    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 */
    017package org.apache.commons.configuration2;
    018
    019import java.io.ByteArrayOutputStream;
    020import java.io.PrintStream;
    021import java.util.ArrayList;
    022import java.util.Collection;
    023import java.util.HashMap;
    024import java.util.HashSet;
    025import java.util.Iterator;
    026import java.util.List;
    027import java.util.Map;
    028import java.util.Set;
    029import java.util.stream.Collectors;
    030
    031import org.apache.commons.configuration2.event.ConfigurationEvent;
    032import org.apache.commons.configuration2.event.EventListener;
    033import org.apache.commons.configuration2.event.EventSource;
    034import org.apache.commons.configuration2.event.EventType;
    035import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
    036import org.apache.commons.configuration2.sync.LockMode;
    037import org.apache.commons.configuration2.tree.DefaultConfigurationKey;
    038import org.apache.commons.configuration2.tree.DefaultExpressionEngine;
    039import org.apache.commons.configuration2.tree.ExpressionEngine;
    040import org.apache.commons.configuration2.tree.ImmutableNode;
    041import org.apache.commons.configuration2.tree.NodeCombiner;
    042import org.apache.commons.configuration2.tree.NodeTreeWalker;
    043import org.apache.commons.configuration2.tree.QueryResult;
    044import org.apache.commons.configuration2.tree.TreeUtils;
    045import org.apache.commons.configuration2.tree.UnionCombiner;
    046import org.apache.commons.lang3.StringUtils;
    047
    048/**
    049 * <p>
    050 * A hierarchical composite configuration class.
    051 * </p>
    052 * <p>
    053 * This class maintains a list of configuration objects, which can be added using the diverse {@code addConfiguration()}
    054 * methods. After that the configurations can be accessed either by name (if one was provided when the configuration was
    055 * added) or by index. For the whole set of managed configurations a logical node structure is constructed. For this
    056 * purpose a {@link org.apache.commons.configuration2.tree.NodeCombiner NodeCombiner} object can be set. This makes it
    057 * possible to specify different algorithms for the combination process.
    058 * </p>
    059 * <p>
    060 * The big advantage of this class is that it creates a truly hierarchical structure of all the properties stored in the
    061 * contained configurations - even if some of them are no hierarchical configurations per se. So all enhanced features
    062 * provided by a hierarchical configuration (for example choosing an expression engine) are applicable.
    063 * </p>
    064 * <p>
    065 * The class works by registering itself as an event listener at all added configurations. So it gets notified whenever
    066 * one of these configurations is changed and can invalidate its internal node structure. The next time a property is
    067 * accessed the node structure will be re-constructed using the current state of the managed configurations. Note that,
    068 * depending on the used {@code NodeCombiner}, this may be a complex operation.
    069 * </p>
    070 * <p>
    071 * Because of the way a {@code CombinedConfiguration} is working it has more or less view character: it provides a logic
    072 * view on the configurations it contains. In this constellation not all methods defined for hierarchical configurations
    073 * - especially methods that update the stored properties - can be implemented in a consistent manner. Using such
    074 * methods (like {@code addProperty()}, or {@code clearProperty()} on a {@code CombinedConfiguration} is not strictly
    075 * forbidden, however, depending on the current {@link NodeCombiner} and the involved properties, the results may be
    076 * different than expected. Some examples may illustrate this:
    077 * </p>
    078 * <ul>
    079 * <li>Imagine a {@code CombinedConfiguration} <em>cc</em> containing two child configurations with the following
    080 * content:
    081 * <dl>
    082 * <dt>user.properties</dt>
    083 * <dd>
    084 *
    085 * <pre>
    086 * gui.background = blue
    087 * gui.position = (10, 10, 400, 200)
    088 * </pre>
    089 *
    090 * </dd>
    091 * <dt>default.properties</dt>
    092 * <dd>
    093 *
    094 * <pre>
    095 * gui.background = black
    096 * gui.foreground = white
    097 * home.dir = /data
    098 * </pre>
    099 *
    100 * </dd>
    101 * </dl>
    102 * As a {@code NodeCombiner} a {@link org.apache.commons.configuration2.tree.OverrideCombiner OverrideCombiner} is used.
