* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached幾個關鍵參數：
1.corePoolSize：核心線程數，將會永久存在線程池中；
2.maximumPoolSize：最大線程數，如果任務隊列滿了，新來的任務將會額外創建臨時線程來處理，但是總的線程數目不會超過這個值；
3.keepAliveTime：指的是額外創建的臨時線程過期閾值；
線程池的參數作用：如果線程數小於corePoolSize，那麼每一個任務進來都會創建線程；如果大於等於corePoolSize，那麼新來的任務將會進入隊列等待處理；如果隊列超過閾值，那麼就會新建臨時線程，直到線程數到達maximumPoolSize。如果到達maximumPoolSize，那麼會根據拒絕策略拒絕後來的任務。
線程池原本的模型就是池化一定數目的線程，然後處理用戶請求，如果用戶請求多了，那麼就扔進隊列裏，等待池化的線程處理完手頭工作再處理 新的請求。那麼如果隊列也滿了，說明系統不堪重負，此時需要臨時添加一下線程救急，一旦過了高峯期，這些臨時線程就會被回收，這就是線程池的概念模型。