System.currentTimeMillis()

测试代码

public static void main(String[] args) {
    long start = System.nanoTime();
    singleThread(100);
    System.out.println("single: " + (System.nanoTime() - start));
    start = System.nanoTime();
    multiThread(100);
    System.out.println("multi: " + (System.nanoTime() - start));
}

public static void singleThread(int size) {
    for (int i = 0; i < size; i++) {
        System.currentTimeMillis();
    }
}

public static void multiThread(int size) {
    CountDownLatch latch = new CountDownLatch(size);
    for (int i = 0; i < size; i++) {
        new Thread(() -> {
            try {
                System.currentTimeMillis();
            } finally {
                latch.countDown();
            }
        }).start();
    }
    try {
        latch.await();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

分析

System.currentTimeMillis()调用会与系统交互，频繁访问或者高并发会造成严重的争用

结果

1 2	single: 17727 multi: 58580004

解决思路

创建一个类，在类中维护一个线程，定时去同步

public class CurrentTimeMillisClock {
    private volatile long now;

    private CurrentTimeMillisClock() {
        this.now = System.currentTimeMillis();
        scheduleTick();
    }

    private void scheduleTick() {
        new ScheduledThreadPoolExecutor(1, runnable -> {
            Thread thread = new Thread(runnable, "current-time-millis");
            thread.setDaemon(true);
            return thread;
        }).scheduleAtFixedRate(() -> {
            now = System.currentTimeMillis();
        }, 1, 1, TimeUnit.MILLISECONDS);
    }

    public long now() {
        return now;
    }

    public static CurrentTimeMillisClock getInstance() {
        return SingletonHolder.INSTANCE;
    }

    private static class SingletonHolder {
        private static final CurrentTimeMillisClock INSTANCE = new
                CurrentTimeMillisClock();
    }
}