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.LinkedHashSet;
025import java.util.LinkedList;
026import java.util.List;
027import java.util.Map;
028import java.util.Objects;
029import java.util.Set;
030import java.util.Stack;
031import java.util.stream.Collectors;
032
033import org.apache.commons.configuration2.event.ConfigurationEvent;
034import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
035import org.apache.commons.configuration2.sync.NoOpSynchronizer;
036import org.apache.commons.configuration2.tree.ConfigurationNodeVisitorAdapter;
037import org.apache.commons.configuration2.tree.DefaultExpressionEngine;
038import org.apache.commons.configuration2.tree.ExpressionEngine;
039import org.apache.commons.configuration2.tree.NodeAddData;
040import org.apache.commons.configuration2.tree.NodeHandler;
041import org.apache.commons.configuration2.tree.NodeKeyResolver;
042import org.apache.commons.configuration2.tree.NodeModel;
043import org.apache.commons.configuration2.tree.NodeTreeWalker;
044import org.apache.commons.configuration2.tree.NodeUpdateData;
045import org.apache.commons.configuration2.tree.QueryResult;
046
047/**
048 * <p>
049 * A specialized configuration class that extends its base class by the ability of keeping more structure in the stored
050 * properties.
051 * </p>
052 * <p>
053 * There are some sources of configuration data that cannot be stored very well in a {@code BaseConfiguration} object
054 * because then their structure is lost. This is for instance true for XML documents. This class can deal with such
055 * structured configuration sources by storing the properties in a tree-like organization. The exact storage structure
056 * of the underlying data does not matter for the configuration instance; it uses a {@link NodeModel} object for
057 * accessing it.
058 * </p>
059 * <p>
060 * The hierarchical organization allows for a more sophisticated access to single properties. As an example consider the
061 * following XML document:
062 * </p>
063 *
064 * <pre>
065 * &lt;database&gt;
066 *   &lt;tables&gt;
067 *     &lt;table&gt;
068 *       &lt;name&gt;users&lt;/name&gt;
069 *       &lt;fields&gt;
070 *         &lt;field&gt;
071 *           &lt;name&gt;lid&lt;/name&gt;
072 *           &lt;type&gt;long&lt;/name&gt;
073 *         &lt;/field&gt;
074 *         &lt;field&gt;
075 *           &lt;name&gt;usrName&lt;/name&gt;
076 *           &lt;type&gt;java.lang.String&lt;/type&gt;
077 *         &lt;/field&gt;
078 *        ...
079 *       &lt;/fields&gt;
080 *     &lt;/table&gt;
081 *     &lt;table&gt;
082 *       &lt;name&gt;documents&lt;/name&gt;
083 *       &lt;fields&gt;
084 *         &lt;field&gt;
085 *           &lt;name&gt;docid&lt;/name&gt;
086 *           &lt;type&gt;long&lt;/type&gt;
087 *         &lt;/field&gt;
088 *         ...
089 *       &lt;/fields&gt;
090 *     &lt;/table&gt;
091 *     ...
092 *   &lt;/tables&gt;
093 * &lt;/database&gt;
094 * </pre>
095 *
096 * <p>
097 * If this document is parsed and stored in a hierarchical configuration object (which can be done by one of the sub
098 * classes), there are enhanced possibilities of accessing properties. Per default, the keys for querying information
099 * can contain indices that select a specific element if there are multiple hits.
100 * </p>
101 * <p>
102 * For instance the key {@code tables.table(0).name} can be used to find out the name of the first table. In opposite
103 * {@code tables.table.name} would return a collection with the names of all available tables. Similarly the key
104 * {@code tables.table(1).fields.field.name} returns a collection with the names of all fields of the second table. If
105 * another index is added after the {@code field} element, a single field can be accessed:
106 * {@code tables.table(1).fields.field(0).name}.
107 * </p>
108 * <p>
109 * There is a {@code getMaxIndex()} method that returns the maximum allowed index that can be added to a given property
110 * key. This method can be used to iterate over all values defined for a certain property.
