code: public void begin() throws NotSupportedException,
                                SystemException
     {
        Transaction tx = registry.getTransaction();
  
        if (tx != null && tx.getStatus() != Status.STATUS_UNKNOWN)
           throw new SystemException();
  
        registry.startTransaction();
  
        if (userTransactionRegistry != null)
           userTransactionRegistry.userTransactionStarted();
     }
docs: {@inheritDoc}