System.currentTimeMillis()

System.currentTimeMillis() 方法的粒度大于1毫秒。如果你重复地调用该方法，在一段时间内你会看到你获取到的都是相同的值，然后突然一下子跳了10/20/30毫秒甚至更多。该方法不是全世界最准确的或者细粒度的计时器。

public class SystemClock {

    // 保证其原子性
    private final AtomicLong now;

    // 时间间隔
    private final int period;

    private SystemClock(int period) {
        this.now = new AtomicLong(System.currentTimeMillis());
        this.period = period;
        clockUpdate();
    }

    private static class InstanceHolder {
        private static SystemClock INSTANCE = new SystemClock(1);
    }

    private void clockUpdate() {
        ScheduledExecutorService service = Executors.newSingleThreadScheduledExecutor(r -> {
            Thread t = new Thread(r, "SystemClock");
            // 主线程退出后，该线程也退出
            t.setDaemon(true);
            return t;
        });
        service.scheduleAtFixedRate(() -> now.set(System.currentTimeMillis()), period, period, TimeUnit.MILLISECONDS);
    }

    public static long now() {
        return InstanceHolder.INSTANCE.now.get();
    }

}