111 * </p>
112 * <p>
113 * Since the 1.3 release of <em>Commons Configuration</em> hierarchical configurations support an <em>expression
114 * engine</em>. This expression engine is responsible for evaluating the passed in configuration keys and map them to
115 * the stored properties. The examples above are valid for the default expression engine, which is used when a new
116 * {@code AbstractHierarchicalConfiguration} instance is created. With the {@code setExpressionEngine()} method a
117 * different expression engine can be set. For instance with
118 * {@link org.apache.commons.configuration2.tree.xpath.XPathExpressionEngine} there is an expression engine available
119 * that supports configuration keys in XPATH syntax.
120 * </p>
121 * <p>
122 * In addition to the events common for all configuration classes, hierarchical configurations support some more events
123 * that correspond to some specific methods and features. For those events specific event type constants in
124 * {@code ConfigurationEvent} exist:
125 * </p>
126 * <dl>
127 * <dt><em>ADD_NODES</em></dt>
128 * <dd>The {@code addNodes()} method was called; the event object contains the key, to which the nodes were added, and a
129 * collection with the new nodes as value.</dd>
130 * <dt><em>CLEAR_TREE</em></dt>
131 * <dd>The {@code clearTree()} method was called; the event object stores the key of the removed sub tree.</dd>
132 * <dt><em>SUBNODE_CHANGED</em></dt>
133 * <dd>A {@code SubnodeConfiguration} that was created from this configuration has been changed. The value property of
134 * the event object contains the original event object as it was sent by the subnode configuration.</dd>
135 * </dl>
136 * <p>
137 * Whether an {@code AbstractHierarchicalConfiguration} object is thread-safe or not depends on the underlying
138 * {@code NodeModel} and the {@link org.apache.commons.configuration2.sync.Synchronizer Synchronizer} it is associated
139 * with. Some {@code NodeModel} implementations are inherently thread-safe; they do not require a special
140 * {@code Synchronizer}. (Per default, a dummy {@code Synchronizer} is used which is not thread-safe!) The methods for
141 * querying or updating configuration data invoke this {@code Synchronizer} accordingly. When accessing the
142 * configuration's root node directly, the client application is responsible for proper synchronization. This is
143 * achieved by calling the methods {@link #lock(org.apache.commons.configuration2.sync.LockMode) lock()}, and
144 * {@link #unlock(org.apache.commons.configuration2.sync.LockMode) unlock()} with a proper
145 * {@link org.apache.commons.configuration2.sync.LockMode LockMode} argument. In any case, it is recommended to not
146 * access the root node directly, but to use corresponding methods for querying or updating configuration data instead.
147 * Direct manipulations of a configuration's node structure circumvent many internal mechanisms and thus can cause
148 * undesired effects. For concrete subclasses dealing with specific node structures, this situation may be different.
149 * </p>
150 *
151 * @since 2.0
152 * @param <T> the type of the nodes managed by this hierarchical configuration
153 */
154public abstract class AbstractHierarchicalConfiguration<T> extends AbstractConfiguration
155    implements Cloneable, NodeKeyResolver<T>, HierarchicalConfiguration<T> {
156
157    /** The model for managing the data stored in this configuration. */
158    private NodeModel<T> nodeModel;
159
160    /** Stores the expression engine for this instance. */
161    private ExpressionEngine expressionEngine;
162
163    /**
164     * Creates a new instance of {@code AbstractHierarchicalConfiguration} and sets the {@code NodeModel} to be used.
165     *
166     * @param nodeModel the {@code NodeModel}
167     */
168    protected AbstractHierarchicalConfiguration(final NodeModel<T> nodeModel) {
169        this.nodeModel = nodeModel;
170    }
171
172    /**
173     * {@inheritDoc} This implementation handles synchronization and delegates to {@code getRootElementNameInternal()}.
174     */
175    @Override
176    public final String getRootElementName() {
177        beginRead(false);
178        try {
179            return getRootElementNameInternal();
180        } finally {
181            endRead();
182        }
183    }
184
185    /**
186     * Actually obtains the name of the root element. This method is called by {@code getRootElementName()}. It just returns
187     * the name of the root node. Subclasses that treat the root element name differently can override this method.
188     *
189     * @return the name of this configuration's root element
190     */
191    protected String getRootElementNameInternal() {
192        final NodeHandler<T> nodeHandler = getModel().getNodeHandler();
193        return nodeHandler.nodeName(nodeHandler.getRootNode());
194    }
195
196    /**
197     * {@inheritDoc} This implementation returns the configuration's {@code NodeModel}. It is guarded by the current
198     * {@code Synchronizer}.
