config.getInitParameter always return null

The canonical way is to just use the inherited GenericServlet#getInitParameter() in the argumentless init() method (and remove any init(config) method).

@Override
public void init() throws ServletException {
    filename = getInitParameter("addressfile");
}

If that still doesn't work, then your web.xml is not properly been deployed, or you have a typo in the parameter name, or you actually accessed a different instance variable than filename to use/test it.

Ensure your servlet is calling super.init(config) on its init method, else it won't work.