code: public Spider thread(ExecutorService executorService, int threadNum) {
          checkIfRunning();
          this.threadNum = threadNum;
          if (threadNum <= 0) {
              throw new IllegalArgumentException("threadNum should be more than one!");
          }
          this.executorService = executorService;
          return this;
      }
docs: start with more than one threads
  
  @param executorService executorService to run the spider
  @param threadNum threadNum
  @return this