Well, first of all an entry in a jar file, executable or otherwise, can't be accessed as a File. It isn't a file as far as the operating system is concerned, so that's that. Besides, a relative path in a File is always relative to the current working directory, not to the classpath.
Second, the classpath of an executable jar is only the jar itself, plus whatever else is mentioned in the Class-Path entry of the jar's manifest.
Third, you can't write to an entry in a jar file. Consider it to be read-only.
So, whatever you are trying to do (it isn't 100% clear to me),
you should probably reconsider in light of those three facts.