线程池（3）Executors.newCachedThreadPool

例子：

ExecutorService es = Executors.newCachedThreadPool();
        try {
            for (int i = 0; i < 20; i++) {
                Runnable syncRunnable = new Runnable() {
                    @Override
                    public void run() {
                        log.info(Thread.currentThread().getName());
                        try {
                            Thread.sleep(2000