/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *     https://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.commons.configuration2;
  
  import java.io.ByteArrayOutputStream;
  import java.io.PrintStream;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.Set;
  import java.util.stream.Collectors;
  
  import org.apache.commons.configuration2.event.ConfigurationEvent;
  import org.apache.commons.configuration2.event.EventListener;
  import org.apache.commons.configuration2.event.EventSource;
  import org.apache.commons.configuration2.event.EventType;
  import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
  import org.apache.commons.configuration2.sync.LockMode;
  import org.apache.commons.configuration2.tree.DefaultConfigurationKey;
  import org.apache.commons.configuration2.tree.DefaultExpressionEngine;
  import org.apache.commons.configuration2.tree.ExpressionEngine;
  import org.apache.commons.configuration2.tree.ImmutableNode;
  import org.apache.commons.configuration2.tree.NodeCombiner;
  import org.apache.commons.configuration2.tree.NodeTreeWalker;
  import org.apache.commons.configuration2.tree.QueryResult;
  import org.apache.commons.configuration2.tree.TreeUtils;
  import org.apache.commons.configuration2.tree.UnionCombiner;
  import org.apache.commons.lang3.StringUtils;
  
  /**
   * <p>
   * A hierarchical composite configuration class.
   * </p>
   * <p>
   * This class maintains a list of configuration objects, which can be added using the diverse {@code addConfiguration()}
   * methods. After that the configurations can be accessed either by name (if one was provided when the configuration was
   * added) or by index. For the whole set of managed configurations a logical node structure is constructed. For this
   * purpose a {@link org.apache.commons.configuration2.tree.NodeCombiner NodeCombiner} object can be set. This makes it
   * possible to specify different algorithms for the combination process.
   * </p>
   * <p>
   * The big advantage of this class is that it creates a truly hierarchical structure of all the properties stored in the
   * contained configurations - even if some of them are no hierarchical configurations per se. So all enhanced features
   * provided by a hierarchical configuration (for example choosing an expression engine) are applicable.
   * </p>
   * <p>
   * The class works by registering itself as an event listener at all added configurations. So it gets notified whenever
   * one of these configurations is changed and can invalidate its internal node structure. The next time a property is
   * accessed the node structure will be re-constructed using the current state of the managed configurations. Note that,
   * depending on the used {@code NodeCombiner}, this may be a complex operation.
   * </p>
   * <p>
   * Because of the way a {@code CombinedConfiguration} is working it has more or less view character: it provides a logic
   * view on the configurations it contains. In this constellation not all methods defined for hierarchical configurations
   * - especially methods that update the stored properties - can be implemented in a consistent manner. Using such
   * methods (like {@code addProperty()}, or {@code clearProperty()} on a {@code CombinedConfiguration} is not strictly
   * forbidden, however, depending on the current {@link NodeCombiner} and the involved properties, the results may be
   * different than expected. Some examples may illustrate this:
   * </p>
   * <ul>
   * <li>Imagine a {@code CombinedConfiguration} <em>cc</em> containing two child configurations with the following
   * content:
   * <dl>
   * <dt>user.properties</dt>
   * <dd>
   *
   * <pre>
   * gui.background = blue
   * gui.position = (10, 10, 400, 200)
   * </pre>
   *
   * </dd>
   * <dt>default.properties</dt>
   * <dd>
   *
   * <pre>
   * gui.background = black
   * gui.foreground = white
   * home.dir = /data
   * </pre>
   *
  * </dd>
  * </dl>
  * As a {@code NodeCombiner} a {@link org.apache.commons.configuration2.tree.OverrideCombiner OverrideCombiner} is used.
  * This combiner will ensure that defined user settings take precedence over the default values. If the resulting
  * {@code CombinedConfiguration} is queried for the background color, {@code blue} will be returned because this value
  * is defined in {@code user.properties}. Now consider what happens if the key {@code gui.background} is removed from
  * the {@code CombinedConfiguration}:
  *
  * <pre>
  * cc.clearProperty(&quot;gui.background&quot;);
  * </pre>
  *
  * Will a {@code cc.containsKey("gui.background")} now return <strong>false</strong>? No, it won't! The {@code clearProperty()}
  * operation is executed on the node set of the combined configuration, which was constructed from the nodes of the two
  * child configurations. It causes the value of the <em>background</em> node to be cleared, which is also part of the
  * first child configuration. This modification of one of its child configurations causes the
  * {@code CombinedConfiguration} to be re-constructed. This time the {@code OverrideCombiner} cannot find a
  * {@code gui.background} property in the first child configuration, but it finds one in the second, and adds it to the
  * resulting combined configuration. So the property is still present (with a different value now).</li>
  * <li>{@code addProperty()} can also be problematic: Most node combiners use special view nodes for linking parts of
  * the original configurations' data together. If new properties are added to such a special node, they do not belong to
  * any of the managed configurations and thus hang in the air. Using the same configurations as in the last example, the
  * statement
  *
  * <pre>
  * addProperty(&quot;database.user&quot;, &quot;scott&quot;);
  * </pre>
  *
  * would cause such a hanging property. If now one of the child configurations is changed and the
  * {@code CombinedConfiguration} is re-constructed, this property will disappear! (Add operations are not problematic if
  * they result in a child configuration being updated. For instance an {@code addProperty("home.url", "localhost");}
  * will alter the second child configuration - because the prefix <em>home</em> is here already present; when the
  * {@code CombinedConfiguration} is re-constructed, this change is taken into account.)</li>
  * </ul>
  * <p>
  * Because of such problems it is recommended to perform updates only on the managed child configurations.
  * </p>
  * <p>
  * Whenever the node structure of a {@code CombinedConfiguration} becomes invalid (either because one of the contained
  * configurations was modified or because the {@code invalidate()} method was directly called) an event is generated. So
  * this can be detected by interested event listeners. This also makes it possible to add a combined configuration into
  * another one.
  * </p>
  * <p>
  * Notes about thread-safety: This configuration implementation uses a {@code Synchronizer} object to protect instances
  * against concurrent access. The concrete {@code Synchronizer} implementation used determines whether an instance of
  * this class is thread-safe or not. In contrast to other implementations derived from
  * {@link BaseHierarchicalConfiguration}, thread-safety is an issue here because the nodes structure used by this
  * configuration has to be constructed dynamically when a child configuration is changed. Therefore, when multiple
  * threads are involved which also manipulate one of the child configurations, a proper {@code Synchronizer} object
  * should be set. Note that the {@code Synchronizer} objects used by the child configurations do not really matter.
  * Because immutable in-memory nodes structures are used for them there is no danger that updates on child
  * configurations could interfere with read operations on the combined configuration.
  * </p>
  *
  * @since 1.3
  */
 public class CombinedConfiguration extends BaseHierarchicalConfiguration implements EventListener<ConfigurationEvent> {
     /**
      * An internal helper class for storing information about contained configurations.
      */
     private final class ConfigData {
         /** Stores a reference to the configuration. */
         private final Configuration configuration;
 
