A simple System.CurrentTimemillis () saves more than 5 times on high concurrency

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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(a) {
        return InstanceHolder.INSTANCE;
    }

    private void scheduleClockUpdating(a) {
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor(new ThreadFactory() {
            @Override
            public Thread newThread(Runnable runnable) {
                Thread thread = new Thread(runnable, "System Clock");
                thread.setDaemon(true);
                returnthread; }}); scheduler.scheduleAtFixedRate(() -> now.set(System.currentTimeMillis()), period, period, TimeUnit.MILLISECONDS); }private long currentTimeMillis(a) {
        return now.get();
    }

    /** * replaces system.currentTimemillis () */
    public static long now(a) {
        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://upload-images.jianshu.io/upload_images/19662588-0e6edfd1236e6043.gif? imageMogr2/auto-orient/strip)

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

# While starting a single thread increases CPU usage, it is also an optimization point in high concurrency.

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