Is Ring written natively in Clojure or does it run on top of a Java web server framework?

ring is a unified clj format to talk to http libs from clojure. It can "run" with various adapters (jetty, undertow, netty etc), but yes the adapters are usually thin wrappers to java libs.

Aleph is arguably the one that has the most lower level bits written in clojure (I think), but it relies heavily on netty.