    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *     http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017
    018package org.apache.commons.configuration2;
    019
    020import java.util.Collection;
    021import java.util.Collections;
    022import java.util.HashMap;
    023import java.util.Iterator;
    024import java.util.LinkedList;
    025import java.util.List;
    026import java.util.Map;
    027import java.util.stream.Collectors;
    028
    029import org.apache.commons.configuration2.event.ConfigurationEvent;
    030import org.apache.commons.configuration2.event.EventListener;
    031import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
    032import org.apache.commons.configuration2.interpol.ConfigurationInterpolator;
    033import org.apache.commons.configuration2.tree.ConfigurationNodeVisitorAdapter;
    034import org.apache.commons.configuration2.tree.ImmutableNode;
    035import org.apache.commons.configuration2.tree.InMemoryNodeModel;
    036import org.apache.commons.configuration2.tree.InMemoryNodeModelSupport;
    037import org.apache.commons.configuration2.tree.NodeHandler;
    038import org.apache.commons.configuration2.tree.NodeModel;
    039import org.apache.commons.configuration2.tree.NodeSelector;
    040import org.apache.commons.configuration2.tree.NodeTreeWalker;
    041import org.apache.commons.configuration2.tree.QueryResult;
    042import org.apache.commons.configuration2.tree.ReferenceNodeHandler;
    043import org.apache.commons.configuration2.tree.TrackedNodeModel;
    044import org.apache.commons.lang3.ObjectUtils;
    045
    046/**
    047 * <p>
    048 * A specialized hierarchical configuration implementation that is based on a structure of {@link ImmutableNode}
    049 * objects.
    050 * </p>
    051 */
    052public class BaseHierarchicalConfiguration extends AbstractHierarchicalConfiguration<ImmutableNode> implements InMemoryNodeModelSupport {
    053
    054    /**
    055     * A specialized visitor base class that can be used for storing the tree of configuration nodes. The basic idea is that
    056     * each node can be associated with a reference object. This reference object has a concrete meaning in a derived class,
    057     * for example an entry in a JNDI context or an XML element. When the configuration tree is set up, the {@code load()} method
    058     * is responsible for setting the reference objects. When the configuration tree is later modified, new nodes do not
    059     * have a defined reference object. This visitor class processes all nodes and finds the ones without a defined
    060     * reference object. For those nodes the {@code insert()} method is called, which must be defined in concrete sub
    061     * classes. This method can perform all steps to integrate the new node into the original structure.
    062     */
    063    protected abstract static class BuilderVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
    064        /**
    065         * Inserts a new node into the structure constructed by this builder. This method is called for each node that has been
    066         * added to the configuration tree after the configuration has been loaded from its source. These new nodes have to be
    067         * inserted into the original structure. The passed in nodes define the position of the node to be inserted: its parent
    068         * and the siblings between to insert.
    069         *
    070         * @param newNode the node to be inserted
    071         * @param parent the parent node
    072         * @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
    073         *        the first child node
    074         * @param sibling2 the sibling before which the node is to be inserted; can be <strong>null</strong> if the new node is going to
    075         *        be the last child node
    076         * @param refHandler the {@code ReferenceNodeHandler}
    077         */
    078        protected abstract void insert(ImmutableNode newNode, ImmutableNode parent, ImmutableNode sibling1, ImmutableNode sibling2,
    079            ReferenceNodeHandler refHandler);
    080
    081        /**
    082         * Inserts new children that have been added to the specified node.
    083         *
    084         * @param node the current node to be processed
    085         * @param refHandler the {@code ReferenceNodeHandler}
    086         */
    087        private void insertNewChildNodes(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
    088            final Collection<ImmutableNode> subNodes = new LinkedList<>(refHandler.getChildren(node));
    089            final Iterator<ImmutableNode> children = subNodes.iterator();
    090            ImmutableNode sibling1;
    091            ImmutableNode nd = null;
    092
    093            while (children.hasNext()) {
    094                // find the next new node
    095                do {
    096                    sibling1 = nd;
    097                    nd = children.next();
    098                } while (refHandler.getReference(nd) != null && children.hasNext());
    099
    100                if (refHandler.getReference(nd) == null) {
    101                    // find all following new nodes
    102                    final List<ImmutableNode> newNodes = new LinkedList<>();
    103                    newNodes.add(nd);
    104                    while (children.hasNext()) {
    105                        nd = children.next();
    106                        if (refHandler.getReference(nd) != null) {
    107                            break;
    108                        }
    109                        newNodes.add(nd);
    110                    }
    111
    112                    // Insert all new nodes
    113                    final ImmutableNode sibling2 = refHandler.getReference(nd) == null ? null : nd;
    114                    for (final ImmutableNode insertNode : newNodes) {
    115                        if (refHandler.getReference(insertNode) == null) {
    116                            insert(insertNode, node, sibling1, sibling2, refHandler);
    117                            sibling1 = insertNode;
    118                        }
    119                    }
    120                }
    121            }
    122        }
    123
    124        /**
    125         * Updates a node that already existed in the original hierarchy. This method is called for each node that has an
    126         * assigned reference object. A concrete implementation should update the reference according to the node's current
    127         * value.
