ThreadPoolExecutor backed by PriorityBlockingQueue doesn't seem to work

Question

I have a large number of pictures to fetch from a server, and I want to fetch some pictures with higher priority than others so I've implemented my own ThreadPoolExecutor that returns a FutureTask that implements Comparable but it doesn't seem to work. The tasks are more or less processed in the order I add them to the queue. I've debugged the BlockingQueue of my ThreadPoolExecutor and found out that when I add my Runnable with a higher priority, it is not shifted all the way up at the top of the queue. Here is the code

public class PriorityThreadPoolExecutor extends ThreadPoolExecutor {

    public PriorityThreadPoolExecutor(int corePoolSize, int maximumPoolSize,
            long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
    }

    protected <T> RunnableFuture<T> newTaskForValue(Runnable runnable, T value) {
        return new ComparableFutureTask<T>(runnable, value);
    }

    protected class ComparableFutureTask<T> 
    extends FutureTask<T> implements Comparable<ComparableFutureTask<T>> {

        private Object object;

        public ComparableFutureTask(Runnable runnable, T result) {
            super(runnable, result);
            object = runnable;
        }

        @Override
        @SuppressWarnings({ "unchecked", "rawtypes" })
        public int compareTo(ComparableFutureTask<T> o) {
            if (this == o) {
                return 0;
            }
            if (o == null) {
                return -1; // this has higher priority than null
            }
            if (object != null && o.object != null) {
                if (object.getClass().equals(o.object.getClass())) {
                    if (object instanceof Comparable) {
                        return ((Comparable) object).compareTo(o.object);
                    }
                }
            }
            return 0;
        }
    }

}

And I add the tasks to the pool in this way:

public BitmapLoader(Context context){
        mThreadPoolExecutor = new PriorityThreadPoolExecutor(10, Integer.MAX_VALUE,//corepool and maxpool
                1L, TimeUnit.SECONDS,//keep alive idle threads
                new PriorityBlockingQueue<Runnable>());//priority queue for jobs
    }

public void queuePhoto(String url, ImageView imageView, int priority) {     
    BitmapToLoad p = new BitmapToLoad(url, imageView, priority);
    final RunnableFuture<Object> futureTask = 
            mThreadPoolExecutor.newTaskForValue(new BitmapLoaderRunnable(p), null);
    Log.d("BitmapLoader", "Scheduling job with priority " + priority);
    mThreadPoolExecutor.execute(futureTask);
}

My BitmapLoaderRunnable implements Comparable and when I debug the compareTo method is being called. What am I doing wrong? Thanks

EDIT: below is the code of my runnables

private class BitmapLoaderRunnable implements Runnable, Comparable<BitmapLoaderRunnable> {
        private BitmapToLoad bitmapToLoad;

        public BitmapLoaderRunnable(BitmapToLoad bitmap) {
            this.bitmapToLoad = bitmap;
        }

        @Override
        public void run() {
            try{
                if(imageViewReused(bitmapToLoad))
                    return;
                Thread.sleep(1000);
                Bitmap bmp = getBitmap(bitmapToLoad.url);
                BitmapCache.put(bitmapToLoad.url, bmp);
                if(imageViewReused(bitmapToLoad))
                    return;
                BitmapDisplayer bd = new BitmapDisplayer(bmp, bitmapToLoad);
                mHandler.post(bd);
            } catch(Throwable th){
                th.printStackTrace();
            }
        }

        @Override
        public int compareTo(BitmapLoaderRunnable other) {
            return this.bitmapToLoad.priority - other.bitmapToLoad.priority;
        }
    }

Answer 1

The head of a PriorityQueue is the least element. so if you want the highest priority first, you need to reverse your comparison.

    @Override
    public int compareTo(BitmapLoaderRunnable other) {
        return other.bitmapToLoad.priority - this.bitmapToLoad.priority;
    }