         /** Stores the name under which the configuration is stored. */
         private final String name;
 
         /** Stores the at information as path of nodes. */
         private final Collection<String> atPath;
 
         /** Stores the at string. */
         private final String at;
 
         /** Stores the root node for this child configuration. */
         private ImmutableNode rootNode;
 
         /**
          * Creates a new instance of {@code ConfigData} and initializes it.
          *
          * @param config the configuration
          * @param n the name
          * @param at the at position
          */
         public ConfigData(final Configuration config, final String n, final String at) {
             configuration = config;
             name = n;
             atPath = parseAt(at);
             this.at = at;
         }
 
         /**
          * Gets the at position of this configuration.
          *
          * @return the at position
          */
         public String getAt() {
             return at;
         }
 
         /**
          * Gets the stored configuration.
          *
          * @return the configuration
          */
         public Configuration getConfiguration() {
             return configuration;
         }
 
         /**
          * Gets the configuration's name.
          *
          * @return the name
          */
         public String getName() {
             return name;
         }
 
         /**
          * Gets the root node for this child configuration.
          *
          * @return the root node of this child configuration
          * @since 1.5
          */
         public ImmutableNode getRootNode() {
             return rootNode;
         }
 
         /**
          * Obtains the root node of the wrapped configuration. If necessary, a hierarchical representation of the configuration
          * has to be created first.
          *
          * @return the root node of the associated configuration
          */
         private ImmutableNode getRootNodeOfConfiguration() {
             getConfiguration().lock(LockMode.READ);
             try {
                 final ImmutableNode root = ConfigurationUtils.convertToHierarchical(getConfiguration(), conversionExpressionEngine).getNodeModel()
                     .getInMemoryRepresentation();
                 rootNode = root;
                 return root;
             } finally {
                 getConfiguration().unlock(LockMode.READ);
             }
         }
 
