    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 *     https://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        /**
    066         * Constructs a new instance.
    067         */
    068        public BuilderVisitor() {
    069            // empty
    070        }
    071
    072        /**
    073         * Inserts a new node into the structure constructed by this builder. This method is called for each node that has been
    074         * added to the configuration tree after the configuration has been loaded from its source. These new nodes have to be
    075         * inserted into the original structure. The passed in nodes define the position of the node to be inserted: its parent
    076         * and the siblings between to insert.
    077         *
    078         * @param newNode the node to be inserted
    079         * @param parent the parent node
    080         * @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
    081         *        the first child node
    082         * @param sibling2 the sibling before which the node is to be inserted; can be <strong>null</strong> if the new node is going to
    083         *        be the last child node
    084         * @param refHandler the {@code ReferenceNodeHandler}
    085         */
    086        protected abstract void insert(ImmutableNode newNode, ImmutableNode parent, ImmutableNode sibling1, ImmutableNode sibling2,
    087            ReferenceNodeHandler refHandler);
    088
    089        /**
    090         * Inserts new children that have been added to the specified node.
    091         *
    092         * @param node the current node to be processed
    093         * @param refHandler the {@code ReferenceNodeHandler}
    094         */
    095        private void insertNewChildNodes(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
    096            final Collection<ImmutableNode> subNodes = new LinkedList<>(refHandler.getChildren(node));
    097            final Iterator<ImmutableNode> children = subNodes.iterator();
    098            ImmutableNode sibling1;
    099            ImmutableNode nd = null;
    100
    101            while (children.hasNext()) {
    102                // find the next new node
    103                do {
    104                    sibling1 = nd;
    105                    nd = children.next();
    106                } while (refHandler.getReference(nd) != null && children.hasNext());
    107
    108                if (refHandler.getReference(nd) == null) {
    109                    // find all following new nodes
    110                    final List<ImmutableNode> newNodes = new LinkedList<>();
    111                    newNodes.add(nd);
    112                    while (children.hasNext()) {
    113                        nd = children.next();
    114                        if (refHandler.getReference(nd) != null) {
    115                            break;
    116                        }
    117                        newNodes.add(nd);
    118                    }
    119
    120                    // Insert all new nodes
    121                    final ImmutableNode sibling2 = refHandler.getReference(nd) == null ? null : nd;
    122                    for (final ImmutableNode insertNode : newNodes) {
    123                        if (refHandler.getReference(insertNode) == null) {
    124                            insert(insertNode, node, sibling1, sibling2, refHandler);
    125                            sibling1 = insertNode;
    126                        }
    127                    }
    128                }
    129            }
    130        }
    131
    132        /**
    133         * Updates a node that already existed in the original hierarchy. This method is called for each node that has an
    134         * assigned reference object. A concrete implementation should update the reference according to the node's current
    135         * value.
    136         *
    137         * @param node the current node to be processed
    138         * @param reference the reference object for this node
    139         * @param refHandler the {@code ReferenceNodeHandler}
    140         */
    141        protected abstract void update(ImmutableNode node, Object reference, ReferenceNodeHandler refHandler);
    142
    143        /**
    144         * Updates the value of a node. If this node is associated with a reference object, the {@code update()} method is
    145         * called.
    146         *
    147         * @param node the current node to be processed
    148         * @param refHandler the {@code ReferenceNodeHandler}
    149         */
    150        private void updateNode(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
    151            final Object reference = refHandler.getReference(node);
    152            if (reference != null) {
    153                update(node, reference, refHandler);
    154            }
    155        }
    156
    157        @Override
    158        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    159            final ReferenceNodeHandler refHandler = (ReferenceNodeHandler) handler;
    160            updateNode(node, refHandler);
    161            insertNewChildNodes(node, refHandler);
    162        }
    163    }
    164
    165    /**
    166     * A specialized visitor implementation which constructs the root node of a configuration with all variables replaced by
    167     * their interpolated values.
