    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.ArrayList;
    021import java.util.Collection;
    022import java.util.Collections;
    023import java.util.HashMap;
    024import java.util.Iterator;
    025import java.util.LinkedList;
    026import java.util.List;
    027import java.util.Map;
    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 *
    052 */
    053public class BaseHierarchicalConfiguration extends AbstractHierarchicalConfiguration<ImmutableNode> implements InMemoryNodeModelSupport {
    054    /** A listener for reacting on changes caused by sub configurations. */
    055    private final EventListener<ConfigurationEvent> changeListener;
    056
    057    /**
    058     * Creates a new instance of {@code BaseHierarchicalConfiguration}.
    059     */
    060    public BaseHierarchicalConfiguration() {
    061        this((HierarchicalConfiguration<ImmutableNode>) null);
    062    }
    063
    064    /**
    065     * Creates a new instance of {@code BaseHierarchicalConfiguration} and copies all data contained in the specified
    066     * configuration into the new one.
    067     *
    068     * @param c the configuration that is to be copied (if <b>null</b>, this constructor will behave like the standard
    069     *        constructor)
    070     * @since 1.4
    071     */
    072    public BaseHierarchicalConfiguration(final HierarchicalConfiguration<ImmutableNode> c) {
    073        this(createNodeModel(c));
    074    }
    075
    076    /**
    077     * Creates a new instance of {@code BaseHierarchicalConfiguration} and initializes it with the given {@code NodeModel}.
    078     *
    079     * @param model the {@code NodeModel}
    080     */
    081    protected BaseHierarchicalConfiguration(final NodeModel<ImmutableNode> model) {
    082        super(model);
    083        changeListener = createChangeListener();
    084    }
    085
    086    /**
    087     * {@inheritDoc} This implementation returns the {@code InMemoryNodeModel} used by this configuration.
    088     */
    089    @Override
    090    public InMemoryNodeModel getNodeModel() {
    091        return (InMemoryNodeModel) super.getNodeModel();
    092    }
    093
    094    /**
    095     * Creates a new {@code Configuration} object containing all keys that start with the specified prefix. This
    096     * implementation will return a {@code BaseHierarchicalConfiguration} object so that the structure of the keys will be
    097     * saved. The nodes selected by the prefix (it is possible that multiple nodes are selected) are mapped to the root node
    098     * of the returned configuration, i.e. their children and attributes will become children and attributes of the new root
    099     * node. However, a value of the root node is only set if exactly one of the selected nodes contain a value (if multiple
    100     * nodes have a value, there is simply no way to decide how these values are merged together). Note that the returned
    101     * {@code Configuration} object is not connected to its source configuration: updates on the source configuration are
    102     * not reflected in the subset and vice versa. The returned configuration uses the same {@code Synchronizer} as this
    103     * configuration.
    104     *
    105     * @param prefix the prefix of the keys for the subset
    106     * @return a new configuration object representing the selected subset
    107     */
    108    @Override
    109    public Configuration subset(final String prefix) {
    110        beginRead(false);
    111        try {
    112            final List<QueryResult<ImmutableNode>> results = fetchNodeList(prefix);
    113            if (results.isEmpty()) {
    114                return new BaseHierarchicalConfiguration();
    115            }
    116
    117            final BaseHierarchicalConfiguration parent = this;
    118            final BaseHierarchicalConfiguration result = new BaseHierarchicalConfiguration() {
    119                // Override interpolate to always interpolate on the parent
    120                @Override
    121                protected Object interpolate(final Object value) {
    122                    return parent.interpolate(value);
    123                }
    124
    125                @Override
    126                public ConfigurationInterpolator getInterpolator() {
    127                    return parent.getInterpolator();
    128                }
    129            };
    130            result.getModel().setRootNode(createSubsetRootNode(results));
    131
    132            if (result.isEmpty()) {
    133                return new BaseHierarchicalConfiguration();
    134            }
    135            result.setSynchronizer(getSynchronizer());
    136            return result;
    137        } finally {
    138            endRead();
    139        }
    140    }
    141
    142    /**
    143     * Creates a root node for a subset configuration based on the passed in query results. This method creates a new root
    144     * node and adds the children and attributes of all result nodes to it. If only a single node value is defined, it is
    145     * assigned as value of the new root node.
