XML Formatter

What this tool does

This formatter re-indents XML and checks that it is well-formed before it writes anything. An element whose content is only text stays on one line, so a feed's <title> is not spread across three, while nested elements are indented one level per generation. Comments, CDATA sections, processing instructions and the DOCTYPE are all preserved exactly as written.

When the document is not well-formed you get the line number and a caret under the problem, with both ends of the mismatch named: which tag was opened, on which line, and which tag tried to close it. Minify does the reverse — every byte of whitespace between tags removed, for embedding in a request body.

Common uses

• Reading a SOAP response that arrived as one 12,000-character line, where the fault string is buried somewhere in the middle.
• Checking an RSS or Atom feed before a validator sees it, or after hand-editing an entry.
• Tidying an SVG exported by a design tool so a diff shows the path that actually changed.
• Fixing a Maven pom.xml, an Android layout or a web.config whose parser complains about line 1.
• Minifying an XML payload before pasting it into a test fixture or a shell command.

A short example

This input:

<channel><title>Changelog</title><item><guid>bc-1</guid></item></channel>

becomes:

<channel>
  <title>Changelog</title>
  <item>
    <guid>bc-1</guid>
  </item>
</channel>

Well-formed is not the same as valid

XML uses those two words for different things, and only the first is checked here. Well-formed means the syntax holds together: one root element, every tag closed by a matching tag in the right order, attribute values quoted, and the reserved characters escaped. Valid means the document also obeys a schema — a DTD, an XSD or a RELAX NG grammar. A perfectly well-formed <invoice> is still rejected by an XSD that requires a <total> child.

Three things break well-formedness more often than anything else:

• A bare ampersand. & must be written & — which is why URLs with query parameters so often break a feed.
• An unescaped < in text content, usually from pasting code without a CDATA section.
• A namespace prefix that was never declared, such as soap:Body in a fragment copied out of its envelope. The prefix is fine syntactically but a namespace-aware parser will refuse it.

Worth knowing

Indenting changes whitespace, and in XML whitespace inside an element is character data that belongs to the document. For SOAP, RSS, config and most data XML nothing depends on it. It matters in mixed content, where text and elements are siblings — reformatting <p>see <b>this</b> page</p> can add spaces a renderer shows — and anywhere xml:space="preserve" is set. Never reformat a signed document: XML Signature hashes a canonical form of the bytes, and reindenting invalidates it.