    103 * This combiner will ensure that defined user settings take precedence over the default values. If the resulting
    104 * {@code CombinedConfiguration} is queried for the background color, {@code blue} will be returned because this value
    105 * is defined in {@code user.properties}. Now consider what happens if the key {@code gui.background} is removed from
    106 * the {@code CombinedConfiguration}:
    107 *
    108 * <pre>
    109 * cc.clearProperty(&quot;gui.background&quot;);
    110 * </pre>
    111 *
    112 * Will a {@code cc.containsKey("gui.background")} now return <strong>false</strong>? No, it won't! The {@code clearProperty()}
    113 * operation is executed on the node set of the combined configuration, which was constructed from the nodes of the two
    114 * child configurations. It causes the value of the <em>background</em> node to be cleared, which is also part of the
    115 * first child configuration. This modification of one of its child configurations causes the
    116 * {@code CombinedConfiguration} to be re-constructed. This time the {@code OverrideCombiner} cannot find a
    117 * {@code gui.background} property in the first child configuration, but it finds one in the second, and adds it to the
    118 * resulting combined configuration. So the property is still present (with a different value now).</li>
    119 * <li>{@code addProperty()} can also be problematic: Most node combiners use special view nodes for linking parts of
    120 * the original configurations' data together. If new properties are added to such a special node, they do not belong to
    121 * any of the managed configurations and thus hang in the air. Using the same configurations as in the last example, the
    122 * statement
    123 *
    124 * <pre>
    125 * addProperty(&quot;database.user&quot;, &quot;scott&quot;);
    126 * </pre>
    127 *
    128 * would cause such a hanging property. If now one of the child configurations is changed and the
    129 * {@code CombinedConfiguration} is re-constructed, this property will disappear! (Add operations are not problematic if
    130 * they result in a child configuration being updated. For instance an {@code addProperty("home.url", "localhost");}
    131 * will alter the second child configuration - because the prefix <em>home</em> is here already present; when the
    132 * {@code CombinedConfiguration} is re-constructed, this change is taken into account.)</li>
    133 * </ul>
    134 * <p>
    135 * Because of such problems it is recommended to perform updates only on the managed child configurations.
    136 * </p>
    137 * <p>
    138 * Whenever the node structure of a {@code CombinedConfiguration} becomes invalid (either because one of the contained
    139 * configurations was modified or because the {@code invalidate()} method was directly called) an event is generated. So
    140 * this can be detected by interested event listeners. This also makes it possible to add a combined configuration into
    141 * another one.
    142 * </p>
    143 * <p>
    144 * Notes about thread-safety: This configuration implementation uses a {@code Synchronizer} object to protect instances
    145 * against concurrent access. The concrete {@code Synchronizer} implementation used determines whether an instance of
    146 * this class is thread-safe or not. In contrast to other implementations derived from
    147 * {@link BaseHierarchicalConfiguration}, thread-safety is an issue here because the nodes structure used by this
    148 * configuration has to be constructed dynamically when a child configuration is changed. Therefore, when multiple
    149 * threads are involved which also manipulate one of the child configurations, a proper {@code Synchronizer} object
    150 * should be set. Note that the {@code Synchronizer} objects used by the child configurations do not really matter.
    151 * Because immutable in-memory nodes structures are used for them there is no danger that updates on child
    152 * configurations could interfere with read operations on the combined configuration.
    153 * </p>
    154 *
    155 * @since 1.3
    156 */
    157public class CombinedConfiguration extends BaseHierarchicalConfiguration implements EventListener<ConfigurationEvent> {
    158    /**
    159     * An internal helper class for storing information about contained configurations.
    160     */
    161    private final class ConfigData {
    162        /** Stores a reference to the configuration. */
    163        private final Configuration configuration;
    164
    165        /** Stores the name under which the configuration is stored. */
    166        private final String name;
    167
    168        /** Stores the at information as path of nodes. */
    169        private final Collection<String> atPath;
    170
    171        /** Stores the at string. */
    172        private final String at;
    173
    174        /** Stores the root node for this child configuration. */
    175        private ImmutableNode rootNode;
    176
    177        /**
    178         * Creates a new instance of {@code ConfigData} and initializes it.
    179         *
    180         * @param config the configuration
    181         * @param n the name
    182         * @param at the at position
    183         */
    184        public ConfigData(final Configuration config, final String n, final String at) {
    185            configuration = config;
    186            name = n;
    187            atPath = parseAt(at);
    188            this.at = at;
    189        }
    190
    191        /**
    192         * Gets the at position of this configuration.
    193         *
    194         * @return the at position
    195         */
    196        public String getAt() {
    197            return at;
    198        }
    199
    200        /**
    201         * Gets the stored configuration.
