通过生产者消费者案例理解等待唤醒机制和虚假唤醒

心已赠人 2022-07-12 04:45 198阅读 0赞

首先引入下面这段生产者和消费者的程序，店员类作为生产产品和消费产品的中介，其中的数据product为共享数据，产品最多只能囤积5个，当产品达到5个还在生产时，就会提示“产品已满！”，类似地，如果产品只有0个了还在消费，会提示“缺货！”：

package concurrent;
    
    //店员类
    class Clerk {
        private int product = 0;
    
        // 进货
        public synchronized void get() {
            if (product >= 5) {
                System.out.println("产品已满！");
            } else {
                System.out.println(Thread.currentThread().getName() + ":" + ++product);
            }
        }
    
        // 售货
        public synchronized void sale() {
            if (product <= 0) {
                System.out.println("缺货！");
            } else {
                System.out.println(Thread.currentThread().getName() + ":" + --product);
            }
        }
    }
    
    // 生产者类
    class Productor implements Runnable {
    
        private Clerk clerk;
    
        public Productor(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                clerk.get();
            }
    
        }
    
    }
    
    //消费者类
    class Consumer implements Runnable {
    
        private Clerk clerk;
    
        public Consumer(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                clerk.sale();
            }
    
        }
    
    }
    
    public class TestProductorAndConsumer { 
    
        public static void main(String[] args) {
            Clerk clerk = new Clerk();
    
            Productor productor = new Productor(clerk);
            Consumer consumer = new Consumer(clerk);
    
            new Thread(productor,"Productor A").start();
            new Thread(consumer,"Consumer B").start();
        }
    
    }

运行程序，结果如下：  
　　  
　　![image\_1b8jssm26hp3moi8ae1f4h1uv49.png-49.3kB][image_1b8jssm26hp3moi8ae1f4h1uv49.png-49.3kB]  
　　  
　　 这是一种不好的情况，因为当产品已满时，还在不停地生产，当缺货时，还在不停地消费。为此，我们引入等待唤醒机制：

package concurrent;
    
    //店员类
    class Clerk {
        private int product = 0;
    
        // 进货
        public synchronized void get() {
            if (product >= 5) {
                System.out.println("产品已满！");
    
                //等待
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } else {
                System.out.println(Thread.currentThread().getName() + ":" + ++product);
                //唤醒
                this.notifyAll();
            }
        }
    
        // 售货
        public synchronized void sale() {
            if (product <= 0) {
                System.out.println("缺货！");
                //等待
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } else {
                System.out.println(Thread.currentThread().getName() + ":" + --product);
                //唤醒
                this.notifyAll();
            }
        }
    }
    
    // 生产者类
    class Productor implements Runnable {
    
        private Clerk clerk;
    
        public Productor(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                clerk.get();
            }
    
        }
    
    }
    
    //消费者类
    class Consumer implements Runnable {
    
        private Clerk clerk;
    
        public Consumer(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                clerk.sale();
            }
    
        }
    
    }
    
    public class TestProductorAndConsumer { 
    
        public static void main(String[] args) {
            Clerk clerk = new Clerk();
    
            Productor productor = new Productor(clerk);
            Consumer consumer = new Consumer(clerk);
    
            new Thread(productor,"Productor A").start();
            new Thread(consumer,"Consumer B").start();
        }
    
