/*
    * 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
    *
    *     http://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.configuration;
  
  import java.io.Serializable;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Set;
  import java.util.Stack;
  
  import org.apache.commons.collections.set.ListOrderedSet;
  import org.apache.commons.collections.iterators.SingletonIterator;
  import org.apache.commons.configuration.event.ConfigurationEvent;
  import org.apache.commons.configuration.event.ConfigurationListener;
  import org.apache.commons.configuration.tree.ConfigurationNode;
  import org.apache.commons.configuration.tree.ConfigurationNodeVisitor;
  import org.apache.commons.configuration.tree.ConfigurationNodeVisitorAdapter;
  import org.apache.commons.configuration.tree.DefaultConfigurationNode;
  import org.apache.commons.configuration.tree.DefaultExpressionEngine;
  import org.apache.commons.configuration.tree.ExpressionEngine;
  import org.apache.commons.configuration.tree.NodeAddData;
  import org.apache.commons.lang.StringUtils;
  
  /**
   * <p>A specialized configuration class that extends its base class by the
   * ability of keeping more structure in the stored properties.</p><p>There
   * are some sources of configuration data that cannot be stored very well in a
   * <code>BaseConfiguration</code> object because then their structure is lost.
   * This is especially true for XML documents. This class can deal with such
   * structured configuration sources by storing the properties in a tree-like
   * organization.</p><p>The internal used storage form allows for a more
   * sophisticated access to single properties. As an example consider the
   * following XML document:</p><p>
   *
   * <pre>
   * &lt;database&gt;
   *   &lt;tables&gt;
   *     &lt;table&gt;
   *       &lt;name&gt;users&lt;/name&gt;
   *       &lt;fields&gt;
   *         &lt;field&gt;
   *           &lt;name&gt;lid&lt;/name&gt;
   *           &lt;type&gt;long&lt;/name&gt;
   *         &lt;/field&gt;
   *         &lt;field&gt;
   *           &lt;name&gt;usrName&lt;/name&gt;
   *           &lt;type&gt;java.lang.String&lt;/type&gt;
   *         &lt;/field&gt;
   *        ...
   *       &lt;/fields&gt;
   *     &lt;/table&gt;
   *     &lt;table&gt;
   *       &lt;name&gt;documents&lt;/name&gt;
   *       &lt;fields&gt;
   *         &lt;field&gt;
   *           &lt;name&gt;docid&lt;/name&gt;
   *           &lt;type&gt;long&lt;/type&gt;
   *         &lt;/field&gt;
   *         ...
   *       &lt;/fields&gt;
   *     &lt;/table&gt;
   *     ...
   *   &lt;/tables&gt;
   * &lt;/database&gt;
   * </pre>
   *
   * </p><p>If this document is parsed and stored in a
   * <code>HierarchicalConfiguration</code> object (which can be done by one of
   * the sub classes), there are enhanced possibilities of accessing properties.
   * The keys for querying information can contain indices that select a certain
   * element if there are multiple hits.</p><p>For instance the key
   * <code>tables.table(0).name</code> can be used to find out the name of the
   * first table. In opposite <code>tables.table.name</code> would return a
   * collection with the names of all available tables. Similarly the key
   * <code>tables.table(1).fields.field.name</code> returns a collection with
   * the names of all fields of the second table. If another index is added after
   * the <code>field</code> element, a single field can be accessed:
   * <code>tables.table(1).fields.field(0).name</code>.</p><p>There is a
   * <code>getMaxIndex()</code> method that returns the maximum allowed index
   * that can be added to a given property key. This method can be used to iterate
   * over all values defined for a certain property.</p>
  * <p>Since the 1.3 release of <em>Commons Configuration</em> hierarchical
  * configurations support an <em>expression engine</em>. This expression engine
  * is responsible for evaluating the passed in configuration keys and map them
  * to the stored properties. The examples above are valid for the default
  * expression engine, which is used when a new <code>HierarchicalConfiguration</code>
  * instance is created. With the <code>setExpressionEngine()</code> method a
  * different expression engine can be set. For instance with
  * <code>{@link org.apache.commons.configuration.tree.xpath.XPathExpressionEngine}</code>
  * there is an expression engine available that supports configuration keys in
  * XPATH syntax.</p>
  * <p>In addition to the events common for all configuration classes hierarchical
  * configurations support some more events that correspond to some specific
  * methods and features:
  * <dl><dt><em>EVENT_ADD_NODES</em></dt><dd>The <code>addNodes()</code> method
  * was called; the event object contains the key, to which the nodes were added,
  * and a collection with the new nodes as value.</dd>
  * <dt><em>EVENT_CLEAR_TREE</em></dt><dd>The <code>clearTree()</code> method was
  * called; the event object stores the key of the removed sub tree.</dd>
  * <dt><em>EVENT_SUBNODE_CHANGED</em></dt><dd>A <code>SubnodeConfiguration</code>
  * that was created from this configuration has been changed. The value property
  * of the event object contains the original event object as it was sent by the
  * subnode configuration.</dd></dl></p>
  * <p><em>Note:</em>Configuration objects of this type can be read concurrently
  * by multiple threads. However if one of these threads modifies the object,
  * synchronization has to be performed manually.</p>
  *
  * @author Oliver Heger
  * @version $Id: HierarchicalConfiguration.java 593319 2007-11-08 21:14:21Z oheger $
  */
 public class HierarchicalConfiguration extends AbstractConfiguration implements Serializable, Cloneable
 {
     /**
      * Constant for the clear tree event.
      * @since 1.3
      */
     public static final int EVENT_CLEAR_TREE = 10;
 