    146     *
    147     * @param results the collection of query results
    148     * @return the root node for the subset configuration
    149     */
    150    private ImmutableNode createSubsetRootNode(final Collection<QueryResult<ImmutableNode>> results) {
    151        final ImmutableNode.Builder builder = new ImmutableNode.Builder();
    152        Object value = null;
    153        int valueCount = 0;
    154
    155        for (final QueryResult<ImmutableNode> result : results) {
    156            if (result.isAttributeResult()) {
    157                builder.addAttribute(result.getAttributeName(), result.getAttributeValue(getModel().getNodeHandler()));
    158            } else {
    159                if (result.getNode().getValue() != null) {
    160                    value = result.getNode().getValue();
    161                    valueCount++;
    162                }
    163                builder.addChildren(result.getNode().getChildren());
    164                builder.addAttributes(result.getNode().getAttributes());
    165            }
    166        }
    167
    168        if (valueCount == 1) {
    169            builder.value(value);
    170        }
    171        return builder.create();
    172    }
    173
    174    /**
    175     * {@inheritDoc} The result of this implementation depends on the {@code supportUpdates} flag: If it is <b>false</b>, a
    176     * plain {@code BaseHierarchicalConfiguration} is returned using the selected node as root node. This is suitable for
    177     * read-only access to properties. Because the configuration returned in this case is not connected to the parent
    178     * configuration, updates on properties made by one configuration are not reflected by the other one. A value of
    179     * <b>true</b> for this parameter causes a tracked node to be created, and result is a {@link SubnodeConfiguration}
    180     * based on this tracked node. This configuration is really connected to its parent, so that updated properties are
    181     * visible on both.
    182     *
    183     * @see SubnodeConfiguration
    184     * @throws ConfigurationRuntimeException if the key does not select a single node
    185     */
    186    @Override
    187    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key, final boolean supportUpdates) {
    188        beginRead(false);
    189        try {
    190            return supportUpdates ? createConnectedSubConfiguration(key) : createIndependentSubConfiguration(key);
    191        } finally {
    192            endRead();
    193        }
    194    }
    195
    196    /**
    197     * Returns the {@code InMemoryNodeModel} to be used as parent model for a new sub configuration. This method is called
    198     * whenever a sub configuration is to be created. This base implementation returns the model of this configuration. Sub
    199     * classes with different requirements for the parent models of sub configurations have to override it.
    200     *
    201     * @return the parent model for a new sub configuration
    202     */
    203    protected InMemoryNodeModel getSubConfigurationParentModel() {
    204        return (InMemoryNodeModel) getModel();
    205    }
    206
    207    /**
    208     * Returns the {@code NodeSelector} to be used for a sub configuration based on the passed in key. This method is called
    209     * whenever a sub configuration is to be created. This base implementation returns a new {@code NodeSelector}
    210     * initialized with the passed in key. Sub classes may override this method if they have a different strategy for
    211     * creating a selector.
    212     *
    213     * @param key the key of the sub configuration
    214     * @return a {@code NodeSelector} for initializing a sub configuration
    215     * @since 2.0
    216     */
    217    protected NodeSelector getSubConfigurationNodeSelector(final String key) {
    218        return new NodeSelector(key);
    219    }
    220
    221    /**
    222     * Creates a connected sub configuration based on a selector for a tracked node.
    223     *
    224     * @param selector the {@code NodeSelector}
    225     * @param parentModelSupport the {@code InMemoryNodeModelSupport} object for the parent node model
    226     * @return the newly created sub configuration
    227     * @since 2.0
    228     */
    229    protected SubnodeConfiguration createSubConfigurationForTrackedNode(final NodeSelector selector, final InMemoryNodeModelSupport parentModelSupport) {
    230        final SubnodeConfiguration subConfig = new SubnodeConfiguration(this, new TrackedNodeModel(parentModelSupport, selector, true));
    231        initSubConfigurationForThisParent(subConfig);
    232        return subConfig;
    233    }
    234
    235    /**
    236     * Initializes a {@code SubnodeConfiguration} object. This method should be called for each sub configuration created
    237     * for this configuration. It ensures that the sub configuration is correctly connected to its parent instance and that
    238     * update events are correctly propagated.
    239     *
    240     * @param subConfig the sub configuration to be initialized
    241     * @since 2.0
    242     */
    243    protected void initSubConfigurationForThisParent(final SubnodeConfiguration subConfig) {
    244        initSubConfiguration(subConfig);
    245        subConfig.addEventListener(ConfigurationEvent.ANY, changeListener);
    246    }
    247
    248    /**
    249     * Creates a sub configuration from the specified key which is connected to this configuration. This implementation
    250     * creates a {@link SubnodeConfiguration} with a tracked node identified by the passed in key.
