I'm not sure - I don't use either go or Java much these days - but some cursory googling implies that light-4j/undertow core is to fasthttp as netty is to net/http?
FastHTTP is a low allocation HTTP library in golang. Netty is a low level event driven framework for writing networking applications and protocols. It's used in many libraries and frameworks which build on top of it like Vertx, Spring webflux, and more recently Undertow.
Things should get more interesting once Java gets green threads, which would make it easier to implement high performance concurrent servers without needing to drop down to event driven code unless for specific needs. That, and with value types, should push the performance bar forward.
https://github.com/networknt/light-4j/