import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

/**
 * 
 * @author rs
 */
public class SystemClock {

    private final int period;

    private final AtomicLong now;

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

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

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

    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(() -> now.set(System.currentTimeMillis()), period, period, TimeUnit.MILLISECONDS);
    }

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

    /**
     * 用来替换原来的System.currentTimeMillis()
     */
    public static long now() {
        return instance().currentTimeMillis();
    }

    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        for (long i = 0; i < Integer.MAX_VALUE; i++) {
            SystemClock.now();
        }
        long end = System.currentTimeMillis();
        System.out.println("SystemClock Time:" + (end - start) + "毫秒");
        long start2 = System.currentTimeMillis();
        for (long i = 0; i < Integer.MAX_VALUE; i++) {![123.gif](https://img.haomeiwen.com/i19662588/0e6edfd1236e6043.gif?imageMogr2/auto-orient/strip)

            System.currentTimeMillis();
        }
        long end2 = System.currentTimeMillis();
        System.out.println("currentTimeMillis Time:" + (end2 - start2) + "毫秒");
        //SystemClock Time:1450毫秒
        //currentTimeMillis Time:7482毫秒
    }
}

虽然单独启动了一个线程，增加了cpu使用率，但是在高并发当中也是一个优化点。 123.gif