Hibernate active transaction

In my service class I would like to have something like:

class ClientService {

  // Authorize
  // Returns true: Authorization successful
  // Returns false: Authorization failed
  public boolean authorize(String id, String password) {

  //Here I would like to check if an active transaction exists. 
  //If it exists, use that one, else create a new session and start 
  //a new transaction.
  //For example:
  Session session = HibernateUtil.getSessionFactory().getCurrentSession();
  if(!session.SOMEMETHOD_CHECK_IF_TRANSACTION_IS_ACTIVE) {
    session.beginTransaction();
  }

  Client client = clientDAO.get(id);

  if (client != null) {
    if (client.getPassword().equals(password)) {
      logger.debug("Authorization successful. ID: " + client.getId() + ", password: " + client.getPassword());
      return true;
    } else {
      logger.error("Authorization unsuccessful");
      return false;    
    } else {
      logger.debug("Authorization unsuccessful. No client exists with ID: " + id);
      return false;
    }
  }
}

Notice the commented text after the method head. I'm not so familiar with Hibernate, but I think it would be great if the service methods check if transaction exists, use it, otherwise create a new one and close it.

If it is possible, is there any performance reasons (or others) that I should have in mind? Or is it any other way to perform this kind of thing?

Best regards