import java.io.StringReader;

import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;

import org.xml.sax.InputSource;

public class XPathTest {

    // This field is set to true as soon as one thread encounters an exception. When this happens, both threads will terminate.
    private static volatile boolean exception;

    private static Object lock = new Object();

    public static void main(String[] args) {
        Thread thread1 = createThread();
        Thread thread2 = createThread();
        thread1.start();
        thread2.start();
        try {
            // wait at most two minutes
            thread1.join(60000);
            thread2.join(60000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private static Thread createThread() {
        String xml = "<test/>";
        return new Thread(new Runnable() {
            @Override
            public void run() {
                while (!exception) {
                    try {
                        XPathFactory xPathFactory;
                        // Synchronize both threads, as XPathFactory.newInstance() is not thread-safe.
                        synchronized (lock) {
                            xPathFactory = XPathFactory.newInstance();
                        }
                        xPathFactory.newXPath().compile("/test").evaluate(new InputSource(new StringReader(xml)));
                    } catch (XPathExpressionException e) {
                        e.printStackTrace();
                        exception = true;
                    }
                }
            }
        });

    }
}

static DocumentBuilderFactory dbf = null;
    static DocumentBuilder db = null;
    static Document d = null;

и далее по тексту:

if ( dbf == null ) {
                dbf = FactoryImpl.getDOMFactory(useServicesMechanism);
                dbf.setNamespaceAware( true );
                dbf.setValidating( false );
            }
            db = dbf.newDocumentBuilder();
            Document document = db.parse( source );

synchronized (GLOBAL_LOCK) {
     XPathFactory.newInstance().newXPath().compile("/test").evaluate(new InputSource(new StringReader(xml)));
}

XPath xPath = null;
     synchronized (GLOBAL_LOCK) {
         xPath = XPathFactory.newInstance().newXPath();
     }
xPath.evaluate("/test", new InputSource(new StringReader(xml)));