    251     *
    252     * @param key the key of the sub configuration
    253     * @return the new sub configuration
    254     */
    255    private BaseHierarchicalConfiguration createConnectedSubConfiguration(final String key) {
    256        final NodeSelector selector = getSubConfigurationNodeSelector(key);
    257        getSubConfigurationParentModel().trackNode(selector, this);
    258        return createSubConfigurationForTrackedNode(selector, this);
    259    }
    260
    261    /**
    262     * Creates a list of connected sub configurations based on a passed in list of node selectors.
    263     *
    264     * @param parentModelSupport the parent node model support object
    265     * @param selectors the list of {@code NodeSelector} objects
    266     * @return the list with sub configurations
    267     */
    268    private List<HierarchicalConfiguration<ImmutableNode>> createConnectedSubConfigurations(final InMemoryNodeModelSupport parentModelSupport,
    269        final Collection<NodeSelector> selectors) {
    270        final List<HierarchicalConfiguration<ImmutableNode>> configs = new ArrayList<>(selectors.size());
    271        for (final NodeSelector selector : selectors) {
    272            configs.add(createSubConfigurationForTrackedNode(selector, parentModelSupport));
    273        }
    274        return configs;
    275    }
    276
    277    /**
    278     * Creates a sub configuration from the specified key which is independent on this configuration. This means that the
    279     * sub configuration operates on a separate node model (although the nodes are initially shared).
    280     *
    281     * @param key the key of the sub configuration
    282     * @return the new sub configuration
    283     */
    284    private BaseHierarchicalConfiguration createIndependentSubConfiguration(final String key) {
    285        final List<ImmutableNode> targetNodes = fetchFilteredNodeResults(key);
    286        final int size = targetNodes.size();
    287        if (size != 1) {
    288            throw new ConfigurationRuntimeException("Passed in key must select exactly one node (found %,d): %s", size, key);
    289        }
    290        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(targetNodes.get(0)));
    291        initSubConfiguration(sub);
    292        return sub;
    293    }
    294
    295    /**
    296     * Returns an initialized sub configuration for this configuration that is based on another
    297     * {@code BaseHierarchicalConfiguration}. Thus, it is independent from this configuration.
    298     *
    299     * @param node the root node for the sub configuration
    300     * @return the initialized sub configuration
    301     */
    302    private BaseHierarchicalConfiguration createIndependentSubConfigurationForNode(final ImmutableNode node) {
    303        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(node));
    304        initSubConfiguration(sub);
    305        return sub;
    306    }
    307
    308    /**
    309     * Executes a query on the specified key and filters it for node results.
    310     *
    311     * @param key the key
    312     * @return the filtered list with result nodes
    313     */
    314    private List<ImmutableNode> fetchFilteredNodeResults(final String key) {
    315        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    316        return resolveNodeKey(handler.getRootNode(), key, handler);
    317    }
    318
    319    /**
    320     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    321     * Then an immutable wrapper is created and returned.
    322     */
    323    @Override
    324    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key, final boolean supportUpdates) {
    325        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key, supportUpdates));
    326    }
    327
    328    /**
    329     * {@inheritDoc} This is a short form for {@code configurationAt(key,
    330     * <b>false</b>)}.
    331     *
    332     * @throws ConfigurationRuntimeException if the key does not select a single node
    333     */
    334    @Override
    335    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key) {
    336        return configurationAt(key, false);
    337    }
    338
    339    /**
    340     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    341     * Then an immutable wrapper is created and returned.
    342     *
    343     * @throws ConfigurationRuntimeException if the key does not select a single node
    344     */
    345    @Override
    346    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key) {
    347        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key));
    348    }
    349
    350    /**
    351     * {@inheritDoc} This implementation creates sub configurations in the same way as described for
    352     * {@link #configurationAt(String)}.
