对性能有何帮助(读书笔记)

     为每一个线程分配一个独立的对象对系统性能也许是有帮助的,当然了,这也不一定,这完全取决于共享对象的内部逻辑,如果共享对象对于竞争的处理容易引起性能损失,我们还是考虑使用ThreadLocal为每一个线程分配单独的对象.一个经典的案例就是在多线程下产生随机数:

public class RandomDemo {
    public static final int GET_COUNT = 10000000;
    public static final int THREAD_COUNT = 4;
    static ExecutorService exe = Executors.newFixedThreadPool(THREAD_COUNT);
    public static Random rnd = new Random(123);
    public static ThreadLocal<Random> tRnd = new ThreadLocal<Random>() {
        @Override
        protected Random initialValue() {
            return new Random(123);
        }
    };

    public static class RndTask implements Callable<Long> {
        private int mode = 0;

        public RndTask(int mode) {
            this.mode = mode;
        }

        public Random getRondom() {
            if (mode == 0) {
                return rnd;
            } else if (mode == 1) {
                return tRnd.get();
            } else {
                return null;
            }
        }

        /**
         * Computes a result, or throws an exception if unable to do so.
         *
         * @return computed result
         * @throws Exception if unable to compute a result
         */
        @Override
        public Long call() throws Exception {
            long b = System.currentTimeMillis();
            for (long i = 0; i < GET_COUNT; i++) {
                getRondom().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>[] futs = new Future[THREAD_COUNT];
        for (int i = 0; i < THREAD_COUNT; i++) {
            futs[i] = exe.submit(new RndTask(0));
        }
        long totaltime = 0;
        for (int i = 0; i < THREAD_COUNT; i++) {
            totaltime += futs[i].get();
        }
        System.out.println("多线程访问同一个Random实例:" + totaltime + "ms");

        for (int i = 0; i < THREAD_COUNT; i++) {
            futs[i] = exe.submit(new RndTask(1));
        }
        totaltime = 0;
        for (int i = 0; i < THREAD_COUNT; i++) {
            totaltime += futs[i].get();
        }
        System.out.println("使用ThreadLocal包装Random实例:" + totaltime + "ms");
        exe.shutdown();
    }
}

 上述代码运行结果,如下:

     效果是很明显的,