为什么挥发物不能正常工作 [英] Why volatile is not working properly
问题描述
今天，我正在使用TimerTask创建一个超时作业，但是遇到了一个新问题，即我有一个static volatile boolean变量flag.我的理解是，此变量的值一经更改，便会由所有正在运行的thread通知.但是当我运行该程序时，我得到的输出低于可接受的水平.
Today I was creating one timeout job using TimerTask but fell in to a new problem where i have a static volatile boolean variable flag. My understanding is as soon as value of this variable get changed it is notified by all running thread. But when I ran this program I got below output which is not acceptable.
O/P:
--------------
--------------
DD
BB
Exiting process..
CC
我的期望是我的最后打印应该是退出过程.为什么会有这种奇怪的行为?
My expectation is my last print should be Exiting process.. Why is this strange behavior?
我的代码是:
public class TimeOutSort {
static volatile boolean flag = false;
public static void main(String[] args) {
Timer timer = new Timer();
timer.schedule(new TimerTask() {
@Override
public void run() {
flag = true;
System.out.println("Exiting process..");
// System.exit(0);
}
}, 10 * 200);
new Thread(new Runnable() {
@Override
public void run() {
while (!flag)
System.out.println("BB");
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
while (!flag)
System.out.println("CC");
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
while (!flag)
System.out.println("DD");
}
}).start();
}
}
编辑:我该如何实现?
推荐答案
volatile几乎意味着，每次线程访问变量时，必须确保使用对每个线程可见的版本(即，没有按线程缓存)
volatile pretty much means that each time a thread accesses a variable it must ensure to use the version visible to each thread (i.e. no per-thread caching).
在flag设置为true后，此不会强制CC打印线程立即真正运行 .一个线程完全有可能(尤其是在单核计算机上)设置标志 并在CC打印线程有机会运行之前打印消息 .
This doesn't force the CC-printing thread to actually get to run immediately after the flag has been set to true. It's entirely possible (especially on a single-core machine) that one thread sets the flag and prints the message before the CC-printing thread even had a chance to run.
还:请注意，打印到System.out涉及获取锁(在println()调用中的某个位置)，该锁可以修改测试代码的多线程行为.
Also: note that printing to System.out involves acquiring a lock (somewhere inside the println() call), which can modify the multi-threaded behaviour of test code.
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