docs: public synchronized void restart() throws IllegalStateException {
  
          if (this.isShutdown) {
              throw new IllegalStateException("isShutdown==true");
          } else if (this.taskRunnerThread == null) {
              this.taskRunnerThread =
                  this.threadFactory.newThread(this.taskRunner);
  
              this.taskRunnerThread.start();
          } else {
              this.taskQueue.unpark();
          }
      }