GAE-java: Using the Users Service

package guestbook;

import java.io.IOException;

import javax.servlet.http.*;

import com.google.appengine.api.users.User;

import com.google.appengine.api.users.UserService;

import com.google.appengine.api.users.UserServiceFactory;

public class GuestbookServlet extends HttpServlet {

    public void doGet(HttpServletRequest req, HttpServletResponse resp)

              throws IOException {

        UserService userService = UserServiceFactory.getUserService();

        User user = userService.getCurrentUser();

        if (user != null) {

            resp.setContentType("text/plain");

            resp.getWriter().println("Hello, " + user.getNickname());

        } else {

            resp.sendRedirect(userService.createLoginURL(req.getRequestURI()));

        }

    }

}