    353     */
    354    @Override
    355    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key) {
    356        List<ImmutableNode> nodes;
    357        beginRead(false);
    358        try {
    359            nodes = fetchFilteredNodeResults(key);
    360        } finally {
    361            endRead();
    362        }
    363
    364        final List<HierarchicalConfiguration<ImmutableNode>> results = new ArrayList<>(nodes.size());
    365        for (final ImmutableNode node : nodes) {
    366            final BaseHierarchicalConfiguration sub = createIndependentSubConfigurationForNode(node);
    367            results.add(sub);
    368        }
    369
    370        return results;
    371    }
    372
    373    /**
    374     * {@inheritDoc} This implementation creates tracked nodes for the specified key. Then sub configurations for these
    375     * nodes are created and returned.
    376     */
    377    @Override
    378    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key, final boolean supportUpdates) {
    379        if (!supportUpdates) {
    380            return configurationsAt(key);
    381        }
    382
    383        InMemoryNodeModel parentModel;
    384        beginRead(false);
    385        try {
    386            parentModel = getSubConfigurationParentModel();
    387        } finally {
    388            endRead();
    389        }
    390
    391        final Collection<NodeSelector> selectors = parentModel.selectAndTrackNodes(key, this);
    392        return createConnectedSubConfigurations(this, selectors);
    393    }
    394
    395    /**
    396     * {@inheritDoc} This implementation first delegates to {@code configurationsAt()} to create a list of
    397     * {@code SubnodeConfiguration} objects. Then for each element of this list an unmodifiable wrapper is created.
    398     */
    399    @Override
    400    public List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(final String key) {
    401        return toImmutable(configurationsAt(key));
    402    }
    403
    404    /**
    405     * {@inheritDoc} This implementation resolves the node(s) selected by the given key. If not a single node is selected,
    406     * an empty list is returned. Otherwise, sub configurations for each child of the node are created.
    407     */
    408    @Override
    409    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key) {
    410        List<ImmutableNode> nodes;
    411        beginRead(false);
    412        try {
    413            nodes = fetchFilteredNodeResults(key);
    414        } finally {
    415            endRead();
    416        }
    417
    418        if (nodes.size() != 1) {
    419            return Collections.emptyList();
    420        }
    421
    422        final ImmutableNode parent = nodes.get(0);
    423        final List<HierarchicalConfiguration<ImmutableNode>> subs = new ArrayList<>(parent.getChildren().size());
    424        for (final ImmutableNode node : parent) {
    425            subs.add(createIndependentSubConfigurationForNode(node));
    426        }
    427
    428        return subs;
    429    }
    430
    431    /**
    432     * {@inheritDoc} This method works like {@link #childConfigurationsAt(String)}; however, depending on the value of the
    433     * {@code supportUpdates} flag, connected sub configurations may be created.
    434     */
    435    @Override
    436    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key, final boolean supportUpdates) {
    437        if (!supportUpdates) {
    438            return childConfigurationsAt(key);
    439        }
    440
    441        final InMemoryNodeModel parentModel = getSubConfigurationParentModel();
    442        return createConnectedSubConfigurations(this, parentModel.trackChildNodes(key, this));
    443    }
    444
    445    /**
    446     * {@inheritDoc} This implementation first delegates to {@code childConfigurationsAt()} to create a list of mutable
    447     * child configurations. Then a list with immutable wrapper configurations is created.
    448     */
    449    @Override
    450    public List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(final String key) {
    451        return toImmutable(childConfigurationsAt(key));
    452    }
    453
    454    /**
    455     * This method is always called when a subnode configuration created from this configuration has been modified. This
    456     * implementation transforms the received event into an event of type {@code SUBNODE_CHANGED} and notifies the
    457     * registered listeners.
    458     *
    459     * @param event the event describing the change
    460     * @since 1.5
    461     */
    462    protected void subnodeConfigurationChanged(final ConfigurationEvent event) {
    463        fireEvent(ConfigurationEvent.SUBNODE_CHANGED, null, event, event.isBeforeUpdate());
    464    }
    465
    466    /**
    467     * Initializes properties of a sub configuration. A sub configuration inherits some settings from its parent, e.g. the
    468     * expression engine or the synchronizer. The corresponding values are copied by this method.
    469     *
    470     * @param sub the sub configuration to be initialized
    471     */
    472    private void initSubConfiguration(final BaseHierarchicalConfiguration sub) {
    473        sub.setSynchronizer(getSynchronizer());
    474        sub.setExpressionEngine(getExpressionEngine());
    475        sub.setListDelimiterHandler(getListDelimiterHandler());
    476        sub.setThrowExceptionOnMissing(isThrowExceptionOnMissing());
    477        sub.getInterpolator().setParentInterpolator(getInterpolator());
    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     * Returns a configuration with the same content as this configuration, but with all variables replaced by their actual
    492     * values. This implementation is specific for hierarchical configurations. It clones the current configuration and runs
    493     * a specialized visitor on the clone, which performs interpolation on the single configuration nodes.
