Source code
Description: source: package: org.apache.commons.configuration2, class: BaseHierarchicalConfiguration, class: BuilderVisitor

    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
    171        /** A stack for managing node builder instances. */
    172        private final List<ImmutableNode.Builder> builderStack;
    173
    174        /** The resulting root node. */
    175        private ImmutableNode interpolatedRoot;
    176
    177        /**
    178         * Creates a new instance of {@code InterpolatedVisitor}.
    179         */
    180        public InterpolatedVisitor() {
    181            builderStack = new LinkedList<>();
    182        }
    183
    184        /**
    185         * Gets the result of this builder: the root node of the interpolated nodes hierarchy.
    186         *
    187         * @return the resulting root node
    188         */
    189        public ImmutableNode getInterpolatedRoot() {
    190            return interpolatedRoot;
    191        }
    192
    193        /**
    194         * Handles interpolation for a node with no children. If interpolation does not change this node, it is copied as is to
    195         * the resulting structure. Otherwise, a new node is created with the interpolated values.
    196         *
    197         * @param node the current node to be processed
    198         * @param handler the {@code NodeHandler}
    199         */
    200        private void handleLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    201            final Object value = interpolate(node.getValue());
    202            final Map<String, Object> interpolatedAttributes = new HashMap<>();
    203            final boolean attributeChanged = interpolateAttributes(node, handler, interpolatedAttributes);
    204            final ImmutableNode newNode = valueChanged(value, handler.getValue(node)) || attributeChanged
    205                ? new ImmutableNode.Builder().name(handler.nodeName(node)).value(value).addAttributes(interpolatedAttributes).create()
    206                : node;
    207            storeInterpolatedNode(newNode);
    208        }
    209
    210        /**
    211         * Returns a map with interpolated attributes of the passed in node.
    212         *
    213         * @param node the current node to be processed
    214         * @param handler the {@code NodeHandler}
    215         * @return the map with interpolated attributes
    216         */
    217        private Map<String, Object> interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    218            final Map<String, Object> attributes = new HashMap<>();
    219            interpolateAttributes(node, handler, attributes);
    220            return attributes;
    221        }
    222
    223        /**
    224         * Populates a map with interpolated attributes of the passed in node.
    225         *
    226         * @param node the current node to be processed
    227         * @param handler the {@code NodeHandler}
    228         * @param interpolatedAttributes a map for storing the results
    229         * @return a flag whether an attribute value was changed by interpolation
    230         */
    231        private boolean interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler,
    232            final Map<String, Object> interpolatedAttributes) {
    233            boolean attributeChanged = false;
    234            for (final String attr : handler.getAttributes(node)) {
    235                final Object attrValue = interpolate(handler.getAttributeValue(node, attr));
    236                if (valueChanged(attrValue, handler.getAttributeValue(node, attr))) {
    237                    attributeChanged = true;
    238                }
    239                interpolatedAttributes.put(attr, attrValue);
    240            }
    241            return attributeChanged;
    242        }
    243
    244        /**
    245         * Returns a flag whether the given node is a leaf. This is the case if it does not have children.
    246         *
    247         * @param node the node in question
    248         * @param handler the {@code NodeHandler}
    249         * @return a flag whether this is a leaf node
    250         */
    251        private boolean isLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    252            return handler.getChildren(node).isEmpty();
    253        }
    254
    255        /**
    256         * Returns the top-level element from the stack without removing it.
    257         *
    258         * @return the top-level element from the stack
    259         */
    260        private ImmutableNode.Builder peek() {
    261            return builderStack.get(0);
    262        }
    263
    264        /**
    265         * Pops the top-level element from the stack.
    266         *
    267         * @return the element popped from the stack
    268         */
    269        private ImmutableNode.Builder pop() {
    270            return builderStack.remove(0);
    271        }
    272
    273        /**
    274         * Pushes a new builder on the stack.
    275         *
    276         * @param builder the builder
    277         */
    278        private void push(final ImmutableNode.Builder builder) {
    279            builderStack.add(0, builder);
    280        }
    281
    282        /**
    283         * Stores a processed node. Per default, the node is added to the current builder on the stack. If no such builder
    284         * exists, this is the result node.
