Vyuudha-eventing is a project mainly written in Java, it's free.
Implementation of the Observer pattern for Vyuudha for Event Driven Architecture
The Observer pattern allows an object (an Observer) to watch another object (a Subject). The subject and observer to have a publish/subscribe relationship. Observers can register to receive events from the Subject. When the subject can update the Observers when certain expected events occur.
TBD
http://java-x.blogspot.com/2007/01/implementing-observer-pattern-in-java.html http://www.javaworld.com/javaworld/javaqa/2001-05/04-qa-0525-observer.html