Spring MVC : Sign-in with google+ by using API

I am working on a Spring-MVC application in which I would like to integrate Google+ signin functionality. Before this, with some help form nice people on SO, I managed to implement Google Calendar functionality.

Now, when I am trying to use Google+ sign in, I have 2 options, either use PHP code, or Javascript coupled with some other backend options.

My point is, Google Calendar implementation also required authorization by the user, but I was able to do that purely in Java, while I am having trouble doing that in Java as I cannot find any relevant code. I would just like to call the java method, which opens a new tab in browser, user clicks accept, and I am redirected to URI(Similar to Google calendar I will post below)

Can anyone help me understand why the only code I found runs in embedded server, and I cannot get it running without it. Thanks a lot.

Calendar code :

public void authorizeAndTest(){

        try {
            httpTransport = new NetHttpTransport();
            dataStoreFactory = new FileDataStoreFactory(DATA_STORE_DIR);
            Credential credential = authorize();

            client = new com.google.api.services.calendar.Calendar.Builder(httpTransport,JSON_FACTORY,credential).setApplicationName(APPLICATION_NAME).build();
            showCalenders();
            addCalendarsUsingBatch();
            Calendar calendar = addCalendar();
            updateCalendar(calendar);
            addEvent(calendar);
            showEvents(calendar);
            deleteCalendarInBatch();
            deleteCalendar(calendar);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

 private static Credential authorize() throws Exception{

        AuthorizationCodeFlow codeFlow = new GoogleAuthorizationCodeFlow(httpTransport,JSON_FACTORY,
                "clientid",
                "clientsecret", Collections.singleton(CalendarScopes.CALENDAR));


 return new AuthorizationCodeInstalledApp(codeFlow,new LocalServerReceiver()).authorize("user@gmail.com");
        }

Now the above code works like a charm, I just call my controller method, which calls authorizeAndTest, no problems at all, it opens a tab, I click accept, and it starts the process.

Welcome to Google+ sign in : Below code is from sample :

 public static void main(String[] args) throws Exception {
    Server server = new Server(4567);
    ServletHandler servletHandler = new ServletHandler();
    SessionHandler sessionHandler = new SessionHandler();
    sessionHandler.setHandler(servletHandler);
    server.setHandler(sessionHandler);
    servletHandler.addServletWithMapping(ConnectServlet.class, "/connect");
    servletHandler.addServletWithMapping(DisconnectServlet.class, "/disconnect");
    servletHandler.addServletWithMapping(PeopleServlet.class, "/people");
    servletHandler.addServletWithMapping(MainServlet.class, "/");
    server.start();
    server.join();
  }

public static class MainServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
      // This check prevents the "/" handler from handling all requests by default
      if (!"/".equals(request.getServletPath())) {
        response.setStatus(HttpServletResponse.SC_NOT_FOUND);
        return;
      }

      response.setContentType("text/html");
      try {
        // Create a state token to prevent request forgery.
        // Store it in the session for later validation.
        String state = new BigInteger(130, new SecureRandom()).toString(32);
        request.getSession().setAttribute("state", state);
        // Fancy way to read index.html into memory, and set the client ID
        // and state values in the HTML before serving it.
        response.getWriter().print(new Scanner(new File("index.html"), "UTF-8")
            .useDelimiter("\\A").next()
            .replaceAll("[{]{2}\\s*CLIENT_ID\\s*[}]{2}", CLIENT_ID)
            .replaceAll("[{]{2}\\s*STATE\\s*[}]{2}", state)
            .replaceAll("[{]{2}\\s*APPLICATION_NAME\\s*[}]{2}",
                APPLICATION_NAME)
            .toString());
        response.setStatus(HttpServletResponse.SC_OK);
      } catch (FileNotFoundException e) {
        // When running the quickstart, there was some path issue in finding
        // index.html.  Double check the quickstart guide.
        e.printStackTrace();
        response.setStatus(HttpServletResponse.SC_NOT_FOUND);
        response.getWriter().print(e.toString());
      }
    }
  }

So, I cannot call classes in Controller as far as I know, and the dependency over index.html is a problem, so I modified the code a bit.

GoogleSignin.java

  public void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            // This check prevents the "/" handler from handling all requests by default
            if (!"/".equals(request.getServletPath())) {
                response.setStatus(HttpServletResponse.SC_NOT_FOUND);
                return;
            }

            response.setContentType("text/html");
            try {
   String state = new BigInteger(130, new SecureRandom()).toString(32);
                request.getSession().setAttribute("state", state);
   response.setStatus(HttpServletResponse.SC_OK);
            } catch (Exception e) {
                // When running the quickstart, there was some path issue in finding
                // index.html.  Double check the quickstart guide.
                e.printStackTrace();
                response.setStatus(HttpServletResponse.SC_NOT_FOUND);
                response.getWriter().print(e.toString());
            }
        }

  public void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            response.setContentType("application/json");


            String tokenData = (String) request.getSession().getAttribute("token");
            System.out.println("Tokendata is "+tokenData);
            if (tokenData != null) {
                response.setStatus(HttpServletResponse.SC_OK);
                response.getWriter().print(GSON.toJson("Current user is already connected."));
                return;
            }

            if (!request.getParameter("state").equals(request.getSession().getAttribute("state"))) {
                response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
                response.getWriter().print(GSON.toJson("Invalid state parameter."));
                return;
            }

            System.out.println("Request is "+request.toString());
            ByteArrayOutputStream resultStream = new ByteArrayOutputStream();
            getContent(request.getInputStream(), resultStream);
            String code = new String(resultStream.toByteArray(), "UTF-8");

            try {
                GoogleTokenResponse tokenResponse =
                        new GoogleAuthorizationCodeTokenRequest(TRANSPORT, JSON_FACTORY,
                                CLIENT_ID, CLIENT_SECRET, code, "postmessage").execute();

                GoogleIdToken idToken = tokenResponse.parseIdToken();
                String gplusId = idToken.getPayload().getSubject();
                System.out.println("Response from google plus is "+gplusId);

                request.getSession().setAttribute("token", tokenResponse.toString());
                response.setStatus(HttpServletResponse.SC_OK);
                response.getWriter().print(GSON.toJson("Successfully connected user."));
            } catch (TokenResponseException e) {
                response.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
                response.getWriter().print(GSON.toJson("Failed to upgrade the authorization code."));
            } catch (IOException e) {
                response.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
                response.getWriter().print(GSON.toJson("Failed to read token data from Google. " +
                        e.getMessage()));
            }
        }

Controller code goes like this :

@RequestMapping(value = "/authorize")
    public String googleReplyManagment(HttpServletRequest request, HttpServletResponse response){
        //OAuth oAuth = new OAuth();
        //oAuth.authorizeAndTest();
        response.setContentType("application/json");

        GoogleSignIn googleSignIn = new GoogleSignIn();
        try {
            googleSignIn.doGet(request,response);
            googleSignIn.doPost(request,response);

        } catch (Exception e) {
            e.printStackTrace();
        }
        return "redirect:/";
    }

And I get the following error :

Error code :

java.lang.NullPointerException
    at com.journaldev.spring.utility.GoogleSignIn.doPost(GoogleSignIn.java:127)
    at com.journaldev.spring.controller.PersonController.googleReplyManagment(PersonController.java:1453)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:215)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:749)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:689)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:83)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:938)

What can I do about this, any help would be nice. Thanks alot.