     /**
      * Constant for the add nodes event.
      * @since 1.3
      */
     public static final int EVENT_ADD_NODES = 11;
 
     /**
      * Constant for the subnode configuration modified event.
      * @since 1.5
      */
     public static final int EVENT_SUBNODE_CHANGED = 12;
 
     /**
      * The serial version UID.
      */
     private static final long serialVersionUID = 3373812230395363192L;
 
     /** Stores the default expression engine to be used for new objects.*/
     private static ExpressionEngine defaultExpressionEngine;
 
     /** Stores the root node of this configuration. This field is required for
      * backwards compatibility only.
      */
     private Node root;
 
     /** Stores the root configuration node.*/
     private ConfigurationNode rootNode;
 
     /** Stores the expression engine for this instance.*/
     private transient ExpressionEngine expressionEngine;
 
     /**
      * Creates a new instance of <code>HierarchicalConfiguration</code>.
      */
     public HierarchicalConfiguration()
     {
         setRootNode(new Node());
     }
 
     /**
      * Creates a new instance of <code>HierarchicalConfiguration</code> and
      * copies all data contained in the specified configuration into the new
      * one.
      *
      * @param c the configuration that is to be copied (if <b>null</b>, this
      * constructor will behave like the standard constructor)
      * @since 1.4
      */
     public HierarchicalConfiguration(HierarchicalConfiguration c)
     {
         this();
         if (c != null)
         {
             CloneVisitor visitor = new CloneVisitor();
             c.getRootNode().visit(visitor);
             setRootNode(visitor.getClone());
         }
     }
 
     /**
      * Returns the root node of this hierarchical configuration. This method
      * exists for backwards compatibility only. New code should use the
      * <code>{@link #getRootNode()}</code> method instead, which operates on
      * the preferred data type <code>ConfigurationNode</code>.
      *
      * @return the root node
      */
     public Node getRoot()
     {
         return root;
     }
 
     /**
      * Sets the root node of this hierarchical configuration. This method
      * exists for backwards compatibility only. New code should use the
      * <code>{@link #setRootNode(ConfigurationNode)}</code> method instead,
      * which operates on the preferred data type <code>ConfigurationNode</code>.
      *
      * @param node the root node
      */
     public void setRoot(Node node)
     {
         if (node == null)
         {
             throw new IllegalArgumentException("Root node must not be null!");
         }
         root = node;
         rootNode = null;
     }
 
     /**
      * Returns the root node of this hierarchical configuration.
      *
      * @return the root node
      * @since 1.3
      */
     public ConfigurationNode getRootNode()
     {
         return (rootNode != null) ? rootNode : root;
     }
 
     /**
      * Sets the root node of this hierarchical configuration.
      *
      * @param rootNode the root node
      * @since 1.3
      */
     public void setRootNode(ConfigurationNode rootNode)
     {
         if (rootNode == null)
         {
             throw new IllegalArgumentException("Root node must not be null!");
         }
         this.rootNode = rootNode;
 
         // For backward compatibility also set the old root field.
         root = (rootNode instanceof Node) ? (Node) rootNode : new Node(rootNode);
     }
 
     /**
      * Returns the default expression engine.
      *
      * @return the default expression engine
      * @since 1.3
      */
     public static synchronized ExpressionEngine getDefaultExpressionEngine()
     {
         if (defaultExpressionEngine == null)
         {
             defaultExpressionEngine = new DefaultExpressionEngine();
         }
         return defaultExpressionEngine;
     }
 
     /**
      * Sets the default expression engine. This expression engine will be used
      * if no specific engine was set for an instance. It is shared between all
      * hierarchical configuration instances. So modifying its properties will
      * impact all instances, for which no specific engine is set.
      *
      * @param engine the new default expression engine
      * @since 1.3
      */
     public static synchronized void setDefaultExpressionEngine(ExpressionEngine engine)
     {
         if (engine == null)
         {
             throw new IllegalArgumentException(
                     "Default expression engine must not be null!");
         }
         defaultExpressionEngine = engine;
     }
 
