" already exists, choose a different identifier");
" already exists, choose a different identifier or unregister current listener");  return;
    public void registerListener(Serializable identifier, MessageListener l) {        if(l == null || identifier == null) {            if(log.isErrorEnabled()) log.error("message listener or identifier is null");            return;        }        if(listeners.containsKey(identifier)) {            if(log.isErrorEnabled()) log.error("listener with identifier=" + identifier +                    " already exists, choose a different identifier");        }        listeners.put(identifier, l);    }