使用多线程同时访问同一个Random实例发现的耗时问题，求解答

public class RandomTimeTest {

    // 要产生的随机数数量

    public static final int GEN_COUNT = 10000000;

    // 参与工作的线程数量

    public static final int THREAD_COUNT = 4;

    // 定义线程池

    static ExecutorService exe = Executors.newFixedThreadPool(THREAD_COUNT);



    public static Random rnd = new Random(123);

    public static ThreadLocalRandom tlRnd = ThreadLocalRandom.current();



    // 由ThreadLocal封装的Random

    public static ThreadLocal<Random> tRnd = ThreadLocal.withInitial(() -> new Random(123));



    public static class RndTask implements Callable<Long> {

        private int mode = 0;



        public RndTask(int mode) {

            this.mode = mode;

        }



        // 创建随机数的方式

        public Random getRandom() {

            if (mode == 0)

                return rnd;

            else if (mode == 1)

                return tRnd.get();

            else if (mode == 2)

                return tlRnd;

            else

                return null;

        }



        /**

         * 每个线程产生若干随机数，完成工作后记录幷返回所消耗的时间

         */

        @Override

        public Long call() throws Exception {

            long b = System.currentTimeMillis();

            for (int i = 0; i < GEN_COUNT; i++)

                getRandom().nextInt();

            long e = System.currentTimeMillis();

            System.out.println(Thread.currentThread().getName()+ " spend " + (e - b) + "ms");

            return e - b;

        }

    }



    public static void main(String[] args) throws ExecutionException, InterruptedException {

        Future<Long>[] futures = new Future[THREAD_COUNT];

        // 多线程共享Random实例

        for (int i = 0; i < THREAD_COUNT; i++)

            futures[i] = exe.submit(new RndTask(0));

        long totaltime = 0;

        for (int i = 0; i < THREAD_COUNT; i++)

            totaltime += futures[i].get();

        System.out.println("多线程同时访问同一个Random实例：" + totaltime + "ms");





        // 使用ThreadLocal情况

        for (int i = 0; i < THREAD_COUNT; i++)

            futures[i] = exe.submit(new RndTask(1));

        totaltime = 0;

        for (int i = 0; i < THREAD_COUNT; i++)

            totaltime += futures[i].get();

        System.out.println("使用ThreadLocal包装Random实例: " + totaltime + "ms");



        // 使用ThreadLocalRandom情况

        for (int i = 0; i < THREAD_COUNT; i++)

            futures[i] = exe.submit(new RndTask(2));

        totaltime = 0;

        for (int i = 0; i < THREAD_COUNT; i++)

            totaltime += futures[i].get();

        System.out.println("使用ThreadLocalRandom实例: " + totaltime + "ms");

        exe.shutdown();





        /*long totaltime = 0;

        for (int i = 0; i < THREAD_COUNT; i++)

            totaltime += exe.submit(new RndTask(0)).get();

        System.out.println("多线程同时访问同一个Random实例：" + totaltime + "ms");



        totaltime = 0;

        for (int i = 0; i < THREAD_COUNT; i++)

            totaltime += exe.submit(new RndTask(1)).get();

        System.out.println("使用ThreadLocal包装Random实例：" + totaltime + "ms");



        totaltime = 0;

        for (int i = 0; i < THREAD_COUNT; i++)

            totaltime += exe.submit(new RndTask(2)).get();

        System.out.println("使用ThreadLocalRandom实例：" + totaltime + "ms");

        exe.shutdown();*/

    }

}