Implementing an Event Listener - The J2EE 1.4 Tutorial

Implementing an Event Listener

As explained in Event and Listener Model, JavaServer Faces technology supports action events and value-change events.

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

Value-change events occur when the user changes the value of a UIInput component or a component whose class extends UIInput. These events are represented by the javax.faces.event.ValueChangeEvent class.

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

This section explains how to implement the two listener classes.

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

Note: You need not 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 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.

The NameChanged listener implementation is registered on the name UIInput component on the bookcashier.jsp page. This listener stores into session scope the name the user entered in the text field corresponding to the name component. When the bookreceipt.jsp page is loaded, it displays the first 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 {

   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 text 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. Next, 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

An 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 application does not use any ActionListener implementations. Instead, it uses method-binding expressions from actionListener attributes to refer to backing bean methods that handle events. This section explains how to turn one of these methods into an ActionListener implementation.

The chooselocale.jsp page allows the user to select a locale for the application by clicking on one of a set of hyperlinks. When the user clicks one of the hyperlinks, an action event is generated, and the chooseLocaleFromLink(ActionEvent) method of LocaleBean is invoked. Instead of implementing a bean method to handle this event, you can create a listener implementation to handle it. To do this, you do the following:

Move the chooseLocaleFromLink(ActionEvent) method to a class that implements ActionListener

Rename the method to processAction(ActionEvent)

The listener implementation would look something like this:
...
public class LocaleChangeListener extends Object implements 
  ActionListener {

  private Map locales = null;
   
  public LocaleChangeListener() {
    locales = new HashMap();
    locales.put("NAmerica", new Locale("en", "US"));
    locales.put("SAmerica", new Locale("es", "MX"));
    locales.put("Germany", new Locale("de", "DE"));
    locales.put("France", new Locale("fr", "FR"));
  }

  public void processAction(ActionEvent event)
    throws AbortProcessingException {
    
    String current = event.getComponent().getId();
    FacesContext context = FacesContext.getCurrentInstance();
    context.getViewRoot().setLocale((Locale) 
    locales.get(current)); 
  }
} 
Registering an Action Listener on a Component explains how to register this listener onto a component.