    168     */
    169    private final class InterpolatedVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
    170        /** A stack for managing node builder instances. */
    171        private final List<ImmutableNode.Builder> builderStack;
    172
    173        /** The resulting root node. */
    174        private ImmutableNode interpolatedRoot;
    175
    176        /**
    177         * Creates a new instance of {@code InterpolatedVisitor}.
    178         */
    179        public InterpolatedVisitor() {
    180            builderStack = new LinkedList<>();
    181        }
    182
    183        /**
    184         * Gets the result of this builder: the root node of the interpolated nodes hierarchy.
    185         *
    186         * @return the resulting root node
    187         */
    188        public ImmutableNode getInterpolatedRoot() {
    189            return interpolatedRoot;
    190        }
    191
    192        /**
    193         * Handles interpolation for a node with no children. If interpolation does not change this node, it is copied as is to
    194         * the resulting structure. Otherwise, a new node is created with the interpolated values.
    195         *
    196         * @param node the current node to be processed
    197         * @param handler the {@code NodeHandler}
    198         */
    199        private void handleLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    200            final Object value = interpolate(node.getValue());
    201            final Map<String, Object> interpolatedAttributes = new HashMap<>();
    202            final boolean attributeChanged = interpolateAttributes(node, handler, interpolatedAttributes);
    203            final ImmutableNode newNode = valueChanged(value, handler.getValue(node)) || attributeChanged
    204                ? new ImmutableNode.Builder().name(handler.nodeName(node)).value(value).addAttributes(interpolatedAttributes).create()
    205                : node;
    206            storeInterpolatedNode(newNode);
    207        }
    208
    209        /**
    210         * Returns a map with interpolated attributes of the passed in node.
    211         *
    212         * @param node the current node to be processed
    213         * @param handler the {@code NodeHandler}
    214         * @return the map with interpolated attributes
    215         */
    216        private Map<String, Object> interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    217            final Map<String, Object> attributes = new HashMap<>();
    218            interpolateAttributes(node, handler, attributes);
    219            return attributes;
    220        }
    221
    222        /**
    223         * Populates a map with interpolated attributes of the passed in node.
    224         *
    225         * @param node the current node to be processed
    226         * @param handler the {@code NodeHandler}
    227         * @param interpolatedAttributes a map for storing the results
    228         * @return a flag whether an attribute value was changed by interpolation
    229         */
    230        private boolean interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler,
    231            final Map<String, Object> interpolatedAttributes) {
    232            boolean attributeChanged = false;
    233            for (final String attr : handler.getAttributes(node)) {
    234                final Object attrValue = interpolate(handler.getAttributeValue(node, attr));
    235                if (valueChanged(attrValue, handler.getAttributeValue(node, attr))) {
    236                    attributeChanged = true;
    237                }
    238                interpolatedAttributes.put(attr, attrValue);
    239            }
    240            return attributeChanged;
    241        }
    242
    243        /**
    244         * Returns a flag whether the given node is a leaf. This is the case if it does not have children.
    245         *
    246         * @param node the node in question
    247         * @param handler the {@code NodeHandler}
    248         * @return a flag whether this is a leaf node
    249         */
    250        private boolean isLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    251            return handler.getChildren(node).isEmpty();
    252        }
    253
    254        /**
    255         * Returns the top-level element from the stack without removing it.
    256         *
    257         * @return the top-level element from the stack
    258         */
    259        private ImmutableNode.Builder peek() {
    260            return builderStack.get(0);
    261        }
    262
    263        /**
    264         * Pops the top-level element from the stack.
    265         *
    266         * @return the element popped from the stack
    267         */
    268        private ImmutableNode.Builder pop() {
    269            return builderStack.remove(0);
    270        }
    271
    272        /**
    273         * Pushes a new builder on the stack.
    274         *
    275         * @param builder the builder
    276         */
    277        private void push(final ImmutableNode.Builder builder) {
    278            builderStack.add(0, builder);
    279        }
    280
    281        /**
    282         * Stores a processed node. Per default, the node is added to the current builder on the stack. If no such builder
    283         * exists, this is the result node.
