    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *     http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017
    018package org.apache.commons.configuration2;
    019
    020import java.util.Collection;
    021import java.util.Collections;
    022import java.util.HashMap;
    023import java.util.Iterator;
    024import java.util.LinkedList;
    025import java.util.List;
    026import java.util.Map;
    027import java.util.stream.Collectors;
    028
    029import org.apache.commons.configuration2.event.ConfigurationEvent;
    030import org.apache.commons.configuration2.event.EventListener;
    031import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
    032import org.apache.commons.configuration2.interpol.ConfigurationInterpolator;
    033import org.apache.commons.configuration2.tree.ConfigurationNodeVisitorAdapter;
    034import org.apache.commons.configuration2.tree.ImmutableNode;
    035import org.apache.commons.configuration2.tree.InMemoryNodeModel;
    036import org.apache.commons.configuration2.tree.InMemoryNodeModelSupport;
    037import org.apache.commons.configuration2.tree.NodeHandler;
    038import org.apache.commons.configuration2.tree.NodeModel;
    039import org.apache.commons.configuration2.tree.NodeSelector;
    040import org.apache.commons.configuration2.tree.NodeTreeWalker;
    041import org.apache.commons.configuration2.tree.QueryResult;
    042import org.apache.commons.configuration2.tree.ReferenceNodeHandler;
    043import org.apache.commons.configuration2.tree.TrackedNodeModel;
    044import org.apache.commons.lang3.ObjectUtils;
    045
    046/**
    047 * <p>
    048 * A specialized hierarchical configuration implementation that is based on a structure of {@link ImmutableNode}
    049 * objects.
    050 * </p>
    051 */
    052public class BaseHierarchicalConfiguration extends AbstractHierarchicalConfiguration<ImmutableNode> implements InMemoryNodeModelSupport {
    053
    054    /** 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     * Gets 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     * Gets 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        return selectors.stream().map(sel -> createSubConfigurationForTrackedNode(sel, parentModelSupport)).collect(Collectors.toList());
    271    }
    272
    273    /**
    274     * Creates a sub configuration from the specified key which is independent on this configuration. This means that the
    275     * sub configuration operates on a separate node model (although the nodes are initially shared).
    276     *
    277     * @param key the key of the sub configuration
    278     * @return the new sub configuration
    279     */
    280    private BaseHierarchicalConfiguration createIndependentSubConfiguration(final String key) {
    281        final List<ImmutableNode> targetNodes = fetchFilteredNodeResults(key);
    282        final int size = targetNodes.size();
    283        if (size != 1) {
    284            throw new ConfigurationRuntimeException("Passed in key must select exactly one node (found %,d): %s", size, key);
    285        }
    286        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(targetNodes.get(0)));
    287        initSubConfiguration(sub);
    288        return sub;
    289    }
    290
    291    /**
    292     * Returns an initialized sub configuration for this configuration that is based on another
    293     * {@code BaseHierarchicalConfiguration}. Thus, it is independent from this configuration.
    294     *
    295     * @param node the root node for the sub configuration
    296     * @return the initialized sub configuration
    297     */
    298    private BaseHierarchicalConfiguration createIndependentSubConfigurationForNode(final ImmutableNode node) {
    299        final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(node));
    300        initSubConfiguration(sub);
    301        return sub;
    302    }
    303
    304    /**
    305     * Executes a query on the specified key and filters it for node results.
    306     *
    307     * @param key the key
    308     * @return the filtered list with result nodes
    309     */
    310    private List<ImmutableNode> fetchFilteredNodeResults(final String key) {
    311        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    312        return resolveNodeKey(handler.getRootNode(), key, handler);
    313    }
    314
    315    /**
    316     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    317     * Then an immutable wrapper is created and returned.
    318     */
    319    @Override
    320    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key, final boolean supportUpdates) {
    321        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key, supportUpdates));
    322    }
    323
    324    /**
    325     * {@inheritDoc} This is a short form for {@code configurationAt(key,
    326     * <b>false</b>)}.
    327     *
    328     * @throws ConfigurationRuntimeException if the key does not select a single node
    329     */
    330    @Override
    331    public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key) {
    332        return configurationAt(key, false);
    333    }
    334
    335    /**
    336     * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
    337     * Then an immutable wrapper is created and returned.
