You can use the ServletContext object's getRealPath() method to find the actual path for a given virtual path. So, to get the actual path for your context root, you'd code the following in your Actoin class:
String contextPath = getServlet().getServletContext().getRealPath("/");
Once you have that, you can get any other path relative to it. For example:
String webInfPath = contextPath + "/WEB-INF";