What is XML?

XML (Extensible Markup Language) is a markup language designed for storing and transmitting data. It allows defining custom tags and document structure.

Key features:

• Extensibility: You can create your own tags, adapting XML for any data.
• Structured: Data is organized in a hierarchical tree of elements.
• Clarity: XML has a strict syntax. A malformed document will not be read.
• Readability: The format is understandable by both computers and humans.
• Platform independence: XML documents can be processed on various operating systems and software platforms.

Example of a simple XML structure:

<!-- Book information -->
<bookstore>
  <book category="fiction">
    <title lang="en">The Lord of the Rings</title>
    <author>J.R.R. Tolkien</author>
    <year>1954</year>
    <price>29.99</price>
  </book>
  <book category="programming">
    <title lang="en">Effective Java</title>
    <author>Joshua Bloch</author>
    <year>2018</year>
    <price>45.00</price>
  </book>
</bookstore>

XML is widely used for:

• Configuration files (e.g., in Java Spring, Maven).
• Data exchange between different systems (SOAP, REST in the past).
• Data storage (e.g., in some databases or file systems).
• Document formats (DocBook, Atom).

For parsing and working with XML in Java, several APIs are available:

• DOM (Document Object Model): Loads the entire XML document into memory as a tree of objects. Convenient for navigation and modification, but can consume a lot of memory for large files.
• SAX (Simple API for XML): Event-driven parser. Processes the document sequentially, calling handler methods upon certain events (e.g., start/end of an element). Memory-efficient for large documents but more complex for navigation.
• StAX (Streaming API for XML): Combines features of DOM and SAX. Allows sequential processing of the document with more control over the process, enabling access to the current element.
• JAXB (Java Architecture for XML Binding): Converts Java objects to XML and vice versa (marshalling/unmarshalling). Simplifies working with XML by providing an object-oriented representation.

Currently, JSON is often preferred for data exchange due to its more compact syntax, but XML remains an important format, especially in enterprise systems and for configurations.