無xmpp基礎的人可能看起來有點複雜;
假設我們注冊了一個使用者，使用者名叫shimiso，那麼我們如何讓shimiso這個使用者一登陸就取到離線消息呢？
運作結果：
這裡我們需要特别當心的是先不要告訴openfire伺服器你上線了，否則永遠也拿不到離線消息，用下面老外的話将就是在你上線之前去擷取離線消息，這麼設計是很有道理的。
the
offlinemessagemanager helps manage offline messages even before the user
has sent an available presence. when a user asks for his offline
messages before sending an available presence then the server will not
send a flood with all the offline messages when the user becomes
online. the server will not send a flood with all the offline messages
to the session that made the offline messages request or to any other
session used by the user that becomes online.
同時拿到離線消息以後删除離線消息offlinemanager.deletemessages();，同是通知伺服器自己上線了。