Java ExecutorService - why does this program keep running?

Question

I'm trying to build something like a background task executor which terminates background tasks after a certain time if there's no answer (background tasks call webservices and they can time-out but I need to make sure they time out under a certain time)

So I have this as an experiment but if I run this the program does not terminate. I wonder if its because a background thread is still active maybe? How can I shut this down?

public class Test {

public static class Task implements Callable<Object> {

    @Override
    public Object call() throws Exception {
        while(true) {}
    }

}

public static void main(String[] args) {
    try {
        Task t = new Task();
        ExecutorService executor = Executors.newSingleThreadExecutor();
        executor.invokeAll(Arrays.asList(t), 5L, TimeUnit.SECONDS);
        executor.shutdown();
        System.out.println("DONE");
    } 
    catch (InterruptedException e) {
        e.printStackTrace();
    }
}

}

Answer 1

The ExecutorService does not kill the running threads, and since threads are created as non-daemon, the JVM doesn't exit.

What happens is that when timeout expires, futures returned by invokeAll() are cancelled, which means that a flag is set on the future object and you get a CancellationException if you try to call future.get(). However neither invokeAll(), nor shutdown() (or shutdownNow()) do anything to kill the thread.

Note that you cannot even kill threads yourself. All you can do is setting some application-specific flag or call Thread.interrupt(), but even that does not guarantee that the thread terminates.

Answer 2

There is a great post by Winterbe on how executors work. This is an excerpt from his tutorial

So basically executors always keep listening to the new tasks or callables/runnables and one way to shutdown the executor or stop the executor from listening is to interrupt whatever task it is executing. One way to do is calling the future.get() which stops when the main thread , suspends it and makes sure that the current thread gets executed completely before handing over the resource to other thread

You could probably have a higher number of threads and write your code to shutdown gracefully in the InterruptedException block

Here is a sample code that I've written and tested:

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class ExecutorTest {

    public static void main(String[] args) {

        ExecutorService service = Executors.newWorkStealingPool(10);

        Callable<AccountClass> newInstance = () -> {
            TimeUnit.SECONDS.sleep(3);
            return getAcc(Thread.currentThread().getId());
        };

        // for now only one instance is added to the list
        // List<Callable<AccountClass>> callablesSingleList = Arrays.asList(newInstance);

        // adding multipleCallalbes
        List<Callable<AccountClass>> callablesMultipleList = Arrays.asList(
                () -> {
                    TimeUnit.SECONDS.sleep(3);
                    return getAcc(Thread.currentThread().getId());
                },
                () -> {
                    TimeUnit.SECONDS.sleep(3);
                    return getAcc(Thread.currentThread().getId());
                },
                () -> {
                    TimeUnit.SECONDS.sleep(3);
                    return getAcc(Thread.currentThread().getId());
                });

        try {
            service.invokeAll(callablesMultipleList).stream().map(future -> {
                AccountClass fuClass = null;
                try {
                    fuClass = future.get();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (ExecutionException e) {
                    e.printStackTrace();
                }
                return fuClass;
            }).forEach(getValue -> {
                System.out.println("retunred value:" + getValue);
            });
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }

    }

    private static AccountClass getAcc(long itr) {
        // probably call DB for every new thread iterator
        System.out.println("getting the current thread" + itr);
        AccountClass ac = new AccountClass();
        ac.setId(itr);
        ac.setName("vv");
        ac.setRole("admin");
        System.out.println("sending the accnt class:" + ac);
        return ac;
    }
}

UPDATE:

Another way of shutting down the executor is using the service.shutDownNow() - > which shutdowns the program even if its the middle of execution. You could use awaitTermination method to specify if you feel that it might take a few minutes to complete execution and then probably shutdown the service

import java.util.List;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class ExecutorScheduleFixedRate {

    public static void main(String[] args) {

        ScheduledExecutorService service = Executors.newScheduledThreadPool(10);

        Runnable task = () -> {
            getAcc(33);
        };

        service.scheduleWithFixedDelay(task, 10, 5, TimeUnit.SECONDS);

        if (!service.isShutdown()) {
            List<Runnable> list2 = service.shutdownNow();
            System.out.println(list2);
            System.out.println("is shutdonw" + service.isShutdown());
            System.out.println("Do something after the thread execution");
        }

    }

    private static AccountClass getAcc(long itr) {
        // probably call DB for every new thread iterator
        System.out.println("getting the current thread" + itr);
        AccountClass ac = new AccountClass();
        ac.setId(itr);
        ac.setName("vv");
        ac.setRole("admin");
        System.out.println("sending the accnt class:" + ac);
        return ac;
    }

}