Returns the current HttpSession associated with this request or, if if there is no
current session and create is true, returns a new session.
If create is false and the request has no valid HttpSession, this method
returns null.
current session and create is true, returns a new session.
If create is false and the request has no valid HttpSession, this method
returns null.
如果存在会话 getSession(false)返回现存的会话,getSession(true)返回现存的会话