/*
 * 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.configuration2;

import java.util.ArrayList;
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 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;

 A specialized hierarchical configuration implementation that is based on a structure of ImmutableNode objects. 
/**
 * <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 listener for reacting on changes caused by sub configurations. /** A listener for reacting on changes caused by sub configurations. */
    private final EventListener<ConfigurationEvent> changeListener;

    
Creates a new instance of BaseHierarchicalConfiguration. /**
     * Creates a new instance of {@code BaseHierarchicalConfiguration}.
     */
    public BaseHierarchicalConfiguration()
    {
        this((HierarchicalConfiguration<ImmutableNode>) null);
    }

    
Creates a new instance of BaseHierarchicalConfiguration and copies all data contained in the specified configuration into the new one. 
Params:
c – the configuration that is to be copied (if null, this
constructor will behave like the standard constructor)
Since:
1.4/**
     * 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 <b>null</b>, 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 BaseHierarchicalConfiguration and initializes it with the given NodeModel. 
Params:
model – the NodeModel/**
     * 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 returns the InMemoryNodeModel used by this configuration. /**
     * {@inheritDoc} This implementation returns the {@code InMemoryNodeModel}
     * used by this configuration.
     */
    @Override
    public InMemoryNodeModel getNodeModel()
    {
        return (InMemoryNodeModel) super.getNodeModel();
    }

    
Creates a new Configuration object containing all keys that start with the specified prefix. This implementation will return a 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 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 Synchronizer as this configuration. 
Params:
prefix – the prefix of the keys for the subset
Returns:
a new configuration object representing the selected subset/**
     * 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)
    {
        beginRead(false);
        try
        {
            final List<QueryResult<ImmutableNode>> results = fetchNodeList(prefix);
            if (results.isEmpty())
            {
                return new BaseHierarchicalConfiguration();
            }

            final BaseHierarchicalConfiguration parent = this;
            final BaseHierarchicalConfiguration result =
                    new BaseHierarchicalConfiguration()
                    {
                        // Override interpolate to always interpolate on the parent
                        @Override
                        protected Object interpolate(final Object value)
                        {
                            return parent.interpolate(value);
                        }

                        @Override
                        public ConfigurationInterpolator getInterpolator()
                        {
                            return parent.getInterpolator();
                        }
                    };
            result.getModel().setRootNode(createSubsetRootNode(results));

            if (result.isEmpty())
            {
                return new BaseHierarchicalConfiguration();
            }
            result.setSynchronizer(getSynchronizer());
            return result;
        }
        finally
        {
            endRead();
        }
    }

    
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.
Params:
results – the collection of query results
Returns:
the root node for the subset configuration/**
     * 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();
    }

    
{@inheritDoc} The result of this implementation depends on the supportUpdates flag: If it is false, a plain 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 true for this parameter causes a tracked node to be created, and result is a SubnodeConfiguration based on this tracked node. This configuration is really connected to its parent, so that updated properties are visible on both. 
Throws:
ConfigurationRuntimeException – if the key does not select a single
        node
See Also:
SubnodeConfiguration/**
     * {@inheritDoc} The result of this implementation depends on the
     * {@code supportUpdates} flag: If it is <b>false</b>, 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 <b>true</b> 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)
    {
        beginRead(false);
        try
        {
            return supportUpdates ? createConnectedSubConfiguration(key)
                    : createIndependentSubConfiguration(key);
        }
        finally
        {
            endRead();
        }
    }

    
Returns the 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. 
Returns:
the parent model for a new sub configuration/**
     * Returns 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();
    }

    
Returns the 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 NodeSelector initialized with the passed in key. Sub classes may override this method if they have a different strategy for creating a selector. 
Params:
key – the key of the sub configuration
Returns:
a NodeSelector for initializing a sub configuration
Since:
2.0/**
     * Returns 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);
    }

    
Creates a connected sub configuration based on a selector for a tracked
node.
Params:
selector – the NodeSelector
parentModelSupport – the InMemoryNodeModelSupport object for the parent node model
Returns:
the newly created sub configuration
Since:
2.0/**
     * 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;
    }

    
Initializes a 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. 
Params:
subConfig – the sub configuration to be initialized
Since:
2.0/**
     * 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);
    }

    
Creates a sub configuration from the specified key which is connected to this configuration. This implementation creates a SubnodeConfiguration with a tracked node identified by the passed in key. 
Params:
key – the key of the sub configuration
Returns:
the new sub configuration/**
     * 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.
Params:
parentModelSupport – the parent node model support object
selectors – the list of NodeSelector objects
Returns:
the list with sub configurations/**
     * 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)
    {
        final List<HierarchicalConfiguration<ImmutableNode>> configs =
                new ArrayList<>(
                        selectors.size());
        for (final NodeSelector selector : selectors)
        {
            configs.add(createSubConfigurationForTrackedNode(selector,
                    parentModelSupport));
        }
        return configs;
    }

