DOM Parser in Java: Reading & Writing XML Trees

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

public class DomReadExample {

    public static void main(String[] args) throws Exception {
        String xml = """
                <library>
                  <book id="1" genre="fiction">
                    <title>Clean Code</title>
                    <author>Robert C. Martin</author>
                    <year>2008</year>
                  </book>
                  <book id="2" genre="technical">
                    <title>Effective Java</title>
                    <author>Joshua Bloch</author>
                    <year>2018</year>
                  </book>
                </library>
                """;

        // 1. Obtain a DocumentBuilder (thread-unsafe — create one per thread or synchronise)
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        factory.setNamespaceAware(true);          // enable namespace support
        DocumentBuilder builder = factory.newDocumentBuilder();

        // 2. Parse from string
        Document doc = builder.parse(new ByteArrayInputStream(xml.getBytes()));

        // 3. Normalise text nodes (merges adjacent text nodes)
        doc.getDocumentElement().normalize();

        // 4. Read the root element
        Element root = doc.getDocumentElement();
        System.out.println("Root: " + root.getTagName());   // library

        // 5. Iterate child elements
        NodeList books = root.getElementsByTagName("book");
        for (int i = 0; i < books.getLength(); i++) {
            Element book = (Element) books.item(i);
            String id    = book.getAttribute("id");
            String title = book.getElementsByTagName("title").item(0).getTextContent();
            String author = book.getElementsByTagName("author").item(0).getTextContent();
            System.out.printf("Book %s: %s by %s%n", id, title, author);
        }
    }
}

Element root = doc.getDocumentElement();

// Get direct children (includes text nodes with whitespace!)
NodeList children = root.getChildNodes();
for (int i = 0; i < children.getLength(); i++) {
    Node child = children.item(i);
    if (child.getNodeType() == Node.ELEMENT_NODE) {
        Element el = (Element) child;
        System.out.println("Tag: " + el.getTagName());
    }
}

// Get all descendants with a tag name
NodeList allBooks = doc.getElementsByTagName("book");

// Navigate up
Node parent = someNode.getParentNode();

// Navigate siblings
Node next = someNode.getNextSibling();
Node prev = someNode.getPreviousSibling();

// Read attributes
NamedNodeMap attrs = element.getAttributes();
for (int i = 0; i < attrs.getLength(); i++) {
    Attr attr = (Attr) attrs.item(i);
    System.out.println(attr.getName() + " = " + attr.getValue());
}

// Text content of a node (includes all descendant text)
String text = element.getTextContent();

// Read a specific attribute
String genre = element.getAttribute("genre");
boolean hasAttr = element.hasAttribute("id");

import javax.xml.parsers.*;
import javax.xml.transform.*;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import org.w3c.dom.*;

public class DomWriteExample {

    public static void main(String[] args) throws Exception {
        DocumentBuilder builder = DocumentBuilderFactory.newInstance()
                .newDocumentBuilder();
        Document doc = builder.newDocument();

        // Create root element
        Element root = doc.createElement("library");
        doc.appendChild(root);

        // Helper to create a book element
        Element book1 = createBook(doc, "1", "fiction", "Clean Code", "Robert C. Martin", "2008");
        Element book2 = createBook(doc, "2", "technical", "Effective Java", "Joshua Bloch", "2018");

        root.appendChild(book1);
        root.appendChild(book2);

        // Serialise to string
        Transformer tf = TransformerFactory.newInstance().newTransformer();
        tf.setOutputProperty(OutputKeys.INDENT, "yes");
        tf.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");
        tf.setOutputProperty(OutputKeys.ENCODING, "UTF-8");

        StringWriter sw = new StringWriter();
        tf.transform(new DOMSource(doc), new StreamResult(sw));
        System.out.println(sw);
    }

    private static Element createBook(Document doc, String id, String genre,
                                       String title, String author, String year) {
        Element book = doc.createElement("book");
        book.setAttribute("id", id);
        book.setAttribute("genre", genre);

        Element titleEl = doc.createElement("title");
        titleEl.setTextContent(title);
        book.appendChild(titleEl);

        Element authorEl = doc.createElement("author");
        authorEl.setTextContent(author);
        book.appendChild(authorEl);

        Element yearEl = doc.createElement("year");
        yearEl.setTextContent(year);
        book.appendChild(yearEl);

        return book;
    }
}

Document doc = builder.parse(xmlFile);

// Add a new child element
Element root   = doc.getDocumentElement();
Element newBook = doc.createElement("book");
newBook.setAttribute("id", "3");
newBook.setTextContent("New Book");
root.appendChild(newBook);

// Modify an existing element's text
NodeList titles = doc.getElementsByTagName("title");
titles.item(0).setTextContent("Updated Title");

// Add/change an attribute
Element book = (Element) doc.getElementsByTagName("book").item(0);
book.setAttribute("available", "true");

// Remove a node
Node toRemove = doc.getElementsByTagName("book").item(1);
toRemove.getParentNode().removeChild(toRemove);

// Replace a node
Element replacement = doc.createElement("book");
replacement.setTextContent("Replacement");
root.replaceChild(replacement, root.getFirstChild());

// Clone a node (deep = true clones children too)
Node clone = book.cloneNode(true);
root.appendChild(clone);

// Enable namespace awareness BEFORE creating the builder
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setNamespaceAware(true);
DocumentBuilder builder = factory.newDocumentBuilder();

Document doc = builder.parse(xmlFile);

// Access namespace-aware elements
NodeList items = doc.getElementsByTagNameNS(
        "http://example.com/books", "book");

Element el = (Element) items.item(0);
System.out.println("Local name : " + el.getLocalName());    // book
System.out.println("Namespace  : " + el.getNamespaceURI()); // http://example.com/books
System.out.println("Prefix     : " + el.getPrefix());       // e.g., "bk"

// Create namespace-aware elements
Element nsEl = doc.createElementNS("http://example.com/books", "bk:book");
nsEl.setAttributeNS("http://example.com/books", "bk:id", "42");

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

// Disable DOCTYPE declarations entirely (most restrictive)
factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);

// Or selectively disable external entities
factory.setFeature("http://xml.org/sax/features/external-general-entities", false);
factory.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
factory.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);
factory.setXIncludeAware(false);
factory.setExpandEntityReferences(false);

DocumentBuilder builder = factory.newDocumentBuilder();