Google OAuth2 / Vaadin Web Application

This tutorial explains how you can use Google OAuth 2.0 to authenticate / authorize your users in Vaadin Web Applications and allow users to login using their Google Id.

1. Logon to Google Developer Console
2. Navigate to API & Auth --> Credentials
3. Create a Client ID. This will return a generate Client ID and Client Secret Key 
4. Click Download JSON to download client_secrets.json.
5. Save client_secrets.json to WAR/WEB-INF directory
6. Define the following methods in a Utils file
1. public static GoogleClientSecrets getGoogleClientSecrets(ServletContext context)            throws InvalidClientSecretsException {
           GoogleClientSecrets secrets;
           try {
               secrets = GoogleClientSecrets.load(JSON_FACTORY,
                       getClientSecretsStream(context));
           } catch (IOException e) {
               throw new InvalidClientSecretsException(
                       "client_secrets.json is missing or invalid.");
           }
           return secrets;
       } 
2. public static Oauth2 getOauth2Service(Credential credentials) {
           HttpTransport httpTransport = new NetHttpTransport();
           JsonFactory jsonFactory = new JacksonFactory();
           return new Oauth2.Builder(httpTransport, jsonFactory, credentials).build();
       }
3. public static AuthorizationCodeFlow getAuthorizationCodeFlow(ServletContext context) {
             final List<String> SCOPE = Arrays.asList(
                   // Required to access and manipulate files.
                   "https://www.googleapis.com/auth/drive.file",
                   // Required to identify the user in our data store.
                   "https://www.googleapis.com/auth/userinfo.email",
                   "https://www.googleapis.com/auth/userinfo.profile");
              HttpTransport httpTransport = new NetHttpTransport();
              JsonFactory jsonFactory = new JacksonFactory();
              GoogleAuthorizationCodeFlow flow = null;
              try {
               GoogleClientSecrets clientSecrets = getGoogleClientSecrets(context);
               GoogleAuthorizationCodeFlow.Builder builder = new GoogleAuthorizationCodeFlow.Builder(
                       httpTransport, jsonFactory, clientSecrets, SCOPE)
                       .setAccessType("online")
                       .setApprovalPrompt("force");
              
               builder.setCredentialStore(credentialStore);
               flow = builder.build();
               } catch (InvalidClientSecretsException e) {
                   e.printStackTrace();
               }
              return flow;
           }
4.  public static String getRedirectURL(ServletContext context) {
           String redirectURL = "";
           GoogleClientSecrets clientSecrets;
           try {
               clientSecrets = getGoogleClientSecrets(context);
               redirectURL = clientSecrets.getWeb().getRedirectUris().get(0);
           } catch (InvalidClientSecretsException e) {
               e.printStackTrace();
           }
           return redirectURL;
    }
5. public static UserDetails loginSuccessful(ServletContext context, HttpServletRequest request) {
           UserDetails userDetails = null;
   
           StringBuffer fullUrlBuf = request.getRequestURL();
           if (request.getQueryString() != null) {
             fullUrlBuf.append('?').append(request.getQueryString());
           }
   
           try {
               AuthorizationCodeResponseUrl authResponse =
                   new AuthorizationCodeResponseUrl(fullUrlBuf.toString());
               // check for user-denied error
               if (authResponse.getError() != null) {
                   login = false;
               } else {
                   String code = authResponse.getCode();
                   if(code != null) {
   
                       /* Get the Token */
                       String redirectURL = getRedirectURL(context);
                       GoogleTokenResponse tokenResponse = (GoogleTokenResponse) Utils.getAuthorizationCodeFlow(context).newTokenRequest(code).setRedirectUri(redirectURL).execute();
                       GoogleCredential credential = new GoogleCredential().setFromTokenResponse(tokenResponse);
   
                       /* Get the UserInfo */
                       Oauth2 oauth = Utils.getOauth2Service(credential);
                       Userinfo about = oauth.userinfo().get().execute();
                       userDetails = new UserDetails();
                       userDetails.setEmail(about.getEmail());
                       userDetails.setName(about.getName());
                       /* Create and Store the Token */
                       Utils.getAuthorizationCodeFlow(context).createAndStoreCredential(tokenResponse, userDetails.getEmail());
                       login = true;
                   }
               }
           } catch (Exception ex) {
               ex.printStackTrace();
           }      
           return userDetails;      
       }
7.  On click of 'Login with Google', call the following snippet of code GoogleAuthorizationCodeFlow flow = (GoogleAuthorizationCodeFlow)Utils.getAuthorizationCodeFlow(context);
              String redirectURL = Utils.getRedirectURL(context);
              String url = flow.newAuthorizationUrl().setRedirectUri(redirectURL).build();
              open(url); //go to the URL
    
8.  In your Servlet, ensure you have the following code   

public class MyServlet extends GAEVaadinServlet {

protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        try {
            VaadinSession vaadinSession = null;
            if(VaadinSession.getCurrent() == null) {
                vaadinSession = getService().findVaadinSession(createVaadinRequest(request));
            } else {
                vaadinSession = VaadinSession.getCurrent();
            }
            ServletContext context = this.getServletContext();
            WrappedSession wSession = vaadinSession.getSession();
            if(wSession != null && wSession.getAttribute("userDetails") == null) {
                UserDetails userDetails = Utils.loginSuccessful(context, request);
                wSession.setAttribute("userDetails", userDetails);
            }
        } catch (ServiceException e) {
            e.printStackTrace();
        } catch (SessionExpiredException e) {
            e.printStackTrace();
        }
        super.service(request, response);
    }
}

Important Links
https://developers.google.com/console/help/new/#generatingoauth2
https://code.google.com/p/google-api-java-client/wiki/OAuth2
https://developers.google.com/accounts/docs/OAuth2

Drop me a comment if you are facing any issues.

Good Luck!