多线程虚假唤醒现象、原因及解决方法

现象

　　资源类

class NumberDemo{
    private int num = 0;
    private Lock lock = new ReentrantLock();
    private Condition condition1 = lock.newCondition();
    private Condition condition2 = lock.newCondition();
    public void incr(){
        lock.lock();
        try{
            //判断
            if(num != 0){
                condition1.await();
            }
            //干活
            num++;
            System.out.println(Thread.currentThread().getName() + "\t:\t" + num);
            //通知
            condition2.signal();
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            lock.unlock();
        }
    }

    public void desc(){
        lock.lock();
        try{
            if(num != 1){
                condition2.await();
            }
            num--;
            System.out.println(Thread.currentThread().getName() + "\t:\t" + num);
            condition1.signal();
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            lock.unlock();
        }
    }
}

 

　　主进程

    public static void main(String[] args) {
        NumberDemo number = new NumberDemo();
        new Thread(()->{
            for (int i = 0; i < 10; i++){
                number.incr();
            }
        },"A").start();
        new Thread(()->{
            for (int i = 0; i < 10; i++){
                number.incr();
            }
        },"B").start();
        new Thread(()->{
            for (int i = 0; i < 10; i++){
                number.desc();
            }
        },"C").start();
        new Thread(()->{
            for (int i = 0; i < 10; i++){
                number.desc();
            }
        },"D").start();
    }

 

　　结果：

　　　　

　　　　出现了意料之外的结果：-1

 

原因：

　　

 

 

解决方法：

　　将if改为while来判断

    public void incr(){
        lock.lock();
        try{
            //判断
            while (num != 0){
                condition1.await();
            }
            //干活
            num++;
            System.out.println(Thread.currentThread().getName() + "\t:\t" + num);
            //通知
            condition2.signal();
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            lock.unlock();
        }
    }