volatile 的一個經典例子

volatile 關鍵字的兩層語義

一旦一個共享變量（類的成員變量、類的靜態成員變量）被 volatile 修飾之后，那么就具備了兩層語義： 1）保證了不同線程對這個變量進行操作時的 可見性，即一個線程修改了某個變量的值，這新值對其他線程來說是立即可見的。 2）禁止進行 指令重排序。

方式一：變量不使用 volatile 修飾

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：變量使用 volatile 修飾

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;
    }
}

運行結果
方式一：線程不會結束

方式二：線程會結束

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() + "線程停止，死循環被打開");
            }
        }).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 為" + 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("進入 run() 方法中...");
        while (isRunning == true) {
            // doSomething()
        }
        System.out.println("線程結束了...");
    }

    public static void main(String[] args) throws InterruptedException {
        RunThread myThread = new RunThread();
        myThread.start();
        Thread.sleep(3000);
        myThread.setRunning(false);
        System.out.println("isRunning 的值已經設置為了 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("進入 run() 方法中...");
        while (isRunning == true) {
            // doSomething()
        }
        System.out.println("線程結束了...");
    }

    public static void main(String[] args) throws InterruptedException {
        RunThread myThread = new RunThread();
        myThread.start();
        Thread.sleep(3000);
        myThread.setRunning(false);
        System.out.println("isRunning 的值已經設置為了 false");
        Thread.sleep(1000);
        System.out.println(myThread.isRunning);
    }
}