RRiBbit is the most advanced, versatile and easy to use Open Source Eventbus in the Java World. You can use it within a Java application, to have different classes/components talk to each other without needing dependencies on one another, and you can use it to communicate with other Java applications on other machines. 

RRiBbit improves upon existing Eventbuses by being compatible with existing code and by allowing the listeners to send something back to the senders. That's why we like to use the term Request-Response-Bus. 

Version 2.3.0 improves the recently added HTTP support by integrating it with the Spring Framework. This means that the RRiBbit Servlet now automatically initializes itself with RRiBbit beans from your Spring Context. 

Another improvement is the support for multiple dispatchers, so that one Request can now be sent to multiple remote destinations in one call. The responses will automatically be aggregated and returned to the sender. 

Check out www.rribbit.org or download the latest version here: http://www.rribbit.org/versions.html