How to stop a Callable submitted to ExecutorService?

Question

I'm trying to implement a sample application to test Callable and ExecutorService interfaces.

In my app I have declared:

ExecutorService exSvc = Executors.newSingleThreadExecutor();

Then:

Future<Integer> test = exSvc.submit(
    new Callable<Integer>() {
        public Integer call() {
            for(int i = 0; i < 1000; i++){
                System.out.println(i);
            }
            return 1;
        }
    });

Now I'm trying to stop the process before it terminate, I'm using exSvc.shutdownNow() but it doesn't work.

To stop gracefully a classical Thread I usually use some kind of condition variable. Which is a common approach to follow with ExecutorService?

Answer 1

Future.cancel(true) and ExecutorService.shutdownNow() use thread interruption. As long as you don't make uninterruptable blocking calls in your task, all you need is to handle interrupted condition correctly, something like this:

for(int i = 0; i < 1000; i++){
    // Uses isInterrupted() to keep interrupted status set
    if (Thread.currentThread().isInterrupted()) {
        // Cannot use InterruptedException since it's checked
        throw new RuntimeException(); 
    }
    System.out.println(i);
}

If you make uninterruptable blocking calls (such as network IO), things become more complex, you need to interrupt them manually somehow, for example, by closing the underlying sockets.

Answer 2

This is how I'd do it with a FixedThreadPool, hope it's of some help.

    ExecutorService pool = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());

    List<Future<Void>> results = new ArrayList<>();

    for (int i = 0; i < numberOfJobs; i++) {
        MyCallableJob job = new MyCallableJob (...);
        results.add(pool.submit(job));
    }

    for (Future<Void> result : results) {
        try { result.get(); }
        catch (InterruptedException | ExecutionException ignorable) { }
    }

    pool.shutdown();