After 3 years of building and maintaining a mission-critical Java backend, I would never do it again... We were using a similar framework, called dropwizard, great product, super lean and reliable.
If you think Rails is magic, then you haven't seen the Java world, all the magic with annotations, dependency injection, glueing libraries together by placing some files in a folder.
If you think Rails 2.3 to 3.0 upgrade was dramatic, then what we went through with a minor update from dropwizard (i think 0.7 to 0.8) was a nightmareish 1-2 months of rewriting half of the code and dependency injection handling.
Java is still super verbose, not just the language itself, but also all the libraries that are often over-engineered.
Hibernate is quiet easy to get started and you don't understand what all the fuss is about. But once you go start adding features it quickly becomes unmaintainable and any change to the models comes with FUD whether that'll make the queries explode in an unpredictable manner.
There's a lot of libraries, but on UI/UX level you don't have anything close to what Rails offers out of the box. Things you take for granted don't exist, like devise.
The good parts were: Rails deployments are from the stone-age compared to dropwizards simple approach of installing java and copying the self-running jar (fat binary) to server and start. Very good instrumentation/metrics out of the box with joda-metrics.
After 3 years, a pivot required us to rewrite large parts of the logic. We took that as a chance to rewrite/migrate all our services to Elixir/Phoenix. And never looked back.
So far Elixir/Phoenix/Ecto to me is the perfect middle ground between a solid Java environment and the speed of development of Rails.