/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *     https://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package org.apache.commons.configuration2;
  
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.Reader;
  import java.io.StringReader;
  import java.io.StringWriter;
  import java.io.Writer;
  import java.net.URL;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.Iterator;
  import java.util.Map;
  
  import javax.xml.parsers.DocumentBuilder;
  import javax.xml.parsers.DocumentBuilderFactory;
  import javax.xml.parsers.ParserConfigurationException;
  import javax.xml.transform.OutputKeys;
  import javax.xml.transform.Result;
  import javax.xml.transform.Source;
  import javax.xml.transform.Transformer;
  import javax.xml.transform.dom.DOMSource;
  import javax.xml.transform.stream.StreamResult;
  
  import org.apache.commons.configuration2.convert.ListDelimiterHandler;
  import org.apache.commons.configuration2.ex.ConfigurationException;
  import org.apache.commons.configuration2.io.ConfigurationLogger;
  import org.apache.commons.configuration2.io.FileLocator;
  import org.apache.commons.configuration2.io.FileLocatorAware;
  import org.apache.commons.configuration2.io.InputStreamSupport;
  import org.apache.commons.configuration2.resolver.DefaultEntityResolver;
  import org.apache.commons.configuration2.tree.ImmutableNode;
  import org.apache.commons.configuration2.tree.NodeTreeWalker;
  import org.apache.commons.configuration2.tree.ReferenceNodeHandler;
  import org.apache.commons.lang3.StringUtils;
  import org.apache.commons.lang3.mutable.MutableObject;
  import org.w3c.dom.Attr;
  import org.w3c.dom.CDATASection;
  import org.w3c.dom.Document;
  import org.w3c.dom.Element;
  import org.w3c.dom.NamedNodeMap;
  import org.w3c.dom.Node;
  import org.w3c.dom.NodeList;
  import org.w3c.dom.Text;
  import org.xml.sax.EntityResolver;
  import org.xml.sax.InputSource;
  import org.xml.sax.SAXException;
  import org.xml.sax.SAXParseException;
  import org.xml.sax.helpers.DefaultHandler;
  
  /**
   * <p>
   * A specialized hierarchical configuration class that is able to parse XML documents.
   * </p>
   * <p>
   * The parsed document will be stored keeping its structure. The class also tries to preserve as much information from
   * the loaded XML document as possible, including comments and processing instructions. These will be contained in
   * documents created by the {@code save()} methods, too.
   * </p>
   * <p>
   * Like other file based configuration classes this class maintains the name and path to the loaded configuration file.
   * These properties can be altered using several setter methods, but they are not modified by {@code save()} and
   * {@code load()} methods. If XML documents contain relative paths to other documents (for example to a DTD), these references
   * are resolved based on the path set for this configuration.
   * </p>
   * <p>
   * By inheriting from {@link AbstractConfiguration} this class provides some extended functionality, for example interpolation
   * of property values. Like in {@link PropertiesConfiguration} property values can contain delimiter characters (the
   * comma ',' per default) and are then split into multiple values. This works for XML attributes and text content of
   * elements as well. The delimiter can be escaped by a backslash. As an example consider the following XML fragment:
   * </p>
   *
   * <pre>
   * &lt;config&gt;
   *   &lt;array&gt;10,20,30,40&lt;/array&gt;
   *   &lt;scalar&gt;3\,1415&lt;/scalar&gt;
   *   &lt;cite text="To be or not to be\, this is the question!"/&gt;
   * &lt;/config&gt;
   * </pre>
   *
  * <p>
  * Here the content of the {@code array} element will be split at the commas, so the {@code array} key will be assigned
  * 4 values. In the {@code scalar} property and the {@code text} attribute of the {@code cite} element the comma is
  * escaped, so that no splitting is performed.
  * </p>
  * <p>
  * The configuration API allows setting multiple values for a single attribute, for example something like the following is
  * legal (assuming that the default expression engine is used):
  * </p>
  *
  * <pre>
  * XMLConfiguration config = new XMLConfiguration();
  * config.addProperty(&quot;test.dir[@name]&quot;, &quot;C:\\Temp\\&quot;);
  * config.addProperty(&quot;test.dir[@name]&quot;, &quot;D:\\Data\\&quot;);
  * </pre>
  *
  * <p>
  * However, in XML such a constellation is not supported; an attribute can appear only once for a single element.
  * Therefore, an attempt to save a configuration which violates this condition will throw an exception.
  * </p>
  * <p>
  * Like other {@code Configuration} implementations, {@code XMLConfiguration} uses a {@link ListDelimiterHandler} object
  * for controlling list split operations. Per default, a list delimiter handler object is set which disables this
  * feature. XML has a built-in support for complex structures including list properties; therefore, list splitting is
  * not that relevant for this configuration type. Nevertheless, by setting an alternative {@code ListDelimiterHandler}
  * implementation, this feature can be enabled. It works as for any other concrete {@code Configuration} implementation.
  * </p>
  * <p>
  * Whitespace in the content of XML documents is trimmed per default. In most cases this is desired. However, sometimes
  * whitespace is indeed important and should be treated as part of the value of a property as in the following example:
  * </p>
  *
  * <pre>
  *   &lt;indent&gt;    &lt;/indent&gt;
  * </pre>
  *
  * <p>
  * Per default the spaces in the {@code indent} element will be trimmed resulting in an empty element. To tell
  * {@code XMLConfiguration} that spaces are relevant the {@code xml:space} attribute can be used, which is defined in
  * the <a href="https://www.w3.org/TR/REC-xml/#sec-white-space">XML specification</a>. This will look as follows:
  * </p>
  *
  * <pre>
  *   &lt;indent <strong>xml:space=&quot;preserve&quot;</strong>&gt;    &lt;/indent&gt;
  * </pre>
  *
  * <p>
  * The value of the {@code indent} property will now contain the spaces.
  * </p>
  * <p>
  * {@code XMLConfiguration} implements the {@link FileBasedConfiguration} interface and thus can be used together with a
  * file-based builder to load XML configuration files from various sources like files, URLs, or streams.
  * </p>
  * <p>
  * Like other {@code Configuration} implementations, this class uses a {@code Synchronizer} object to control concurrent
  * access. By choosing a suitable implementation of the {@code Synchronizer} interface, an instance can be made
  * thread-safe or not. Note that access to most of the properties typically set through a builder is not protected by
  * the {@code Synchronizer}. The intended usage is that these properties are set once at construction time through the
  * builder and after that remain constant. If you wish to change such properties during life time of an instance, you
  * have to use the {@code lock()} and {@code unlock()} methods manually to ensure that other threads see your changes.
  * </p>
  * <p>
  * More information about the basic functionality supported by {@code XMLConfiguration} can be found at the user's guide
  * at <a href="https://commons.apache.org/proper/commons-configuration/userguide/howto_basicfeatures.html"> Basic
  * features and AbstractConfiguration</a>. There is also a separate chapter dealing with
  * <a href="commons.apache.org/proper/commons-configuration/userguide/howto_xml.html"> XML Configurations</a> in
  * special.
  * </p>
  *
  * @since 1.0
  */
 public class XMLConfiguration extends BaseHierarchicalConfiguration implements FileBasedConfiguration, FileLocatorAware, InputStreamSupport {
 