    285         *
    286         * @param node the node to be stored
    287         */
    288        private void storeInterpolatedNode(final ImmutableNode node) {
    289            if (builderStack.isEmpty()) {
    290                interpolatedRoot = node;
    291            } else {
    292                peek().addChild(node);
    293            }
    294        }
    295
    296        /**
    297         * Tests whether a value is changed because of interpolation.
    298         *
    299         * @param interpolatedValue the interpolated value
    300         * @param value the original value
    301         * @return a flag whether the value was changed
    302         */
    303        private boolean valueChanged(final Object interpolatedValue, final Object value) {
    304            return ObjectUtils.notEqual(interpolatedValue, value);
    305        }
    306
    307        @Override
    308        public void visitAfterChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    309            if (!isLeafNode(node, handler)) {
    310                final ImmutableNode newNode = pop().create();
    311                storeInterpolatedNode(newNode);
    312            }
    313        }
    314
    315        @Override
    316        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    317            if (isLeafNode(node, handler)) {
    318                handleLeafNode(node, handler);
    319            } else {
    320                final ImmutableNode.Builder builder = new ImmutableNode.Builder(handler.getChildrenCount(node, null)).name(handler.nodeName(node))
    321                    .value(interpolate(handler.getValue(node))).addAttributes(interpolateAttributes(node, handler));
    322                push(builder);
    323            }
    324        }
    325    }
    326
    327    /**
    328     * Creates the {@code NodeModel} for this configuration based on a passed in source configuration. This implementation
    329     * creates an {@link InMemoryNodeModel}. If the passed in source configuration is defined, its root node also becomes
    330     * the root node of this configuration. Otherwise, a new, empty root node is used.
    331     *
    332     * @param c the configuration that is to be copied
    333     * @return the {@code NodeModel} for the new configuration
    334     */
    335    private static NodeModel<ImmutableNode> createNodeModel(final HierarchicalConfiguration<ImmutableNode> c) {
    336        return new InMemoryNodeModel(obtainRootNode(c));
    337    }
    338
    339    /**
    340     * Obtains the root node from a configuration whose data is to be copied. It has to be ensured that the synchronizer is
    341     * called correctly.
    342     *
    343     * @param c the configuration that is to be copied
    344     * @return the root node of this configuration
    345     */
    346    private static ImmutableNode obtainRootNode(final HierarchicalConfiguration<ImmutableNode> c) {
    347        return c != null ? c.getNodeModel().getNodeHandler().getRootNode() : null;
    348    }
    349
    350    /**
    351     * Creates a list with immutable configurations from the given input list.
    352     *
    353     * @param subs a list with mutable configurations
    354     * @return a list with corresponding immutable configurations
    355     */
    356    private static List<ImmutableHierarchicalConfiguration> toImmutable(final List<? extends HierarchicalConfiguration<?>> subs) {
    357        return subs.stream().map(ConfigurationUtils::unmodifiableConfiguration).collect(Collectors.toList());
    358    }
    359
    360    /** A listener for reacting on changes caused by sub configurations. */
    361    private final EventListener<ConfigurationEvent> changeListener;
    362
    363    /**
    364     * Creates a new instance of {@code BaseHierarchicalConfiguration}.
    365     */
    366    public BaseHierarchicalConfiguration() {
    367        this((HierarchicalConfiguration<ImmutableNode>) null);
    368    }
    369
    370    /**
    371     * Creates a new instance of {@code BaseHierarchicalConfiguration} and copies all data contained in the specified
    372     * configuration into the new one.
    373     *
    374     * @param c the configuration that is to be copied (if <strong>null</strong>, this constructor will behave like the standard
    375     *        constructor)
    376     * @since 1.4
    377     */
    378    public BaseHierarchicalConfiguration(final HierarchicalConfiguration<ImmutableNode> c) {
    379        this(createNodeModel(c));
    380    }
    381
    382    /**
    383     * Creates a new instance of {@code BaseHierarchicalConfiguration} and initializes it with the given {@code NodeModel}.