    338     *
    339     * @throws ConfigurationRuntimeException if the key does not select a single node
    340     */
    341    @Override
    342    public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key) {
    343        return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key));
    344    }
    345
    346    /**
    347     * {@inheritDoc} This implementation creates sub configurations in the same way as described for
    348     * {@link #configurationAt(String)}.
    349     */
    350    @Override
    351    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key) {
    352        List<ImmutableNode> nodes;
    353        beginRead(false);
    354        try {
    355            nodes = fetchFilteredNodeResults(key);
    356        } finally {
    357            endRead();
    358        }
    359        return nodes.stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
    360    }
    361
    362    /**
    363     * {@inheritDoc} This implementation creates tracked nodes for the specified key. Then sub configurations for these
    364     * nodes are created and returned.
    365     */
    366    @Override
    367    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key, final boolean supportUpdates) {
    368        if (!supportUpdates) {
    369            return configurationsAt(key);
    370        }
    371
    372        InMemoryNodeModel parentModel;
    373        beginRead(false);
    374        try {
    375            parentModel = getSubConfigurationParentModel();
    376        } finally {
    377            endRead();
    378        }
    379
    380        final Collection<NodeSelector> selectors = parentModel.selectAndTrackNodes(key, this);
    381        return createConnectedSubConfigurations(this, selectors);
    382    }
    383
    384    /**
    385     * {@inheritDoc} This implementation first delegates to {@code configurationsAt()} to create a list of
    386     * {@code SubnodeConfiguration} objects. Then for each element of this list an unmodifiable wrapper is created.
    387     */
    388    @Override
    389    public List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(final String key) {
    390        return toImmutable(configurationsAt(key));
    391    }
    392
    393    /**
    394     * {@inheritDoc} This implementation resolves the node(s) selected by the given key. If not a single node is selected,
    395     * an empty list is returned. Otherwise, sub configurations for each child of the node are created.
    396     */
    397    @Override
    398    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key) {
    399        List<ImmutableNode> nodes;
    400        beginRead(false);
    401        try {
    402            nodes = fetchFilteredNodeResults(key);
    403        } finally {
    404            endRead();
    405        }
    406
    407        if (nodes.size() != 1) {
    408            return Collections.emptyList();
    409        }
    410
    411        return nodes.get(0).stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
    412    }
    413
    414    /**
    415     * {@inheritDoc} This method works like {@link #childConfigurationsAt(String)}; however, depending on the value of the
    416     * {@code supportUpdates} flag, connected sub configurations may be created.
    417     */
    418    @Override
    419    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key, final boolean supportUpdates) {
    420        if (!supportUpdates) {
    421            return childConfigurationsAt(key);
    422        }
    423
    424        final InMemoryNodeModel parentModel = getSubConfigurationParentModel();
    425        return createConnectedSubConfigurations(this, parentModel.trackChildNodes(key, this));
    426    }
    427
    428    /**
    429     * {@inheritDoc} This implementation first delegates to {@code childConfigurationsAt()} to create a list of mutable
    430     * child configurations. Then a list with immutable wrapper configurations is created.
    431     */
    432    @Override
    433    public List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(final String key) {
    434        return toImmutable(childConfigurationsAt(key));
    435    }
    436
    437    /**
    438     * This method is always called when a subnode configuration created from this configuration has been modified. This
    439     * implementation transforms the received event into an event of type {@code SUBNODE_CHANGED} and notifies the
    440     * registered listeners.
    441     *
    442     * @param event the event describing the change
    443     * @since 1.5
    444     */
    445    protected void subnodeConfigurationChanged(final ConfigurationEvent event) {
    446        fireEvent(ConfigurationEvent.SUBNODE_CHANGED, null, event, event.isBeforeUpdate());
    447    }
    448
    449    /**
    450     * Initializes properties of a sub configuration. A sub configuration inherits some settings from its parent, e.g. the
    451     * expression engine or the synchronizer. The corresponding values are copied by this method.
    452     *
    453     * @param sub the sub configuration to be initialized
    454     */
    455    private void initSubConfiguration(final BaseHierarchicalConfiguration sub) {
    456        sub.setSynchronizer(getSynchronizer());
    457        sub.setExpressionEngine(getExpressionEngine());
    458        sub.setListDelimiterHandler(getListDelimiterHandler());
    459        sub.setThrowExceptionOnMissing(isThrowExceptionOnMissing());
    460        sub.getInterpolator().setParentInterpolator(getInterpolator());
    461    }
    462
    463    /**
    464     * Creates a listener which reacts on all changes on this configuration or one of its {@code SubnodeConfiguration}
    465     * instances. If such a change is detected, some updates have to be performed.
