if (log.isTraceEnabled()) { log.trace("setMessageListener"); }
if (trace) { log.trace("setMessageListener"); }
   public Object handleSetMessageListener(Invocation invocation) throws Throwable   {      if (log.isTraceEnabled()) { log.trace("setMessageListener"); }            MethodInvocation mi = (MethodInvocation)invocation;            MessageListener listener = (MessageListener)mi.getArguments()[0];            if (listener == null)      {         throw new IllegalStateException("Cannot set a null MessageListener on the session");      }            sessionListener = listener;            return null;   }