Executors
创建ThreadPoolExecutor
newFixedThreadPool(nThreads)
newCachedThreadPool()
newSingleThreadExecutor()
创建ScheduledThreadPoolExecutor
newScheduledThreadPool(int)
newSingleThreadScheduledExecutor()
把一个Runnable包装成一个Callable的API
Callable<T> callable(Runnable task)
Callable<T><t> callable(Runnable task, T result)</t>