199     */
200    @Override
201    public NodeModel<T> getNodeModel() {
202        beginRead(false);
203        try {
204            return getModel();
205        } finally {
206            endRead();
207        }
208    }
209
210    /**
211     * Gets the expression engine used by this configuration. This method will never return <b>null</b>; if no specific
212     * expression engine was set, the default expression engine will be returned.
213     *
214     * @return the current expression engine
215     * @since 1.3
216     */
217    @Override
218    public ExpressionEngine getExpressionEngine() {
219        return expressionEngine != null ? expressionEngine : DefaultExpressionEngine.INSTANCE;
220    }
221
222    /**
223     * Sets the expression engine to be used by this configuration. All property keys this configuration has to deal with
224     * will be interpreted by this engine.
225     *
226     * @param expressionEngine the new expression engine; can be <b>null</b>, then the default expression engine will be
227     *        used
228     * @since 1.3
229     */
230    @Override
231    public void setExpressionEngine(final ExpressionEngine expressionEngine) {
232        this.expressionEngine = expressionEngine;
233    }
234
235    /**
236     * Fetches the specified property. This task is delegated to the associated expression engine.
237     *
238     * @param key the key to be looked up
239     * @return the found value
240     */
241    @Override
242    protected Object getPropertyInternal(final String key) {
243        final List<QueryResult<T>> results = fetchNodeList(key);
244
245        if (results.isEmpty()) {
246            return null;
247        }
248        final NodeHandler<T> handler = getModel().getNodeHandler();
249        final List<Object> list = results.stream().map(r -> valueFromResult(r, handler)).filter(Objects::nonNull).collect(Collectors.toList());
250
251        if (list.size() < 1) {
252            return null;
253        }
254        return list.size() == 1 ? list.get(0) : list;
255    }
256
257    /**
258     * Adds the property with the specified key. This task will be delegated to the associated {@code ExpressionEngine}, so
259     * the passed in key must match the requirements of this implementation.
260     *
261     * @param key the key of the new property
262     * @param obj the value of the new property
263     */
264    @Override
265    protected void addPropertyInternal(final String key, final Object obj) {
266        addPropertyToModel(key, getListDelimiterHandler().parse(obj));
267    }
268
269    /**
270     * {@inheritDoc} This method is not called in the normal way (via {@code addProperty()} for hierarchical configurations
271     * because all values to be added for the property have to be passed to the model in a single step. However, to allow
272     * derived classes to add an arbitrary value as an object, a special implementation is provided here. The passed in
273     * object is not parsed as a list, but passed directly as only value to the model.
274     */
275    @Override
276    protected void addPropertyDirect(final String key, final Object value) {
277        addPropertyToModel(key, Collections.singleton(value));
278    }
279
280    /**
281     * Helper method for executing an add property operation on the model.
282     *
283     * @param key the key of the new property
284     * @param values the values to be added for this property
285     */
286    private void addPropertyToModel(final String key, final Iterable<?> values) {
287        getModel().addProperty(key, values, this);
288    }
289
290    /**
291     * Adds a collection of nodes at the specified position of the configuration tree. This method works similar to
292     * {@code addProperty()}, but instead of a single property a whole collection of nodes can be added - and thus complete
293     * configuration sub trees. E.g. with this method it is possible to add parts of another
294     * {@code BaseHierarchicalConfiguration} object to this object. If the passed in key refers to an existing and unique
295     * node, the new nodes are added to this node. Otherwise a new node will be created at the specified position in the
296     * hierarchy. Implementation node: This method performs some book-keeping and then delegates to
297     * {@code addNodesInternal()}.
298     *
299     * @param key the key where the nodes are to be added; can be <b>null</b>, then they are added to the root node
300     * @param nodes a collection with the {@code Node} objects to be added
301     */
302    @Override
303    public final void addNodes(final String key, final Collection<? extends T> nodes) {
304        if (nodes == null || nodes.isEmpty()) {
305            return;
306        }
307
308        beginWrite(false);
309        try {
310            fireEvent(ConfigurationEvent.ADD_NODES, key, nodes, true);
311            addNodesInternal(key, nodes);
312            fireEvent(ConfigurationEvent.ADD_NODES, key, nodes, false);
313        } finally {
314            endWrite();
315        }
316    }
317
318    /**
319     * Actually adds a collection of new nodes to this configuration. This method is called by {@code addNodes()}. It can be
320     * overridden by subclasses that need to adapt this operation.
