A modern programming language that makes developers happier.

kotlinlang

I want to ask about the future: should I continue using `Java` with `Spring`, or will Spring use `Kotlin` more frequently in the future?

I have been using `Kotlin` with `Spring` exclusively since about 2017.  I've let Java go pretty much completely - except for maintenance on some older codebases.  I'd go woth `Kotlin`

Spring has a lot of Kotlin code under the hood already as well. It even has some Kotlin extension functions to make things easier in kotlin that you can't access from Java.