You can probably adopt both for a large existing project. Old files get compiled with JPlus, new files will be written in Kotlin. Old files can also gradually be ported to kotlin.

Exactly, that could work. You can keep the existing files as they are and compile them with JPlus, while writing new modules in Kotlin to adopt it gradually. JPlus can serve as a stepping stone before moving fully to Kotlin. However, converting all Java code in an existing project to Kotlin would not only carry significant risks but also be costly. With JPlus, you can fully leverage the proven existing Java codes.