package com.github.ltsopensource.core.support;

import com.github.ltsopensource.core.factory.NamedThreadFactory;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
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
 */
public class SystemClock {

    private final long period;
    private final AtomicLong now;

    private SystemClock(long period) {
        this.period = period;
        now = new AtomicLong(System.currentTimeMillis());
        scheduleClockUpdating();
    }

    private static class InstanceHolder {
        public static final SystemClock INSTANCE = new SystemClock(1);
    }

    private static SystemClock instance() {
        return InstanceHolder.INSTANCE;
    }

    private void scheduleClockUpdating() {
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor(new NamedThreadFactory("System Clock", true));
        scheduler.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                now.set(System.currentTimeMillis());
            }
        }, period, period, TimeUnit.MILLISECONDS);
    }

    private long currentTimeMillis() {
        return now.get();
    }

    public static long now() {
        return instance().currentTimeMillis();
    }

}