Cancelling Scheduler Executor when response is Success

Question

I have a scheduled executor service. It Will call a rest api every 30 seconds. The response is either WAITING or SUCCESS. i need to cancel the executor once the response is SUCCESS.

ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(2);
ScheduledFuture<?> task = scheduledExecutorService.scheduleAtFixedRate(
    () -> //Calling a RestApi returing a response of SUCCESS or WAITING
, 0, 30, TimeUnit.SECONDS);

Have you checked the Javadoc of ScheduledFuture? I immediately see a method (in super-interface Future) `cancel` and if you read it's Javadoc, you see that it does what you want: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Future.html#cancel-boolean- — Erwin Bolwidt, Jul 13 '19 at 02:37
You call it from the code that checks whether the response equals "SUCCESS". — Erwin Bolwidt, Jul 13 '19 at 02:40

score 1 · Answer 1 · answered Jul 13 '19 at 03:04

The general answer to your question can be found on How to remove a task from ScheduledExecutorService?

However to answer your specific question, "how can I do it from within the task?" - it's slightly trickier. You want to avoid the (unlikely) race condition where your task completes before the scheduleAtFixedRate method completes and makes the reference to the ScheduledFuture available to be stored in a field.

The below code solves this by using a CompletableFuture to store the reference to the ScheduledFuture that represents the task.

public class CancelScheduled {
    private ScheduledExecutorService scheduledExecutorService;
    private CompletableFuture<ScheduledFuture<?>> taskFuture;

    public CancelScheduled() {
        scheduledExecutorService = Executors.newScheduledThreadPool(2);
        ((ScheduledThreadPoolExecutor) scheduledExecutorService).setRemoveOnCancelPolicy(true);
    }

    public void run() {
        taskFuture = new CompletableFuture<>();
        ScheduledFuture<?> task = scheduledExecutorService.scheduleAtFixedRate(
                () -> {
                    // Get the result of the REST call (stubbed with "SUCCESS" below)
                    String result = "SUCCESS";
                    if (result.equals("SUCCESS")) {
                        // Get the reference to my own `ScheduledFuture` in a race-condition free way
                        ScheduledFuture<?> me;
                        try {
                            me = taskFuture.get();
                        } catch (InterruptedException | ExecutionException e) {
                            throw new RuntimeException(e);
                        }
                        me.cancel(false);
                    }
                }, 0, 30, TimeUnit.SECONDS);
        // Supply the reference to the `ScheduledFuture` object to the task itself in a race-condition free way
        taskFuture.complete(task);
    }

Cancelling Scheduler Executor when response is Success

1 Answers1