JavaBeans provides a way for a JavaBean to declare events that can be fired by that bean. Such a bean that fires events is called an event source. Other objects can register with the event source to receive those events when they are fired. These objects are called the event listeners.

The JavaBeans specification explains how to create event sources and listeners. The following is a short example that demonstrates how Nucleus services can fire and listen for events. In this example, a StockPricer service fires stock events whenever it receives an update for a stock price. The StockWatcher service listens for these events and prints them out.