Sams Teach Yourself XML in 21 Days

By Steven Holzner

• Table of Contents
  • About the Author
  • Acknowledgments
  • We Want to Hear from You!
  • Introduction
  • Part I: At a Glance
  • Day 1. Welcome to XML
  • Day 2. Creating XML Documents
  • Day 3. Creating Well-Formed XML Documents
  • Day 4. Creating Valid XML Documents: DTDs
    • All About DTDs
    • Validating a Document by Using a DTD
    • Creating Element Content Models
    • Commenting a DTD
    • Supporting External DTDs
    • Handling Namespaces in DTDs
    • Summary
    • Q&A
    • Workshop
  • Declaring Attributes in DTDs
  • Day 6. Creating Valid XML Documents: XML Schemas
  • Day 7. Creating Types in XML Schemas
  • Part I. In Review
  • Day 8. Formatting XML by Using Cascading Style Sheets
  • Day 9. Formatting XML by Using XSLT
  • Day 10. Working with XSL Formatting Objects
  • Part II. In Review
  • Part III: At a Glance
  • Day 11. Extending HTML with XHTML
  • Day 12. Putting XHTML to Work
  • Day 13. Creating Graphics and Multimedia: SVG and SMIL
  • Day 14. Handling XLinks, XPointers, and XForms
  • Part III. In Review
  • Part IV: At a Glance
  • Day 15. Using JavaScript and XML
  • Day 16. Using Java and .NET: DOM
  • Day 17. Using Java and .NET: SAX
  • Day 18. Working with SOAP and RDF
  • Part IV. In Review
  • Part V: At a Glance
  • Day 19. Handling XML Data Binding
  • Day 20. Working with XML and Databases
  • Day 21. Handling XML in .NET
  • Part V. In Review
  • Appendix A. Quiz Answers

Commenting a DTD

DTDs can get long and involved in complex XML documents. You can use standard XML comments in DTDs, just as you can throughout the body of an XML document, to clarify what you're doing. Here's an example of comments in a DTD:

<!DOCTYPE document [
<!--Create the document element first -->
<!ELEMENT document (employee)*>
<!--Each employee needs a name, hire date, and projects -->
<!ELEMENT employee (name, hiredate, projects)>
<!-- Should we reverse the order of these two? -->
<!ELEMENT name (lastname, firstname)>
<!ELEMENT lastname (#PCDATA)>
<!ELEMENT firstname (#PCDATA)>
<!--Use standard date format for the hiredate element -->
<!ELEMENT hiredate (#PCDATA)>
<!ELEMENT projects (project)*>
<!ELEMENT project (product, id, price)>
<!--Product name should match catalog 8547382 -->
<!ELEMENT product (#PCDATA)>
<!ELEMENT id (#PCDATA)>
<!ELEMENT price (#PCDATA)>
]>

As in any standard XML document, comments in a DTD are stripped out of the DTD by the XML processor. Some XML processors pass those comments on to us, but some don't.

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