    128         *
    129         * @param node the current node to be processed
    130         * @param reference the reference object for this node
    131         * @param refHandler the {@code ReferenceNodeHandler}
    132         */
    133        protected abstract void update(ImmutableNode node, Object reference, ReferenceNodeHandler refHandler);
    134
    135        /**
    136         * Updates the value of a node. If this node is associated with a reference object, the {@code update()} method is
    137         * called.
    138         *
    139         * @param node the current node to be processed
    140         * @param refHandler the {@code ReferenceNodeHandler}
    141         */
    142        private void updateNode(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
    143            final Object reference = refHandler.getReference(node);
    144            if (reference != null) {
    145                update(node, reference, refHandler);
    146            }
    147        }
    148
    149        @Override
    150        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    151            final ReferenceNodeHandler refHandler = (ReferenceNodeHandler) handler;
    152            updateNode(node, refHandler);
    153            insertNewChildNodes(node, refHandler);
    154        }
    155    }
    156
    157    /**
    158     * A specialized visitor implementation which constructs the root node of a configuration with all variables replaced by
    159     * their interpolated values.
    160     */
    161    private final class InterpolatedVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
    162        /** A stack for managing node builder instances. */
    163        private final List<ImmutableNode.Builder> builderStack;
    164
    165        /** The resulting root node. */
    166        private ImmutableNode interpolatedRoot;
    167
    168        /**
    169         * Creates a new instance of {@code InterpolatedVisitor}.
    170         */
    171        public InterpolatedVisitor() {
    172            builderStack = new LinkedList<>();
    173        }
    174
    175        /**
    176         * Gets the result of this builder: the root node of the interpolated nodes hierarchy.
    177         *
    178         * @return the resulting root node
    179         */
    180        public ImmutableNode getInterpolatedRoot() {
    181            return interpolatedRoot;
    182        }
    183
    184        /**
    185         * Handles interpolation for a node with no children. If interpolation does not change this node, it is copied as is to
    186         * the resulting structure. Otherwise, a new node is created with the interpolated values.
    187         *
    188         * @param node the current node to be processed
    189         * @param handler the {@code NodeHandler}
    190         */
    191        private void handleLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    192            final Object value = interpolate(node.getValue());
    193            final Map<String, Object> interpolatedAttributes = new HashMap<>();
    194            final boolean attributeChanged = interpolateAttributes(node, handler, interpolatedAttributes);
    195            final ImmutableNode newNode = valueChanged(value, handler.getValue(node)) || attributeChanged
    196                ? new ImmutableNode.Builder().name(handler.nodeName(node)).value(value).addAttributes(interpolatedAttributes).create()
    197                : node;
    198            storeInterpolatedNode(newNode);
    199        }
    200
    201        /**
    202         * Returns a map with interpolated attributes of the passed in node.
    203         *
    204         * @param node the current node to be processed
    205         * @param handler the {@code NodeHandler}
    206         * @return the map with interpolated attributes
    207         */
    208        private Map<String, Object> interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    209            final Map<String, Object> attributes = new HashMap<>();
    210            interpolateAttributes(node, handler, attributes);
    211            return attributes;
    212        }
    213
    214        /**
    215         * Populates a map with interpolated attributes of the passed in node.
