why doesn't spring boot 2.7.x use kotlin 1.7 or 1.8

Question

I would like to come up with some empirical evidence for, or against, migrating spring boot 2.7.x to kotlin >= 1.7.x.

Looking at the spring boot documentation, kotlin version 1.6.21 is listed. However, in some of the spring examples listed, point to at least kotlin 1.7.X less than one month after release.

Gradle has a pretty good compatibility matrix for kotlin usage, of which doesn't even list the current version that comes standard on the spring Initializer start.spring.io

Kotlin Version	Gradle min and max versions
1.8.20	6.8.3 – 7.6.0
1.8.0	6.8.3 – 7.3.3
1.7.20	6.7.1 – 7.1.1

Answer 1

Spring Boot 2.7.x uses Kotlin 1.6.x because Kotlin 1.6 was the latest version that was available when Spring Boot 2.7 was released. It hasn't been upgraded to Kotlin 1.7 or 1.8 as the policy for managing dependencies prohibits doing so:

Spring Boot provides managed dependencies for many third-party libraries. These libraries are upgraded only at the patch level for any given Spring Boot patch release. Minor and major version upgrades of third-party libraries are only applied in Spring Boot minor or major releases. You should check the EOL policies of projects that you depend on since you may find that you’re using a supported version of Spring Boot against an unsupported third-party library.

As noted in the comments, you can use Kotlin 1.3.x or later with Spring Boot 2.7.x. The Kotlin baseline was raised in Spring Boot 3.0 and it requires 1.7.x or later.