A modern programming language that makes developers happier.

kotlinlang

Coming from Ratpack again, they've made a conscious choice not to support plugins in their framework. Plugin integration (or whatever spring boot calls them) can be a major source of bugs, unexpected behaviours and upgrade woes. Ratpack provides some integrations with external libraries but it mostly comes in the form of static methods that use the same public API you, the developer, are using to program. So you have only one API to learn and if something goes wrong or you want to use another product, it's fairly straightforward to do.