Registering Listeners on Components
Java Platform, Enterprise Edition (Java EE) 8
The Java EE Tutorial

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Registering Listeners on Components

An application developer can implement listeners as classes or as managed bean methods. If a listener is a managed 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 an f:valueChangeListener tag or an f:actionListener tag and nest the tag inside the component tag 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 explain how a page author uses the valueChangeListener and actionListener attributes to reference managed bean methods that handle events.

This section explains how to register a NameChanged value-change listener and a BookChange action listener implementation on components. The Duke’s Bookstore case study includes both of these listeners.

Registering a Value-Change Listener on a Component

A page author can register a ValueChangeListener implementation on a component that implements EditableValueHolder by nesting an f:valueChangeListener tag within the component’s tag on the page. The f:valueChangeListener tag supports the attributes shown in Table 11-5, one of which must be used.

Table 11-5 Attributes for the f:valueChangeListener Tag

Attribute

Description

type

References the fully qualified class name of a ValueChangeListener implementation. Can accept a literal or a value expression.

binding

References an object that implements ValueChangeListener. Can accept only a value expression, which must point to a managed bean property that accepts and returns a ValueChangeListener implementation.

The following example shows a value-change listener registered on a component:

<h:inputText id="name"
             size="30"
             value="#{cashierBean.name}"
             required="true"
             requiredMessage="#{bundle.ReqCustomerName}">
    <f:valueChangeListener
        type="javaeetutorial.dukesbookstore.listeners.NameChanged" />
</h:inputText>

In the example, the core tag type attribute specifies the custom NameChanged listener as the ValueChangeListener implementation registered 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 managed bean property. This attribute works in a similar way to the binding attribute supported by the standard converter tags. See Binding Component Values and Instances to Managed Bean Properties for more information.

Registering an Action Listener on a Component

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

Here is an example of an h:commandLink tag that references an ActionListener implementation:

<h:commandLink id="Duke" action="bookstore">
    <f:actionListener
        type="javaeetutorial.dukesbookstore.listeners.LinkBookChangeListener" />
    <h:outputText value="#{bundle.Book201}"/>
</h:commandLink>

The type attribute of the f:actionListener tag specifies the fully qualified class name of the ActionListener implementation. Similarly to the f:valueChangeListener tag, the f:actionListener tag also supports the binding attribute. See Binding Converters, Listeners, and Validators to Managed Bean Properties for more information about binding listeners to managed bean properties.

In addition to the actionListener tag that allows you register a custom listener onto a component, the core tag library includes the f: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.xhtml page of the Duke’s Bookstore application uses f:setPropertyActionListener with two components: the h:commandLink component used to link to the bookdetails.xhtml page and the h:commandButton component used to add a book to the cart:

<h:dataTable id="books"
    value="#{store.books}"
    var="book"
    headerClass="list-header"
    styleClass="list-background"
    rowClasses="list-row-even, list-row-odd"
    border="1"
    summary="#{bundle.BookCatalog}" >
    ...
    <h:column>
        <f:facet name="header">
            <h:outputText value="#{bundle.ItemTitle}"/>
        </f:facet>
        <h:commandLink action="#{catalog.details}"
                       value="#{book.title}">
            <f:setPropertyActionListener target="#{requestScope.book}"
                                         value="#{book}"/>
        </h:commandLink>
    </h:column>
    ...
    <h:column>
        <f:facet name="header">
            <h:outputText value="#{bundle.CartAdd}"/>
        </f:facet>
        <h:commandButton id="add"
                         action="#{catalog.add}"
                         value="#{bundle.CartAdd}">
            <f:setPropertyActionListener target="#{requestScope.book}"
                                         value="#{book}"/>
        </h:commandButton>
    </h:column>

The h:commandLink and h:commandButton tags are within an h:dataTable 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 an h:dataTable 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.xhtml or activates the commandButton component to go to bookcatalog.xhtml, the book data is available to those pages. Therefore, the f: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 f:setPropertyActionListener tag’s value attribute references the book object. The f: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.


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