     /**
      * Returns the expression engine used by this configuration. This method
      * will never return <b>null</b>; if no specific expression engine was set,
      * the default expression engine will be returned.
      *
      * @return the current expression engine
      * @since 1.3
      */
     public ExpressionEngine getExpressionEngine()
     {
         return (expressionEngine != null) ? expressionEngine
                 : getDefaultExpressionEngine();
     }
 
     /**
      * Sets the expression engine to be used by this configuration. All property
      * keys this configuration has to deal with will be interpreted by this
      * engine.
      *
      * @param expressionEngine the new expression engine; can be <b>null</b>,
      * then the default expression engine will be used
      * @since 1.3
      */
     public void setExpressionEngine(ExpressionEngine expressionEngine)
     {
         this.expressionEngine = expressionEngine;
     }
 
     /**
      * Fetches the specified property. This task is delegated to the associated
      * expression engine.
      *
      * @param key the key to be looked up
      * @return the found value
      */
     public Object getProperty(String key)
     {
         List nodes = fetchNodeList(key);
 
         if (nodes.size() == 0)
         {
             return null;
         }
         else
         {
             List list = new ArrayList();
             for (Iterator it = nodes.iterator(); it.hasNext();)
             {
                 ConfigurationNode node = (ConfigurationNode) it.next();
                 if (node.getValue() != null)
                 {
                     list.add(node.getValue());
                 }
             }
 
             if (list.size() < 1)
             {
                 return null;
             }
             else
             {
                 return (list.size() == 1) ? list.get(0) : list;
             }
         }
     }
 
     /**
      * Adds the property with the specified key. This task will be delegated to
      * the associated <code>ExpressionEngine</code>, so the passed in key
      * must match the requirements of this implementation.
      *
      * @param key the key of the new property
      * @param obj the value of the new property
      */
     protected void addPropertyDirect(String key, Object obj)
     {
         NodeAddData data = getExpressionEngine().prepareAdd(getRootNode(), key);
         ConfigurationNode node = processNodeAddData(data);
         node.setValue(obj);
     }
 
     /**
      * Adds a collection of nodes at the specified position of the configuration
      * tree. This method works similar to <code>addProperty()</code>, but
      * instead of a single property a whole collection of nodes can be added -
      * and thus complete configuration sub trees. E.g. with this method it is
      * possible to add parts of another <code>HierarchicalConfiguration</code>
      * object to this object. (However be aware that a
      * <code>ConfigurationNode</code> object can only belong to a single
      * configuration. So if nodes from one configuration are directly added to
      * another one using this method, the structure of the source configuration
      * will be broken. In this case you should clone the nodes to be added
      * before calling <code>addNodes()</code>.) If the passed in key refers to
      * an existing and unique node, the new nodes are added to this node.
      * Otherwise a new node will be created at the specified position in the
      * hierarchy.
      *
      * @param key the key where the nodes are to be added; can be <b>null </b>,
      * then they are added to the root node
      * @param nodes a collection with the <code>Node</code> objects to be
      * added
      */
     public void addNodes(String key, Collection nodes)
     {
         if (nodes == null || nodes.isEmpty())
         {
             return;
         }
 
         fireEvent(EVENT_ADD_NODES, key, nodes, true);
         ConfigurationNode parent;
         List target = fetchNodeList(key);
         if (target.size() == 1)
         {
             // existing unique key
             parent = (ConfigurationNode) target.get(0);
         }
         else
         {
             // otherwise perform an add operation
             parent = processNodeAddData(getExpressionEngine().prepareAdd(
                     getRootNode(), key));
         }
 
         if (parent.isAttribute())
         {
             throw new IllegalArgumentException(
                     "Cannot add nodes to an attribute node!");
         }
 
         // a visitor to ensure that the nodes' references are cleared; this is
         // necessary if the nodes are moved from another configuration
         ConfigurationNodeVisitor clearRefVisitor = new ConfigurationNodeVisitorAdapter()
         {
             public void visitBeforeChildren(ConfigurationNode node)
             {
                 node.setReference(null);
             }
         };
 
         for (Iterator it = nodes.iterator(); it.hasNext();)
         {
             ConfigurationNode child = (ConfigurationNode) it.next();
             if (child.isAttribute())
             {
                 parent.addAttribute(child);
             }
             else
             {
                 parent.addChild(child);
             }
             child.visit(clearRefVisitor);
         }
         fireEvent(EVENT_ADD_NODES, key, nodes, false);
     }
 
