code: private void sendNotModified(ChannelHandlerContext ctx) {
          FullHttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, NOT_MODIFIED);
          setDateHeader(response);
  
          this.sendAndCleanupConnection(ctx, response);
      }
docs: When file timestamp is the same as what the browser is sending up, send a "304 Not Modified"
  
  @param ctx
  Context