The jar: protocol is a way to build a URI to a resource in a JAR archive:
jar:http://www.example.com/bar/baz.jar!/path/to/file
See the API docs for JarURLConnection: http://java.sun.com/javase/6/docs/api/java/net/JarURLConnection.html
Between the jar: and !/ can be any URL, including a file: URL.