线程：忙等待 - 空循环 [英] Threads: Busy Waiting - Empty While-Loop

问题描述

在我们上大学的课程中，我们学习了主题并使用忙碌等待方法作为 Car 等待 TrafficLight 。对于此任务，我们构建了三个类：

During our lessons in the university, we learned about Threads and used the "Busy Waiting" method for an example of a Car waiting at a TrafficLight. For this task we build three classes:

• TrafficLight（实现Runnable）


• Car（实现Runnable）


• Main

• TrafficLight (implements Runnable)
• Car (implements Runnable)
• Main

在我们的 Main 类中，我们开始两个线程 s， Car 之一，以及 TrafficLight 之一。 Car 具有布尔属性 hasToWait 。这个类中的 run（）方法的工作方式是，它通过而循环工作，只要 hasToWait == true 。要更改此设置，我们在 Car 类中有 notifyCar（）方法，该方法由<$ c使用$ C>交通灯。 TrafficLight 中的 run（）方法通过 Thread.sleep（）模拟一定的等待时间。

In our Main class we start two Threads, one of Car, and one of TrafficLight. The Car has the boolean attribute hasToWait. The run() method in this class works the way, that it works through a while loop as long as hasToWait == true. To change this, we have the notifyCar() method in the Car class, which is used by the TrafficLight. The run() method in TrafficLight runs through a Thread.sleep() to simulate a certain time of waiting.

我的教授一切正常，但最终我遇到了严重问题。只要 Car 类中的 while 循环为空。当我输入 System.out.println（） - 不为空时，它可以正常工作。但是如果Syso为空，则结果是不显示 Run 方法的文本。
当 TrafficLight 中的 Thread.sleep（）为 0时，它也正常工作。比使用空而循环。

Everything works fine at my Prof's but eventually I have serious problems with it. As long as the while loop in the Car class is empty. When I put in a System.out.println() - which is not empty, it works. But if the Syso is empty, the result is no displaying of the Text of the Run method. Also it's working when the Thread.sleep() in TrafficLight is 0. Than it works with an empty while loop.

这是我的代码：

package trafficlight;

public class Car implements Runnable {

    private boolean hasToWait = true;

    public void run() {
        this.crossTrafficLight();
    }

    public void crossTrafficLight() {
        while(hasToWait){ for(int i = 0; i<20; i++){System.out.println("123");}} // Busy waiting
        System.out.println("Auto fährt über Ampel");
    }

    public void notifyCar() {
        this.hasToWait = false;
        System.out.println("Test");
    }
}

TrafficLight.java：

TrafficLight.java:

package trafficlight;

public class TrafficLight implements Runnable {
    private Car car;

    public TrafficLight(Car car) {
        this.car = car;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        this.car.notifyCar();
    }
}

Main.java：

Main.java:

package trafficlight;

public class Main {

    public static void main(String[] args){
        Car car = new Car();
        TrafficLight tl = new TrafficLight(car);

        new Thread(car).start();
        new Thread(tl).start();
    }

}

问题出在哪里？为什么它适用于我的教授而不是我的电脑？我在Eclipse Juno中获得了代码1：1，使用 JRE 1.7

Where is the problem? Why does it work at my profs but not at my computer? I got the code 1:1 in my Eclipse Juno, using JRE 1.7

推荐答案

除了其他答案中所说的一切之外（只需替换你的 hasToWait 在该答案中已完成，代码在添加 println时开始工作的原因如下：

In addition to everything said in this other answer (just substitute your hasToWait for finished in that answer), the reason why the code starts working when you add a println is as follows:

• println 是 synchronized 方法;


• 你在两个线程中调用它;


• 这会在之前发生 >两个线程之间的关系;


• 因此对子线程的布尔标志的写入变得可见。

• println is a synchronized method;
• you call it in both threads;
• this creates a happens-before relationship between the two threads;
• therefore the write to the boolean flag becomes visible to the child thread.

你可以说它主要是开始工作：你正在捎带 println 中正在进行的同步。

You could say that it starts working mostly by accident: you are piggybacking on the synchronization going on in println.

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