// 以生产和消费烤鸭为例

class Resource

{

    private String name;

    private int count = 1; // 记录烤鸭的编号

    private boolean flag = false;

    public synchronized void set(String name)

    {

        if(flag)

            try{this.wait();}catch(InterruptedException e){}

        this.name = name + count;

        count++;

        System.out.println(Thread.currentThread().getName()+"..生产者.."+this.name);

        flag = true;

        this.notify();

    }

    public synchronized void out()

    {

        if(!flag)

            try{this.wait();}catch(InterruptedException e){}

        Sytem.out.println(Thread.currentThread().getName()+ "...消费者.."+ this.name);

        flag = false;

        this.notify();

    }

}

class Producer implements Runnable

{

    Resource r;

    Producer(Resource r)

    {

        this.r = r;

    }

    public void run()

    {

        while(true)

        {

            r.set("烤鸭");

        }

    }

}

class Consumer implements Runnable

{

    Resource r;

    Consumer(Resource r)

    {

        this.r = r;

    }

    public void run()

    {

        while(true)

        {

            r.out();

        }

    }

}

class ProducerConsumerDemo

{

    public static void main(String[] args)

    {

        // 创建资源

        Resource r = new Resource();

        // 创建任务

        Producer pro = new Producer(r);

        Consumer con = new Consumer(r);

        // 多生产者

        Thread t0 = new Thread(pro);

        Thread t1 = new Thread(pro);

        // 多消费者

        Thread t2 = new Thread(con);

        Thread t3 = new Thread(con);

        Thread t4 = new Thread(con);

        Thread t5 = new Thread(con);

        // 开启线程

        t0.start();

        t1.start();

        t2.start();

        t3.start();

        t4.start();

        t5.start();

    }

}

出现错误的两种情况:

1. 线程安全问题(虚假唤醒): 线程1 生产的烤鸭被线程3 和线程5 两个线程同时消费
   • if 只能判断标记一次, 会导致不该运行的线程运行了, 出现数据错误的情况
   • while 可以多次判断标记, 解决了线程获取执行权后, 是否要运行的问题!

1. 死锁
   • notify() 一次只能唤醒一个线程, 如果本方唤醒类本方, 没有意义. 而且 while 判断标记多次,
     
     会导致死锁.
   • notifyAll() 解决了本方线程一定会唤醒对方线程的问题.

// 升级版代码

class Resource

{

    private String name;

    private int count = 1; // 记录烤鸭的编号

    private boolean flag = false;

    public synchronized void set(String name)

    {

        // 将 if 换为 while, 线程从冻结状态被唤醒后,需要判断 flag 标记之后, 确定是否继续生产"烤鸭"

        while(flag)

            try{this.wait();}catch(InterruptedException e){}

        this.name = name + count;

        count++;

        System.out.println(Thread.currentThread().getName()+"..生产者.."+this.name);

        flag = true;

        this.notifyAll(); // 肯定会唤醒对方的线程, 解决了死锁问题

    }

    public synchronized void out()

    {

        while(!flag)

            try{this.wait();}catch(InterruptedException e){}

        Sytem.out.println(Thread.currentThread().getName()+ "...消费者.."+ this.name);

        flag = false;

        this.notifyAll();

    }

}

class Producer implements Runnable

{

    Resource r;

    Producer(Resource r)

    {

        this.r = r;

    }

    public void run()

    {

        while(true)

        {

            r.set("烤鸭");

        }

    }

}

- [JavaSE 基础视频(毕向东)](https://www.bilibili.com/video/av3106510/#page=4)