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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *     http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  
18  package org.apache.commons.configuration2;
19  
20  import java.util.Collection;
21  import java.util.Collections;
22  import java.util.HashMap;
23  import java.util.Iterator;
24  import java.util.LinkedList;
25  import java.util.List;
26  import java.util.Map;
27  import java.util.stream.Collectors;
28  
29  import org.apache.commons.configuration2.event.ConfigurationEvent;
30  import org.apache.commons.configuration2.event.EventListener;
31  import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
32  import org.apache.commons.configuration2.interpol.ConfigurationInterpolator;
33  import org.apache.commons.configuration2.tree.ConfigurationNodeVisitorAdapter;
34  import org.apache.commons.configuration2.tree.ImmutableNode;
35  import org.apache.commons.configuration2.tree.InMemoryNodeModel;
36  import org.apache.commons.configuration2.tree.InMemoryNodeModelSupport;
37  import org.apache.commons.configuration2.tree.NodeHandler;
38  import org.apache.commons.configuration2.tree.NodeModel;
39  import org.apache.commons.configuration2.tree.NodeSelector;
40  import org.apache.commons.configuration2.tree.NodeTreeWalker;
41  import org.apache.commons.configuration2.tree.QueryResult;
42  import org.apache.commons.configuration2.tree.ReferenceNodeHandler;
43  import org.apache.commons.configuration2.tree.TrackedNodeModel;
44  import org.apache.commons.lang3.ObjectUtils;
45  
46  /**
47   * <p>
48   * A specialized hierarchical configuration implementation that is based on a structure of {@link ImmutableNode}
49   * objects.
50   * </p>
51   */
52  public class BaseHierarchicalConfiguration extends AbstractHierarchicalConfiguration<ImmutableNode> implements InMemoryNodeModelSupport {
53  
54      /**
55       * A specialized visitor base class that can be used for storing the tree of configuration nodes. The basic idea is that
56       * each node can be associated with a reference object. This reference object has a concrete meaning in a derived class,
57       * for example an entry in a JNDI context or an XML element. When the configuration tree is set up, the {@code load()} method
58       * is responsible for setting the reference objects. When the configuration tree is later modified, new nodes do not
59       * have a defined reference object. This visitor class processes all nodes and finds the ones without a defined
60       * reference object. For those nodes the {@code insert()} method is called, which must be defined in concrete sub
61       * classes. This method can perform all steps to integrate the new node into the original structure.
62       */
63      protected abstract static class BuilderVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
64          /**
65           * Inserts a new node into the structure constructed by this builder. This method is called for each node that has been
66           * added to the configuration tree after the configuration has been loaded from its source. These new nodes have to be
67           * inserted into the original structure. The passed in nodes define the position of the node to be inserted: its parent
68           * and the siblings between to insert.
69           *
70           * @param newNode the node to be inserted
71           * @param parent the parent node
72           * @param sibling1 the sibling after which the node is to be inserted; can be <strong>null</strong> if the new node is going to be
73           *        the first child node
74           * @param sibling2 the sibling before which the node is to be inserted; can be <strong>null</strong> if the new node is going to
75           *        be the last child node
76           * @param refHandler the {@code ReferenceNodeHandler}
77           */
78          protected abstract void insert(ImmutableNode newNode, ImmutableNode parent, ImmutableNode sibling1, ImmutableNode sibling2,
79              ReferenceNodeHandler refHandler);
80  
81          /**
82           * Inserts new children that have been added to the specified node.
83           *
84           * @param node the current node to be processed
85           * @param refHandler the {@code ReferenceNodeHandler}
86           */
87          private void insertNewChildNodes(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
88              final Collection<ImmutableNode> subNodes = new LinkedList<>(refHandler.getChildren(node));
89              final Iterator<ImmutableNode> children = subNodes.iterator();
90              ImmutableNode sibling1;
91              ImmutableNode nd = null;
92  
93              while (children.hasNext()) {
94                  // find the next new node
95                  do {
96                      sibling1 = nd;
97                      nd = children.next();
98                  } while (refHandler.getReference(nd) != null && children.hasNext());
99  
100                 if (refHandler.getReference(nd) == null) {
101                     // find all following new nodes
102                     final List<ImmutableNode> newNodes = new LinkedList<>();
103                     newNodes.add(nd);
104                     while (children.hasNext()) {
105                         nd = children.next();
106                         if (refHandler.getReference(nd) != null) {
107                             break;
108                         }
109                         newNodes.add(nd);
110                     }
111 
112                     // Insert all new nodes
113                     final ImmutableNode sibling2 = refHandler.getReference(nd) == null ? null : nd;
114                     for (final ImmutableNode insertNode : newNodes) {
115                         if (refHandler.getReference(insertNode) == null) {
116                             insert(insertNode, node, sibling1, sibling2, refHandler);
117                             sibling1 = insertNode;
118                         }
119                     }
120                 }
121             }
122         }
123 
124         /**
125          * Updates a node that already existed in the original hierarchy. This method is called for each node that has an
126          * assigned reference object. A concrete implementation should update the reference according to the node's current
127          * value.
