netty里面HttpServerCodec放在newWebsocketDecoder后面是否会无效掉

Question

代码里面建立websocket连接的时候通过

WebSocketServerHandshakerFactory wsFactory = new WebSocketServerHandshakerFactory("ws://" + request.headers().get(HttpHeaders.Names.HOST), null, false);
        handshaker = wsFactory.newHandshaker(request);
        if (handshaker == null) { // 无法处理的websocket版本
            WebSocketServerHandshakerFactory.sendUnsupportedWebSocketVersionResponse(ctx.channel());
        } else { // 向客户端发送websocket握手,完成握手
            handshaker.handshake(ctx.channel(), request);
            // 记录管道处理上下文，便于服务器推送数据到客户端
            this.ctx = ctx;
        }

发送握手，而在下面handshaker.handshake的netty源代码里面的方法里面放置了websocket的编解码器，却没有移除HttpServerCodec让我想不明白,这里的netty是netty-all-5.0.0.Alpha1.jar里面的代码

 ChannelHandlerContext ctx = p.context(HttpRequestDecoder.class);
        final String encoderName;
        if (ctx == null) {
            // this means the user use a HttpServerCodec
            ctx = p.context(HttpServerCodec.class);
            if (ctx == null) {
                promise.setFailure(
                        new IllegalStateException("No HttpDecoder and no HttpServerCodec in the pipeline"));
                return promise;
            }
            p.addBefore(ctx.name(), "wsdecoder", newWebsocketDecoder());
            p.addBefore(ctx.name(), "wsencoder", newWebSocketEncoder());
            encoderName = ctx.name();
        } else {
            p.replace(ctx.name(), "wsdecoder", newWebsocketDecoder());

            encoderName = p.context(HttpResponseEncoder.class).name();
            p.addBefore(encoderName, "wsencoder", newWebSocketEncoder());
        }

把编解码器放在了服务器http编解码器HttpServerCodec的前面而没有移除httpHttpServerCodec。为什么这里没有移除httpHttpServerCodec？