XML

• eXtensible Markup Language
• Derived from SGML, which is also the original parent language of HTML.
• XML is a "meta" language, that can be used to defined an unlimited number of custom hierarchical data formats.
• Example:

    <?xml version="1.0" ?>
    <sandwich price="2.00">
      <meat>Pastrami</meat>
      <bread>Rye</bread>
      <cheese slices="1">Swiss</cheese>
      <condiments>
        <condiment>mustard</condiment>
        <condiment>pepper</condiment>
      </condiments>
    </sandwich>
  

• In the example above <sandwich>, <condiment>, and all tags within <> are elements.
• Elements can be nested within each other infinitely deep.
• "price" and "slices" are attributes of their respective elements.
• XML must be well formed to be porcessed:
  • All tags are closed: <bread>Rye</bread>,
    or <bread/> which denotes an empty element.
  • Elements must be closed on the same level they start.
    <sandwich><meat></sandwich></meat> is not well formed.
  • All attributes are within single or double quotes.
• The structure of a particular XML document can be described with Document Type Definitions (DTD) or schemas. Instances of that document can then be validated against the corresponding DTD or schema.
• Why use XML?
  • Flexible hierarchical data formatting language that can easily be read by humans as well as programs.
  • Readily available tools for document creation, parsing, and validation.
  • Plaform and programming language neutral.
• The Java API for XML Processing (JAXP) defines a standard interface for using XML in Java programs. There are several implementations of this API, including Xerces from the Apache Group and Crimson from Sun.