    494     *
    495     * @return a configuration with all variables interpolated
    496     * @since 1.5
    497     */
    498    @Override
    499    public Configuration interpolatedConfiguration() {
    500        final InterpolatedVisitor visitor = new InterpolatedVisitor();
    501        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    502        NodeTreeWalker.INSTANCE.walkDFS(handler.getRootNode(), visitor, handler);
    503
    504        final BaseHierarchicalConfiguration c = (BaseHierarchicalConfiguration) clone();
    505        c.getNodeModel().setRootNode(visitor.getInterpolatedRoot());
    506        return c;
    507    }
    508
    509    /**
    510     * {@inheritDoc} This implementation creates a new instance of {@link InMemoryNodeModel}, initialized with this
    511     * configuration's root node. This has the effect that although the same nodes are used, the original and copied
    512     * configurations are independent on each other.
    513     */
    514    @Override
    515    protected NodeModel<ImmutableNode> cloneNodeModel() {
    516        return new InMemoryNodeModel(getModel().getNodeHandler().getRootNode());
    517    }
    518
    519    /**
    520     * Creates a list with immutable configurations from the given input list.
    521     *
    522     * @param subs a list with mutable configurations
    523     * @return a list with corresponding immutable configurations
    524     */
    525    private static List<ImmutableHierarchicalConfiguration> toImmutable(final List<? extends HierarchicalConfiguration<?>> subs) {
    526        final List<ImmutableHierarchicalConfiguration> res = new ArrayList<>(subs.size());
    527        for (final HierarchicalConfiguration<?> sub : subs) {
    528            res.add(ConfigurationUtils.unmodifiableConfiguration(sub));
    529        }
    530        return res;
    531    }
    532
    533    /**
    534     * Creates the {@code NodeModel} for this configuration based on a passed in source configuration. This implementation
    535     * creates an {@link InMemoryNodeModel}. If the passed in source configuration is defined, its root node also becomes
    536     * the root node of this configuration. Otherwise, a new, empty root node is used.
    537     *
    538     * @param c the configuration that is to be copied
    539     * @return the {@code NodeModel} for the new configuration
    540     */
    541    private static NodeModel<ImmutableNode> createNodeModel(final HierarchicalConfiguration<ImmutableNode> c) {
    542        final ImmutableNode root = c != null ? obtainRootNode(c) : null;
    543        return new InMemoryNodeModel(root);
    544    }
    545
    546    /**
    547     * Obtains the root node from a configuration whose data is to be copied. It has to be ensured that the synchronizer is
    548     * called correctly.
    549     *
    550     * @param c the configuration that is to be copied
    551     * @return the root node of this configuration
    552     */
    553    private static ImmutableNode obtainRootNode(final HierarchicalConfiguration<ImmutableNode> c) {
    554        return c.getNodeModel().getNodeHandler().getRootNode();
    555    }
    556
    557    /**
    558     * A specialized visitor base class that can be used for storing the tree of configuration nodes. The basic idea is that
    559     * each node can be associated with a reference object. This reference object has a concrete meaning in a derived class,
    560     * e.g. an entry in a JNDI context or an XML element. When the configuration tree is set up, the {@code load()} method
    561     * is responsible for setting the reference objects. When the configuration tree is later modified, new nodes do not
    562     * have a defined reference object. This visitor class processes all nodes and finds the ones without a defined
    563     * reference object. For those nodes the {@code insert()} method is called, which must be defined in concrete sub
    564     * classes. This method can perform all steps to integrate the new node into the original structure.
    565     */
    566    protected abstract static class BuilderVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
    567        @Override
    568        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    569            final ReferenceNodeHandler refHandler = (ReferenceNodeHandler) handler;
    570            updateNode(node, refHandler);
    571            insertNewChildNodes(node, refHandler);
    572        }
    573
    574        /**
    575         * Inserts a new node into the structure constructed by this builder. This method is called for each node that has been
    576         * added to the configuration tree after the configuration has been loaded from its source. These new nodes have to be
    577         * inserted into the original structure. The passed in nodes define the position of the node to be inserted: its parent
    578         * and the siblings between to insert.