     /**
      * Checks if this configuration is empty. Empty means that there are no keys
      * with any values, though there can be some (empty) nodes.
      *
      * @return a flag if this configuration is empty
      */
     public boolean isEmpty()
     {
         return !nodeDefined(getRootNode());
     }
 
     /**
      * Creates a new <code>Configuration</code> object containing all keys
      * that start with the specified prefix. This implementation will return a
      * <code>HierarchicalConfiguration</code> object so that the structure of
      * the keys will be saved. The nodes selected by the prefix (it is possible
      * that multiple nodes are selected) are mapped to the root node of the
      * returned configuration, i.e. their children and attributes will become
      * children and attributes of the new root node. However a value of the root
      * node is only set if exactly one of the selected nodes contain a value (if
      * multiple nodes have a value, there is simply no way to decide how these
      * values are merged together). Note that the returned
      * <code>Configuration</code> object is not connected to its source
      * configuration: updates on the source configuration are not reflected in
      * the subset and vice versa.
      *
      * @param prefix the prefix of the keys for the subset
      * @return a new configuration object representing the selected subset
      */
     public Configuration subset(String prefix)
     {
         Collection nodes = fetchNodeList(prefix);
         if (nodes.isEmpty())
         {
             return new HierarchicalConfiguration();
         }
 
         final HierarchicalConfiguration parent = this;
         HierarchicalConfiguration result = new HierarchicalConfiguration()
         {
             // Override interpolate to always interpolate on the parent
             protected Object interpolate(Object value)
             {
                 return parent.interpolate(value);
             }
         };
         CloneVisitor visitor = new CloneVisitor();
 
         // Initialize the new root node
         Object value = null;
         int valueCount = 0;
         for (Iterator it = nodes.iterator(); it.hasNext();)
         {
             ConfigurationNode nd = (ConfigurationNode) it.next();
             if (nd.getValue() != null)
             {
                 value = nd.getValue();
                 valueCount++;
             }
             nd.visit(visitor);
 
             for (Iterator it2 = visitor.getClone().getChildren().iterator(); it2
                     .hasNext();)
             {
                 result.getRootNode().addChild((ConfigurationNode) it2.next());
             }
             for (Iterator it2 = visitor.getClone().getAttributes().iterator(); it2
                     .hasNext();)
             {
                 result.getRootNode().addAttribute(
                         (ConfigurationNode) it2.next());
             }
         }
 
         // Determine the value of the new root
         if (valueCount == 1)
         {
             result.getRootNode().setValue(value);
         }
         return (result.isEmpty()) ? new HierarchicalConfiguration() : result;
     }
 
     /**
      * <p>
      * Returns a hierarchical subnode configuration object that wraps the
      * configuration node specified by the given key. This method provides an
      * easy means of accessing sub trees of a hierarchical configuration. In the
      * returned configuration the sub tree can directly be accessed, it becomes
      * the root node of this configuration. Because of this the passed in key
      * must select exactly one configuration node; otherwise an
      * <code>IllegalArgumentException</code> will be thrown.
      * </p>
      * <p>
      * The difference between this method and the
      * <code>{@link #subset(String)}</code> method is that
      * <code>subset()</code> supports arbitrary subsets of configuration nodes
      * while <code>configurationAt()</code> only returns a single sub tree.
      * Please refer to the documentation of the
      * <code>SubnodeConfiguration</code> class to obtain further information
      * about subnode configurations and when they should be used.
      * </p>
      * <p>
      * With the <code>supportUpdate</code> flag the behavior of the returned
      * <code>SubnodeConfiguration</code> regarding updates of its parent
      * configuration can be determined. A subnode configuration operates on the
      * same nodes as its parent, so changes at one configuration are normally
      * directly visible for the other configuration. There are however changes
      * of the parent configuration, which are not recognized by the subnode
      * configuration per default. An example for this is a reload operation (for
      * file-based configurations): Here the complete node set of the parent
      * configuration is replaced, but the subnode configuration still references
      * the old nodes. If such changes should be detected by the subnode
      * configuration, the <code>supportUpdates</code> flag must be set to
      * <b>true</b>. This causes the subnode configuration to reevaluate the key
      * used for its creation each time it is accessed. This guarantees that the
      * subnode configuration always stays in sync with its key, even if the
      * parent configuration's data significantly changes. If such a change
      * makes the key invalid - because it now no longer points to exactly one
      * node -, the subnode configuration is not reconstructed, but keeps its
      * old data. It is then quasi detached from its parent.
      * </p>
      *
      * @param key the key that selects the sub tree
      * @param supportUpdates a flag whether the returned subnode configuration
      * should be able to handle updates of its parent
      * @return a hierarchical configuration that contains this sub tree
      * @see SubnodeConfiguration
      * @since 1.5
      */
     public SubnodeConfiguration configurationAt(String key,
             boolean supportUpdates)
     {
         List nodes = fetchNodeList(key);
         if (nodes.size() != 1)
         {
             throw new IllegalArgumentException(
                     "Passed in key must select exactly one node: " + key);
         }
         return supportUpdates ? createSubnodeConfiguration(
                 (ConfigurationNode) nodes.get(0), key)
                 : createSubnodeConfiguration((ConfigurationNode) nodes.get(0));
     }
 
