为什么我的程序有时会卡住？

Question

所以我使用监视器在 java 中编写了一个有界缓冲区问题，但我无法弄清楚我的程序有什么问题。有时它会在第三个循环结束之前一直在无限循环中运行。大多数时候它运行完美。 该程序很简单，关于一个生产者和多个消费者。我将不胜感激任何帮助。这是我的 github 的链接，您可以在其中找到整个代码。 the full code

有界缓冲区

public class BoundedBuffer {
    public BoundedBuffer ()
    {
        int numWorms = 10;
        int numBirds = 5;

        Dish dish = new Dish (numWorms);
        Parent parent = new Parent(dish);
        parent.start();
        Child child[] = new Child[numBirds];
        for (int i = 0; i < numBirds; i++)
        {
            child[i] = new Child (dish);
            child[i].start();
        }       
        for (int i = 0; i < numBirds; i++)
        {
            try {child[i].join();}
            catch (Exception ie) {System.out.println (ie.getMessage());}
        }

        System.out.println("bids done eating :D");
    }
}

菜

public class Dish 
{
    int worms;
    int copy;
    public Dish (int worms)
    {
        this.worms = worms;
        copy = worms;
    }
    public synchronized void eat ()
    {
        if (worms <= 0)
        {

            waitForFull();
        }
        worms --;
        System.out.println("Bird " + Thread.currentThread().getName() + " has 
eaten."
                        + " The number of worms left is " + worms);

    }
    public synchronized void fill()
    {
        if (worms > 0)
        {
            waitForEmpty();
        }
        worms = copy;
        System.out.println ("Parent filled the dish");
        notifyAll();
    }
    public synchronized void waitForEmpty ()
    {
        while (worms > 0)
        {
            notifyAll();
            try {wait();}
            catch (Exception ie) {System.out.println (ie.getMessage());}
            }
}
    public synchronized void waitForFull ()
    {
        while (worms <= 0)
        {
            notifyAll();
            try {wait();}
            catch (Exception ie) {System.out.println (ie.getMessage());}
        }
    }
}

Answer 1

我无法重现stuck 的情况。但是我在您的代码中发现了一些其他问题：

• Parent 在三个循环后立即设置done = 1，此时 时间，Dish中还有Worms。
• waitForEmpty 和 fill 不是自动的，这可能会导致 有些矛盾。
• waitForFull 和 eat 不是自动的，这可能会导致 有些矛盾。

两个解决这些问题，我觉得你需要：

• 将waitForEmpty 和fill 合并为一个方法，以及waitForFull 和eat。
• 使用shutdown 和isTerminate 作为信号。 Parent 将 shutdown Dish，而最后一个 Child 将 terminate Dish。

代码如下：

主要

public class Main {
    public static void main(String[] args) {
        new BoundedBuffer ();
    }
}

有界缓冲区

public class BoundedBuffer {
    public BoundedBuffer () {
        int numWorms = 10;
        int numBirds = 5;

        Dish dish = new Dish (numWorms);
        Parent parent = new Parent(dish);
        parent.start();
        Child child[] = new Child[numBirds];
        for (int i = 0; i < numBirds; i++) {
            child[i] = new Child (dish);
            child[i].start();
        }       
        for (int i = 0; i < numBirds; i++) {
            try {child[i].join();}
            catch (Exception ie) {System.out.println (ie.getMessage());}
        }

        System.out.println("bids done eating :D");
    }
}

菜

public class Dish {
    int worms;
    int copy;
    boolean shutDown;
    boolean isTerminated;

    public Dish (int worms) {
        this.worms = worms;
        copy = worms;
    }

    public synchronized void waitForEmptyToFill() {
        while (worms > 0) {
            try {
                notifyAll();
                wait();
            } catch (Exception ie) {
                System.out.println (ie.getMessage());
            }
        }
        worms = copy;
        System.out.println ("Parent filled the dish");
        notifyAll();
    }

