Is there any (unbounded) fair blocking queue in java?

Question

Is there any implementation of blocking queue which guarantees fair take() operation if multiple consumers are removing element from the same queue. I checked LinkedBlocking

Answer 1

We can implement an unbounded fair blocking queue using an unbounded queue like ConcurrentLinked queue and a fair Semaphore. The class below doesn't implement all methods from the BlockingQueue interface but just a few of them for demonstration purposes. The main() method is written as a test only.

public class FairBlockingQueue<T> {

    private final Queue<T> queue;
    private final Semaphore takeSemaphore;

    public FairBlockingQueue() {
        queue = new ConcurrentLinkedQueue<T>();
        takeSemaphore = new Semaphore(0, true);
    }

    public FairBlockingQueue(Collection<T> c) {
        queue = new ConcurrentLinkedQueue<T>(c);
        takeSemaphore = new Semaphore(c.size(), true);
    }

    public T poll() {
        if (!takeSemaphore.tryAcquire()) {
            return null;
        }
        return queue.poll();
    }

    public T poll(long millis) throws InterruptedException {
        if (!takeSemaphore.tryAcquire(millis, TimeUnit.MILLISECONDS)) {
            return null;
        }
        return queue.poll();
    }

    public T take() throws InterruptedException {
        takeSemaphore.acquire();
        return queue.poll();
    }

    public void add(T t) {
        queue.add(t);
        takeSemaphore.release();
    }

    public static void main(String[] args) throws Exception {
        FairBlockingQueue<Object> q = new FairBlockingQueue<Object>();
        Object o = q.poll();
        assert o == null;
        o = q.poll(1000);
        assert o == null;

        q.add(new Object());
        q.add(new Object());
        q.add(new Object());

        o = q.take();
        assert o != null;
        o = q.poll();
        assert o != null;
        o = q.poll(1000);
        assert o != null;

        o = q.poll();
        assert o == null;
    }
}

Answer 2

Fairness policy may be specified for SynchronousQueue:

a queue constructed with fairness set to true grants threads access in FIFO order

Answer 3

PriorityBlockingQueue