【Java多线程】多线程虚假唤醒

public class CustomerDemo {
    public static void main(String[] args) {
        /**
         * 要求一个线程增长，一个线程减小
         */
        Number number= new Number();
        new Thread(() -> {
            for (int i = 0; i < 10 ;i ++) {
                try {
                    number.decrease();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "A").start();

        new Thread(() -> {
            for (int i = 0; i < 10 ;i ++) {
                try {
                    number.increase();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "B").start();

        new Thread(() -> {
            for (int i = 0; i < 10 ;i ++) {
                try {
                    number.decrease();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "C").start();


        new Thread(() -> {
            for (int i = 0; i < 10 ;i ++) {
                try {
                    number.increase();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "D").start();
    }
}


/**
 * 线程之间的通讯
 */
class Number {

    private int num = 0;

    public synchronized void decrease() throws InterruptedException {
        if (num == 0) {
            this.wait();
        }
        num --;
        System.out.println(Thread.currentThread().getName() + " ... " + num);
        this.notifyAll();
    }

    public synchronized void increase() throws InterruptedException {
        if (num != 0) {
            this.wait();
        }
        num ++;
        System.out.println(Thread.currentThread().getName() + " ... " + num);
        this.notifyAll();
    }

}

上面程序最理想的状态是0 ,1,0,1间隔执行，可是多执行几回就有可能会出现如下情形

B ... 1
A ... 0
B ... 1
.....
.....
A ... -1
A ... -2
A ... -3
A ... -4

这是由于多线程的虚假唤醒致使的，在多线程中判断必须使用while
若是使用if

if (num != 0) {
// 假设此时有B,D线程在这里wait,若是被唤醒后，不会再次进行num != 0判断，这样就会致使num ++ 两次。若是使用while的话，线程被唤醒之后会再次进行条件判断
   this.wait();
 }