private static boolean flag = true;

public static void main(String[] args) throws InterruptedException {
    new Thread(() -> {
        while (flag) {
            // 一直循环
        }
        System.out.println("线程退出");
    }).start();

    Thread.sleep(1000);
    flag = false;
    System.out.println("主线程已将 flag 设为 false");
}