     /**
      * Returns a hierarchical subnode configuration for the node specified by
      * the given key. This is a short form for <code>configurationAt(key,
      * <b>false</b>)</code>.
      *
      * @param key the key that selects the sub tree
      * @return a hierarchical configuration that contains this sub tree
      * @see SubnodeConfiguration
      * @since 1.3
      */
     public SubnodeConfiguration configurationAt(String key)
     {
         return configurationAt(key, false);
     }
 
     /**
      * Returns a list of sub configurations for all configuration nodes selected
      * by the given key. This method will evaluate the passed in key (using the
      * current <code>ExpressionEngine</code>) and then create a subnode
      * configuration for each returned node (like
      * <code>{@link #configurationAt(String)}</code>}). This is especially
      * useful when dealing with list-like structures. As an example consider the
      * configuration that contains data about database tables and their fields.
      * If you need access to all fields of a certain table, you can simply do
      *
      * <pre>
      * List fields = config.configurationsAt("tables.table(0).fields.field");
      * for(Iterator it = fields.iterator(); it.hasNext();)
      * {
      *     HierarchicalConfiguration sub = (HierarchicalConfiguration) it.next();
      *     // now the children and attributes of the field node can be
      *     // directly accessed
      *     String fieldName = sub.getString("name");
      *     String fieldType = sub.getString("type");
      *     ...
      * </pre>
      *
      * @param key the key for selecting the desired nodes
      * @return a list with hierarchical configuration objects; each
      * configuration represents one of the nodes selected by the passed in key
      * @since 1.3
      */
     public List configurationsAt(String key)
     {
         List nodes = fetchNodeList(key);
         List configs = new ArrayList(nodes.size());
         for (Iterator it = nodes.iterator(); it.hasNext();)
         {
             configs.add(createSubnodeConfiguration((ConfigurationNode) it.next()));
         }
         return configs;
     }
 
     /**
      * Creates a subnode configuration for the specified node. This method is
      * called by <code>configurationAt()</code> and
      * <code>configurationsAt()</code>.
      *
      * @param node the node, for which a subnode configuration is to be created
      * @return the configuration for the given node
      * @since 1.3
      */
     protected SubnodeConfiguration createSubnodeConfiguration(ConfigurationNode node)
     {
         SubnodeConfiguration result = new SubnodeConfiguration(this, node);
         registerSubnodeConfiguration(result);
         return result;
     }
 
     /**
      * Creates a new subnode configuration for the specified node and sets its
      * construction key. A subnode configuration created this way will be aware
      * of structural changes of its parent.
      *
      * @param node the node, for which a subnode configuration is to be created
      * @param subnodeKey the key used to construct the configuration
      * @return the configuration for the given node
      * @since 1.5
      */
     protected SubnodeConfiguration createSubnodeConfiguration(
             ConfigurationNode node, String subnodeKey)
     {
         SubnodeConfiguration result = createSubnodeConfiguration(node);
         result.setSubnodeKey(subnodeKey);
         return result;
     }
 
     /**
      * This method is always called when a subnode configuration created from
      * this configuration has been modified. This implementation transforms the
      * received event into an event of type <code>EVENT_SUBNODE_CHANGED</code>
      * and notifies the registered listeners.
      *
      * @param event the event describing the change
      * @since 1.5
      */
     protected void subnodeConfigurationChanged(ConfigurationEvent event)
     {
         fireEvent(EVENT_SUBNODE_CHANGED, null, event, event.isBeforeUpdate());
     }
 
     /**
      * Registers this instance at the given subnode configuration. This
      * implementation will register a change listener, so that modifications of
      * the subnode configuration can be tracked.
      *
      * @param config the subnode configuration
      * @since 1.5
      */
     void registerSubnodeConfiguration(SubnodeConfiguration config)
     {
         config.addConfigurationListener(new ConfigurationListener()
         {
             public void configurationChanged(ConfigurationEvent event)
             {
                 subnodeConfigurationChanged(event);
             }
         });
     }
 
