Question

I have the following problem that the standard library doesn’t solve well, and I’m wondering if anybody has seen another library out there than can do it so I don’t need to hack together a custom solution. I have a task that is currently scheduled on a thread pool using scheduleWithFixedDelay(), and I need to modify the code to handle requests for “urgent” execution of the task related to asynchronous events. Thus, if the task is scheduled to occur with a delay of 5 minutes between executions, and an event occurs 2 minutes after the last completed execution, I would like to execute the task immediately and then have it wait for 5 minutes after the completion of the urgent execution before it runs again. Right now the best solution that I can come up with is to have the event handler call cancel() on the ScheduledFuture object returned by scheduleWithFixedDelay() and execute the task immediately, and then set a flag in the task to tell it to reschedule itself with the same delay parameters. Is this functionality available already and I’m just missing something in the documentation?

Answer 1

If you are using ScheduledThreadPoolExecutor there is a method decorateTask (well in fact there are two, for Runnable and Callable tasks) that you can override to store a reference to the task somewhere.

When you need urgent execution, you just call run() on that reference which makes it run and rescheduled with same delay.

A quick hack-up attempt:

public class UrgentScheduledThreadPoolExecutor extends
        ScheduledThreadPoolExecutor {
    RunnableScheduledFuture scheduledTask;

    public UrgentScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize);
    }

    @Override
    protected  RunnableScheduledFuture decorateTask(Runnable runnable,
            RunnableScheduledFuture task) {
        scheduledTask = task;
        return super.decorateTask(runnable, task);
    }

    public void runUrgently() {
        this.scheduledTask.run();
    }
}

which can be used like this:

public class UrgentExecutionTest {

    public static void main(String[] args) throws Exception {
        UrgentScheduledThreadPoolExecutor pool = new UrgentScheduledThreadPoolExecutor(5);

        pool.scheduleWithFixedDelay(new Runnable() {
            SimpleDateFormat format = new SimpleDateFormat("ss"); 

            @Override
            public void run() {
                System.out.println(format.format(new Date()));
            }
        }, 0, 2L, TimeUnit.SECONDS);
        Thread.sleep(7000);
        pool.runUrgently();
        pool.awaitTermination(600, TimeUnit.SECONDS);
    }
}

and produces the following output:
06
08
10
11
13
15