How to create LIFO executor?

Question

I would like to create a thread pool which will execute the most recently submitted task. Any advice on how to accomplish this?

Thank you

Answer 1

You could probably just implement your own BlockingQueue wrapper that maps offer/poll to a stack. Then use this as the BlockingQueue implementation you pass to a ThreadPoolExecutor. My suggestion would be to wrap one of the existing Deque implementations such as ArrayDeque.

• This is not synchronized, so you'll need to wrap each of the BlockingQueue methods with a synchronizer (if not something more exotic).
• You'll also need to introduce wait/notify conditions for the blocking operations.
• Finally, you'll need to map one set of the BlockingQueue polarities (either the "put" or the "take" side) to the same end of the dequeue as the other (to treat it like a stack).

Note that there is some work (see Herlihy's book on The Art of Multiprocessor Programming) on faster concurrent stacks, but I don't think there are any implementations in the JDK and I'm not sure if Herlihy's implementations offer blocking flavors.

An Implementation atop Deque

I checked the Android documentation, which suggests that Deque is around for you, so here's an implementation. It's a fairly easy step to do a wrapper around a stack, too, but Deque is preferred.

import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.BlockingDeque;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.TimeUnit;


public final class BlockingLifoQueue<T> implements BlockingQueue<T>
{
  // we add and remove only from the end of the queue
  private final BlockingDeque<T> deque; 

  public BlockingLifoQueue()
  { deque = new LinkedBlockingDeque<T>(); }

  public boolean add(T e) {
    deque.addLast(e);
    return true;
  }

  public boolean contains(Object o)
  { return deque.contains(o); }

  public int drainTo(Collection<? super T> c)
  { return deque.drainTo(c); }

  public int drainTo(Collection<? super T> c, int maxElements)
  { return deque.drainTo(c,maxElements); }

  public boolean offer(T e)
  { return deque.offerLast(e); }

  public boolean offer(T e, long timeout, TimeUnit unit)
      throws InterruptedException
  { return deque.offerLast(e,timeout,unit); }

  public T poll(long timeout, TimeUnit unit) throws InterruptedException
  { return deque.pollLast(timeout, unit); }

  public void put(T e) throws InterruptedException
  { deque.putLast(e); }

  public int remainingCapacity()
  { return deque.size(); }

  public boolean remove(Object o)
  { return deque.remove(o); }

  public T take() throws InterruptedException
  { return deque.takeLast(); }

  public T element()
  {
    if (deque.isEmpty()) { 
      throw new NoSuchElementException("empty stack");
    }

    return deque.pollLast();
  }

  public T peek()
  { return deque.peekLast(); }

  public T poll()
  { return deque.pollLast(); } // deque.peekLast(); } -- fixed typo.

  public T remove()
  {
    if (deque.isEmpty()) { 
      throw new NoSuchElementException("empty stack");
    }

    return deque.pollLast();
  }

  public boolean addAll(Collection<? extends T> c)
  { 
    for (T e : c) { deque.add(e); }
    return true;
  }

  public void clear()
  { deque.clear();}

  public boolean containsAll(Collection<?> c)
  { return deque.containsAll(c); }

  public boolean isEmpty()
  {  return deque.isEmpty(); }

  public Iterator<T> iterator()
  { return deque.descendingIterator(); }

  public boolean removeAll(Collection<?> c)
  { return deque.removeAll(c); }

  public boolean retainAll(Collection<?> c)
  { return deque.retainAll(c); }

  public int size()
  { return deque.size(); }

  public Object[] toArray()
  { return deque.toArray(); }

  public <T> T[] toArray(T[] a)
  { return deque.toArray(a); }
}