321     *
322     * @param key the key where the nodes are to be added; can be <b>null</b>, then they are added to the root node
323     * @param nodes a collection with the {@code Node} objects to be added
324     * @since 2.0
325     */
326    protected void addNodesInternal(final String key, final Collection<? extends T> nodes) {
327        getModel().addNodes(key, nodes, this);
328    }
329
330    /**
331     * Checks if this configuration is empty. Empty means that there are no keys with any values, though there can be some
332     * (empty) nodes.
333     *
334     * @return a flag if this configuration is empty
335     */
336    @Override
337    protected boolean isEmptyInternal() {
338        return !nodeDefined(getModel().getNodeHandler().getRootNode());
339    }
340
341    /**
342     * Checks if the specified key is contained in this configuration. Note that for this configuration the term
343     * &quot;contained&quot; means that the key has an associated value. If there is a node for this key that has no value
344     * but children (either defined or undefined), this method will still return <b>false </b>.
345     *
346     * @param key the key to be checked
347     * @return a flag if this key is contained in this configuration
348     */
349    @Override
350    protected boolean containsKeyInternal(final String key) {
351        return getPropertyInternal(key) != null;
352    }
353
354    /**
355     * Sets the value of the specified property.
356     *
357     * @param key the key of the property to set
358     * @param value the new value of this property
359     */
360    @Override
361    protected void setPropertyInternal(final String key, final Object value) {
362        getModel().setProperty(key, value, this);
363    }
364
365    /**
366     * {@inheritDoc} This implementation delegates to the expression engine.
367     */
368    @Override
369    public List<QueryResult<T>> resolveKey(final T root, final String key, final NodeHandler<T> handler) {
370        return getExpressionEngine().query(root, key, handler);
371    }
372
373    /**
374     * {@inheritDoc} This implementation delegates to {@code resolveKey()} and then filters out attribute results.
375     */
376    @Override
377    public List<T> resolveNodeKey(final T root, final String key, final NodeHandler<T> handler) {
378        return resolveKey(root, key, handler).stream().filter(r -> !r.isAttributeResult()).map(QueryResult::getNode)
379            .collect(Collectors.toCollection(LinkedList::new));
380    }
381
382    /**
383     * {@inheritDoc} This implementation delegates to the expression engine.
384     */
385    @Override
386    public NodeAddData<T> resolveAddKey(final T root, final String key, final NodeHandler<T> handler) {
387        return getExpressionEngine().prepareAdd(root, key, handler);
388    }
389
390    /**
391     * {@inheritDoc} This implementation executes a query for the given key and constructs a {@code NodeUpdateData} object
392     * based on the results. It determines which nodes need to be changed and whether new ones need to be added or existing
393     * ones need to be removed.
394     */
395    @Override
396    public NodeUpdateData<T> resolveUpdateKey(final T root, final String key, final Object newValue, final NodeHandler<T> handler) {
397        final Iterator<QueryResult<T>> itNodes = fetchNodeList(key).iterator();
398        final Iterator<?> itValues = getListDelimiterHandler().parse(newValue).iterator();
399        final Map<QueryResult<T>, Object> changedValues = new HashMap<>();
400        Collection<Object> additionalValues = null;
401        Collection<QueryResult<T>> removedItems = null;
402
403        while (itNodes.hasNext() && itValues.hasNext()) {
404            changedValues.put(itNodes.next(), itValues.next());
405        }
406
407        // Add additional nodes if necessary
408        if (itValues.hasNext()) {
409            additionalValues = new LinkedList<>();
410            itValues.forEachRemaining(additionalValues::add);
411        }
412
413        // Remove remaining nodes
414        if (itNodes.hasNext()) {
415            removedItems = new LinkedList<>();
416            itNodes.forEachRemaining(removedItems::add);
417        }
418
419        return new NodeUpdateData<>(changedValues, additionalValues, removedItems, key);
420    }
421
422    /**
423     * {@inheritDoc} This implementation uses the expression engine to generate a canonical key for the passed in node. For
424     * this purpose, the path to the root node has to be traversed. The cache is used to store and access keys for nodes
425     * encountered on the path.
