How do I implement task prioritization using an ExecutorService in Java 5?

Question

I am implementing a thread pooling mechanism in which I'd like to execute tasks of varying priorities. I'd like to have a nice mechanism whereby I can submit a high priority task to the service and have it be scheduled before other tasks. The priority of the task is an intrinsic property of the task itself (whether I express that task as a Callable or a Runnable is not important to me).

Now, superficially it looks like I could use a PriorityBlockingQueue as the task queue in my ThreadPoolExecutor, but that queue contains Runnable objects, which may or may not be the Runnable tasks I've submitted to it. Moreover, if I've submitted Callable tasks, it's not clear how this would ever map.

Is there a way to do this? I'd really rather not roll my own for this, since I'm far more likely to get it wrong that way.

(An aside; yes, I'm aware of the possibility of starvation for lower-priority jobs in something like this. Extra points (?!) for solutions that have a reasonable guarantee of fairness)

Answer 1

Would it be possible to have one ThreadPoolExecutor for each level of priority? A ThreadPoolExecutor can be instanciated with a ThreadFactory and you could have your own implementation of a ThreadFactory to set the different priority levels.

 class MaxPriorityThreadFactory implements ThreadFactory {
     public Thread newThread(Runnable r) {
         Thread thread = new Thread(r);
         thread.setPriority(Thread.MAX_PRIORITY);
     }
 }

Answer 2

At first blush it would seem you could define an interface for your tasks that extends Runnable or Callable<T> and Comparable. Then wrap a ThreadPoolExecutor with a PriorityBlockingQueue as the queue, and only accept tasks that implement your interface.

Taking your comment into account, it looks like one option is to extend ThreadPoolExecutor, and override the submit() methods. Refer to AbstractExecutorService to see what the default ones look like; all they do is wrap the Runnable or Callable in a FutureTask and execute() it. I'd probably do this by writing a wrapper class that implements ExecutorService and delegates to an anonymous inner ThreadPoolExecutor. Wrap them in something that has your priority, so that your Comparator can get at it.

Answer 3

Hi Chris

I am facing the same challenge which you had faced.Did you find any solution for this. If this din't worked what had you done to achieve the goal.

Answer 4

the correct answer is here

http://binkley.blogspot.com/2009/04/jumping-work-queue-in-executor.html