    579         *
    580         * @param newNode the node to be inserted
    581         * @param parent the parent node
    582         * @param sibling1 the sibling after which the node is to be inserted; can be <b>null</b> if the new node is going to be
    583         *        the first child node
    584         * @param sibling2 the sibling before which the node is to be inserted; can be <b>null</b> if the new node is going to
    585         *        be the last child node
    586         * @param refHandler the {@code ReferenceNodeHandler}
    587         */
    588        protected abstract void insert(ImmutableNode newNode, ImmutableNode parent, ImmutableNode sibling1, ImmutableNode sibling2,
    589            ReferenceNodeHandler refHandler);
    590
    591        /**
    592         * Updates a node that already existed in the original hierarchy. This method is called for each node that has an
    593         * assigned reference object. A concrete implementation should update the reference according to the node's current
    594         * value.
    595         *
    596         * @param node the current node to be processed
    597         * @param reference the reference object for this node
    598         * @param refHandler the {@code ReferenceNodeHandler}
    599         */
    600        protected abstract void update(ImmutableNode node, Object reference, ReferenceNodeHandler refHandler);
    601
    602        /**
    603         * Updates the value of a node. If this node is associated with a reference object, the {@code update()} method is
    604         * called.
    605         *
    606         * @param node the current node to be processed
    607         * @param refHandler the {@code ReferenceNodeHandler}
    608         */
    609        private void updateNode(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
    610            final Object reference = refHandler.getReference(node);
    611            if (reference != null) {
    612                update(node, reference, refHandler);
    613            }
    614        }
    615
    616        /**
    617         * Inserts new children that have been added to the specified node.
    618         *
    619         * @param node the current node to be processed
    620         * @param refHandler the {@code ReferenceNodeHandler}
    621         */
    622        private void insertNewChildNodes(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
    623            final Collection<ImmutableNode> subNodes = new LinkedList<>(refHandler.getChildren(node));
    624            final Iterator<ImmutableNode> children = subNodes.iterator();
    625            ImmutableNode sibling1;
    626            ImmutableNode nd = null;
    627
    628            while (children.hasNext()) {
    629                // find the next new node
    630                do {
    631                    sibling1 = nd;
    632                    nd = children.next();
    633                } while (refHandler.getReference(nd) != null && children.hasNext());
    634
    635                if (refHandler.getReference(nd) == null) {
    636                    // find all following new nodes
    637                    final List<ImmutableNode> newNodes = new LinkedList<>();
    638                    newNodes.add(nd);
    639                    while (children.hasNext()) {
    640                        nd = children.next();
    641                        if (refHandler.getReference(nd) != null) {
    642                            break;
    643                        }
    644                        newNodes.add(nd);
    645                    }
    646
    647                    // Insert all new nodes
    648                    final ImmutableNode sibling2 = refHandler.getReference(nd) == null ? null : nd;
    649                    for (final ImmutableNode insertNode : newNodes) {
    650                        if (refHandler.getReference(insertNode) == null) {
    651                            insert(insertNode, node, sibling1, sibling2, refHandler);
    652                            sibling1 = insertNode;
    653                        }
    654                    }
    655                }
    656            }
    657        }
    658    }
    659
    660    /**
    661     * A specialized visitor implementation which constructs the root node of a configuration with all variables replaced by
    662     * their interpolated values.
    663     */
    664    private class InterpolatedVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
    665        /** A stack for managing node builder instances. */
    666        private final List<ImmutableNode.Builder> builderStack;
    667
    668        /** The resulting root node. */
    669        private ImmutableNode interpolatedRoot;
    670
    671        /**
    672         * Creates a new instance of {@code InterpolatedVisitor}.
    673         */
    674        public InterpolatedVisitor() {
    675            builderStack = new LinkedList<>();
    676        }
    677
    678        /**
    679         * Returns the result of this builder: the root node of the interpolated nodes hierarchy.
    680         *
    681         * @return the resulting root node
    682         */
    683        public ImmutableNode getInterpolatedRoot() {
    684            return interpolatedRoot;
    685        }
    686
    687        @Override
    688        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    689            if (isLeafNode(node, handler)) {
    690                handleLeafNode(node, handler);
    691            } else {
    692                final ImmutableNode.Builder builder = new ImmutableNode.Builder(handler.getChildrenCount(node, null)).name(handler.nodeName(node))
    693                    .value(interpolate(handler.getValue(node))).addAttributes(interpolateAttributes(node, handler));
    694                push(builder);
    695            }
    696        }
    697
    698        @Override
    699        public void visitAfterChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    700            if (!isLeafNode(node, handler)) {
    701                final ImmutableNode newNode = pop().create();
    702                storeInterpolatedNode(newNode);
    703            }
    704        }
    705
    706        /**
    707         * Pushes a new builder on the stack.
