Question

Please assume the following:

public static void main(String[] args) {
    ThreadPoolExecutor pool = new ThreadPoolExecutor(0, 5, 1L, TimeUnit.SECONDS, new PriorityBlockingQueue<Runnable>());
    DashboardHtmlExport d = new DashboardHtmlExport();
    for (int i = 0; i < 40; i++) {
        System.out.println("Submitting: " + i);
        try {
            Thread.sleep(cooldown);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        pool.submit(d.new A(i));
    }
}

private class A implements Runnable, Comparable<A> {
    private int order;

    public A(int order) {
        this.order = order;
    }

    public void run() {
        try {
            Thread.sleep(busyTime);
        } catch (InterruptedException e) {
            System.out.println(e.getMessage());
        }
        System.out.println(new Date());
    }

    public int compareTo(A o) {
        return Integer.valueOf(o.order).compareTo(Integer.valueOf(order));
    }
}

This produced the following error:

Submitting: 0
Submitting: 1
Exception in thread "main" java.lang.ClassCastException: java.util.concurrent.FutureTask cannot be cast to java.lang.Comparable
    at java.util.PriorityQueue.siftUpComparable(PriorityQueue.java:578)
    at java.util.PriorityQueue.siftUp(PriorityQueue.java:574)
    at java.util.PriorityQueue.offer(PriorityQueue.java:274)
    at java.util.concurrent.PriorityBlockingQueue.offer(PriorityBlockingQueue.java:164)
    at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:653)
    at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:78)
    at com.skycomm.cth.pages.test.DashboardHtmlExport.main(DashboardHtmlExport.java:133)
Tue Jul 10 23:48:45 GMT+02:00 2012

Why is this happening ?!

This only works if the busyTime is less that the cooldown time. Meaning that the queue can’t handle more than 2 submitted tasks !!

Basically what I’m trying to do is to have a pool with UNBOUND size and sortable whether by comparable or a comparator. I can only achieve an UNBOUND queue using a LinkedBlockingQueue but that’s of course not sortable !

Thank you.

Answer 1

The ThreadPoolExecutor transforms the Runnable you submit into a RunnableFuture using the newTaskFor() method, and adds this RunnableFuture to the queue. So if you want to use a PriorityQueue, you should override the newTaskFor() method to make sure the RunnableFuture instances it creates are comparable.

Or you could also use the execute() method instead of the submit() method, to bypass the task creation.