     /**
      * A concrete {@code BuilderVisitor} that constructs XML documents.
      */
     static class XMLBuilderVisitor extends BuilderVisitor {
 
         /**
          * Removes all attributes of the given element.
          *
          * @param elem the element.
          */
         private static void clearAttributes(final Element elem) {
             final NamedNodeMap attributes = elem.getAttributes();
             for (int i = 0; i < attributes.getLength(); i++) {
                 elem.removeAttribute(attributes.item(i).getNodeName());
             }
         }
 
         /**
          * Returns the only text node of an element for update. This method is called when the element's text changes. Then all
          * text nodes except for the first are removed. A reference to the first is returned or <strong>null</strong> if there is no text
          * node at all.
          *
          * @param elem the element.
          * @return the first and only text node.
          */
         private static Text findTextNodeForUpdate(final Element elem) {
             Text result = null;
             // Find all Text nodes
             final NodeList children = elem.getChildNodes();
             final Collection<Node> textNodes = new ArrayList<>();
             for (int i = 0; i < children.getLength(); i++) {
                 final Node nd = children.item(i);
                 if (nd instanceof Text) {
                     if (result == null) {
                         result = (Text) nd;
                     } else {
                         textNodes.add(nd);
                     }
                 }
             }
 
             // We don't want CDATAs
             if (result instanceof CDATASection) {
                 textNodes.add(result);
                 result = null;
             }
 
             // Remove all but the first Text node
             textNodes.forEach(elem::removeChild);
             return result;
         }
 
         /**
          * Helper method for updating the values of all attributes of the specified node.
          *
          * @param node the affected node.
          * @param elem the element that is associated with this node.
          */
         private static void updateAttributes(final ImmutableNode node, final Element elem) {
             if (node != null && elem != null) {
                 clearAttributes(elem);
                 node.getAttributes().forEach((k, v) -> {
                     if (v != null) {
                         elem.setAttribute(k, v.toString());
                     }
                 });
             }
         }
 
         /** Stores the document to be constructed. */
         private final Document document;
 
         /** The element mapping. */
         private final Map<Node, Node> elementMapping;
 
         /** A mapping for the references for new nodes. */
         private final Map<ImmutableNode, Element> newElements;
 
         /** Stores the list delimiter handler . */
         private final ListDelimiterHandler listDelimiterHandler;
 
         /**
          * Creates a new instance of {@code XMLBuilderVisitor}.
          *
          * @param docHelper the document helper
          * @param handler the delimiter handler for properties with multiple values
          */
         public XMLBuilderVisitor(final XMLDocumentHelper docHelper, final ListDelimiterHandler handler) {
             document = docHelper.getDocument();
             elementMapping = docHelper.getElementMapping();
             listDelimiterHandler = handler;
             newElements = new HashMap<>();
         }
 
         /**
          * Helper method for accessing the element of the specified node.
          *
          * @param node the node.
          * @param refHandler the {@code ReferenceNodeHandler}.
          * @return the element of this node.
          */
         private Element getElement(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
             final Element elementNew = newElements.get(node);
             if (elementNew != null) {
                 return elementNew;
             }
 
