/*
    * 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.util.Collection;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.Iterator;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Map;
  import java.util.stream.Collectors;
  
  import org.apache.commons.configuration2.event.ConfigurationEvent;
  import org.apache.commons.configuration2.event.EventListener;
  import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
  import org.apache.commons.configuration2.interpol.ConfigurationInterpolator;
  import org.apache.commons.configuration2.tree.ConfigurationNodeVisitorAdapter;
  import org.apache.commons.configuration2.tree.ImmutableNode;
  import org.apache.commons.configuration2.tree.InMemoryNodeModel;
  import org.apache.commons.configuration2.tree.InMemoryNodeModelSupport;
  import org.apache.commons.configuration2.tree.NodeHandler;
  import org.apache.commons.configuration2.tree.NodeModel;
  import org.apache.commons.configuration2.tree.NodeSelector;
  import org.apache.commons.configuration2.tree.NodeTreeWalker;
  import org.apache.commons.configuration2.tree.QueryResult;
  import org.apache.commons.configuration2.tree.ReferenceNodeHandler;
  import org.apache.commons.configuration2.tree.TrackedNodeModel;
  import org.apache.commons.lang3.ObjectUtils;
  
  /**
   * <p>
   * A specialized hierarchical configuration implementation that is based on a structure of {@link ImmutableNode}
   * objects.
   * </p>
   */
  public class BaseHierarchicalConfiguration extends AbstractHierarchicalConfiguration<ImmutableNode> implements InMemoryNodeModelSupport {
  
      /**
       * A specialized visitor base class that can be used for storing the tree of configuration nodes. The basic idea is that
       * each node can be associated with a reference object. This reference object has a concrete meaning in a derived class,
       * for example an entry in a JNDI context or an XML element. When the configuration tree is set up, the {@code load()} method
       * is responsible for setting the reference objects. When the configuration tree is later modified, new nodes do not
       * have a defined reference object. This visitor class processes all nodes and finds the ones without a defined
       * reference object. For those nodes the {@code insert()} method is called, which must be defined in concrete sub
       * classes. This method can perform all steps to integrate the new node into the original structure.
       */
      protected abstract static class BuilderVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
  
          /**
           * Constructs a new instance.
           */
          public BuilderVisitor() {
              // empty
          }
  
          /**
           * Inserts a new node into the structure constructed by this builder. This method is called for each node that has been
           * added to the configuration tree after the configuration has been loaded from its source. These new nodes have to be
           * inserted into the original structure. The passed in nodes define the position of the node to be inserted: its parent
           * and the siblings between to insert.
           *
           * @param newNode the node to be inserted
           * @param parent the parent node
           * @param sibling1 the sibling after which the node is to be inserted; can be <strong>null</strong> if the new node is going to be
           *        the first child node
           * @param sibling2 the sibling before which the node is to be inserted; can be <strong>null</strong> if the new node is going to
           *        be the last child node
           * @param refHandler the {@code ReferenceNodeHandler}
           */
          protected abstract void insert(ImmutableNode newNode, ImmutableNode parent, ImmutableNode sibling1, ImmutableNode sibling2,
              ReferenceNodeHandler refHandler);
  
          /**
           * Inserts new children that have been added to the specified node.
           *
           * @param node the current node to be processed
           * @param refHandler the {@code ReferenceNodeHandler}
           */
          private void insertNewChildNodes(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
              final Collection<ImmutableNode> subNodes = new LinkedList<>(refHandler.getChildren(node));
              final Iterator<ImmutableNode> children = subNodes.iterator();
              ImmutableNode sibling1;
              ImmutableNode nd = null;
 
             while (children.hasNext()) {
                 // find the next new node
                 do {
                     sibling1 = nd;
                     nd = children.next();
                 } while (refHandler.getReference(nd) != null && children.hasNext());
 
                 if (refHandler.getReference(nd) == null) {
                     // find all following new nodes
                     final List<ImmutableNode> newNodes = new LinkedList<>();
                     newNodes.add(nd);
                     while (children.hasNext()) {
                         nd = children.next();
                         if (refHandler.getReference(nd) != null) {
                             break;
                         }
                         newNodes.add(nd);
                     }
 
                     // Insert all new nodes
                     final ImmutableNode sibling2 = refHandler.getReference(nd) == null ? null : nd;
                     for (final ImmutableNode insertNode : newNodes) {
                         if (refHandler.getReference(insertNode) == null) {
                             insert(insertNode, node, sibling1, sibling2, refHandler);
                             sibling1 = insertNode;
                         }
                     }
                 }
             }
         }
 
