c# - Java 和 C# 线程处理数据同步的方式有何不同？

Question

在下面的 C# 代码中，t1 总是(对于我尝试过的次数)完成。

class MainClass
{
    static void DoExperiment ()
    {
        int value = 0;

        Thread t1 = new Thread (() => {
            Console.WriteLine ("T one is running now");
            while (value == 0) {
                //do nothing
            }
            Console.WriteLine ("T one is done now");
        });

        Thread t2 = new Thread (() => {
            Console.WriteLine ("T two is running now");
            Thread.Sleep (1000);
            value = 1;
            Console.WriteLine ("T two changed value to 1");
            Console.WriteLine ("T two is done now");
        });

        t1.Start ();
        t2.Start ();

        t1.Join ();
        t1.Join ();
    }

    public static void Main (string[] args)
    {
        for (int i=0; i<10; i++) {
            DoExperiment ();
            Console.WriteLine ("------------------------");
        }
    }
} 

但是在非常相似的Java代码中，t1永远不会(我尝试过的时候)退出:

public class MainClass {
static class Experiment {
    private int value = 0;

    public void doExperiment() throws InterruptedException {

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {

                System.out.println("T one is running now");
                while (value == 0) {
                    //do nothing
                }
                System.out.println("T one is done now");
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("T two is running now");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                value = 1;
                System.out.println("T two changed value to 1");
                System.out.println("T two is done now");
            }
        }
        );

        t1.start();
        t2.start();

        t1.join();
        t1.join();
    }
}


public static void main(String[] args) throws InterruptedException {
    for (int i = 0; i < 10; i++) {
        new Experiment().doExperiment();
        System.out.println("------------------------");
    }
}

}

这是为什么？

Answer 1

我不确定 C# 中是如何发生的，但 Java 中发生的是 JVM 优化。 value 的值在 while 循环内不会改变，并且 JVM 会识别它并跳过测试并将咬合代码更改为类似的内容这个:

while (true) {
    // do nothing
}

为了在 Java 中修复此问题，您需要将 value 声明为 volatile :

private volatile int value = 0;

这将使JVM不会优化此while循环并检查value的实际值> 在每次迭代开始时。