Program never ends on TimeoutException in java

Question

I am trying a java program to understand working of Future.

I wrote following program and it never ends. If I put a value that is less than 10 in Thread.sleep(), then it works but not for values >=10.

I understood the part that is causing problem is probably the future.get call.

However, on further analysis, what I tried was, to handle all the exceptions and not letting jvm handle them.

eg:

Now it terminated fine.

I did a further check and saw that if I throw ExecutionException and InterruptedException and handle TimeoutException it works fine again. Here strange part is I have to compulsorily handle TimeoutException, else it will not work. I am not so sure why this strange behaviour persists.

I am using OpenJDK 15.

If anybody wants to try code snippet its here:

import java.util.concurrent.*;

public class FixedThreadPoolExecutorDemo2 {

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        workWithFutureCallable(executorService);
        executorService.shutdownNow();
    }

    private static void workWithFutureCallable(ExecutorService executorService) throws ExecutionException, InterruptedException, TimeoutException {

        Future<Integer> myOtherFuture = executorService.submit(() -> {
            try {
                Thread.sleep(109);
            } catch (InterruptedException e) {
            }
            return 1000;
        });
        System.out.println("myOtherFuture  should be cancelled if running for more than specified time. ->" + myOtherFuture.get(10, TimeUnit.MILLISECONDS));

    }
}

as the Exception is passed on, the executor service is not shut down and it is using a non-daemon Thread. Doc of newFixedThreadPool: "The threads in the pool will exist until it is explicitly shutdown". Use (create) a ThreadFactory that create daemon Threads or (better IMO) use try-finally to always shut down — user15244370
– user15244370, Commented Mar 18, 2021 at 18:29
` executorService.shutdownNow();` is called in the code if you see. — swapyonubuntu
– swapyonubuntu, Commented Mar 18, 2021 at 18:34
You must be able to catch the thread interruption in the main method and that should exit the application. — aksappy
– aksappy, Commented Mar 18, 2021 at 18:34
additionally, I saw that it goes in uncaughtException method of threadgroup, there it prints stacktrace and moves ahed if it does not see instanceof ThreadDeath — swapyonubuntu
– swapyonubuntu, Commented Mar 18, 2021 at 18:36

4 revs user15244370 · Accepted Answer · 2021-03-18 19:11:57Z

1

The TimeoutException is not being caught in main, so main is also terminating abruptly without calling shutdownNow. The main thread is terminated but the threads created by Executors are non-daemon by default, so the virtual machine is not shut down, the Executors thread(s) continue running.

Solution:

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        try {
            workWithFutureCallable(executorService);
        } finally {
            executorService.shutdownNow();
        }
    }

or

implement a ThreadFactory to create daemon threads and use it to get the service:

        ExecutorService executorService = Executors.newFixedThreadPool(2, r -> {
            var thread = new Thread(r);
            thread.setDaemon(true);
            return thread;
        });

Even better, at least in production code, catch and handle the Exceptions (together with the finally block.)

edited Mar 18, 2021 at 19:11

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4 revs
user15244370

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1 Comment

swapyonubuntu Over a year ago

Hi @user15244370 , I am sorry, I could not understand same thing via your comments. I added finally block and it worked. Indeed, I did not think that threads created by executorservice are non-daemon threads. Thank you very much. Also, here I am studying about concurrency, so yes, production grade code will handle and will also note that shutdown of executorservice should be in finally block only.

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