How to only execute Scheduler if it is not started?

Question

ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
if(executor.isShutdown()) {
    executor.scheduleAtFixedRate(helloRunnable, 0, 1, TimeUnit.SECONDS);
}

This piece of code is executed everytime a event happens, how do i make it only execute for the first time?(like, if the scheduler is starter, do not execute)

I tried using executor.isShutdown() but it does not work

[ScheduledExecutorService - Check if scheduled task has already been completed](https://stackoverflow.com/questions/21442322/scheduledexecutorservice-check-if-scheduled-task-has-already-been-completed) may be helpful. — Eskandar Abedini, Jun 17 '22 at 18:37

score 0 · Answer 1 · answered Jun 18 '22 at 15:40

You can use AtomicBoolean to manage access to the executor:

    private final ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
    private static final AtomicBoolean IS_ACTIVE = new AtomicBoolean(false);

    private void startScheduler() {
        if (IS_ACTIVE.compareAndSet(false, true)) {
            executor.scheduleAtFixedRate(() -> {}, 0L, 5L, TimeUnit.SECONDS);
        }
    }

Or you can use synchronized and lazy initialization to make it work:

    private volatile ScheduledExecutorService scheduler;

    public void startScheduler() {
        if (scheduler == null) {
            this.initAndStart();
        }
    }

    private synchronized void initAndStart() {
        if (scheduler == null) {
            scheduler = Executors.newSingleThreadScheduledExecutor();
            scheduler.schedule(() -> log.info("Started, Thread: {}", Thread.currentThread().getName()), 0L, TimeUnit.SECONDS);
        }
    }

Note: It will try to block current class and threads which managed to call initAndStart will also cause blocking, but as soon as scheduler is initialized there will be no blocking

How to only execute Scheduler if it is not started?

1 Answers1