Apparently, dynamic languages in Java are a hot topic. At least if you judge by the size of the audience in the JSR223 - Dynamic Languages on the Java Platform University talk at JavaPolis today. The room was quite packed with people.
The talk was quite interesting. Geert Bevin did the first part on scripting features in Java (he claimed that he was just a stand in for someone else who had made the slides - he did a good job anyway.) Charles Nutter and Thomas Enebo presented their work with JRuby and presented a short demo with JRuby on Rails which is pretty much up and running. Finally, Dierk König presented some of the features in Groovy which is finally approaching its 1.0 release in the end of December - this year :-).
The ability to actually run Rails in a JVM is also quite interesting. This might make Rails more acceptable for some organizations which would never allow something like Rails on its production environments. When deployed on a Java application server it is basically just Java. Furthermore, JRuby comes with a database adapter for JDBC which adds database support for even more databases than Rails itself. As Thomas and Charles mentioned, you can now run Rails on a mainframe - quite an interesting thought :-)
Groovy is also an interesting language. I have used Groovy in small scale for some time now and have been quite happy with it (apart from the many changes since its inception). Groovy has the advantage over JRuby (and most other scripting for Java) that it is much more Java-like. Also, the Groovy scripts can be compiled into Java classes which allow them to be used from Java code without you noticing it. JRuby doesn't have this ability yet (although the feature is planned.)
There is a talk on Grails on Thursday that I consider seeing. I could hope that Groovy will get the same momentum with its companion web framework, Grails, as Ruby has had with Rails (although I doubt it.)