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some utility class for java develop
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package com.star.time;

import java.sql.Timestamp;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;

/**
 * 系统时钟

 * 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
 * 
 * see： http://git.oschina.net/yu120/sequence
 *
 * @author lry, looly
 */
public class SystemClock {

    /**
     * 时钟更新间隔，单位毫秒
     */
    private final long period;
    /**
     * 现在时刻的毫秒数
     */
    private volatile long now;

    /**
     * 构造
     *
     * @param period
     */
    private SystemClock(long period) {
        this.period = period;
        this.now = System.currentTimeMillis();
        scheduleClockUpdating();
    }

    /**
     * 单例实例
     *
     * @return 单例实例
     */
    private static SystemClock instance() {
        return InstanceHolder.INSTANCE;
    }

    /**
     * @return 当前时间
     */
    public static long now() {
        return instance().currentTimeMillis();
    }

    //------------------------------------------------------------------------ static

    /**
     * @return 当前时间字符串表现形式
     */
    public static String nowDate() {
        return new Timestamp(instance().currentTimeMillis()).toString();
    }

    /**
     * 开启计时器线程
     */
    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 = System.currentTimeMillis();
            }
        }, period, period, TimeUnit.MILLISECONDS);
    }

    /**
     * @return 当前时间毫秒数
     */
    private long currentTimeMillis() {
        return now;
    }

    /**
     * 单例
     *
     * @author Looly
     */
    private static class InstanceHolder {
        public static final SystemClock INSTANCE = new SystemClock(1);
    }
}