    216         *
    217         * @param node the current node to be processed
    218         * @param handler the {@code NodeHandler}
    219         * @param interpolatedAttributes a map for storing the results
    220         * @return a flag whether an attribute value was changed by interpolation
    221         */
    222        private boolean interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler,
    223            final Map<String, Object> interpolatedAttributes) {
    224            boolean attributeChanged = false;
    225            for (final String attr : handler.getAttributes(node)) {
    226                final Object attrValue = interpolate(handler.getAttributeValue(node, attr));
    227                if (valueChanged(attrValue, handler.getAttributeValue(node, attr))) {
    228                    attributeChanged = true;
    229                }
    230                interpolatedAttributes.put(attr, attrValue);
    231            }
    232            return attributeChanged;
    233        }
    234
    235        /**
    236         * Returns a flag whether the given node is a leaf. This is the case if it does not have children.
    237         *
    238         * @param node the node in question
    239         * @param handler the {@code NodeHandler}
    240         * @return a flag whether this is a leaf node
    241         */
    242        private boolean isLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    243            return handler.getChildren(node).isEmpty();
    244        }
    245
    246        /**
    247         * Returns the top-level element from the stack without removing it.
    248         *
    249         * @return the top-level element from the stack
    250         */
    251        private ImmutableNode.Builder peek() {
    252            return builderStack.get(0);
    253        }
    254
    255        /**
    256         * Pops the top-level element from the stack.
    257         *
    258         * @return the element popped from the stack
    259         */
    260        private ImmutableNode.Builder pop() {
    261            return builderStack.remove(0);
    262        }
    263
    264        /**
    265         * Pushes a new builder on the stack.
    266         *
    267         * @param builder the builder
    268         */
    269        private void push(final ImmutableNode.Builder builder) {
    270            builderStack.add(0, builder);
    271        }
    272
    273        /**
    274         * Stores a processed node. Per default, the node is added to the current builder on the stack. If no such builder
    275         * exists, this is the result node.
    276         *
    277         * @param node the node to be stored
    278         */
    279        private void storeInterpolatedNode(final ImmutableNode node) {
    280            if (builderStack.isEmpty()) {
    281                interpolatedRoot = node;
    282            } else {
    283                peek().addChild(node);
    284            }
    285        }
    286
    287        /**
    288         * Tests whether a value is changed because of interpolation.
    289         *
    290         * @param interpolatedValue the interpolated value
    291         * @param value the original value
    292         * @return a flag whether the value was changed
    293         */
    294        private boolean valueChanged(final Object interpolatedValue, final Object value) {
    295            return ObjectUtils.notEqual(interpolatedValue, value);
    296        }
    297
    298        @Override
    299        public void visitAfterChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    300            if (!isLeafNode(node, handler)) {
    301                final ImmutableNode newNode = pop().create();
    302                storeInterpolatedNode(newNode);
    303            }
    304        }
    305
    306        @Override
    307        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    308            if (isLeafNode(node, handler)) {
    309                handleLeafNode(node, handler);
    310            } else {
    311                final ImmutableNode.Builder builder = new ImmutableNode.Builder(handler.getChildrenCount(node, null)).name(handler.nodeName(node))
    312                    .value(interpolate(handler.getValue(node))).addAttributes(interpolateAttributes(node, handler));
    313                push(builder);
    314            }
    315        }
    316    }
    317
    318    /**
    319     * Creates the {@code NodeModel} for this configuration based on a passed in source configuration. This implementation
    320     * creates an {@link InMemoryNodeModel}. If the passed in source configuration is defined, its root node also becomes
    321     * the root node of this configuration. Otherwise, a new, empty root node is used.
    322     *
    323     * @param c the configuration that is to be copied
    324     * @return the {@code NodeModel} for the new configuration
    325     */
    326    private static NodeModel<ImmutableNode> createNodeModel(final HierarchicalConfiguration<ImmutableNode> c) {
    327        return new InMemoryNodeModel(obtainRootNode(c));
    328    }
    329
    330    /**
    331     * Obtains the root node from a configuration whose data is to be copied. It has to be ensured that the synchronizer is
    332     * called correctly.
    333     *
    334     * @param c the configuration that is to be copied
    335     * @return the root node of this configuration
    336     */
    337    private static ImmutableNode obtainRootNode(final HierarchicalConfiguration<ImmutableNode> c) {
    338        return c != null ? c.getNodeModel().getNodeHandler().getRootNode() : null;
    339    }
    340
    341    /**
    342     * Creates a list with immutable configurations from the given input list.
