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This article introduces a way to handle XML documents that goes beyond the standard Java APIs for manipulating XML. We'll see that in many cases XPath and XSLT provide simpler, more elegant ways of solving application problems. In some simple samples, we will compare a pure Java/XML solution with one that utilizes XPath and/or XSLT.
Both XSLT and XPath are part of the Extensible Stylesheet Language (XSL) specification (see Resources). XSL consists of three parts: the XSL language specification itself, XSL Transformations (XSLT), and XML Path Language (XPath). XSL is a language for transforming XML documents; it includes a definition -- Formatting Objects -- of how XML documents can be formatted for presentation. XSLT specifies a vocabulary for transforming one XML document into another. You can consider XSLT to be XSL minus Formatting Objects. The XPath language addresses specific parts of XML documents and is intended to be used from within an XSLT stylesheet.
For the purposes of this article, it is assumed that you are familiar with the basics of XML and XSLT, as well as the DOM APIs. (For information and tutorials on these topics, see Resources.)
Note: This article's code samples were compiled and tested with the Apache Xerces XML parser and the Apache Xalan XSL processor (see Resources).
Many articles and papers that deal with XML state that it is the perfect vehicle to accomplish a good design practice in Web programming: the Model-View-Controller pattern (MVC), or, in simpler terms, the separation of application data from presentation data. If the application data is formatted in XML, it can easily be bound -- typically in a servlet or Java ServerPage -- to, say, HTML templates by using an XSL stylesheet.
But XML can do much more than merely help with model-view separation for an application's frontend. We currently observe more and more widespread use of components (for example, components developed using the EJB standard) that can be used to assemble applications, thus enhancing developer productivity. Component reusability can be improved by formatting the data that components deal with in a standard way. Indeed, we can expect to see more and more published components that use XML to describe their interfaces.