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EJB & JSP: Java On The Edge, Unlimited Edition by Lou Marco ISBN: 0764548026 Your Guide to Cutting-Edge J2EE Programming Techniques. Appendix D: XML Overview Throughout this book, you ve read about JSP elements and EJB files coded in XML syntax. Although the examples of such elements and files presented in the book convey the essential flavor of XML syntax, a more thorough presentation is called for. Thus, the purpose of this appendix is to present the essentials of XML syntax. This appendix provides an overview of XML, or Extensible Markup Language, a universal document format for structuring data for presentation on the Web. The appendix starts with an overview of XML features that overcome existing problems with HTML. Next, an extremely simple XML document is provided along with a discussion of XML document components. The important XML terms, well-formed documents and valid documents, are covered, as are XML Document Type Definitions (DTDs). Finally, a brief description of related technologies wraps up this appendix. XML Features XML does not have a fixed set of markup tags, overcoming HTML s greatest deficiency, according to some experts. XML is not a markup language per se; XML is a meta-markup language that enables document authors to define their own tags. As a result, authors can create markup languages peculiar to their particular industries, and XML document authors can use this markup language to encode data in industry-specific terminology. XML requires document authors to follow certain rules in creating what is known as well-formed XML documents. If these rules are not followed, the XML document is useless. This XML specification prohibits XML tools from trying to fix problems with the document. The intent is to stop the browser madness prevalent in HTML, in which different browsers attempt to fix broken HTML and, of course, parse and display this HTML differently. For example, an HTML document author can write HTML with missing end tags, which the major browsers parse and display. Such foolishness cannot fly with XML; if an XML document is broken, the document cannot be rendered. Therefore, an XML author can confidently create XML documents, knowing that these documents are parsed identically with different pieces of compliant software. XML stresses the separation of data content from data presentation. Over time, HTML has blurred the distinction between organizing document content and displaying the content. A typical HTML document has tags that describe relationships among document content (such as