This new Java World article compares the different Java scripting languages (Groovy, JudoScript, Pnuts, JRuby, Jacl, Jython, Rhino, and BeanShell), and list the issues that you have to select the good one...
In this article that I like I would like to read a little about the JSR-223 that should help with the integration part.
I will be very interested to know how you use Java Scripting language in your projects, so feel free to drop me a comment or mail ( tugdual at gmail)