    284         *
    285         * @param node the node to be stored
    286         */
    287        private void storeInterpolatedNode(final ImmutableNode node) {
    288            if (builderStack.isEmpty()) {
    289                interpolatedRoot = node;
    290            } else {
    291                peek().addChild(node);
    292            }
    293        }
    294
    295        /**
    296         * Tests whether a value is changed because of interpolation.
    297         *
    298         * @param interpolatedValue the interpolated value
    299         * @param value the original value
    300         * @return a flag whether the value was changed
    301         */
    302        private boolean valueChanged(final Object interpolatedValue, final Object value) {
    303            return ObjectUtils.notEqual(interpolatedValue, value);
    304        }
    305
    306        @Override
    307        public void visitAfterChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    308            if (!isLeafNode(node, handler)) {
    309                final ImmutableNode newNode = pop().create();
    310                storeInterpolatedNode(newNode);
    311            }
    312        }
    313
    314        @Override
    315        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    316            if (isLeafNode(node, handler)) {
    317                handleLeafNode(node, handler);
    318            } else {
    319                final ImmutableNode.Builder builder = new ImmutableNode.Builder(handler.getChildrenCount(node, null)).name(handler.nodeName(node))
    320                    .value(interpolate(handler.getValue(node))).addAttributes(interpolateAttributes(node, handler));
    321                push(builder);
    322            }
    323        }
    324    }
    325
    326    /**
    327     * Creates the {@code NodeModel} for this configuration based on a passed in source configuration. This implementation
    328     * creates an {@link InMemoryNodeModel}. If the passed in source configuration is defined, its root node also becomes
    329     * the root node of this configuration. Otherwise, a new, empty root node is used.
    330     *
    331     * @param c the configuration that is to be copied
    332     * @return the {@code NodeModel} for the new configuration
    333     */
    334    private static NodeModel<ImmutableNode> createNodeModel(final HierarchicalConfiguration<ImmutableNode> c) {
    335        return new InMemoryNodeModel(obtainRootNode(c));
    336    }
    337
    338    /**
    339     * Obtains the root node from a configuration whose data is to be copied. It has to be ensured that the synchronizer is
    340     * called correctly.
    341     *
    342     * @param c the configuration that is to be copied
    343     * @return the root node of this configuration
    344     */
    345    private static ImmutableNode obtainRootNode(final HierarchicalConfiguration<ImmutableNode> c) {
    346        return c != null ? c.getNodeModel().getNodeHandler().getRootNode() : null;
    347    }
    348
    349    /**
    350     * Creates a list with immutable configurations from the given input list.
    351     *
    352     * @param subs a list with mutable configurations
    353     * @return a list with corresponding immutable configurations
    354     */
    355    private static List<ImmutableHierarchicalConfiguration> toImmutable(final List<? extends HierarchicalConfiguration<?>> subs) {
    356        return subs.stream().map(ConfigurationUtils::unmodifiableConfiguration).collect(Collectors.toList());
    357    }
    358
    359    /** A listener for reacting on changes caused by sub configurations. */
    360    private final EventListener<ConfigurationEvent> changeListener;
    361
    362    /**
    363     * Creates a new instance of {@code BaseHierarchicalConfiguration}.
    364     */
    365    public BaseHierarchicalConfiguration() {
    366        this((HierarchicalConfiguration<ImmutableNode>) null);
    367    }
    368
    369    /**
    370     * Creates a new instance of {@code BaseHierarchicalConfiguration} and copies all data contained in the specified
    371     * configuration into the new one.
    372     *
    373     * @param c the configuration that is to be copied (if <strong>null</strong>, this constructor will behave like the standard
    374     *        constructor)
    375     * @since 1.4
    376     */
    377    public BaseHierarchicalConfiguration(final HierarchicalConfiguration<ImmutableNode> c) {
    378        this(createNodeModel(c));
    379    }
    380
    381    /**
    382     * Creates a new instance of {@code BaseHierarchicalConfiguration} and initializes it with the given {@code NodeModel}.
