A versatile markup language for storing and transporting structured data
XML (Extensible Markup Language) is a versatile markup language designed to store and transport data. Developed by the World Wide Web Consortium (W3C), XML provides a text-based format for representing structured information that is both human-readable and machine-readable.
Unlike HTML, which focuses on displaying data, XML is designed for carrying data with a focus on what that data represents. It allows users to define their own custom tags and document structures, making it highly adaptable to different types of information and industries.
Since its recommendation by the W3C in 1998, XML has become a fundamental technology for data exchange on the web and between different systems and applications. It serves as the foundation for numerous other formats and protocols, including RSS, SOAP, XHTML, and many industry-specific standards.
XML documents consist of elements defined by tags (similar to HTML but customizable), attributes that provide additional information about elements, and content contained within element tags. All XML documents must have a single root element that contains all other elements, creating a hierarchical structure. XML strictly enforces syntax rules like proper nesting and closing of tags, making it more rigorous than HTML.
XML excels as a format for exchanging data between different systems, particularly in enterprise environments. Its platform independence and self-describing nature make it ideal for integration scenarios where different applications, potentially using different technologies, need to communicate structured information reliably.
Many applications and frameworks use XML for configuration files due to its hierarchical structure and ability to represent complex relationships. From web servers (Apache, Tomcat) to build tools (Maven, Ant) to application frameworks (Spring), XML configuration files are widespread in software development.
XML serves as the foundation for numerous document formats including DOCX (Microsoft Word), ODF (OpenDocument), SVG (graphics), and DITA (technical documentation). These formats leverage XML's ability to represent structured content with metadata while enabling transformation for different presentation contexts.
XML is fundamental to many web service protocols including SOAP, XML-RPC, and various REST implementations. While newer services often use JSON, XML remains important in enterprise environments and legacy systems, particularly where formal contracts (WSDL) and validation are required.
Numerous industries have developed XML-based standards for specialized data exchange. Examples include HL7 in healthcare, FpML in financial services, NIEM in government information exchange, and UBL in e-commerce. These standards leverage XML's extensibility and validation capabilities to ensure reliable data interchange.
XML enjoys broad support across programming languages:
Many applications can work with XML files:
XML works across all major platforms:
XML has a rich ecosystem of related technologies:
| Feature | XML | JSON | YAML | HTML | CSV |
|---|---|---|---|---|---|
| Hierarchical Data | |||||
| File Size Efficiency | |||||
| Human Readability | |||||
| Validation Support | |||||
| Ease of Parsing | |||||
| Mixed Content Support |
XML excels in representing complex, hierarchical data with strong validation capabilities and support for mixed content, but it's more verbose and complex to parse than JSON. JSON offers better parsing performance and smaller file sizes, making it preferable for web APIs. YAML provides the best human readability but with less formal validation. HTML is specialized for web presentation, while CSV is optimal for simple tabular data but limited for complex structures.
Converting JSON to XML is straightforward since both are hierarchical formats. Use specialized conversion tools or libraries available in most programming languages. Be aware that JSON doesn't have concepts like attributes or namespaces, so you'll need to decide how to represent these in the resulting XML. Also consider how to handle JSON arrays, which can be represented in XML either as repeated elements or with numeric attribute identifiers.
When converting tabular data to XML, first determine the appropriate hierarchical structure. Simple approaches map each row to an element and each column to a nested element or attribute. More complex mappings might group related data into nested structures. For Excel files with multiple sheets, consider representing each sheet as a separate section in the XML hierarchy.
Many database systems offer direct XML export capabilities. When designing the XML structure, consider whether to map tables directly to elements or create a more semantic representation of the data model. Handling relationships between tables requires decisions about nesting versus referencing. For large datasets, consider streaming approaches to manage memory usage during conversion.
XML to JSON conversion requires decisions about how to handle XML-specific features. XML attributes can be prefixed (e.g., "@name") or placed in a separate attributes object. Text content might be represented as a special property like "#text". Namespaces typically get dropped or simplified. Use established conventions or libraries that implement them, such as the BadgerFish or Parker conventions.
For converting XML to HTML, XSLT (Extensible Stylesheet Language Transformations) is the most powerful approach. XSLT stylesheets can transform XML into HTML with complete control over the output structure. Alternatively, for simple conversions, DOM manipulation in a programming language can be used to transform the XML tree into an HTML document.
Converting hierarchical XML to flat CSV requires decisions about which elements to include and how to represent nesting. For complex XML, you may need multiple CSV files to represent different sections of the hierarchy. Conversion tools typically require configuration to specify the mapping between XML paths and CSV columns.