Found the nice example from Brian Clozel about HTTP/2 and Spring Boot, especially with Server Push: https://github.com/bclozel/http2-experiments.
There is an index.html which has some image resources. However, those image are pushed to the server, but I have no idea where exactly this push is initiated from. I don't see any push filters or any other indicators which could lead to push the resources inside the index.html. Any ideas?
Jetty's PushCacheFilter is configured here in the example.
PushCacheFilter will automatically "learn" what secondary resources are associated to any resource, and will automatically push them.
The example also shows a way to programmatically push a resource.
However, that API has been deprecated in favor of using a PushBuilder.
PushBuilder will be part of Servlet 4.0, becoming a standard API.