高並發場景下System.currentTimeMillis()的性能問題的優化

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，不過聽說在100倍左右），然而該方法又是一個常用方法，有時不得不使用，比如生成wokerId、打印日志什么的，在高並發情形下肯定存在性能問題的，但怎么做才好呢？ System.currentTimeMillis()之所以慢是因為去跟系統打了一次交道。那什么快？內存！如果該方法從內存直接取數，那不就美滋滋了。

代碼實現

package com.nyvi.support.util;

import java.sql.Timestamp;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

/**
 * <p>
 * 高並發場景下System.currentTimeMillis()的性能問題的優化
 * </p>
 * @author czk
 */
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。

測試

寫了一個簡單的測試代碼:

	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
看着結果效率提升還是挺明顯的。