The Java EE 5 Tutorial

Registering Listeners on Components

An application developer can implement listeners as classes or as backing bean methods. If a listener is a backing bean method, the page author references the method from either the component’s valueChangeListener attribute or its actionListener attribute. If the listener is a class, the page author can reference the listener from either a valueChangeListener tag or an actionListener tag and nest the tag inside the component tag in order to register the listener on the component.

Referencing a Method That Handles an Action Event and Referencing a Method That Handles a Value-change Event describe how a page author uses the valueChangeListener and actionListener attributes to reference backing bean methods that handle events.

The Duke’s Bookstore application includes a ValueChangeListener implementation class but does not use an ActionListener implementation class. This section explains how to register the NameChanged value-change listener and a hypothetical LocaleChange action listener implementation on components. Implementing Value-Change Listeners explains how to implement NameChanged. Implementing Action Listeners explains how to implement the hypothetical LocaleChange listener.

Registering a Value-Change Listener on a Component

A page author can register a ValueChangeListener implementation on a component that implements EditableValueHolder by nesting a valueChangeListener tag within the component’s tag on the page. The valueChangeListener tag supports two attributes:

• type: References the fully qualified class name of a ValueChangeListener implementation

• binding: References an object that implements ValueChangeListener

A page author must use one of these attributes to reference the value-change listener. The type attribute accepts a literal or a value expression. The binding attribute only accepts a value expression, which must point to a backing bean property that accepts and returns a ValueChangeListener implementation.

Following is the tag corresponding to the name component from the bookcashier.jsp page. It uses the type attribute to reference a value-change listener:

<h:inputText  id="name" size="50" value="#{cashier.name}"
     required="true">    
     <f:valueChangeListener type="listeners.NameChanged" />
</h:inputText>

The type attribute specifies the custom NameChanged listener as the ValueChangeListener implementation to register on the name component.

After this component tag is processed and local values have been validated, its corresponding component instance will queue the ValueChangeEvent associated with the specified ValueChangeListener to the component.

The binding attribute is used to bind a ValueChangeListener implementation to a backing bean property. It works in a similar way to the binding attribute supported by the standard converter tags. Binding Component Values and Instances to External Data Sources explains more about binding listeners to backing bean properties.

Registering an Action Listener on a Component

A page author can register an ActionListener implementation on a UICommand component by nesting an actionListener tag within the component’s tag on the page. Similarly to the valueChangeListener tag, the actionListener tag supports both the type and binding attributes. A page author must use one of these attributes to reference the action listener.

Duke’s Bookstore does not use any ActionListener implementations. Here is one of the commandLink tags on the chooselocale.jsp page, changed to reference an ActionListener implementation rather than a backing bean method:

<h:commandLink id="NAmerica" action="bookstore">
     <f:actionListener type="listeners.LocaleChange" />
</h:commandLink>

The type attribute of the actionListener tag specifies the fully qualified class name of the ActionListener implementation. Similarly to the valueChangeListener tag, the actionListener tag also supports the binding attribute. Binding Converters, Listeners, and Validators to Backing Bean Properties explains more about how to bind listeners to backing bean properties.

When this tag’s component is activated, the component’s decode method (or its associated Renderer) automatically queues the ActionEvent implementation associated with the specified ActionListener implementation onto the component.

In addition to the actionListener tag that allows you register a custom listener onto a component, the core tag library includes the setPropertyActionListener tag. You use this tag to register a special action listener onto the ActionSource instance associated with a component. When the component is activated, the listener will store the object referenced by the tag’s value attribute into the object referenced by the tag’s target attribute.

The bookcatalog.jsp page uses setPropertyActionListener with two components: the commandLink component used to link to the bookdetails.jsp page and the commandButton component used to add a book to the cart:

<c:forEach items="#{bookDBAO.books}" var="book"
     varStatus="stat">
    <c:set var="book" scope="request" value="${book}"/>
        ...
    <h:commandLink action="#{catalog.details}"
        value="#{book.title}">
        <f:setPropertyActionListener
             target="#{requestScope.book}" value="#{book}"/>
    </h:commandLink>
    ...
    <h:commandButton id="add"
        action="#{catalog.add}" value="#{bundle.CartAdd}">
        <f:setPropertyActionListener
             target="#{requestScope.book}" value="#{book}"/>
    </h:commandButton>
    <c:remove var="book" scope="request"/>
</c:forEach>

As shown in the preceding code, the commandLink and commandButton components are within a forEach tag, which iterates over the list of books. The var attribute refers to a single book in the list of books.

The object referenced by the var attribute of a forEach tag is in page scope. However, in this case, you need to put this object into request scope so that when the user activates the commandLink component to go to bookdetails.jsp or activates the commandButton component to go to bookcatalog.jsp, the book data is available to those pages. Therefore, the setPropertyActionListener tag is used to set the current book object into request scope when the commandLink or commandButton component is activated.

In the preceding example, the setPropertyActionListener tag’s value attribute references the book object. The setPropertyActionListener tag’s target attribute references the value expression requestScope.book, which is where the book object referenced by the value attribute is stored when the commandLink or the commandButton component is activated.