    public synchronized void waitForFullToEat () {
        while (worms <= 0 && !isTerminated()) {
            try {
                notifyAll();
                wait();
            } catch (Exception ie) {
                System.out.println (ie.getMessage());
            }
        }
        if (worms > 0) {
            worms--;
            System.out.println("Bird " + Thread.currentThread().getName() + " has eaten."
                    + " The number of worms left is " + worms);
            if (worms == 0 && isShutDown()) {
                setTerminated(true);
                notifyAll();
            }
        }
    }


    public synchronized boolean isShutDown() {
        return shutDown;
    }

    public synchronized void setShutDown(boolean shutDown) {
        this.shutDown = shutDown;
    }

    public synchronized boolean isTerminated() {
        return isTerminated;
    }

    public synchronized void setTerminated(boolean terminated) {
        isTerminated = terminated;
    }
}

父母

public class Parent extends Thread {

    private Dish dish;

    public Parent (Dish dish) {
        this.dish = dish;
    }

    public void run() {
        for (int i = 0; i < 3; i++) {
            dish.waitForEmptyToFill();
        }
        dish.setShutDown(true);
    }
}

孩子

public class Child extends Thread {
    private Dish dish;
    public Child (Dish dish)
    {
        this.dish = dish;
    }
    public void run () {
        while (!dish.isTerminated()) {
            dish.waitForFullToEat();
            try {
                sleep(100);
            } catch (Exception ie) {
                System.out.println (ie.getMessage());
            }
        }
    }
}

输出：

Bird Thread-4 has eaten. The number of worms left is 9
Bird Thread-3 has eaten. The number of worms left is 8
Bird Thread-2 has eaten. The number of worms left is 7
Bird Thread-5 has eaten. The number of worms left is 6
Bird Thread-1 has eaten. The number of worms left is 5
Bird Thread-2 has eaten. The number of worms left is 4
Bird Thread-3 has eaten. The number of worms left is 3
Bird Thread-4 has eaten. The number of worms left is 2
Bird Thread-1 has eaten. The number of worms left is 1
Bird Thread-5 has eaten. The number of worms left is 0
Parent filled the dish
Bird Thread-4 has eaten. The number of worms left is 9
Bird Thread-5 has eaten. The number of worms left is 8
Bird Thread-3 has eaten. The number of worms left is 7
Bird Thread-1 has eaten. The number of worms left is 6
Bird Thread-2 has eaten. The number of worms left is 5
Bird Thread-4 has eaten. The number of worms left is 4
Bird Thread-5 has eaten. The number of worms left is 3
Bird Thread-1 has eaten. The number of worms left is 2
Bird Thread-2 has eaten. The number of worms left is 1
Bird Thread-3 has eaten. The number of worms left is 0
Parent filled the dish
Bird Thread-1 has eaten. The number of worms left is 9
Bird Thread-5 has eaten. The number of worms left is 8
Bird Thread-3 has eaten. The number of worms left is 7
Bird Thread-2 has eaten. The number of worms left is 6
Bird Thread-4 has eaten. The number of worms left is 5
Bird Thread-2 has eaten. The number of worms left is 4
Bird Thread-3 has eaten. The number of worms left is 3
Bird Thread-1 has eaten. The number of worms left is 2
Bird Thread-5 has eaten. The number of worms left is 1
Bird Thread-4 has eaten. The number of worms left is 0
Parent filled the dish
Bird Thread-2 has eaten. The number of worms left is 9
Bird Thread-3 has eaten. The number of worms left is 8
Bird Thread-1 has eaten. The number of worms left is 7
Bird Thread-5 has eaten. The number of worms left is 6
Bird Thread-4 has eaten. The number of worms left is 5
Bird Thread-2 has eaten. The number of worms left is 4
Bird Thread-3 has eaten. The number of worms left is 3
Bird Thread-1 has eaten. The number of worms left is 2
Bird Thread-5 has eaten. The number of worms left is 1
Bird Thread-4 has eaten. The number of worms left is 0
bids done eating :D