XML (eXtensible Markup Language) is a text format for storing and transporting structured data. Despite JSON's dominance in modern APIs, XML remains essential in enterprise systems, SOAP web services, configuration files (Maven, Android resources, Spring), document formats (Word's .docx, Excel's .xlsx), RSS feeds, SVG graphics, and many other contexts. Knowing how to work with XML is a necessary skill in most development environments.
What Well-Formed XML Requires
XML has strict well-formedness rules. Every opening tag must have a matching closing tag. Tags are case-sensitive (so Item and item are different elements). Attribute values must be quoted. Special characters within text content must be escaped as HTML entities (the less-than sign as <, the ampersand as &, etc.). There must be exactly one root element.
A well-formed XML document passes the parser's syntax check. A valid XML document additionally conforms to a schema (either a DTD or an XSD schema) that defines the allowed structure and content.
Common XML Errors
Unclosed tags are the most common error in hand-written XML. Forgetting to close a tag or mistyping a closing tag name causes the parser to fail.
Unescaped special characters in text content break the parser because the less-than sign is interpreted as the start of a new tag.
Multiple root elements cause a parse error because XML requires a single root element that contains everything else.
Encoding mismatches occur when the XML declaration specifies UTF-8 but the file is saved in a different encoding, causing the parser to misinterpret non-ASCII characters.
Using the DevHexLab XML Formatter
Open the tool at /tools/developer/xml-formatter. Paste your XML or upload an .xml file. The tool validates the XML and reports any syntax errors with the line number. If the XML is valid, it produces a pretty-printed version with consistent indentation. Copy or download the result.
Frequently Asked Questions
What is the difference between XML and HTML?
HTML has predefined tags and is lenient about unclosed tags (the browser tries to fix malformed HTML). XML allows custom tags and is strict: an unclosed tag is a fatal error. XHTML is a strict version of HTML that follows XML rules.
What is SOAP and why does it use XML?
SOAP (Simple Object Access Protocol) is a protocol for web services that uses XML envelopes for request and response messages. It predates REST and is still widely used in enterprise and legacy systems. SOAP messages are verbose XML documents with defined namespaces and structure.
Validate and format XML before using it in any integration.