    202         *
    203         * @return the configuration
    204         */
    205        public Configuration getConfiguration() {
    206            return configuration;
    207        }
    208
    209        /**
    210         * Gets the configuration's name.
    211         *
    212         * @return the name
    213         */
    214        public String getName() {
    215            return name;
    216        }
    217
    218        /**
    219         * Gets the root node for this child configuration.
    220         *
    221         * @return the root node of this child configuration
    222         * @since 1.5
    223         */
    224        public ImmutableNode getRootNode() {
    225            return rootNode;
    226        }
    227
    228        /**
    229         * Obtains the root node of the wrapped configuration. If necessary, a hierarchical representation of the configuration
    230         * has to be created first.
    231         *
    232         * @return the root node of the associated configuration
    233         */
    234        private ImmutableNode getRootNodeOfConfiguration() {
    235            getConfiguration().lock(LockMode.READ);
    236            try {
    237                final ImmutableNode root = ConfigurationUtils.convertToHierarchical(getConfiguration(), conversionExpressionEngine).getNodeModel()
    238                    .getInMemoryRepresentation();
    239                rootNode = root;
    240                return root;
    241            } finally {
    242                getConfiguration().unlock(LockMode.READ);
    243            }
    244        }
    245
    246        /**
    247         * Gets the transformed root node of the stored configuration. The term &quot;transformed&quot; means that an
    248         * eventually defined at path has been applied.
    249         *
    250         * @return the transformed root node
    251         */
    252        public ImmutableNode getTransformedRoot() {
    253            final ImmutableNode configRoot = getRootNodeOfConfiguration();
    254            return atPath == null ? configRoot : prependAtPath(configRoot);
    255        }
    256
    257        /**
    258         * Splits the at path into its components.
    259         *
    260         * @param at the at string
    261         * @return a collection with the names of the single components
    262         */
    263        private Collection<String> parseAt(final String at) {
    264            if (StringUtils.isEmpty(at)) {
    265                return null;
    266            }
    267
    268            final Collection<String> result = new ArrayList<>();
    269            final DefaultConfigurationKey.KeyIterator it = new DefaultConfigurationKey(AT_ENGINE, at).iterator();
    270            while (it.hasNext()) {
    271                result.add(it.nextKey());
    272            }
    273            return result;
    274        }
    275
    276        /**
    277         * Prepends the at path to the given node.
    278         *
    279         * @param node the root node of the represented configuration
    280         * @return the new root node including the at path
    281         */
    282        private ImmutableNode prependAtPath(final ImmutableNode node) {
    283            final ImmutableNode.Builder pathBuilder = new ImmutableNode.Builder();
    284            final Iterator<String> pathIterator = atPath.iterator();
    285            prependAtPathComponent(pathBuilder, pathIterator.next(), pathIterator, node);
    286            return new ImmutableNode.Builder(1).addChild(pathBuilder.create()).create();
    287        }
    288
    289        /**
    290         * Handles a single component of the at path. A corresponding node is created and added to the hierarchical path to the
    291         * original root node of the configuration.
    292         *
    293         * @param builder the current node builder object
    294         * @param currentComponent the name of the current path component
    295         * @param components an iterator with all components of the at path
    296         * @param orgRoot the original root node of the wrapped configuration
    297         */
    298        private void prependAtPathComponent(final ImmutableNode.Builder builder, final String currentComponent, final Iterator<String> components,
    299            final ImmutableNode orgRoot) {
    300            builder.name(currentComponent);
    301            if (components.hasNext()) {
    302                final ImmutableNode.Builder childBuilder = new ImmutableNode.Builder();
    303                prependAtPathComponent(childBuilder, components.next(), components, orgRoot);
    304                builder.addChild(childBuilder.create());
    305            } else {
    306                builder.addChildren(orgRoot.getChildren());
    307                builder.addAttributes(orgRoot.getAttributes());
    308                builder.value(orgRoot.getValue());
    309            }
    310        }
    311    }
    312
    313    /**
    314     * Constant for the event type fired when the internal node structure of a combined configuration becomes invalid.