    }

再运行程序，就不会再出现上述的情况：  
　　  
　　![image\_1b8jtiigv1rgb1d9h10kaisj1jojm.png-52.7kB][image_1b8jtiigv1rgb1d9h10kaisj1jojm.png-52.7kB]  
　　  
　　但是，现在，我们将产品的囤积上限设定为1（这种情况在现实中也是有可能出现的）：  
　　  
　　![image\_1b8ju98t384gsair4uhbv1d3k13.png-19kB][image_1b8ju98t384gsair4uhbv1d3k13.png-19kB]  
　　  
　　然后运行程序：  
　　  
　　![image\_1b8juau5v2il1rjd12gq1uph1gbp1g.png-87kB][image_1b8juau5v2il1rjd12gq1uph1gbp1g.png-87kB]  
　　  
　　程序的输出貌似没有问题，但请注意图中箭头所指的地方，这表示程序没有结束，还一直在执行。这是因为，当循坏到最后一轮时，由于产品已满引发了wait()操作，然后生产者线程等待，随后消费者消费了一份产品，并唤醒等待的生产者线程，此时，被唤醒的生产者线程由于循环结束，直接结束了线程的执行，但是另一边，消费者线程没有结束，而且由于将产品消费完后再次进入了等待，但是生产者线程此时已经结束了，不能再唤醒消费者线程，所以便进入了死循环。  
　　解决这种问题的方法时去掉Clerk类中get方法和sale方法的else，并将原来else中的代码直接提出，这样，就算线程结束，也会先再次唤醒等待的线程：

package concurrent;
    
    //店员类
    class Clerk {
        private int product = 0;
    
        // 进货
        public synchronized void get() {
            if (product >= 1) {
                System.out.println("产品已满！");
    
                // 等待
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + ":" + ++product);
            // 唤醒
            this.notifyAll();
        }
    
        // 售货
        public synchronized void sale() {
            if (product <= 0) {
                System.out.println("缺货！");
                // 等待
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + ":" + --product);
            // 唤醒
            this.notifyAll();
        }
    }
    
    // 生产者类
    class Productor implements Runnable {
    
        private Clerk clerk;
    
        public Productor(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                clerk.get();
            }
    
        }
    
    }
    
    // 消费者类
    class Consumer implements Runnable {
    
        private Clerk clerk;
    
        public Consumer(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                clerk.sale();
            }
    
        }
    
    }
    
    public class TestProductorAndConsumer { 
    
        public static void main(String[] args) {
            Clerk clerk = new Clerk();
    
            Productor productor = new Productor(clerk);
            Consumer consumer = new Consumer(clerk);
    
            new Thread(productor, "Productor A").start();
            new Thread(consumer, "Consumer B").start();
        }
    
    }

运行程序，不再死循环：  
　　  
　　![image\_1b8jun7qj1jikn2gf013oa12mo1t.png-91.3kB][image_1b8jun7qj1jikn2gf013oa12mo1t.png-91.3kB]  
　　  
　　但是，如果现在有两个（多个）消费者线程和生产者线程，并且我们在生产者类的run方法中添加一个sleep()方法的执行，情况会如何呢？

package concurrent;
    
    //店员类
    class Clerk {
        private int product = 0;
    
        // 进货
        public synchronized void get() {
            if (product >= 1) {
                System.out.println("产品已满！");
    
                // 等待
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + ":" + ++product);
            // 唤醒
            this.notifyAll();
        }
    
        // 售货
        public synchronized void sale() {
            if (product <= 0) {
                System.out.println("缺货！");
                // 等待
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + ":" + --product);
            // 唤醒
            this.notifyAll();
        }
    }
    
    // 生产者类
    class Productor implements Runnable {
    
        private Clerk clerk;
    
        public Productor(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                clerk.get();
            }
    
        }
    
    }
    
    // 消费者类
    class Consumer implements Runnable {
    
        private Clerk clerk;
    
        public Consumer(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                clerk.sale();
            }
    
        }
    
    }
    
    public class TestProductorAndConsumer { 
    
        public static void main(String[] args) {
            Clerk clerk = new Clerk();
    
            Productor productor = new Productor(clerk);
            Consumer consumer = new Consumer(clerk);
    
            new Thread(productor, "Productor A").start();
            new Thread(consumer, "Consumer B").start();
            new Thread(productor, "Productor C").start();
            new Thread(consumer, "Consumer D").start();
        }
    