         /**
          * Updates a node that already existed in the original hierarchy. This method is called for each node that has an
          * assigned reference object. A concrete implementation should update the reference according to the node's current
          * value.
          *
          * @param node the current node to be processed
          * @param reference the reference object for this node
          * @param refHandler the {@code ReferenceNodeHandler}
          */
         protected abstract void update(ImmutableNode node, Object reference, ReferenceNodeHandler refHandler);
 
         /**
          * Updates the value of a node. If this node is associated with a reference object, the {@code update()} method is
          * called.
          *
          * @param node the current node to be processed
          * @param refHandler the {@code ReferenceNodeHandler}
          */
         private void updateNode(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
             final Object reference = refHandler.getReference(node);
             if (reference != null) {
                 update(node, reference, refHandler);
             }
         }
 
         @Override
         public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
             final ReferenceNodeHandler refHandler = (ReferenceNodeHandler) handler;
             updateNode(node, refHandler);
             insertNewChildNodes(node, refHandler);
         }
     }
 
     /**
      * A specialized visitor implementation which constructs the root node of a configuration with all variables replaced by
      * their interpolated values.
      */
     private final class InterpolatedVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
 
         /** A stack for managing node builder instances. */
         private final List<ImmutableNode.Builder> builderStack;
 
         /** The resulting root node. */
         private ImmutableNode interpolatedRoot;
 
         /**
          * Creates a new instance of {@code InterpolatedVisitor}.
          */
         public InterpolatedVisitor() {
             builderStack = new LinkedList<>();
         }
 
         /**
          * Gets the result of this builder: the root node of the interpolated nodes hierarchy.
          *
          * @return the resulting root node
          */
         public ImmutableNode getInterpolatedRoot() {
             return interpolatedRoot;
         }
 
         /**
          * Handles interpolation for a node with no children. If interpolation does not change this node, it is copied as is to
          * the resulting structure. Otherwise, a new node is created with the interpolated values.
          *
          * @param node the current node to be processed
          * @param handler the {@code NodeHandler}
          */
         private void handleLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
             final Object value = interpolate(node.getValue());
             final Map<String, Object> interpolatedAttributes = new HashMap<>();
             final boolean attributeChanged = interpolateAttributes(node, handler, interpolatedAttributes);
             final ImmutableNode newNode = valueChanged(value, handler.getValue(node)) || attributeChanged
                 ? new ImmutableNode.Builder().name(handler.nodeName(node)).value(value).addAttributes(interpolatedAttributes).create()
                 : node;
             storeInterpolatedNode(newNode);
         }
 
         /**
          * Returns a map with interpolated attributes of the passed in node.
          *
          * @param node the current node to be processed
          * @param handler the {@code NodeHandler}
          * @return the map with interpolated attributes
          */
         private Map<String, Object> interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
             final Map<String, Object> attributes = new HashMap<>();
             interpolateAttributes(node, handler, attributes);
             return attributes;
         }
 
         /**
          * Populates a map with interpolated attributes of the passed in node.
          *
          * @param node the current node to be processed
          * @param handler the {@code NodeHandler}
          * @param interpolatedAttributes a map for storing the results
          * @return a flag whether an attribute value was changed by interpolation
          */
         private boolean interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler,
             final Map<String, Object> interpolatedAttributes) {
             boolean attributeChanged = false;
             for (final String attr : handler.getAttributes(node)) {
                 final Object attrValue = interpolate(handler.getAttributeValue(node, attr));
                 if (valueChanged(attrValue, handler.getAttributeValue(node, attr))) {
                     attributeChanged = true;
                 }
                 interpolatedAttributes.put(attr, attrValue);
             }
             return attributeChanged;
         }
 
         /**
          * Returns a flag whether the given node is a leaf. This is the case if it does not have children.
          *
          * @param node the node in question
          * @param handler the {@code NodeHandler}
          * @return a flag whether this is a leaf node
          */
         private boolean isLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
             return handler.getChildren(node).isEmpty();
         }
 
         /**
          * Returns the top-level element from the stack without removing it.
          *
          * @return the top-level element from the stack
          */
         private ImmutableNode.Builder peek() {
             return builderStack.get(0);
         }
 
         /**
          * Pops the top-level element from the stack.
          *
          * @return the element popped from the stack
          */
         private ImmutableNode.Builder pop() {
             return builderStack.remove(0);
         }
 
