多线程-生产者消费者Exception in thread “Thread-12“ java.lang.IllegalMonitorStateException

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博客内容讲述了在实现多线程的生产者消费者模型时遇到的`java.lang.IllegalMonitorStateException`错误。错误源于线程调用`wait()`方法时未持有正确的监视器对象。通过分析JDK源码，指出只有对象监视器的所有者线程才能调用`wait()`，并提供了将代码修改为`queue.wait()`的解决方案，以解决这个问题。

首先建了一个简单的Product类，用来表示生产和消费的产品

package com.spring.boot.entity;

public class Product {
	
	private String name;

    public Product(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

主函数里我设定了两类线程，并且这里选择用普通的ArrayDeque 来实现Queue，更简单的方式是直接用Java 中的BlockingQueue来实现。

BlockingQueue是阻塞队列，它有一系列的方法可以让线程实现自动阻塞，常用的。这里为了更好的理解并发协同的这个过程，我们先自己处理。

package com.spring.boot.entity;

import java.util.ArrayDeque;
import java.util.Queue;

public class ProducerAndConsumer {
	public static void main(String[] args) {
		Queue<Product> queue = new ArrayDeque<>();

        for (int i = 0; i < 100; i++) {
            new Thread(new Producer(queue, 100)).start();
            new Thread(new Consumer(queue, 100)).start();
        }

	}
}

然后就是Producer和Consumer 了。

package com.spring.boot.entity;

import java.util.Queue;
import java.util.Random;

public class Producer implements Runnable{
		private Queue<Product> queue;
	    private int maxCapacity;

	    public Producer(Queue queue, int maxCapacity) {
	        this.queue = queue;
	        this.maxCapacity = maxCapacity;
	    }

	    @Override
	    public void run() {
	    	while(true) {
	    		synchronized (queue) {
		            while (queue.size() == maxCapacity) { //一定要用 while，而不是 if，下文解释
		                try {
		                    System.out.println("生产者" + Thread.currentThread().getName() + "等待中... Queue 已达到最大容量，无法生产");
		                    wait();
		                    System.out.println("生产者" + Thread.currentThread().getName() + "退出等待");
		                } catch (InterruptedException e) {
		                    e.printStackTrace();
		                }
		            }
		            if (queue.size() == 0) { //队列里的产品从无到有，需要通知在等待的消费者
		                queue.notifyAll();
		            }
		            Random random = new Random();
		            Integer i = random.nextInt();
		            queue.offer(new Product("产品"  + i.toString()));
		            System.out.println("生产者" + Thread.currentThread().getName() + "生产了产品：" + i.toString());
		        }
	    	}
	        
	    }
	    
	  
}

package com.spring.boot.entity;

import java.util.Queue;

public class Consumer implements Runnable{
	
		private Queue<Product> queue;
	    private int maxCapacity;

	    public Consumer(Queue queue, int maxCapacity) {
	        this.queue = queue;
	        this.maxCapacity = maxCapacity;
	    }

	    @Override
	    public void run() {
	    	while(true){
	    		synchronized (queue) {
		            while (queue.isEmpty()) {
		                try {
		                    System.out.println("消费者" + Thread.currentThread().getName() + "等待中... Queue 已缺货，无法消费");
		                    wait();
		                    System.out.println("消费者" + Thread.currentThread().getName() + "退出等待");
		                } catch (InterruptedException e) {
		                    e.printStackTrace();
		                }
		            }
		            Product product = queue.poll();
		            System.out.println("消费者" + Thread.currentThread().getName() + "消费了：" + product.getName());
		            if (queue.size() == maxCapacity) {
		                queue.notifyAll();
		            }

		            
		        }
	    	}
	        
	    }
}

运行，报错：Exception in thread "Thread-12" java.lang.IllegalMonitorStateException

抛出： IllegalMonitorStateException - 如果当前线程不是此对象监视器的所有者。

原因：根据jdk源码

synchronized (obj) {
      while (<condition does not hold>)
           obj.wait(); 
         // Perform action appropriate to condition
}

此方法只应由作为此对象监视器的所有者的线程来调用。

所以代码wait()改成：queue.wait()

package com.spring.boot.entity;

import java.util.Queue;
import java.util.Random;

public class Producer implements Runnable{
		private Queue<Product> queue;
	    private int maxCapacity;

	    public Producer(Queue queue, int maxCapacity) {
	        this.queue = queue;
	        this.maxCapacity = maxCapacity;
	    }

	    @Override
	    public void run() {
	    	while(true) {
	    		synchronized (queue) {
		            while (queue.size() == maxCapacity) { //一定要用 while，而不是 if，下文解释
		                try {
		                    System.out.println("生产者" + Thread.currentThread().getName() + "等待中... Queue 已达到最大容量，无法生产");
		                    queue.wait();
		                    System.out.println("生产者" + Thread.currentThread().getName() + "退出等待");
		                } catch (InterruptedException e) {
		                    e.printStackTrace();
		                }
		            }
		            if (queue.size() == 0) { //队列里的产品从无到有，需要通知在等待的消费者
		                queue.notifyAll();
		            }
		            Random random = new Random();
		            Integer i = random.nextInt();
		            queue.offer(new Product("产品"  + i.toString()));
		            System.out.println("生产者" + Thread.currentThread().getName() + "生产了产品：" + i.toString());
		        }
	    	}
	        
	    }
	    
	  
}



package com.spring.boot.entity;

import java.util.Queue;

public class Consumer implements Runnable{
	
		private Queue<Product> queue;
	    private int maxCapacity;

	    public Consumer(Queue queue, int maxCapacity) {
	        this.queue = queue;
	        this.maxCapacity = maxCapacity;
	    }

	    @Override
	    public void run() {
	    	while(true){
	    		synchronized (queue) {
		            while (queue.isEmpty()) {
		                try {
		                    System.out.println("消费者" + Thread.currentThread().getName() + "等待中... Queue 已缺货，无法消费");
		                    queue.wait();
		                    System.out.println("消费者" + Thread.currentThread().getName() + "退出等待");
		                } catch (InterruptedException e) {
		                    e.printStackTrace();
		                }
		            }
		            Product product = queue.poll();
		            System.out.println("消费者" + Thread.currentThread().getName() + "消费了：" + product.getName());
		            if (queue.size() == maxCapacity) {
		                queue.notifyAll();
		            }

		            
		        }
	    	}
	        
	    }
}

https://blog.youkuaiyun.com/zhouxiaoyun0228/article/details/7757313?utm_medium=distribute.pc_relevant_t0.none-task-blog-2%7Edefault%7EBlogCommendFromMachineLearnPai2%7Edefault-1.control&depth_1-utm_source=distribute.pc_relevant_t0.none-task-blog-2%7Edefault%7EBlogCommendFromMachineLearnPai2%7Edefault-1.control

https://blog.youkuaiyun.com/hch814/article/details/106296929/