    383     *
    384     * @param model the {@code NodeModel}
    385     */
    386    protected BaseHierarchicalConfiguration(final NodeModel<ImmutableNode> model) {
    387        super(model);
    388        changeListener = createChangeListener();
    389    }
    390
    391    /**
    392     * {@inheritDoc} This implementation resolves the node(s) selected by the given key. If not a single node is selected,
    393     * an empty list is returned. Otherwise, sub configurations for each child of the node are created.
    394     */
    395    @Override
    396    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key) {
    397        final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
    398        if (nodes.size() != 1) {
    399            return Collections.emptyList();
    400        }
    401        return nodes.get(0).stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
    402    }
    403
    404    /**
    405     * {@inheritDoc} This method works like {@link #childConfigurationsAt(String)}; however, depending on the value of the
    406     * {@code supportUpdates} flag, connected sub configurations may be created.
    407     */
    408    @Override
    409    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key, final boolean supportUpdates) {
    410        if (!supportUpdates) {
    411            return childConfigurationsAt(key);
    412        }
    413
    414        final InMemoryNodeModel parentModel = getSubConfigurationParentModel();
    415        return createConnectedSubConfigurations(this, parentModel.trackChildNodes(key, this));
    416    }
    417
    418    /**
    419     * {@inheritDoc} This implementation creates a new instance of {@link InMemoryNodeModel}, initialized with this
    420     * configuration's root node. This has the effect that although the same nodes are used, the original and copied
    421     * configurations are independent on each other.
    422     */
    423    @Override
    424    protected NodeModel<ImmutableNode> cloneNodeModel() {
    425        return new InMemoryNodeModel(getModel().getNodeHandler().getRootNode());
    426    }
    427
    428    /**
    429     * {@inheritDoc} This is a short form for {@code configurationAt(key,
    430     * <strong>false</strong>)}.
    431     *
    432     * @throws ConfigurationRuntimeException if the key does not select a single node
    433     */
    434    @Override
    435    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key) {
    436        return configurationAt(key, false);
    437    }
    438
    439    /**
    440     * {@inheritDoc} The result of this implementation depends on the {@code supportUpdates} flag: If it is <strong>false</strong>, a
    441     * plain {@code BaseHierarchicalConfiguration} is returned using the selected node as root node. This is suitable for
    442     * read-only access to properties. Because the configuration returned in this case is not connected to the parent
    443     * configuration, updates on properties made by one configuration are not reflected by the other one. A value of
    444     * <strong>true</strong> for this parameter causes a tracked node to be created, and result is a {@link SubnodeConfiguration}
    445     * based on this tracked node. This configuration is really connected to its parent, so that updated properties are
    446     * visible on both.
    447     *
    448     * @see SubnodeConfiguration
    449     * @throws ConfigurationRuntimeException if the key does not select a single node
    450     */
    451    @Override
    452    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key, final boolean supportUpdates) {
    453        return syncRead(() -> supportUpdates ? createConnectedSubConfiguration(key) : createIndependentSubConfiguration(key), false);
    454    }
    455
    456    /**
    457     * {@inheritDoc} This implementation creates sub configurations in the same way as described for
    458     * {@link #configurationAt(String)}.
    459     */
    460    @Override
    461    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key) {
    462        final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
    463        return nodes.stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
    464    }
    465
    466    /**
    467     * {@inheritDoc} This implementation creates tracked nodes for the specified key. Then sub configurations for these
    468     * nodes are created and returned.
    469     */
    470    @Override
    471    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key, final boolean supportUpdates) {
    472        if (!supportUpdates) {
    473            return configurationsAt(key);
    474        }
    475        final InMemoryNodeModel parentModel = syncRead(this::getSubConfigurationParentModel, false);
    476        return createConnectedSubConfigurations(this, parentModel.selectAndTrackNodes(key, this));
    477    }
    478
    479    /**
    480     * Creates a listener which reacts on all changes on this configuration or one of its {@code SubnodeConfiguration}
    481     * instances. If such a change is detected, some updates have to be performed.
    482     *
    483     * @return the newly created change listener
    484     */
    485    private EventListener<ConfigurationEvent> createChangeListener() {
    486        return this::subnodeConfigurationChanged;
    487    }
    488
    489    /**
    490     * Creates a sub configuration from the specified key which is connected to this configuration. This implementation
    491     * creates a {@link SubnodeConfiguration} with a tracked node identified by the passed in key.
