@leethaksor I've worked in projects that have had java files in them, you can put any java class you like under src/main/groovy. With the Java files you can wire them up as beans using resources.groovy, just as you would manually wire up any class or you can set up class scanning like this: https://mrhaki.blogspot.com/2014/08/grails-goodness-use-spring-java.html As for Kotlin you have Jefss demo which looks like it puts its source in src/main/kotlin: https://github.com/jeffbrown/langdemo Although personally I usually go the other way, convert Java to statically compiled Groovy, because I like to be able to have simpler code and take advantage of AST Transforms. One of the things you lose when going to other languages in Grails, is the convention over configuration because you have to start wiring thing up manually, and putting things under src/main/groovy, rather than under the Grails artefact folders(grails-app/*). At which point you could just use Spring Book, or even better yet Micronaut.