java - 为什么指令重新排序的 volatile 发生前顺序失败？

Question

我有以下代码来测试 volatile 。 bEnd 和 nCount 被定义为 volatile 的。

nCount = 0, bEnd = false

Writer 线程将设置

nCount = 100, bEnd = true

Reader 线程读取这些 viviable 并打印它们。基于 Java Happens-before 顺序，在我看来，当 bEnd = true 时，volatile 确保 nCount = 100。但有时程序会打印以下内容:

main thread done.
thread Reader running ...
thread Writer running ...
SharedData nCount = 0, bEnd = false
thread Writer bEnd = true
thread Reader nCount = 0, bEnd = true
thread Reader nCount = 100, bEnd = true
thread Reader nCount = 100, bEnd = true
thread Reader done.

Reader如何获得“nCount = 0，bEnd = true”？？？

以下代码在windows10、jdk1.8.0_131上运行

public class HappensBeforeWithVolatile {

    public static void main(String[] args) {

        Thread threadWriter = new Thread(new Writer());
        Thread threadReader = new Thread(new Reader());
        threadWriter.start();
        threadReader.start();

        System.out.println("main thread done.");
    }
}

class Writer implements Runnable {

    @Override
    public void run() {
        System.out.println("thread Writer running ...");
        SharedData.nCount = 100;
//        System.out.println("thread Writer nCount = 100");
        SharedData.bEnd = true;
        System.out.println("thread Writer bEnd = true");
    }
}

class Reader implements Runnable {

    @Override
    public void run() {
        System.out.println("thread Reader running ...");
        System.out.println("thread Reader nCount = " + SharedData.nCount + ", bEnd = " + SharedData.bEnd);
        System.out.println("thread Reader nCount = " + SharedData.nCount + ", bEnd = " + SharedData.bEnd);
        if (SharedData.nCount == 0 && SharedData.bEnd) {
            System.out.println("thread Reader CODE REORDER !!!");
        }
        System.out.println("thread Reader nCount = " + SharedData.nCount + ", bEnd = " + SharedData.bEnd);
        System.out.println("thread Reader done.");
    }
}

class SharedData {
    volatile public static boolean bEnd = false;
    volatile public static int nCount = 0;

    static {
        System.out.println("SharedData nCount = " + nCount + ", bEnd = " + bEnd);
    }
}

Answer 1

当 bEnd = true 时， volatile 确保 nCount = 100

从技术上来说，是的。但读者并没有自动地阅读它们。因此它可能会打印 nCount = 0 和 bEnd = true。

这是一个例子:

1. 读取器读取nCount 0
2. Wirter 写道 nCount = 100
3. Wirter 写道 bEnd = true
4. Writer 打印 thread Writer bEnd = true
5. 读者读取bEnd true