Java™ Application Development on Linux - Dator

9.4 A Sample Ant Buildfile221standards, such as W3C’s XML Schema. In either case, the DTD or schemaspecify what tags may be used in the markup, where they may exist with respectto one another, what attributes tags may have, and how many times a given tagmay appear in a given place. A document can thus be validated—that is,checked against the corresponding DTD or schema. It’s not necessary, however;in many situations, documents can also be used without validation so long asthey are well-formed—that is, conform to the basic syntax of XML.HTML, with which even nonprogrammers are familiar, is an instance ofa markup language defined in terms of SGML (and XHTML is its reformulationin terms of XML). This book itself was written in Docbook, which isanother SGML markup language.So, if SGML is such a wonder, why is XML all the rage? Well, SGML isone of those standards that attempt to “subsume the world.” SGML has verycomplex and flexible syntax, with many different ways to represent a simplemarkup construct. Thus, to completely implement an SGML parser is difficult.Recognizing that 90% of the complexity of SGML is needed in only about 1%of cases, the designers of XML realized that they could make a markup specificationlanguage only 10% as complicated that would cover 99% of cases (ofcourse, like 85% of statistics, we’re making these numbers up, but you getthe point).Implementing an XML parser, while not exactly trivial, is much easierthan implementing an SGML parser.SGML/DSSSL and XML/XSLT are efforts to make the transformationand presentation of hierarchical data easier and more standardized. If whatyou have read here is all that you know about XML (or SGML), you shouldcertainly consider getting yourself a book on these important standards.For now, we can say that XML consists of tags which are set off from datacontent by the familiar less-than and greater-than brackets we are used to seeingin HTML:Just as in HTML, the tags may have start tag and end tag forms:Sample XML tagged dataThe entire construct, including the pair of matching tags and everythinginside them, is called an element. The start tags may also, like in HTML, carrydata inside them in the form of attributes: