signing out appengine app without signing out from google.com

Question

The following code will sign out the user from both appengine app (custom domain in this case) and google.com:

public class MyServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws IOException {
    UserService userService = UserServiceFactory.getUserService();

    String thisURL = req.getRequestURI();

    resp.setContentType("text/html");
    if (req.getUserPrincipal() != null) {
        resp.getWriter().println("<p>Hello, " +
                                 req.getUserPrincipal().getName() +
                                 "!  You can <a href=\"" +
                                 userService.createLogoutURL(thisURL) +
                                 "\">sign out</a>.</p>");
    }
}
}

How can we sign out only from appengine app and keep user signing in google.com?

Answer 1

There is no way to do so directly from the UserService API.

Instead of using the UserService API to logout, you can manually remove the AppEngine specific cookies that are set. Check out this blog post that discusses how to so (written in Python, but you should be able to modify it for Java). This should effectively log the user out from your own app but not from other Google services (though I haven't tested this myself).

The more robust approach would be to create your own User class and manage your own session cookies, while wrapping the UserService API. The downside of this approach is the extra work that is required to set it up, compared to the very easy to use UserService API. However, the advantage of maintaining your own Users is that you will be able to use other methods of authentication besides for Google (eg now you will be able to use a Facebook login as well, or even a native login if you choose to set that up).