    466     *
    467     * @return the newly created change listener
    468     */
    469    private EventListener<ConfigurationEvent> createChangeListener() {
    470        return this::subnodeConfigurationChanged;
    471    }
    472
    473    /**
    474     * Returns a configuration with the same content as this configuration, but with all variables replaced by their actual
    475     * values. This implementation is specific for hierarchical configurations. It clones the current configuration and runs
    476     * a specialized visitor on the clone, which performs interpolation on the single configuration nodes.
    477     *
    478     * @return a configuration with all variables interpolated
    479     * @since 1.5
    480     */
    481    @Override
    482    public Configuration interpolatedConfiguration() {
    483        final InterpolatedVisitor visitor = new InterpolatedVisitor();
    484        final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
    485        NodeTreeWalker.INSTANCE.walkDFS(handler.getRootNode(), visitor, handler);
    486
    487        final BaseHierarchicalConfiguration c = (BaseHierarchicalConfiguration) clone();
    488        c.getNodeModel().setRootNode(visitor.getInterpolatedRoot());
    489        return c;
    490    }
    491
    492    /**
    493     * {@inheritDoc} This implementation creates a new instance of {@link InMemoryNodeModel}, initialized with this
    494     * configuration's root node. This has the effect that although the same nodes are used, the original and copied
    495     * configurations are independent on each other.
    496     */
    497    @Override
    498    protected NodeModel<ImmutableNode> cloneNodeModel() {
    499        return new InMemoryNodeModel(getModel().getNodeHandler().getRootNode());
    500    }
    501
    502    /**
    503     * Creates a list with immutable configurations from the given input list.
    504     *
    505     * @param subs a list with mutable configurations
    506     * @return a list with corresponding immutable configurations
    507     */
    508    private static List<ImmutableHierarchicalConfiguration> toImmutable(final List<? extends HierarchicalConfiguration<?>> subs) {
    509        return subs.stream().map(ConfigurationUtils::unmodifiableConfiguration).collect(Collectors.toList());
    510    }
    511
    512    /**
    513     * Creates the {@code NodeModel} for this configuration based on a passed in source configuration. This implementation
    514     * creates an {@link InMemoryNodeModel}. If the passed in source configuration is defined, its root node also becomes
    515     * the root node of this configuration. Otherwise, a new, empty root node is used.
    516     *
    517     * @param c the configuration that is to be copied
    518     * @return the {@code NodeModel} for the new configuration
    519     */
    520    private static NodeModel<ImmutableNode> createNodeModel(final HierarchicalConfiguration<ImmutableNode> c) {
    521        final ImmutableNode root = c != null ? obtainRootNode(c) : null;
    522        return new InMemoryNodeModel(root);
    523    }
    524
    525    /**
    526     * Obtains the root node from a configuration whose data is to be copied. It has to be ensured that the synchronizer is
    527     * called correctly.
    528     *
    529     * @param c the configuration that is to be copied
    530     * @return the root node of this configuration
    531     */
    532    private static ImmutableNode obtainRootNode(final HierarchicalConfiguration<ImmutableNode> c) {
    533        return c.getNodeModel().getNodeHandler().getRootNode();
    534    }
    535
    536    /**
    537     * A specialized visitor base class that can be used for storing the tree of configuration nodes. The basic idea is that
    538     * each node can be associated with a reference object. This reference object has a concrete meaning in a derived class,
    539     * e.g. an entry in a JNDI context or an XML element. When the configuration tree is set up, the {@code load()} method
    540     * is responsible for setting the reference objects. When the configuration tree is later modified, new nodes do not
    541     * have a defined reference object. This visitor class processes all nodes and finds the ones without a defined
    542     * reference object. For those nodes the {@code insert()} method is called, which must be defined in concrete sub
    543     * classes. This method can perform all steps to integrate the new node into the original structure.
    544     */
    545    protected abstract static class BuilderVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
    546        @Override
    547        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    548            final ReferenceNodeHandler refHandler = (ReferenceNodeHandler) handler;
    549            updateNode(node, refHandler);
    550            insertNewChildNodes(node, refHandler);
    551        }
    552
    553        /**
    554         * Inserts a new node into the structure constructed by this builder. This method is called for each node that has been
    555         * added to the configuration tree after the configuration has been loaded from its source. These new nodes have to be
    556         * inserted into the original structure. The passed in nodes define the position of the node to be inserted: its parent
    557         * and the siblings between to insert.
