There are plenty of resources available to a Java developer for getting a jump-start into Ruby/Rails development. The reverse doesn't appear to be true.
What resources would you suggest for getting up-to-date on the current state of java technologies? How about learning how to approach DRY (don't repeat yourself) without the use of metaprogramming? Or how to approach various scenarios where a ruby developer is used to passing in a function (proc/lambda/block) as an argument (callbacks, etc)?
You might start out by learning Groovy and Grails, which may seem a bit more familiar to you. Then you can start learning more of the Java side of Groovy to get familiar with what Java can do. Eventually (if you need to) you can move into 100% Java.
Groovy is in many ways similar to Ruby (at least from my Ruby-novice view), but you also have full access to Java libraries and coding.
I guess you might also look into JRuby first rather than Groovy since it is a Java-based implementation of Ruby.
The best thing to do is consider what you would want to do with Java and try to dig deeper into that area. Java as a whole has gotten pretty unwieldy over that last few years.
Good luck and I hope this helps a little.
I wouldn't start with the "current state" of Java. Like @chstehno said, Java is huge, and a bit of a mess. Focus on learning the basics, then delve in the the libraries as they become relevant to what you're working on.
I went from Java (and a bunch of other old school languages) to Ruby, so I can't speak to going the other way, except to say that Java is a lot more "by the book" and you can figure out a lot just by reading and searching through code, whereas that can be tough in Ruby with all of the mixins, lambdas, and convention-based magic in frameworks like Rails.