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XML 101
To understand XML, it's helpful to compare and contrast it with another technology with which a great many of us are familiar - HyperText Markup Language (HTML).
If you've used or read about HTML, you know that it was created so that users could format and display information on the Web. HTML uses a fixed and finite set of tags, elements, and attributes that allow it to communicate to a user's browser how that browser should display the document. We see HTML everywhere, and it has for some time served as the lingua franca for displaying information on the Web. It is a proven technology that well serves its purpose in most scenarios. What if, however, the current version of HTML doesn't allow me to do something that I want to do? I have two choices: I can either write my own browser that understands my tags (bad idea) or I can put my project on hold for a year or so and hope that the next version of HTML includes the functionality that I need (even worse idea). Try selling either of these options to your boss or client and see if you still have a job by the time you end your discussion. So concludes our one-paragraph, in-depth investigation of HTML.
Now, if I may lapse into my days of standardized test taking, HTML is to displaying information as XML is to defining information. They both are text-based, and they both consist of tags, elements, and attributes. Unlike HTML, however, XML allows users to structure and define the information in their documents. While technically it is a markup language (it allows you to use tags to "mark-up" the contents of your document), it more appropriately is a meta-language. By meta-language, I mean that it allows users to create their own collection of tags, elements, and attributes as needed and in so doing to accurately describe the physical contents of a document. Unlike HTML with its finite collection of tags, XML allows users to create their own to meet their own requirements (thus the eXtensibility).
I've made several references to tags, elements, and attributes. These are the core building blocks of an XML document. Consider the following HTML fragment. It should be painfully familiar to anyone who's ever looked at an HTML document and will prove useful in understanding XML syntax.
Here is the first group of text< d>
Here is the second group of text< d>
< r>
< able>
This document contains a table element (" ") with a table row element (" "). The table row element, in turn contains two table cell elements (" "). Each of these elements has both an opening tag (" ") and a closing tag ("< able>"). While this is fairly straightforward, it also is somewhat inflexible. What if, for example, I need to create a document that describes my company's employee roster for the Annual InfoStrat softball tournament? With XML, it's as easy as replacing the element and attribute names from the previous HTML document with my own custom tags that describe my company and its employees. Here is what such a document might look like:
<?xml version="1.0"?>
With this XML document, I have defined my company and two of its employees and have described the relationship between company (parent) and employees (children). I have shown that my company has two employees, but I easily could add new employee elements to reflect new hires that we bring on to ensure that we don't lose this year's tournament:
After creating my XML document, I can display its contents in my format of choice. The same XML document could easily be displayed as HTML, a Microsoft Word document, an Adobe .pdf file, or even as text in the body of an e-mail message. As long as the XML document is well formed (meaning that it follows the appropriate XML format and syntax), you can choose your method of preference (or necessity) for displaying its content.
Let's dissect the pieces of my company roster XML document to see each piece's role and responsibility.
Header:
The header tells the document's user that this is an XML document - using version 1.0 of the XML specification in this case.
<?xml version="1.0"?>
Tags (brackets, greater than, less than):
Just like in HTML, you use greater than (">") and less than ("<") signs called tags to indicate the opening and closing of an element.
<?xml version="1.0"?>
Elements:
Elements are the basic building blocks of XML. They may contain text, comments, or other elements, and consist of a start tag and an end tag. Typically, XML elements are akin to nouns in the real world. They represent people, places, or things.
<?xml version="1.0"?>
Note that in XML, every opening element (i.e. "
Attributes:
Where elements represent the nouns contained in an XML document, attributes represent the adjectives that describe the elements. The following document tells me that Hank Aaron's id is "1" and that Babe Ruth's is "2". This helps to describe these two employees.
<?xml version="1.0"?>
Note that in order to be well formed, all attribute values must be contained within quotation marks. id="1" is correct, while id=1 is not acceptable. This is a marked difference from standard HTML formatting that places much looser restrictions on what is acceptable.
Text/Content:
Elements contain contents that give critical information about them. This information represents that entity itself in an XML document. In the following document, Hank Aaron is the employee; Babe Ruth is the employee.
<?xml version="1.0"?>
As you can see, XML and HTML are practically identical with the exception that XML is far less lenient when it comes to case-sensitivity, using closing tags, and properly nesting parent/child elements. This is excellent news for Web developers everywhere as it ensures that if you write well-formed HTML, you'll find the transition to XML virtually seamless.
To summarize, XML is a text-based meta-language that uses tags, elements, and attributes to add structure and definition to documents. It is similar to HTML in syntax and implementation, but different with regard to functionality. Where HTML allows users to control how documents are displayed, XML allows them to describe the actual contents of the documents. It is a markup language because it uses tags to mark-up documents and it is a meta-language because it uses these tags to give structure to documents that it in turn uses as a means of communication. XML is extensible because it enables users to create their own collection of tags (unlike HTML).
Now, why should you care about XML? If for now other reason, consider that the World Wide Web Consortium (W3C), the Internet's governing body, is considering a proposal to rewrite the HTML 4 language in XML 1.0. As of the time this article was written, XHTML had received endorsement by the director of the W3C as a recommendation. This proposal, known as XHTML will require well formedness in all HTML documents. The W3C is a neutral standards body responsible for defining the future of the Internet. They do not support every new idea that comes along, and we should view their full support of XML (or any technology), as a harbinger of where tomorrow's Internet will take us. Ignore XML if you will, but know that it is most definitely a legitimate technology that will revolutionize the way that we program applications for the Web
About the Author
Jeff Jones
For more information on XHTML, XML, and the W3C, check out the W3C website at http://www.w3c.org.