4.3.3 thread对性能有何帮助

public class ThreadLocalDemo {
    public static final int GE_COUNT = 10000000;
    public static final int THREAD_COUT = 4;

    static ExecutorService executorService = Executors.newFixedThreadPool(THREAD_COUT);
    public static Random random = new Random(123);

    public static ThreadLocal<Random> randomThreadLocal = new ThreadLocal<Random>() {
        protected Random initialValue() {
            return new Random(123);
        }
    };

    public static class RandomTask implements Callable<Long> {

        private int mode = 0;

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

        public Random getRandom() {
            if (mode == 0) {
                return random;
            } else if (mode == 1) {
                return randomThreadLocal.get();
            } else {
                return null;
            }
        }

        public Long call() throws Exception {
            long b = System.currentTimeMillis();

            for (int i = 0; i < GE_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_COUT];
        for (int i = 0; i < THREAD_COUT; i++) {
            futures[i] = executorService.submit(new RandomTask(0));
        }

        long totalTime = 0;

        for (int i = 0; i < THREAD_COUT; i++) {
            totalTime += futures[i].get();
        }
        System.out.println("多线程访问同一个Random实例:" + totalTime + "ms");
         executorService.shutdown();
        executorService=Executors.newFixedThreadPool(4);
        //ThreadLocal的情况
        for (int i = 0; i < THREAD_COUT; i++) {
            futures[i] = executorService.submit(new RandomTask(1));
        }
        totalTime = 0;
        for (int i = 0; i < THREAD_COUT; i++) {
            totalTime += futures[i].get();
        }
        executorService.shutdown();
        System.out.println("使用ThreadLocal包装Random实例:" + totalTime + "ms");

    }

运行结果：

pool-1-thread-3 spend1353ms
pool-1-thread-1 spend1542ms
pool-1-thread-2 spend1573ms
pool-1-thread-4 spend1593ms
多线程访问同一个Random实例:6061ms
pool-2-thread-1 spend531ms
pool-2-thread-2 spend512ms
pool-2-thread-3 spend450ms
pool-2-thread-4 spend414ms
使用ThreadLocal包装Random实例:1907ms