    
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).
Params:
key – the key of the sub configuration
Returns:
the new sub configuration/**
     * 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 BaseHierarchicalConfiguration. Thus, it is independent from this configuration. 
Params:
node – the root node for the sub configuration
Returns:
the initialized sub configuration/**
     * 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;
    }

    
Executes a query on the specified key and filters it for node results.
Params:
key – the key
Returns:
the filtered list with result nodes/**
     * 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 creates a SubnodeConfiguration by delegating to configurationAt(). Then an immutable wrapper is created and returned. /**
     * {@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 is a short form for configurationAt(key,
<b>false</b>). 
Throws:
ConfigurationRuntimeException – if the key does not select a single node/**
     * {@inheritDoc} This is a short form for {@code configurationAt(key,
     * <b>false</b>)}.
     * @throws ConfigurationRuntimeException if the key does not select a single node
     */
    @Override
    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key)
    {
        return configurationAt(key, false);
    }

    
{@inheritDoc} This implementation creates a SubnodeConfiguration by delegating to configurationAt(). Then an immutable wrapper is created and returned. 
Throws:
ConfigurationRuntimeException – if the key does not select a single node/**
     * {@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 sub configurations in the same way as described for configurationAt(String). /**
     * {@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)
    {
        List<ImmutableNode> nodes;
        beginRead(false);
        try
        {
            nodes = fetchFilteredNodeResults(key);
        }
        finally
        {
            endRead();
        }

        final List<HierarchicalConfiguration<ImmutableNode>> results =
                new ArrayList<>(
                        nodes.size());
        for (final ImmutableNode node : nodes)
        {
            final BaseHierarchicalConfiguration sub =
                    createIndependentSubConfigurationForNode(node);
            results.add(sub);
        }

        return results;
    }

    
{@inheritDoc} This implementation creates tracked nodes for the specified
key. Then sub configurations for these nodes are created and returned.
/**
     * {@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);
        }

        InMemoryNodeModel parentModel;
        beginRead(false);
        try
        {
            parentModel = getSubConfigurationParentModel();
        }
        finally
        {
            endRead();
        }

        final Collection<NodeSelector> selectors =
                parentModel.selectAndTrackNodes(key, this);
        return createConnectedSubConfigurations(this, selectors);
    }

    
{@inheritDoc} This implementation first delegates to configurationsAt() to create a list of SubnodeConfiguration objects. Then for each element of this list an unmodifiable wrapper is created. /**
     * {@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));
    }

    
{@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.
/**
     * {@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)
    {
        List<ImmutableNode> nodes;
        beginRead(false);
        try
        {
            nodes = fetchFilteredNodeResults(key);
        }
        finally
        {
            endRead();
        }

        if (nodes.size() != 1)
        {
            return Collections.emptyList();
        }

        final ImmutableNode parent = nodes.get(0);
        final List<HierarchicalConfiguration<ImmutableNode>> subs =
                new ArrayList<>(parent
                        .getChildren().size());
        for (final ImmutableNode node : parent.getChildren())
        {
            subs.add(createIndependentSubConfigurationForNode(node));
        }

        return subs;
    }

    
{@inheritDoc} This method works like childConfigurationsAt(String); however, depending on the value of the supportUpdates flag, connected sub configurations may be created. /**
     * {@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 first delegates to childConfigurationsAt() to create a list of mutable child configurations. Then a list with immutable wrapper configurations is created. /**
     * {@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));
    }

    
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 SUBNODE_CHANGED and notifies the registered listeners. 
Params:
event – the event describing the change
Since:
1.5/**
     * 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());
    }

    
Initializes properties of a sub configuration. A sub configuration
inherits some settings from its parent, e.g. the expression engine or the
synchronizer. The corresponding values are copied by this method.
Params:
sub – the sub configuration to be initialized/**
     * Initializes properties of a sub configuration. A sub configuration
     * inherits some settings from its parent, e.g. 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());
    }

    
Creates a listener which reacts on all changes on this configuration or one of its SubnodeConfiguration instances. If such a change is detected, some updates have to be performed. 
Returns:
the newly created change listener/**
     * 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 event -> subnodeConfigurationChanged(event);
    }

    
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.
Returns:
a configuration with all variables interpolated
Since:
1.5/**
     * 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;
    }

    
{@inheritDoc} This implementation creates a new instance of 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. /**
     * {@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());
    }

    
Creates a list with immutable configurations from the given input list.
Params:
subs – a list with mutable configurations
Returns:
a list with corresponding immutable configurations/**
     * 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)
    {
        final List<ImmutableHierarchicalConfiguration> res =
                new ArrayList<>(subs.size());
        for (final HierarchicalConfiguration<?> sub : subs)
        {
            res.add(ConfigurationUtils.unmodifiableConfiguration(sub));
        }
        return res;
    }

