XML Parsing: DOM and SAX

What is XML Parsing?

XML parsing is the process of reading an XML document and making its data available to a program. Two primary parsing models:

1. DOM (Document Object Model) – load entire document into memory as a tree
2. SAX (Simple API for XML) – event-driven, processes XML sequentially

DOM Parsing

DOM parsing loads the entire XML into memory as a tree structure.

DOM Parsing in JavaScript (Browser)

const xmlString = `<?xml version="1.0"?>
<students>
  <student id="1">
    <name>Alice</name>
    <marks>92</marks>
  </student>
  <student id="2">
    <name>Bob</name>
    <marks>85</marks>
  </student>
</students>`;

const parser = new DOMParser();
const xmlDoc = parser.parseFromString(xmlString, "application/xml");

// Access elements
const students = xmlDoc.getElementsByTagName("student");
for (let i = 0; i < students.length; i++) {
    const name = students[i].getElementsByTagName("name")[0].textContent;
    const marks = students[i].getElementsByTagName("marks")[0].textContent;
    const id = students[i].getAttribute("id");
    console.log(`ID: ${id}, Name: ${name}, Marks: ${marks}`);
}

DOM Parsing in Java

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document doc = builder.parse(new File("students.xml"));
doc.getDocumentElement().normalize();

NodeList nodeList = doc.getElementsByTagName("student");
for (int i = 0; i < nodeList.getLength(); i++) {
    Node node = nodeList.item(i);
    if (node.getNodeType() == Node.ELEMENT_NODE) {
        Element element = (Element) node;
        System.out.println(element.getElementsByTagName("name")
                                  .item(0).getTextContent());
    }
}

SAX Parsing

SAX is event-driven and does not load the entire document into memory.

// SAX Handler in Java
public class StudentHandler extends DefaultHandler {
    boolean inName = false;

    @Override
    public void startElement(String uri, String localName,
                             String qName, Attributes attributes) {
        if (qName.equalsIgnoreCase("name")) {
            inName = true;
        }
        if (qName.equalsIgnoreCase("student")) {
            System.out.println("ID: " + attributes.getValue("id"));
        }
    }

    @Override
    public void characters(char[] ch, int start, int length) {
        if (inName) {
            System.out.println("Name: " + new String(ch, start, length));
            inName = false;
        }
    }
}

DOM vs SAX Comparison

When to Use Which

• Use DOM when you need to navigate, search, or modify XML freely
• Use SAX when processing very large XML files where memory is a concern

Key Takeaway: DOM parsing creates a full in-memory tree (easy to navigate), while SAX parsing is event-driven and memory-efficient. Choose based on file size and whether you need random access to the XML structure.