Extensible Markup Language

Extensible Markup Language
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Basics

What is it?
- A subset of the Standard Generalized Markup Language (SGML)
- A meta-language (i.e., it is used to define other languages)
Comparison with HTML:
- HTML is an application of SGML/XML (i.e, a language defined using SGML/XML)

Our View of XML in this Course

HTML is a way of "marking up" papers/articles/books
XML is a way of "marking up" hierarchical data (and papers/articles/books each contain hierarchical data)

XML Elements without Components

Syntax:
- < tag [ attribute =" value " ] ... />
An Example:
- <Caboose id="857-931" type="limited" />

XML Elements with Components

Syntax:
- < tag [ attribute =" value " ] ... > [ component ... ] </ tag >
An Example:
- <Employee job="Engineer">John Smith</Employee>
Note:
- Tags and attributes are case-sensitive

Attribute or Element?

In General:
- It depends on how the information will need to be processed
A Rule of Thumb:
- Use attributes for all types that have literals except for large blocks of text
- Use elements for everything else (avoiding text nodes where possible)

Well-Formed XML Documents

Must only contain elements that have a start tag and a close tag (or must use the abbreviated syntax for empty elements)
Must have a single root element
Must have the XML declaration <?xml version="1.0" ?> as the first line

An Example

A Gradebook

xmlexamples/basics/grades-1977.xml

<?xml version="1.0"?>
<GRADES>
  <STUDENT>
     <NAME>Kim Maschino</NAME>
     <ID>321-00-0021</ID>
     <GRADE>A+</GRADE>
  </STUDENT>
  <STUDENT>
     <NAME>Doug Sideler</NAME>
     <ID>001-31-9998</ID>
     <GRADE>B</GRADE>
  </STUDENT>
  <STUDENT>
     <NAME>Wayne Tromer</NAME>
     <ID>100-23-4167</ID>
     <GRADE>C</GRADE>
  </STUDENT>
</GRADES>

A Potential Problem

Imagine that you had developed an accounts payable database that contained the following:

xmlexamples/namespaces/accounts-payable.xml

<?xml version="1.0"?>
<payments>
  <payment>
    <amount>21.35</amount>
    <check bank="157-1" >150</check>
  </payment>
</payments>

Also, imagine someone at another organization developed a testing database that included the following:

xmlexamples/namespaces/testing.xml

<?xml version="1.0"?>
<testing>
  <validation>
    <check location="store">
      <checker>Mary Jones</checker>
    </check>
  </validation>
</testing>

What problem would arise if you wanted to combine the two systems so that payments could be validated?

The two systems use both use a "check" tag but use it in different ways.

A Potential Problem (cont.)

Namespaces:
- Can be used by developers to ensure that tags and attributes are unique
- Are implemented using the xmlns attribute
Types:
- Unqualified - applies to the element that has the xmlns attribute and all of its children
- Qualified - applied to a particular element

An Example of an Unqualified Namespace

All of the Tags are Within the Namespace

xmlexamples/namespaces/example-unqualified.xml

<?xml version="1.0"?>
<syllabus xmlns="http://www.cs.jmu.edu/users/bernstdh/">
  <header>
    <coursename>Introduction to XML</coursename>
    <coursenumber>CS000</coursenumber>
  </header>
  <body>
    <title>Course Overview</title>
  </body>
</syllabus>

An Example of a Qualified Namespace

The Namespace does not Apply to the coursename and coursenumber Elements

xmlexamples/namespaces/example-qualified.xml

<?xml version="1.0"?>
<syllabus xmlns:dhb="http://www.cs.jmu.edu/users/bernstdh/">
  <dhb:header>
    <coursename>Introduction to XML</coursename>
    <coursenumber>CS000</coursenumber>
  </dhb:header>
  <dhb:body>
    <dhb:title>Course Overview</dhb:title>
  </dhb:body>
</dhb:syllabus>