Event and Listener Model

The JavaServer Faces event and listener model is similar to the JavaBeans event model in that it has strongly typed event classes and listener interfaces that an application can use to handle events generated by UI components.

An Event object identifies the component that generated the event and stores information about the event. To be notified of an event, an application must provide an implementation of the Listener class and must register it on the component that generates the event. When the user activates a component, such as by clicking a button, an event is fired. This causes the JavaServer Faces implementation to invoke the listener method that processes the event.

JavaServer Faces technology supports three kinds of events: value-change events, action events, and data-model events.

An action event occurs when the user activates a component that implements ActionSource. These components include buttons and hyperlinks.

A value-change event occurs when the user changes the value of a component represented by UIInput or one of its subclasses. An example is selecting a check box, an action that results in the component’s value changing to true. The component types that can generate these types of events are the UIInput, UISelectOne, UISelectMany, and UISelectBoolean components. Value-change events are fired only if no validation errors were detected.

Depending on the value of the immediate property (see The immediate Attribute) of the component emitting the event, action events can be processed during the invoke application phase or the apply request values phase, and value-change events can be processed during the process validations phase or the apply request values phase.

A data-model event occurs when a new row of a UIData component is selected. The discussion of data-model events is an advanced topic. It is not covered in this tutorial but may be discussed in future versions of this tutorial.

There are two ways to cause your application to react to action events or value-change events emitted by a standard component:

• Implement an event listener class to handle the event and register the listener on the component by nesting either a valueChangeListener tag or an actionListener tag inside the component tag.

• Implement a method of a backing bean to handle the event and refer to the method with a method expression from the appropriate attribute of the component’s tag.

See Implementing an Event Listener for information on how to implement an event listener. See Registering Listeners on Components for information on how to register the listener on a component.

See Writing a Method to Handle an Action Event and Writing a Method to Handle a Value-Change Event for information on how to implement backing bean methods that handle these events.

See Referencing a Backing Bean Method for information on how to refer to the backing bean method from the component tag.

When emitting events from custom components, you must implement the appropriate Event class and manually queue the event on the component in addition to implementing an event listener class or a backing bean method that handles the event. Handling Events for Custom Components explains how to do this.