Read and write xml with whitespace in an attribute.

WriteXml.java

import org.w3c.dom.Document;
import org.w3c.dom.Element;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamReader;
import javax.xml.stream.XMLStreamWriter;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.Writer;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;

/**
 * See <a href="https://stackoverflow.com/q/79614120">Why do the stax and the dom api write xml attributes with newlines differently?</a>
 * <p>
 * The XOM parts require this dependency:
 *
 * <pre>
 * &lt;dependency&gt;
 *   &lt;groupId&gt;xom&lt;/groupId&gt;
 *   &lt;artifactId&gt;xom&lt;/artifactId&gt;
 *   &lt;version&gt;1.3.9&lt;/version&gt;
 *   &lt;exclusions&gt;
 *     &lt;exclusion&gt;
 *       &lt;groupId&gt;xerces&lt;/groupId&gt;
 *       &lt;artifactId&gt;xercesImpl&lt;/artifactId&gt;
 *     &lt;/exclusion&gt;
 *   &lt;/exclusions&gt;
 * &lt;/dependency&gt;
 * </pre>
 */
public class WriteXml {

    public static void main(String[] args) throws Exception {

        // returns "foo SPACE bar" because writeWithStax literally writes the newline
        Path staxFile = writeWithStax("foo\nbar");
        System.out.println("staxFile = " + Files.readString(staxFile));
        System.out.println("attribute read with stax = " + readWithStax(staxFile));
        System.out.println("attribute read with dom = " + readWithDom(staxFile));
//        System.out.println("attribute read with xom = " + readWithXom(staxFile));
        System.out.println();

        // returns "foo NEWLINE bar" because writeWithDom escapes the newline to "&#10;"
        Path domFile = writeWithDom("foo\nbar");
        System.out.println("domFile = " + Files.readString(domFile));
        System.out.println("attribute read with stax = " + readWithStax(domFile));
        System.out.println("attribute read with dom = " + readWithDom(domFile));
//        System.out.println("attribute read with xom = " + readWithXom(domFile));
        System.out.println();

//        // returns "foo NEWLINE bar" because writeWithXom escapes the newline to "&#0A;"
//        Path xomFile = writeWithXom("foo\nbar");
//        System.out.println("xomFile = " + Files.readString(xomFile));
//        System.out.println("attribute read with stax = " + readWithStax(xomFile));
//        System.out.println("attribute read with dom  = " + readWithDom(xomFile));
//        System.out.println("attribute read with xom  = " + readWithXom(xomFile));
//        System.out.println();

    }

//    private static Path writeWithStax(String text) throws Exception {
//        Path file = Files.createTempFile("stax", ".xml");
//        try (OutputStream out = Files.newOutputStream(file)) {
//            XMLOutputFactory factory = XMLOutputFactory.newInstance();
//            XMLStreamWriter writer = factory.createXMLStreamWriter(out);
//            writer.writeStartDocument();
//            writer.writeStartElement("element");
//            writer.writeAttribute("attribute", text);
//            writer.writeEndElement();
//            writer.writeEndDocument();
//            writer.flush();
//            writer.close();
//        }
//        return file;
//    }

    private static Path writeWithStax(String text) throws Exception {
        Path file = Files.createTempFile("stax", ".xml");
        try (Writer out = Files.newBufferedWriter(file, StandardCharsets.UTF_8)) {
            XMLOutputFactory factory = XMLOutputFactory.newInstance();
            XMLStreamWriter writer = factory.createXMLStreamWriter(out);
            writer.writeStartDocument();
            writer.writeStartElement("element");

            // hack to preserve whitespace
            writeAttributePreservingWhitespace(out, writer, "attribute", text);

            writer.writeEndElement();
            writer.writeEndDocument();
            writer.flush();
            writer.close();
        }
        return file;
    }

    private static void writeAttributePreservingWhitespace(Writer out, XMLStreamWriter writer, String name, String value) throws Exception {
        boolean preserveWhitespace = true;
        // see https://www.w3.org/TR/xml/#AVNormalize
        if (value.contains("\t") || value.contains("\n") || value.contains("\r")) {
            writer.flush();
            out.write(" ");
            writeXMLContent(out, name);
            out.write("=\"");
            writeXMLContent(out, value);
            out.write("\"");
        } else {
            writer.writeAttribute(name, value);
        }
    }

    private static void writeXMLContent(Writer out, String text) throws Exception {
        for (char ch : text.toCharArray()) {
            switch (ch) {
                case '<' -> out.write("&lt;");
                case '&' -> out.write("&amp;");
                case '>' -> out.write("&gt;");
                case '"' -> out.write("&quot;");
                case '\t' -> out.write("&#x9;");
                case '\n' -> out.write("&#xA;");
                case '\r' -> out.write("&#xD;");
                default -> out.write(ch);
            }
        }
    }

    private static Path writeWithDom(String text) throws Exception {
        Path file = Files.createTempFile("dom", ".xml");
        try (OutputStream out = Files.newOutputStream(file)) {
            DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
            DocumentBuilder builder = factory.newDocumentBuilder();
            Document document = builder.newDocument();
            Element warningElement = document.createElement("element");
            warningElement.setAttribute("attribute", text);
            document.appendChild(warningElement);
            TransformerFactory transformerFactory = TransformerFactory.newInstance();
            Transformer transformer = transformerFactory.newTransformer();
            transformer.transform(new DOMSource(document), new StreamResult(out));
        }
        return file;
    }

//    private static Path writeWithXom(String text) throws Exception {
//        Path file = Files.createTempFile("xom", ".xml");
//        var element = new nu.xom.Element("element");
//        var attribute = new nu.xom.Attribute("attribute", text);
//        element.addAttribute(attribute);
//        var doc = new nu.xom.Document(element);
//        String result = doc.toXML();
//        Files.writeString(file, result);
//        return file;
//    }

    private static String readWithStax(Path file) throws Exception {
        try (InputStream in = Files.newInputStream(file)) {
            XMLInputFactory factory = XMLInputFactory.newInstance();
            XMLStreamReader reader = factory.createXMLStreamReader(in);
            while (reader.hasNext()) {
                int event = reader.next();
                if (event == XMLStreamReader.START_ELEMENT
                    && "element".equals(reader.getLocalName())) {
                    return reader.getAttributeValue(null, "attribute");
                }
            }
        }
        return null;
    }

    private static String readWithDom(Path file) throws Exception {
        try (InputStream in = Files.newInputStream(file)) {
            DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
            DocumentBuilder builder = factory.newDocumentBuilder();
            Document document = builder.parse(in);
            return document.getDocumentElement()
                    .getAttribute("attribute");
        }
    }

//    private static String readWithXom(Path file) throws Exception {
//        try (InputStream in = Files.newInputStream(file)) {
//            var parser = new nu.xom.Builder();
//            var document = parser.build(in);
//            return document.getRootElement()
//                    .getAttribute("attribute")
//                    .getValue();
//        }
//    }

}