Looks nice and clean. What use cases make this a better choice than Ktor? This is an important question to answer because Ktor seems a bit more complete, has more contributors, and is backed by IntelliJ.
One big point in Javalin's favor: it does have more documentation than Ktor, which is currently missing some rather important parts (e.g. having no documentation for authentication and authorization is kinda unforgivable).