         /**
          * Gets the transformed root node of the stored configuration. The term &quot;transformed&quot; means that an
          * eventually defined at path has been applied.
          *
          * @return the transformed root node
          */
         public ImmutableNode getTransformedRoot() {
             final ImmutableNode configRoot = getRootNodeOfConfiguration();
             return atPath == null ? configRoot : prependAtPath(configRoot);
         }
 
         /**
          * Splits the at path into its components.
          *
          * @param at the at string
          * @return a collection with the names of the single components
          */
         private Collection<String> parseAt(final String at) {
             if (StringUtils.isEmpty(at)) {
                 return null;
             }
 
             final Collection<String> result = new ArrayList<>();
             final DefaultConfigurationKey.KeyIterator it = new DefaultConfigurationKey(AT_ENGINE, at).iterator();
             while (it.hasNext()) {
                 result.add(it.nextKey());
             }
             return result;
         }
 
         /**
          * Prepends the at path to the given node.
          *
          * @param node the root node of the represented configuration
          * @return the new root node including the at path
          */
         private ImmutableNode prependAtPath(final ImmutableNode node) {
             final ImmutableNode.Builder pathBuilder = new ImmutableNode.Builder();
             final Iterator<String> pathIterator = atPath.iterator();
             prependAtPathComponent(pathBuilder, pathIterator.next(), pathIterator, node);
             return new ImmutableNode.Builder(1).addChild(pathBuilder.create()).create();
         }
 
         /**
          * Handles a single component of the at path. A corresponding node is created and added to the hierarchical path to the
          * original root node of the configuration.
          *
          * @param builder the current node builder object
          * @param currentComponent the name of the current path component
          * @param components an iterator with all components of the at path
          * @param orgRoot the original root node of the wrapped configuration
          */
         private void prependAtPathComponent(final ImmutableNode.Builder builder, final String currentComponent, final Iterator<String> components,
             final ImmutableNode orgRoot) {
             builder.name(currentComponent);
             if (components.hasNext()) {
                 final ImmutableNode.Builder childBuilder = new ImmutableNode.Builder();
                 prependAtPathComponent(childBuilder, components.next(), components, orgRoot);
                 builder.addChild(childBuilder.create());
             } else {
                 builder.addChildren(orgRoot.getChildren());
                 builder.addAttributes(orgRoot.getAttributes());
                 builder.value(orgRoot.getValue());
             }
         }
     }
 
     /**
      * Constant for the event type fired when the internal node structure of a combined configuration becomes invalid.
      *
      * @since 2.0
      */
     public static final EventType<ConfigurationEvent> COMBINED_INVALIDATE = new EventType<>(ConfigurationEvent.ANY, "COMBINED_INVALIDATE");
 
     /** Constant for the expression engine for parsing the at path. */
     private static final DefaultExpressionEngine AT_ENGINE = DefaultExpressionEngine.INSTANCE;
 
     /** Constant for the default node combiner. */
     private static final NodeCombiner DEFAULT_COMBINER = new UnionCombiner();
 
     /** Constant for a root node for an empty configuration. */
     private static final ImmutableNode EMPTY_ROOT = new ImmutableNode.Builder().create();
 
     /** Stores the combiner. */
     private NodeCombiner nodeCombiner;
 
     /** Stores a list with the contained configurations. */
     private List<ConfigData> configurations;
 
     /** Stores a map with the named configurations. */
     private Map<String, Configuration> namedConfigurations;
 
     /**
      * An expression engine used for converting child configurations to hierarchical ones.
      */
     private ExpressionEngine conversionExpressionEngine;
 
     /** A flag whether this configuration is up-to-date. */
     private boolean upToDate;
 
     /**
      * Creates a new instance of {@code CombinedConfiguration} that uses a union combiner.
      *
      * @see org.apache.commons.configuration2.tree.UnionCombiner
      */
     public CombinedConfiguration() {
         this(null);
     }
 
     /**
      * Creates a new instance of {@code CombinedConfiguration} and initializes the combiner to be used.
      *
      * @param comb the node combiner (can be <strong>null</strong>, then a union combiner is used as default)
      */
     public CombinedConfiguration(final NodeCombiner comb) {
         nodeCombiner = comb != null ? comb : DEFAULT_COMBINER;
         initChildCollections();
     }
 
