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

 1 package cn.ucaner.alpaca.common.util.key;
 2 
 3 import java.sql.Timestamp;
 4 import java.util.concurrent.*;
 5 import java.util.concurrent.atomic.AtomicLong;
 6 
 7 /**
 8  * 高并发场景下System.currentTimeMillis()的性能问题的优化
 9  * <p><p>
10  * 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>
11  * System.currentTimeMillis()之所以慢是因为去跟系统打了一次交道<p>
12  * 后台定时更新时钟，JVM退出时，线程自动回收<p>
13  * 10亿：43410,206,210.72815533980582%<p>
14  * 1亿：4699,29,162.0344827586207%<p>
15  * 1000万：480,12,40.0%<p>
16  * 100万：50,10,5.0%<p>
17  * @author lry
18  */
19 public class SystemClock {
20     
21     private final long period;
22     
23     private final AtomicLong now;
24     
25     ExecutorService executor = Executors.newSingleThreadExecutor();
26 
27     private SystemClock(long period) {
28         this.period = period;
29         this.now = new AtomicLong(System.currentTimeMillis());
30         scheduleClockUpdating();
31     }
32 
33     private static class InstanceHolder {
34         public static final SystemClock INSTANCE = new SystemClock(1);
35     }
36 
37     private static SystemClock instance() {
38         return InstanceHolder.INSTANCE;
39     }
40 
41     private void scheduleClockUpdating() {
42         ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor(new ThreadFactory() {
43             @Override
44             public Thread newThread(Runnable runnable) {
45                 Thread thread = new Thread(runnable, "System Clock");
46                 thread.setDaemon(true);
47                 return thread;
48             }
49         });
50         scheduler.scheduleAtFixedRate(new Runnable() {
51             @Override
52             public void run() {
53                 now.set(System.currentTimeMillis());
54             }
55         }, period, period, TimeUnit.MILLISECONDS);
56     }
57 
58     private long currentTimeMillis() {
59         return now.get();
60     }
61 
62     public static long now() {
63         return instance().currentTimeMillis();
64     }
65 
66     public static String nowDate() {
67         return new Timestamp(instance().currentTimeMillis()).toString();
68     }
69     
70     /**
71      * @Description: Just for test
72      * @param args void
73      * @throws InterruptedException 
74      * @Autor: Jason - [email protected]
75      */
76     public static void main(String[] args) throws InterruptedException {
77         for (int i = 0; i < 100; i++) {
78             System.out.println(nowDate());
79             Thread.sleep(1000);
80         }
81     }
82 }
83 //Outputs
84 //2018-05-10 15:37:18.774
85 //2018-05-10 15:37:19.784
86 //2018-05-10 15:37:20.784
87 //2018-05-10 15:37:21.785
88 //2018-05-10 15:37:22.784
89 //2018-05-10 15:37:23.784
90 //2018-05-10 15:37:24.785
91 //2018-05-10 15:37:25.784
92 //2018-05-10 15:37:26.785
93 //2018-05-10 15:37:27.786
94 //2018-05-10 15:37:28.785
95 //2018-05-10 15:37:29.785
96 //2018-05-10 15:37:30.785
97 //2018-05-10 15:37:31.785