426     */
427    @Override
428    public String nodeKey(final T node, final Map<T, String> cache, final NodeHandler<T> handler) {
429        final List<T> paths = new LinkedList<>();
430        T currentNode = node;
431        String key = cache.get(node);
432        while (key == null && currentNode != null) {
433            paths.add(0, currentNode);
434            currentNode = handler.getParent(currentNode);
435            key = cache.get(currentNode);
436        }
437
438        for (final T n : paths) {
439            final String currentKey = getExpressionEngine().canonicalKey(n, key, handler);
440            cache.put(n, currentKey);
441            key = currentKey;
442        }
443
444        return key;
445    }
446
447    /**
448     * Clears this configuration. This is a more efficient implementation than the one inherited from the base class. It
449     * delegates to the node model.
450     */
451    @Override
452    protected void clearInternal() {
453        getModel().clear(this);
454    }
455
456    /**
457     * Removes all values of the property with the given name and of keys that start with this name. So if there is a
458     * property with the key &quot;foo&quot; and a property with the key &quot;foo.bar&quot;, a call of
459     * {@code clearTree("foo")} would remove both properties.
460     *
461     * @param key the key of the property to be removed
462     */
463    @Override
464    public final void clearTree(final String key) {
465        beginWrite(false);
466        try {
467            fireEvent(ConfigurationEvent.CLEAR_TREE, key, null, true);
468            fireEvent(ConfigurationEvent.CLEAR_TREE, key, clearTreeInternal(key), false);
469        } finally {
470            endWrite();
471        }
472    }
473
474    /**
475     * Actually clears the tree of elements referenced by the given key. This method is called by {@code clearTree()}.
476     * Subclasses that need to adapt this operation can override this method. This base implementation delegates to the node
477     * model.
478     *
479     * @param key the key of the property to be removed
480     * @return an object with information about the nodes that have been removed (this is needed for firing a meaningful
481     *         event of type CLEAR_TREE)
482     * @since 2.0
483     */
484    protected Object clearTreeInternal(final String key) {
485        return getModel().clearTree(key, this);
486    }
487
488    /**
489     * Removes the property with the given key. Properties with names that start with the given key (i.e. properties below
490     * the specified key in the hierarchy) won't be affected. This implementation delegates to the node+ model.
491     *
492     * @param key the key of the property to be removed
493     */
494    @Override
495    protected void clearPropertyDirect(final String key) {
496        getModel().clearProperty(key, this);
497    }
498
499    /**
500     * {@inheritDoc} This implementation is slightly more efficient than the default implementation. It does not iterate
501     * over the key set, but directly queries its size after it has been constructed. Note that constructing the key set is
502     * still an O(n) operation.
503     */
504    @Override
505    protected int sizeInternal() {
506        return visitDefinedKeys().getKeyList().size();
507    }
508
509    /**
510     * Gets an iterator with all keys defined in this configuration. Note that the keys returned by this method will not
511     * contain any indices. This means that some structure will be lost.
512     *
513     * @return an iterator with the defined keys in this configuration
514     */
515    @Override
516    protected Iterator<String> getKeysInternal() {
517        return visitDefinedKeys().getKeyList().iterator();
518    }
519
520    /**
521     * Creates a {@code DefinedKeysVisitor} and visits all defined keys with it.
522     *
523     * @return the visitor after all keys have been visited
524     */
525    private DefinedKeysVisitor visitDefinedKeys() {
526        final DefinedKeysVisitor visitor = new DefinedKeysVisitor();
527        final NodeHandler<T> nodeHandler = getModel().getNodeHandler();
528        NodeTreeWalker.INSTANCE.walkDFS(nodeHandler.getRootNode(), visitor, nodeHandler);
529        return visitor;
530    }
531
532    /**
533     * Gets an iterator with all keys defined in this configuration that start with the given prefix. The returned keys
534     * will not contain any indices. This implementation tries to locate a node whose key is the same as the passed in
535     * prefix. Then the subtree of this node is traversed, and the keys of all nodes encountered (including attributes) are
536     * added to the result set.
