Newsletter sign-up

View all newsletters

Sign up for our technology specific newsletters.

Enterprise Java

Email Address:

Programming XML in Java, Part 3

DOMination: Take control of structured documents with the Document Object Model

By Mark Johnson, JavaWorld.com, 07/07/00

The Simple API for XML (SAX) is an excellent interface for many XML applications. It is intuitive, extremely easy to learn, and, as its name implies, simple. Any Java programmer can, in just an hour or two, learn to use and develop an application using SAX. It is especially useful in situations where the data in an XML file is already in a form that is structurally similar to the desired output. For instance, the recipe example in Part 2 of this series formatted Recipe XML into an HTML representation of a recipe page and a shopping list. The structure of the output HTML was very similar to the structure of the input XML. The ingredients in the Recipe XML were grouped together in an <Ingredients> element; the ingredients in the output HTML were grouped together in an unordered list (<ul>). The tags were somewhat different, but the basic structure was the same.

TEXTBOX:

TEXTBOX_HEAD: Programming XML in Java: Read the whole series!

• Part 1. Use the Simple API for XML (SAX) to process XML in Java easily
• Part 2. Learn about SAX and XML validation through illustrative examples
• Part 3. DOMination: Take control of structured documents with the Document Object Model

:END_TEXTBOX

In real data-processing situations, however, the structure of the input data often differs greatly from the eventual output structure. Since SAX passes SAX events to a programmer-defined handler in the order in which they appear in the input XML, as the programmer you are responsible for any data restructuring or reordering. Also, if the same data is to be used in more than one place in the output, you must either perform multiple passes over the XML or arrange for the handler to "remember" that data while producing output. One example of this was the recipe title in Part 2, which the handler maintained in an internal variable for use both in the browser title bar and in the Webpage.

For tasks of low and intermediate complexity, SAX works just fine. As an application's complexity (and functionality) increases, however, the SAX handler code can become extremely difficult to understand. SAX code can spend most of its time storing information from the input in an internal form usable for producing the desired output. When using SAX, you are generally responsible for creating an internal object model of your application's information.

DOM to the rescue

The Document Object Model, or DOM, is a standardized object model for XML documents. DOM is a set of interfaces describing an abstract structure for an XML document. Programs that access document structures through the DOM interface can arbitrarily insert, delete, and rearrange the nodes of an XML document programmatically.

DOM and SAX parsers work in different ways. A SAX parser processes the XML document as it parses the XML input stream, passing SAX events to a programmer-defined handler method. A DOM parser, on the other hand, parses the entire input XML stream and returns a Document object. Document is the programmatic, language-neutral interface that represents a document. The Document returned by the DOM parser has an API that lets you manipulate a (virtual) tree of Node objects; this tree represents the structure of the input XML. Figures 1 and 2 illustrate this difference between the APIs.

 

About Us | Advertise | Contact Us | Terms of Service/Privacy

Copyright, 2006-2008 Network World, Inc. All rights reserved.