BaseHierarchicalConfiguration.java

/*
 * 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.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> {
        /**
         * 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);
    }
}