128          *
129          * @param node the current node to be processed
130          * @param reference the reference object for this node
131          * @param refHandler the {@code ReferenceNodeHandler}
132          */
133         protected abstract void update(ImmutableNode node, Object reference, ReferenceNodeHandler refHandler);
134 
135         /**
136          * Updates the value of a node. If this node is associated with a reference object, the {@code update()} method is
137          * called.
138          *
139          * @param node the current node to be processed
140          * @param refHandler the {@code ReferenceNodeHandler}
141          */
142         private void updateNode(final ImmutableNode node, final ReferenceNodeHandler refHandler) {
143             final Object reference = refHandler.getReference(node);
144             if (reference != null) {
145                 update(node, reference, refHandler);
146             }
147         }
148 
149         @Override
150         public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
151             final ReferenceNodeHandler refHandler = (ReferenceNodeHandler) handler;
152             updateNode(node, refHandler);
153             insertNewChildNodes(node, refHandler);
154         }
155     }
156 
157     /**
158      * A specialized visitor implementation which constructs the root node of a configuration with all variables replaced by
159      * their interpolated values.
160      */
161     private final class InterpolatedVisitor extends ConfigurationNodeVisitorAdapter<ImmutableNode> {
162         /** A stack for managing node builder instances. */
163         private final List<ImmutableNode.Builder> builderStack;
164 
165         /** The resulting root node. */
166         private ImmutableNode interpolatedRoot;
167 
168         /**
169          * Creates a new instance of {@code InterpolatedVisitor}.
170          */
171         public InterpolatedVisitor() {
172             builderStack = new LinkedList<>();
173         }
174 
175         /**
176          * Gets the result of this builder: the root node of the interpolated nodes hierarchy.
177          *
178          * @return the resulting root node
179          */
180         public ImmutableNode getInterpolatedRoot() {
181             return interpolatedRoot;
182         }
183 
184         /**
185          * Handles interpolation for a node with no children. If interpolation does not change this node, it is copied as is to
186          * the resulting structure. Otherwise, a new node is created with the interpolated values.
187          *
188          * @param node the current node to be processed
189          * @param handler the {@code NodeHandler}
190          */
191         private void handleLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
192             final Object value = interpolate(node.getValue());
193             final Map<String, Object> interpolatedAttributes = new HashMap<>();
194             final boolean attributeChanged = interpolateAttributes(node, handler, interpolatedAttributes);
195             final ImmutableNode newNode = valueChanged(value, handler.getValue(node)) || attributeChanged
196                 ? new ImmutableNode.Builder().name(handler.nodeName(node)).value(value).addAttributes(interpolatedAttributes).create()
197                 : node;
198             storeInterpolatedNode(newNode);
199         }
200 
201         /**
202          * Returns a map with interpolated attributes of the passed in node.
203          *
204          * @param node the current node to be processed
205          * @param handler the {@code NodeHandler}
206          * @return the map with interpolated attributes
207          */
208         private Map<String, Object> interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
209             final Map<String, Object> attributes = new HashMap<>();
210             interpolateAttributes(node, handler, attributes);
211             return attributes;
212         }
213 
214         /**
215          * Populates a map with interpolated attributes of the passed in node.