537     *
538     * @param prefix the prefix of the keys to start with
539     * @return an iterator with the found keys
540     */
541    @Override
542    protected Iterator<String> getKeysInternal(final String prefix) {
543        final DefinedKeysVisitor visitor = new DefinedKeysVisitor(prefix);
544        if (containsKey(prefix)) {
545            // explicitly add the prefix
546            visitor.getKeyList().add(prefix);
547        }
548
549        final List<QueryResult<T>> results = fetchNodeList(prefix);
550        final NodeHandler<T> handler = getModel().getNodeHandler();
551
552        results.forEach(result -> {
553            if (!result.isAttributeResult()) {
554                handler.getChildren(result.getNode()).forEach(c -> NodeTreeWalker.INSTANCE.walkDFS(c, visitor, handler));
555                visitor.handleAttributeKeys(prefix, result.getNode(), handler);
556            }
557        });
558
559        return visitor.getKeyList().iterator();
560    }
561
562    /**
563     * Gets the maximum defined index for the given key. This is useful if there are multiple values for this key. They
564     * can then be addressed separately by specifying indices from 0 to the return value of this method. If the passed in
565     * key is not contained in this configuration, result is -1.
566     *
567     * @param key the key to be checked
568     * @return the maximum defined index for this key
569     */
570    @Override
571    public final int getMaxIndex(final String key) {
572        beginRead(false);
573        try {
574            return getMaxIndexInternal(key);
575        } finally {
576            endRead();
577        }
578    }
579
580    /**
581     * Actually retrieves the maximum defined index for the given key. This method is called by {@code getMaxIndex()}.
582     * Subclasses that need to adapt this operation have to override this method.
583     *
584     * @param key the key to be checked
585     * @return the maximum defined index for this key
586     * @since 2.0
587     */
588    protected int getMaxIndexInternal(final String key) {
589        return fetchNodeList(key).size() - 1;
590    }
591
592    /**
593     * Creates a copy of this object. This new configuration object will contain copies of all nodes in the same structure.
594     * Registered event listeners won't be cloned; so they are not registered at the returned copy.
595     *
596     * @return the copy
597     * @since 1.2
598     */
599    @Override
600    public Object clone() {
601        beginRead(false);
602        try {
603            @SuppressWarnings("unchecked") // clone returns the same type
604            final AbstractHierarchicalConfiguration<T> copy = (AbstractHierarchicalConfiguration<T>) super.clone();
605            copy.setSynchronizer(NoOpSynchronizer.INSTANCE);
606            copy.cloneInterpolator(this);
607            copy.setSynchronizer(ConfigurationUtils.cloneSynchronizer(getSynchronizer()));
608            copy.nodeModel = cloneNodeModel();
609
610            return copy;
611        } catch (final CloneNotSupportedException cex) {
612            // should not happen
613            throw new ConfigurationRuntimeException(cex);
614        } finally {
615            endRead();
616        }
617    }
618
619    /**
620     * Creates a clone of the node model. This method is called by {@code clone()}.
621     *
622     * @return the clone of the {@code NodeModel}
623     * @since 2.0
624     */
625    protected abstract NodeModel<T> cloneNodeModel();
626
627    /**
628     * Helper method for resolving the specified key.
629     *
630     * @param key the key
631     * @return a list with all results selected by this key
632     */
633    protected List<QueryResult<T>> fetchNodeList(final String key) {
634        final NodeHandler<T> nodeHandler = getModel().getNodeHandler();
635        return resolveKey(nodeHandler.getRootNode(), key, nodeHandler);
636    }
637
638    /**
639     * Checks if the specified node is defined.
640     *
641     * @param node the node to be checked
642     * @return a flag if this node is defined
643     */
644    protected boolean nodeDefined(final T node) {
645        final DefinedVisitor<T> visitor = new DefinedVisitor<>();
646        NodeTreeWalker.INSTANCE.walkBFS(node, visitor, getModel().getNodeHandler());
647        return visitor.isDefined();
648    }
649
650    /**
651     * Gets the {@code NodeModel} used by this configuration. This method is intended for internal use only. Access to
652     * the model is granted without any synchronization. This is in contrast to the &quot;official&quot;
653     * {@code getNodeModel()} method which is guarded by the configuration's {@code Synchronizer}.
