A PushCacheFilter contains the logic to associate secondary resources
to primary resources.
PushCacheFilter calls a Jetty-specific API on the request dispatcher:
Dispatcher.push(ServletRequest). This is a technology preview of the
push functionality slated for Servlet 4.0.
The push() invocation arrives to the transport and it is converted to
HTTP/2 specific PUSH_PROMISE, along with the mechanism to simulate
the request for the secondary resource.
Greg Wilkins
Simone Bordet