Due to a few changes in `WebSocketServerFactory` and `Session` API, our `JettyRequestUpgradeStrategy` and `JettyWebSocketSession` needed to adapt. As of 9.3.15+ and 9.4.0+, some reflection is required to support previous versions. Spring 4.3 websocket officially supports Jetty 9.1 to 9.3. Issue: SPR-14940