Java 生产者消费者 IllegalMonitorStateException

Question

我已经使用 java 线程构建了生产者-消费者。生产者和消费者是两个不同的类，指的是单链表和对象锁。下面的实现有什么问题？

Item Produced by Thread-0 Item 1
Item Consumed by Thread-1 Item 1

    Exception in thread "Thread-0" Exception in thread "Thread-1" java.lang.IllegalMonitorStateException
        at java.lang.Object.notify(Native Method)
        at ProducerConsumer$Producer.produce(ProducerConsumer.java:35)
        at ProducerConsumer$Producer.run(ProducerConsumer.java:19)
        at java.lang.Thread.run(Thread.java:745)
    java.lang.IllegalMonitorStateException
        at java.lang.Object.notify(Native Method)
        at ProducerConsumer$Consumer.consume(ProducerConsumer.java:63)
        at ProducerConsumer$Consumer.run(ProducerConsumer.java:50)
        at java.lang.Thread.run(Thread.java:745)

代码实现

import java.util.LinkedList;

public class ProducerConsumer {
    LinkedList<Integer> items =  new LinkedList<>();
    Object lock = new Object();
    int capacity = 10;
    public static void main(String[] args) {
        ProducerConsumer m = new ProducerConsumer();
        Thread p = new Thread(m.new Producer());
        Thread c =  new Thread(m.new Consumer());
        p.start();
        c.start();
    }

    class Producer implements Runnable{

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

        public void produce(){
            int value =0;
            while(true){
                synchronized (lock){
                    while(items.size() == capacity){
                        try{
                            wait();
                        }catch (InterruptedException e){
                            e.printStackTrace();
                        }
                    }
                    items.add(++value);
                    System.out.println("Item Produced by "+Thread.currentThread().getName()+" Item "+value);
                    notify();
                    try{
                        Thread.sleep(1000);
                    }catch (InterruptedException e){
                        e.printStackTrace();
                    }

                }
            }
        }
    }

    class Consumer implements Runnable{
        @Override
        public void run() {
            consume();
        }
        public void consume(){
            while (true){
                synchronized (lock){
                    while (items.size() == 0){
                        try{
                            wait();
                        }catch (InterruptedException e){
                            e.printStackTrace();
                        }
                    }
                    System.out.println("Item Consumed by "+Thread.currentThread().getName()+" Item "+items.remove());
                    notify();
                }
            }
        }
    }
}

Answer 1

Exception in thread "Thread-0" Exception in thread "Thread-1" java.lang.IllegalMonitorStateException
        at java.lang.Object.notify(Native Method)
        at ProducerConsumer$Producer.produce(ProducerConsumer.java:35)
        at ProducerConsumer$Producer.run(ProducerConsumer.java:19)
        at java.lang.Thread.run(Thread.java:745)
    java.lang.IllegalMonitorStateException
        at java.lang.Object.notify(Native Method)
        at ProducerConsumer$Consumer.consume(ProducerConsumer.java:63)
        at ProducerConsumer$Consumer.run(ProducerConsumer.java:50)
        at java.lang.Thread.run(Thread.java:745)

这是因为对 notify(); 方法的调用不遵守其 contract: 的事实。

Wakes up a single thread that is waiting on this object's monitor.(...)This method should only be called by a thread that is the owner ofthis object's monitor.

同样适用于 wait 方法:

This method should only be called by a thread that is the owner of this object's monitor.

TL:DR

您在错误的锁上调用wait 和notify()(即 this 实例的隐式 锁返回。

分别将这些调用更改为:

lock.notify(); 和 lock.wait();

基于所提供代码的运行示例:

import java.util.LinkedList;

public class ProducerConsumer {
    final LinkedList<Integer> items =  new LinkedList<>();
    final Object lock = new Object();
    final int capacity = 10;
    public static void main(String[] args) {
        ProducerConsumer m = new ProducerConsumer();
        Thread p = new Thread(m.new Producer());
        Thread c = new Thread(m.new Consumer());
        p.start();
        c.start();
    }

    class Producer implements Runnable{

        @Override
        public void run() {
            try {
                produce();
            } catch (InterruptedException e) { /** do something **/ }
        }

        public void produce() throws InterruptedException {
            int value =0;
            while(true){
                synchronized (lock){
                    while(items.size() == capacity)
                        lock.wait();
                    items.add(++value);
                    System.out.println("Item Produced by "+Thread.currentThread().getName()+" Item "+value);
                    lock.notify();
                    Thread.sleep(1000);
                }
            }
        }
    }

    class Consumer implements Runnable{
        @Override
        public void run() {
            try {
                consume();
            } catch (InterruptedException e) { /** do something **/ }
        }
        public void consume() throws InterruptedException {
            while (true){
                synchronized (lock){
                    while (items.size() == 0)
                            lock.wait();
                    System.out.println( "Item Consumed by "
                                        +Thread.currentThread().getName()+
                                        " Item "+items.remove());
                    lock.notify();
                }
            }
        }
    }
}