为什么将来的.isDone（）必须在executorService.shutdown（）之后

Question

Why future .isDone() must after the executorService.sutdown() .

This can work:

Future<Integer> submit1 = executorService.submit(callable);
executorService.shutdown();
while (submit1.isDone()){
   System.out.println(submit1.get());
}

But after I commented this line:

Future<Integer> submit1 = executorService.submit(callable);
//executorService.shutdown();
while (submit1.isDone()){
   System.out.println(submit1.get());
}

It can't print any result.

Answer 1

You shouldn't be calling submit1.isDone() in the first place, and definitely not in a while loop. As Future.get() is a blocking call, you'll get the same behaviour from both when you remove the while() part.

The reason you're not getting anything printed in the second case is because the callable hasn't had a chance to finish, so the condition of while is false.

What you can call after shutdown() is ExecutorService.awaitTermination() . It will wait up to the given amount of time for all tasks to finish. If you don't want the tasks to finish, you can call shutdownNow() .

Answer 2

Because when you call .shutdown() thread's state will change which makes isDone return true. Look at the ThreadPoolExecutor implementation how it changes your threads state:

public void shutdown() {
    //...
    tryTerminate();
}


/**
 * Transitions to TERMINATED state if either (SHUTDOWN and pool
 * and queue empty) or (STOP and pool empty). 
 * ....
 * ....
 */
final void tryTerminate() {
....
}

And isDone() documentation says that this method returns true due to normal termination, exception or cancellation . So:

1) You submitted Callable

2) You shutDown process, so its state is now TERMINATED

3) isDone returns true, you see the out.

In other cases, your Callable never terminates, throws an exception or canceled which makes isDone always return false.