为什么指令重排序的 volatile Happens-Before 指令失败？

Question

我有以下代码来测试 volatile。 bEnd 和 nCount 定义为可变的。

nCount = 0, bEnd = false

Writer 线程将设置

nCount = 100, bEnd = true

Reader 线程读取这些变量并打印它们。基于 Java Happens-before 顺序，在我看来，当 bEnd = true 时，volatile 确保 nCount = 100。但有时程序会打印这个：

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如何得到"nCount = 0, bEnd = true"???

下面代码运行上windows10,jdk1.8.0_131

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

volatile 确保当 bEnd = true 时 nCount = 100

从技术上讲，是的。但是 reader 并没有自动读取它们。所以它可能会打印 nCount = 0 and bEnd = true.

这是一个例子：

Reader 读取 nCount 0
维特写道 nCount = 100
维特写道 bEnd = true
作者打印 thread Writer bEnd = true
Reader 读取 bEnd true

Answer 2

顺便说一句，您的整个示例都有点缺陷。要测试之前发生的情况，您需要测试后续操作，即使 bEnd 易变且 nCount 不是易变的，然后将您的示例简化为：

static class Reader implements Runnable {
    @Override
    public void run() {

        while (true) {
            if (SharedData.bEnd) {
                System.out.println(SharedData.nCount);
                break;
            } else {
                System.out.println("Not yet seen as true");
                LockSupport.parkNanos(TimeUnit.MILLISECONDS.toNanos(100));
            }
        }
    }
}

static class Writer implements Runnable {
    @Override
    public void run() {
        LockSupport.parkNanos(TimeUnit.MILLISECONDS.toNanos(1000));
        SharedData.nCount = 100;
        SharedData.bEnd = true;

    }
}

这将始终输出 100（至少在这种情况下）。正确的解释是，如果 Reader 线程看到 Write 线程对 volatile 变量的更新，它将看到之前完成的所有操作，因此 100.

为什么指令重排序的 volatile Happens-Before 指令失败？

Why the volatile Happens-Before order for Instruction Reordering fails?

java

multithreading

volatile