使用信号量的监控程序在 java 中无法按预期工作

Question

我正在尝试在 java 中创建一个多生产者多消费者问题的解决方案（即监视器）。

我决定使用 Semaphore class 并同步如下。

import java.util.concurrent.Semaphore;

public class PC implements Runnable {
    Semaphore s;
    Object lock;
    boolean isProducer;
    final int ProcessCount = 4;
    final int producerDelay = 1500;
    final int consumerDelay = 1000;

    public static void main(String[] args) {
        new PC();
    }

    PC() {
        this.s = new Semaphore(10);
        this.lock = new Object();
        this.isProducer = true;
        // create few consumers and producers
        for (int i = 0; i < ProcessCount; i++) {
            new Thread(this).start();
        }
    }

    public void run() {    //alternately create producers and consumers
        if (isProducer) {
            isProducer = false;
            producer();
        } else {
            isProducer = true;
            consumer();
        }
    }

    void producer() {
        try {
            while (true) {
                synchronized (this.lock) {
                    s.release(); // produce an resource
                }
                System.out.println("produced:\tAvailable now:\t" + s.availablePermits());
                Thread.sleep(producerDelay);
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

    void consumer() {
        try {
            while (true) {
                synchronized (this.lock) {
                    s.acquire(); // consume resource
                }
                System.out.println("consumed:\tAvailable now:\t" + s.availablePermits());
                Thread.sleep(consumerDelay);
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

但是输出并不像预期的那样。当生产者延迟小于消费者延迟时，信号量可用资源的数量超过 10。另一方面，当生产者延迟更大时，当资源变为 0 时程序似乎陷入死锁（而我希望消费者线程暂停, 直到资源线程产生一些东西)

示例输出：

当消费者延迟较低时：

consumed:       Available now:  10
consumed:       Available now:  10
produced:       Available now:  11
produced:       Available now:  10
consumed:       Available now:  8
consumed:       Available now:  8
produced:       Available now:  10
produced:       Available now:  10
consumed:       Available now:  9
consumed:       Available now:  8
produced:       Available now:  9
consumed:       Available now:  8
produced:       Available now:  9
consumed:       Available now:  8
consumed:       Available now:  7
consumed:       Available now:  6
produced:       Available now:  7
produced:       Available now:  8
consumed:       Available now:  7
consumed:       Available now:  6
produced:       Available now:  7
produced:       Available now:  8
consumed:       Available now:  7
consumed:       Available now:  6
consumed:       Available now:  5
consumed:       Available now:  4
produced:       Available now:  5
produced:       Available now:  6
consumed:       Available now:  5
consumed:       Available now:  4
produced:       Available now:  5
produced:       Available now:  6
consumed:       Available now:  5
consumed:       Available now:  4
consumed:       Available now:  3
consumed:       Available now:  2
produced:       Available now:  3
produced:       Available now:  4
consumed:       Available now:  3
consumed:       Available now:  2
produced:       Available now:  3
produced:       Available now:  4
consumed:       Available now:  3
consumed:       Available now:  2
consumed:       Available now:  0
consumed:       Available now:  0
produced:       Available now:  2
produced:       Available now:  2
consumed:       Available now:  1
consumed:       Available now:  0
produced:       Available now:  1
produced:       Available now:  2
consumed:       Available now:  1
consumed:       Available now:  0

当生产者延迟较低时：

produced:       Available now:  11
consumed:       Available now:  10
produced:       Available now:  11
consumed:       Available now:  10
produced:       Available now:  11
produced:       Available now:  12
consumed:       Available now:  11
consumed:       Available now:  10
produced:       Available now:  11
produced:       Available now:  12
produced:       Available now:  13
produced:       Available now:  12
consumed:       Available now:  12
consumed:       Available now:  11
produced:       Available now:  13
produced:       Available now:  14
consumed:       Available now:  13
consumed:       Available now:  12
produced:       Available now:  13
produced:       Available now:  14
produced:       Available now:  15
produced:       Available now:  16
consumed:       Available now:  15
consumed:       Available now:  14
produced:       Available now:  15
produced:       Available now:  16
consumed:       Available now:  15
consumed:       Available now:  14
produced:       Available now:  15
produced:       Available now:  16
produced:       Available now:  17
consumed:       Available now:  16
consumed:       Available now:  17
produced:       Available now:  18
produced:       Available now:  17
produced:       Available now:  18
consumed:       Available now:  17
consumed:       Available now:  16
produced:       Available now:  17
produced:       Available now:  18
consumed:       Available now:  16
consumed:       Available now:  17
produced:       Available now:  17
produced:       Available now:  18
produced:       Available now:  19
produced:       Available now:  20
consumed:       Available now:  19
consumed:       Available now:  18
produced:       Available now:  19
produced:       Available now:  20
consumed:       Available now:  18
produced:       Available now:  19
consumed:       Available now:  18
produced:       Available now:  20
produced:       Available now:  21
produced:       Available now:  22
consumed:       Available now:  21
consumed:       Available now:  20
produced:       Available now:  21
produced:       Available now:  22
consumed:       Available now:  21
consumed:       Available now:  20
produced:       Available now:  21
produced:       Available now:  22
produced:       Available now:  23
produced:       Available now:  24
consumed:       Available now:  23
consumed:       Available now:  22
produced:       Available now:  23
produced:       Available now:  24

程序有什么问题？

Answer 1

首先，您创建一个具有 10 个初始许可的信号量，如 here 所述。这不是上限，所以release()可以使可用许可证的数量超过10个。

另外，请看下面的代码：

synchronized (this.lock) {
    s.acquire(); // consume resource
}

您获得了锁。如果没有可用资源，线程会暂停，但不会释放锁。因此，下面的代码无法获取锁也无法执行。消费者等待许可，生产者等待锁，导致死锁。

synchronized (this.lock) {
    s.release(); // produce an resource
}

您可以通过不同步 s.acquire() 和 s.release() 来解决这个问题。

Answer 2

这就是将要发生的事情

    void producer() {
        try {
            while (true) {

                //***All producers are stuck here because "first consumer" is having the lock.
                synchronized (this.lock) {
                    s.release(); // produce an resource
                }
                System.out.println("produced:\tAvailable now:\t" + s.availablePermits());
                Thread.sleep(producerDelay);
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

    void consumer() {
        try {
            while (true) {
                //***All other consumers are stuck here because of line below.
                synchronized (this.lock) {
                    //***"First consumer" is stuck here waiting s.release()
                    s.acquire(); // consume resource
                }
                System.out.println("consumed:\tAvailable now:\t" + s.availablePermits());
                Thread.sleep(consumerDelay);
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

使用信号量的监控程序在 java 中无法按预期工作

Monitor program using semaphore does not work as expected in java

java

multithreading

semaphore

synchronized

producer-consumer