    708         *
    709         * @param builder the builder
    710         */
    711        private void push(final ImmutableNode.Builder builder) {
    712            builderStack.add(0, builder);
    713        }
    714
    715        /**
    716         * Pops the top-level element from the stack.
    717         *
    718         * @return the element popped from the stack
    719         */
    720        private ImmutableNode.Builder pop() {
    721            return builderStack.remove(0);
    722        }
    723
    724        /**
    725         * Returns the top-level element from the stack without removing it.
    726         *
    727         * @return the top-level element from the stack
    728         */
    729        private ImmutableNode.Builder peek() {
    730            return builderStack.get(0);
    731        }
    732
    733        /**
    734         * Returns a flag whether the given node is a leaf. This is the case if it does not have children.
    735         *
    736         * @param node the node in question
    737         * @param handler the {@code NodeHandler}
    738         * @return a flag whether this is a leaf node
    739         */
    740        private boolean isLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    741            return handler.getChildren(node).isEmpty();
    742        }
    743
    744        /**
    745         * Handles interpolation for a node with no children. If interpolation does not change this node, it is copied as is to
    746         * the resulting structure. Otherwise, a new node is created with the interpolated values.
    747         *
    748         * @param node the current node to be processed
    749         * @param handler the {@code NodeHandler}
    750         */
    751        private void handleLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    752            final Object value = interpolate(node.getValue());
    753            final Map<String, Object> interpolatedAttributes = new HashMap<>();
    754            final boolean attributeChanged = interpolateAttributes(node, handler, interpolatedAttributes);
    755            final ImmutableNode newNode = valueChanged(value, handler.getValue(node)) || attributeChanged
    756                ? new ImmutableNode.Builder().name(handler.nodeName(node)).value(value).addAttributes(interpolatedAttributes).create()
    757                : node;
    758            storeInterpolatedNode(newNode);
    759        }
    760
    761        /**
    762         * Stores a processed node. Per default, the node is added to the current builder on the stack. If no such builder
    763         * exists, this is the result node.
    764         *
    765         * @param node the node to be stored
    766         */
    767        private void storeInterpolatedNode(final ImmutableNode node) {
    768            if (builderStack.isEmpty()) {
    769                interpolatedRoot = node;
    770            } else {
    771                peek().addChild(node);
    772            }
    773        }
    774
    775        /**
    776         * Populates a map with interpolated attributes of the passed in node.
    777         *
    778         * @param node the current node to be processed
    779         * @param handler the {@code NodeHandler}
    780         * @param interpolatedAttributes a map for storing the results
    781         * @return a flag whether an attribute value was changed by interpolation
    782         */
    783        private boolean interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler,
    784            final Map<String, Object> interpolatedAttributes) {
    785            boolean attributeChanged = false;
    786            for (final String attr : handler.getAttributes(node)) {
    787                final Object attrValue = interpolate(handler.getAttributeValue(node, attr));
    788                if (valueChanged(attrValue, handler.getAttributeValue(node, attr))) {
    789                    attributeChanged = true;
    790                }
    791                interpolatedAttributes.put(attr, attrValue);
    792            }
    793            return attributeChanged;
    794        }
    795
    796        /**
    797         * Returns a map with interpolated attributes of the passed in node.
    798         *
    799         * @param node the current node to be processed
    800         * @param handler the {@code NodeHandler}
    801         * @return the map with interpolated attributes
    802         */
    803        private Map<String, Object> interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    804            final Map<String, Object> attributes = new HashMap<>();
    805            interpolateAttributes(node, handler, attributes);
    806            return attributes;
    807        }
    808
    809        /**
    810         * Tests whether a value is changed because of interpolation.
    811         *
    812         * @param interpolatedValue the interpolated value
    813         * @param value the original value
    814         * @return a flag whether the value was changed
    815         */
    816        private boolean valueChanged(final Object interpolatedValue, final Object value) {
    817            return ObjectUtils.notEqual(interpolatedValue, value);
    818        }
    819    }
    820}