Java: load User-defined interface implementation (from config file)

I need to allow a user to specify the implementation of an interface at runtime via a config file, similar to in this question: Specify which implementation of Java interface to use in command line argument

However, my situation is different in that the implementations are not known at compile time, so I will have to use reflection to instantiate the class. My question is ... how do I structure my application such that my class can see the new implementation's .jar, so that it can load the class when I call:

Class.forName(fileObject.getClassName()).newInstance()

?