216          *
217          * @param node the current node to be processed
218          * @param handler the {@code NodeHandler}
219          * @param interpolatedAttributes a map for storing the results
220          * @return a flag whether an attribute value was changed by interpolation
221          */
222         private boolean interpolateAttributes(final ImmutableNode node, final NodeHandler<ImmutableNode> handler,
223             final Map<String, Object> interpolatedAttributes) {
224             boolean attributeChanged = false;
225             for (final String attr : handler.getAttributes(node)) {
226                 final Object attrValue = interpolate(handler.getAttributeValue(node, attr));
227                 if (valueChanged(attrValue, handler.getAttributeValue(node, attr))) {
228                     attributeChanged = true;
229                 }
230                 interpolatedAttributes.put(attr, attrValue);
231             }
232             return attributeChanged;
233         }
234 
235         /**
236          * Returns a flag whether the given node is a leaf. This is the case if it does not have children.
237          *
238          * @param node the node in question
239          * @param handler the {@code NodeHandler}
240          * @return a flag whether this is a leaf node
241          */
242         private boolean isLeafNode(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
243             return handler.getChildren(node).isEmpty();
244         }
245 
246         /**
247          * Returns the top-level element from the stack without removing it.
248          *
249          * @return the top-level element from the stack
250          */
251         private ImmutableNode.Builder peek() {
252             return builderStack.get(0);
253         }
254 
255         /**
256          * Pops the top-level element from the stack.
257          *
258          * @return the element popped from the stack
259          */
260         private ImmutableNode.Builder pop() {
261             return builderStack.remove(0);
262         }
263 
264         /**
265          * Pushes a new builder on the stack.
266          *
267          * @param builder the builder
268          */
269         private void push(final ImmutableNode.Builder builder) {
270             builderStack.add(0, builder);
271         }
272 
273         /**
274          * Stores a processed node. Per default, the node is added to the current builder on the stack. If no such builder
275          * exists, this is the result node.
276          *
277          * @param node the node to be stored
278          */
279         private void storeInterpolatedNode(final ImmutableNode node) {
280             if (builderStack.isEmpty()) {
281                 interpolatedRoot = node;
282             } else {
283                 peek().addChild(node);
284             }
285         }
286 
287         /**
288          * Tests whether a value is changed because of interpolation.
289          *
290          * @param interpolatedValue the interpolated value
291          * @param value the original value
292          * @return a flag whether the value was changed
293          */
294         private boolean valueChanged(final Object interpolatedValue, final Object value) {
295             return ObjectUtils.notEqual(interpolatedValue, value);
296         }
297 
298         @Override
299         public void visitAfterChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
300             if (!isLeafNode(node, handler)) {
301                 final ImmutableNode newNode = pop().create();
302                 storeInterpolatedNode(newNode);
303             }
304         }
305 
306         @Override
307         public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
308             if (isLeafNode(node, handler)) {
309                 handleLeafNode(node, handler);
310             } else {
311                 final ImmutableNode.Builder builder = new ImmutableNode.Builder(handler.getChildrenCount(node, null)).name(handler.nodeName(node))
312                     .value(interpolate(handler.getValue(node))).addAttributes(interpolateAttributes(node, handler));
313                 push(builder);
314             }
315         }
316     }
317 
318     /**
319      * Creates the {@code NodeModel} for this configuration based on a passed in source configuration. This implementation
320      * creates an {@link InMemoryNodeModel}. If the passed in source configuration is defined, its root node also becomes
321      * the root node of this configuration. Otherwise, a new, empty root node is used.
322      *
323      * @param c the configuration that is to be copied
324      * @return the {@code NodeModel} for the new configuration
325      */
326     private static NodeModel<ImmutableNode> createNodeModel(final HierarchicalConfiguration<ImmutableNode> c) {
327         return new InMemoryNodeModel(obtainRootNode(c));
328     }
329 
330     /**
331      * Obtains the root node from a configuration whose data is to be copied. It has to be ensured that the synchronizer is
332      * called correctly.
333      *
334      * @param c the configuration that is to be copied
335      * @return the root node of this configuration
336      */
337     private static ImmutableNode obtainRootNode(final HierarchicalConfiguration<ImmutableNode> c) {
338         return c != null ? c.getNodeModel().getNodeHandler().getRootNode() : null;
339     }
340 
341     /**
342      * Creates a list with immutable configurations from the given input list.
343      *
344      * @param subs a list with mutable configurations
345      * @return a list with corresponding immutable configurations
346      */
347     private static List<ImmutableHierarchicalConfiguration> toImmutable(final List<? extends HierarchicalConfiguration<?>> subs) {
348         return subs.stream().map(ConfigurationUtils::unmodifiableConfiguration).collect(Collectors.toList());
349     }
350 
351     /** A listener for reacting on changes caused by sub configurations. */
352     private final EventListener<ConfigurationEvent> changeListener;
353 
354     /**
355      * Creates a new instance of {@code BaseHierarchicalConfiguration}.
