可以增加的闩锁

Question

有谁知道是否有任何执行以下操作的闩锁实现：

• 有一种减少锁存器值的方法，或者如果该值为零则等待
• 具有等待锁存器值为零的方法
• 具有将数字添加到锁存器值的方法

Answer 1

除了从AQS开始，您可以使用下面的简单实现。它有些天真（它是同步的，而AQS是无锁算法），但是除非您希望在满足条件的情况下使用它，否则它可能已经足够好了。

public class CountUpAndDownLatch {
    private CountDownLatch latch;
    private final Object lock = new Object();

    public CountUpAndDownLatch(int count) {
        this.latch = new CountDownLatch(count);
    }

    public void countDownOrWaitIfZero() throws InterruptedException {
        synchronized(lock) {
            while(latch.getCount() == 0) {
                lock.wait();
            }
            latch.countDown();
            lock.notifyAll();
        }
    }

    public void waitUntilZero() throws InterruptedException {
        synchronized(lock) {
            while(latch.getCount() != 0) {
                lock.wait();
            }
        }
    }

    public void countUp() { //should probably check for Integer.MAX_VALUE
        synchronized(lock) {
            latch = new CountDownLatch((int) latch.getCount() + 1);
            lock.notifyAll();
        }
    }

    public int getCount() {
        synchronized(lock) {
            return (int) latch.getCount();
        }
    }
}

注意：我尚未对其进行深入测试，但它的行为似乎与预期的一样：

public static void main(String[] args) throws InterruptedException {
    final CountUpAndDownLatch latch = new CountUpAndDownLatch(1);
    Runnable up = new Runnable() {
        @Override
        public void run() {
            try {
                System.out.println("IN UP " + latch.getCount());
                latch.countUp();
                System.out.println("UP " + latch.getCount());
            } catch (InterruptedException ex) {
            }
        }
    };

    Runnable downOrWait = new Runnable() {
        @Override
        public void run() {
            try {
                System.out.println("IN DOWN " + latch.getCount());
                latch.countDownOrWaitIfZero();
                System.out.println("DOWN " + latch.getCount());
            } catch (InterruptedException ex) {
            }
        }
    };

    Runnable waitFor0 = new Runnable() {
        @Override
        public void run() {
            try {
                System.out.println("WAIT FOR ZERO " + latch.getCount());
                latch.waitUntilZero();
                System.out.println("ZERO " + latch.getCount());
            } catch (InterruptedException ex) {
            }
        }
    };
    new Thread(waitFor0).start();
    up.run();
    downOrWait.run();
    Thread.sleep(100);
    downOrWait.run();
    new Thread(up).start();
    downOrWait.run();
}

输出：

IN UP 1
UP 2
WAIT FOR ZERO 1
IN DOWN 2
DOWN 1
IN DOWN 1
ZERO 0
DOWN 0
IN DOWN 0
IN UP 0
DOWN 0
UP 0