Best practice on using GAE with external jabber server?

How to use Java GAE with开发者_运维知识库 an external jabber server like Openfire? Any guide?

That depends on what you mean by "use with". If you want to send messages to an external server and receive messages from it, you can do that just fine by setting up federation the usual way. If you want some other form of interaction, you're out of luck - App Engine doesn't use XMPP as a client, but is instead tightly integrated with the server.