java - 生产者/消费者死锁原因及调试

Question

虽然以下是众所周知的话题，但我想请您提供意见。我写了一个小程序如下:所有生产者和消费者都排队。我不明白为什么会这样。什么场景下可以完全阻塞。

让我们考虑一下生产者/消费者正在等待数组上的锁，以及是什么让消费者/生产者退出同步块(synchronized block)。我的意思是它必须至少缓慢地移动，但决不能发生僵局。我相信。

这里我有两个问题:1. 死锁发生的场景有哪些。2. 如何理解幕后发生的事情。我的意思是如何调试。

public class ProducerConsumer implements Runnable {

    boolean producer = false;

    private volatile int i = 0;

    int[] array = new int[10];

    public static String getThreadName() {
        return Thread.currentThread().getName();
    }

    public void producer() {
        try {
            synchronized (array) {
                while (i > 9) {
                    System.out.println("Producer of " + getThreadName()
                            + " is waiting i " + i);
                    array.wait();
                    System.out.println("Producer of " + getThreadName()
                            + " came out of sleep i " + i);
                }
                System.out.println("Producer of " + getThreadName()
                        + " in synchronized block i" + i);
                array[i] = generateRandom();
                System.out.println("Producer of " + getThreadName()
                        + " inserted in array " + array[i] + " index " + i);
                i++;
                array.notifyAll();
            }
            Thread.sleep(100);
        } catch (InterruptedException e) {
            System.out.println("Producer of " + getThreadName()
                    + " interrupted " + e);
        }

    }

    public void consumer() {
        try {
            synchronized (array) {
                while (i < 0) {
                    System.out.println("Consumer of " + getThreadName()
                            + " is waiting i " + i);
                    array.wait();
                    System.out.println("Consumer of " + getThreadName()
                            + " came out of sleep i " + i);
                }
                System.out.println("Consumer of " + getThreadName()
                        + " in synchronized block extracted value " + array[i]
                        + " of index " + i);
                i--;
                array.notifyAll();
            }
            Thread.sleep(100);
        } catch (InterruptedException e) {
            System.out.println("Consumer of " + getThreadName()
                    + " interrupted " + e);
        }

    }

    public static int generateRandom() {
        Random random = new Random();
        return random.nextInt(10);
    }

    public static void main(String[] args) {
        ProducerConsumer pc = new ProducerConsumer();
        for (int i = 0; i < 4; i++) {
            if (i % 2 == 0)
                new Thread(pc, "producer thread").start();
            else {
                new Thread(pc, "consumer thread").start();
            }
        }
    }

    public void run() {
        while (true) {
            if (getThreadName().equalsIgnoreCase("producer thread")) {
                producer();
            } else {
                consumer();
            }
        }

    }
}

它的输出如下:

Consumer of consumer thread in synchronized block extracted value 0 of index 0
Producer of producer thread in synchronized block i-1
Producer of producer thread in synchronized block i-1
Consumer of consumer thread is waiting i -1
Consumer of consumer thread is waiting i -1

Answer 1

您的代码在很多地方都不正确。

我希望所有线程都以异常结束，要么是因为

• IllegalMonitorException(在 ProducerConsumer 对象上调用 notification())但此 ProducerConsumer 对象上没有同步块(synchronized block))
• ArrayIndexOfBoundsException(i 在 Produce() 方法中可以变为 10)

您检查过错误输出吗？