         /**
          * Pushes a new builder on the stack.
          *
          * @param builder the builder
          */
         private void push(final ImmutableNode.Builder builder) {
             builderStack.add(0, builder);
         }
 
         /**
          * Stores a processed node. Per default, the node is added to the current builder on the stack. If no such builder
          * exists, this is the result node.
          *
          * @param node the node to be stored
          */
         private void storeInterpolatedNode(final ImmutableNode node) {
             if (builderStack.isEmpty()) {
                 interpolatedRoot = node;
             } else {
                 peek().addChild(node);
             }
         }
 
         /**
          * Tests whether a value is changed because of interpolation.
          *
          * @param interpolatedValue the interpolated value
          * @param value the original value
          * @return a flag whether the value was changed
          */
         private boolean valueChanged(final Object interpolatedValue, final Object value) {
             return ObjectUtils.notEqual(interpolatedValue, value);
         }
 
         @Override
         public void visitAfterChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
             if (!isLeafNode(node, handler)) {
                 final ImmutableNode newNode = pop().create();
                 storeInterpolatedNode(newNode);
             }
         }
 
         @Override
         public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
             if (isLeafNode(node, handler)) {
                 handleLeafNode(node, handler);
             } else {
                 final ImmutableNode.Builder builder = new ImmutableNode.Builder(handler.getChildrenCount(node, null)).name(handler.nodeName(node))
                     .value(interpolate(handler.getValue(node))).addAttributes(interpolateAttributes(node, handler));
                 push(builder);
             }
         }
     }
 
     /**
      * Creates the {@code NodeModel} for this configuration based on a passed in source configuration. This implementation
      * creates an {@link InMemoryNodeModel}. If the passed in source configuration is defined, its root node also becomes
      * the root node of this configuration. Otherwise, a new, empty root node is used.
      *
      * @param c the configuration that is to be copied
      * @return the {@code NodeModel} for the new configuration
      */
     private static NodeModel<ImmutableNode> createNodeModel(final HierarchicalConfiguration<ImmutableNode> c) {
         return new InMemoryNodeModel(obtainRootNode(c));
     }
 
     /**
      * Obtains the root node from a configuration whose data is to be copied. It has to be ensured that the synchronizer is
      * called correctly.
      *
      * @param c the configuration that is to be copied
      * @return the root node of this configuration
      */
     private static ImmutableNode obtainRootNode(final HierarchicalConfiguration<ImmutableNode> c) {
         return c != null ? c.getNodeModel().getNodeHandler().getRootNode() : null;
     }
 
     /**
      * Creates a list with immutable configurations from the given input list.
      *
      * @param subs a list with mutable configurations
      * @return a list with corresponding immutable configurations
      */
     private static List<ImmutableHierarchicalConfiguration> toImmutable(final List<? extends HierarchicalConfiguration<?>> subs) {
         return subs.stream().map(ConfigurationUtils::unmodifiableConfiguration).collect(Collectors.toList());
     }
 
     /** A listener for reacting on changes caused by sub configurations. */
     private final EventListener<ConfigurationEvent> changeListener;
 
     /**
      * Creates a new instance of {@code BaseHierarchicalConfiguration}.
      */
     public BaseHierarchicalConfiguration() {
         this((HierarchicalConfiguration<ImmutableNode>) null);
     }
 
     /**
      * Creates a new instance of {@code BaseHierarchicalConfiguration} and copies all data contained in the specified
      * configuration into the new one.
      *
      * @param c the configuration that is to be copied (if <strong>null</strong>, this constructor will behave like the standard
      *        constructor)
      * @since 1.4
      */
     public BaseHierarchicalConfiguration(final HierarchicalConfiguration<ImmutableNode> c) {
         this(createNodeModel(c));
     }
 
     /**
      * Creates a new instance of {@code BaseHierarchicalConfiguration} and initializes it with the given {@code NodeModel}.
      *
      * @param model the {@code NodeModel}
      */
     protected BaseHierarchicalConfiguration(final NodeModel<ImmutableNode> model) {
         super(model);
         changeListener = createChangeListener();
     }
 
     /**
      * {@inheritDoc} This implementation resolves the node(s) selected by the given key. If not a single node is selected,
      * an empty list is returned. Otherwise, sub configurations for each child of the node are created.
      */
     @Override
     public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key) {
         final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
         if (nodes.size() != 1) {
             return Collections.emptyList();
         }
         return nodes.get(0).stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
     }
 