     /**
      * Adds a new configuration to this combined configuration. The new configuration is not given a name. Its properties
      * will be added under the root of the combined node structure.
      *
      * @param config the configuration to add (must not be <strong>null</strong>)
      */
     public void addConfiguration(final Configuration config) {
         addConfiguration(config, null, null);
     }
 
     /**
      * Adds a new configuration to this combined configuration with an optional name. The new configuration's properties
      * will be added under the root of the combined node structure.
      *
      * @param config the configuration to add (must not be <strong>null</strong>)
      * @param name the name of this configuration (can be <strong>null</strong>)
      */
     public void addConfiguration(final Configuration config, final String name) {
         addConfiguration(config, name, null);
     }
 
     /**
      * Adds a new configuration to this combined configuration. It is possible (but not mandatory) to give the new
      * configuration a name. This name must be unique, otherwise a {@code ConfigurationRuntimeException} will be thrown.
      * With the optional {@code at} argument you can specify where in the resulting node structure the content of the added
      * configuration should appear. This is a string that uses dots as property delimiters (independent on the current
      * expression engine). For instance if you pass in the string {@code "database.tables"}, all properties of the added
      * configuration will occur in this branch.
      *
      * @param config the configuration to add (must not be <strong>null</strong>)
      * @param name the name of this configuration (can be <strong>null</strong>)
      * @param at the position of this configuration in the combined tree (can be <strong>null</strong>)
      */
     public void addConfiguration(final Configuration config, final String name, final String at) {
         if (config == null) {
             throw new IllegalArgumentException("Added configuration must not be null!");
         }
 
         beginWrite(true);
         try {
             if (name != null && namedConfigurations.containsKey(name)) {
                 throw new ConfigurationRuntimeException("A configuration with the name '" + name + "' already exists in this combined configuration!");
             }
 
             final ConfigData cd = new ConfigData(config, name, at);
             if (getLogger().isDebugEnabled()) {
                 getLogger().debug("Adding configuration " + config + " with name " + name);
             }
             configurations.add(cd);
             if (name != null) {
                 namedConfigurations.put(name, config);
             }
 
             invalidateInternal();
         } finally {
             endWrite();
         }
         registerListenerAt(config);
     }
 
     /**
      * {@inheritDoc} This implementation checks whether a combined root node is available. If not, it is constructed by
      * requesting a write lock.
      */
     @Override
     protected void beginRead(final boolean optimize) {
         if (optimize) {
             // just need a lock, don't construct configuration
             super.beginRead(true);
             return;
         }
 
         boolean lockObtained = false;
         do {
             super.beginRead(false);
             if (isUpToDate()) {
                 lockObtained = true;
             } else {
                 // release read lock and try to obtain a write lock
                 endRead();
                 beginWrite(false); // this constructs the root node
                 endWrite();
             }
         } while (!lockObtained);
     }
 
     /**
      * {@inheritDoc} This implementation checks whether a combined root node is available. If not, it is constructed now.
      */
     @Override
     protected void beginWrite(final boolean optimize) {
         super.beginWrite(true);
         if (optimize) {
             // just need a lock, don't construct configuration
             return;
         }
 
         boolean success = false;
         try {
             if (!isUpToDate()) {
                 getSubConfigurationParentModel().replaceRoot(constructCombinedNode(), this);
                 upToDate = true;
             }
             success = true;
         } finally {
             if (!success) {
                 endWrite();
             }
         }
     }
 
     /**
      * Clears this configuration. All contained configurations will be removed.
      */
     @Override
     protected void clearInternal() {
         unregisterListenerAtChildren();
         initChildCollections();
         invalidateInternal();
     }
 
