docs: public synchronized void setExecutorService(ExecutorService executorService) throws InvalidArgumentException {
  
          if (executorService == null) {
              throw new InvalidArgumentException("Executor service can not be null.");
          }
          if (this.executorService != null && this.executorService != executorService) {
              throw new InvalidArgumentException("Executor service has already been set.");
          }
  
          this.executorService = executorService;
  
      }