             // special treatment for root node of the hierarchy
             final Object reference = refHandler.getReference(node);
             final Node element;
             if (reference instanceof XMLDocumentHelper) {
                 element = ((XMLDocumentHelper) reference).getDocument().getDocumentElement();
             } else if (reference instanceof XMLListReference) {
                 element = ((XMLListReference) reference).getElement();
             } else {
                 element = (Node) reference;
             }
             return element != null ? (Element) elementMapping.get(element) : document.getDocumentElement();
         }
 
         /**
          * Updates the current XML document regarding removed nodes. The elements associated with removed nodes are removed from
          * the document.
          *
          * @param refHandler the {@code ReferenceNodeHandler}.
          */
         public void handleRemovedNodes(final ReferenceNodeHandler refHandler) {
             refHandler.removedReferences().stream().filter(Node.class::isInstance).forEach(ref -> removeReference(elementMapping.get(ref)));
         }
 
         /**
          * {@inheritDoc} This implementation ensures that the correct XML element is created and inserted between the given
          * siblings.
          */
         @Override
         protected void insert(final ImmutableNode newNode, final ImmutableNode parent, final ImmutableNode sibling1, final ImmutableNode sibling2,
             final ReferenceNodeHandler refHandler) {
             if (XMLListReference.isListNode(newNode, refHandler)) {
                 return;
             }
 
             final Element elem = document.createElement(newNode.getNodeName());
             newElements.put(newNode, elem);
             updateAttributes(newNode, elem);
             if (newNode.getValue() != null) {
                 final String txt = String.valueOf(listDelimiterHandler.escape(newNode.getValue(), ListDelimiterHandler.NOOP_TRANSFORMER));
                 elem.appendChild(document.createTextNode(txt));
             }
             if (sibling2 == null) {
                 getElement(parent, refHandler).appendChild(elem);
             } else if (sibling1 != null) {
                 getElement(parent, refHandler).insertBefore(elem, getElement(sibling1, refHandler).getNextSibling());
             } else {
                 getElement(parent, refHandler).insertBefore(elem, getElement(parent, refHandler).getFirstChild());
             }
         }
 
         /**
          * Processes the specified document, updates element values, and adds new nodes to the hierarchy.
          *
          * @param refHandler the {@code ReferenceNodeHandler}.
          */
         public void processDocument(final ReferenceNodeHandler refHandler) {
             updateAttributes(refHandler.getRootNode(), document.getDocumentElement());
             NodeTreeWalker.INSTANCE.walkDFS(refHandler.getRootNode(), this, refHandler);
         }
 
         /**
          * Updates the associated XML elements when a node is removed.
          *
          * @param element the element to be removed.
          */
         private void removeReference(final Node element) {
             final Node parentElem = element.getParentNode();
             if (parentElem != null) {
                 parentElem.removeChild(element);
             }
         }
 
         /**
          * {@inheritDoc} This implementation determines the XML element associated with the given node. Then this element's
          * value and attributes are set accordingly.
          */
         @Override
         protected void update(final ImmutableNode node, final Object reference, final ReferenceNodeHandler refHandler) {
             if (XMLListReference.isListNode(node, refHandler)) {
                 if (XMLListReference.isFirstListItem(node, refHandler)) {
                     final String value = XMLListReference.listValue(node, refHandler, listDelimiterHandler);
                     updateElement(node, refHandler, value);
                 }
             } else {
                 final Object value = listDelimiterHandler.escape(refHandler.getValue(node), ListDelimiterHandler.NOOP_TRANSFORMER);
                 updateElement(node, refHandler, value);
             }
         }
 
         /**
          * Updates the node's value if it represents an element node.
          *
          * @param element the element.
          * @param value the new value.
          */
         private void updateElement(final Element element, final Object value) {
             Text txtNode = findTextNodeForUpdate(element);
             if (value == null) {
                 // remove text
                 if (txtNode != null) {
                     element.removeChild(txtNode);
                 }
             } else {
                 final String newValue = String.valueOf(value);
                 if (txtNode == null) {
                     txtNode = document.createTextNode(newValue);
                     if (element.getFirstChild() != null) {
                         element.insertBefore(txtNode, element.getFirstChild());
                     } else {
                         element.appendChild(txtNode);
                     }
                 } else {
                     txtNode.setNodeValue(newValue);
                 }
             }
         }
 
         private void updateElement(final ImmutableNode node, final ReferenceNodeHandler refHandler, final Object value) {
             final Element element = getElement(node, refHandler);
             updateElement(element, value);
             updateAttributes(node, element);
         }
     }
 
     /** Constant for the default indent size. */
     static final int DEFAULT_INDENT_SIZE = 2;
 
     /** Constant for output property name used on a transformer to specify the indent amount. */
     static final String INDENT_AMOUNT_PROPERTY = "{http://xml.apache.org/xslt}indent-amount";
 
     /** Constant for the default root element name. */
     private static final String DEFAULT_ROOT_NAME = "configuration";
 
     /** Constant for the name of the space attribute. */
     private static final String ATTR_SPACE = "xml:space";
 
