XML App Dev. & Cross-Server Compatibility

[Rich Joslin]

Let's say...
 
I am developing a JSP app that uses an XML/XSLT parser to parse flat XML
files into HTML/JSP pages.  I want this app to be easily portable from
my environment (Tomcat) to the production environment (JRun [ugh]).
What API should I use?  And even regardless of portability, does anyone
prefer a certain API for XML/XSLT stuff in JSP?  I prefer ease of use
over features.  My stuff is very basic.
 
Thanks for any advice.
 
- Rich