Veamos cómo se valida un documento XML.

import javax.xml.parsers.*;
import org.xml.sax.*;
import org.xml.sax.ext.LexicalHandler;
import org.xml.sax.helpers.DefaultHandler;

public class Validate {

    public Validate(String filename) {
        StringBuffer buff = new StringBuffer();

        try {
            File f = new File(".", filename);
            StringBuffer errorBuff = new StringBuffer();
            InputSource input = new InputSource(new FileInputStream(f));

            // Set systemID so parser can find the dtd with a relative URL in the source document.
            input.setSystemId(f.toString());
            SAXParserFactory spfact = SAXParserFactory.newInstance();

            spfact.setValidating(true);
            spfact.setNamespaceAware(true);

            SAXParser parser = spfact.newSAXParser();
            XMLReader reader = parser.getXMLReader();

            //Instantiate inner-class error and lexical handler.
            Handler handler = new Handler(filename, errorBuff);
            reader.setProperty("http://xml.org/sax/properties/lexical-handler ", handler);
            parser.parse(input, handler);

            if (handler.containsDTD && !handler.errorOrWarning) { // valid
                buff.append("VALID " + filename +"\n");
            }
            else if (handler.containsDTD) { // not valid
                buff.append ("NOT VALID " + filename + "\n");
                buff.append(errorBuff.toString());
            }
            else buff.append("NO DOCTYPE DECLARATION " + filename + "\n");
        }
        catch (Exception e) {
            buff.append("NOT WELL-FORMED " + filename + ". " + e.getMessage() + "\n");
        }

        System.out.print(buff.toString());
    }

    class Handler extends DefaultHandler implements LexicalHandler {
        boolean errorOrWarning;
        boolean containsDTD;
        String sourceFile;
        StringBuffer errorBuff;

        Handler(String sourceFile, StringBuffer errorBuff) {
            super();
            this.sourceFile = sourceFile;
            this.errorBuff = errorBuff;
            errorOrWarning = false;
            containsDTD = false;
        }

        public void error(SAXParseException exc) {
            errorBuff.append(sourceFile + " Error: " + exc.getMessage()+ "\n");
            errorOrWarning = true;
        }

        public void warning(SAXParseException exc) {
            errorBuff.append(sourceFile + " Warning:" + exc.getMessage()+ "\n");
            errorOrWarning = true;
        }

        public void startDTD (String name, String publicId, String systemId) throws SAXException {
            containsDTD = true;
        }

        public void endDTD () throws SAXException {}
        public void startEntity (String name) throws SAXException {}
        public void endEntity (String name) throws SAXException {}
        public void startCDATA () throws SAXException {}
        public void endCDATA () throws SAXException {}
        public void comment (char ch[], int start, int length) throws SAXException {}
    }

    public static void main(String[] args) {
        Validate v = new Validate("birds.xml");
    }
}