    384     *
    385     * @param model the {@code NodeModel}
    386     */
    387    protected BaseHierarchicalConfiguration(final NodeModel<ImmutableNode> model) {
    388        super(model);
    389        changeListener = createChangeListener();
    390    }
    391
    392    /**
    393     * {@inheritDoc} This implementation resolves the node(s) selected by the given key. If not a single node is selected,
    394     * an empty list is returned. Otherwise, sub configurations for each child of the node are created.
    395     */
    396    @Override
    397    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key) {
    398        final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
    399        if (nodes.size() != 1) {
    400            return Collections.emptyList();
    401        }
    402        return nodes.get(0).stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
    403    }
    404
    405    /**
    406     * {@inheritDoc} This method works like {@link #childConfigurationsAt(String)}; however, depending on the value of the
    407     * {@code supportUpdates} flag, connected sub configurations may be created.
    408     */
    409    @Override
    410    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key, final boolean supportUpdates) {
    411        if (!supportUpdates) {
    412            return childConfigurationsAt(key);
    413        }
    414
    415        final InMemoryNodeModel parentModel = getSubConfigurationParentModel();
    416        return createConnectedSubConfigurations(this, parentModel.trackChildNodes(key, this));
    417    }
    418
    419    /**
    420     * {@inheritDoc} This implementation creates a new instance of {@link InMemoryNodeModel}, initialized with this
    421     * configuration's root node. This has the effect that although the same nodes are used, the original and copied
    422     * configurations are independent on each other.
    423     */
    424    @Override
    425    protected NodeModel<ImmutableNode> cloneNodeModel() {
    426        return new InMemoryNodeModel(getModel().getNodeHandler().getRootNode());
    427    }
    428
    429    /**
    430     * {@inheritDoc} This is a short form for {@code configurationAt(key,
    431     * <strong>false</strong>)}.
    432     *
    433     * @throws ConfigurationRuntimeException if the key does not select a single node
    434     */
    435    @Override
    436    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key) {
    437        return configurationAt(key, false);
    438    }
    439
    440    /**
    441     * {@inheritDoc} The result of this implementation depends on the {@code supportUpdates} flag: If it is <strong>false</strong>, a
    442     * plain {@code BaseHierarchicalConfiguration} is returned using the selected node as root node. This is suitable for
    443     * read-only access to properties. Because the configuration returned in this case is not connected to the parent
    444     * configuration, updates on properties made by one configuration are not reflected by the other one. A value of
    445     * <strong>true</strong> for this parameter causes a tracked node to be created, and result is a {@link SubnodeConfiguration}
    446     * based on this tracked node. This configuration is really connected to its parent, so that updated properties are
    447     * visible on both.
    448     *
    449     * @see SubnodeConfiguration
    450     * @throws ConfigurationRuntimeException if the key does not select a single node
    451     */
    452    @Override
    453    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key, final boolean supportUpdates) {
    454        return syncRead(() -> supportUpdates ? createConnectedSubConfiguration(key) : createIndependentSubConfiguration(key), false);
    455    }
    456
    457    /**
    458     * {@inheritDoc} This implementation creates sub configurations in the same way as described for
    459     * {@link #configurationAt(String)}.
    460     */
    461    @Override
    462    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key) {
    463        final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
    464        return nodes.stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
    465    }
    466
    467    /**
    468     * {@inheritDoc} This implementation creates tracked nodes for the specified key. Then sub configurations for these
    469     * nodes are created and returned.
    470     */
    471    @Override
    472    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key, final boolean supportUpdates) {
    473        if (!supportUpdates) {
    474            return configurationsAt(key);
    475        }
    476        final InMemoryNodeModel parentModel = syncRead(this::getSubConfigurationParentModel, false);
    477        return createConnectedSubConfigurations(this, parentModel.selectAndTrackNodes(key, this));
    478    }
    479
    480    /**
    481     * Creates a listener which reacts on all changes on this configuration or one of its {@code SubnodeConfiguration}
    482     * instances. If such a change is detected, some updates have to be performed.