    343     *
    344     * @param subs a list with mutable configurations
    345     * @return a list with corresponding immutable configurations
    346     */
    347    private static List<ImmutableHierarchicalConfiguration> toImmutable(final List<? extends HierarchicalConfiguration<?>> subs) {
    348        return subs.stream().map(ConfigurationUtils::unmodifiableConfiguration).collect(Collectors.toList());
    349    }
    350
    351    /** A listener for reacting on changes caused by sub configurations. */
    352    private final EventListener<ConfigurationEvent> changeListener;
    353
    354    /**
    355     * Creates a new instance of {@code BaseHierarchicalConfiguration}.
    356     */
    357    public BaseHierarchicalConfiguration() {
    358        this((HierarchicalConfiguration<ImmutableNode>) null);
    359    }
    360
    361    /**
    362     * Creates a new instance of {@code BaseHierarchicalConfiguration} and copies all data contained in the specified
    363     * configuration into the new one.
    364     *
    365     * @param c the configuration that is to be copied (if <strong>null</strong>, this constructor will behave like the standard
    366     *        constructor)
    367     * @since 1.4
    368     */
    369    public BaseHierarchicalConfiguration(final HierarchicalConfiguration<ImmutableNode> c) {
    370        this(createNodeModel(c));
    371    }
    372
    373    /**
    374     * Creates a new instance of {@code BaseHierarchicalConfiguration} and initializes it with the given {@code NodeModel}.
    375     *
    376     * @param model the {@code NodeModel}
    377     */
    378    protected BaseHierarchicalConfiguration(final NodeModel<ImmutableNode> model) {
    379        super(model);
    380        changeListener = createChangeListener();
    381    }
    382
    383    /**
    384     * {@inheritDoc} This implementation resolves the node(s) selected by the given key. If not a single node is selected,
    385     * an empty list is returned. Otherwise, sub configurations for each child of the node are created.
    386     */
    387    @Override
    388    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key) {
    389        final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
    390        if (nodes.size() != 1) {
    391            return Collections.emptyList();
    392        }
    393        return nodes.get(0).stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
    394    }
    395
    396    /**
    397     * {@inheritDoc} This method works like {@link #childConfigurationsAt(String)}; however, depending on the value of the
    398     * {@code supportUpdates} flag, connected sub configurations may be created.
    399     */
    400    @Override
    401    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key, final boolean supportUpdates) {
    402        if (!supportUpdates) {
    403            return childConfigurationsAt(key);
    404        }
    405
    406        final InMemoryNodeModel parentModel = getSubConfigurationParentModel();
    407        return createConnectedSubConfigurations(this, parentModel.trackChildNodes(key, this));
    408    }
    409
    410    /**
    411     * {@inheritDoc} This implementation creates a new instance of {@link InMemoryNodeModel}, initialized with this
    412     * configuration's root node. This has the effect that although the same nodes are used, the original and copied
    413     * configurations are independent on each other.
    414     */
    415    @Override
    416    protected NodeModel<ImmutableNode> cloneNodeModel() {
    417        return new InMemoryNodeModel(getModel().getNodeHandler().getRootNode());
    418    }
    419
    420    /**
    421     * {@inheritDoc} This is a short form for {@code configurationAt(key,
    422     * <strong>false</strong>)}.
    423     *
    424     * @throws ConfigurationRuntimeException if the key does not select a single node
    425     */
    426    @Override
    427    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key) {
    428        return configurationAt(key, false);
    429    }
    430
    431    /**
    432     * {@inheritDoc} The result of this implementation depends on the {@code supportUpdates} flag: If it is <strong>false</strong>, a
    433     * plain {@code BaseHierarchicalConfiguration} is returned using the selected node as root node. This is suitable for
    434     * read-only access to properties. Because the configuration returned in this case is not connected to the parent
    435     * configuration, updates on properties made by one configuration are not reflected by the other one. A value of
    436     * <strong>true</strong> for this parameter causes a tracked node to be created, and result is a {@link SubnodeConfiguration}
    437     * based on this tracked node. This configuration is really connected to its parent, so that updated properties are
    438     * visible on both.
    439     *
    440     * @see SubnodeConfiguration
    441     * @throws ConfigurationRuntimeException if the key does not select a single node
    442     */
    443    @Override
    444    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key, final boolean supportUpdates) {
    445        return syncRead(() -> supportUpdates ? createConnectedSubConfiguration(key) : createIndependentSubConfiguration(key), false);
    446    }
    447
    448    /**
    449     * {@inheritDoc} This implementation creates sub configurations in the same way as described for
    450     * {@link #configurationAt(String)}.
