@Override public Mono<Void> handleRequest(ServerWebExchange exchange, WebSocketHandler handler) { ServerHttpRequest request = exchange.getRequest(); HttpMethod method = request.getMethod(); HttpHeaders headers = request.getHeaders(); if (HttpMethod.GET != method) { return Mono.error(new MethodNotAllowedException( request.getMethodValue(), Collections.singleton(HttpMethod.GET))); } if (!"WebSocket".equalsIgnoreCase(headers.getUpgrade())) { return handleBadRequest(exchange, "Invalid 'Upgrade' header: " + headers); } List<String> connectionValue = headers.getConnection(); if (!connectionValue.contains("Upgrade") && !connectionValue.contains("upgrade")) { return handleBadRequest(exchange, "Invalid 'Connection' header: " + headers); } String key = headers.getFirst(SEC_WEBSOCKET_KEY); if (key == null) { return handleBadRequest(exchange, "Missing \"Sec-WebSocket-Key\" header"); } String protocol = selectProtocol(headers, handler); return initAttributes(exchange).flatMap(attributes -> this.upgradeStrategy.upgrade(exchange, handler, protocol, () -> createHandshakeInfo(exchange, request, protocol, attributes)) ); }