In addition to these strong dependencies, applications often use a “weak” relationship to describe notifications and messages. These are encapsulated by JavaBean events. An event is designed into a source Bean when one or more listener Beans wish to be notified of some event that takes place on the source Bean. This is described as a “weak” relationship because neither Bean actually needs to know about the other in order to run at all. Of course, the application may require that connection to be in place to work properly, but the components themselves do not require the connection. Logging, for instance, uses JavaBean events: individual components do not require anyone to be listening for their log events, but the application as a whole usually requires certain logging connections to be in place. For more information, see Events and Event Listeners in the Core Dynamo Services chapter.
Event source/listener relationships can be established through Nucleus configuration files. The event source’s configuration file specifies which components will act as event listeners, and Nucleus automatically makes the connections.
After establishing the architecture, components, dependencies, and configuration of the application, the developer can then hand the whole application over to Nucleus and watch it run.