    451     */
    452    @Override
    453    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key) {
    454        final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
    455        return nodes.stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
    456    }
    457
    458    /**
    459     * {@inheritDoc} This implementation creates tracked nodes for the specified key. Then sub configurations for these
    460     * nodes are created and returned.
    461     */
    462    @Override
    463    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key, final boolean supportUpdates) {
    464        if (!supportUpdates) {
    465            return configurationsAt(key);
    466        }
    467        final InMemoryNodeModel parentModel = syncRead(this::getSubConfigurationParentModel, false);
    468        return createConnectedSubConfigurations(this, parentModel.selectAndTrackNodes(key, this));
    469    }
    470
    471    /**
    472     * Creates a listener which reacts on all changes on this configuration or one of its {@code SubnodeConfiguration}
    473     * instances. If such a change is detected, some updates have to be performed.
    474     *
    475     * @return the newly created change listener
    476     */
    477    private EventListener<ConfigurationEvent> createChangeListener() {
    478        return this::subnodeConfigurationChanged;
    479    }
    480
    481    /**
    482     * Creates a sub configuration from the specified key which is connected to this configuration. This implementation
    483     * creates a {@link SubnodeConfiguration} with a tracked node identified by the passed in key.
    484     *
    485     * @param key the key of the sub configuration
    486     * @return the new sub configuration
    487     */
    488    private BaseHierarchicalConfiguration createConnectedSubConfiguration(final String key) {
    489        final NodeSelector selector = getSubConfigurationNodeSelector(key);
    490        getSubConfigurationParentModel().trackNode(selector, this);
    491        return createSubConfigurationForTrackedNode(selector, this);
    492    }
    493
    494    /**
    495     * Creates a list of connected sub configurations based on a passed in list of node selectors.
    496     *
    497     * @param parentModelSupport the parent node model support object
    498     * @param selectors the list of {@code NodeSelector} objects
    499     * @return the list with sub configurations
    500     */
    501    private List<HierarchicalConfiguration<ImmutableNode>> createConnectedSubConfigurations(final InMemoryNodeModelSupport parentModelSupport,
    502        final Collection<NodeSelector> selectors) {
    503        return selectors.stream().map(sel -> createSubConfigurationForTrackedNode(sel, parentModelSupport)).collect(Collectors.toList());
    504    }
    505
    506    /**
    507     * Creates a sub configuration from the specified key which is independent on this configuration. This means that the
    508     * sub configuration operates on a separate node model (although the nodes are initially shared).
    509     *
    510     * @param key the key of the sub configuration
    511     * @return the new sub configuration
    512     */
    513    private BaseHierarchicalConfiguration createIndependentSubConfiguration(final String key) {
    514        final List<ImmutableNode> targetNodes = fetchFilteredNodeResults(key);
    515        final int size = targetNodes.size();
    516        if (size != 1) {
    517            throw new ConfigurationRuntimeException("Passed in key must select exactly one node (found %,d): %s", size, key);
    518        }
    519        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(targetNodes.get(0)));
    520        initSubConfiguration(sub);
    521        return sub;
    522    }
    523
    524    /**
    525     * Returns an initialized sub configuration for this configuration that is based on another
    526     * {@code BaseHierarchicalConfiguration}. Thus, it is independent from this configuration.
    527     *
    528     * @param node the root node for the sub configuration
    529     * @return the initialized sub configuration
    530     */
    531    private BaseHierarchicalConfiguration createIndependentSubConfigurationForNode(final ImmutableNode node) {
    532        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(node));
    533        initSubConfiguration(sub);
    534        return sub;
    535    }
    536
    537    /**
    538     * Creates a connected sub configuration based on a selector for a tracked node.
    539     *
    540     * @param selector the {@code NodeSelector}
    541     * @param parentModelSupport the {@code InMemoryNodeModelSupport} object for the parent node model
    542     * @return the newly created sub configuration
    543     * @since 2.0
    544     */
    545    protected SubnodeConfiguration createSubConfigurationForTrackedNode(final NodeSelector selector, final InMemoryNodeModelSupport parentModelSupport) {
    546        final SubnodeConfiguration subConfig = new SubnodeConfiguration(this, new TrackedNodeModel(parentModelSupport, selector, true));
    547        initSubConfigurationForThisParent(subConfig);
    548        return subConfig;
    549    }
    550
    551    /**
    552     * Creates a root node for a subset configuration based on the passed in query results. This method creates a new root
    553     * node and adds the children and attributes of all result nodes to it. If only a single node value is defined, it is
    554     * assigned as value of the new root node.