    }

运行程序：  
　　  
　　![image\_1b8juu3hg3a91pfn191n1m75h012a.png-75.6kB][image_1b8juu3hg3a91pfn191n1m75h012a.png-75.6kB]  
　　  
　　产品数量出现了负数，这肯定是错误的。错误的原因在于，当一个消费者线程遇到产品为0时，等待，并释放锁标志，然后另外一个消费者线程获取到该锁标志，由于产品仍然为0，也等待，并释放锁标志。这时候，生产者线程获取到锁，在生产一个产品后，执行notifyAll()唤醒所有线程，这时候，一个消费者线程消费一个产品使得产品为0，另外一个消费者线程再消费一个产品使得产品变为了负数，这种现象称为虚假唤醒。在Object.wait()方法的javadoc中叙述了该如何解决这种问题：  
　　  
　　![image\_1b8k018lqm9i2khi6ontn118p2n.png-14kB][image_1b8k018lqm9i2khi6ontn118p2n.png-14kB]  
　　  
　　即，将get和sale方法中的if都改为while，这样，每次被唤醒后，都会再次判断产品数是否>=0：

package concurrent;
    
    //店员类
    class Clerk {
        private int product = 0;
    
        // 进货
        public synchronized void get() {
            while (product >= 1) {
                System.out.println("产品已满！");
    
                // 等待
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + ":" + ++product);
            // 唤醒
            this.notifyAll();
        }
    
        // 售货
        public synchronized void sale() {
            while (product <= 0) {
                System.out.println("缺货！");
                // 等待
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + ":" + --product);
            // 唤醒
            this.notifyAll();
        }
    }
    
    // 生产者类
    class Productor implements Runnable {
    
        private Clerk clerk;
    
        public Productor(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                clerk.get();
            }
    
        }
    
    }
    
    // 消费者类
    class Consumer implements Runnable {
    
        private Clerk clerk;
    
        public Consumer(Clerk clerk) {
            this.clerk = clerk;
        }
    
        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                clerk.sale();
            }
    
        }
    
    }
    
    public class TestProductorAndConsumer { 
    
        public static void main(String[] args) {
            Clerk clerk = new Clerk();
    
            Productor productor = new Productor(clerk);
            Consumer consumer = new Consumer(clerk);
    
            new Thread(productor, "Productor A").start();
            new Thread(consumer, "Consumer B").start();
            new Thread(productor, "Productor C").start();
            new Thread(consumer, "Consumer D").start();
        }
    
    }

运行程序，发现结果终于正常了：  
　　  
　　![image\_1b8k0490b9ln1bqb9ae12tt2f734.png-87.5kB][image_1b8k0490b9ln1bqb9ae12tt2f734.png-87.5kB]

[image_1b8jssm26hp3moi8ae1f4h1uv49.png-49.3kB]: /images/20220711/9a4f275fbad44615b2e2bbaed6527398.png
[image_1b8jtiigv1rgb1d9h10kaisj1jojm.png-52.7kB]: /images/20220711/d27dac7535584ce49669a45b9be31690.png
[image_1b8ju98t384gsair4uhbv1d3k13.png-19kB]: /images/20220711/86f8f371782d4521bc58fb259f9e7904.png
[image_1b8juau5v2il1rjd12gq1uph1gbp1g.png-87kB]: /images/20220711/84d37233d28a48fdb3f1026e32b73693.png
[image_1b8jun7qj1jikn2gf013oa12mo1t.png-91.3kB]: /images/20220711/6d104b609fc846b296a67fe0032a9275.png
[image_1b8juu3hg3a91pfn191n1m75h012a.png-75.6kB]: /images/20220711/9fcb7bff6ee24ee197580e96fbcdd918.png
[image_1b8k018lqm9i2khi6ontn118p2n.png-14kB]: /images/20220711/6c42d725f9934114bc32ff4e4932b1ef.png
[image_1b8k0490b9ln1bqb9ae12tt2f734.png-87.5kB]: /images/20220711/88194931b4ee4a1484979965b87fd058.png