     /**
      * Checks if the specified key is contained in this configuration. Note that
      * for this configuration the term &quot;contained&quot; means that the key
      * has an associated value. If there is a node for this key that has no
      * value but children (either defined or undefined), this method will still
      * return <b>false </b>.
      *
      * @param key the key to be chekced
      * @return a flag if this key is contained in this configuration
      */
     public boolean containsKey(String key)
     {
         return getProperty(key) != null;
     }
 
     /**
      * Sets the value of the specified property.
      *
      * @param key the key of the property to set
      * @param value the new value of this property
      */
     public void setProperty(String key, Object value)
     {
         fireEvent(EVENT_SET_PROPERTY, key, value, true);
 
         // Update the existing nodes for this property
         Iterator itNodes = fetchNodeList(key).iterator();
         Iterator itValues;
         if (!isDelimiterParsingDisabled())
         {
             itValues = PropertyConverter.toIterator(value, getListDelimiter());
         }
         else
         {
             itValues = new SingletonIterator(value);
         }
 
         while (itNodes.hasNext() && itValues.hasNext())
         {
             ((ConfigurationNode) itNodes.next()).setValue(itValues.next());
         }
 
         // Add additional nodes if necessary
         while (itValues.hasNext())
         {
             addPropertyDirect(key, itValues.next());
         }
 
         // Remove remaining nodes
         while (itNodes.hasNext())
         {
             clearNode((ConfigurationNode) itNodes.next());
         }
 
         fireEvent(EVENT_SET_PROPERTY, key, value, false);
     }
 
     /**
      * Removes all values of the property with the given name and of keys that
      * start with this name. So if there is a property with the key
      * &quot;foo&quot; and a property with the key &quot;foo.bar&quot;, a call
      * of <code>clearTree("foo")</code> would remove both properties.
      *
      * @param key the key of the property to be removed
      */
     public void clearTree(String key)
     {
         fireEvent(EVENT_CLEAR_TREE, key, null, true);
         List nodes = fetchNodeList(key);
 
         for (Iterator it = nodes.iterator(); it.hasNext();)
         {
             removeNode((ConfigurationNode) it.next());
         }
         fireEvent(EVENT_CLEAR_TREE, key, nodes, false);
     }
 
     /**
      * Removes the property with the given key. Properties with names that start
      * with the given key (i.e. properties below the specified key in the
      * hierarchy) won't be affected.
      *
      * @param key the key of the property to be removed
      */
     public void clearProperty(String key)
     {
         fireEvent(EVENT_CLEAR_PROPERTY, key, null, true);
         List nodes = fetchNodeList(key);
 
         for (Iterator it = nodes.iterator(); it.hasNext();)
         {
             clearNode((ConfigurationNode) it.next());
         }
 
         fireEvent(EVENT_CLEAR_PROPERTY, key, null, false);
     }
 
     /**
      * Returns an iterator with all keys defined in this configuration.
      * Note that the keys returned by this method will not contain any
      * indices. This means that some structure will be lost.</p>
      *
      * @return an iterator with the defined keys in this configuration
      */
     public Iterator getKeys()
     {
         DefinedKeysVisitor visitor = new DefinedKeysVisitor();
         getRootNode().visit(visitor);
 
         return visitor.getKeyList().iterator();
     }
 
     /**
      * Returns an iterator with all keys defined in this configuration that
      * start with the given prefix. The returned keys will not contain any
      * indices.
      *
      * @param prefix the prefix of the keys to start with
      * @return an iterator with the found keys
      */
     public Iterator getKeys(String prefix)
     {
         DefinedKeysVisitor visitor = new DefinedKeysVisitor(prefix);
         List nodes = fetchNodeList(prefix);
 
         for (Iterator itNodes = nodes.iterator(); itNodes.hasNext();)
         {
             ConfigurationNode node = (ConfigurationNode) itNodes.next();
             for (Iterator it = node.getChildren().iterator(); it.hasNext();)
             {
                 ((ConfigurationNode) it.next()).visit(visitor);
             }
             for (Iterator it = node.getAttributes().iterator(); it.hasNext();)
             {
                 ((ConfigurationNode) it.next()).visit(visitor);
             }
         }
 
         return visitor.getKeyList().iterator();
     }
 
     /**
      * Returns the maximum defined index for the given key. This is useful if
      * there are multiple values for this key. They can then be addressed
      * separately by specifying indices from 0 to the return value of this
      * method.
      *
      * @param key the key to be checked
      * @return the maximum defined index for this key
      */
     public int getMaxIndex(String key)
     {
         return fetchNodeList(key).size() - 1;
     }
 
     /**
      * Creates a copy of this object. This new configuration object will contain
      * copies of all nodes in the same structure. Registered event listeners
      * won't be cloned; so they are not registered at the returned copy.
      *
      * @return the copy
      * @since 1.2
      */
     public Object clone()
     {
         try
         {
             HierarchicalConfiguration copy = (HierarchicalConfiguration) super
                     .clone();
 
