Can I "replace" a Java class from outside of a static code?

I have an application I want to test:

import foo.ExtClass;
public class App {
  public static void main(String[] args) {
     ExtClass ext = new ExtClass();
     ...
  }
}

I want to write a unit test for this application, however I do not want to use the foo.ExtClass, but use another mock implementation for the class.

Normally I would use a factory to instantiate the class according to some configuration that can be controlled in the unit test. However, in this case, I cannot modify the tested app.

I was thinking in the direction of writing a custom class loader to load the mock class instead of the real class - not sure if this is possible without any modification to the tested app, and how.