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

There's an implementation of this in the Google Collections/Guava library: MinMaxPriorityQueue.

A min-max priority queue can be configured with a maximum size. If so, each time the size of the queue exceeds that value, the queue automatically removes its greatest element according to its comparator (which might be the element that was just added). This is different from conventional bounded queues, which either block or reject new elements when full.

Answer 2

Of the top of my head, I'd subclass it and overwrite the put method to enforce this. If it goes over throw an exception or do whatever seems appropriate.

Something like:

public class LimitedPBQ extends PriorityBlockingQueue {

    private int maxItems;
    public LimitedPBQ(int maxItems){
        this.maxItems = maxItems;
    }

    @Override
    public boolean offer(Object e) {
        boolean success = super.offer(e);
        if(!success){
            return false;
        } else if (this.size()>maxItems){
            // Need to drop last item in queue
            // The array is not guaranteed to be in order, 
            // so you should sort it to be sure, even though Sun's Java 6 
            // version will return it in order
            this.remove(this.toArray()[this.size()-1]);
        }
        return true;
    }
}

Edit: Both add and put invoke offer, so overriding it should be enough

Edit 2: Should now remove the last element if over maxItems. There may be a more elegant way of doing it though.

Answer 3

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<T> toRetain = new ArrayList<T>(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.

Answer 4

There is an implementation in the ConcurrencyUtils repo.

Answer 5

After implementing a BoundedPriorityBlockingQueue according to what Frank V suggested I realized it didn't do quite what I wanted. The main problem is that the item which I have to insert into the queue may be a higher priority than everything already in the queue. Thus what I really want is a 'pivot' method, if I put an object into the queue, when the queue is full, I want to get back the lowest priority object, rather than blocking.

To flesh out Frank V's suggestions I used the following fragments...

public class BoundedPriorityBlockingQueue<E> 
   implements 
     Serializable, 
     Iterable<E>, 
     Collection<E>, 
     BlockingQueue<E>, 
     Queue<E>, 
     InstrumentedQueue 
{

... private final ReentrantLock lock; // = new ReentrantLock(); private final Condition notFull;

final private int capacity;
final private PriorityBlockingQueue<E> queue;

public BoundedPriorityBlockingQueue(int capacity) 
  throws IllegalArgumentException, 
         NoSuchFieldException, 
         IllegalAccessException 
{
   if (capacity < 1) throw 
       new IllegalArgumentException("capacity must be greater than zero");      
   this.capacity = capacity;
   this.queue = new PriorityBlockingQueue<E>();

   // gaining access to private field
   Field reqField;
   try {
    reqField = PriorityBlockingQueue.class.getDeclaredField("lock");
    reqField.setAccessible(true);
    this.lock = (ReentrantLock)reqField.get(ReentrantLock.class);
    this.notFull = this.lock.newCondition();

   } catch (SecurityException ex) {
    ex.printStackTrace();
    throw ex;
   } catch (NoSuchFieldException ex) {
    ex.printStackTrace();
    throw ex;
   } catch (IllegalAccessException ex) {
    ex.printStackTrace();
    throw ex;
   }

...

@Override
public boolean offer(E e) {
    this.lock.lock();
    try {
        while (this.size() == this.capacity)
            notFull.await();
        boolean success = this.queue.offer(e);
        return success;
    } catch (InterruptedException ie) {
        notFull.signal(); // propagate to a non-interrupted thread
        return false;

    } finally {
        this.lock.unlock();
    }
}
...

This also has some instrumentation so I can check the effectiveness of the queue. I am still working on 'PivotPriorityBlockingQueue', if anyone is interested I can post it.

Answer 6

I actually wouldn't subclass it. While I can't put together example code right now, I'd suggest a version of the decorator pattern.

Create a new class and implement the interfaces implemented by your class of interest: PriorityBlockingQueue. I've found the following interfaces used by this class:

Serializable, Iterable<E>, Collection<E>, BlockingQueue<E>, Queue<E>

In the constructor for a class, accept a PriorityBlockingQueue as a constructor parameter.

Then implement all the methods required by the interfaces via the instances of the PriorityblockingQueue. Add any code required to make it Bounded. This is a fairly standard implementation of a Decorator pattern.