356      */
357     public BaseHierarchicalConfiguration() {
358         this((HierarchicalConfiguration<ImmutableNode>) null);
359     }
360 
361     /**
362      * Creates a new instance of {@code BaseHierarchicalConfiguration} and copies all data contained in the specified
363      * configuration into the new one.
364      *
365      * @param c the configuration that is to be copied (if <strong>null</strong>, this constructor will behave like the standard
366      *        constructor)
367      * @since 1.4
368      */
369     public BaseHierarchicalConfiguration(final HierarchicalConfiguration<ImmutableNode> c) {
370         this(createNodeModel(c));
371     }
372 
373     /**
374      * Creates a new instance of {@code BaseHierarchicalConfiguration} and initializes it with the given {@code NodeModel}.
375      *
376      * @param model the {@code NodeModel}
377      */
378     protected BaseHierarchicalConfiguration(final NodeModel<ImmutableNode> model) {
379         super(model);
380         changeListener = createChangeListener();
381     }
382 
383     /**
384      * {@inheritDoc} This implementation resolves the node(s) selected by the given key. If not a single node is selected,
385      * an empty list is returned. Otherwise, sub configurations for each child of the node are created.
386      */
387     @Override
388     public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key) {
389         final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
390         if (nodes.size() != 1) {
391             return Collections.emptyList();
392         }
393         return nodes.get(0).stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
394     }
395 
396     /**
397      * {@inheritDoc} This method works like {@link #childConfigurationsAt(String)}; however, depending on the value of the
398      * {@code supportUpdates} flag, connected sub configurations may be created.
399      */
400     @Override
401     public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(final String key, final boolean supportUpdates) {
402         if (!supportUpdates) {
403             return childConfigurationsAt(key);
404         }
405 
406         final InMemoryNodeModel parentModel = getSubConfigurationParentModel();
407         return createConnectedSubConfigurations(this, parentModel.trackChildNodes(key, this));
408     }
409 
410     /**
411      * {@inheritDoc} This implementation creates a new instance of {@link InMemoryNodeModel}, initialized with this
412      * configuration's root node. This has the effect that although the same nodes are used, the original and copied
413      * configurations are independent on each other.
414      */
415     @Override
416     protected NodeModel<ImmutableNode> cloneNodeModel() {
417         return new InMemoryNodeModel(getModel().getNodeHandler().getRootNode());
418     }
419 
420     /**
421      * {@inheritDoc} This is a short form for {@code configurationAt(key,
422      * <strong>false</strong>)}.
423      *
424      * @throws ConfigurationRuntimeException if the key does not select a single node
425      */
426     @Override
427     public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key) {
428         return configurationAt(key, false);
429     }
430 
431     /**
432      * {@inheritDoc} The result of this implementation depends on the {@code supportUpdates} flag: If it is <strong>false</strong>, a
433      * plain {@code BaseHierarchicalConfiguration} is returned using the selected node as root node. This is suitable for
434      * read-only access to properties. Because the configuration returned in this case is not connected to the parent
435      * configuration, updates on properties made by one configuration are not reflected by the other one. A value of
436      * <strong>true</strong> for this parameter causes a tracked node to be created, and result is a {@link SubnodeConfiguration}
437      * based on this tracked node. This configuration is really connected to its parent, so that updated properties are
438      * visible on both.
439      *
440      * @see SubnodeConfiguration
441      * @throws ConfigurationRuntimeException if the key does not select a single node
442      */
443     @Override
444     public HierarchicalConfiguration<ImmutableNode> configurationAt(final String key, final boolean supportUpdates) {
445         return syncRead(() -> supportUpdates ? createConnectedSubConfiguration(key) : createIndependentSubConfiguration(key), false);
446     }
447 
448     /**
449      * {@inheritDoc} This implementation creates sub configurations in the same way as described for
450      * {@link #configurationAt(String)}.
