一个java多线程的问题

为什么代码中线程VolatileExample 不会终止

public class VolatileExample extends Thread{
    private static boolean flag = false;
    @Override
    public void run() {
        while (!flag) {
            //System.out.println(1);
        }
    }
    public static void main(String[] args) throws InterruptedException {
        VolatileExample v = new VolatileExample();
        v.start();
        Thread.sleep(1000);
        flag = true;

    }
}

而

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

    @Override
    public void run() {
        while (!flag) {
            System.out.println(1);
        }
    }
    public static void main(String[] args) throws InterruptedException {
        VolatileExample v = new VolatileExample();
        v.start();
        Thread.sleep(1000);
        flag = true;

    }
}

却可以正常终止。
还有这样

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

    @Override
    public void run() {
        while (!flag) {
            //System.out.println(1);
        }
    }
    public static void main(String[] args) throws InterruptedException {
        VolatileExample v = new VolatileExample();
        v.start();
        Thread.sleep(1000);
        flag = true;

    }
}

线程也可以正常终止。