Grails VS Ratpack

And frameworks like Grails build conventions and helpers on top of Spring.

I don't have any direct experience and am only suggesting it because you mentioned RoR...But Grails (https://grails.org/) is basically the JVM version of RoR (Groovy on Rails -> Grails).

Grails - Spring under the hood. Much less boilerplate. Opinionated, which helps keep things consistent. Uses Spring-Security plugin for authentication.

Also, Grails, which a Rails like framework build on Groovy, a JVM scripting language.

Any JVM language to the rescue here? There’s one, but it’s not the one you’re thinking about. In a sign that this index may not accurately reflect our project reality, Groovy saw a meteoric rise of 0.86% to 1.04% last year! That was good for place 17. Yep, Groovy! Are people writing Gradle plugins in Groovy? Or is Grails having a resurgence? I’m as baffled as you are.

There are a huge set of web frameworks in Java that have the same example as shown on this site; I have no idea how this differs from, say, https://ratpack.io or https://sparkjava.com. This really needs some key features on the site to sell it.

Ratpack is also the name of a Java web framework - https://ratpack.io

If you want something really small that simply let's you expose REST APIs using plain Java, without the IoC containers, you might want to check out Javalin, Ratpack or Armeria