     /** Constant for an internally used space attribute. */
     private static final String ATTR_SPACE_INTERNAL = "config-xml:space";
 
     /** Constant for the xml:space value for preserving whitespace. */
     private static final String VALUE_PRESERVE = "preserve";
 
     /** Schema Langauge key for the parser */
     private static final String JAXP_SCHEMA_LANGUAGE = "http://java.sun.com/xml/jaxp/properties/schemaLanguage";
 
     /** Schema Language for the parser */
     private static final String W3C_XML_SCHEMA = "http://www.w3.org/2001/XMLSchema";
 
     /**
      * Determines the number of child elements of this given node with the specified node name.
      *
      * @param parent the parent node.
      * @param name the name in question.
      * @return the number of child elements with this name.
      */
     private static int countChildElements(final Node parent, final String name) {
         final NodeList childNodes = parent.getChildNodes();
         int count = 0;
         for (int i = 0; i < childNodes.getLength(); i++) {
             final Node item = childNodes.item(i);
             if (item instanceof Element && name.equals(((Element) item).getTagName())) {
                 count++;
             }
         }
         return count;
     }
 
     /**
      * Determines the value of a configuration node. This method mainly checks whether the text value is to be trimmed or
      * not. This is normally defined by the trim flag. However, if the node has children and its content is only whitespace,
      * then it makes no sense to store any value; this would only scramble layout when the configuration is saved again.
      *
      * @param content the text content of this node.
      * @param hasChildren a flag whether the node has children.
      * @param trimFlag the trim flag.
      * @return the value to be stored for this node.
      */
     private static String determineValue(final String content, final boolean hasChildren, final boolean trimFlag) {
         final boolean shouldTrim = trimFlag || StringUtils.isBlank(content) && hasChildren;
         return shouldTrim ? content.trim() : content;
     }
 
     /**
      * Checks whether an element defines a complete list. If this is the case, extended list handling can be applied.
      *
      * @param element the element to be checked.
      * @return a flag whether this is the only element defining the list.
      */
     private static boolean isSingleElementList(final Element element) {
         final Node parentNode = element.getParentNode();
         return countChildElements(parentNode, element.getTagName()) == 1;
     }
 
     /**
      * Helper method for initializing the attributes of a configuration node from the given XML element.
      *
      * @param element the current XML element
      * @return a map with all attribute values extracted for the current node
      */
     private static Map<String, String> processAttributes(final Node element) {
         final NamedNodeMap attributes = element.getAttributes();
         final Map<String, String> attrmap = new HashMap<>();
         for (int i = 0; i < attributes.getLength(); ++i) {
             final Node w3cNode = attributes.item(i);
             if (w3cNode instanceof Attr) {
                 final Attr attr = (Attr) w3cNode;
                 attrmap.put(attr.getName(), attr.getValue());
             }
         }
         return attrmap;
     }
 
     /**
      * Checks whether the content of the current XML element should be trimmed. This method checks whether a
      * {@code xml:space} attribute is present and evaluates its value. See
      * <a href="https://www.w3.org/TR/REC-xml/#sec-white-space"> http://www.w3.org/TR/REC-xml/#sec-white-space</a> for more
      * details.
      *
      * @param element the current XML element
      * @param currentTrim the current trim flag
      * @return a flag whether the content of this element should be trimmed
      */
     private static boolean shouldTrim(final Element element, final boolean currentTrim) {
         final Attr attr = element.getAttributeNode(ATTR_SPACE);
         if (attr == null) {
             return currentTrim;
         }
         return !VALUE_PRESERVE.equals(attr.getValue());
     }
 
     /** Stores the name of the root element. */
     private String rootElementName;
 
     /** Stores the public ID from the DOCTYPE. */
     private String publicID;
 
     /** Stores the system ID from the DOCTYPE. */
     private String systemID;
 
     /** Stores the document builder that should be used for loading. */
     private DocumentBuilder documentBuilder;
 
     /** Stores a flag whether DTD or Schema validation should be performed. */
     private boolean validating;
 
     /** Stores a flag whether DTD or Schema validation is used. */
     private boolean schemaValidation;
 
     /** The EntityResolver to use. */
     private EntityResolver entityResolver = new DefaultEntityResolver();
 
     /** The current file locator. */
     private FileLocator locator;
 
     /**
      * Creates a new instance of {@code XMLConfiguration}.
      */
     public XMLConfiguration() {
         initLogger(new ConfigurationLogger(XMLConfiguration.class));
     }
 
     /**
      * Creates a new instance of {@code XMLConfiguration} and copies the content of the passed in configuration into this
      * object. Note that only the data of the passed in configuration will be copied. If, for instance, the other
      * configuration is a {@code XMLConfiguration}, too, things like comments or processing instructions will be lost.
      *
      * @param c the configuration to copy.
      * @since 1.4
      */
     public XMLConfiguration(final HierarchicalConfiguration<ImmutableNode> c) {
         super(c);
         rootElementName = c != null ? c.getRootElementName() : null;
         initLogger(new ConfigurationLogger(XMLConfiguration.class));
     }
 