    492     *
    493     * @param key the key of the sub configuration
    494     * @return the new sub configuration
    495     */
    496    private BaseHierarchicalConfiguration createConnectedSubConfiguration(final String key) {
    497        final NodeSelector selector = getSubConfigurationNodeSelector(key);
    498        getSubConfigurationParentModel().trackNode(selector, this);
    499        return createSubConfigurationForTrackedNode(selector, this);
    500    }
    501
    502    /**
    503     * Creates a list of connected sub configurations based on a passed in list of node selectors.
    504     *
    505     * @param parentModelSupport the parent node model support object
    506     * @param selectors the list of {@code NodeSelector} objects
    507     * @return the list with sub configurations
    508     */
    509    private List<HierarchicalConfiguration<ImmutableNode>> createConnectedSubConfigurations(final InMemoryNodeModelSupport parentModelSupport,
    510        final Collection<NodeSelector> selectors) {
    511        return selectors.stream().map(sel -> createSubConfigurationForTrackedNode(sel, parentModelSupport)).collect(Collectors.toList());
    512    }
    513
    514    /**
    515     * Creates a sub configuration from the specified key which is independent on this configuration. This means that the
    516     * sub configuration operates on a separate node model (although the nodes are initially shared).
    517     *
    518     * @param key the key of the sub configuration
    519     * @return the new sub configuration
    520     */
    521    private BaseHierarchicalConfiguration createIndependentSubConfiguration(final String key) {
    522        final List<ImmutableNode> targetNodes = fetchFilteredNodeResults(key);
    523        final int size = targetNodes.size();
    524        if (size != 1) {
    525            throw new ConfigurationRuntimeException("Passed in key must select exactly one node (found %,d): %s", size, key);
    526        }
    527        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(targetNodes.get(0)));
    528        initSubConfiguration(sub);
    529        return sub;
    530    }
    531
    532    /**
    533     * Returns an initialized sub configuration for this configuration that is based on another
    534     * {@code BaseHierarchicalConfiguration}. Thus, it is independent from this configuration.
    535     *
    536     * @param node the root node for the sub configuration
    537     * @return the initialized sub configuration
    538     */
    539    private BaseHierarchicalConfiguration createIndependentSubConfigurationForNode(final ImmutableNode node) {
    540        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(node));
    541        initSubConfiguration(sub);
    542        return sub;
    543    }
    544
    545    /**
    546     * Creates a connected sub configuration based on a selector for a tracked node.
    547     *
    548     * @param selector the {@code NodeSelector}
    549     * @param parentModelSupport the {@code InMemoryNodeModelSupport} object for the parent node model
    550     * @return the newly created sub configuration
    551     * @since 2.0
    552     */
    553    protected SubnodeConfiguration createSubConfigurationForTrackedNode(final NodeSelector selector, final InMemoryNodeModelSupport parentModelSupport) {
    554        final SubnodeConfiguration subConfig = new SubnodeConfiguration(this, new TrackedNodeModel(parentModelSupport, selector, true));
    555        initSubConfigurationForThisParent(subConfig);
    556        return subConfig;
    557    }
    558
    559    /**
    560     * Creates a root node for a subset configuration based on the passed in query results. This method creates a new root
    561     * node and adds the children and attributes of all result nodes to it. If only a single node value is defined, it is
    562     * assigned as value of the new root node.
    563     *
    564     * @param results the collection of query results
    565     * @return the root node for the subset configuration
    566     */
    567    private ImmutableNode createSubsetRootNode(final Collection<QueryResult<ImmutableNode>> results) {
    568        final ImmutableNode.Builder builder = new ImmutableNode.Builder();
    569        Object value = null;
    570        int valueCount = 0;
    571
    572        for (final QueryResult<ImmutableNode> result : results) {
    573            if (result.isAttributeResult()) {
    574                builder.addAttribute(result.getAttributeName(), result.getAttributeValue(getModel().getNodeHandler()));
    575            } else {
    576                if (result.getNode().getValue() != null) {
    577                    value = result.getNode().getValue();
    578                    valueCount++;
    579                }
    580                builder.addChildren(result.getNode().getChildren());
    581                builder.addAttributes(result.getNode().getAttributes());
    582            }
    583        }
    584
    585        if (valueCount == 1) {
    586            builder.value(value);
    587        }
    588        return builder.create();
    589    }
    590
    591    /**
    592     * Executes a query on the specified key and filters it for node results.
