I've gotten the impression over the years that Grails is written in Java and only bundles Groovy for user scripting, but I haven't checked up recently. I certainly looked through the Gradle codebase when its version 2.0 came out and there was hardly a line of Groovy code in there, only Java. It, too, only uses Groovy for user scripting. If I'm right about the Grails codebase, static-typed Groovy isn't actually used to build any systems of note. Perhaps Cedric Champeau, who wrote the static-typed Groovy codebase, will use it to build the Groovy for Android he's presently working on. It'll be interesting to see whether he uses it or uses Java because it'll show whether he has enough faith in what he built to actually use it to build something with it. Until static-typed Groovy is used to build something, Groovy will remain a dynamically-typed scripting language for testing, Gradle builds, webpages, etc.