Bounded PriorityBlockingQueue

Question

PriorityBlockingQueue is unbounded, but I need to bound it somehow. What is the best way to achieve that?

For information, the bounded PriorityBlo

Answer 1

If the order of the Runnables you want to execute is not strict (as is: it may occur that some lower priority tasks are executed even though higher priority tasks exist), then I would suggest the following, which boils down to periodically cutting the PriorityQueue down in size:

if (queue.size() > MIN_RETAIN * 2){
    ArrayList toRetain = new ArrayList(MIN_RETAIN);
    queue.drainTo(toRetain, MIN_RETAIN);
    queue.clear();
    for (T t : toRetain){
      queue.offer(t);
    }
}

This will obviously fail if the order needs to be strict, as draining will lead to a moment, wenn low priority task will retrieved from the queue using concurrent access.

The advantages are, that this is thread-safe and likely to get as fast as you can do with the priority queue design.