volatile修饰符

Volatile 修饰的成员变量在每次被线程访问时，都强制从共享内存中重新读取该成员变量的值。而且，当成员变量发生变化时，会强制线程将变化值回写到共享内存。这样在任何时刻，两个不同的线程总是看到某个成员变量的同一个值。

一个volatile对象引用可能是null。

（1）Java程序代码

package niukewang;

public class volatile_Test {

    public static void main(String args[]) {
        MyRunnable r = new MyRunnable();
        Thread t1 = new Thread(r);

        t1.start();
        
        try {
            Thread.sleep(2);
            r.stopThread();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

class MyRunnable implements Runnable {
    private volatile boolean active;

    public void run() {
        active = true;
        while (active) // 第一行
        {
            // 代码
            System.out.println("Print the line");
        }
    }

    public void stopThread() {
        System.out.println("Set to false");
        active = false; // 第二行
    }
}

只有当调用stopThread时候，设置了ative为false的时候，然后线程获取了false的值，会停止第一行中内容的输出。

（2）输出结果。

Print the line
Print the line
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Print the line
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Print the line
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Print the line
Set to false
Print the line