public class SpinLock {

    private AtomicReference<Thread> sign = new AtomicReference<>();

    public void lock() {
        Thread current = Thread.currentThread();
        // 自旋
        while (!sign.compareAndSet(null, current)) {

        }
    }

    public void unlock() {
        Thread current = Thread.currentThread();
        sign.compareAndSet(current, null);
    }

    public static void main(String[] args) {
        SpinLock spinLock = new SpinLock();
        Runnable r = () -> {
            System.out.println(Thread.currentThread().getName() + "开始尝试获取自旋锁");
            spinLock.lock();
            try {
                System.out.println(Thread.currentThread().getName() + "获取到了自旋锁");
                Thread.sleep(300);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                spinLock.unlock();
                System.out.println(Thread.currentThread().getName() + "释放了自旋锁");
            }
        };

        new Thread(r).start();
        new Thread(r).start();
    }
}

会有同时获取到锁的情况，JDK版本是11：