     /**
      * {@inheritDoc} This method works like {@link #childConfigurationsAt(String)}; however, depending on the value of the
      * {@code supportUpdates} flag, connected sub configurations may be created.
      */
     @Override
     public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key, final boolean supportUpdates) {
         if (!supportUpdates) {
             return childConfigurationsAt(key);
         }
 
         final InMemoryNodeModel parentModel = getSubConfigurationParentModel();
         return createConnectedSubConfigurations(this, parentModel.trackChildNodes(key, this));
     }
 
     /**
      * {@inheritDoc} This implementation creates a new instance of {@link InMemoryNodeModel}, initialized with this
      * configuration's root node. This has the effect that although the same nodes are used, the original and copied
      * configurations are independent on each other.
      */
     @Override
     protected NodeModel<ImmutableNode> cloneNodeModel() {
         return new InMemoryNodeModel(getModel().getNodeHandler().getRootNode());
     }
 
     /**
      * {@inheritDoc} This is a short form for {@code configurationAt(key,
      * <strong>false</strong>)}.
      *
      * @throws ConfigurationRuntimeException if the key does not select a single node
      */
     @Override
     public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key) {
         return configurationAt(key, false);
     }
 
     /**
      * {@inheritDoc} The result of this implementation depends on the {@code supportUpdates} flag: If it is <strong>false</strong>, a
      * plain {@code BaseHierarchicalConfiguration} is returned using the selected node as root node. This is suitable for
      * read-only access to properties. Because the configuration returned in this case is not connected to the parent
      * configuration, updates on properties made by one configuration are not reflected by the other one. A value of
      * <strong>true</strong> for this parameter causes a tracked node to be created, and result is a {@link SubnodeConfiguration}
      * based on this tracked node. This configuration is really connected to its parent, so that updated properties are
      * visible on both.
      *
      * @see SubnodeConfiguration
      * @throws ConfigurationRuntimeException if the key does not select a single node
      */
     @Override
     public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key, final boolean supportUpdates) {
         return syncRead(() -> supportUpdates ? createConnectedSubConfiguration(key) : createIndependentSubConfiguration(key), false);
     }
 
     /**
      * {@inheritDoc} This implementation creates sub configurations in the same way as described for
      * {@link #configurationAt(String)}.
      */
     @Override
     public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key) {
         final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
         return nodes.stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
     }
 
     /**
      * {@inheritDoc} This implementation creates tracked nodes for the specified key. Then sub configurations for these
      * nodes are created and returned.
      */
     @Override
     public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key, final boolean supportUpdates) {
         if (!supportUpdates) {
             return configurationsAt(key);
         }
         final InMemoryNodeModel parentModel = syncRead(this::getSubConfigurationParentModel, false);
         return createConnectedSubConfigurations(this, parentModel.selectAndTrackNodes(key, this));
     }
 
     /**
      * Creates a listener which reacts on all changes on this configuration or one of its {@code SubnodeConfiguration}
      * instances. If such a change is detected, some updates have to be performed.
      *
      * @return the newly created change listener
      */
     private EventListener<ConfigurationEvent> createChangeListener() {
         return this::subnodeConfigurationChanged;
     }
 
     /**
      * Creates a sub configuration from the specified key which is connected to this configuration. This implementation
      * creates a {@link SubnodeConfiguration} with a tracked node identified by the passed in key.
      *
      * @param key the key of the sub configuration
      * @return the new sub configuration
      */
     private BaseHierarchicalConfiguration createConnectedSubConfiguration(final String key) {
         final NodeSelector selector = getSubConfigurationNodeSelector(key);
         getSubConfigurationParentModel().trackNode(selector, this);
         return createSubConfigurationForTrackedNode(selector, this);
     }
 
     /**
      * Creates a list of connected sub configurations based on a passed in list of node selectors.
      *
      * @param parentModelSupport the parent node model support object
      * @param selectors the list of {@code NodeSelector} objects
      * @return the list with sub configurations
      */
     private List<HierarchicalConfiguration<ImmutableNode>> createConnectedSubConfigurations(final InMemoryNodeModelSupport parentModelSupport,
         final Collection<NodeSelector> selectors) {
         return selectors.stream().map(sel -> createSubConfigurationForTrackedNode(sel, parentModelSupport)).collect(Collectors.toList());
     }
 
