What is XML?
.xml · application/xml
XML (Extensible Markup Language) is a flexible, self-describing markup language for structured data used in web services, configuration, and document formats.
Overview
XML (Extensible Markup Language) is a markup language designed to store and transport data in a format that is both human-readable and machine-readable. Unlike HTML, XML has no predefined tags — users define their own tags to describe data structure. XML is the foundation for many other formats including XHTML, SVG, RSS, SOAP, and Office Open XML (DOCX, XLSX). It supports schemas for validation, namespaces for avoiding conflicts, and XSLT for transformations.
History
XML was developed by a working group of the World Wide Web Consortium (W3C) and became a W3C Recommendation in February 1998. It was derived from SGML (Standard Generalized Markup Language), simplified for use on the web. XML 1.0 has been revised multiple times, with the fifth edition published in 2008. While JSON has replaced XML for many web API use cases, XML remains dominant in enterprise systems, document formats, and configuration.
File Structure
An XML document starts with an optional XML declaration (<?xml version="1.0"?>), followed by a single root element. Elements can contain text, attributes, child elements, or be empty. Elements must be properly nested and closed. Attributes provide metadata on elements. CDATA sections allow unescaped text. Comments use <!-- --> syntax. Namespaces prevent naming conflicts in combined documents.
Common Use Cases
- Enterprise web services (SOAP)
- Configuration files (Maven, Spring, Android)
- Document formats (DOCX, XLSX, SVG, RSS)
- Data interchange in enterprise systems
- Structured document storage
- Feed syndication (RSS, Atom)
- Schema-validated data exchange
- Transformation pipelines (XSLT)
Advantages
- Self-describing with meaningful tag names
- Strong validation with XSD schemas
- Namespace support for complex documents
- Mature ecosystem of tools and standards
- XSLT enables powerful transformations
- Widely supported in enterprise systems
Disadvantages
- Verbose syntax with high overhead
- Slower to parse than JSON
- Complex specification and tooling
- Larger file sizes than equivalent JSON
- Steep learning curve for advanced features
- Declining use in modern web APIs
Frequently Asked Questions
What is an XML file?
An XML (Extensible Markup Language) file is a text file that uses custom tags to describe the structure and meaning of data. It's used for configuration files, web services, document formats, and data exchange between systems.
What is the difference between XML and HTML?
HTML has predefined tags for displaying content in browsers, while XML uses custom tags to describe data structure. HTML is lenient with syntax; XML requires strict well-formedness. XML is for data transport; HTML is for data presentation.
Is XML still used?
Yes, XML is widely used in enterprise systems (SOAP, Spring), document formats (DOCX, SVG), configuration (Maven, Android), and data interchange. While JSON dominates web APIs, XML remains essential in many domains.
How do I validate an XML file?
XML can be validated for well-formedness (proper syntax) using any XML parser, or against a schema (XSD, DTD, RelaxNG) for structural validation. Online validators, IDE plugins, and command-line tools like xmllint can validate XML.
Technical Details
- Extension
- .xml
- MIME Type
- application/xml
- Magic Bytes
- <?xml (typical, not required)
- Encoding
- UTF-8 (default), UTF-16, others via declaration
- Compression
- None (use gzip externally)
- Specification
- W3C XML 1.0 (Fifth Edition)
- Max Size
- No specification limit