceptors

OkHttpClient okHttpClient = new OkHttpClient();
okHttpClient.setAuthenticator(authAuthenticator);

Use this client when creating your Retrofit RestAdapter

RestAdapter restAdapter = new RestAdapter.Builder()
                .setEndpoint(ENDPOINT)
                .setClient(new OkClient(okHttpClient))
                .build();
return restAdapter.create(API.class);