public Crawler(OkHttpClient client) {
    this.client = client;
  }

  private void parallelDrainQueue(int threadCount) {
    ExecutorService executor = Executors.newFixedThreadPool(threadCount);
    for (int i = 0; i < threadCount; i++) {
      executor.execute(() -> {
        try {
          drainQueue();
        } catch (Exception e) {
          e.printStackTrace();
        }