private static synchronized DocumentBuilder getDocumentBuilder() {
        try {
            return factory.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            log.log(Level.WARNING, "No XML parser available - " + e);
            return null;
        }
    }
Creates a new XML document builder.

@return a new DocumentBuilder instance, or null if we fail to get one.
config-application-package/src/main/java/com/yahoo/config/application/Xml.java