The JettyWebSocketAdapter class doesn't set the configured WebClient proxy on Jetty's WebSocketClient instance.

Jetty supports proxies on websockets since 9.40 through their httpClient api. If support for older versions of Jetty is required, implementation of this feature should be possibile by adding the following lines of code during the WebSocketClient object initialization while still using the deprecated constructor (WARNING: Untested):

client_.getHttpClient()
    .getProxyConfiguration()
    .getProxies()
    .add(new HttpProxy("localhost", 8080));