Here is a really simple library to send notifications to Growl from any Java program.
This implementation is originally based on the Growl Jenkins plugin and uses the network feature provided by Growl to publish notifications.
Under OS X preferences, go to the Growl options and then activate the network feature. Set a passwork in the server password text field if necessary. This password will be required in the Java code.
Using JavaGrowl is quite simple, once you have activated the network feature in Growl, you can use it like this: