JUC学习之虚假唤醒

比眉伴天荒 2022-05-19 05:10 201阅读 0赞

## 虚假唤醒 ##

public class Test { 
    
        public static void main(String[] args) throws ExecutionException, InterruptedException {
            Seller seller = new Seller();
            Productor productor = new Productor(seller);
            Consumer consumer = new Consumer(seller);
            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();
        }
    }
    
    // 销售员
    class Seller {
        private int product = 0;
    
        // 进货
        public synchronized void get() {
            if (product >= 1) {
                System.out.println("货物已满");
                try {
                    // 对应Object中的 wait()
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + " : " + ++product);
            // 对应Object中的notifyAll()
            this.notifyAll();
        }
    
        // 销售
        public synchronized void sale() {
            if (product <= 0) {
                System.out.println("货物卖完");
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + " : " + --product);
            this.notifyAll();
        }
    }
    
    // 生产者
    class Productor implements Runnable {
        private Seller seller;
    
        public Productor(Seller seller) {
            this.seller = seller;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 20; i++) {
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                seller.get();
            }
        }
    }
    
    class Consumer implements Runnable {
        private Seller seller;
    
        public Consumer(Seller seller) {
            this.seller = seller;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 20; i++) {
                seller.sale();
            }
        }
    }

## 解决虚假唤醒 ##

对于Object.wait()方法，JDK开发文档是这样说的

> As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop(对于某一个参数的版本，实现中断和虚假唤醒是可能的，而且此方法应始终在循环中使用)：
> 
>     synchronized (obj) {
>          while (<condition does not hold>)
>              obj.wait();
>          ... // Perform action appropriate to condition
>      }

一、使用synchronized关键字

public class Test { 
    
        public static void main(String[] args) throws ExecutionException, InterruptedException {
            Seller seller = new Seller();
            Productor productor = new Productor(seller);
            Consumer consumer = new Consumer(seller);
            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();
        }
    }
    
    // 销售员
    class Seller {
        private int product = 0;
    
        // 进货
        public synchronized void get() {
            while (product >= 1) {
                System.out.println("货物已满");
                try {
                    this.wait(); // wait()方法应该总是使用在while里面可以防止虚假唤醒
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + " : " + ++product);
            this.notifyAll();
        }
    
        // 销售
        public synchronized void sale() {
            while (product <= 0) {
                System.out.println("货物卖完");
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + " : " + --product);
            this.notifyAll();
        }
    }
    
    // 生产者
    class Productor implements Runnable {
        private Seller seller;
    
        public Productor(Seller seller) {
            this.seller = seller;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 20; i++) {
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                seller.get();
            }
        }
    }
    
    class Consumer implements Runnable {
        private Seller seller;
    
        public Consumer(Seller seller) {
            this.seller = seller;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 20; i++) {
                seller.sale();
            }
        }
    }

二、使用同步锁

public class Test { 
    
        public static void main(String[] args) throws ExecutionException, InterruptedException {
            Seller seller = new Seller();
            Productor productor = new Productor(seller);
            Consumer consumer = new Consumer(seller);
            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();
        }
    }
    
    // 销售员
    class Seller {
        private int product = 0;
    
        private Lock lock = new ReentrantLock();
        private Condition condition = lock.newCondition();
    
        // 进货
        public void get() {
            lock.lock();
            try {
                while (product >= 1) {
                    System.out.println("货物已满");
                    try {
                        // 对应Object中的 wait()
                        condition.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println(Thread.currentThread().getName() + " : " + ++product);
                // 对应Object中的notifyAll()
                condition.signalAll();
            } finally {
                lock.unlock();
            }
        }
    
        // 销售
        public synchronized void sale() {
            lock.lock();
            try {
                while (product <= 0) {
                    System.out.println("货物卖完");
                    try {
                        condition.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println(Thread.currentThread().getName() + " : " + --product);
                condition.signalAll();
            } finally {
                lock.unlock();
            }
        }
    }
    
    // 生产者
    class Productor implements Runnable {
        private Seller seller;
    
        public Productor(Seller seller) {
            this.seller = seller;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 20; i++) {
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                seller.get();
            }
        }
    }
    
    class Consumer implements Runnable {
        private Seller seller;
    
        public Consumer(Seller seller) {
            this.seller = seller;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 20; i++) {
                seller.sale();
            }
        }
    }