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

## Delegating Rendering to a Renderer

Both `MapComponent` and `AreaComponent` delegate all of their rendering to a separate renderer. The section Performing Encoding explains how `MapRenderer` performs the encoding for `MapComponent`. This section explains in detail the process of delegating rendering to a renderer using `AreaRenderer`, which performs the rendering for `AreaComponent`.

To delegate rendering, you perform the tasks described in the following topics:

- Creating the Renderer Class

- Identifying the Renderer Type

### Creating the Renderer Class

When delegating rendering to a renderer, you can delegate all encoding and decoding to the renderer, or you can choose to do part of it in the component class. The `AreaComponent` class delegates encoding to the `AreaRenderer` class.

The renderer class begins with a `@FacesRenderer` annotation:

    @FacesRenderer(componentFamily = "Area", rendererType = "DemoArea")
    public class AreaRenderer extends Renderer {

The `@FacesRenderer` annotation registers the renderer class with the JavaServer Faces implementation as a renderer class. The annotation identifies the component family as well as the renderer type.

To perform the rendering for `AreaComponent`, `AreaRenderer` must implement an `encodeEnd` method. The `encodeEnd` method of `AreaRenderer` retrieves the shape, coordinates, and alternative text values stored in the `ImageArea` bean that is bound to `AreaComponent`. Suppose that the `area` tag currently being rendered has a `value` attribute value of `"book203"`. The following line from `encodeEnd` gets the value of the attribute `"book203"` from the `FacesContext` instance:

    ImageArea ia = (ImageArea)area.getValue();

The attribute value is the `ImageArea` bean instance, which contains the `shape`, `coords`, and `alt` values associated with the `book203` `AreaComponent` instance. Configuring Model Data describes how the application stores these values.

After retrieving the `ImageArea` object, the method renders the values for `shape`, `coords`, and `alt` by simply calling the associated accessor methods and passing the returned values to the `ResponseWriter` instance, as shown by these lines of code, which write out the shape and coordinates:

    writer.startElement("area", area);
    writer.writeAttribute("alt", iarea.getAlt(), "alt");
    writer.writeAttribute("coords", iarea.getCoords(), "coords");
    writer.writeAttribute("shape", iarea.getShape(), "shape");

The `encodeEnd` method also renders the JavaScript for the `onmouseout`, `onmouseover`, and `onclick` attributes. The Facelets page needs to provide only the path to the images that are to be loaded during an `onmouseover` or `onmouseout` action:

    <bookstore:area id="map3" value="#{Book203}"
                    onmouseover="resources/images/book_203.jpg"
                    onmouseout="resources/images/book_all.jpg"
                    targetImage="mapImage"/>

The `AreaRenderer` class takes care of generating the JavaScript for these actions, as shown in the following code from `encodeEnd`. The JavaScript that `AreaRenderer` generates for the `onclick` action sets the value of the hidden field to the value of the current area’s component ID and submits the page.

    sb = new StringBuffer("document.forms[0]['").append(targetImageId).
            append("'].src='");
    sb.append(
            getURI(context,
            (String) area.getAttributes().get("onmouseout")));
    sb.append("'");
    writer.writeAttribute("onmouseout", sb.toString(), "onmouseout");
    sb = new StringBuffer("document.forms[0]['").append(targetImageId).
            append("'].src='");
    sb.append(
            getURI(context,
            (String) area.getAttributes().get("onmouseover")));
    sb.append("'");
    writer.writeAttribute("onmouseover", sb.toString(), "onmouseover");
    sb = new StringBuffer("document.forms[0]['");
    sb.append(getName(context, area));
    sb.append("'].value='");
    sb.append(iarea.getAlt());
    sb.append("'; document.forms[0].submit()");
    writer.writeAttribute("onclick", sb.toString(), "value");
    writer.endElement("area");

By submitting the page, this code causes the JavaServer Faces lifecycle to return back to the Restore View phase. This phase saves any state information, including the value of the hidden field, so that a new request component tree is constructed. This value is retrieved by the `decode` method of the `MapComponent` class. This decode method is called by the JavaServer Faces implementation during the Apply Request Values phase, which follows the Restore View phase.

In addition to the `encodeEnd` method, `AreaRenderer` contains an empty constructor. This is used to create an instance of `AreaRenderer` so that it can be added to the render kit.

The `@FacesRenderer` annotation registers the renderer class with the JavaServer Faces implementation as a renderer class. The annotation identifies the component family as well as the renderer type.

### Identifying the Renderer Type

Register the renderer with a render kit by using the `@FacesRenderer` annotation (or by using the application configuration resource file, as explained in Registering a Custom Renderer with a Render Kit). During the Render Response phase, the JavaServer Faces implementation calls the `getRendererType` method of the component’s tag handler to determine which renderer to invoke, if there is one.

You identify the type associated with the renderer in the `rendererType` element of the `@FacesRenderer` annotation for `AreaRenderer` as well as in the `renderer-type` element of the tag library descriptor.

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