UserService in GWT

Question

I am writing a test application for using Google's App Engine. I would like to provide login functionality using the default UserService. To login, a request i made from the client to the server:

public class ShoppingList implements EntryPoint {

...

private ShoppingListServiceAsync service = (ShoppingListServiceAsync) GWT.create(ShoppingListService.class);

public void onModuleLoad() {
    login();
}

private void login() {
    AsyncCallback callback = new AsyncCallback() {
        @Override
        public void onFailure(Throwable caught) {
            Window.alert("Login failed.");
        }

        @Override
        public void onSuccess(Object result) {
            Window.open((String) result, "_self", "");
        }
    };
    // Get login URL; after login, return to current page.
    service.getLoginURL(GWT.getHostPageBaseURL(), callback);
}
}

The server generates the login URL which is returned to the client:

public class ShoppingListServiceImpl extends RemoteServiceServlet implements ShoppingListService {

...

@Override
public String getLoginURL(String returnURL) {
    UserService userService =  UserServiceFactory.getUserService();
    Boolean loggedIn = userService.isUserLoggedIn();
    String url = userService.createLoginURL("http://google.com");
    User user = userService.getCurrentUser();
    return returnURL;
}
}

The code compiles and runs. Now, my problem is that the userService object is somehow not loaded correctly. When i call any method on it (e.g. isUserLoggedIn()) a null pointer exception is thrown. What am i missing?

Answer 1

Since you didn't post the Code of UserServiceFactory, it's hard to figure out the source of your problem. But I guess that the UserService-Instance is not instanciated correctly in your Factory.