Question

我正在尝试使用以下代码的线程执行一些生产者消费者 poc，一轮后，两个线程都处于等待状态。但我希望它们继续循环，其中一个线程不断增加计数器而其他线程不断减少。

有人可以建议我缺少什么吗？

public class ProducerConsumerWithThreads {

        synchronized void withdrawBoxConsumer() {
            if(box > 0){
                box --;
                System.out.println("Took one box now boxes left "+  box);
            }
            if(box == 0) {
                System.out.println("Please put more boxes");
                notify();
                try{wait();}catch(Exception e){
                    System.out.println("Exception occured" + e.fillInStackTrace());
                }
            } else {
                withdrawBoxConsumer();
            }
        }

        synchronized void putBoxProducer() {
            if(box < 10){
                box ++;
                System.out.println("Put one box now boxes are "+  box);
            }
            if(box == 10) {
                System.out.println("Please Consume boxes");
                notify();
                try{wait();}catch(Exception e){
                    System.out.println("Exception occured" + e.fillInStackTrace());
                }
            } else {
                putBoxProducer();
            }
        }

        public int box = 5;

        public static void main(String[] args) throws InterruptedException {
            //pipeline of 10 boxes
            //consumer takes one at a time .. till its empty

            int boxLimit = 10;
            final int box = 5;
            final ProducerConsumerWithThreads c=new ProducerConsumerWithThreads();

            new Thread(){
                public void run(){c.withdrawBoxConsumer();}
            }.start();
            new Thread(){
                public void run(){c.putBoxProducer();}
            }.start();
        }
}

我得到的输出是：

Took one box now boxes left 4
Took one box now boxes left 3
Took one box now boxes left 2
Took one box now boxes left 1
Took one box now boxes left 0
Please put more boxes
Put one box now boxes are 1
Put one box now boxes are 2
Put one box now boxes are 3
Put one box now boxes are 4
Put one box now boxes are 5
Put one box now boxes are 6
Put one box now boxes are 7
Put one box now boxes are 8
Put one box now boxes are 9
Put one box now boxes are 10
Please Consume boxes

我期待它按照逻辑继续循环下去！有人可以帮忙吗？

Answer 1

您的问题是一个非常基本的流程问题：当调用“notify”并且等待的线程再次启动时，该方法完成并且线程停止运行。

请注意，您的两个方法在同一个对象上同步，因此它们永远不会同时运行。例如。一个线程将获得监视器，并递增/递减该框，直到它最终等待。然后另一个线程会去，直到它等待。

在使用等待和通知时，您还有其他一些非常常见的问题。

    synchronized void withdrawBoxConsumer() {
        while( !Thread.currentThread().isInterrupted() ) { 
            if(box > 0){
                box --;
                System.out.println("Took one box now boxes left "+  box);
            }
            while(box == 0) {
                System.out.println("Please put more boxes");
                notifyAll();
                try{
                    wait();
                }catch(Exception e){
                    throw new RuntimeException(e);
                }
            } 
        }
    }

    synchronized void putBoxProducer() {
        while( !Thread.currentThread().isInterrupted() ) { 
            if(box < 10){
                box ++;
                System.out.println("Put one box now boxes are "+  box);
            }
            while(box == 10) {
                System.out.println("Please Consume boxes");
                notifyAll();
                try{
                    wait();
                }catch(Exception e){
                   throw new RuntimeException(e);
                }
            }
        }
    }

我将其设为非递归，因为按照您的做法，堆栈会溢出。
由于虚假唤醒，等待条件处于循环中。
我切换到 notifyAll 通知只会唤醒一个等待线程，在这种情况下应该没问题，但最好是安全的。
box 理想情况下应该是一个并发类或 volatile，但由于您总是在同步方法中工作，所以应该没问题。
同样，box++ 和 box-- 是竞争条件。
e.fillInStackTrace() 不是您想要使用的。

Answer 2

public class ProducerConsumerWithThreads {

        synchronized void withdrawBoxConsumer() {
            if(box > 0){
                box --;
                System.out.println("Took one box now boxes left "+  box);
            }
            if(box == 0) {
                System.out.println("Please put more boxes");
                notify();
                try{wait();}catch(Exception e){
                    System.out.println("Exception occured" + e.fillInStackTrace());
                }
            }
            withdrawBoxConsumer();
        }

        synchronized void putBoxProducer() {
            if(box < 10){
                box ++;
                System.out.println("Put one box now boxes are "+  box);
            }
            if(box == 10) {
                System.out.println("Please Consume boxes");
                notify();
                try{wait();}catch(Exception e){
                    System.out.println("Exception occured" + e.fillInStackTrace());
                }
            }
            putBoxProducer();
        }

        public int box = 5;

        public static void main(String[] args) throws InterruptedException {
            //pipeline of 10 boxes
            //consumer takes one at a time .. till its empty

            int boxLimit = 10;
            final int box = 5;
            final ProducerConsumerWithThreads c=new ProducerConsumerWithThreads();

            new Thread(){
                public void run(){c.withdrawBoxConsumer();}
            }.start();
            new Thread(){
                public void run(){c.putBoxProducer();}
            }.start();
        }
}

线程间通信生产者消费者问题

2 个答案: