code: public void exceptionCaught(IoSession session, Throwable cause)
              throws Exception {
          SingleSessionIoHandler handler = (SingleSessionIoHandler) session
                  .getAttribute(HANDLER);
          handler.exceptionCaught(cause);
      }
docs: Delegates the method call to the
  {@link SingleSessionIoHandler#exceptionCaught(Throwable)} method of the
  handler assigned to this session.