高并发场景下System.currentTimeMillis()的性能问题的优化

高并发场景下System.currentTimeMillis()的性能问题的优化

package cn.ucaner.alpaca.common.util.key;

import java.sql.Timestamp;
import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicLong;

/**
 * 高并发场景下System.currentTimeMillis()的性能问题的优化
 * <p><p>
 * System.currentTimeMillis()的调用比new一个普通对象要耗时的多（具体耗时高出多少我还没测试过，有人说是100倍左右）<p>
 * System.currentTimeMillis()之所以慢是因为去跟系统打了一次交道<p>
 * 后台定时更新时钟，JVM退出时，线程自动回收<p>
 * 10亿：43410,206,210.72815533980582%<p>
 * 1亿：4699,29,162.0344827586207%<p>
 * 1000万：480,12,40.0%<p>
 * 100万：50,10,5.0%<p>
 * @author lry
 */
public class SystemClock {
    
    private final long period;
    
    private final AtomicLong now;
    
    ExecutorService executor = Executors.newSingleThreadExecutor();

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

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

    private static SystemClock instance() {
        return InstanceHolder.INSTANCE;
    }

    private void scheduleClockUpdating() {
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor(new ThreadFactory() {
            @Override
            public Thread newThread(Runnable runnable) {
                Thread thread = new Thread(runnable, "System Clock");
                thread.setDaemon(true);
                return thread;
            }
        });
        scheduler.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                now.set(System.currentTimeMillis());
            }
        }, period, period, TimeUnit.MILLISECONDS);
    }

    private long currentTimeMillis() {
        return now.get();
    }

    public static long now() {
        return instance().currentTimeMillis();
    }

    public static String nowDate() {
        return new Timestamp(instance().currentTimeMillis()).toString();
    }
    
    /**
     * @Description: Just for test
     * @param args void
     * @throws InterruptedException 
     * @Autor: Jason - jasonandy@hotmail.com
     */
    public static void main(String[] args) throws InterruptedException {
        for (int i = 0; i < 100; i++) {
            System.out.println(nowDate());
            Thread.sleep(1000);
        }
    }
}
//Outputs
//2018-05-10 15:37:18.774
//2018-05-10 15:37:19.784
//2018-05-10 15:37:20.784
//2018-05-10 15:37:21.785
//2018-05-10 15:37:22.784
//2018-05-10 15:37:23.784
//2018-05-10 15:37:24.785
//2018-05-10 15:37:25.784
//2018-05-10 15:37:26.785
//2018-05-10 15:37:27.786
//2018-05-10 15:37:28.785
//2018-05-10 15:37:29.785
//2018-05-10 15:37:30.785
//2018-05-10 15:37:31.785