open fun setSockJsClientLibraryUrl(clientLibraryUrl: String): Unit
Transports with no native cross-domain communication (e.g. "eventsource", "htmlfile") must get a simple page from the "foreign" domain in an invisible iframe so that code in the iframe can run from a domain local to the SockJS server. Since the iframe needs to load the SockJS javascript client library, this property allows specifying where to load it from.
By default this is set to point to "https://cdn.jsdelivr.net/sockjs/1.0.0/sockjs.min.js". However, it can also be set to point to a URL served by the application.
Note that it's possible to specify a relative URL in which case the URL must be relative to the iframe URL. For example assuming a SockJS endpoint mapped to "/sockjs", and resulting iframe URL "/sockjs/iframe.html", then the the relative URL must start with "../../" to traverse up to the location above the SockJS mapping. In case of a prefix-based Servlet mapping one more traversal may be needed.