Jeremy Rayner created a groovy-mode for (X)Emacs, which has syntax hilighting, recognises curly brace indentation (just use the tab key), and doesn't panic about optional semicolons. He tested it in Emacs on Mac OSX 10.3, and others use it and it isn't painful.
Russel Winder has begun an alternative version of groovy-mode as a derived mode in CC Mode. Currently, this has some problems with indenting when semicolons are not used as statement terminators but this is being actively worked on -- CC Mode has support for languages like Awk and Groovy that do not require semicolons.
Stuart Clayman has created a "Groovy inferior mode" (nothing inferior about Stuart's code but this is the jargon for an interpreter execution mode) which allows groovysh to be run from within (X)Emacs.
(NB Links go to the latest versions in Subversion)
Download Jeremy's groovy-mode.el file and place it somewhere like (on OSX)
your mileage may vary...
Download Russel's groovy-mode.el and place it in your (X)Emacs' load path.
NB As both Jeremy's and Russel's are called groovy-mode you have to have one or the other, you can't have both.
Download Stuart's inf-groovy.el and place it in your (X)Emacs load path.
add the following lines to your ~/.emacs file:
Comments regarding Jeremy's mode:
Comments regarding Russel's mode: