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Dahua ICC Open API SDK for Java

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package com.dahuatech.hutool.core.date;

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 时钟更新间隔，单位毫秒
   */
  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);
  }
}