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 - [e-mail]
      */
     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