JUC(六) - Lock 同步锁(2)

6.2 案例二

1.生产者与消费者互动

1. 方式1

1.店员

class Clerk {
    private int product = 0;

    /**
     * 进货
     */
    public synchronized void get() {
        if (product >= 10) {
            System.out.println("产品已满");
        } else {
            System.out.println(Thread.currentThread().getName()+" : " + ++product);
        }
    }

    /**
     * 卖货
     */
    public synchronized void sale() {
        if (product <= 0) {
            System.out.println("缺货");
        } else {
            System.out.println(Thread.currentThread().getName()+" : " + --product);
        }
    }
}

2.生产者

class Productor implements Runnable {
    private Clerk clerk;

    public Productor(Clerk clerk) {
        this.clerk = clerk;
    }

    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            clerk.get();
        }
    }
}

3.消费者

class Consumer implements Runnable {
    private Clerk clerk;

    public Consumer(Clerk clerk) {
        this.clerk = clerk;
    }

    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            clerk.sale();
        }
    }
}

结果

image.png

以上问题:重复补货,重复消费,缺乏唤醒机制

2. 方式2

增加唤醒机制

image.png image.png

虽然解决了唤醒问题,但是有一直等待的风险

3. 方式3

当有多个生产者/消费者,会出现虚假唤醒

代码

    public static void main(String[] args) {
        Clerk clerk = new Clerk();

        Productor productor = new Productor(clerk);
        Consumer consumer = new Consumer(clerk);

        new Thread(productor, "生产者A").start();
        new Thread(consumer, "消费者B").start();

        new Thread(productor, "生产者C").start();
        new Thread(consumer, "消费者D").start();
    }

结果

image.png

解决方法:

image.png

    /**
     * 进货
     */
    public synchronized void get() {
        while (product >= 1) {
            System.out.println("产品已满");
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(Thread.currentThread().getName() + " : " + ++product);
        this.notifyAll();
    }

    /**
     * 卖货
     */
    public synchronized void sale() {
        while (product <= 0) {
            System.out.println("缺货");
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(Thread.currentThread().getName() + " : " + --product);
        this.notifyAll();
    }

if 变为 while