Google Plus SignIn / oAuth2 - server-side jitter TokenResponseException: 401 Unauthorized

Question

So, I am building a simple system consisting of an Android application and Java EE RESTful service, and I had terrible authorization problems with Google. I implement Google+ Sign-in, and it works well on the client side, that is, I can get a user email address, JWT idToken and server authentication code, which I would like to exchange for access and update tokens and save them in my database. This is done as follows:

mGoogleApiClient = new GoogleApiClient.Builder(this)
    .addApi(Plus.API)
    .addScope(Plus.SCOPE_PLUS_LOGIN)
    .addScope(Plus.SCOPE_PLUS_PROFILE)
    .addConnectionCallbacks(this)
    .addOnConnectionFailedListener(this)
    .requestServerAuthCode(SERVICE_CLIENT_ID, this)
    .build();
        // ...
        mGoogleApiClient.connect();

After a successful connection, onUploadServerAuthCode is called.

@Override
public boolean onUploadServerAuthCode(String idToken, String serverAuthCode) {
   //...
   //When it called, I send serverAuthCode to my server.
}

Then on my server side I use the code: https://developers.google.com/drive/web/credentials

String CLIENTSECRET_LOCATION = "/WEB-INF/classes/client_secret.json";
String REDIRECT_URI = "urn:ietf:wg:oauth:2.0:oob";
List<String> SCOPES = Arrays.asList("https://www.googleapis.com/auth/plus.login");

GoogleAuthorizationCodeFlow getFlow() throws IOException {
    if (flow == null) {
        InputStream in = context.getResourceAsStream(CLIENTSECRET_LOCATION);
        GoogleClientSecrets clientSecret = GoogleClientSecrets.load(
                JSON_FACTORY, new InputStreamReader(in));
        flow = new GoogleAuthorizationCodeFlow.Builder(HTTP_TRANSPORT,
                JSON_FACTORY, clientSecret, SCOPES)
                .setAccessType("offline").setApprovalPrompt("force")
                .build();
    }
    return flow;
}

Credential exchangeCode(String authorizationCode)
        throws CodeExchangeException {
    try {
        GoogleAuthorizationCodeFlow flow = getFlow();
        GoogleTokenResponse response = flow
                .newTokenRequest(authorizationCode)
                .setRedirectUri(REDIRECT_URI).execute();
        return flow.createAndStoreCredential(response, null);
    } catch (IOException e) {
        System.err.println("An error occurred: " + e);
        throw new CodeExchangeException(null);
    }
}

.

: client_secret.json Android-:

{
   "installed":{
       "auth_uri":"https://accounts.google.com/o/oauth2/auth",
       "token_uri":"https://accounts.google.com/o/oauth2/token",
       "client_email":"",
       "redirect_uris":["urn:ietf:wg:oauth:2.0:oob","oob"],
       "client_x509_cert_url":"",
       "client_id":"243714256753-lqcm63mXXXXXXXXXXXXXXXXXXXXXXXXX.apps.googleusercontent.com",
       "auth_provider_x509_cert_url":"https://www.googleapis.com/oauth2/v1/certs"
   }
}

in exchangeCode(String) GoogleAuthorizationCodeTokenRequest.execute() throws TokenResponseException: 401 Unauthorized

: client_secret.json :

{
  "private_key_id": "77bee9dXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
  "private_key": "-----BEGIN PRIVATE KEY-----\nMIICdgIBADANBgk ... \u003d\u003d\n-----END PRIVATE KEY-----\n",
      "client_email": "[e-mail]",
      "client_id": "243714256753-g21p1XXXXXXXXXXXXXXXXXXXXXXXXXXX.apps.googleusercontent.com",
  "type": "service_account"
}

{
    "web": {
      "private_key_id": "77bee9dXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
      "private_key": "-----BEGIN PRIVATE KEY-----\nMIICdgI ... \u003d\u003d\n-----END PRIVATE KEY-----\n",
      "client_email": "[e-mail]",
      "client_id": "243714256753-g21p1XXXXXXXXXXXXXXXXXXXXXXXXXXX.apps.googleusercontent.com",
      "type": "service_account"
    }
}

in exchangeCode() GoogleAuthorizationCodeTokenRequest.execute() throws

com.google.api.client.auth.oauth2.TokenResponseException: 400 Bad Request
{
  "error" : "invalid_request",
  "error_description" : "client_secret is missing."
}

1) ? 2) JWT, onUploadServerAuthCode?