Pretty URLs in Google App Engine

I want to pass a parameter 'A1B2C3' to a GWT application based on Google App Engine. I do it like www.example.com/index.html?key=A1B2C3. Although it's working, I'd like to use pretty URLs. Is it possible to do URL rewriting on Google App Engine? I couldn't find out how.

www.example.com/A1B2C3

instead of

www.example.com/index.html?key=A1B2C3

I'm using Google App Engine and GWT. All in Java.