Gmail forwards WebHook URL - api

Question

I am a Java developer, how can I automatically forward the unread emails received by my personal gmail mailbox to the specified webhook URL.I tried to do this in Java but couldn't get it to work.
E.g:
public class GmailQuickstart {
/** Application name. */
private static final String APPLICATION_NAME = "Gmail API Java Quickstart";
/** Global instance of the JSON factory. */
private static final JsonFactory JSON_FACTORY = GsonFactory.getDefaultInstance();
/** Directory to store authorization tokens for this application. */
private static final String TOKENS_DIRECTORY_PATH = "tokens";
/**
* Global instance of the scopes required by this quickstart.
* If modifying these scopes, delete your previously saved tokens/ folder.
*/
private static final List<String> SCOPES = Collections.singletonList(GmailScopes.GMAIL_LABELS);
private static final String CREDENTIALS_FILE_PATH = "/credentials.json";
/**
* Creates an authorized Credential object.
* #param HTTP_TRANSPORT The network HTTP Transport.
* #return An authorized Credential object.
* #throws IOException If the credentials.json file cannot be found.
*/
private static Credential getCredentials(final NetHttpTransport HTTP_TRANSPORT) throws IOException {
// Load client secrets.
InputStream in = GmailQuickstart.class.getResourceAsStream(CREDENTIALS_FILE_PATH);
if (in == null) {
throw new FileNotFoundException("Resource not found: " + CREDENTIALS_FILE_PATH);
}
GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(in));
// Build flow and trigger user authorization request.
GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
HTTP_TRANSPORT, JSON_FACTORY, clientSecrets, SCOPES)
.setDataStoreFactory(new FileDataStoreFactory(new java.io.File(TOKENS_DIRECTORY_PATH)))
.setAccessType("offline")
.build();
LocalServerReceiver receiver = new LocalServerReceiver.Builder().setPort(8888).build();
Credential credential = new AuthorizationCodeInstalledApp(flow, receiver).authorize("user");
//returns an authorized Credential object.
return credential;
}
public static void main(String... args) throws IOException, GeneralSecurityException {
// Build a new authorized API client service.
final NetHttpTransport HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
Gmail service = new Gmail.Builder(HTTP_TRANSPORT, JSON_FACTORY, getCredentials(HTTP_TRANSPORT))
.setApplicationName(APPLICATION_NAME)
.build();
// Print the labels in the user's account.
String user = "me";
ListLabelsResponse listResponse = service.users().labels().list(user).execute();
List<Label> labels = listResponse.getLabels();
if (labels.isEmpty()) {
System.out.println("No labels found.");
} else {
System.out.println("Labels:");
for (Label label : labels) {
System.out.printf("- %s\n", label.getName());
}
}
}
}
This is the official example I use. The result is a failure.
failure picture