     /**
      * Creates a sub configuration from the specified key which is independent on this configuration. This means that the
      * sub configuration operates on a separate node model (although the nodes are initially shared).
      *
      * @param key the key of the sub configuration
      * @return the new sub configuration
      */
     private BaseHierarchicalConfiguration createIndependentSubConfiguration(final String key) {
         final List<ImmutableNode> targetNodes = fetchFilteredNodeResults(key);
         final int size = targetNodes.size();
         if (size != 1) {
             throw new ConfigurationRuntimeException("Passed in key must select exactly one node (found %,d): %s", size, key);
         }
         final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(targetNodes.get(0)));
         initSubConfiguration(sub);
         return sub;
     }
 
     /**
      * Returns an initialized sub configuration for this configuration that is based on another
      * {@code BaseHierarchicalConfiguration}. Thus, it is independent from this configuration.
      *
      * @param node the root node for the sub configuration
      * @return the initialized sub configuration
      */
     private BaseHierarchicalConfiguration createIndependentSubConfigurationForNode(final ImmutableNode node) {
         final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(node));
         initSubConfiguration(sub);
         return sub;
     }
 
     /**
      * Creates a connected sub configuration based on a selector for a tracked node.
      *
      * @param selector the {@code NodeSelector}
      * @param parentModelSupport the {@code InMemoryNodeModelSupport} object for the parent node model
      * @return the newly created sub configuration
      * @since 2.0
      */
     protected SubnodeConfiguration createSubConfigurationForTrackedNode(final NodeSelector selector, final InMemoryNodeModelSupport parentModelSupport) {
         final SubnodeConfiguration subConfig = new SubnodeConfiguration(this, new TrackedNodeModel(parentModelSupport, selector, true));
         initSubConfigurationForThisParent(subConfig);
         return subConfig;
     }
 
     /**
      * Creates a root node for a subset configuration based on the passed in query results. This method creates a new root
      * node and adds the children and attributes of all result nodes to it. If only a single node value is defined, it is
      * assigned as value of the new root node.
      *
      * @param results the collection of query results
      * @return the root node for the subset configuration
      */
     private ImmutableNode createSubsetRootNode(final Collection<QueryResult<ImmutableNode>> results) {
         final ImmutableNode.Builder builder = new ImmutableNode.Builder();
         Object value = null;
         int valueCount = 0;
 
         for (final QueryResult<ImmutableNode> result : results) {
             if (result.isAttributeResult()) {
                 builder.addAttribute(result.getAttributeName(), result.getAttributeValue(getModel().getNodeHandler()));
             } else {
                 if (result.getNode().getValue() != null) {
                     value = result.getNode().getValue();
                     valueCount++;
                 }
                 builder.addChildren(result.getNode().getChildren());
                 builder.addAttributes(result.getNode().getAttributes());
             }
         }
 
         if (valueCount == 1) {
             builder.value(value);
         }
         return builder.create();
     }
 
     /**
      * Executes a query on the specified key and filters it for node results.
      *
      * @param key the key
      * @return the filtered list with result nodes
      */
     private List<ImmutableNode> fetchFilteredNodeResults(final String key) {
         final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
         return resolveNodeKey(handler.getRootNode(), key, handler);
     }
 
     /**
      * {@inheritDoc} This implementation returns the {@code InMemoryNodeModel} used by this configuration.
      */
     @Override
     public InMemoryNodeModel getNodeModel() {
         return (InMemoryNodeModel) super.getNodeModel();
     }
 
     /**
      * Gets the {@code NodeSelector} to be used for a sub configuration based on the passed in key. This method is called
      * whenever a sub configuration is to be created. This base implementation returns a new {@code NodeSelector}
      * initialized with the passed in key. Sub classes may override this method if they have a different strategy for
      * creating a selector.
      *
      * @param key the key of the sub configuration
      * @return a {@code NodeSelector} for initializing a sub configuration
      * @since 2.0
      */
     protected NodeSelector getSubConfigurationNodeSelector(final String key) {
         return new NodeSelector(key);
     }
 
     /**
      * Gets the {@code InMemoryNodeModel} to be used as parent model for a new sub configuration. This method is called
      * whenever a sub configuration is to be created. This base implementation returns the model of this configuration. Sub
      * classes with different requirements for the parent models of sub configurations have to override it.
      *
      * @return the parent model for a new sub configuration
      */
     protected InMemoryNodeModel getSubConfigurationParentModel() {
         return (InMemoryNodeModel) getModel();
     }
 