     /**
      * Returns a copy of this object. This implementation performs a deep clone, i.e. all contained configurations will be
      * cloned, too. For this to work, all contained configurations must be cloneable. Registered event listeners won't be
      * cloned. The clone will use the same node combiner than the original.
      *
      * @return the copied object
      */
     @Override
     public Object clone() {
         beginRead(false);
         try {
             final CombinedConfiguration copy = (CombinedConfiguration) super.clone();
             copy.initChildCollections();
             configurations.forEach(cd -> copy.addConfiguration(ConfigurationUtils.cloneConfiguration(cd.getConfiguration()), cd.getName(), cd.getAt()));
 
             return copy;
         } finally {
             endRead();
         }
     }
 
     /**
      * Creates the root node of this combined configuration.
      *
      * @return the combined root node
      */
     private ImmutableNode constructCombinedNode() {
         if (getNumberOfConfigurationsInternal() < 1) {
             if (getLogger().isDebugEnabled()) {
                 getLogger().debug("No configurations defined for " + this);
             }
             return EMPTY_ROOT;
         }
         final Iterator<ConfigData> it = configurations.iterator();
         ImmutableNode node = it.next().getTransformedRoot();
         while (it.hasNext()) {
             node = nodeCombiner.combine(node, it.next().getTransformedRoot());
         }
         if (getLogger().isDebugEnabled()) {
             final ByteArrayOutputStream os = new ByteArrayOutputStream();
             final PrintStream stream = new PrintStream(os);
             TreeUtils.printTree(stream, node);
             getLogger().debug(os.toString());
         }
         return node;
     }
 
     /**
      * Determines the configurations to which the specified node belongs. This is done by inspecting the nodes structures of
      * all child configurations.
      *
      * @param node the node
      * @return a set with the owning configurations
      */
     private Set<Configuration> findSourceConfigurations(final ImmutableNode node) {
         final Set<Configuration> result = new HashSet<>();
         final FindNodeVisitor<ImmutableNode> visitor = new FindNodeVisitor<>(node);
 
         configurations.forEach(cd -> {
             NodeTreeWalker.INSTANCE.walkBFS(cd.getRootNode(), visitor, getModel().getNodeHandler());
             if (visitor.isFound()) {
                 result.add(cd.getConfiguration());
                 visitor.reset();
             }
         });
 
         return result;
     }
 
     /**
      * Gets the configuration at the specified index. The contained configurations are numbered in the order they were
      * added to this combined configuration. The index of the first configuration is 0.
      *
      * @param index the index
      * @return the configuration at this index
      */
     public Configuration getConfiguration(final int index) {
         beginRead(true);
         try {
             final ConfigData cd = configurations.get(index);
             return cd.getConfiguration();
         } finally {
             endRead();
         }
     }
 
     /**
      * Gets the configuration with the given name. This can be <strong>null</strong> if no such configuration exists.
      *
      * @param name the name of the configuration
      * @return the configuration with this name
      */
     public Configuration getConfiguration(final String name) {
         beginRead(true);
         try {
             return namedConfigurations.get(name);
         } finally {
             endRead();
         }
     }
 
     /**
      * Gets a List of the names of all the configurations that have been added in the order they were added. A NULL value
      * will be present in the list for each configuration that was added without a name.
      *
      * @return A List of all the configuration names.
      * @since 1.7
      */
     public List<String> getConfigurationNameList() {
         beginRead(true);
         try {
             return configurations.stream().map(ConfigData::getName).collect(Collectors.toList());
         } finally {
             endRead();
         }
     }
 
     /**
      * Gets a set with the names of all configurations contained in this combined configuration. Of course here are only
      * these configurations listed, for which a name was specified when they were added.
      *
      * @return a set with the names of the contained configurations (never <strong>null</strong>)
      */
     public Set<String> getConfigurationNames() {
         beginRead(true);
         try {
             return namedConfigurations.keySet();
         } finally {
             endRead();
         }
     }
 
     /**
      * Gets a List of all the configurations that have been added.
      *
      * @return A List of all the configurations.
      * @since 1.7
      */
     public List<Configuration> getConfigurations() {
         beginRead(true);
         try {
             return configurations.stream().map(ConfigData::getConfiguration).collect(Collectors.toList());
         } finally {
             endRead();
         }
     }
 
     /**
      * Gets the {@code ExpressionEngine} for converting flat child configurations to hierarchical ones.
      *
      * @return the conversion expression engine
      * @since 1.6
      */
     public ExpressionEngine getConversionExpressionEngine() {
         beginRead(true);
         try {
             return conversionExpressionEngine;
         } finally {
             endRead();
         }
     }
 
     /**
      * Gets the node combiner that is used for creating the combined node structure.
      *
      * @return the node combiner
      */
     public NodeCombiner getNodeCombiner() {
         beginRead(true);
         try {
             return nodeCombiner;
         } finally {
             endRead();
         }
     }
 