    593     *
    594     * @param key the key
    595     * @return the filtered list with result nodes
    596     */
    597    private List<ImmutableNode> fetchFilteredNodeResults(final String key) {
    598        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    599        return resolveNodeKey(handler.getRootNode(), key, handler);
    600    }
    601
    602    /**
    603     * {@inheritDoc} This implementation returns the {@code InMemoryNodeModel} used by this configuration.
    604     */
    605    @Override
    606    public InMemoryNodeModel getNodeModel() {
    607        return (InMemoryNodeModel) super.getNodeModel();
    608    }
    609
    610    /**
    611     * Gets the {@code NodeSelector} to be used for a sub configuration based on the passed in key. This method is called
    612     * whenever a sub configuration is to be created. This base implementation returns a new {@code NodeSelector}
    613     * initialized with the passed in key. Sub classes may override this method if they have a different strategy for
    614     * creating a selector.
    615     *
    616     * @param key the key of the sub configuration
    617     * @return a {@code NodeSelector} for initializing a sub configuration
    618     * @since 2.0
    619     */
    620    protected NodeSelector getSubConfigurationNodeSelector(final String key) {
    621        return new NodeSelector(key);
    622    }
    623
    624    /**
    625     * Gets the {@code InMemoryNodeModel} to be used as parent model for a new sub configuration. This method is called
    626     * whenever a sub configuration is to be created. This base implementation returns the model of this configuration. Sub
    627     * classes with different requirements for the parent models of sub configurations have to override it.
    628     *
    629     * @return the parent model for a new sub configuration
    630     */
    631    protected InMemoryNodeModel getSubConfigurationParentModel() {
    632        return (InMemoryNodeModel) getModel();
    633    }
    634
    635    /**
    636     * {@inheritDoc} This implementation first delegates to {@code childConfigurationsAt()} to create a list of mutable
    637     * child configurations. Then a list with immutable wrapper configurations is created.
    638     */
    639    @Override
    640    public List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(final String key) {
    641        return toImmutable(childConfigurationsAt(key));
    642    }
    643
    644    /**
    645     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    646     * Then an immutable wrapper is created and returned.
    647     *
    648     * @throws ConfigurationRuntimeException if the key does not select a single node
    649     */
    650    @Override
    651    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key) {
    652        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key));
    653    }
    654
    655    /**
    656     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    657     * Then an immutable wrapper is created and returned.
    658     */
    659    @Override
    660    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key, final boolean supportUpdates) {
    661        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key, supportUpdates));
    662    }
    663
    664    /**
    665     * {@inheritDoc} This implementation first delegates to {@code configurationsAt()} to create a list of
    666     * {@code SubnodeConfiguration} objects. Then for each element of this list an unmodifiable wrapper is created.
    667     */
    668    @Override
    669    public List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(final String key) {
    670        return toImmutable(configurationsAt(key));
    671    }
    672
    673    /**
    674     * Initializes properties of a sub configuration. A sub configuration inherits some settings from its parent, for example the
    675     * expression engine or the synchronizer. The corresponding values are copied by this method.
    676     *
    677     * @param sub the sub configuration to be initialized
    678     */
    679    private void initSubConfiguration(final BaseHierarchicalConfiguration sub) {
    680        sub.setSynchronizer(getSynchronizer());
    681        sub.setExpressionEngine(getExpressionEngine());
    682        sub.setListDelimiterHandler(getListDelimiterHandler());
    683        sub.setThrowExceptionOnMissing(isThrowExceptionOnMissing());
    684        sub.getInterpolator().setParentInterpolator(getInterpolator());
    685    }
    686
    687    /**
    688     * Initializes a {@code SubnodeConfiguration} object. This method should be called for each sub configuration created
    689     * for this configuration. It ensures that the sub configuration is correctly connected to its parent instance and that
    690     * update events are correctly propagated.
