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This article is based on, and contains excerpts from, the book Pro JSF: Building Rich Internet Components by Jonas Jacobi and John Fallows, published by Apress. Book is now available on fine bookstores and Amazon as of February 25, 2006.

JavaServer Faces (JSF) standardizes the server-side component model for Web application development but doesn't standardize the presentation layer at the browser. In a series of articles we are going to look at how JSF can fulfill new presentation requirements without sacrificing application developer productivity building Rich Internet Applications (RIA).

Consumer Requirements
It's always the end user who feels the effect of any chosen technology, especially at the presentation layer. The end-user experience should be top priority for any developer building applications, whether they are Web applications or desktop applications. End users demand feature-rich and highly interactive user interfaces.

Introduction to Rich Internet Technologies
Web application developers today are faced with higher demand for richer functionality using technologies such as HTML, CSS, JavaScript, and DOM. These technologies were not developed with enterprise applications in mind and, with increasing pressure from consumers to provide applications with features not fully described or supported by these technologies, developers are looking for alternative solutions or extensions to these standards.

From a combination of both consumer requirements and developer needs, a new breed of Web technologies has evolved. Often referred to as Rich Internet Technologies, these technologies enhance the traditionally static content provided by Web applications.

In the absence of sufficiently powerful standards, new technologies such as Mozilla's XUL, Microsoft's HTC, Java applets, Flex, and OpenLaszlo have emerged to meet the requirements. Such technologies support application-specific extensions to traditional HTML markup while still leveraging the benefits of deploying an application to a central HTTP server. XMLHTTP has also returned under a newly branded name, AJAX (Asynchronous JavaScript and XML). Applications built with these technologies are often referred to as Rich Internet Applications (RIA).

In this article series we are going to focus on AJAX and XUL to illustrate the potential that JSF brings to the JavaEE developer.

AJAX
AJAX has gained momentum primarily due to the XMLHttpRequest browser object, which supports asynchronous communication with any business services used by the Web application. Popular sites such as Google Mail and Google Suggest use AJAX to deliver RIA.

With the XMLHttpRequest object, developers can now send requests to the Web server to retrieve only the data needed and use JavaScript to process the response.This ability to reduce the amount of data transferred between client and Web server reduces the bandwidth to a minimum and saves processing time on the server since most of the processing is done on the client using JavaScript.

It is important to note that the XMLHttpRequest object, although widely used, could at best be called a "de facto" standard, since most browsers, including Firefox, Inter-net Explorer, Opera, and Safari, support it. It is also worth noting that the XMLHttpRequest object is not exclusive to AJAX; potentially the XMLHttpRequest object can be used by any HTML/XML-based Web technology such as XUL or HTC.

Desktop vs Web
Traditional Web applications are in most cases slower than their counterpart desktop applications. With AJAX we can now send requests to the Web server to retrieve only the data needed and use JavaScript to process the response, which leads to a more responsive Web application.

A typical AJAX application leverages standard HTML/XHTML at the presentation layer and uses JavaScript to dynamically change the DOM. This creates an effect of "richness" in the user interface with no dependency on additional browser plug-ins.

Mozilla XUL
XUL (pronounced zool) was created by the Mozilla organization (Mozilla.org) as an open source project in 1998. With XUL, developers can build rich user interfaces that may be deployed either as "thin client" Web applications, locally on a desktop, or as Internet-enabled "thick client" desktop applications. To fully provide the consumer with a rich user interfaces, XUL needs to execute in the Mozilla Gecko Runtime Environment (GRE). There are many examples of applications using XUL, including the Firefox browser, the Thunderbird e-mail client, and numerous plug-ins to each.

XUL Structure
The base idea behind XUL is to provide a markup for building user interfaces much like HTML, and to leverage technologies such as CSS for look-and-feel and JavaScript for events and behavior. There are even APIs available to access files systems and Web services over the network. As an XML language, XUL can be also be used in combination with other XML languages, such as XHTML and SVG.

XUL Components
XUL comes with a base set of components that are available through the Mozilla GRE, and has the added benefit of not needing to download components to display an application in the browser. You can also design your own components with XUL; these will need to be downloaded on request and cached in the browser.

XUL uses a language called XBL (eXtensible Binding Language) to define new components. XBL is also used to bridge the gap between XUL and HTML, making it easy to attach behavior to traditional HTML markup.

Listing 1 is a sample XUL file that embeds XUL elements and standard, name-spaced HTML elements.

XUL Event Handling
Using XUL event handling is not that different from using HTML event handling. The GRE implementation supports DOM Level 2 (and partially DOM Level 3) and is virtually the same for HTML and XUL, and changes to the state and events are propagated through a range of DOM calls. XUL elements come with predefined event handlers, much like the event handlers provided with the standard HTML elements (see Listing 2).

As in HTML, developers can use JavaScript functions located in external libraries, or embedded in the page.

<script type="text/javascript"src="http://
www.bob.org/js/myscript.js">

There is a large set of built-in XUL components, each with its own set of event handler attributes. For example, the XUL window element delivers events such as load and paint.

Creating Custom XUL Components Using XBL
XBL is an XML language that allows developers to extend XUL by adding "custom" components to the extensive set of existing XUL elements. In XUL, developers can define the look and feel using CSS, and they can define the behavior using XBL.

Developers can look at XUL as the "implementation" that comes with a pre-set of components, or tag libraries that can be used to build a user interface, much like the JSF Reference Implementation. XBL is the language developers use to extend XUL components and enable integration with HTML, in much the same way that Java is used to extend JSF components.

XBL Structure
An XBL file contains a set of bindings, each describing the behavior, properties, and methods of an XUL component.

A quick look at Listing 3 shows the root <bindings> element containing two binding elements. A <bindings> element can contain any number of <binding> elements. The namespace in the <bindings> element defines which syntax will be used, and in Listing 3 it is XBL - xmlns=http://www.mozilla.org/xbl. The file also contains a combination of XUL and HTML elements <xul:text/> and <html:input/>. This is extremely useful for simplifying development by encapsulating several components as a single, reusable component. The xbl:inherits attribute allows the text element to inherit values from the bound element by defining a variable called name and assigns it to the value attribute. If no value is defined when the component is used in a page, the text field's value will default to "Guest".

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