     /**
      * Helper method for building the internal storage hierarchy. The XML elements are transformed into node objects.
      *
      * @param node     a builder for the current node.
      * @param refValue stores the text value of the element.
      * @param element  the current XML element.
      * @param elemRefs a map for assigning references objects to nodes; can be <strong>null</strong>, then reference objects are irrelevant.
      * @param trim     a flag whether the text content of elements should be trimmed; this controls the whitespace handling.
      * @param level    the current level in the hierarchy.
      * @return a map with all attribute values extracted for the current node; this map also contains the value of the trim flag for this node under the key
      *         {@value #ATTR_SPACE}.
      */
     private Map<String, String> constructHierarchy(final ImmutableNode.Builder node, final MutableObject<String> refValue, final Element element,
         final Map<ImmutableNode, Object> elemRefs, final boolean trim, final int level) {
         final boolean trimFlag = shouldTrim(element, trim);
         final Map<String, String> attributes = processAttributes(element);
         attributes.put(ATTR_SPACE_INTERNAL, String.valueOf(trimFlag));
         final StringBuilder buffer = new StringBuilder();
         final NodeList list = element.getChildNodes();
         boolean hasChildren = false;
 
         for (int i = 0; i < list.getLength(); i++) {
             final Node w3cNode = list.item(i);
             if (w3cNode instanceof Element) {
                 final Element child = (Element) w3cNode;
                 final ImmutableNode.Builder childNode = new ImmutableNode.Builder();
                 childNode.name(child.getTagName());
                 final MutableObject<String> refChildValue = new MutableObject<>();
                 final Map<String, String> attrmap = constructHierarchy(childNode, refChildValue, child, elemRefs, trimFlag, level + 1);
                 final boolean childTrim = Boolean.parseBoolean(attrmap.remove(ATTR_SPACE_INTERNAL));
                 childNode.addAttributes(attrmap);
                 final ImmutableNode newChild = createChildNodeWithValue(node, childNode, child, refChildValue.get(), childTrim, attrmap, elemRefs);
                 if (elemRefs != null && !elemRefs.containsKey(newChild)) {
                     elemRefs.put(newChild, child);
                 }
                 hasChildren = true;
             } else if (w3cNode instanceof Text) {
                 final Text data = (Text) w3cNode;
                 buffer.append(data.getData());
             }
         }
 
         boolean childrenFlag = false;
         if (hasChildren || trimFlag) {
             childrenFlag = hasChildren || attributes.size() > 1;
         }
         final String text = determineValue(buffer.toString(), childrenFlag, trimFlag);
         if (!text.isEmpty() || !childrenFlag && level != 0) {
             refValue.setValue(text);
         }
         return attributes;
     }
 
     /**
      * Creates a new child node, assigns its value, and adds it to its parent. This method also deals with elements whose value is a list. In this case multiple
      * child elements must be added. The return value is the first child node which was added.
      *
      * @param parent   the builder for the parent element.
      * @param child    the builder for the child element.
      * @param elem     the associated XML element.
      * @param value    the value of the child element.
      * @param trim     flag whether texts of elements should be trimmed.
      * @param attrmap  a map with the attributes of the current node.
      * @param elemRefs a map for assigning references objects to nodes; can be <strong>null</strong>, then reference objects are irrelevant.
      * @return the first child node added to the parent.
      */
     private ImmutableNode createChildNodeWithValue(final ImmutableNode.Builder parent, final ImmutableNode.Builder child, final Element elem,
         final String value, final boolean trim, final Map<String, String> attrmap, final Map<ImmutableNode, Object> elemRefs) {
         final ImmutableNode addedChildNode;
         final Collection<String> values;
 
         if (value != null) {
             values = getListDelimiterHandler().split(value, trim);
         } else {
             values = Collections.emptyList();
         }
 
         if (values.size() > 1) {
             final Map<ImmutableNode, Object> refs = isSingleElementList(elem) ? elemRefs : null;
             final Iterator<String> it = values.iterator();
             // Create new node for the original child's first value
             child.value(it.next());
             addedChildNode = child.create();
             parent.addChild(addedChildNode);
             XMLListReference.assignListReference(refs, addedChildNode, elem);
 
             // add multiple new children
             while (it.hasNext()) {
                 final ImmutableNode.Builder c = new ImmutableNode.Builder();
                 c.name(addedChildNode.getNodeName());
                 c.value(it.next());
                 c.addAttributes(attrmap);
                 final ImmutableNode newChild = c.create();
                 parent.addChild(newChild);
                 XMLListReference.assignListReference(refs, newChild, null);
             }
         } else {
             if (values.size() == 1) {
                 // we will have to replace the value because it might
                 // contain escaped delimiters
                 child.value(values.iterator().next());
             }
             addedChildNode = child.create();
             parent.addChild(addedChildNode);
         }
 
         return addedChildNode;
     }
 
     /**
      * Creates a DOM document from the internal tree of configuration nodes.
      *
      * @return the new document.
      * @throws ConfigurationException if an error occurs.
      */
     private Document createDocument() throws ConfigurationException {
         final ReferenceNodeHandler handler = getReferenceHandler();
         final XMLDocumentHelper docHelper = (XMLDocumentHelper) handler.getReference(handler.getRootNode());
         final XMLDocumentHelper newHelper = docHelper == null ? XMLDocumentHelper.forNewDocument(getRootElementName()) : docHelper.createCopy();
 
         final XMLBuilderVisitor builder = new XMLBuilderVisitor(newHelper, getListDelimiterHandler());
         builder.handleRemovedNodes(handler);
         builder.processDocument(handler);
         initRootElementText(newHelper.getDocument(), getModel().getNodeHandler().getRootNode().getValue());
         return newHelper.getDocument();
     }
 
