java - 消费者生产者在java中不起作用

Question

我尝试根据官方的oracle示例(http://docs.oracle.com/javase/tutorial/essential/concurrency/guardmeth.html)创建一个消费者生产者。

这个解决方案给我带来了一个问题，因为不知何故消耗(或生产)被执行了两次。

示例输出:

0.27621192120612414

0.24838246527492802、0.30404137713732027

0.8848100139189661

0.9910138279470992、0.778606199890833

0.17368370874476935、0.5661899414440023 ......

我的代码与发布的预言机非常相似:

import java.util.Random;

public class Consumer implements Runnable {

    private SharedData sharedData;

    public Consumer(SharedData sharedData) {
    this.sharedData = sharedData;
    }

    @
    Override
    public void run() {
    // TODO Auto-generated method stub
    for (int i = 0; i < 10; i++) {

        double result = this.sharedData.calc();
        System.out.println(result);

        try {
        Thread.sleep(20);
        } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        }
    }
    }

public class Producer implements Runnable {

    private SharedData sharedData;

    private Random numGenerator = new Random();

    public Producer(SharedData sharedData) {
        this.sharedData = sharedData;
    }

    @
    Override
    public void run() {
        // TODO Auto-generated method stub
        for (int i = 0; i < 10; i++) {
        double firstNum = numGenerator.nextDouble();
        double secondNum = numGenerator.nextDouble();

        this.sharedData.store(firstNum, secondNum);
        System.out.println(firstNum + ", " + secondNum);

        try {
            Thread.sleep(20);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        }
    }
}

public class SharedData {

    private double x = 0;
    private double y = 0;

    private boolean empty = true;

    public synchronized double calc() {
        while (empty) {
        try {
            wait();
        } catch (InterruptedException e) {}
        }
        empty = true;
        notifyAll();
        return (x + y) / 2;
    }

    public synchronized void store(double x, double y) {
        while (!empty) {
        try {
            wait();
        } catch (InterruptedException e) {}
        }
        // Toggle status.
        empty = false;
        this.x = x;
        this.y = y;
        notifyAll();
    }
}

Answer 1

您的 System.out.println 调用不在任何同步部分中，因此即使生产者在消费者消费之前进行生产，也无法保证其输出将在消费者输出之前出现:

1. 生产者线程生产，消费者线程等待
2. 消费者线程消费
3. 消费者线程打印结果
4. 生产者线程打印结果

将 System.out.println 调用放入同步部分中，您应该会看到消费者和生产者输出之间的严格交替。