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. 

Check out www.rribbit.org or download the latest version here. RRiBbit is also available from the Maven Central.