com.nyvi.support.util.SystemClock Maven / Gradle / Ivy

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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;

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

 * System.currentTimeMillis()之所以慢是因为去跟系统打了一次交道

 * 后台定时更新时钟,JVM退出时,线程自动回收

 * 10亿：43410,206,210.72815533980582%

 * 1亿：4699,29,162.0344827586207%

 * 1000万：480,12,40.0%

 * 100万：50,10,5.0%

 * 
 * @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);
	}
}