     /**
      * {@inheritDoc} This implementation first delegates to {@code childConfigurationsAt()} to create a list of mutable
      * child configurations. Then a list with immutable wrapper configurations is created.
      */
     @Override
     public List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(final String key) {
         return toImmutable(childConfigurationsAt(key));
     }
 
     /**
      * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
      * Then an immutable wrapper is created and returned.
      *
      * @throws ConfigurationRuntimeException if the key does not select a single node
      */
     @Override
     public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key) {
         return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key));
     }
 
     /**
      * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
      * Then an immutable wrapper is created and returned.
      */
     @Override
     public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key, final boolean supportUpdates) {
         return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key, supportUpdates));
     }
 
     /**
      * {@inheritDoc} This implementation first delegates to {@code configurationsAt()} to create a list of
      * {@code SubnodeConfiguration} objects. Then for each element of this list an unmodifiable wrapper is created.
      */
     @Override
     public List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(final String key) {
         return toImmutable(configurationsAt(key));
     }
 
     /**
      * Initializes properties of a sub configuration. A sub configuration inherits some settings from its parent, for example the
      * expression engine or the synchronizer. The corresponding values are copied by this method.
      *
      * @param sub the sub configuration to be initialized
      */
     private void initSubConfiguration(final BaseHierarchicalConfiguration sub) {
         sub.setSynchronizer(getSynchronizer());
         sub.setExpressionEngine(getExpressionEngine());
         sub.setListDelimiterHandler(getListDelimiterHandler());
         sub.setThrowExceptionOnMissing(isThrowExceptionOnMissing());
         sub.getInterpolator().setParentInterpolator(getInterpolator());
     }
 
     /**
      * Initializes a {@code SubnodeConfiguration} object. This method should be called for each sub configuration created
      * for this configuration. It ensures that the sub configuration is correctly connected to its parent instance and that
      * update events are correctly propagated.
      *
      * @param subConfig the sub configuration to be initialized
      * @since 2.0
      */
     protected void initSubConfigurationForThisParent(final SubnodeConfiguration subConfig) {
         initSubConfiguration(subConfig);
         subConfig.addEventListener(ConfigurationEvent.ANY, changeListener);
     }
 
     /**
      * 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
      */
     @Override
     public Configuration interpolatedConfiguration() {
         final InterpolatedVisitor visitor = new InterpolatedVisitor();
         final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
         NodeTreeWalker.INSTANCE.walkDFS(handler.getRootNode(), visitor, handler);
 
         final BaseHierarchicalConfiguration c = (BaseHierarchicalConfiguration) clone();
         c.getNodeModel().setRootNode(visitor.getInterpolatedRoot());
         return c;
     }
 
     /**
      * 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 SUBNODE_CHANGED} and notifies the
      * registered listeners.
      *
      * @param event the event describing the change
      * @since 1.5
      */
     protected void subnodeConfigurationChanged(final ConfigurationEvent event) {
         fireEvent(ConfigurationEvent.SUBNODE_CHANGED, null, event, event.isBeforeUpdate());
     }
 
     /**
      * Creates a new {@code Configuration} object containing all keys that start with the specified prefix. This
      * implementation will return a {@code BaseHierarchicalConfiguration} 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} object is not connected to its source configuration: updates on the source configuration are
      * not reflected in the subset and vice versa. The returned configuration uses the same {@code Synchronizer} as this
      * configuration.
      *
      * @param prefix the prefix of the keys for the subset
      * @return a new configuration object representing the selected subset
      */
     @Override
     public Configuration subset(final String prefix) {
         return syncRead(() -> {
             final List<QueryResult<ImmutableNode>> results = fetchNodeList(prefix);
             if (results.isEmpty()) {
                 return new BaseHierarchicalConfiguration();
             }
             final BaseHierarchicalConfiguration parent = this;
             final BaseHierarchicalConfiguration result = new BaseHierarchicalConfiguration() {
 
                 @Override
                 public ConfigurationInterpolator getInterpolator() {
                     return parent.getInterpolator();
                 }
 
                 // Override interpolate to always interpolate on the parent
                 @Override
                 protected Object interpolate(final Object value) {
                     return parent.interpolate(value);
                 }
             };
             result.getModel().setRootNode(createSubsetRootNode(results));
             if (result.isEmpty()) {
                 return new BaseHierarchicalConfiguration();
             }
             result.setSynchronizer(getSynchronizer());
             return result;
         }, false);
     }
 }