Как мокать то, на что нет ссылки?

public class RegistrationServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException, ServletException {
        User userInfo = collectUserBeanParameters(req);
        RegistrationValidator validator = new RegistrationValidator();
        validator.validateUserInfo(userInfo);
 
        List<String> errorMessages = validator.getErrorMessages();
 
        HttpSession session = req.getSession();
        Captcha captcha = (Captcha) session.getAttribute("simpleCaptcha");
        req.setCharacterEncoding("UTF-8");
 
        String answer = req.getParameter("answer");
        if (!captcha.isCorrect(answer)) {
            errorMessages.add("Captcha is wrong!");
        }
 
        if (errorMessages.isEmpty()) {
            resp.sendRedirect("/");
        } else {
            req.setAttribute("userInfo", userInfo);
            req.setAttribute("errorMessages", errorMessages);
            req.getRequestDispatcher("/tabs/registration.jsp").forward(req, resp);
        }
 
    }
 
    private User collectUserBeanParameters(HttpServletRequest req) {
        String username = req.getParameter("username");
        String name = req.getParameter("name");
        String surname = req.getParameter("surname");
 
        String email = req.getParameter("email");
        String password = req.getParameter("password");
        String confirmPassword = req.getParameter("password_confirm");
        boolean interestedInNewsFeed = Boolean.parseBoolean(req.getParameter("news_feed"));
 
        return new User(username, name, surname, email, password, confirmPassword, interestedInNewsFeed);
    }
}

public class RegistrationServletTest {
    @Test
    public void testServlet() throws IOException, ServletException {
        HttpServletRequest req = mock(HttpServletRequest.class);
        HttpServletResponse resp = mock(HttpServletResponse.class);
 
        when(req.getParameter("username")).thenReturn("new_user");
        when(req.getParameter("name")).thenReturn("Vasya");
        when(req.getParameter("surname")).thenReturn("Pupkin");
        when(req.getParameter("email")).thenReturn("artmalchik@gmail.com");
        when(req.getParameter("password")).thenReturn("qwerty1234");
        when(req.getParameter("confirm_password")).thenReturn("qwerty1234");
        when(req.getParameter("news_feed")).thenReturn(String.valueOf(true));
        when(req.getParameter("answer")).thenReturn(anyString());
 
        new RegistrationServlet().doPost(req, resp);
        verify(req, calls(1)).getParameter("username");
    }
}

HttpSession session = req.getSession();

when(req.getSession()).thenReturn(mock(HttpSession.class));

Captcha captcha = (Captcha) session.getAttribute("simpleCaptcha");

        HttpSession session = mock(HttpSession.class);
        when(req.getSession()).thenReturn(session);
        when(session.getAttribute("captcha")).thenReturn(mock(Captcha.class));

public class RegistrationServlet extends HttpServlet {
    private RegistrationValidator validator;
 
    @Override
    public void init() {
        validator = (RegistrationValidator) getServletContext().getAttribute("registrationValidator");
    }
 
    ...
}