虚假唤醒问题

不安全代码

/**
 * 线程之间的通信问题: 生产者和消费者问题! 等待唤醒,通知唤醒
 * 线程交替执行  A  B  操作同一个变量 num=0
 * A num+1
 * B num-1
 */
public class A {
    public static void main(String[] args) {
        Data data = new Data();

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

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

// 判断等待,业务,通知
class Data{// 数字 资源类

    private int number = 0;

    // +1
    public synchronized void increment() throws InterruptedException {
        if (number != 0) {
            // 等待
            this.wait();
        }
        // 业务
        number++;
        System.out.println(Thread.currentThread().getName() + "=>" + number);
        // 通知其他线程,我+1完毕了
        this.notifyAll();
    }

    // -1
    public synchronized void decrement() throws InterruptedException {
        if (number == 0) {
            // 等待
            this.wait();
        }
        // 业务
        number--;
        System.out.println(Thread.currentThread().getName() + "=>" + number);
        // 通知其他线程,我-1完毕了
        this.notifyAll();
    }

}

四个线程还安全么?解决方法: 把if改成while

安全代码

/**
 * 线程之间的通信问题: 生产者和消费者问题! 等待唤醒,通知唤醒
 * 线程交替执行  A  B  操作同一个变量 num=0
 * A num+1
 * B num-1
 */
public class A {
    public static void main(String[] args) {
        Data data = new Data();

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

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

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

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

// 循环等待,业务,通知
class Data{// 数字 资源类

    private int number = 0;

    // +1
    public synchronized void increment() throws InterruptedException {
        while (number != 0) {
            // 等待
            this.wait();
        }
        // 业务
        number++;
        System.out.println(Thread.currentThread().getName() + "=>" + number);
        // 通知其他线程,我+1完毕了
        this.notifyAll();
    }

    // -1
    public synchronized void decrement() throws InterruptedException {
        while (number == 0) {
            // 等待
            this.wait();
        }
        // 业务
        number--;
        System.out.println(Thread.currentThread().getName() + "=>" + number);
        // 通知其他线程,我-1完毕了
        this.notifyAll();
    }

}