             // clone the nodes, too
             CloneVisitor v = new CloneVisitor();
             getRootNode().visit(v);
             copy.setRootNode(v.getClone());
 
             return copy;
         }
         catch (CloneNotSupportedException cex)
         {
             // should not happen
             throw new ConfigurationRuntimeException(cex);
         }
     }
 
     /**
      * Returns a configuration with the same content as this configuration, but
      * with all variables replaced by their actual values. This implementation
      * is specific for hierarchical configurations. It clones the current
      * configuration and runs a specialized visitor on the clone, which performs
      * interpolation on the single configuration nodes.
      *
      * @return a configuration with all variables interpolated
      * @since 1.5
      */
     public Configuration interpolatedConfiguration()
     {
         HierarchicalConfiguration c = (HierarchicalConfiguration) clone();
         c.getRootNode().visit(new ConfigurationNodeVisitorAdapter()
         {
             public void visitAfterChildren(ConfigurationNode node)
             {
                 node.setValue(interpolate(node.getValue()));
             }
         });
         return c;
     }
 
     /**
      * Helper method for fetching a list of all nodes that are addressed by the
      * specified key.
      *
      * @param key the key
      * @return a list with all affected nodes (never <b>null </b>)
      */
     protected List fetchNodeList(String key)
     {
         return getExpressionEngine().query(getRootNode(), key);
     }
 
     /**
      * Recursive helper method for fetching a property. This method processes
      * all facets of a configuration key, traverses the tree of properties and
      * fetches the the nodes of all matching properties.
      *
      * @param keyPart the configuration key iterator
      * @param node the actual node
      * @param nodes here the found nodes are stored
      * @deprecated Property keys are now evaluated by the expression engine
      * associated with the configuration; this method will no longer be called.
      * If you want to modify the way properties are looked up, consider
      * implementing you own <code>ExpressionEngine</code> implementation.
      */
     protected void findPropertyNodes(ConfigurationKey.KeyIterator keyPart,
             Node node, Collection nodes)
     {
     }
 
     /**
      * Checks if the specified node is defined.
      *
      * @param node the node to be checked
      * @return a flag if this node is defined
      * @deprecated Use the method <code>{@link #nodeDefined(ConfigurationNode)}</code>
      * instead.
      */
     protected boolean nodeDefined(Node node)
     {
         return nodeDefined((ConfigurationNode) node);
     }
 
     /**
      * Checks if the specified node is defined.
      *
      * @param node the node to be checked
      * @return a flag if this node is defined
      */
     protected boolean nodeDefined(ConfigurationNode node)
     {
         DefinedVisitor visitor = new DefinedVisitor();
         node.visit(visitor);
         return visitor.isDefined();
     }
 
     /**
      * Removes the specified node from this configuration. This method ensures
      * that parent nodes that become undefined by this operation are also
      * removed.
      *
      * @param node the node to be removed
      * @deprecated Use the method <code>{@link #removeNode(ConfigurationNode)}</code>
      * instead.
      */
     protected void removeNode(Node node)
     {
         removeNode((ConfigurationNode) node);
     }
 
     /**
      * Removes the specified node from this configuration. This method ensures
      * that parent nodes that become undefined by this operation are also
      * removed.
      *
      * @param node the node to be removed
      */
     protected void removeNode(ConfigurationNode node)
     {
         ConfigurationNode parent = node.getParentNode();
         if (parent != null)
         {
             parent.removeChild(node);
             if (!nodeDefined(parent))
             {
                 removeNode(parent);
             }
         }
     }
 
     /**
      * Clears the value of the specified node. If the node becomes undefined by
      * this operation, it is removed from the hierarchy.
      *
      * @param node the node to be cleared
      * @deprecated Use the method <code>{@link #clearNode(ConfigurationNode)}</code>
      * instead
      */
     protected void clearNode(Node node)
     {
         clearNode((ConfigurationNode) node);
     }
 
     /**
      * Clears the value of the specified node. If the node becomes undefined by
      * this operation, it is removed from the hierarchy.
      *
      * @param node the node to be cleared
      */
     protected void clearNode(ConfigurationNode node)
     {
         node.setValue(null);
         if (!nodeDefined(node))
         {
             removeNode(node);
         }
     }
 
