在java中有没有（无界）公平阻塞队列？ [英] Is there any (unbounded) fair blocking queue in java?

问题描述

是否有阻塞队列的任何实现，如果多个消费者从同一队列中删除元素，则该队列保证公平的take（）操作。我检查LinkedBlockingQueue，LinkedTransferQueue和看起来像他们都不公平。 ArrayBlockingQueue提供公平的操作，但它的有界。

Is there any implementation of blocking queue which guarantees fair take() operation if multiple consumers are removing element from the same queue. I checked LinkedBlockingQueue, LinkedTransferQueue and looks like both of them are unfair. ArrayBlockingQueue provides fair operation but its bounded.

推荐答案

我们可以使用无界队列像ConcurrentLinked队列公平的信号量。下面的类不实现BlockingQueue接口的所有方法，但只是其中的一些用于演示的目的。 main（）方法仅作为测试。

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;
    }
}

这篇关于在java中有没有（无界）公平阻塞队列？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！