为什么这段代码会导致illegalMonitorState异常?

时间:2012-11-11 20:55:53

标签: java java.util.concurrent

下面的代码试图将随机值插入循环队列并将其删除。但是,存在一些同步问题。我知道我可以使用更高级别的例程,我将为生产代码执行此操作,但我很好奇为什么这不起作用?我在这里错过了什么?

public class CircularQueue {
int count;
int rear;
int front;
Object lock = new Object();
int size;
int[] array;
CircularQueue(int size)
{
    this.size= size;
    array = new int[size];
}

void enqueue(int number) throws InterruptedException
{
    if(isFull())
        lock.wait();

    synchronized(lock)
    {

        array[rear] = number;
        System.out.println("Rear is:"+ rear+ "value is:"+number+"Size is:"+size);

        rear = (rear+1)%size;
        count++;
    }
    lock.notify();

}

void dequeue() throws InterruptedException
{
    if(isEmpty())
        lock.wait();

    synchronized(lock)
    {
        int retVal = 0;
        retVal = array[front];
        System.out.println("Front is:"+ front+ "value is:"+retVal);

        front = (front+1)%size;
        count--;
    }

    lock.notify();

}

boolean isFull()
{
    if(count == size)
    {
        return true;
    }
    else
        return false;

}

boolean isEmpty()
{
    return count == 0;
}
}

//测试类

import java.util.Random;
public class App {

    public static void main(String[] args) throws InterruptedException
    {
       final Random random = new Random();
       final CircularQueue circularQueue = new CircularQueue(10);
       Thread t1 = new Thread(new Runnable(){

        @Override
        public void run() {
            try {
                circularQueue.enqueue(random.nextInt(100));
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

       });
       Thread t2 = new Thread(new Runnable(){

            @Override
            public void run() {
                try {
                    circularQueue.dequeue();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

           });

       t1.start();
       t2.start();
       t1.join();
       t2.join();

    }

}

2 个答案:

答案 0 :(得分:6)

因为必须从synchronized块调用java.lang.Object#waitjava.lang.Object#notifyjava.lang.Object#notifyAll

作为解决方案(需要检查),您应该将条件置于同步块中:

void enqueue(int number) throws InterruptedException
{

    synchronized(lock)
    {
        if(isFull())
           lock.wait();

        array[rear] = number;
        System.out.println("Rear is:"+ rear+ "value is:"+number+"Size is:"+size);

        rear = (rear+1)%size;
        count++;
        lock.notify();
    }
}

void dequeue() throws InterruptedException
{
    synchronized(lock)
    {
        if(isEmpty())
           lock.wait();

        int retVal = 0;
        retVal = array[front];
        System.out.println("Front is:"+ front+ "value is:"+retVal);

        front = (front+1)%size;
        count--;
        lock.notify();
    }

}

答案 1 :(得分:0)

此代码中的另一个问题是即使isEmpty / isFull返回true - 在调用相邻等待之前,队列的状态可能会发生变化。
例如:
- 队列是空的 - 线程1调用isEmpty()
- 上下文切换
- 线程2调用入队(现在队列不为空)
- 上下文切换
- 虽然队列不为空,但线程1不会调用lock.wait()事件

当将wait()/ notify()的调用放在同步块中时,这个问题当然会得到解决。