System.currentTimeMillis的性能真有如此不堪嗎？

/** * 弱精度的計(jì)時(shí)器，考慮性能不使用同步策略。 *  * @author mycat */public class TimeUtil {    //當(dāng)前毫秒數(shù)的緩存    private static volatile long CURRENT_TIME = System.currentTimeMillis();
    public static final long currentTimeMillis() { return CURRENT_TIME; }    public static final long currentTimeNanos() { return System.nanoTime(); } //更新緩存    public static final void update() { CURRENT_TIME = System.currentTimeMillis(); }
}//使用定時(shí)任務(wù)調(diào)度線程池，定期（每1s）調(diào)用update方法更新緩存時(shí)鐘heartbeatScheduler.scheduleAtFixedRate(processorCheck(), 0L, 1000, TimeUnit.MILLISECONDS)；

long begin = System.nanoTime();//單次調(diào)用System.currrentTimeMillis()long end = System.nanoTime();sum += end - begin;

public class CurrentTimeMillisTest {
    public static void main(String[] args) {        int num = 10000000;        System.out.print("單線程"+num+"次System.currentTimeMillis調(diào)用總耗時(shí)：");        System.out.println(singleThreadTest(() -> {            long l = System.currentTimeMillis();        },num));        System.out.print("單線程"+num+"次CacheClock.currentTimeMillis調(diào)用總耗時(shí)：");        System.out.println(singleThreadTest(() -> {            long l = CacheClock.currentTimeMillis();        },num));        System.out.print("并發(fā)"+num+"次System.currentTimeMillis調(diào)用總耗時(shí)：");        System.out.println(concurrentTest(() -> {            long l = System.currentTimeMillis();        },num));        System.out.print("并發(fā)"+num+"次CacheClock.currentTimeMillis調(diào)用總耗時(shí)：");        System.out.println(concurrentTest(() -> {            long l = CacheClock.currentTimeMillis();        },num));    }


    /**     * 單線程測(cè)試     * @return     */    private static long singleThreadTest(Runnable runnable,int num) {        long sum = 0;        for (int i = 0; i < num; i++) {            long begin = System.nanoTime();            runnable.run();            long end = System.nanoTime();            sum += end - begin;        }        return sum;    }
    /**     * 并發(fā)測(cè)試     * @return     */    private static long concurrentTest(Runnable runnable,int num) {        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(200,200,60, TimeUnit.SECONDS,new LinkedBlockingQueue<>(num));        long[] sum = new long[]{0};        //閉鎖基于CAS實(shí)現(xiàn)，并不適合當(dāng)前的計(jì)算密集型場(chǎng)景，可能導(dǎo)致等待時(shí)間較長(zhǎng)        CountDownLatch countDownLatch = new CountDownLatch(num);        for (int i = 0; i < num; i++) {            threadPoolExecutor.submit(() -> {                long begin = System.nanoTime();                runnable.run();                long end = System.nanoTime();                //計(jì)算復(fù)雜型場(chǎng)景更適合使用悲觀鎖                synchronized(CurrentTimeMillisTest.class) {                    sum[0] += end - begin;                }                countDownLatch.countDown();            });        }        try {            countDownLatch.await();        } catch (InterruptedException e) {            e.printStackTrace();        }        return sum[0];    }

    /**     * 緩存時(shí)鐘，緩存System.currentTimeMillis()的值，每隔20ms更新一次     */    public static class CacheClock{        //定時(shí)任務(wù)調(diào)度線程池        private static ScheduledExecutorService timer = new ScheduledThreadPoolExecutor(1);        //毫秒緩存        private static volatile long timeMilis;              static {            //每秒更新毫秒緩存            timer.scheduleAtFixedRate(new Runnable() {                @Override                public void run() {                    timeMilis = System.currentTimeMillis();                }            },0,1000,TimeUnit.MILLISECONDS);        }                public static long currentTimeMillis() {            return timeMilis;        }    }}

@BenchmarkMode(Mode.AverageTime)@OutputTimeUnit(TimeUnit.MICROSECONDS)//120輪預(yù)熱，充分利用JIT的編譯優(yōu)化技術(shù)@Warmup(iterations = 120,time = 1,timeUnit = TimeUnit.MILLISECONDS)@Measurement(time = 1,timeUnit = TimeUnit.MICROSECONDS)//線程數(shù)：CPU*2（計(jì)算復(fù)雜型，也有CPU+1的說法）@Threads(8)@Fork(1)@State(Scope.Benchmark)public class JMHTest {
    public static void main(String[] args) throws RunnerException {        testNTime(10000);    }
    private static void testNTime(int num) throws RunnerException {        Options options = new OptionsBuilder()                .include(JMHTest.class.getSimpleName())                .measurementIterations(num)                .output("E://testRecord.log")                .build();        new Runner(options).run();    }

    /**     * System.currentMillisTime測(cè)試     * @return 將結(jié)果返回是為了防止死碼消除（編譯器將 無引用的變量 當(dāng)成無用代碼優(yōu)化掉）     */    @Benchmark    public long testSystem() {        return System.currentTimeMillis();    }
    /**     * 緩存時(shí)鐘測(cè)試     * @return     */    @Benchmark    public long testCacheClock() {        return JMHTest.CacheClock.currentTimeMillis();    }
    /**     * 緩存時(shí)鐘，緩存System.currentTimeMillis()的值，每隔1s更新一次     */    public static class CacheClock{        private static ScheduledExecutorService timer = new ScheduledThreadPoolExecutor(1);        private static volatile long timeMilis;        static {            timer.scheduleAtFixedRate(new Runnable() {                @Override                public void run() {                    timeMilis = System.currentTimeMillis();                }            },0,1000,TimeUnit.MILLISECONDS);        }        public static long currentTimeMillis() {            return timeMilis;        }    }}