I have this hard requirement that one of my classes, ClassX is instantiated in a reflective manner by a third party module Modul3P, calling ClassX‘s default constructor. Then, Modul3P calls one of the instance’s method, aMethod():

//inaccessible, approximate code
void Modul3P(String className) {
    Class<?> clazz = Class.forName(className);
    InterfaceX object = (InterfaceX) clazz.newInstance();
    object.aMethod();
}
interface InterfaceX {
    void aMethod();
}

My ClassX, however, depends badly a parameter to be properly set up:

class ClassX implements InterfaceX {
    ClassX(Object param){...} //required for proper setup   
    ClassX(){...};            //called by Modul3P
    void aMethod(){...}
}

How do I overcome this issue?

My failed attempt (with Javassist) was to create a proper instance inst, then create on-the-fly a class derived from ClassX, which can be default-initialised, and which during initialisation would be injected inst:

class ClassX implements InterfaceX {
   //...
   Class<? extends ClassX> defaultConstructibleOnTheFlyClass(){...}
}
//...
ClassX inst = new ClassX(aParam); //proper initialisation
Modul3P(inst.defaultConstructibleOnTheFlyClass().getName());