     /**
      * Gets the number of configurations that are contained in this combined configuration.
      *
      * @return the number of contained configurations
      */
     public int getNumberOfConfigurations() {
         beginRead(true);
         try {
             return getNumberOfConfigurationsInternal();
         } finally {
             endRead();
         }
     }
 
     /**
      * Gets the number of child configurations in this combined configuration. The internal list of child configurations
      * is accessed without synchronization.
      *
      * @return the number of child configurations
      */
     private int getNumberOfConfigurationsInternal() {
         return configurations.size();
     }
 
     /**
      * Gets the configuration source, in which the specified key is defined. This method will determine the configuration
      * node that is identified by the given key. The following constellations are possible:
      * <ul>
      * <li>If no node object is found for this key, <strong>null</strong> is returned.</li>
      * <li>If the key maps to multiple nodes belonging to different configuration sources, a
      * {@code IllegalArgumentException} is thrown (in this case no unique source can be determined).</li>
      * <li>If exactly one node is found for the key, the (child) configuration object, to which the node belongs is
      * determined and returned.</li>
      * <li>For keys that have been added directly to this combined configuration and that do not belong to the namespaces
      * defined by existing child configurations this configuration will be returned.</li>
      * </ul>
      *
      * @param key the key of a configuration property
      * @return the configuration, to which this property belongs or <strong>null</strong> if the key cannot be resolved
      * @throws IllegalArgumentException if the key maps to multiple properties and the source cannot be determined, or if
      *         the key is <strong>null</strong>
      * @since 1.5
      */
     public Configuration getSource(final String key) {
         if (key == null) {
             throw new IllegalArgumentException("Key must not be null!");
         }
 
         final Set<Configuration> sources = getSources(key);
         if (sources.isEmpty()) {
             return null;
         }
         final Iterator<Configuration> iterator = sources.iterator();
         final Configuration source = iterator.next();
         if (iterator.hasNext()) {
             throw new IllegalArgumentException("The key " + key + " is defined by multiple sources!");
         }
         return source;
     }
 
     /**
      * Gets a set with the configuration sources, in which the specified key is defined. This method determines the
      * configuration nodes that are identified by the given key. It then determines the configuration sources to which these
      * nodes belong and adds them to the result set. Note the following points:
      * <ul>
      * <li>If no node object is found for this key, an empty set is returned.</li>
      * <li>For keys that have been added directly to this combined configuration and that do not belong to the namespaces
      * defined by existing child configurations this combined configuration is contained in the result set.</li>
      * </ul>
      *
      * @param key the key of a configuration property
      * @return a set with the configuration sources, which contain this property
      * @since 2.0
      */
     public Set<Configuration> getSources(final String key) {
         beginRead(false);
         try {
             final List<QueryResult<ImmutableNode>> results = fetchNodeList(key);
             final Set<Configuration> sources = new HashSet<>();
 
             results.forEach(result -> {
                 final Set<Configuration> resultSources = findSourceConfigurations(result.getNode());
                 if (resultSources.isEmpty()) {
                     // key must be defined in combined configuration
                     sources.add(this);
                 } else {
                     sources.addAll(resultSources);
                 }
             });
 
             return sources;
         } finally {
             endRead();
         }
     }
 
     /**
      * Initializes internal data structures for storing information about child configurations.
      */
     private void initChildCollections() {
         configurations = new ArrayList<>();
         namedConfigurations = new HashMap<>();
     }
 
     /**
      * Invalidates this combined configuration. This means that the next time a property is accessed the combined node
      * structure must be re-constructed. Invalidation of a combined configuration also means that an event of type
      * {@code EVENT_COMBINED_INVALIDATE} is fired. Note that while other events most times appear twice (once before and
      * once after an update), this event is only fired once (after update).
      */
     public void invalidate() {
         beginWrite(true);
         try {
             invalidateInternal();
         } finally {
             endWrite();
         }
     }
 
     /**
      * Marks this configuration as invalid. This means that the next access re-creates the root node. An invalidate event is
      * also fired. Note: This implementation expects that an exclusive (write) lock is held on this instance.
      */
     private void invalidateInternal() {
         upToDate = false;
         fireEvent(COMBINED_INVALIDATE, null, null, false);
     }
 
