Testing PriorityBlockingQueue in ThreadPoolExecutor

Question

I realized my ThreadPoolExecutor with PriorityBlockingQueue like in this example: https://stackoverflow.com/a/12722648/2206775

and wrote a test:

Prio         


        

Answer 1

The priority is only taken into account if the pool is fully busy and you submit several new tasks. If you define your pool with only one thread, you should get the expected output. In your example, all tasks get executed concurrently and which one finishes first is somewhat random.

By the way the linked implementation has a problem and throws an exception if your queue is full and you submit new tasks.

See below a working example of what you are trying to achieve (I have overriden newTaskFor in a simplistic way, just to make it work - you might want to improve that part).

It prints: 1 2 3 4 5.

public class Test {

    public static void main(String[] args) {
        PriorityExecutor executorService = (PriorityExecutor) PriorityExecutor.newFixedThreadPool(1);
        executorService.submit(getRunnable("1"), 1);
        executorService.submit(getRunnable("3"), 3);
        executorService.submit(getRunnable("2"), 2);
        executorService.submit(getRunnable("5"), 5);
        executorService.submit(getRunnable("4"), 4);

        executorService.shutdown();
        try {
            executorService.awaitTermination(30, TimeUnit.MINUTES);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static Runnable getRunnable(final String id) {
        return new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                    System.out.println(id);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };
    }

    static class PriorityExecutor extends ThreadPoolExecutor {

        public PriorityExecutor(int corePoolSize, int maximumPoolSize,
                                long keepAliveTime, TimeUnit unit, BlockingQueue workQueue) {
            super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
        }
        //Utitlity method to create thread pool easily

        public static ExecutorService newFixedThreadPool(int nThreads) {
            return new PriorityExecutor(nThreads, nThreads, 0L,
                                        TimeUnit.MILLISECONDS, new PriorityBlockingQueue());
        }
        //Submit with New comparable task

        public Future submit(Runnable task, int priority) {
            return super.submit(new ComparableFutureTask(task, null, priority));
        }
        //execute with New comparable task

        public void execute(Runnable command, int priority) {
            super.execute(new ComparableFutureTask(command, null, priority));
        }

        @Override
        protected  RunnableFuture newTaskFor(Callable callable) {
            return (RunnableFuture) callable;
        }

        @Override
        protected  RunnableFuture newTaskFor(Runnable runnable, T value) {
            return (RunnableFuture) runnable;
        }
    }

    static class ComparableFutureTask extends FutureTask implements Comparable> {

        volatile int priority = 0;

        public ComparableFutureTask(Runnable runnable, T result, int priority) {
            super(runnable, result);
            this.priority = priority;
        }

        public ComparableFutureTask(Callable callable, int priority) {
            super(callable);
            this.priority = priority;
        }

        @Override
        public int compareTo(ComparableFutureTask o) {
            return Integer.valueOf(priority).compareTo(o.priority);
        }
    }
}