public class Test {

static Boolean flag = true;

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

    Thread thread1 = new Thread(() -> {
        int a = 0;
        while (true) {
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(a++ + flag.toString());
        }
    });

    Thread thread2 = new Thread(()->{
        while (true){
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            flag = !flag;
        }
    });

    thread1.start();
    thread2.start();
}

}

无论是否加volatile关键字，thread2对flag变量的修改对thread1始终都是可见的。