What is XML?

XML stands for eXtensible Markup Language. It is a markup language designed to store and transport data in a human-readable, self-describing format. Unlike HTML, XML has no predefined tags — you define your own tags to describe the structure and meaning of your data. XML is a W3C standard and forms the backbone of countless file formats and enterprise systems.

History

XML was developed by the XML Working Group of the World Wide Web Consortium (W3C) as a simplified subset of SGML (Standard Generalized Markup Language). It became a W3C Recommendation on 10 February 1998. In the early 2000s XML was the dominant format for web data exchange and web services (SOAP). Although JSON has taken over for most REST APIs, XML remains essential in document formats (DOCX, SVG, RSS), enterprise middleware, and configuration-heavy ecosystems like Java's Maven.

Basic Syntax

An XML document begins with an optional declaration that specifies the version and character encoding. The rest of the document is a tree of elements. Every element has an opening tag, optional content or child elements, and a closing tag. Elements can carry attributes inside the opening tag.

<?xml version="1.0" encoding="UTF-8"?>
<library>
  <book id="1" available="true">
    <title>Clean Code</title>
    <author>Robert C. Martin</author>
    <year>2008</year>
  </book>
  <book id="2" available="false">
    <title>The Pragmatic Programmer</title>
    <author>Andrew Hunt</author>
    <year>1999</year>
  </book>
</library>

Core Concepts

• Declaration — <?xml version="1.0" encoding="UTF-8"?> — optional but recommended; specifies XML version and encoding
• Element — The basic building block. Every element has an opening tag <tag> and a closing tag </tag>. Empty elements can be self-closing: <br />
• Root element — Every XML document must have exactly one root element that contains all other elements
• Attribute — Name-value pairs inside an opening tag: <book id="1">
• Text content — The text between an element's opening and closing tags
• Comment — <!-- This is a comment -->
• CDATA section — Wraps raw text that should not be parsed: <![CDATA[ <raw> content ]]>
• Namespace — Prevents name conflicts when combining XML from different sources: xmlns:ns="http://example.com"

Well-Formed vs Valid XML

An XML document is well-formed when it follows all syntax rules: exactly one root element, all tags properly closed and nested, attributes quoted, and special characters escaped. A well-formed document can be parsed by any XML parser.

A document is also valid when it conforms to a schema — either a DTD (Document Type Definition) or an XSD (XML Schema Definition). Schemas define which elements and attributes are allowed, their data types, and their order.

Special Characters and Entities

Five characters have special meaning in XML and must be escaped when used inside text content or attribute values:

• & — & (ampersand)
• < — < (less-than)
• > — > (greater-than)
• " — " (double quote, in attributes)
• ' — ' (apostrophe, in attributes)

Common Use Cases

• Document formats — Microsoft Office (DOCX, XLSX, PPTX), OpenDocument (ODT), SVG images and RSS/Atom feeds are all XML-based
• Configuration files — Java's Maven (pom.xml), Android layouts, Spring Framework beans, Ant build scripts
• Web services — SOAP (Simple Object Access Protocol) uses XML for request and response messages
• Enterprise data exchange — EDI, banking, healthcare (HL7 FHIR), and government systems still rely heavily on XML
• Sitemap and feeds — sitemap.xml for search engines, RSS feeds for content syndication

XML vs JSON

JSON has largely replaced XML for REST APIs, but XML retains advantages in specific areas:

• JSON advantages — Shorter syntax, easier to read, natively supported in JavaScript, faster to parse, simpler tooling
• XML advantages — Supports attributes and mixed content, has namespaces, allows comments, has mature schema languages (XSD/DTD), and is self-documenting through tags

Choose JSON for web APIs and application data. Choose XML when you need document-centric formats, schemas, SOAP services, or when integrating with enterprise systems that mandate XML.

Rules to Remember

• XML is case-sensitive — <Book> and <book> are different elements
• Every opening tag must have a matching closing tag (or be self-closing)
• Elements must be properly nested — overlapping tags are not allowed
• Attribute values must always be quoted (single or double quotes)
• There can only be one root element
• The five special characters (&, <, >, ", ') must be escaped