Custom User login in AppEngine - authentication

Question

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?

Answer 1

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.