We use it for (non-Spring) backend development. It's lovely and always my first choice over Java.

I like to avoid mixing Java/Kotlin within the same module when I can, but it still works, and parts of our codebase are mixed this way. (by module I mean e.g. the same Maven or Gradle module, i.e. try to avoid a situation where you have a `src/main/java` and `src/main/kotlin` next to each other)

You can put kotlin code in src/main/java