1、前言
System.currentTimeMillis()的調用比new一個普通對象要耗時的多（具體耗時高出多少我也不知道，不過據說在100倍左右），然而該方法又是一個經常使用方法，html
有時不得不使用，好比生成wokerId、打印日誌什麼的，在高併發情形下確定存在性能問題的，但怎麼作纔好呢？ System.currentTimeMillis()之因此慢是由於java
去跟系統打了一次交道。那什麼快？內存！若是該方法從內存直接取數，那不就美滋滋了。markdown
2、代碼實現
public class SystemClock { private final long period; private final AtomicLong now; private SystemClock(long period) { this.period = period; this.now = new AtomicLong(System.currentTimeMillis()); scheduleClockUpdating(); } private static SystemClock instance() { return InstanceHolder.INSTANCE; } public static long now() { return instance().currentTimeMillis(); } public static String nowDate() { return new Timestamp(instance().currentTimeMillis()).toString(); } private void scheduleClockUpdating() { ScheduledThreadPoolExecutor scheduler = new ScheduledThreadPoolExecutor(1, new ThreadFactory() { @Override public Thread newThread(Runnable r) { Thread thread = new Thread(r, "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(); } private static class InstanceHolder { public static final SystemClock INSTANCE = new SystemClock(1); } }
用的時候直接調用SystemClock.now();就ok了。併發
測試
寫了一個簡單的測試代碼:ide
public static void main(String[] args) { long start = System.currentTimeMillis(); for (long i = 0; i < Integer.MAX_VALUE; i++) { SystemClock.now(); } long end = System.currentTimeMillis(); System.out.println("SystemClock Time:" + (end - start) + "毫秒"); long start2 = System.currentTimeMillis(); for (long i = 0; i < Integer.MAX_VALUE; i++) { System.currentTimeMillis(); } long end2 = System.currentTimeMillis(); System.out.println("currentTimeMillis Time:" + (end2 - start2) + "毫秒"); }
輸出結果是:
SystemClock Time:1787毫秒
currentTimeMillis Time:33851毫秒
看着結果效率提高仍是挺明顯的。高併發