    483     *
    484     * @return the newly created change listener
    485     */
    486    private EventListener<ConfigurationEvent> createChangeListener() {
    487        return this::subnodeConfigurationChanged;
    488    }
    489
    490    /**
    491     * Creates a sub configuration from the specified key which is connected to this configuration. This implementation
    492     * creates a {@link SubnodeConfiguration} with a tracked node identified by the passed in key.
    493     *
    494     * @param key the key of the sub configuration
    495     * @return the new sub configuration
    496     */
    497    private BaseHierarchicalConfiguration createConnectedSubConfiguration(final String key) {
    498        final NodeSelector selector = getSubConfigurationNodeSelector(key);
    499        getSubConfigurationParentModel().trackNode(selector, this);
    500        return createSubConfigurationForTrackedNode(selector, this);
    501    }
    502
    503    /**
    504     * Creates a list of connected sub configurations based on a passed in list of node selectors.
    505     *
    506     * @param parentModelSupport the parent node model support object
    507     * @param selectors the list of {@code NodeSelector} objects
    508     * @return the list with sub configurations
    509     */
    510    private List<HierarchicalConfiguration<ImmutableNode>> createConnectedSubConfigurations(final InMemoryNodeModelSupport parentModelSupport,
    511        final Collection<NodeSelector> selectors) {
    512        return selectors.stream().map(sel -> createSubConfigurationForTrackedNode(sel, parentModelSupport)).collect(Collectors.toList());
    513    }
    514
    515    /**
    516     * Creates a sub configuration from the specified key which is independent on this configuration. This means that the
    517     * sub configuration operates on a separate node model (although the nodes are initially shared).
    518     *
    519     * @param key the key of the sub configuration
    520     * @return the new sub configuration
    521     */
    522    private BaseHierarchicalConfiguration createIndependentSubConfiguration(final String key) {
    523        final List<ImmutableNode> targetNodes = fetchFilteredNodeResults(key);
    524        final int size = targetNodes.size();
    525        if (size != 1) {
    526            throw new ConfigurationRuntimeException("Passed in key must select exactly one node (found %,d): %s", size, key);
    527        }
    528        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(targetNodes.get(0)));
    529        initSubConfiguration(sub);
    530        return sub;
    531    }
    532
    533    /**
    534     * Returns an initialized sub configuration for this configuration that is based on another
    535     * {@code BaseHierarchicalConfiguration}. Thus, it is independent from this configuration.
    536     *
    537     * @param node the root node for the sub configuration
    538     * @return the initialized sub configuration
    539     */
    540    private BaseHierarchicalConfiguration createIndependentSubConfigurationForNode(final ImmutableNode node) {
    541        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(node));
    542        initSubConfiguration(sub);
    543        return sub;
    544    }
    545
    546    /**
    547     * Creates a connected sub configuration based on a selector for a tracked node.
    548     *
    549     * @param selector the {@code NodeSelector}
    550     * @param parentModelSupport the {@code InMemoryNodeModelSupport} object for the parent node model
    551     * @return the newly created sub configuration
    552     * @since 2.0
    553     */
    554    protected SubnodeConfiguration createSubConfigurationForTrackedNode(final NodeSelector selector, final InMemoryNodeModelSupport parentModelSupport) {
    555        final SubnodeConfiguration subConfig = new SubnodeConfiguration(this, new TrackedNodeModel(parentModelSupport, selector, true));
    556        initSubConfigurationForThisParent(subConfig);
    557        return subConfig;
    558    }
    559
    560    /**
    561     * Creates a root node for a subset configuration based on the passed in query results. This method creates a new root
    562     * node and adds the children and attributes of all result nodes to it. If only a single node value is defined, it is
    563     * assigned as value of the new root node.
