code: public BatcherBuilder listenerService(ExecutorService listenerService) {
          checkState(this.listenerService == null, "A listener service has already been set");
          requireNonNull(listenerService);
          this.listenerService = listenerService;
          return this;
      }
docs: Provide a configured {@link ExecutorService} to use for running the {@link BatchListener} on.
  This {@link ExecutorService} will be shutdown when the created batcher is closed.  By default a cached
  thread pool is used to handle calls to the {@link BatchListener}.