451      */
452     @Override
453     public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key) {
454         final List<ImmutableNode> nodes = syncRead(() -> fetchFilteredNodeResults(key), false);
455         return nodes.stream().map(this::createIndependentSubConfigurationForNode).collect(Collectors.toList());
456     }
457 
458     /**
459      * {@inheritDoc} This implementation creates tracked nodes for the specified key. Then sub configurations for these
460      * nodes are created and returned.
461      */
462     @Override
463     public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(final String key, final boolean supportUpdates) {
464         if (!supportUpdates) {
465             return configurationsAt(key);
466         }
467         final InMemoryNodeModel parentModel = syncRead(this::getSubConfigurationParentModel, false);
468         return createConnectedSubConfigurations(this, parentModel.selectAndTrackNodes(key, this));
469     }
470 
471     /**
472      * Creates a listener which reacts on all changes on this configuration or one of its {@code SubnodeConfiguration}
473      * instances. If such a change is detected, some updates have to be performed.
474      *
475      * @return the newly created change listener
476      */
477     private EventListener<ConfigurationEvent> createChangeListener() {
478         return this::subnodeConfigurationChanged;
479     }
480 
481     /**
482      * Creates a sub configuration from the specified key which is connected to this configuration. This implementation
483      * creates a {@link SubnodeConfiguration} with a tracked node identified by the passed in key.
484      *
485      * @param key the key of the sub configuration
486      * @return the new sub configuration
487      */
488     private BaseHierarchicalConfiguration createConnectedSubConfiguration(final String key) {
489         final NodeSelector selector = getSubConfigurationNodeSelector(key);
490         getSubConfigurationParentModel().trackNode(selector, this);
491         return createSubConfigurationForTrackedNode(selector, this);
492     }
493 
494     /**
495      * Creates a list of connected sub configurations based on a passed in list of node selectors.
496      *
497      * @param parentModelSupport the parent node model support object
498      * @param selectors the list of {@code NodeSelector} objects
499      * @return the list with sub configurations
500      */
501     private List<HierarchicalConfiguration<ImmutableNode>> createConnectedSubConfigurations(final InMemoryNodeModelSupport parentModelSupport,
502         final Collection<NodeSelector> selectors) {
503         return selectors.stream().map(sel -> createSubConfigurationForTrackedNode(sel, parentModelSupport)).collect(Collectors.toList());
504     }
505 
506     /**
507      * Creates a sub configuration from the specified key which is independent on this configuration. This means that the
508      * sub configuration operates on a separate node model (although the nodes are initially shared).
509      *
510      * @param key the key of the sub configuration
511      * @return the new sub configuration
512      */
513     private BaseHierarchicalConfiguration createIndependentSubConfiguration(final String key) {
514         final List<ImmutableNode> targetNodes = fetchFilteredNodeResults(key);
515         final int size = targetNodes.size();
516         if (size != 1) {
517             throw new ConfigurationRuntimeException("Passed in key must select exactly one node (found %,d): %s", size, key);
518         }
519         final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(targetNodes.get(0)));
520         initSubConfiguration(sub);
521         return sub;
522     }
523 
524     /**
525      * Returns an initialized sub configuration for this configuration that is based on another
526      * {@code BaseHierarchicalConfiguration}. Thus, it is independent from this configuration.
527      *
528      * @param node the root node for the sub configuration
529      * @return the initialized sub configuration
530      */
531     private BaseHierarchicalConfiguration createIndependentSubConfigurationForNode(final ImmutableNode node) {
532         final BaseHierarchicalConfiguration sub = new BaseHierarchicalConfiguration(new InMemoryNodeModel(node));
533         initSubConfiguration(sub);
534         return sub;
535     }
536 
537     /**
538      * Creates a connected sub configuration based on a selector for a tracked node.
539      *
540      * @param selector the {@code NodeSelector}
541      * @param parentModelSupport the {@code InMemoryNodeModelSupport} object for the parent node model
542      * @return the newly created sub configuration
543      * @since 2.0
544      */
545     protected SubnodeConfiguration createSubConfigurationForTrackedNode(final NodeSelector selector, final InMemoryNodeModelSupport parentModelSupport) {
546         final SubnodeConfiguration subConfig = new SubnodeConfiguration(this, new TrackedNodeModel(parentModelSupport, selector, true));
547         initSubConfigurationForThisParent(subConfig);
548         return subConfig;
549     }
550 
551     /**
552      * Creates a root node for a subset configuration based on the passed in query results. This method creates a new root
553      * node and adds the children and attributes of all result nodes to it. If only a single node value is defined, it is
554      * assigned as value of the new root node.
