为什么这个简单的线程程序卡住？ [英] Why does this simple threaded program get stuck?

问题描述

看看这个简单的Java程序：

Take a look at this simple Java program:

import java.lang.*;

class A {
    static boolean done;
    public static void main(String args[]) {
        done = false;
        new Thread() {
            public void run() {
            try {
                Thread.sleep(1000); // dummy work load
            } catch (Exception e) {
                done = true;
            }
            done = true;
            }
        }.start();
        while (!done);
        System.out.println("bye");
    }
}

在一台机器上打印bye在另一台机器上，它不打印任何东西，永远坐在那里。为什么？

On one machine, it prints "bye" and exits right away, while on another machine, it doesn't print anything and sits there forever. Why?

推荐答案

这是因为 boolean 不是 volatile ，因此线程允许缓存它的副本，从不更新它们。我会推荐一个 AtomicBoolean - 这将防止您可能有的任何问题。

This is because your boolean is not volatile, therefore Threads are allowed to cache copies of it and never update them. I would recommend an AtomicBoolean - that will prevent any issues you may have.

public static void main(String args[]) {
    final AtomicBoolean done = new AtomicBoolean(false);
    new Thread() {
        public void run() {
            done.set(true);
        }
    }.start();
    while (!done.get());
    System.out.println("bye");
}

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