Introduction to XML

Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format which is both human-readable and machine-readable

 

Example:

<?xml version="1.0" encoding="UTF-8"?>
<Employee>
  <id>001</id>
  <name>Jacob</name>
  <age>31</age>
  <salary>1000000</salary>
</Employee>

 

Important points about XML

  1. XML is both human-readable and machine-readable

  2. XML was designed to store and transport data.

  3. XML is a W3C Recommendation

  4. XML is similar to HTML, but they are different

    • XML focus on structure of data, while HTML focus on displaying data

    • HTML has predefined tags, but XML has no predefined tags.

    • XML has a strict syntax compared to HTML.

  5. An XML document is a string of characters. Almost every legal Unicode character may appear in an XML document.

    • Unicode is a computing industry standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems.

    •  The latest version of Unicode contains a repertoire of more than 120,000 characters.

    • The standard consists of a set of code charts for visual reference, an encoding method and set of standard character encodings, a set of reference data files, and a number of related items

      • UTF-8 is the default character encoding for XML documents.

 

Syntax summary

  • The characters making up an XML document are divided into markup and content.

    • Generally, strings that constitute markup either begin with the character < and end with a >, or they begin with the character & and end with a ;.

    • Strings of characters that are not markup are content. 

    • E.g. In the above example, id is part of markup while 001 is content.

  • A markup construct (tags) that begins with < and ends with >.

    • Tags come in three flavors:

      • start-tags

        • e.g. <section>

      • end-tags

        • e.g. </section>

      • empty-element tags

        • e.g. <line-break />

  • A logical document component which either begins with a start-tag and ends with a matching end-tag is called as a element.

    • May consist only of an empty-element tag. 

  • A start-tag or an empty-element tag, may contain key/value pairs of data called as attributes.

    • <Employee id ="001">

  • XML documents may have a description about itself called as prolog.

    • E.g. <?xml version="1.0" encoding="UTF-8"?>

    • If it exists, it must come first in the document.

    • UTF-8 is the default character encoding for XML documents.

  • XML has strict set of rules

    • All XML Elements Must Have a Closing Tag

    • XML Tags are Case Sensitive

    • XML Elements Must be Properly Nested

    • XML Attribute Values Must be Quoted

    • Some characters have a special meaning in XML (e.g. <). They should be replaced with an entity reference.

      • E.g. You can replace the "<" character with an entity reference: &lt;

 

Please go throgh the attached reference links to learn more about XML.

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