code: public void afterConnectionClosed(WebSocketSession session, CloseStatus status) {
          logger.debug("WebSocket closed: {}, Close Status: {}", session, status.toString());
      }
docs: Invoked after the web socket connection is closed.
  You can override this method in the child classes.
  
  @param session websocket session between bot and slack
  @param status  websocket close status
  @see WebSocketHandler#afterConnectionClosed