How to remove a task from ScheduledExecutorService?

Question

I have a ScheduledExecutorService that times a few different task periodically with scheduleAtFixedRate(Runnable, INIT_DELAY, ACTION_DELAY, TimeUnit.SECONDS);

I also have a different Runnable that I'm using with this scheduler. the problem starts when I want to remove one of the tasks from the scheduler.

Is there a way to do this?

Am I doing the right thing using one scheduler for different tasks? What is the best way to implement this?

Answer 1

Simply cancel the future returned by scheduledAtFixedRate() :

// Create the scheduler
ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1);
// Create the task to execute
Runnable r = new Runnable() {
    @Override
    public void run() {
        System.out.println("Hello");
    }
};
// Schedule the task such that it will be executed every second
ScheduledFuture<?> scheduledFuture =
    scheduledExecutorService.scheduleAtFixedRate(r, 1L, 1L, TimeUnit.SECONDS);
// Wait 5 seconds
Thread.sleep(5000L);
// Cancel the task
scheduledFuture.cancel(false);

Another thing to note is that cancel does not remove the task from scheduler. All it ensures is that isDone method always return true . This may lead to memory leaks if you keep adding such tasks. For eg: if you start a task based on some client activity or UI button click, repeat it n-times and exit. If that button is clicked too many times, you might end up with big pool of threads that cannot be garbage collected as scheduler still has a reference.

You may want to use setRemoveOnCancelPolicy(true) in ScheduledThreadPoolExecutor class available in Java 7 onwards. For backward compatibility, default is set to false.

Answer 2

如果您的ScheduledExecutorService实例扩展了ThreadPoolExecutor （例如ScheduledThreadPoolExecutor ），您可以使用remove(Runnable) （但请参阅其 javadoc 中的注释：“在放入内部队列之前，它可能无法删除已转换为其他形式的任务。” ) 或purge() 。