What is XML?

XML (Extensible Markup Language) is a markup language designed to store and transport data. It uses custom tags to define data structure and is both human-readable and machine-readable. XML is widely used in enterprise systems, web services, and configuration files.

XML Syntax

XML syntax is strict and well-defined. Here are the key rules:

• All elements must have a closing tag
• Tags are case-sensitive
• Elements must be properly nested
• Attribute values must be quoted
• There must be exactly one root element

Basic XML Example

<?xml version="1.0" encoding="UTF-8"?>
<user id="123" active="true">
  <name>John Doe</name>
  <email>john@example.com</email>
  <age>30</age>
  <address>
    <city>New York</city>
    <country>USA</country>
  </address>
  <hobbies>
    <hobby>reading</hobby>
    <hobby>gaming</hobby>
    <hobby>hiking</hobby>
  </hobbies>
</user>

XML Components

XML Declaration

The optional declaration at the beginning specifies version and encoding:

<?xml version="1.0" encoding="UTF-8"?>

Elements

Elements are the building blocks of XML. They have opening and closing tags:

<element>content</element>

<!-- Empty element (self-closing) -->
<empty />

<!-- Element with child elements -->
<parent>
  <child>value</child>
</parent>

Attributes

Attributes provide additional information about elements:

<book isbn="978-0-123456-78-9" category="fiction">
  <title>The Great Story</title>
  <author>Jane Smith</author>
</book>

Comments

<!-- This is a comment -->
<data>
  <!-- Comments can span
       multiple lines -->
  <value>123</value>
</data>

CDATA Sections

CDATA sections contain text that should not be parsed:

<script>
  <![CDATA[
    function compare(a, b) {
      return a < b && a > 0;
    }
  ]]>
</script>

Common XML Use Cases

1. Configuration Files

<!-- Maven pom.xml -->
<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example</groupId>
  <artifactId>my-app</artifactId>
  <version>1.0.0</version>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.13.2</version>
    </dependency>
  </dependencies>
</project>

2. SOAP Web Services

<?xml version="1.0"?>
<soap:Envelope
  xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetUser xmlns="http://example.com/api">
      <userId>123</userId>
    </GetUser>
  </soap:Body>
</soap:Envelope>

3. RSS Feeds

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0">
  <channel>
    <title>My Blog</title>
    <link>https://example.com</link>
    <item>
      <title>First Post</title>
      <link>https://example.com/post-1</link>
      <pubDate>Mon, 01 Jan 2024 00:00:00 GMT</pubDate>
    </item>
  </channel>
</rss>

XML Validation

XML documents can be validated against schemas to ensure correct structure:

DTD (Document Type Definition)

The older, simpler validation method:

<!DOCTYPE user [
  <!ELEMENT user (name, email)>
  <!ELEMENT name (#PCDATA)>
  <!ELEMENT email (#PCDATA)>
]>

XSD (XML Schema Definition)

The modern, more powerful validation method with data types:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="user">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="name" type="xs:string"/>
        <xs:element name="age" type="xs:integer"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

Common XML Mistakes

Special Characters in XML

Learn more about other formats:

• What is JSON?
• What is YAML?
• JSON vs XML Comparison