getRealPath() returning null

I have Tomcat installed in Windows XP. In my web application deployed in tomcat, I have the following code line in my action class

String strFilePath = getPageContext().getServletContext().getRealPath(File.separator);

this gives me the value

C:\cvs_repo\Documentum\webtop\src\

Now this same application I have deployed on Weblogic (hosted on windows 2003 server). On this server, the above code line returns null

Now this same application is deployed on Weblogic (hosted on AIX box). On this machine this code line returns the correct file path.

Can somone please let me know why it is returning null on windows 2003 Weblogic.

Thank you