     /**
      * Creates the {@code DocumentBuilder} to be used for loading files. This implementation checks whether a specific
      * {@code DocumentBuilder} has been set. If this is the case, this one is used. Otherwise a default builder is created.
      * Depending on the value of the validating flag this builder will be a validating or a non validating
      * {@code DocumentBuilder}.
      *
      * @return the {@code DocumentBuilder} for loading configuration files.
      * @throws ParserConfigurationException if an error occurs.
      * @since 1.2
      */
     protected DocumentBuilder createDocumentBuilder() throws ParserConfigurationException {
         if (getDocumentBuilder() != null) {
             return getDocumentBuilder();
         }
         final DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
         if (isValidating()) {
             factory.setValidating(true);
             if (isSchemaValidation()) {
                 factory.setNamespaceAware(true);
                 factory.setAttribute(JAXP_SCHEMA_LANGUAGE, W3C_XML_SCHEMA);
             }
         }
 
         final DocumentBuilder result = factory.newDocumentBuilder();
         result.setEntityResolver(this.entityResolver);
 
         if (isValidating()) {
             // register an error handler which detects validation errors
             result.setErrorHandler(new DefaultHandler() {
                 @Override
                 public void error(final SAXParseException ex) throws SAXException {
                     throw ex;
                 }
             });
         }
         return result;
     }
 
     /**
      * Creates and initializes the transformer used for save operations. This base implementation initializes all of the
      * default settings like indentation mode and the DOCTYPE. Derived classes may overload this method if they have
      * specific needs.
      *
      * @return the transformer to use for a save operation.
      * @throws ConfigurationException if an error occurs.
      * @since 1.3
      */
     protected Transformer createTransformer() throws ConfigurationException {
         final Transformer transformer = XMLDocumentHelper.createTransformer();
 
         transformer.setOutputProperty(OutputKeys.INDENT, "yes");
         transformer.setOutputProperty(INDENT_AMOUNT_PROPERTY, Integer.toString(DEFAULT_INDENT_SIZE));
         if (locator != null && locator.getEncoding() != null) {
             transformer.setOutputProperty(OutputKeys.ENCODING, locator.getEncoding());
         }
         if (publicID != null) {
             transformer.setOutputProperty(OutputKeys.DOCTYPE_PUBLIC, publicID);
         }
         if (systemID != null) {
             transformer.setOutputProperty(OutputKeys.DOCTYPE_SYSTEM, systemID);
         }
 
         return transformer;
     }
 
     /**
      * Gets the XML document this configuration was loaded from. The return value is <strong>null</strong> if this configuration
      * was not loaded from a XML document.
      *
      * @return the XML document this configuration was loaded from.
      */
     public Document getDocument() {
         final XMLDocumentHelper docHelper = getDocumentHelper();
         return docHelper != null ? docHelper.getDocument() : null;
     }
 
     /**
      * Gets the {@code DocumentBuilder} object that is used for loading documents. If no specific builder has been set,
      * this method returns <strong>null</strong>.
      *
      * @return the {@code DocumentBuilder} for loading new documents.
      * @since 1.2
      */
     public DocumentBuilder getDocumentBuilder() {
         return documentBuilder;
     }
 
     /**
      * Gets the helper object for managing the underlying document.
      *
      * @return the {@code XMLDocumentHelper}.
      */
     private XMLDocumentHelper getDocumentHelper() {
         final ReferenceNodeHandler handler = getReferenceHandler();
         return (XMLDocumentHelper) handler.getReference(handler.getRootNode());
     }
 
     /**
      * Gets the EntityResolver.
      *
      * @return The EntityResolver.
      * @since 1.7
      */
     public EntityResolver getEntityResolver() {
         return this.entityResolver;
     }
 
     /**
      * Gets the public ID of the DOCTYPE declaration from the loaded XML document. This is <strong>null</strong> if no document has
      * been loaded yet or if the document does not contain a DOCTYPE declaration with a public ID.
      *
      * @return the public ID.
      * @since 1.3
      */
     public String getPublicID() {
         return syncReadValue(publicID, false);
     }
 
     /**
      * Gets the extended node handler with support for references.
      *
      * @return the {@code ReferenceNodeHandler}.
      */
     private ReferenceNodeHandler getReferenceHandler() {
         return getSubConfigurationParentModel().getReferenceNodeHandler();
     }
 
     /**
      * Gets the name of the root element. If this configuration was loaded from a XML document, the name of this
      * document's root element is returned. Otherwise it is possible to set a name for the root element that will be used
      * when this configuration is stored.
      *
      * @return the name of the root element.
      */
     @Override
     protected String getRootElementNameInternal() {
         final Document doc = getDocument();
         if (doc == null) {
             return rootElementName == null ? DEFAULT_ROOT_NAME : rootElementName;
         }
         return doc.getDocumentElement().getNodeName();
     }
 