     /**
      * Returns a flag whether this configuration has been invalidated. This means that the combined nodes structure has to
      * be rebuilt before the configuration can be accessed.
      *
      * @return a flag whether this configuration is invalid
      */
     private boolean isUpToDate() {
         return upToDate;
     }
 
     /**
      * Event listener call back for configuration update events. This method is called whenever one of the contained
      * configurations was modified. It invalidates this combined configuration.
      *
      * @param event the update event
      */
     @Override
     public void onEvent(final ConfigurationEvent event) {
         if (event.isBeforeUpdate()) {
             invalidate();
         }
     }
 
     /**
      * Registers this combined configuration as listener at the given child configuration.
      *
      * @param configuration the child configuration
      */
     private void registerListenerAt(final Configuration configuration) {
         if (configuration instanceof EventSource) {
             ((EventSource) configuration).addEventListener(ConfigurationEvent.ANY, this);
         }
     }
 
     /**
      * Removes the specified configuration from this combined configuration.
      *
      * @param config the configuration to be removed
      * @return a flag whether this configuration was found and could be removed
      */
     public boolean removeConfiguration(final Configuration config) {
         for (int index = 0; index < getNumberOfConfigurations(); index++) {
             if (configurations.get(index).getConfiguration() == config) {
                 removeConfigurationAt(index);
                 return true;
             }
         }
 
         return false;
     }
 
     /**
      * Removes the configuration with the specified name.
      *
      * @param name the name of the configuration to be removed
      * @return the removed configuration (<strong>null</strong> if this configuration was not found)
      */
     public Configuration removeConfiguration(final String name) {
         final Configuration conf = getConfiguration(name);
         if (conf != null) {
             removeConfiguration(conf);
         }
         return conf;
     }
 
     /**
      * Removes the configuration at the specified index.
      *
      * @param index the index
      * @return the removed configuration
      */
     public Configuration removeConfigurationAt(final int index) {
         final ConfigData cd = configurations.remove(index);
         if (cd.getName() != null) {
             namedConfigurations.remove(cd.getName());
         }
         unregisterListenerAt(cd.getConfiguration());
         invalidateInternal();
         return cd.getConfiguration();
     }
 
     /**
      * Sets the {@code ExpressionEngine} for converting flat child configurations to hierarchical ones. When constructing
      * the root node for this combined configuration the properties of all child configurations must be combined to a single
      * hierarchical node structure. In this process, non hierarchical configurations are converted to hierarchical ones
      * first. This can be problematic if a child configuration contains keys that are no compatible with the default
      * expression engine used by hierarchical configurations. Therefore it is possible to specify a specific expression
      * engine to be used for this purpose.
      *
      * @param conversionExpressionEngine the conversion expression engine
      * @see ConfigurationUtils#convertToHierarchical(Configuration, ExpressionEngine)
      * @since 1.6
      */
     public void setConversionExpressionEngine(final ExpressionEngine conversionExpressionEngine) {
         beginWrite(true);
         try {
             this.conversionExpressionEngine = conversionExpressionEngine;
         } finally {
             endWrite();
         }
     }
 
     /**
      * Sets the node combiner. This object will be used when the combined node structure is to be constructed. It must not
      * be <strong>null</strong>, otherwise an {@code IllegalArgumentException} exception is thrown. Changing the node combiner causes
      * an invalidation of this combined configuration, so that the new combiner immediately takes effect.
      *
      * @param nodeCombiner the node combiner
      */
     public void setNodeCombiner(final NodeCombiner nodeCombiner) {
         if (nodeCombiner == null) {
             throw new IllegalArgumentException("Node combiner must not be null!");
         }
 
         beginWrite(true);
         try {
             this.nodeCombiner = nodeCombiner;
             invalidateInternal();
         } finally {
             endWrite();
         }
     }
 
     /**
      * Removes this combined configuration as listener from the given child configuration.
      *
      * @param configuration the child configuration
      */
     private void unregisterListenerAt(final Configuration configuration) {
         if (configuration instanceof EventSource) {
             ((EventSource) configuration).removeEventListener(ConfigurationEvent.ANY, this);
         }
     }
 
     /**
      * Removes this combined configuration as listener from all child configurations. This method is called on a clear()
      * operation.
      */
     private void unregisterListenerAtChildren() {
         if (configurations != null) {
             configurations.forEach(child -> unregisterListenerAt(child.getConfiguration()));
         }
     }
 }