    564     *
    565     * @param results the collection of query results
    566     * @return the root node for the subset configuration
    567     */
    568    private ImmutableNode createSubsetRootNode(final Collection<QueryResult<ImmutableNode>> results) {
    569        final ImmutableNode.Builder builder = new ImmutableNode.Builder();
    570        Object value = null;
    571        int valueCount = 0;
    572
    573        for (final QueryResult<ImmutableNode> result : results) {
    574            if (result.isAttributeResult()) {
    575                builder.addAttribute(result.getAttributeName(), result.getAttributeValue(getModel().getNodeHandler()));
    576            } else {
    577                if (result.getNode().getValue() != null) {
    578                    value = result.getNode().getValue();
    579                    valueCount++;
    580                }
    581                builder.addChildren(result.getNode().getChildren());
    582                builder.addAttributes(result.getNode().getAttributes());
    583            }
    584        }
    585
    586        if (valueCount == 1) {
    587            builder.value(value);
    588        }
    589        return builder.create();
    590    }
    591
    592    /**
    593     * Executes a query on the specified key and filters it for node results.
    594     *
    595     * @param key the key
    596     * @return the filtered list with result nodes
    597     */
    598    private List<ImmutableNode> fetchFilteredNodeResults(final String key) {
    599        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    600        return resolveNodeKey(handler.getRootNode(), key, handler);
    601    }
    602
    603    /**
    604     * {@inheritDoc} This implementation returns the {@code InMemoryNodeModel} used by this configuration.
    605     */
    606    @Override
    607    public InMemoryNodeModel getNodeModel() {
    608        return (InMemoryNodeModel) super.getNodeModel();
    609    }
    610
    611    /**
    612     * Gets the {@code NodeSelector} to be used for a sub configuration based on the passed in key. This method is called
    613     * whenever a sub configuration is to be created. This base implementation returns a new {@code NodeSelector}
    614     * initialized with the passed in key. Sub classes may override this method if they have a different strategy for
    615     * creating a selector.
    616     *
    617     * @param key the key of the sub configuration
    618     * @return a {@code NodeSelector} for initializing a sub configuration
    619     * @since 2.0
    620     */
    621    protected NodeSelector getSubConfigurationNodeSelector(final String key) {
    622        return new NodeSelector(key);
    623    }
    624
    625    /**
    626     * Gets the {@code InMemoryNodeModel} to be used as parent model for a new sub configuration. This method is called
    627     * whenever a sub configuration is to be created. This base implementation returns the model of this configuration. Sub
    628     * classes with different requirements for the parent models of sub configurations have to override it.
    629     *
    630     * @return the parent model for a new sub configuration
    631     */
    632    protected InMemoryNodeModel getSubConfigurationParentModel() {
    633        return (InMemoryNodeModel) getModel();
    634    }
    635
    636    /**
    637     * {@inheritDoc} This implementation first delegates to {@code childConfigurationsAt()} to create a list of mutable
    638     * child configurations. Then a list with immutable wrapper configurations is created.
    639     */
    640    @Override
    641    public List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(final String key) {
    642        return toImmutable(childConfigurationsAt(key));
    643    }
    644
    645    /**
    646     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    647     * Then an immutable wrapper is created and returned.
    648     *
    649     * @throws ConfigurationRuntimeException if the key does not select a single node
    650     */
    651    @Override
    652    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key) {
    653        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key));
    654    }
    655
    656    /**
    657     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    658     * Then an immutable wrapper is created and returned.
    659     */
    660    @Override
    661    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key, final boolean supportUpdates) {
    662        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key, supportUpdates));
    663    }
    664
    665    /**
    666     * {@inheritDoc} This implementation first delegates to {@code configurationsAt()} to create a list of
    667     * {@code SubnodeConfiguration} objects. Then for each element of this list an unmodifiable wrapper is created.
    668     */
    669    @Override
    670    public List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(final String key) {
    671        return toImmutable(configurationsAt(key));
    672    }
    673
    674    /**
    675     * Initializes properties of a sub configuration. A sub configuration inherits some settings from its parent, for example the
    676     * expression engine or the synchronizer. The corresponding values are copied by this method.
    677     *
    678     * @param sub the sub configuration to be initialized
    679     */
    680    private void initSubConfiguration(final BaseHierarchicalConfiguration sub) {
    681        sub.setSynchronizer(getSynchronizer());
    682        sub.setExpressionEngine(getExpressionEngine());
    683        sub.setListDelimiterHandler(getListDelimiterHandler());
    684        sub.setThrowExceptionOnMissing(isThrowExceptionOnMissing());
    685        sub.getInterpolator().setParentInterpolator(getInterpolator());
    686    }
    687
    688    /**
    689     * Initializes a {@code SubnodeConfiguration} object. This method should be called for each sub configuration created
    690     * for this configuration. It ensures that the sub configuration is correctly connected to its parent instance and that
    691     * update events are correctly propagated.