    558         *
    559         * @param newNode the node to be inserted
    560         * @param parent the parent node
    561         * @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
    562         *        the first child node
    563         * @param sibling2 the sibling before which the node is to be inserted; can be <b>null</b> if the new node is going to
    564         *        be the last child node
    565         * @param refHandler the {@code ReferenceNodeHandler}
    566         */
    567        protected abstract void insert(ImmutableNode newNode, ImmutableNode parent, ImmutableNode sibling1, ImmutableNode sibling2,
    568            ReferenceNodeHandler refHandler);
    569
    570        /**
    571         * Updates a node that already existed in the original hierarchy. This method is called for each node that has an
    572         * assigned reference object. A concrete implementation should update the reference according to the node's current
    573         * value.
    574         *
    575         * @param node the current node to be processed
    576         * @param reference the reference object for this node
    577         * @param refHandler the {@code ReferenceNodeHandler}
    578         */
    579        protected abstract void update(ImmutableNode node, Object reference, ReferenceNodeHandler refHandler);
    580
    581        /**
    582         * Updates the value of a node. If this node is associated with a reference object, the {@code update()} method is
    583         * called.
    584         *
    585         * @param node the current node to be processed
    586         * @param refHandler the {@code ReferenceNodeHandler}
    587         */
    588        private void updateNode(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
    589            final Object reference = refHandler.getReference(node);
    590            if (reference != null) {
    591                update(node, reference, refHandler);
    592            }
    593        }
    594
    595        /**
    596         * Inserts new children that have been added to the specified node.
    597         *
    598         * @param node the current node to be processed
    599         * @param refHandler the {@code ReferenceNodeHandler}
    600         */
    601        private void insertNewChildNodes(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
    602            final Collection<ImmutableNode> subNodes = new LinkedList<>(refHandler.getChildren(node));
    603            final Iterator<ImmutableNode> children = subNodes.iterator();
    604            ImmutableNode sibling1;
    605            ImmutableNode nd = null;
    606
    607            while (children.hasNext()) {
    608                // find the next new node
    609                do {
    610                    sibling1 = nd;
    611                    nd = children.next();
    612                } while (refHandler.getReference(nd) != null && children.hasNext());
    613
    614                if (refHandler.getReference(nd) == null) {
    615                    // find all following new nodes
    616                    final List<ImmutableNode> newNodes = new LinkedList<>();
    617                    newNodes.add(nd);
    618                    while (children.hasNext()) {
    619                        nd = children.next();
    620                        if (refHandler.getReference(nd) != null) {
    621                            break;
    622                        }
    623                        newNodes.add(nd);
    624                    }
    625
    626                    // Insert all new nodes
    627                    final ImmutableNode sibling2 = refHandler.getReference(nd) == null ? null : nd;
    628                    for (final ImmutableNode insertNode : newNodes) {
    629                        if (refHandler.getReference(insertNode) == null) {
    630                            insert(insertNode, node, sibling1, sibling2, refHandler);
    631                            sibling1 = insertNode;
    632                        }
    633                    }
    634                }
    635            }
    636        }
    637    }
    638
    639    /**
    640     * A specialized visitor implementation which constructs the root node of a configuration with all variables replaced by
    641     * their interpolated values.
    642     */
    643    private final class InterpolatedVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
    644        /** A stack for managing node builder instances. */
    645        private final List<ImmutableNode.Builder> builderStack;
    646
    647        /** The resulting root node. */
    648        private ImmutableNode interpolatedRoot;
    649
    650        /**
    651         * Creates a new instance of {@code InterpolatedVisitor}.
    652         */
    653        public InterpolatedVisitor() {
    654            builderStack = new LinkedList<>();
    655        }
    656
    657        /**
    658         * Gets the result of this builder: the root node of the interpolated nodes hierarchy.
    659         *
    660         * @return the resulting root node
    661         */
    662        public ImmutableNode getInterpolatedRoot() {
    663            return interpolatedRoot;
    664        }
    665
    666        @Override
    667        public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    668            if (isLeafNode(node, handler)) {
    669                handleLeafNode(node, handler);
    670            } else {
    671                final ImmutableNode.Builder builder = new ImmutableNode.Builder(handler.getChildrenCount(node, null)).name(handler.nodeName(node))
    672                    .value(interpolate(handler.getValue(node))).addAttributes(interpolateAttributes(node, handler));
    673                push(builder);
    674            }
    675        }
    676
    677        @Override
    678        public void visitAfterChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    679            if (!isLeafNode(node, handler)) {
    680                final ImmutableNode newNode = pop().create();
    681                storeInterpolatedNode(newNode);
    682            }
    683        }
    684
    685        /**
    686         * Pushes a new builder on the stack.
    687         *
    688         * @param builder the builder
    689         */
    690        private void push(final ImmutableNode.Builder builder) {
    691            builderStack.add(0, builder);
    692        }
    693
    694        /**
    695         * Pops the top-level element from the stack.
    696         *
    697         * @return the element popped from the stack
    698         */
    699        private ImmutableNode.Builder pop() {
    700            return builderStack.remove(0);
    701        }
    702
    703        /**
    704         * Returns the top-level element from the stack without removing it.
    705         *
    706         * @return the top-level element from the stack
    707         */
    708        private ImmutableNode.Builder peek() {
    709            return builderStack.get(0);
    710        }
    711
    712        /**
    713         * Returns a flag whether the given node is a leaf. This is the case if it does not have children.
    714         *
    715         * @param node the node in question
    716         * @param handler the {@code NodeHandler}
    717         * @return a flag whether this is a leaf node
    718         */
    719        private boolean isLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    720            return handler.getChildren(node).isEmpty();
    721        }
    722
    723        /**
    724         * Handles interpolation for a node with no children. If interpolation does not change this node, it is copied as is to
    725         * the resulting structure. Otherwise, a new node is created with the interpolated values.
    726         *
    727         * @param node the current node to be processed
    728         * @param handler the {@code NodeHandler}
    729         */
    730        private void handleLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    731            final Object value = interpolate(node.getValue());
    732            final Map<String, Object> interpolatedAttributes = new HashMap<>();
    733            final boolean attributeChanged = interpolateAttributes(node, handler, interpolatedAttributes);
    734            final ImmutableNode newNode = valueChanged(value, handler.getValue(node)) || attributeChanged
    735                ? new ImmutableNode.Builder().name(handler.nodeName(node)).value(value).addAttributes(interpolatedAttributes).create()
    736                : node;
    737            storeInterpolatedNode(newNode);
    738        }
    739
    740        /**
    741         * Stores a processed node. Per default, the node is added to the current builder on the stack. If no such builder
    742         * exists, this is the result node.
    743         *
    744         * @param node the node to be stored
    745         */
    746        private void storeInterpolatedNode(final ImmutableNode node) {
    747            if (builderStack.isEmpty()) {
    748                interpolatedRoot = node;
    749            } else {
    750                peek().addChild(node);
    751            }
    752        }
    753
    754        /**
    755         * Populates a map with interpolated attributes of the passed in node.
    756         *
    757         * @param node the current node to be processed
    758         * @param handler the {@code NodeHandler}
    759         * @param interpolatedAttributes a map for storing the results
    760         * @return a flag whether an attribute value was changed by interpolation
    761         */
    762        private boolean interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler,
    763            final Map<String, Object> interpolatedAttributes) {
    764            boolean attributeChanged = false;
    765            for (final String attr : handler.getAttributes(node)) {
    766                final Object attrValue = interpolate(handler.getAttributeValue(node, attr));
    767                if (valueChanged(attrValue, handler.getAttributeValue(node, attr))) {
    768                    attributeChanged = true;
    769                }
    770                interpolatedAttributes.put(attr, attrValue);
    771            }
    772            return attributeChanged;
    773        }
    774
    775        /**
    776         * Returns 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         * @return the map with interpolated attributes
    781         */
    782        private Map<String, Object> interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
    783            final Map<String, Object> attributes = new HashMap<>();
    784            interpolateAttributes(node, handler, attributes);
    785            return attributes;
    786        }
    787
    788        /**
    789         * Tests whether a value is changed because of interpolation.
    790         *
    791         * @param interpolatedValue the interpolated value
    792         * @param value the original value
    793         * @return a flag whether the value was changed
    794         */
    795        private boolean valueChanged(final Object interpolatedValue, final Object value) {
    796            return ObjectUtils.notEqual(interpolatedValue, value);
    797        }
    798    }
    799}