getSession(boolean para)返回当前http会话，如果不存在，则创建一个新的会话
getSession() 调用getSession(true)的简化版

【官方解释】
getSession
public HttpSession getSession(boolean create)
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.
To
make sure the session is properly maintained, you must call this method
before the response is committed. If the container is using cookies to
maintain session integrity and is asked to create a new session when the
response is committed, an IllegalStateException is thrown.
Parameters: true - to create a new session for this request if necessary; false to return null if there's no current session
Returns: the HttpSession associated with this request or null if create is false and the request has no valid session
译：
getSession(boolean create)意思是返回当前reqeust中的HttpSession ，如果当前reqeust为false，并且当前没有HttpSession，那么久返回为null，如果create为true，没有Session时就会创建一个新的Session；
简而言之：
HttpServletRequest.getSession(ture) 等同于 HttpServletRequest.getSession()
HttpServletRequest.getSession(false) 等同于 如果当前Session没有就为null；