    692     *
    693     * @param subConfig the sub configuration to be initialized
    694     * @since 2.0
    695     */
    696    protected void initSubConfigurationForThisParent(final SubnodeConfiguration subConfig) {
    697        initSubConfiguration(subConfig);
    698        subConfig.addEventListener(ConfigurationEvent.ANY, changeListener);
    699    }
    700
    701    /**
    702     * Returns a configuration with the same content as this configuration, but with all variables replaced by their actual
    703     * values. This implementation is specific for hierarchical configurations. It clones the current configuration and runs
    704     * a specialized visitor on the clone, which performs interpolation on the single configuration nodes.
    705     *
    706     * @return a configuration with all variables interpolated
    707     * @since 1.5
    708     */
    709    @Override
    710    public Configuration interpolatedConfiguration() {
    711        final InterpolatedVisitor visitor = new InterpolatedVisitor();
    712        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    713        NodeTreeWalker.INSTANCE.walkDFS(handler.getRootNode(), visitor, handler);
    714
    715        final BaseHierarchicalConfiguration c = (BaseHierarchicalConfiguration) clone();
    716        c.getNodeModel().setRootNode(visitor.getInterpolatedRoot());
    717        return c;
    718    }
    719
    720    /**
    721     * This method is always called when a subnode configuration created from this configuration has been modified. This
    722     * implementation transforms the received event into an event of type {@code SUBNODE_CHANGED} and notifies the
    723     * registered listeners.
    724     *
    725     * @param event the event describing the change
    726     * @since 1.5
    727     */
    728    protected void subnodeConfigurationChanged(final ConfigurationEvent event) {
    729        fireEvent(ConfigurationEvent.SUBNODE_CHANGED, null, event, event.isBeforeUpdate());
    730    }
    731
    732    /**
    733     * Creates a new {@code Configuration} object containing all keys that start with the specified prefix. This
    734     * implementation will return a {@code BaseHierarchicalConfiguration} object so that the structure of the keys will be
    735     * saved. The nodes selected by the prefix (it is possible that multiple nodes are selected) are mapped to the root node
    736     * of the returned configuration, i.e. their children and attributes will become children and attributes of the new root
    737     * node. However, a value of the root node is only set if exactly one of the selected nodes contain a value (if multiple
    738     * nodes have a value, there is simply no way to decide how these values are merged together). Note that the returned
    739     * {@code Configuration} object is not connected to its source configuration: updates on the source configuration are
    740     * not reflected in the subset and vice versa. The returned configuration uses the same {@code Synchronizer} as this
    741     * configuration.
    742     *
    743     * @param prefix the prefix of the keys for the subset
    744     * @return a new configuration object representing the selected subset
    745     */
    746    @Override
    747    public Configuration subset(final String prefix) {
    748        return syncRead(() -> {
    749            final List<QueryResult<ImmutableNode>> results = fetchNodeList(prefix);
    750            if (results.isEmpty()) {
    751                return new BaseHierarchicalConfiguration();
    752            }
    753            final BaseHierarchicalConfiguration parent = this;
    754            final BaseHierarchicalConfiguration result = new BaseHierarchicalConfiguration() {
    755
    756                @Override
    757                public ConfigurationInterpolator getInterpolator() {
    758                    return parent.getInterpolator();
    759                }
    760
    761                // Override interpolate to always interpolate on the parent
    762                @Override
    763                protected Object interpolate(final Object value) {
    764                    return parent.interpolate(value);
    765                }
    766            };
    767            result.getModel().setRootNode(createSubsetRootNode(results));
    768            if (result.isEmpty()) {
    769                return new BaseHierarchicalConfiguration();
    770            }
    771            result.setSynchronizer(getSynchronizer());
    772            return result;
    773        }, false);
    774    }
    775}