654     *
655     * @return the node model
656     */
657    protected NodeModel<T> getModel() {
658        return nodeModel;
659    }
660
661    /**
662     * Extracts the value from a query result.
663     *
664     * @param result the {@code QueryResult}
665     * @param handler the {@code NodeHandler}
666     * @return the value of this result (may be <b>null</b>)
667     */
668    private Object valueFromResult(final QueryResult<T> result, final NodeHandler<T> handler) {
669        return result.isAttributeResult() ? result.getAttributeValue(handler) : handler.getValue(result.getNode());
670    }
671
672    /**
673     * A specialized visitor that checks if a node is defined. &quot;Defined&quot; in this terms means that the node or at
674     * least one of its sub nodes is associated with a value.
675     *
676     * @param <T> the type of the nodes managed by this hierarchical configuration
677     */
678    private static final class DefinedVisitor<T> extends ConfigurationNodeVisitorAdapter<T> {
679
680        /** Stores the defined flag. */
681        private boolean defined;
682
683        /**
684         * Checks if iteration should be stopped. This can be done if the first defined node is found.
685         *
686         * @return a flag if iteration should be stopped
687         */
688        @Override
689        public boolean terminate() {
690            return isDefined();
691        }
692
693        /**
694         * Visits the node. Checks if a value is defined.
695         *
696         * @param node the actual node
697         */
698        @Override
699        public void visitBeforeChildren(final T node, final NodeHandler<T> handler) {
700            defined = handler.getValue(node) != null || !handler.getAttributes(node).isEmpty();
701        }
702
703        /**
704         * Returns the defined flag.
705         *
706         * @return the defined flag
707         */
708        public boolean isDefined() {
709            return defined;
710        }
711    }
712
713    /**
714     * A specialized visitor that fills a list with keys that are defined in a node hierarchy.
715     */
716    private final class DefinedKeysVisitor extends ConfigurationNodeVisitorAdapter<T> {
717
718        /** Stores the list to be filled. */
719        private final Set<String> keyList;
720
721        /** A stack with the keys of the already processed nodes. */
722        private final Stack<String> parentKeys;
723
724        /**
725         * Default constructor.
726         */
727        public DefinedKeysVisitor() {
728            keyList = new LinkedHashSet<>();
729            parentKeys = new Stack<>();
730        }
731
732        /**
733         * Creates a new {@code DefinedKeysVisitor} instance and sets the prefix for the keys to fetch.
734         *
735         * @param prefix the prefix
736         */
737        public DefinedKeysVisitor(final String prefix) {
738            this();
739            parentKeys.push(prefix);
740        }
741
742        /**
743         * Gets the list with all defined keys.
744         *
745         * @return the list with the defined keys
746         */
747        public Set<String> getKeyList() {
748            return keyList;
749        }
750
751        /**
752         * {@inheritDoc} This implementation removes this node's key from the stack.
753         */
754        @Override
755        public void visitAfterChildren(final T node, final NodeHandler<T> handler) {
756            parentKeys.pop();
757        }
758
759        /**
760         * {@inheritDoc} If this node has a value, its key is added to the internal list.
761         */
762        @Override
763        public void visitBeforeChildren(final T node, final NodeHandler<T> handler) {
764            final String parentKey = parentKeys.isEmpty() ? null : parentKeys.peek();
765            final String key = getExpressionEngine().nodeKey(node, parentKey, handler);
766            parentKeys.push(key);
767            if (handler.getValue(node) != null) {
768                keyList.add(key);
769            }
770            handleAttributeKeys(key, node, handler);
771        }
772
773        /**
774         * Appends all attribute keys of the current node.
775         *
776         * @param parentKey the parent key
777         * @param node the current node
778         * @param handler the {@code NodeHandler}
779         */
780        public void handleAttributeKeys(final String parentKey, final T node, final NodeHandler<T> handler) {
781            handler.getAttributes(node).forEach(attr -> keyList.add(getExpressionEngine().attributeKey(parentKey, attr)));
782        }
783    }
784
785    @Override
786    public String toString() {
787        return super.toString() + "(" + getRootElementNameInternal() + ")";
788    }
789}