Understanding the Image Map Example

The cardemo application now includes a custom image map component on the ImageMap.jsp page. This image map displays a map of the world. When the user clicks on one of a particular set of regions in the map, the application sets the locale in the FacesContext to the language spoken in the selected region. The hot spots of the map are: the United States, Spanish-speaking Central and South America, France, and Germany.

Why Use JavaServer Faces Technology to Implement an Image Map?

JavaServer Faces technology is an ideal framework to use for implementing this kind of image map because it can perform the work that must be done on the server without requiring you to create a server-side image map.

In general, client-side image maps are preferred over server-side image maps for a few reasons. One reason is that the client-side image map allows the browser to provide immediate feedback when a user positions her mouse over a hot spot. Another reason is that client-side image maps perform better because they don't require round-trips to the server. However, in some situations, your image map might need to access the server to retrieve some data or to change the appearance of non-form controls, which a client-side image map cannot do.

The image map custom component--because it uses JavaServer Faces technology--has the best of both style of image maps: It can handle the parts of the application that need to be performed on the server, while allowing the other parts of the application to be performed on the client side.

Understanding the Rendered HTML

Here is an abbreviated version of the form part of the HTML page that the application needs to render:

<form METHOD="post" ACTION="/cardemo/faces/...">
  <table> <tr> <td> Welcome to JavaServer Faces</td></tr>
    <tr><td>
      <img id="mapImage" src="world.jpg" usemap="#worldMap"> 
        <map name="worldMap">
          <area shape="poly" 
            coords="6,15,6,28,2,30,6,34,13,28,17,..."
            onclick="document.forms[0].selectedArea.value=
              'NAmericas';
              document.forms[0].submit()"
            onmouseover="document.forms[0].mapImage.src=
              'world_namer.jpg';"
            onmouseout="document.forms[0].mapImage.src=
              'world.jpg';"
            alt="NAmericas">
            ...
          <input type="hidden" name="selectedArea"></map>
    </td></tr>
  </table>
</form>

The img tag associates an image (world.jpg) with an image map, referenced in the usemap attribute value.

Each area tag specifies a region of the image map. The onmouseover, onmouseout, and onmouseclick attributes define which JavaScript code is executed when these events occur. When the user moves her mouse over a region, the onmouseover function associated with the region displays the map with that region highlighted. When the user moves her mouse out of a region, the onmouseout function redisplays the original image. If the user clicks on a region, the onclick function sets the value of the input tag to the id of the selected area and submits the page.

The input tag represents a hidden control that stores the value of the currently-selected area between client/server exchanges so that the server-side component classes can retrieve the value.

The server side objects retrieve the value of selectedArea and set the locale in the FacesContext according to what region was selected.

Understanding the JSP Page

Here is an abbreviated form of the JSP page that the image map component will use to generate the HTML page shown in the previous section:

<f:use_faces> 
  <h:form formName="imageMapForm" >
  ...
    <h:graphic_image id="mapImage" url="/world.jpg"
      usemap="#worldMap" />  
    <d:map id="worldMap" currentArea="NAmericas" >
      <f:action_listener 
        type="cardemo.ImageMapEventHandler" />
      <d:area id="NAmericas" valueRef="NA" 
        onmouseover="/cardemo/world_namer.jpg" 
        onmouseout="/cardemo/world.jpg" />
      ...
    </d:map>
    ...
  </h:form>
</f:use_faces>

The action_listener tag nested inside the map tag causes the ImageMapEventHandler to be registered on the component corresponding to map. This handler changes the locale according to the area selected from the image map. The way this event is handled is explained more in Handling Events for Custom Components.

Notice that the area tags do not contain any of the JavaScript, coordinate, or shape data that is displayed on the HTML page. The JavaScript is generated by the AreaRenderer class. The onmouseover and onmouseout attribute values indicate the image to be loaded when these events occur. How the JavaScript is generated is explained more in Performing Encoding.

