That depends entirely on what your ASP.NET app does. Simple apps can be fairly easily ported into
servlets and JSPs. More complicated apps will need a lot more work. I recommend that you first draw up the complete architecture of the current app, in a lot of detail (you will need to know the work flow for every request), and then build a
Java app to match that architecture.
Needless to say, that is a lot of work. Most companies that I work with won't even bother - they would keep the current ASP.NET app as it is and build new apps using Java EE.