What's nice is that all the main Java server frameworks have embraced Kotlin. That said, they do basic REST API stuff, but what is important is how easy it is to add other functionality that's expected nowadays: scaling, social login, distributed logging, metrics, etc. Not everyone needs all of these, so it really depends on what you need 🙂 Vert.x and Spring Boot both do everything (scaling is simpler w/ Vert.x). Leaner frameworks like Ratpack and SparkJava don't but are slimmer.