     /**
      * Gets the system ID of the DOCTYPE declaration from the loaded XML document. This is <strong>null</strong> if no document has
      * been loaded yet or if the document does not contain a DOCTYPE declaration with a system ID.
      *
      * @return the system ID.
      * @since 1.3
      */
     public String getSystemID() {
         return syncReadValue(systemID, false);
     }
 
     /**
      * {@inheritDoc} Stores the passed in locator for the upcoming IO operation.
      */
     @Override
     public void initFileLocator(final FileLocator loc) {
         locator = loc;
     }
 
     /**
      * Initializes this configuration from an XML document.
      *
      * @param docHelper the helper object with the document to be parsed.
      * @param elemRefs a flag whether references to the XML elements should be set.
      */
     private void initProperties(final XMLDocumentHelper docHelper, final boolean elemRefs) {
         setPublicID(docHelper.getSourcePublicID());
         setSystemID(docHelper.getSourceSystemID());
         initProperties(docHelper, elemRefs, docHelper.getDocument().getDocumentElement());
     }
 
     private void initProperties(final XMLDocumentHelper docHelper, final boolean elemRefs, final Element element) {
         final ImmutableNode.Builder rootBuilder = new ImmutableNode.Builder();
         final MutableObject<String> rootValue = new MutableObject<>();
         final Map<ImmutableNode, Object> elemRefMap = elemRefs ? new HashMap<>() : null;
         final Map<String, String> attributes = constructHierarchy(rootBuilder, rootValue, element, elemRefMap, true, 0);
         attributes.remove(ATTR_SPACE_INTERNAL);
         final ImmutableNode top = rootBuilder.value(rootValue.get()).addAttributes(attributes).create();
         getSubConfigurationParentModel().mergeRoot(top, element.getTagName(), elemRefMap, elemRefs ? docHelper : null, this);
     }
 
     /**
      * Sets the text of the root element of a newly created XML Document.
      *
      * @param doc the document.
      * @param value the new text to be set.
      */
     private void initRootElementText(final Document doc, final Object value) {
         final Element elem = doc.getDocumentElement();
         final NodeList children = elem.getChildNodes();
 
         // Remove all existing text nodes
         for (int i = 0; i < children.getLength(); i++) {
             final Node nd = children.item(i);
             if (nd.getNodeType() == Node.TEXT_NODE) {
                 elem.removeChild(nd);
             }
         }
 
         if (value != null) {
             // Add a new text node
             elem.appendChild(doc.createTextNode(String.valueOf(value)));
         }
     }
 
     /**
      * Returns the value of the schemaValidation flag.
      *
      * @return the schemaValidation flag.
      * @since 1.7
      */
     public boolean isSchemaValidation() {
         return schemaValidation;
     }
 
     /**
      * Returns the value of the validating flag.
      *
      * @return the validating flag.
      * @since 1.2
      */
     public boolean isValidating() {
         return validating;
     }
 
     /**
      * Loads a configuration file from the specified input source.
      *
      * @param source the input source.
      * @throws ConfigurationException if an error occurs.
      */
     private void load(final InputSource source) throws ConfigurationException {
         if (locator == null) {
             throw new ConfigurationException(
                 "Load operation not properly initialized! Do not call read(InputStream) directly, but use a FileHandler to load a configuration.");
         }
 
         try {
             final URL sourceURL = locator.getSourceURL();
             if (sourceURL != null) {
                 source.setSystemId(sourceURL.toString());
             }
 
             final DocumentBuilder builder = createDocumentBuilder();
             final Document newDocument = builder.parse(source);
             final Document oldDocument = getDocument();
             initProperties(XMLDocumentHelper.forSourceDocument(newDocument), oldDocument == null);
         } catch (final SAXParseException spe) {
             throw new ConfigurationException(spe, "Error parsing system ID %s", source.getSystemId());
         } catch (final Exception e) {
             getLogger().debug("Unable to load the configuration: " + e);
             throw new ConfigurationException("Unable to load the configuration", e);
         }
     }
 
     /**
      * Loads the configuration from the given XML DOM Element.
      * <p>
      * This method can be used to initialize the configuration from an XML element in a document that has already been parsed. This is especially useful if the
      * configuration is only a part of a larger XML document.
      * </p>
      *
      * @param element the input element.
      * @throws ConfigurationException if an error occurs.
      * @since 2.14.0
      */
     public void read(final Element element) throws ConfigurationException {
         try {
             initProperties(getDocumentHelper(), getDocument() == null, element);
         } catch (final Exception e) {
             getLogger().debug("Unable to load the configuration: " + e);
             throw new ConfigurationException(e, "Unable to load the configuration %s", element);
         }
     }
 
     /**
      * Loads the configuration from the given input stream. This is analogous to {@link #read(Reader)}, but data is read
      * from a stream. Note that this method will be called most time when reading an XML configuration source. By reading
      * XML documents directly from an input stream, the file's encoding can be correctly dealt with.
      *
      * @param in the input stream.
      * @throws ConfigurationException if an error occurs.
      * @throws IOException if an IO error occurs.
      */
     @Override
     public void read(final InputStream in) throws ConfigurationException, IOException {
         load(new InputSource(in));
     }
 
