Most of Javalin was re-written to Kotlin in version 0.2.0. The SAM interfaces were left as Java, as well as the main Javalin-class which has method-declarations with SAM parameters. This had to be done due to limitations in Kotlin itself (https://youtrack.jetbrains.com/issue/KT-14151, https://devnet.jetbrains.com/thread/461516)
The rest of the library will be ported to Kotlin if this issue is resolved, or and alternative solution is discovered.