In ASP.NET, IHttpModule is like a filter while IHttpHandler is like a servlet. It seemed obvious to modify the existing ServletModule to execute the filter.
The first problem I found is the async design of ASP.NET. The IHttpModule is an event sink and need to implement the event handlers for events like BeginRequest, EndRequest. In contrast, Servlet Filter is using FilterChain to call the filter one by one. I tried to execute the filter chain in BeginRequest event. However, I won't be able to gain the access of Session. Having google around, I found the comprehensive events execute sequence in IHttpModule (http://msdn.microsoft.com/en-us/library/ms178473.aspx):
There is code to wrap the HttpContext.Current.Handler around (http://stackoverflow.com/questions/276355/can-i-access-session-state-from-an-httpmodule). It worked fine but it does not solve the async problem.
From Module to Handler
I managed to run the filter chain in PreRequestHandlerExecute event. It turned out that not only it run the JSP code, but also the source code of the JSP was also append to the result by the default handler. Response.End() has to be invoked. And then some checking for the whether it is my own servlet handler, etc. The code is a bit weird.
Since it does not matter where the filter chain is executed, I moved to code to ServletHandler and revert the changes in ServletModule, it compiled and seemed working with the Hello World JSP. No more Response.End() and checking. Async problem is solved since executing the filter and servlet are in one place. It matched the semantics of Java webapp.
The simplest way to test is to get a filter that GZIP the output and use fiddler to see if the response is compressed. I grabbed filter from http://onjava.com/pub/a/onjava/2003/11/19/filters.html. Added the setting in web.xml. Somehow it does not work. It turned out it's because the default XML namespace in web.xml generated by Eclipse caused the problem. Removing the namespace made enabled the support of filter.
The default XML namespace is quite annoying. I am not sure why it is there as it is meaningless. I tried a few way to remove it with the .Net XML API but the namespace somehow stayed there. The final resort was invoking string.Replace() to get rid of it.
The latest source code has been checked-in in http://jeans.codeplex.com/.