15.1.1 When to Use a Custom Component

A component class defines the state and behavior of a UI component. This behavior includes converting the value of a component to the appropriate markup, queuing events on components, performing validation, and any other behavior related to how the component interacts with the browser and the request-processing lifecycle.

You need to create a custom component in the following situations.

• You need to add new behavior to a standard component, such as generating an additional type of event (for example, notifying another part of the page that something changed in this component as a result of user interaction).

• You need to take a different action in the request processing of the value of a component from what is available in any of the existing standard components.

• You want to take advantage of an HTML capability offered by your target browser, but none of the standard JavaServer Faces components take advantage of the capability in the way you want, if at all. The current release does not contain standard components for complex HTML components, such as frames; however, because of the extensibility of the component architecture, you can use JavaServer Faces technology to create components like these. The Duke's Bookstore case study creates custom components that correspond to the HTML map and area tags.

• You need to render to a non-HTML client that requires extra components not supported by HTML. Eventually, the standard HTML render kit will provide support for all standard HTML components. However, if you are rendering to a different client, such as a phone, you might need to create custom components to represent the controls uniquely supported by the client. For example, some component architectures for wireless clients include support for tickers and progress bars, which are not available on an HTML client. In this case, you might also need a custom renderer along with the component, or you might need only a custom renderer.

You do not need to create a custom component in the following cases.

• You need to aggregate components to create a new component that has its own unique behavior. In this situation, you can use a composite component to combine existing standard components. For more information on composite components, see Composite Components and Chapter 14, "Composite Components: Advanced Topics and an Example".

• You simply need to manipulate data on the component or add application-specific functionality to it. In this situation, you should create a managed bean for this purpose and bind it to the standard component rather than create a custom component. See Managed Beans in JavaServer Faces Technology for more information on managed beans.

• You need to convert a component's data to a type not supported by its renderer. See Using the Standard Converters for more information about converting a component's data.

• You need to perform validation on the component data. Standard validators and custom validators can be added to a component by using the validator tags from the page. See Using the Standard Validators and Creating and Using a Custom Validator for more information about validating a component's data.

• You need to register event listeners on components. You can either register event listeners on components using the f:valueChangeListener and f:actionListener tags, or you can point at an event-processing method on a managed bean using the component's actionListener or valueChangeListener attributes. See Implementing an Event Listener and Writing Managed Bean Methods for more information.

15.1.2 When to Use a Custom Renderer

A renderer, which generates the markup to display a component on a web page, allows you to separate the semantics of a component from its appearance. By keeping this separation, you can support different kinds of client devices with the same kind of authoring experience. You can think of a renderer as a "client adapter." It produces output suitable for consumption and display by the client and accepts input from the client when the user interacts with that component.

If you are creating a custom component, you need to ensure, among other things, that your component class performs these operations that are central to rendering the component:

• Decoding: Converting the incoming request parameters to the local value of the component

• Encoding: Converting the current local value of the component into the corresponding markup that represents it in the response

The JavaServer Faces specification supports two programming models for handling encoding and decoding.

• Direct implementation: The component class itself implements the decoding and encoding.

• Delegated implementation: The component class delegates the implementation of encoding and decoding to a separate renderer.

By delegating the operations to the renderer, you have the option of associating your custom component with different renderers so that you can render the component on different clients. If you don't plan to render a particular component on different clients, it may be simpler to let the component class handle the rendering. However, a separate renderer enables you to preserve the separation of semantics from appearance. The Duke's Bookstore application separates the renderers from the components, although it renders only to HTML 4 web browsers.

If you aren't sure whether you will need the flexibility offered by separate renderers but you want to use the simpler direct-implementation approach, you can actually use both models. Your component class can include some default rendering code, but it can delegate rendering to a renderer if there is one.

15.1.3 Component, Renderer, and Tag Combinations

When you create a custom component, you can create a custom renderer to go with it. To associate the component with the renderer and to reference the component from the page, you will also need a custom tag.

Although you need to write the custom component and renderer, there is no need to write code for a custom tag (called a tag handler). If you specify the component and renderer combination, Facelets creates the tag handler automatically.

In rare situations, you might use a custom renderer with a standard component rather than a custom component. Or you might use a custom tag without a renderer or a component. This section gives examples of these situations and summarizes what is required for a custom component, renderer, and tag.

You would use a custom renderer without a custom component if you wanted to add some client-side validation on a standard component. You would implement the validation code with a client-side scripting language, such as JavaScript, and then render the JavaScript with the custom renderer. In this situation, you need a custom tag to go with the renderer so that its tag handler can register the renderer on the standard component.

Custom components as well as custom renderers need custom tags associated with them. However, you can have a custom tag without a custom renderer or custom component. For example, suppose that you need to create a custom validator that requires extra attributes on the validator tag. In this case, the custom tag corresponds to a custom validator and not to a custom component or custom renderer. In any case, you still need to associate the custom tag with a server-side object.

Table 15-1 summarizes what you must or can associate with a custom component, custom renderer, or custom tag.