Java EE (formerly j2ee) webapps can be deployed as a working file system but they don't have to be. They can also be run as packed war files. Also, parts of a webapp can be bundled into jar files.
If your app has been deployed as an exploded file system, ServletContext.getRealPath("/") will return the path to the root of the web application. If not, then there really isn't a filesystem to access. In this case getRealPath will return null.
As Bear mentioned, the best place to find this information is either the API or the Servlet Spec (links to both are in my signature.).
Good-Luck -Ben [ September 01, 2006: Message edited by: Ben Souther ]