     /**
      * Returns a reference to the parent node of an add operation. Nodes for new
      * properties can be added as children of this node. If the path for the
      * specified key does not exist so far, it is created now.
      *
      * @param keyIt the iterator for the key of the new property
      * @param startNode the node to start the search with
      * @return the parent node for the add operation
      * @deprecated Adding new properties is now to a major part delegated to the
      * <code>ExpressionEngine</code> associated with this configuration instance.
      * This method will no longer be called. Developers who want to modify the
      * process of adding new properties should consider implementing their own
      * expression engine.
      */
     protected Node fetchAddNode(ConfigurationKey.KeyIterator keyIt, Node startNode)
     {
         return null;
     }
 
     /**
      * Finds the last existing node for an add operation. This method traverses
      * the configuration tree along the specified key. The last existing node on
      * this path is returned.
      *
      * @param keyIt the key iterator
      * @param node the actual node
      * @return the last existing node on the given path
      * @deprecated Adding new properties is now to a major part delegated to the
      * <code>ExpressionEngine</code> associated with this configuration instance.
      * This method will no longer be called. Developers who want to modify the
      * process of adding new properties should consider implementing their own
      * expression engine.
      */
     protected Node findLastPathNode(ConfigurationKey.KeyIterator keyIt, Node node)
     {
         return null;
     }
 
     /**
      * Creates the missing nodes for adding a new property. This method ensures
      * that there are corresponding nodes for all components of the specified
      * configuration key.
      *
      * @param keyIt the key iterator
      * @param root the base node of the path to be created
      * @return the last node of the path
      * @deprecated Adding new properties is now to a major part delegated to the
      * <code>ExpressionEngine</code> associated with this configuration instance.
      * This method will no longer be called. Developers who want to modify the
      * process of adding new properties should consider implementing their own
      * expression engine.
      */
     protected Node createAddPath(ConfigurationKey.KeyIterator keyIt, Node root)
     {
         return null;
     }
 
     /**
      * Creates a new <code>Node</code> object with the specified name. This
      * method can be overloaded in derived classes if a specific node type is
      * needed. This base implementation always returns a new object of the
      * <code>Node</code> class.
      *
      * @param name the name of the new node
      * @return the new node
      */
     protected Node createNode(String name)
     {
         return new Node(name);
     }
 
     /**
      * Helper method for processing a node add data object obtained from the
      * expression engine. This method will create all new nodes.
      *
      * @param data the data object
      * @return the new node
      * @since 1.3
      */
     private ConfigurationNode processNodeAddData(NodeAddData data)
     {
         ConfigurationNode node = data.getParent();
 
         // Create missing nodes on the path
         for (Iterator it = data.getPathNodes().iterator(); it.hasNext();)
         {
             ConfigurationNode child = createNode((String) it.next());
             node.addChild(child);
             node = child;
         }
 
         // Add new target node
         ConfigurationNode child = createNode(data.getNewNodeName());
         if (data.isAttribute())
         {
             node.addAttribute(child);
         }
         else
         {
             node.addChild(child);
         }
         return child;
     }
 
     /**
      * Clears all reference fields in a node structure. A configuration node can
      * store a so-called &quot;reference&quot;. The meaning of this data is
      * determined by a concrete sub class. Typically such references are
      * specific for a configuration instance. If this instance is cloned or
      * copied, they must be cleared. This can be done using this method.
      *
      * @param node the root node of the node hierarchy, in which the references
      * are to be cleared
      * @since 1.4
      */
     protected static void clearReferences(ConfigurationNode node)
     {
         node.visit(new ConfigurationNodeVisitorAdapter()
         {
             public void visitBeforeChildren(ConfigurationNode node)
             {
                 node.setReference(null);
             }
         });
     }
 
     /**
      * A data class for storing (hierarchical) property information. A property
      * can have a value and an arbitrary number of child properties. From
      * version 1.3 on this class is only a thin wrapper over the
      * <code>{@link org.apache.commons.configuration.tree.DefaultConfigurationNode DefaultconfigurationNode}</code>
      * class that exists mainly for the purpose of backwards compatibility.
      */
     public static class Node extends DefaultConfigurationNode implements Serializable
     {
         /**
          * The serial version UID.
          */
         private static final long serialVersionUID = -6357500633536941775L;
 
         /**
          * Creates a new instance of <code>Node</code>.
          */
         public Node()
         {
             super();
         }
 
         /**
          * Creates a new instance of <code>Node</code> and sets the name.
          *
          * @param name the node's name
          */
         public Node(String name)
         {
             super(name);
         }
 
         /**
          * Creates a new instance of <code>Node</code> and sets the name and the value.
          *
          * @param name the node's name
          * @param value the value
          */
         public Node(String name, Object value)
         {
             super(name, value);
         }
 
         /**
          * Creates a new instance of <code>Node</code> based on the given
          * source node. All properties of the source node, including its
          * children and attributes, will be copied.
          *
          * @param src the node to be copied
          */
         public