This OkHttp application interceptor will replace the destination hostname in the request URL.

HostSelectionInterceptor.java

import java.io.IOException;
import okhttp3.HttpUrl;
import okhttp3.Interceptor;
import okhttp3.OkHttpClient;
import okhttp3.Request;

/** An interceptor that allows runtime changes to the URL hostname. */
public final class HostSelectionInterceptor implements Interceptor {
  private volatile String host;

  public void setHost(String host) {
    this.host = host;
  }

  @Override public okhttp3.Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    String host = this.host;
    if (host != null) {
      HttpUrl newUrl = request.url().newBuilder()
          .host(host)
          .build();
      request = request.newBuilder()
          .url(newUrl)
          .build();
    }
    return chain.proceed(request);
  }

  public static void main(String[] args) throws Exception {
    HostSelectionInterceptor interceptor = new HostSelectionInterceptor();

    OkHttpClient okHttpClient = new OkHttpClient.Builder()
        .addInterceptor(interceptor)
        .build();

    Request request = new Request.Builder()
        .url("http://www.coca-cola.com/robots.txt")
        .build();

    okhttp3.Call call1 = okHttpClient.newCall(request);
    okhttp3.Response response1 = call1.execute();
    System.out.println("RESPONSE FROM: " + response1.request().url());
    System.out.println(response1.body().string());

    interceptor.setHost("www.pepsi.com");

    okhttp3.Call call2 = okHttpClient.newCall(request);
    okhttp3.Response response2 = call2.execute();
    System.out.println("RESPONSE FROM: " + response2.request().url());
    System.out.println(response2.body().string());
  }
}

@JessYanCoding
I request two api with different baseUrl at the same time, one of the request's baseUrl is wrong.

mHost.setHost(Urls.DOUBAN_FILM_URL_HOST)
        mRetrofit.create(FilmsApi::class.java)
                .getInTheaters()
                .flatMap { filmsData -> Observable.fromIterable(filmsData.films) }
                .compose(TransformScheduler.applyNewThreadScheduler())
                .subscribe(
                        { film ->
                            val filmEntity = FilmEntity(text = film.title, comment = film.originalTitle, date = Date())

                            Log.e("TAG", "put success " + filmEntity.text)
                        },
                        { e ->
                            Log.e("TAG", e.message)
                        })

        mHost.setHost(Urls.GANK_GIRLS_URL_HOST)
        mRetrofit.create(GirlsApi::class.java)
                .accessGirls(1)
                .compose(TransformScheduler.applyNewThreadScheduler())
                .subscribe({ girl ->
                    Log.e("TAG", "size " + girl.results.size)
                })

The two baseurl are: http://gank.io/ and https://api.douban.com/
And occur this:
D/OkHttp: <-- 404 Not Found http://gank.io/v2/movie/in_theaters (308ms)
This is wrong because the base url is https://api.douban.com/ but not http://gank.io/
I think it course by concurrency with the Volatile , how solve it?

@iBotasky Because the network request is asynchronous, try https://github.com/JessYanCoding/RetrofitUrlManager

I have a good implementation plan, which will be easier to use and solve the problem more thoroughly. Welcome everyone to try https://github.com/uni-cstar/oknet