Currently, I'm developing a website and I want to use XML with JSP so that it can show not only html, but also xhtml, wml and other formats in future.

I have a problem with this as I need to be able to create XML documents on the fly by extracting data from a database. Has anyone tried to do this? Too difficult? Any help would be appreciated. Thanks.

Of course it isn't difficult, you can just dynamically generate the XML. If you know how XML works, it isn't really different from outputting HTML.