555      *
556      * @param results the collection of query results
557      * @return the root node for the subset configuration
558      */
559     private ImmutableNode createSubsetRootNode(final Collection<QueryResult<ImmutableNode>> results) {
560         final ImmutableNode.Builder builder = new ImmutableNode.Builder();
561         Object value = null;
562         int valueCount = 0;
563 
564         for (final QueryResult<ImmutableNode> result : results) {
565             if (result.isAttributeResult()) {
566                 builder.addAttribute(result.getAttributeName(), result.getAttributeValue(getModel().getNodeHandler()));
567             } else {
568                 if (result.getNode().getValue() != null) {
569                     value = result.getNode().getValue();
570                     valueCount++;
571                 }
572                 builder.addChildren(result.getNode().getChildren());
573                 builder.addAttributes(result.getNode().getAttributes());
574             }
575         }
576 
577         if (valueCount == 1) {
578             builder.value(value);
579         }
580         return builder.create();
581     }
582 
583     /**
584      * Executes a query on the specified key and filters it for node results.
585      *
586      * @param key the key
587      * @return the filtered list with result nodes
588      */
589     private List<ImmutableNode> fetchFilteredNodeResults(final String key) {
590         final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
591         return resolveNodeKey(handler.getRootNode(), key, handler);
592     }
593 
594     /**
595      * {@inheritDoc} This implementation returns the {@code InMemoryNodeModel} used by this configuration.
596      */
597     @Override
598     public InMemoryNodeModel getNodeModel() {
599         return (InMemoryNodeModel) super.getNodeModel();
600     }
601 
602     /**
603      * Gets the {@code NodeSelector} to be used for a sub configuration based on the passed in key. This method is called
604      * whenever a sub configuration is to be created. This base implementation returns a new {@code NodeSelector}
605      * initialized with the passed in key. Sub classes may override this method if they have a different strategy for
606      * creating a selector.
607      *
608      * @param key the key of the sub configuration
609      * @return a {@code NodeSelector} for initializing a sub configuration
610      * @since 2.0
611      */
612     protected NodeSelector getSubConfigurationNodeSelector(final String key) {
613         return new NodeSelector(key);
614     }
615 
616     /**
617      * Gets the {@code InMemoryNodeModel} to be used as parent model for a new sub configuration. This method is called
618      * whenever a sub configuration is to be created. This base implementation returns the model of this configuration. Sub
619      * classes with different requirements for the parent models of sub configurations have to override it.
620      *
621      * @return the parent model for a new sub configuration
622      */
623     protected InMemoryNodeModel getSubConfigurationParentModel() {
624         return (InMemoryNodeModel) getModel();
625     }
626 
627     /**
628      * {@inheritDoc} This implementation first delegates to {@code childConfigurationsAt()} to create a list of mutable
629      * child configurations. Then a list with immutable wrapper configurations is created.
630      */
631     @Override
632     public List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(final String key) {
633         return toImmutable(childConfigurationsAt(key));
634     }
635 
636     /**
637      * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
638      * Then an immutable wrapper is created and returned.
639      *
640      * @throws ConfigurationRuntimeException if the key does not select a single node
641      */
642     @Override
643     public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key) {
644         return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key));
645     }
646 
647     /**
648      * {@inheritDoc} This implementation creates a {@code SubnodeConfiguration} by delegating to {@code configurationAt()}.
649      * Then an immutable wrapper is created and returned.
650      */
651     @Override
652     public ImmutableHierarchicalConfiguration immutableConfigurationAt(final String key, final boolean supportUpdates) {
653         return ConfigurationUtils.unmodifiableConfiguration(configurationAt(key, supportUpdates));
654     }
655 
656     /**
657      * {@inheritDoc} This implementation first delegates to {@code configurationsAt()} to create a list of
658      * {@code SubnodeConfiguration} objects. Then for each element of this list an unmodifiable wrapper is created.
659      */
660     @Override
661     public List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(final String key) {
662         return toImmutable(configurationsAt(key));
663     }
664 
665     /**
666      * Initializes properties of a sub configuration. A sub configuration inherits some settings from its parent, for example the
667      * expression engine or the synchronizer. The corresponding values are copied by this method.
