I posted a similar question regarding gradle but this question is without gradle or maven.
I can not get Kotlin working properly using Eclipse IDE. This works great
I'm working on a project with Spring Boot and Kotlin (some controllers/mappers/classes in Java and others in Kotlin) and after trying a lot of approaches, the only that worked was to use Eclipse 03-2020 and Kotlin Plugin for Eclipse V0.8.19.
https://dl.bintray.com/jetbrains/kotlin/eclipse-plugin/0.8.19/