JavaServer Faces technology offers a basic set of standard, reusable UI components that enable quick and easy construction of user interfaces for web applications. These components mostly map one-to-one to the elements in HTML 4. But often an application requires a component that has additional functionality or requires a completely new component. JavaServer Faces technology allows extension of standard components to enhance their functionality or to create custom components. A rich ecosystem of third-party component libraries is built on this extension capability, but it is beyond the scope of this tutorial to examine them. A web search for "JSF Component Libraries" is a good starting point to learn more about this important aspect of using JavaServer Faces technology.

In addition to extending the functionality of standard components, a component writer might want to give a page author the ability to change the appearance of the component on the page or to alter listener behavior. Alternatively, the component writer might want to render a component to a different kind of client device type, such as a smartphone or a tablet instead of a desktop computer. Enabled by the flexible JavaServer Faces architecture, a component writer can separate the definition of the component behavior from its appearance by delegating the rendering of the component to a separate renderer. In this way, a component writer can define the behavior of a custom component once but create multiple renderers, each of which defines a different way to render the component to a particular kind of client device.

A javax.faces.component.UIComponent is a Java class that is responsible for representing a self-contained piece of the user interface during the request-processing lifecycle. It is intended to represent the meaning of the component; the visual representation of the component is the responsibility of the javax.faces.render.Renderer. There can be multiple instances of the same UIComponent class in any given JavaServer Faces view, just as there can be multiple instances of any Java class in any given Java program.

JavaServer Faces technology provides the ability to create custom components by extending the UIComponent class, the base class for all standard UI components. A custom component can be used anywhere an ordinary component can be used, such as within a composite component. A UIComponent is identified by two names: component-family specifies the general purpose of the component (input or output, for instance), and component-type indicates the specific purpose of a component, such as a text input field or a command button.

A Renderer is a helper to the UIComponent that deals with how that specific UIComponent class should appear in a specific kind of client device. Renderers are identified by two names: render-kit-id and renderer-type. A render kit is just a bucket into which a particular group of renderers is placed, and the render-kit-id identifies the group. Most JavaServer Faces component libraries provide their own render kits.

A javax.faces.view.facelets.Tag object is a helper to the UIComponent and Renderer that allows the page author to include an instance of a UIComponent in a JavaServer Faces view. A tag represents a specific combination of component-type and renderer-type.

See Component, Renderer, and Tag Combinations for information on how components, renderers, and tags interact.

This chapter uses the image map component from the Duke’s Bookstore case study example to explain how you can create simple custom components, custom renderers, and associated custom tags, and take care of all the other details associated with using the components and renderers in an application. See Chapter 60, "Duke’s Bookstore Case Study Example" for more information about this example.

The chapter also describes how to create other custom objects: custom converters, custom listeners, and custom validators. It also describes how to bind component values and instances to data objects and how to bind custom objects to managed bean properties.