    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> model;
    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        model = 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.model = 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 model;
    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 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 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}