Groovy has been bundled with the Jalview desktop since circa 2012. The program supports interactive execution of groovy scripts via the Groovy Console, and command line execution via the '-groovy' option. The main source for documentation about Groovy in Jalview is the online help pages.
Here are some scripts to get you started:
def alf = Jalview.getAlignFrames(); print alf[0].getTitle(); def alignment = alf[0].viewport.alignment; def seq = alignment.getSequenceAt(0);
Using Java class methods from Groovy is straightforward, but currently, there isn't a set of easy to use methods for the jalview objects. A Jalview Scripting API needs to be developed to make this easier.
jalview.bin.JalviewScript could be a top level jalview instance of a script execution thread, creating and maintaining the context for scripts operating on the jalview datamodel and interfacing with the Jalview GUI.