Java でグローバル変数を使用してスレッドを終了するにはどうすればよいですか?

public class ThreadDemo {
 
    // 全局自定义变量
    private static boolean flag = false;
 
    public static void main(String[] args) throws InterruptedException {
        // 转账线程
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                while (!flag) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("正在讲话...");
                }
                System.out.println("停止说话");
            }
        });
        t1.start();
 
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(310);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                // 改变变量的值来终止线程
                System.out.println("停止说话，有要事发生。");
                flag = true;
            }
        });
        t2.start();
        t1.join(); //对于用户线程而言，join()可以不写
        t2.join();
    }
 
}