Chapter 7. Events, interceptors and exception handling

To complement the contextual component model, there are two further basic concepts that facilitate the extremely loose coupling distinctive of Seam applications. The first is a strong event model, where events are mapped to event listeners with method-binding expressions like those in JavaServer Faces (JSF). The second is the pervasive use of annotations and interceptors to apply cross-cutting concerns to components that implement business logic.

7.1. Seam events

The Seam component model was developed for use with event-driven applications, specifically to enable the development of fine-grained, loosely-coupled components in a fine-grained eventing model. There are several event types in Seam:
  • JSF events
  • jBPM transition events
  • Seam page actions
  • Seam component-driven events
  • Seam contextual events
Each of these events is mapped to Seam components with JSF EL method-binding expressions. For a JSF event, this is defined in the JSF template: 
<h:commandButton value="Click me!" action="#{helloWorld.sayHello}"/>
For a jBPM transition event, it is specified in the jBPM process definition or pageflow definition: 
<start-page name="hello" view-id="/hello.jsp"> 
    <transition to="hello"> 
    <action expression="#{helloWorld.sayHello}"/> 
    </transition> 
    </start-page>
More information about JSF events and jBPM events is available elsewhere. For now, we will concentrate upon the two additional event types defined by Seam.