public WebSocketServiceBuilder fallbackService(HttpService fallbackService) {
        this.fallbackService = requireNonNull(fallbackService, "fallbackService");
        checkArgument(!(fallbackService instanceof WebSocketService),
                      "fallbackService must not be a WebSocketService.");
        return this;
    }
Sets the fallback {@link HttpService} to use when the request is not a valid WebSocket upgrade request.
This is useful when you want to serve both WebSocket and HTTP requests at the same path.
core/src/main/java/com/linecorp/armeria/server/websocket/WebSocketServiceBuilder.java