XML Introduction — XML Tutorial

What is XML?

XML (eXtensible Markup Language) is a text format for structured data. You invent the tags that describe your data, nest them into a tree, and any XML parser in any language can read the result:

<?xml version="1.0" encoding="UTF-8"?>
<order id="1042">
  <customer>Ada Lovelace</customer>
  <item sku="TEA-01" qty="2">Green tea</item>
  <item sku="MUG-07" qty="1">Camel mug</item>
</order>

XML became a W3C standard in 1998 and is still everywhere, often out of sight:

Documents: Word (.docx), Excel (.xlsx) and PowerPoint files are zipped XML; so are EPUB books and SVG images.
Configuration: Maven pom.xml, Android manifests, .NET project files, Spring and many enterprise tools.
Integration: SOAP web services, banking (ISO 20022 payment messages), healthcare (HL7 CDA), e-invoicing (UBL), government and publishing (DocBook, DITA, JATS).
The web: RSS and Atom feeds, sitemaps (sitemap.xml) and XHTML.

XML vs HTML vs JSON

	XML	HTML	JSON
Purpose	Describe any data or document	Display web pages	Exchange data between programs
Tags	You define them	Fixed set (`<p>`, `<div>`)	No tags: objects and arrays
Strictness	Must be well-formed or parsing fails	Browsers repair mistakes	Must be valid JSON
Schemas	DTD, XSD, RELAX NG, Schematron	No	JSON Schema
Query and transform	XPath, XSLT, XQuery	CSS selectors	JSONPath, jq
Mixed content (text with markup)	Yes	Yes	Awkward

Reading XML in code

The same task in three languages: parse the order above and list its items. Python uses lxml, PHP its built-in SimpleXML, Java the DOM parser that ships with the JDK. Switch tabs to compare; Python and PHP run in your browser.

Read an orderTry it yourself

from lxml import etree

xml = b"""<order id="1042">
  <customer>Ada Lovelace</customer>
  <item sku="TEA-01" qty="2">Green tea</item>
  <item sku="MUG-07" qty="1">Camel mug</item>
</order>"""

order = etree.fromstring(xml)
print("Order", order.get("id"), "for", order.findtext("customer"))
for item in order.findall("item"):
    print(f"- {item.get('qty')} x {item.text} ({item.get('sku')})")

Output

Order 1042 for Ada Lovelace
- 2 x Green tea (TEA-01)
- 1 x Camel mug (MUG-07)

Read an orderTry it yourself

<?php
$xml = '<order id="1042">
  <customer>Ada Lovelace</customer>
  <item sku="TEA-01" qty="2">Green tea</item>
  <item sku="MUG-07" qty="1">Camel mug</item>
</order>';

$order = simplexml_load_string($xml);
echo "Order {$order['id']} for {$order->customer}\n";
foreach ($order->item as $item) {
    echo "- {$item['qty']} x {$item} ({$item['sku']})\n";
}

Output

Order 1042 for Ada Lovelace
- 2 x Green tea (TEA-01)
- 1 x Camel mug (MUG-07)

Read an order

import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.*;
import java.io.ByteArrayInputStream;

public class Main {
    public static void main(String[] args) throws Exception {
        String xml = """
            <order id="1042">
              <customer>Ada Lovelace</customer>
              <item sku="TEA-01" qty="2">Green tea</item>
              <item sku="MUG-07" qty="1">Camel mug</item>
            </order>""";
        Element order = DocumentBuilderFactory.newInstance().newDocumentBuilder()
            .parse(new ByteArrayInputStream(xml.getBytes("UTF-8"))).getDocumentElement();
        String customer = order.getElementsByTagName("customer").item(0).getTextContent();
        System.out.println("Order " + order.getAttribute("id") + " for " + customer);
        NodeList items = order.getElementsByTagName("item");
        for (int i = 0; i < items.getLength(); i++) {
            Element item = (Element) items.item(i);
            System.out.println("- " + item.getAttribute("qty") + " x " + item.getTextContent()
                + " (" + item.getAttribute("sku") + ")");
        }
    }
}

import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.*;
import java.io.ByteArrayInputStream;

public class Main {
    public static void main(String[] args) throws Exception {
        String xml = """
            <order id="1042">
              <customer>Ada Lovelace</customer>
              <item sku="TEA-01" qty="2">Green tea</item>
              <item sku="MUG-07" qty="1">Camel mug</item>
            </order>""";
        Element order = DocumentBuilderFactory.newInstance().newDocumentBuilder()
            .parse(new ByteArrayInputStream(xml.getBytes("UTF-8"))).getDocumentElement();
        String customer = order.getElementsByTagName("customer").item(0).getTextContent();
        System.out.println("Order " + order.getAttribute("id") + " for " + customer);
        NodeList items = order.getElementsByTagName("item");
        for (int i = 0; i < items.getLength(); i++) {
            Element item = (Element) items.item(i);
            System.out.println("- " + item.getAttribute("qty") + " x " + item.getTextContent()
                + " (" + item.getAttribute("sku") + ")");
        }
    }
}

Output

Order 1042 for Ada Lovelace
- 2 x Green tea (TEA-01)
- 1 x Camel mug (MUG-07)

Run this on your computer — see “Get Started” for setup.

JavaScript in the browser has XML built in too (DOMParser); the XML in JavaScript page has live examples.

What this tutorial covers

Basics: syntax, elements and attributes, well-formedness, special characters, namespaces.
Validation: DTDs, XML Schema (XSD), RELAX NG and Schematron.
Querying and transforming: XPath, XSLT and XQuery.
XML in code: Python, PHP, Java and JavaScript; DOM vs streaming; security (XXE).
XML in the real world: SOAP, feeds and sitemaps, SVG and Office files, and when to choose XML, JSON or YAML.

What is XML?

XML vs HTML vs JSON

Reading XML in code

What this tutorial covers

Test yourself