    
Creates the NodeModel for this configuration based on a passed in source configuration. This implementation creates an 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. 
Params:
c – the configuration that is to be copied
Returns:
the NodeModel for the new configuration/**
     * 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)
    {
        final ImmutableNode root = c != null ? obtainRootNode(c) : null;
        return new InMemoryNodeModel(root);
    }

    
Obtains the root node from a configuration whose data is to be copied. It
has to be ensured that the synchronizer is called correctly.
Params:
c – the configuration that is to be copied
Returns:
the root node of this configuration/**
     * 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.getNodeModel().getNodeHandler().getRootNode();
    }

    
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, e.g. an entry in a JNDI context or an XML element. When the configuration tree is set up, the 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 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. /**
     * 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, e.g. 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>
    {
        @Override
        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler)
        {
            final ReferenceNodeHandler refHandler = (ReferenceNodeHandler) handler;
            updateNode(node, refHandler);
            insertNewChildNodes(node, refHandler);
        }

        
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.
Params:
newNode – the node to be inserted
parent – the parent node
sibling1 – the sibling after which the node is to be inserted;
       can be null if the new node is going to be the first
       child node
sibling2 – the sibling before which the node is to be inserted;
       can be null if the new node is going to be the last
       child node
refHandler – the ReferenceNodeHandler/**
         * 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 <b>null</b> 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 <b>null</b> 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);

        
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.
Params:
node – the current node to be processed
reference – the reference object for this node
refHandler – the ReferenceNodeHandler/**
         * 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 update() method is called. 
Params:
node – the current node to be processed
refHandler – the ReferenceNodeHandler/**
         * 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);
            }
        }

        
Inserts new children that have been added to the specified node.
Params:
node – the current node to be processed
refHandler – the ReferenceNodeHandler/**
         * 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)
                        {
                            newNodes.add(nd);
                        }
                        else
                        {
                            break;
                        }
                    }

                    // 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;
                        }
                    }
                }
            }
        }
    }

    
A specialized visitor implementation which constructs the root node of a
configuration with all variables replaced by their interpolated values.
/**
     * A specialized visitor implementation which constructs the root node of a
     * configuration with all variables replaced by their interpolated values.
     */
    private class InterpolatedVisitor extends
            ConfigurationNodeVisitorAdapter<ImmutableNode>
    {
        
A stack for managing node builder instances. /** A stack for managing node builder instances. */
        private final List<ImmutableNode.Builder> builderStack;

        
The resulting root node. /** The resulting root node. */
        private ImmutableNode interpolatedRoot;

        
Creates a new instance of InterpolatedVisitor. /**
         * Creates a new instance of {@code InterpolatedVisitor}.
         */
        public InterpolatedVisitor()
        {
            builderStack = new LinkedList<>();
        }

        
Returns the result of this builder: the root node of the interpolated
nodes hierarchy.
Returns:
the resulting root node/**
         * Returns the result of this builder: the root node of the interpolated
         * nodes hierarchy.
         *
         * @return the resulting root node
         */
        public ImmutableNode getInterpolatedRoot()
        {
            return interpolatedRoot;
        }

        @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);
            }
        }

        @Override
        public void visitAfterChildren(final ImmutableNode node,
                final NodeHandler<ImmutableNode> handler)
        {
            if (!isLeafNode(node, handler))
            {
                final ImmutableNode newNode = pop().create();
                storeInterpolatedNode(newNode);
            }
        }

        
Pushes a new builder on the stack.
Params:
builder – the builder/**
         * Pushes a new builder on the stack.
         *
         * @param builder the builder
         */
        private void push(final ImmutableNode.Builder builder)
        {
            builderStack.add(0, builder);
        }

        
Pops the top-level element from the stack.
Returns:
the element popped from the stack/**
         * Pops the top-level element from the stack.
         *
         * @return the element popped from the stack
         */
        private ImmutableNode.Builder pop()
        {
            return builderStack.remove(0);
        }

        
Returns the top-level element from the stack without removing it.
Returns:
the top-level element from the stack/**
         * 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);
        }

        
Returns a flag whether the given node is a leaf. This is the case if
it does not have children.
Params:
node – the node in question
handler – the NodeHandler
Returns:
a flag whether this is a leaf node/**
         * 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();
        }

        
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.
Params:
node – the current node to be processed
handler – the NodeHandler/**
         * 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);
        }

        
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.
Params:
node – the node to be stored/**
         * 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);
            }
        }

        
Populates a map with interpolated attributes of the passed in node.
Params:
node – the current node to be processed
handler – the NodeHandler
interpolatedAttributes – a map for storing the results
Returns:
a flag whether an attribute value was changed by
        interpolation/**
         * 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 map with interpolated attributes of the passed in node.
Params:
node – the current node to be processed
handler – the NodeHandler
Returns:
the map with interpolated attributes/**
         * 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;
        }

        
Tests whether a value is changed because of interpolation.
Params:
interpolatedValue – the interpolated value
value – the original value
Returns:
a flag whether the value was changed/**
         * 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);
        }
    }
}