這個在構造函數裏面竟然有一個Throwable，感覺有些奇怪。

1.

　　Throwable cause 這裏cause要傳一個Throwable的子類異常進去麽？

　　是引起這個異常的異常，如果這個值是空值，那麽這個異常就是源頭；如果這個值等於自己，那麽這個異常還沒被初始化。

　　/**
* The throwable that caused this throwable to get thrown, or null if this
* throwable was not caused by another throwable, or if the causative
* throwable is unknown. If this field is equal to this throwable itself,
* it indicates that the cause of this throwable has not yet been
* initialized.
*
* @serial
* @since 1.4
*/

異常類Exception(String message, Throwable cause)中的cause理解