volatile 的一个经典例子

volatile 关键字的两层语义

一旦一个共享变量（类的成员变量、类的静态成员变量）被 volatile 修饰之后，那么就具备了两层语义： 1）保证了不同线程对这个变量进行操作时的可见性，即一个线程修改了某个变量的值，这新值对其他线程来说是立即可见的。 2）禁止进行指令重排序。

方式一：变量不使用 volatile 修饰

public class VolatileTest extends Thread {
    private static boolean flag = false;

    public void run() {
        while (!flag) ;
    }

    public static void main(String[] args) throws Exception {
        new VolatileTest().start();
        Thread.sleep(2000);
        flag = true;
    }
}

方式二：变量使用 volatile 修饰

public class VolatileTest extends Thread {
    private static volatile boolean flag = false;

    public void run() {
        while (!flag) ;
    }

    public static void main(String[] args) throws Exception {
        new VolatileTest().start();
        Thread.sleep(2000);
        flag = true;
    }
}

运行结果
方式一：线程不会结束

方式二：线程会结束

public class TestMain {
    public static volatile boolean flag = true;

    public static void main(String[] args) throws InterruptedException {

        new Thread(new Runnable() {
            @Override
            public void run() {
                while (flag) {
                }
                System.out.println(Thread.currentThread().getName() + "线程停止，死循环被打开");
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                flag = false;
                System.out.println(Thread.currentThread().getName() + "修改 flag 为" + flag);
            }
        }).start();

        Thread.sleep(Integer.MAX_VALUE);
    }
}

public class RunThread extends Thread {
    /** volatile */
    private volatile boolean isRunning = true;

    private void setRunning(boolean isRunning) {
        this.isRunning = isRunning;
    }

    public void run() {
        System.out.println("进入 run() 方法中...");
        while (isRunning == true) {
            // doSomething()
        }
        System.out.println("线程结束了...");
    }

    public static void main(String[] args) throws InterruptedException {
        RunThread myThread = new RunThread();
        myThread.start();
        Thread.sleep(3000);
        myThread.setRunning(false);
        System.out.println("isRunning 的值已经设置为了 false");
        Thread.sleep(1000);
        System.out.println(myThread.isRunning);
    }
}

public class RunThread extends Thread {
    /** volatile */
    private boolean isRunning = true;

    private void setRunning(boolean isRunning) {
        this.isRunning = isRunning;
    }

    public void run() {
        System.out.println("进入 run() 方法中...");
        while (isRunning == true) {
            // doSomething()
        }
        System.out.println("线程结束了...");
    }

    public static void main(String[] args) throws InterruptedException {
        RunThread myThread = new RunThread();
        myThread.start();
        Thread.sleep(3000);
        myThread.setRunning(false);
        System.out.println("isRunning 的值已经设置为了 false");
        Thread.sleep(1000);
        System.out.println(myThread.isRunning);
    }
}