Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Implementing an Event Listener

The JavaServer Faces technology supports action events and value-change events for components.

Action events occur when the user activates a component that implements `javax.faces.component.ActionSource`. These events are represented by the class `javax.faces.event.ActionEvent`.

Value-change events occur when the user changes the value of a component that implements `javax.faces.component.EditableValueHolder`. These events are represented by the class `javax.faces.event.ValueChangeEvent`.

One way to handle events is to implement the appropriate listener classes. Listener classes that handle the action events in an application must implement the interface `javax.faces.event.ActionListener`. Similarly, listeners that handle the value-change events must implement the interface `javax.faces.event.ValueChangeListener`.

This section explains how to implement the two listener classes.

To handle events generated by custom components, you must implement an event listener and an event handler and manually queue the event on the component. See Handling Events for Custom Components for more information.

Note: You do not need to create an `ActionListener` implementation to handle an event that results solely in navigating to a page and does not perform any other application-specific processing. See Writing a Method to Handle Navigation for information on how to manage page navigation.

### Implementing Value-Change Listeners

A `javax.faces.event.ValueChangeListener` implementation must include a `processValueChange(ValueChangeEvent)` method. This method processes the specified value-change event and is invoked by the JavaServer Faces implementation when the value-change event occurs. The `ValueChangeEvent` instance stores the old and the new values of the component that fired the event.

In the Duke’s Bookstore case study, the `NameChanged` listener implementation is registered on the `name` `UIInput` component on the `bookcashier.xhtml` page. This listener stores into session scope the name the user entered in the field corresponding to the name component.

The `bookreceipt.xhtml` subsequently retrieves the name from the session scope:

    <h:outputFormat title="thanks"
                    value="#{bundle.ThankYouParam}">
        <f:param value="#{sessionScope.name}"/>
    </h:outputFormat>

When the `bookreceipt.xhtml` page is loaded, it displays the name inside the message:

    "Thank you, {0}, for purchasing your books from us."

Here is part of the `NameChanged` listener implementation:

    public class NameChanged extends Object implements ValueChangeListener {

        @Override
        public void processValueChange(ValueChangeEvent event)
                throws AbortProcessingException {

            if (null != event.getNewValue()) {
                FacesContext.getCurrentInstance().getExternalContext().
                    getSessionMap().put("name", event.getNewValue());
            }
        }
    }

When the user enters the name in the field, a value-change event is generated, and the `processValueChange(ValueChangeEvent)` method of the `NameChanged` listener implementation is invoked. This method first gets the ID of the component that fired the event from the `ValueChangeEvent` object, and it puts the value, along with an attribute name, into the session map of the `FacesContext` instance.

Registering a Value-Change Listener on a Component explains how to register this listener onto a component.

### Implementing Action Listeners

A `javax.faces.event.ActionListener` implementation must include a `processAction(ActionEvent)` method. The `processAction(ActionEvent)` method processes the specified action event. The JavaServer Faces implementation invokes the `processAction(ActionEvent)` method when the `ActionEvent` occurs.

The Duke’s Bookstore case study uses two `ActionListener` implementations, `LinkBookChangeListener` and `MapBookChangeListener`. See Handling Events for Custom Components for details on `MapBookChangeListener`.

Registering an Action Listener on a Component explains how to register this listener onto a component.

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