    555     *
    556     * @param results the collection of query results
    557     * @return the root node for the subset configuration
    558     */
    559    private ImmutableNode createSubsetRootNode(final Collection<QueryResult<ImmutableNode>> results) {
    560        final ImmutableNode.Builder builder = new ImmutableNode.Builder();
    561        Object value = null;
    562        int valueCount = 0;
    563
    564        for (final QueryResult<ImmutableNode> result : results) {
    565            if (result.isAttributeResult()) {
    566                builder.addAttribute(result.getAttributeName(), result.getAttributeValue(getModel().getNodeHandler()));
    567            } else {
    568                if (result.getNode().getValue() != null) {
    569                    value = result.getNode().getValue();
    570                    valueCount++;
    571                }
    572                builder.addChildren(result.getNode().getChildren());
    573                builder.addAttributes(result.getNode().getAttributes());
    574            }
    575        }
    576
    577        if (valueCount == 1) {
    578            builder.value(value);
    579        }
    580        return builder.create();
    581    }
    582
    583    /**
    584     * Executes a query on the specified key and filters it for node results.
    585     *
    586     * @param key the key
    587     * @return the filtered list with result nodes
    588     */
    589    private List<ImmutableNode> fetchFilteredNodeResults(final String key) {
    590        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    591        return resolveNodeKey(handler.getRootNode(), key, handler);
    592    }
    593
    594    /**
    595     * {@inheritDoc} This implementation returns the {@code InMemoryNodeModel} used by this configuration.
    596     */
    597    @Override
    598    public InMemoryNodeModel getNodeModel() {
    599        return (InMemoryNodeModel) super.getNodeModel();
    600    }
    601
    602    /**
    603     * Gets the {@code NodeSelector} to be used for a sub configuration based on the passed in key. This method is called
    604     * whenever a sub configuration is to be created. This base implementation returns a new {@code NodeSelector}
    605     * initialized with the passed in key. Sub classes may override this method if they have a different strategy for
    606     * creating a selector.
    607     *
    608     * @param key the key of the sub configuration
    609     * @return a {@code NodeSelector} for initializing a sub configuration
    610     * @since 2.0
    611     */
    612    protected NodeSelector getSubConfigurationNodeSelector(final String key) {
    613        return new NodeSelector(key);
    614    }
    615
    616    /**
    617     * Gets the {@code InMemoryNodeModel} to be used as parent model for a new sub configuration. This method is called
    618     * whenever a sub configuration is to be created. This base implementation returns the model of this configuration. Sub
    619     * classes with different requirements for the parent models of sub configurations have to override it.
    620     *
    621     * @return the parent model for a new sub configuration
    622     */
    623    protected InMemoryNodeModel getSubConfigurationParentModel() {
    624        return (InMemoryNodeModel) getModel();
    625    }
    626
    627    /**
    628     * {@inheritDoc} This implementation first delegates to {@code childConfigurationsAt()} to create a list of mutable
    629     * child configurations. Then a list with immutable wrapper configurations is created.
    630     */
    631    @Override
    632    public List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(final String key) {
    633        return toImmutable(childConfigurationsAt(key));
    634    }
    635
    636    /**
    637     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    638     * Then an immutable wrapper is created and returned.
    639     *
    640     * @throws ConfigurationRuntimeException if the key does not select a single node
    641     */
    642    @Override
    643    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key) {
    644        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key));
    645    }
    646
    647    /**
    648     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    649     * Then an immutable wrapper is created and returned.
    650     */
    651    @Override
    652    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key, final boolean supportUpdates) {
    653        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key, supportUpdates));
    654    }
    655
    656    /**
    657     * {@inheritDoc} This implementation first delegates to {@code configurationsAt()} to create a list of
    658     * {@code SubnodeConfiguration} objects. Then for each element of this list an unmodifiable wrapper is created.
    659     */
    660    @Override
    661    public List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(final String key) {
    662        return toImmutable(configurationsAt(key));
    663    }
    664
    665    /**
    666     * Initializes properties of a sub configuration. A sub configuration inherits some settings from its parent, for example the
    667     * expression engine or the synchronizer. The corresponding values are copied by this method.
