Java生产者-消费者:生产者不通知消费者

Question

我试着用一个生产者和一个消费者来实现一个生产者-消费者问题。生产者不能创造超过五个产品。如果没有，消费者就不能消费产品。

当需要的时候，我把他们都锁在一个叫做“监视器”的领域里。

这是我的代码：

import java.util.concurrent.TimeUnit;

public class ConsumerProducer {

private static final Object monitor = new Object();
private final int MAX_PRODUCTS = 5;
private String[] products = new String[MAX_PRODUCTS];

int slotToProduce = 0;
int slotToConsume = 0;

private Thread producer = new Thread(new Producer());
private Thread consumer = new Thread(new Consumer());

class Producer implements Runnable {

    private synchronized void produce() {
        synchronized (monitor) {
            // Acquiring access to produce a product
            while (slotToProduce - slotToConsume == MAX_PRODUCTS) {
                try {
                    this.wait();
                } catch (InterruptedException e) {
                }
            }

            // Producing a product
            System.out.println("Will now produce product " + slotToProduce);
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
            }
            products[slotToProduce % MAX_PRODUCTS] = "Teddy Bear "
                    + Integer.toString(slotToProduce);
            System.out.println("Successfully produced product "
                    + products[slotToProduce % MAX_PRODUCTS]);
            slotToProduce++;

            // Notifying consumers if there were no products before
            if (slotToProduce - slotToConsume == 1) {
                notify();
            }
        }
    }

    @Override
    public void run() {
        while (true) {
            produce();
        }
    }

}

class Consumer implements Runnable {

    private synchronized void consume() {
        synchronized (monitor) {
            // Acquiring access to consume a product
            while (slotToProduce == slotToConsume) {
                try {
                    this.wait();
                } catch (InterruptedException e) {
                }
            }

            // Consuming a product
            System.out.println("Will now consume product "
                    + products[slotToConsume % MAX_PRODUCTS]);
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
            }
            System.out.println("Successfully consumed product "
                    + products[slotToConsume % MAX_PRODUCTS]);
            slotToConsume++;

            // Notifying producers if before there were no spaces to produce
            // a product
            if (slotToProduce - slotToConsume == MAX_PRODUCTS - 1) {
                notify();
            }
        }
    }

    @Override
    public void run() {
        while (true) {
            consume();
        }
    }
}

public static void main(String args[]) {
    final ConsumerProducer cp = new ConsumerProducer();
    cp.producer.start();
    cp.consumer.start();
}

}

但是，当我运行我的程序时，输出是：

• 现在将生产0产品
• 成功生产泰迪熊0
• 将生产产品1
• 成功生产泰迪熊1号
• 将生产产品2
• 成功生产泰迪熊2
• 将生产产品3
• 成功生产泰迪熊3
• 将生产产品4
• 成功生产泰迪熊4

从现在起节目停止了。

所以问题是:即使我在同一个对象“监视器”上同步了消费者和生产者，为什么消费者总是在睡觉呢？

Answer 1

你在监视对象上同步。但是，您可以在生产者和使用者对象上调用睡眠和通知。

所需更改：

1. 调用wait()，在监视器上通知()
2. 去除方法上的同步。仅在监视器对象上保持同步。
3. 永远不要忽视被打断的异常！
4. 考虑使用队列。查看接口java.util.concurrent.BlockingQueue