JUC学习之虚假唤醒

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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();
        }
    }
}