    668     *
    669     * @param sub the sub configuration to be initialized
    670     */
    671    private void initSubConfiguration(final BaseHierarchicalConfiguration sub) {
    672        sub.setSynchronizer(getSynchronizer());
    673        sub.setExpressionEngine(getExpressionEngine());
    674        sub.setListDelimiterHandler(getListDelimiterHandler());
    675        sub.setThrowExceptionOnMissing(isThrowExceptionOnMissing());
    676        sub.getInterpolator().setParentInterpolator(getInterpolator());
    677    }
    678
    679    /**
    680     * Initializes a {@code SubnodeConfiguration} object. This method should be called for each sub configuration created
    681     * for this configuration. It ensures that the sub configuration is correctly connected to its parent instance and that
    682     * update events are correctly propagated.
    683     *
    684     * @param subConfig the sub configuration to be initialized
    685     * @since 2.0
    686     */
    687    protected void initSubConfigurationForThisParent(final SubnodeConfiguration subConfig) {
    688        initSubConfiguration(subConfig);
    689        subConfig.addEventListener(ConfigurationEvent.ANY, changeListener);
    690    }
    691
    692    /**
    693     * Returns a configuration with the same content as this configuration, but with all variables replaced by their actual
    694     * values. This implementation is specific for hierarchical configurations. It clones the current configuration and runs
    695     * a specialized visitor on the clone, which performs interpolation on the single configuration nodes.
    696     *
    697     * @return a configuration with all variables interpolated
    698     * @since 1.5
    699     */
    700    @Override
    701    public Configuration interpolatedConfiguration() {
    702        final InterpolatedVisitor visitor = new InterpolatedVisitor();
    703        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    704        NodeTreeWalker.INSTANCE.walkDFS(handler.getRootNode(), visitor, handler);
    705
    706        final BaseHierarchicalConfiguration c = (BaseHierarchicalConfiguration) clone();
    707        c.getNodeModel().setRootNode(visitor.getInterpolatedRoot());
    708        return c;
    709    }
    710
    711    /**
    712     * This method is always called when a subnode configuration created from this configuration has been modified. This
    713     * implementation transforms the received event into an event of type {@code SUBNODE_CHANGED} and notifies the
    714     * registered listeners.
    715     *
    716     * @param event the event describing the change
    717     * @since 1.5
    718     */
    719    protected void subnodeConfigurationChanged(final ConfigurationEvent event) {
    720        fireEvent(ConfigurationEvent.SUBNODE_CHANGED, null, event, event.isBeforeUpdate());
    721    }
    722
    723    /**
    724     * Creates a new {@code Configuration} object containing all keys that start with the specified prefix. This
    725     * implementation will return a {@code BaseHierarchicalConfiguration} object so that the structure of the keys will be
    726     * saved. The nodes selected by the prefix (it is possible that multiple nodes are selected) are mapped to the root node
    727     * of the returned configuration, i.e. their children and attributes will become children and attributes of the new root
    728     * node. However, a value of the root node is only set if exactly one of the selected nodes contain a value (if multiple
    729     * nodes have a value, there is simply no way to decide how these values are merged together). Note that the returned
    730     * {@code Configuration} object is not connected to its source configuration: updates on the source configuration are
    731     * not reflected in the subset and vice versa. The returned configuration uses the same {@code Synchronizer} as this
    732     * configuration.
    733     *
    734     * @param prefix the prefix of the keys for the subset
    735     * @return a new configuration object representing the selected subset
    736     */
    737    @Override
    738    public Configuration subset(final String prefix) {
    739        return syncRead(() -> {
    740            final List<QueryResult<ImmutableNode>> results = fetchNodeList(prefix);
    741            if (results.isEmpty()) {
    742                return new BaseHierarchicalConfiguration();
    743            }
    744            final BaseHierarchicalConfiguration parent = this;
    745            final BaseHierarchicalConfiguration result = new BaseHierarchicalConfiguration() {
    746
    747                @Override
    748                public ConfigurationInterpolator getInterpolator() {
    749                    return parent.getInterpolator();
    750                }
    751
    752                // Override interpolate to always interpolate on the parent
    753                @Override
    754                protected Object interpolate(final Object value) {
    755                    return parent.interpolate(value);
    756                }
    757            };
    758            result.getModel().setRootNode(createSubsetRootNode(results));
    759            if (result.isEmpty()) {
    760                return new BaseHierarchicalConfiguration();
    761            }
    762            result.setSynchronizer(getSynchronizer());
    763            return result;
    764        }, false);
    765    }
    766}