As BalusC points out, you do not want to save files to a destination found with getRealPath. Not only will such files be wiped out when you redeploy, but they will also be available for download by any user who can reach your web site, which, depending on what you're up to, may be a serious security problem.
When I'm faced with this problem, I normally create a properties file, and put the directory in which to save the files in the properties file. Then I can have a different path for Linux and Windows, or for server A and server B. It may work for you to just invent a path and hardcode it, but I often find that I need different paths on different servers. Like, in my present project we have three instances of Tomcat running on the same physical box for different stages of testing. We don't want files they write to all go to the same directory: each should have its own.