How do I interrupt a thread given a Future object?

Question

I want to start a thread and cancel it if it doesn't finish within 5 seconds:

private final class HelloWorker implements Callable<String> {
    public String call() throws Exception {
        while(true) {
            if (Thread.isInterrupted()) {
                return null;
            }
        }
        return performExpensiveComputation();
    }

    private String performExpensiveComputation() {
        // some blocking expensive computation that may or may not take a very long time
    }
}

private ExecutorService executorService = Executors.newFixedThreadPool(threadPoolSize);
Future<String> future = executorService.submit(new HelloWorker());

try {  
    String s = future.get(5, TimeUnit.SECONDS);
} catch (TimeoutException e) {
    future.cancel(true);

    System.out.println("cancelled: " + future.isCancelled() + "done: " + future.isDone());

    executorService.shutdown();

    try {
        System.out.println("try to terminate: " + executorService.awaitTermination(60, TimeUnit.SECONDS));
    } catch (Exception ex) {
        // ignore
    }
}

However it looks like the awaitTermination returns false. Is there a way for me to check why an ExecutorService won't terminate? Can I figure out what threads are still running?

Answer 1

There is no safe way to stop a running thread without disturbing the stability of the rest of the process. This is why Thread#stop has been deprecated a long time ago, and why Executor Services only use the soft, cooperative Thread#interrupt mechanism.

Your thread will have to actively check if an interrupt has been requested and perform proper cleanup before ending. Alternatively, the thread will call some interruptible JDK methods, which will throw InterruptedException, which the tread will properly honor and end itself.

Answer 2

Why Future.cancel() doesn't work the way you think it does

Future cancel removes the task from the running queue. If your task is already running it won't stop it. So cancel() is a different concept that interrupting. As the Javadocs say:

Attempts to cancel execution of this task. This attempt will fail if the task has already completed, has already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task. https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/util/concurrent/Future.html#cancel(boolean)

What you are asking is how to interrupt. Fortunately, when you call Future.cancel() it will call the interrupt method. But you'll need to allow it with the mayInterruptIfRunning flag and will need to handle interrupts correctly (see below).

Why interrupt?

Interrupting threads in Java is useful when you have a long running task that you now need to stop, or when you have a daemon that you need to turn off, and other examples.

How to interrupt

To interrupt you call interrupt() on the thread. This is a cooperative process, so your code has to be ready for it. Like this:

myThread.interrupt();

Responsible code

Your code's responsibility is to be ready for any interruptions. I'd go so far to say that whenever you have a long running task, that you insert some interrupt ready code like this:

while (... something long...) {

     ... do something long

     if (Thread.interrupted()) {
         ... stop doing what I'm doing...
     }
}

How to stop what I'm doing?

You have several options:

1. If your you are in Runnable.run() just return or break out of the loop and finish the method.
2. You may be in some other method deep in the code. It may make sense at that point for that method to throw InterruptedException so you would just do that (leaving the flag cleared).
3. But maybe deep in your code it doesn't make sense to throw InterruptedException. In that case you should throw some other exception, but before that mark your thread interrupted again so the code that catches knows that an interrupt was in progress. Here's an example:

private void someMethodDeepDown() {
    while (.. long running task .. ) {
          ... do lots of work ...
    
          if (Thread.interrupted()) {
             // oh no! an interrupt!
             Thread.currentThread().interrupt();
             throw new SomeOtherException();
          }
     }
}

Now the exception can propagate an either terminate the thread or be caught, but the receiving code hopefully notices that an interrupt is in progress.

Should I use isInterrupted() or interrupted()

You should prefer interrupted() because:

1. Your code should reset the interrupt flag because if you don't the thread you are using could go back to a thread pool with an interrupted state causing problems (of course, that's a bug in the thread pool code, you won't get that behavior if you use Executors.newFixedThreadPool() for example. But other threading code could have it.
2. As another answer stated, the clearing of the interrupted flag indicates that you've received the message and are taking action. If you leave it on true, the after a while caller can assume you won't respond to it in a timely manner.

Why interrupt() why not some other flag in my code?

Interrupt is the best mechanism for interruption because our code can be ready for it. If we find code that is just catching and ignoring the InterruptExceptions or not checking for interrupted() in its body then we can correct those mistakes and make our code always cleanly interruptible without creating arcane dependencies on non-standard mechanisms in your code.

Unfortunately Joshua Block proposed the opposite in his famous book Effective Java, Second Edition. But enabling the interrupt() method to work as intended is much better, because this code is used by the Java standard library as explained above. A custom interruption method would not be.