668      *
669      * @param sub the sub configuration to be initialized
670      */
671     private void initSubConfiguration(final BaseHierarchicalConfiguration sub) {
672         sub.setSynchronizer(getSynchronizer());
673         sub.setExpressionEngine(getExpressionEngine());
674         sub.setListDelimiterHandler(getListDelimiterHandler());
675         sub.setThrowExceptionOnMissing(isThrowExceptionOnMissing());
676         sub.getInterpolator().setParentInterpolator(getInterpolator());
677     }
678 
679     /**
680      * Initializes a {@code SubnodeConfiguration} object. This method should be called for each sub configuration created
681      * for this configuration. It ensures that the sub configuration is correctly connected to its parent instance and that
682      * update events are correctly propagated.
683      *
684      * @param subConfig the sub configuration to be initialized
685      * @since 2.0
686      */
687     protected void initSubConfigurationForThisParent(final SubnodeConfiguration subConfig) {
688         initSubConfiguration(subConfig);
689         subConfig.addEventListener(ConfigurationEvent.ANY, changeListener);
690     }
691 
692     /**
693      * Returns a configuration with the same content as this configuration, but with all variables replaced by their actual
694      * values. This implementation is specific for hierarchical configurations. It clones the current configuration and runs
695      * a specialized visitor on the clone, which performs interpolation on the single configuration nodes.
696      *
697      * @return a configuration with all variables interpolated
698      * @since 1.5
699      */
700     @Override
701     public Configuration interpolatedConfiguration() {
702         final InterpolatedVisitor visitor = new InterpolatedVisitor();
703         final NodeHandler<ImmutableNode> handler = getModel().getNodeHandler();
704         NodeTreeWalker.INSTANCE.walkDFS(handler.getRootNode(), visitor, handler);
705 
706         final BaseHierarchicalConfiguration c = (BaseHierarchicalConfiguration) clone();
707         c.getNodeModel().setRootNode(visitor.getInterpolatedRoot());
708         return c;
709     }
710 
711     /**
712      * This method is always called when a subnode configuration created from this configuration has been modified. This
713      * implementation transforms the received event into an event of type {@code SUBNODE_CHANGED} and notifies the
714      * registered listeners.
715      *
716      * @param event the event describing the change
717      * @since 1.5
718      */
719     protected void subnodeConfigurationChanged(final ConfigurationEvent event) {
720         fireEvent(ConfigurationEvent.SUBNODE_CHANGED, null, event, event.isBeforeUpdate());
721     }
722 
723     /**
724      * Creates a new {@code Configuration} object containing all keys that start with the specified prefix. This
725      * implementation will return a {@code BaseHierarchicalConfiguration} object so that the structure of the keys will be
726      * saved. The nodes selected by the prefix (it is possible that multiple nodes are selected) are mapped to the root node
727      * of the returned configuration, i.e. their children and attributes will become children and attributes of the new root
728      * node. However, a value of the root node is only set if exactly one of the selected nodes contain a value (if multiple
729      * nodes have a value, there is simply no way to decide how these values are merged together). Note that the returned
730      * {@code Configuration} object is not connected to its source configuration: updates on the source configuration are
731      * not reflected in the subset and vice versa. The returned configuration uses the same {@code Synchronizer} as this
732      * configuration.
733      *
734      * @param prefix the prefix of the keys for the subset
735      * @return a new configuration object representing the selected subset
736      */
737     @Override
738     public Configuration subset(final String prefix) {
739         return syncRead(() -> {
740             final List<QueryResult<ImmutableNode>> results = fetchNodeList(prefix);
741             if (results.isEmpty()) {
742                 return new BaseHierarchicalConfiguration();
743             }
744             final BaseHierarchicalConfiguration parent = this;
745             final BaseHierarchicalConfiguration result = new BaseHierarchicalConfiguration() {
746 
747                 @Override
748                 public ConfigurationInterpolator getInterpolator() {
749                     return parent.getInterpolator();
750                 }
751 
752                 // Override interpolate to always interpolate on the parent
753                 @Override
754                 protected Object interpolate(final Object value) {
755                     return parent.interpolate(value);
756                 }
757             };
758             result.getModel().setRootNode(createSubsetRootNode(results));
759             if (result.isEmpty()) {
760                 return new BaseHierarchicalConfiguration();
761             }
762             result.setSynchronizer(getSynchronizer());
763             return result;
764         }, false);
765     }
766 }