Authenticate user using the google accounts

It's very easy to use the google accounts login system in your website.

First let's create a class to represent the User state in the view.

public class LoginInfo implements Serializable {    private boolean loggedIn = false;   private String loginUrl;   private String logoutUrl;   private String emailAddress;   private String nickname;

(we hided the getters and setters)

Now we must create the service interface that will fill this object:

import com.google.gwt.user.client.rpc.RemoteService; import com.google.gwt.user.client.rpc.RemoteServiceRelativePath;  @RemoteServiceRelativePath("login") public interface LoginService extends RemoteService {   public LoginInfo login(String requestUri); }

the service Async interface:

import com.google.gwt.user.client.rpc.AsyncCallback;  public interface LoginServiceAsync {   public void login(String requestUri, AsyncCallback async); }

and the service implementation:

import com.google.appengine.api.users.User; import com.google.appengine.api.users.UserService; import com.google.appengine.api.users.UserServiceFactory; import com.google.gwt.sample.stockwatcher.client.LoginInfo; import com.google.gwt.sample.stockwatcher.client.LoginService; import com.google.gwt.user.server.rpc.RemoteServiceServlet;  public class LoginServiceImpl extends RemoteServiceServlet implements     LoginService {    public LoginInfo login(String requestUri) {     UserService userService = UserServiceFactory.getUserService();     User user = userService.getCurrentUser();     LoginInfo loginInfo = new LoginInfo();      if (user != null) {       loginInfo.setLoggedIn(true);       loginInfo.setEmailAddress(user.getEmail());       loginInfo.setNickname(user.getNickname());       loginInfo.setLogoutUrl(userService.createLogoutURL(requestUri));     } else {       loginInfo.setLoggedIn(false);       loginInfo.setLoginUrl(userService.createLoginURL(requestUri));     }     return loginInfo;   }  }

Note that to use the UserService of google accounts you need to instantiate a user Object. If the user is already logged in, the call of userService.getCurrentUser(); will return this user. Else it will return null and we will need to redirect the user to login. We create the login URL using the:userService.createLoginURL(requestUri) . It will return the login url for your application. Also to get the logout url you just need to call:userService.createLogoutURL(requestUri) .

Don't forget to configure the LoginServiceImpl in the web.xml file (you can see an example of how to do this here).

To call this service in the view use:

LoginServiceAsync loginService = GWT.create(LoginService.class);     loginService.login(GWT.getHostPageBaseURL(), new AsyncCallback() {       public void onFailure(Throwable error) {       }        public void onSuccess(LoginInfo result) {         loginInfo = result;         if(loginInfo.isLoggedIn()) {           loadUserData();         } else {           loadLogin();         }       }     });

That's it.

You may also save this LoginInfo object to the database, because it does not contain any password or anything like that.

To learn more you may visit the google code tutorial. All the source code of this post was taken from this tutorial.