This question already has an answer here:
How to get the path of a running JAR file?
You can ask the JVM about the location of a given class. If you know it is inside the jar you refer to, then you can then extract the location of the jar file, and then construct your file objects relative to that.
See http://stackoverflow.com/a/320595/53897 for details about
return new File(MyClass.class.getProtectionDomain().getCodeSource().getLocation().getPath());