code: public static ThreadPoolExecutor newFixedThreadPool(int corePoolSize) {
          return new ThreadPoolExecutor(corePoolSize,
              corePoolSize,
              0,
              TimeUnit.MILLISECONDS,
              new SynchronousQueue<Runnable>());
      }
docs: 固定大小线程池，无队列
  
  @param corePoolSize 初始化线程池
  @return the thread pool executor