    691     *
    692     * @param subConfig the sub configuration to be initialized
    693     * @since 2.0
    694     */
    695    protected void initSubConfigurationForThisParent(final SubnodeConfiguration subConfig) {
    696        initSubConfiguration(subConfig);
    697        subConfig.addEventListener(ConfigurationEvent.ANY, changeListener);
    698    }
    699
    700    /**
    701     * Returns a configuration with the same content as this configuration, but with all variables replaced by their actual
    702     * values. This implementation is specific for hierarchical configurations. It clones the current configuration and runs
    703     * a specialized visitor on the clone, which performs interpolation on the single configuration nodes.
    704     *
    705     * @return a configuration with all variables interpolated
    706     * @since 1.5
    707     */
    708    @Override
    709    public Configuration interpolatedConfiguration() {
    710        final InterpolatedVisitor visitor = new InterpolatedVisitor();
    711        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    712        NodeTreeWalker.INSTANCE.walkDFS(handler.getRootNode(), visitor, handler);
    713
    714        final BaseHierarchicalConfiguration c = (BaseHierarchicalConfiguration) clone();
    715        c.getNodeModel().setRootNode(visitor.getInterpolatedRoot());
    716        return c;
    717    }
    718
    719    /**
    720     * This method is always called when a subnode configuration created from this configuration has been modified. This
    721     * implementation transforms the received event into an event of type {@code SUBNODE_CHANGED} and notifies the
    722     * registered listeners.
    723     *
    724     * @param event the event describing the change
    725     * @since 1.5
    726     */
    727    protected void subnodeConfigurationChanged(final ConfigurationEvent event) {
    728        fireEvent(ConfigurationEvent.SUBNODE_CHANGED, null, event, event.isBeforeUpdate());
    729    }
    730
    731    /**
    732     * Creates a new {@code Configuration} object containing all keys that start with the specified prefix. This
    733     * implementation will return a {@code BaseHierarchicalConfiguration} object so that the structure of the keys will be
    734     * saved. The nodes selected by the prefix (it is possible that multiple nodes are selected) are mapped to the root node
    735     * of the returned configuration, i.e. their children and attributes will become children and attributes of the new root
    736     * node. However, a value of the root node is only set if exactly one of the selected nodes contain a value (if multiple
    737     * nodes have a value, there is simply no way to decide how these values are merged together). Note that the returned
    738     * {@code Configuration} object is not connected to its source configuration: updates on the source configuration are
    739     * not reflected in the subset and vice versa. The returned configuration uses the same {@code Synchronizer} as this
    740     * configuration.
    741     *
    742     * @param prefix the prefix of the keys for the subset
    743     * @return a new configuration object representing the selected subset
    744     */
    745    @Override
    746    public Configuration subset(final String prefix) {
    747        return syncRead(() -> {
    748            final List<QueryResult<ImmutableNode>> results = fetchNodeList(prefix);
    749            if (results.isEmpty()) {
    750                return new BaseHierarchicalConfiguration();
    751            }
    752            final BaseHierarchicalConfiguration parent = this;
    753            final BaseHierarchicalConfiguration result = new BaseHierarchicalConfiguration() {
    754
    755                @Override
    756                public ConfigurationInterpolator getInterpolator() {
    757                    return parent.getInterpolator();
    758                }
    759
    760                // Override interpolate to always interpolate on the parent
    761                @Override
    762                protected Object interpolate(final Object value) {
    763                    return parent.interpolate(value);
    764                }
    765            };
    766            result.getModel().setRootNode(createSubsetRootNode(results));
    767            if (result.isEmpty()) {
    768                return new BaseHierarchicalConfiguration();
    769            }
    770            result.setSynchronizer(getSynchronizer());
    771            return result;
    772        }, false);
    773    }
    774}