publicstaticCredentialauthorize(List<String>scopes,StringcredentialDatastore)throwsIOException {// Load client secrets.ReaderclientSecretReader =newInputStreamReader(Auth.class.getResourceAsStream("/client_secrets.json"));GoogleClientSecretsclientSecrets =GoogleClientSecrets.load(JSON_FACTORY,clientSecretReader);// Checks that the defaults have been replaced (Default = "Enter X here").if (clientSecrets.getDetails().getClientId().startsWith("Enter")                ||clientSecrets.getDetails().getClientSecret().startsWith("Enter ")) {System.out.println("Enter Client ID and Secret from https://console.developers.google.com/project/_/apiui/credential "                            +"into src/main/resources/client_secrets.json");System.exit(1);        }// This creates the credentials datastore at ~/.oauth-credentials/${credentialDatastore}FileDataStoreFactoryfileDataStoreFactory =newFileDataStoreFactory(newFile(System.getProperty("user.home") +"/" +CREDENTIALS_DIRECTORY));DataStore<StoredCredential>datastore =fileDataStoreFactory.getDataStore(credentialDatastore);GoogleAuthorizationCodeFlowflow =newGoogleAuthorizationCodeFlow.Builder(HTTP_TRANSPORT,JSON_FACTORY,clientSecrets,scopes).setCredentialDataStore(datastore)                .build();// Build the local server and bind it to port 8080LocalServerReceiverlocalReceiver =newLocalServerReceiver.Builder().setPort(8080).build();// Authorize.returnnewAuthorizationCodeInstalledApp(flow,localReceiver).authorize("user");    }

 1. An instance of  **jetty server** instance  which will be listening constantly until the response       is coming from Google as mentioned in the redirect url. 2. Though there is a function called `setHost()` inside `new LocalServerReceiver.Builder()` class      which responsible for creating a local jetty server instance, was throughing a **Cannot assign       requested address** error everytime a host name was given irrespective of the port which did not      matter.

publicCredentialauthorize(StringuserId)throwsIOException {try {//get the credentials from the stored credentials fileCredentialcredential =flow.loadCredential(userId);if (credential !=null                    && (credential.getRefreshToken() !=null ||credential.getExpiresInSeconds() >60)) {returncredential;            }//otherwise get from the google authentication// open in browserStringredirectUri =receiver.getRedirectUri();//set your custom name-value pairAuthorizationCodeRequestUrlauthorizationUrl =flow.newAuthorizationUrl().setRedirectUri(redirectUri);onAuthorization(authorizationUrl);// receive authorization code and exchange it for an access tokenStringcode =receiver.waitForCode();TokenResponseresponse =flow.newTokenRequest(code).setRedirectUri(redirectUri).execute();// store credential and return itreturnflow.createAndStoreCredential(response,userId);        }finally {receiver.stop();        }    }

/**     * Get access token from stored credentials     * @param userId lookup the credential file by the userid specified.     * @return credential object or null if there is no such userid exits     */publicCredentialgetAuthorizationFromStorage(StringuserId){try {//get credentials from storageCredentialcredential=flow.loadCredential(userId);/* check whether the credential object and refresh token is not null as then it can generate new accesstoken             * other wise of the access token is not expired  */if(credential!=null && (credential.getRefreshToken() !=null ||credential.getExpiresInSeconds() >60))returncredential;        }catch (IOExceptione) {e.printStackTrace();        }returnnull;    }

/**     * Get the authentication with the credentials from Google     * creates  the url that will lead the user to the authentication page     * and creating a custom defined name-value pair in the state parameter.     * The value should be encoded with base64 encoder so we can receive the     * same code redirected from google after authentication.     * @link http://stackoverflow.com/questions/7722062/google-oauth2-redirect-uri-with-several-parameters     * @param userid     * @param credentialDatastore     * @author soumik 2016-09-27T11:00:40.312Z     */publicvoidgetAuthorizationFromGoogle(Stringuserid,StringcredentialDatastore)throwsIOException {//callback url after google authenticationStringredirectUri=GOOGLE_CALLBACK;//create the Json String that will send along with the other parametersGoogleRequestJSONrequestJSON=newGoogleRequestJSON();requestJSON.setUserid(userid);requestJSON.setCredentialDatastore(credentialDatastore);Stringrequest=newGson().toJson(requestJSON);/*Inside state parameter we can encode our custom parameter that          * will be received after callback*/AuthorizationCodeRequestUrlauthorizationUrl =flow.newAuthorizationUrl()                .setRedirectUri(redirectUri)                .set("state",Base64.encodeBase64String(request.getBytes()));onAuthorization(authorizationUrl);    }

/**     * Save the credentials that we get from  google.     * 1.create the GoogleAuthorizationCodeFlow object from the credentialDatastore     *   from the response received from the google after authentication.     * 2.Hit google to get the permanent refresh-token that can be used to get the     *   accesstoken of the user any time .     * 3.Store the tokens like accesstoken and refreshtoken in the filename as userid     * @param userid it will be send in the googlecallback url     * @param credentialDatastore it will be send in the googlecallback url     * @param code it will be send in the googlecallback url     * @return the credential object     * @throws URISyntaxException     * @throws IOException     * @author soumik 2016-09-27T11:00:54.747Z     */publicCredentialsaveAuthorizationFromGoogle(Stringuserid,StringcredentialDatastore,Stringcode)throwsURISyntaxException,IOException {// Load client secrets.URIfilePath =newURI(GOOGLE_APIKEY);ReaderclientSecretReader =newInputStreamReader(newFileInputStream(filePath.toString()));//Reader clientSecretReader = new InputStreamReader(Auth.class.getResourceAsStream(GOOGLE_APIKEY));GoogleClientSecretsclientSecrets =GoogleClientSecrets.load(JSON_FACTORY,clientSecretReader);// This creates the credentials datastore at ~/.oauth-credentials/${credentialDatastore}FileDataStoreFactoryfileDataStoreFactory =newFileDataStoreFactory(newFile(System.getProperty("user.home") +"/" +CREDENTIALS_DIRECTORY));DataStore<StoredCredential>datastore =fileDataStoreFactory.getDataStore(credentialDatastore);//Builds a login URL based on client ID, secret, callback URI, and scopesGoogleAuthorizationCodeFlowflow =newGoogleAuthorizationCodeFlow                .Builder(HTTP_TRANSPORT,JSON_FACTORY,clientSecrets,SCOPES)                .setAccessType("offline")                .setApprovalPrompt("auto")                .setCredentialDataStore(datastore)                .build();//get the refreshtoken from google as soon as we get the code from googleTokenResponseresponse =flow.newTokenRequest(code).setRedirectUri(GOOGLE_CALLBACK).execute();//save the credentials that we got the tokenresponse.Credentialcredential=flow.createAndStoreCredential(response,userid);returncredential;    }