     /**
      * Loads the configuration from the given reader. Note that the {@code clear()} method is not called, so the properties
      * contained in the loaded file will be added to the current set of properties.
      *
      * @param in the reader.
      * @throws ConfigurationException if an error occurs.
      * @throws IOException if an IO error occurs.
      */
     @Override
     public void read(final Reader in) throws ConfigurationException, IOException {
         load(new InputSource(in));
     }
 
     /**
      * Sets the {@code DocumentBuilder} object to be used for loading documents. This method makes it possible to specify
      * the exact document builder. So an application can create a builder, configure it for its special needs, and then pass
      * it to this method.
      *
      * @param documentBuilder the document builder to be used; if undefined, a default builder will be used.
      * @since 1.2
      */
     public void setDocumentBuilder(final DocumentBuilder documentBuilder) {
         this.documentBuilder = documentBuilder;
     }
 
     /**
      * Sets a new EntityResolver. Setting this will cause RegisterEntityId to have no effect.
      *
      * @param resolver The EntityResolver to use.
      * @since 1.7
      */
     public void setEntityResolver(final EntityResolver resolver) {
         this.entityResolver = resolver;
     }
 
     /**
      * Sets the public ID of the DOCTYPE declaration. When this configuration is saved, a DOCTYPE declaration will be
      * constructed that contains this public ID.
      *
      * @param publicID the public ID.
      * @since 1.3
      */
     public void setPublicID(final String publicID) {
         syncWrite(() -> this.publicID = publicID, false);
     }
 
     /**
      * Sets the name of the root element. This name is used when this configuration object is stored in an XML file. Note
      * that setting the name of the root element works only if this configuration has been newly created. If the
      * configuration was loaded from an XML file, the name cannot be changed and an {@code UnsupportedOperationException}
      * exception is thrown. Whether this configuration has been loaded from an XML document or not can be found out using
      * the {@code getDocument()} method.
      *
      * @param name the name of the root element.
      */
     public void setRootElementName(final String name) {
         beginRead(true);
         try {
             if (getDocument() != null) {
                 throw new UnsupportedOperationException("The name of the root element cannot be changed when loaded from an XML document!");
             }
             rootElementName = name;
         } finally {
             endRead();
         }
     }
 
     /**
      * Sets the value of the schemaValidation flag. This flag determines whether DTD or Schema validation should be used.
      * This flag is evaluated only if no custom {@code DocumentBuilder} was set. If set to true the XML document must
      * contain a schemaLocation definition that provides resolvable hints to the required schemas.
      *
      * @param schemaValidation the validating flag.
      * @since 1.7
      */
     public void setSchemaValidation(final boolean schemaValidation) {
         this.schemaValidation = schemaValidation;
         if (schemaValidation) {
             this.validating = true;
         }
     }
 
     /**
      * Sets the system ID of the DOCTYPE declaration. When this configuration is saved, a DOCTYPE declaration will be
      * constructed that contains this system ID.
      *
      * @param systemID the system ID.
      * @since 1.3
      */
     public void setSystemID(final String systemID) {
         syncWrite(() -> this.systemID = systemID, false);
     }
 
     /**
      * Sets the value of the validating flag. This flag determines whether DTD/Schema validation should be performed when
      * loading XML documents. This flag is evaluated only if no custom {@code DocumentBuilder} was set.
      *
      * @param validating the validating flag.
      * @since 1.2
      */
     public void setValidating(final boolean validating) {
         if (!schemaValidation) {
             this.validating = validating;
         }
     }
 
     /**
      * Validate the document against the Schema.
      *
      * @throws ConfigurationException if the validation fails.
      */
     public void validate() throws ConfigurationException {
         syncWrite(() -> {
             try {
                 final StringWriter writer = new StringWriter();
                 final Result result = new StreamResult(writer);
                 XMLDocumentHelper.transform(createTransformer(), new DOMSource(createDocument()), result);
                 final Reader reader = new StringReader(writer.getBuffer().toString());
                 createDocumentBuilder().parse(new InputSource(reader));
             } catch (final SAXException | IOException | ParserConfigurationException pce) {
                 throw new ConfigurationException("Validation failed", pce);
             }
         }, false);
     }
 
     /**
      * Saves the configuration to the specified writer.
      *
      * @param writer the writer used to save the configuration.
      * @throws ConfigurationException if an error occurs.
      * @throws IOException if an IO error occurs.
      */
     @Override
     public void write(final Writer writer) throws ConfigurationException, IOException {
         write(writer, createTransformer());
     }
 
     /**
      * Saves the configuration to the specified writer.
      *
      * @param writer the writer used to save the configuration.
      * @param transformer How to transform this configuration.
      * @throws ConfigurationException if an error occurs.
      * @since 2.7.0
      */
     public void write(final Writer writer, final Transformer transformer) throws ConfigurationException {
         final Source source = new DOMSource(createDocument());
         final Result result = new StreamResult(writer);
         XMLDocumentHelper.transform(transformer, source, result);
     }
 }