Java overriding classes in jar for blackbox testing (with maven)

I have a java maven project in a jar file (bigfat jar, the whole project in one file). I want to blackbox test it. The areas that I am most interested on checking are the input and outputs, alas, those interfaces are APIs and sockets which are hardcoded inside the jar to communicate with a specific port or with a specific website.

What I want to do is to override the classes inside the jar file that are related to these interfaces, and code them with my model interfaces.

The whole new testing project will not go into production, this is purely for testing purposes.

I am using maven.

Any ideas are more than welcome