java - 在 Java 中使用 wait() 和 notify() 的生产者消费者程序

Question

我正在使用低级同步以及 wait() 和 notify() 在 Java 中解决经典的生产者-消费者问题。我知道使用 java.util.concurrent 包中的结构有更好的实现，但我的问题围绕低级实现:

private static ArrayList<Integer> list = new ArrayList<Integer>();

    static Object obj = new Object();

    public static void producer() throws InterruptedException {
        synchronized (obj) {
            while (true) {
                if (list.size() == 10) {
                    System.out.println("Queue full.. Waiting to Add");
                    obj.wait();
                } else {
                    int value = new Random().nextInt(100);
                    if (value <= 10) {
                        Thread.sleep(200);
                        System.out.println("The element added was : " + value);
                        list.add(value);
                        obj.notify();
                    }
                }
            }
        }

    }

    public static void consumer() throws InterruptedException {
        synchronized (obj) {
            while (true) {
                Thread.sleep(500);
                if (list.size() == 0) {
                    System.out.println("Queue is empty...Waiting to remove");
                    obj.wait();
                } else {
                    System.out.println("The element removed was : "
                            + list.remove(0));
                    obj.notify();
                }
            }
        }

    }

程序中有2个线程，生产者和消费者各1个。代码工作得很好。

唯一的问题 是生产者继续生产消息直到一次达到最大值(直到列表的大小为 10)，而消费者一次消费所有 10 个。

如何让生产者和消费者同时工作？

这是示例输出:

The element added was : 4
The element added was : 0
The element added was : 0
The element added was : 4
The element added was : 3
The element added was : 1
The element added was : 10
The element added was : 10
The element added was : 3
The element added was : 9
Queue full.. Waiting to Add
The element removed was : 4
The element removed was : 0
The element removed was : 0
The element removed was : 4
The element removed was : 3
The element removed was : 1
The element removed was : 10
The element removed was : 10
The element removed was : 3
The element removed was : 9
Queue is empty...Waiting to remove

编辑:这是更正后的代码:

private static ArrayList<Integer> list = new ArrayList<Integer>();
    private static Object obj = new Object();

    public static void producer() throws InterruptedException {
        while (true) {
            Thread.sleep(500);
            if (list.size() == 10) {
                System.out.println("Waiting to add");
                synchronized (obj) {
                    obj.wait();
                }
            }
            synchronized (obj) {
                int value = new Random().nextInt(10);
                list.add(value);
                System.out.println("Added to list: " + value);
                obj.notify();
            }
        }
    }

    public static void consumer() throws InterruptedException {
        while (true) {
            Thread.sleep(500);
            if (list.size() == 0) {
                System.out.println("Waiting to remove");
                synchronized (obj) {
                    obj.wait();
                }
            }
            synchronized (obj) {
                int removed = list.remove(0);
                System.out.println("Removed from list: " + removed);
                obj.notify();
            }
        }
    }

Answer 1

您不能在具有相同对象的同步块(synchronized block)中运行两个线程。当一个方法正在运行时，另一个方法无法运行，直到另一个线程调用wait方法。

要解决这个问题，您应该将 add 和 remove 放在 synchronized block 中。有关详细信息，请参阅 this .