The coordinate, shape, and alt data are obtained through the valueRef attribute, whose value refers to an attribute in application scope. The value of this attribute is a model object, which stores the coordinate, shape, and alt data. How these model objects are stored in the application scope is explained more in Simplifying the JSP Page.

Simplifying the JSP Page

One of the primary goals of JavaServer Faces technology is ease-of-use. This includes separating out the code from the page so that a wider range of page authors can easily contribute to the Web development process. For this reason, all JavaScript is rendered by the component classes rather than being included in the page.

Ease-of-use also includes compartmentalizing the tasks of developing a Web application. For example, rather than requiring the page author to hardcode the coordinates of the hot spots in the page, the application should allow the coordinates to be retrieved from a database or generated by one of the many image map tools available.

In a JavaServer Faces application, data such as coordinates would be retrieved via a model object from the valueRef attribute. However, the shape and coordinates of a hotspot should be defined together because the coordinates are interpreted differently depending on what shape the hotspot is. Since a component's valueRef can only be bound to one property, the valueRef attribute cannot refer to both the shape and the coordinates.

To solve this problem, the application encapsulates all of this information in a set of ImageArea objects. These objects are initialized into application scope by the Managed Bean Facility (Managed Bean Creation). Here is part of the managed-bean declaration for the ImageArea bean corresponding to the South America hotspot:

<managed-bean>
  ...
  <managed-bean-name>SA</managed-bean-name>
  <managed-bean-class>
    components.model.ImageArea
  </managed-bean-class>
  <managed-bean-scope>application</managed-bean-scope>
  <managed-property>
    <property-name>shape</property-name>
    <value>poly</value>
  </managed-property>
  <managed-property>
    <property-name>alt</property-name>
  <value>SAmerica</value>
  </managed-property>
  <managed-property>
    <property-name>coords</property-name>
    <value>89,217,95,100...</value>  
  </managed-property>
</managed-bean>

For more information on initializing managed beans with the Managed Bean Facility, see section Creating Model Objects.

The valueRef attributes of the area tags refer to the beans in the application scope, as shown in this area tag from ImageMap.jsp:

<d:area id="NAmericas" 
    valueRef="NA" 
    onmouseover="/cardemo/world_namer.jpg" 
    onmouseout="/cardemo/world.jpg" />

To reference the ImageArea model object values from the component class, you need to call getvalueRef from your component class. This returns the name of the attribute that stores the ImageArea object associated with the tag being processed. Next, you need to pass the attribute to the getValueRef method of the Util class, which is a reference implementation helper class that contains various factories for resources. This will return a ValueBinding, which uses the expression from the valueRef attribute to locate the ImageArea object containing the values associated with the current UIArea component. Here is the line from AreaRenderer that does all of this:

ImageArea is just a simple bean, so you can access the shape, coordinates, and alt values by calling the appropriate accessor methods of ImageArea. Performing Encoding explains how to do this in the AreaRenderer class.

Summary of the Application Classes

Table 22-2 summarizes all of the classes needed to implement the image map component.

Table 22-2 Image Map Classes
Class	Function
`AreaTag`	The tag handler that implements the area custom tag
`MapTag`	The tag handler that implements the map custom tag
`UIArea`	The class that defines the `UIArea` component, corresponding to the area custom tag
`UIMap`	The class that defines the `UIMap` component, corresponding to the map custom tag
`AreaRenderer`	This `Renderer` performs the delegated rendering for the `UIArea` component
`ImageArea`	The model object that stores the shape and coordinates of the hot spots
`ImageMapEventHandler`	The listener interface for handling the action event generated by the `map` component

AreaTag and MapTag are located in <JWSDP_HOME>/jsf/samples/components/src/components/taglib/.

UIArea and UIMap are located in <JWSDP_HOME>/jsf/samples/components/src/components/components/.

AreaRenderer is located in <JWSDP_HOME>/jsf/samples/components/src/components/renderkit/.

ImageArea is located in <JWSDP_HOME>/jsf/samples/components/src/components/model/.

ImageMapEventHandler is located in <JWSDP_HOME>/jsf/samples/cardemo/src/cardemo/.