Make WebSocketDecoderConfig not final

[faceless2]

Motivation:

WebSocketDecoderConfig is currently final, which means that some aspects (eg maxFramePayloadLength) cannot be changed as a result of - for example - an Authorization header sent with the initial handshake, or with some sort of initial "login" websocket message.

I can't see any reason why this field in particular has to be final; it's not like the value for one received frame has to be identical to a frame received for a different message.

Modification:

I've made the class non-final so it can be extended, but - because there is a builder class - I've made the new zero-argument constructor protected, so it can only be called by someone explicitly subclassing this class.

Result:

Addresses #13600

[normanmaurer]

I dont understand why this is needed... do you want to change things on the fly ?

[faceless2]

Yes, exactly. The initial limit of 65535 (bytes in a frame) may need to be raised after authentication.

[normanmaurer]

Can you show me how you would do this with the api change ?

[faceless2]

Certainly. This is extracted from a patched version, which I've been running here successfully.

// In the ChannelInitializer.initChannel() code, adding the various handlers to the ChannelPipeline
MyWebSocketDecoderConfig config = new MyWebSocketDecoderConfig();
pipeline.addLast(new WebSocketServerProtocolHandler(contextPath, null, false, false, 10000, config);
pipeline.addLast(new MyWebsocketHandler(engine, processorPool, config));

// MyWebsocketHandler is a SimpleChannelInboundHandler<WebsocketFrame>
// it processes the WebSocket messages and can call config.setMaxFramePayloadLength(...)
// when it's able to authenticate the session, ie. after receiving a "login" message.

// Finally MyWebSocketDecoderConfig
private static class MyWebSocketDecoderConfig extends WebSocketDecoderConfig {
    private int maxFramePayloadLength;
    MyWebSocketDecoderConfig() {
        super();
        maxFramePayloadLength = super.maxFramePayloadLength();
    }
    @Override public int maxFramePayloadLength() {
        return maxFramePayloadLength;
    }
    public void setMaxFramePayloadLength(int len) {
        maxFramePayloadLength = len;
    }
}