I know there are some ActiveResource Client libraries for Java, like RAPA and JactiveResource
The question is: is there any easy way to create the Server Side of ActiveResource in Java?
In RubyOnRails all you need to do is to use the Inherited Resources gem. Is there any similar thing in Java?
inherited resource only make your code DRY, nothing more. All magic makes Ruby on Rails. If you ask how to do REST with JAVA like Rails does than check this.
1) you can use JRuby and RoR together.
2) you can look at Groovy on Grails
I don't know if there is a specific library for creating RESTful webservices that implements what you need, I'm no ruby expert but it seems that active resource is something like active record but over REST, what I'd suggest you to do is use Jersey and ActiveJDBC together and maybe you'll get the flavor of it. If it's generic enough you can even make a framework out of it.
Hope it helps!
Check out ActiveWeb: http://code.google.com/p/activeweb/, this is a Java framework inspired by RoR
