System.currentTimeMillis()高并发下性能优化 (原创)

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

思路：使用一个单例 SystemClock对象，对象中开启一个后台线程，每隔一毫秒更新一次clock时间.

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
 * 时间戳打印建议使用
 */
public class SystemClock {

    private static final String THREAD_NAME = "system.clock";
    private static final SystemClock MILLIS_CLOCK = new SystemClock(1);
    private final long precision;
    private final AtomicLong now;

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

    public static SystemClock millisClock() {
        return MILLIS_CLOCK;
    }

    private void scheduleClockUpdating() {
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor(runnable -> {
            Thread thread = new Thread(runnable, THREAD_NAME);
            thread.setDaemon(true);
            return thread;
        });
        scheduler.scheduleAtFixedRate(() ->
                now.set(System.currentTimeMillis()),
                precision, precision, TimeUnit.MILLISECONDS);
    }

    public long now() {
        return now.get();
    }
}