I created a thread pool, and submitted two tasks. Why does my application hang without any exceptions after print `task one ,result: null`???

```
   private final static ThreadPoolExecutor executorService = new
        ThreadPoolExecutor(1, 1, 1L, TimeUnit.MINUTES,
        new SynchronousQueue<Runnable>(), new ThreadPoolExecutor.DiscardPolicy());

    public static void main(String[] args) throws Exception {
        Future taskOne = executorService.submit(() -> {
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Future taskTwo = executorService.submit(() -> System.out.println("task two is working"));;
        System.out.println("task one ,result: " + taskOne.get());
        System.out.println("task two, result: " + taskTwo.get());
        executorService.shutdown();
    }

```