Estoy usando el ejemplo de "aplicaciones de servidor web" que se muestra aquí. En mi servlet de devolución de llamada quiero recuperar un token de acceso pero no puedo hacerlo con éxito. Siguiente es mi codigo

  @Override
  protected String getUserId(HttpServletRequest req) throws ServletException, IOException {
    // return user ID
      Iterator<String> iterator = req.getParameterMap().keySet().iterator();
        while (iterator.hasNext()) {
            String string = iterator.next();

            System.out.println(string+"---++--"+req.getParameter(string));
        }
        GoogleAuthorizationCodeTokenRequest newTokenRequest = new GoogleAuthorizationCodeFlow.Builder(new NetHttpTransport(), new JacksonFactory(),
                                                             "2XXXXXX7218.apps.googleusercontent.com", "KugD_XXX_7vqnGZVXXXXX1M",
                                                              Collections.singleton("https://gdata.youtube.com"))
                                                              .build().newTokenRequest(req.getParameter("code"));
        //GoogleAuth
        GoogleTokenResponse token = newTokenRequest.setRedirectUri("/").execute();
        String accessToken = token.getAccessToken();
        System.out.println("accesstoken:"+accessToken);
        return "";
  }

Después de ejecutar este código me aparece el siguiente error

com.google.api.client.auth.oauth2.TokenResponseException: 400 Bad Request
{
  "error" : "invalid_request"
}

En pocas palabras, obtengo el parámetro "código" (código de solicitud) No estoy seguro de cómo convertirlo en token de acceso. Ya he vistoAPI de Google: la solicitud de token de Oauth2 devuelve "invalid_request" Google OAuth 2: response_type error en solicitud de token Pero no habría mucho que pudiera usar en mi código

EDITAR

La API de Google no me funcionó, así que escribí el siguiente código sin usar ninguna biblioteca.

 URL url = new URL("https://accounts.google.com/o/oauth2/token");
    connection = (HttpURLConnection) url.openConnection();
    String urlParameters = "code=" + req.getParameter("code") + "&client_id=29852.apps.googleusercontent.com&client_secret=KugD_LVi_7vqnssssxxxNRBz1M"+
            "&redirect_uri=https://flixomnia.com/oauth2callback&grant_type=authorization_code&scope=https://gdata.youtube.com&response_type=token";
    connection.setDoOutput(true);
    connection.setDoInput(true);
    connection.setInstanceFollowRedirects(true);
    connection.setRequestMethod("POST");
    connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
    connection.setRequestProperty("Authorization", "Bearer 4/q3Xh_pJI458XXXXXXXkh-lxe3-8.cmaD6o7V5BkQXE-XXXXX-edgI");
    connection.setRequestProperty("Content-Length", "" + Integer.toString(urlParameters.getBytes().length));
    connection.setRequestProperty("X-GData-Key", "key=AI39siXXXXXXM7tyHBvXEM1lLcORetit6QSArQ3sjelBxXXXXXXtgLSPdZPxvsF_vkntOQMnAEYAuVFqhN7oUw");
    connection.setRequestProperty("GData-Version", "2");

    connection.setUseCaches(false);
    //com.google.gdata.client.youtube.YouTubeService service = new com.google.gdata.client.youtube.YouTubeService("","");
    //YouTubeMediaGroup g = new YouTubeMediaGroup();


    DataOutputStream wr = new DataOutputStream(connection.getOutputStream());

    wr.writeBytes(urlParameters);
    wr.flush();
    InputStream inputStream = connection.getInputStream();
    byte[] b = new byte[1024];
    while (inputStream.read(b) != -1) {
        System.out.print(new String(b));

    }
    System.out.println("");
    wr.close();
    connection.disconnect();

Pero aun así me sale el siguiente error.

    {  "error" : "invalid_request"}
java.io.IOException: Server returned HTTP response code: 400 for URL: https://accounts.google.com/o/oauth2/token
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1615)
    at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:254)
    at com.broadcastr.servlets.YouTubeCallbackService.getUserId(YouTubeCallbackService.java:168)
    at com.google.api.client.extensions.servlet.auth.oauth2.AbstractAuthorizationCodeCallbackServlet.doGet(AbstractAuthorizationCodeCallbackServlet.java:130)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:225)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:927)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1001)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:579)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:312)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:722)