IN the API for WicketFIlter, we read:
"The advantage of a filter is that, unlike a servlet, it can choose not to process the request and let whatever is next in chain try. So when using a Wicket filter and a request comes in for foo.gif the filter can choose not to process it because it knows it is not a wicket-related request. Since the filter didn't process it, it falls on to the application server to try, and then it works."
I have adopted Wicket recently to upgrade an old application where most of the classes are still subclasses of HttpServlet.
I think the wicket filter does not process classes that extend HttpServlet, but only those that extend Wicket WebPage, etc.