Custom User login in AppEngine

I have a Java Servlet backend with a datastore connected to my app; I am trying to implement a login system using the Android Studio LoginActivity template, using the user's email and password (not the PlusBaseActivity handling the Google Account login), but I don't know how to proceed from here:

How can you say that a User is logged in? and how can I make it so persistently using my datastore? I've read here: How to login User using UserService on AppEngine Java that I just need to call the method resp.sendRedirect(userService.createLoginURL(req.getRequestURI())), and I've done so:

  @Override
  public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException {
      int size = checkDatastore(); // 0 if empty, > 0 if not empty
      if(size==0){
          populateDatastore();
      }

      String asyncMessage = req.getParameter("order");

      if(asyncMessage.equals("login")){
          mail = req.getParameter("email");
          psw = req.getParameter("password");

          UserService userService = UserServiceFactory.getUserService();
          User user = userService.getCurrentUser();

          String message="";
          resp.setContentType("text/plain");
          PrintWriter out = resp.getWriter();
          if(user == null) {

             //Sends a temporary redirect response to the client using the
             // specified redirect location URL and clears the buffer.
             String uri = userService.createLoginURL(req.getRequestURI());
             resp.sendRedirect(uri);
             User user1 = userService.getCurrentUser();
             message="No one is logged in!\n" + "Sent from App Engine at " + new Date();

            out.println(message);
            out.flush();
        }if(user !=null) {
            //  login(user);
            message = "Hello, " + user.getEmail() +
                    ", "+user.getNickname()+"!" + "\nSent from App Engine at "+ new Date();

            out.println(message);
            out.flush();
        }
    }
}

but the sendRedirect() method only gives me a URI. What for?

Moreover, the User user = userService.getCurrentUser() always returns null. How come?

That's because the resp.sendRedirect(userService.createLoginURL(req.getRequestURI())) of UserService only works when integrating the Login with Google Accounts as shown in this documentation. If you want to implement a personalised login system you can do that in many ways. Surely you will need a Servlet checking new users' data and a datastore to persistently store new account registrations.