    315     *
    316     * @since 2.0
    317     */
    318    public static final EventType<ConfigurationEvent> COMBINED_INVALIDATE = new EventType<>(ConfigurationEvent.ANY, "COMBINED_INVALIDATE");
    319
    320    /** Constant for the expression engine for parsing the at path. */
    321    private static final DefaultExpressionEngine AT_ENGINE = DefaultExpressionEngine.INSTANCE;
    322
    323    /** Constant for the default node combiner. */
    324    private static final NodeCombiner DEFAULT_COMBINER = new UnionCombiner();
    325
    326    /** Constant for a root node for an empty configuration. */
    327    private static final ImmutableNode EMPTY_ROOT = new ImmutableNode.Builder().create();
    328
    329    /** Stores the combiner. */
    330    private NodeCombiner nodeCombiner;
    331
    332    /** Stores a list with the contained configurations. */
    333    private List<ConfigData> configurations;
    334
    335    /** Stores a map with the named configurations. */
    336    private Map<String, Configuration> namedConfigurations;
    337
    338    /**
    339     * An expression engine used for converting child configurations to hierarchical ones.
    340     */
    341    private ExpressionEngine conversionExpressionEngine;
    342
    343    /** A flag whether this configuration is up-to-date. */
    344    private boolean upToDate;
    345
    346    /**
    347     * Creates a new instance of {@code CombinedConfiguration} that uses a union combiner.
    348     *
    349     * @see org.apache.commons.configuration2.tree.UnionCombiner
    350     */
    351    public CombinedConfiguration() {
    352        this(null);
    353    }
    354
    355    /**
    356     * Creates a new instance of {@code CombinedConfiguration} and initializes the combiner to be used.
    357     *
    358     * @param comb the node combiner (can be <strong>null</strong>, then a union combiner is used as default)
    359     */
    360    public CombinedConfiguration(final NodeCombiner comb) {
    361        nodeCombiner = comb != null ? comb : DEFAULT_COMBINER;
    362        initChildCollections();
    363    }
    364
    365    /**
    366     * Adds a new configuration to this combined configuration. The new configuration is not given a name. Its properties
    367     * will be added under the root of the combined node structure.
    368     *
    369     * @param config the configuration to add (must not be <strong>null</strong>)
    370     */
    371    public void addConfiguration(final Configuration config) {
    372        addConfiguration(config, null, null);
    373    }
    374
    375    /**
    376     * Adds a new configuration to this combined configuration with an optional name. The new configuration's properties
    377     * will be added under the root of the combined node structure.
    378     *
    379     * @param config the configuration to add (must not be <strong>null</strong>)
    380     * @param name the name of this configuration (can be <strong>null</strong>)
    381     */
    382    public void addConfiguration(final Configuration config, final String name) {
    383        addConfiguration(config, name, null);
    384    }
    385
    386    /**
    387     * Adds a new configuration to this combined configuration. It is possible (but not mandatory) to give the new
    388     * configuration a name. This name must be unique, otherwise a {@code ConfigurationRuntimeException} will be thrown.
    389     * With the optional {@code at} argument you can specify where in the resulting node structure the content of the added
    390     * configuration should appear. This is a string that uses dots as property delimiters (independent on the current
    391     * expression engine). For instance if you pass in the string {@code "database.tables"}, all properties of the added
    392     * configuration will occur in this branch.
    393     *
    394     * @param config the configuration to add (must not be <strong>null</strong>)
    395     * @param name the name of this configuration (can be <strong>null</strong>)
    396     * @param at the position of this configuration in the combined tree (can be <strong>null</strong>)
    397     */
    398    public void addConfiguration(final Configuration config, final String name, final String at) {
    399        if (config == null) {
    400            throw new IllegalArgumentException("Added configuration must not be null!");
    401        }
    402
    403        beginWrite(true);
    404        try {
    405            if (name != null && namedConfigurations.containsKey(name)) {
    406                throw new ConfigurationRuntimeException("A configuration with the name '" + name + "' already exists in this combined configuration!");
    407            }
    408
    409            final ConfigData cd = new ConfigData(config, name, at);
    410            if (getLogger().isDebugEnabled()) {
    411                getLogger().debug("Adding configuration " + config + " with name " + name);
    412            }
    413            configurations.add(cd);
    414            if (name != null) {
    415                namedConfigurations.put(name, config);
    416            }
    417
    418            invalidateInternal();
    419        } finally {
    420            endWrite();
    421        }
    422        registerListenerAt(config);
    423    }
    424
    425    /**
    426     * {@inheritDoc} This implementation checks whether a combined root node is available. If not, it is constructed by
    427     * requesting a write lock.
    428     */
    429    @Override
    430    protected void beginRead(final boolean optimize) {
    431        if (optimize) {
    432            // just need a lock, don't construct configuration
    433            super.beginRead(true);
    434            return;
    435        }
    436
    437        boolean lockObtained = false;
    438        do {
    439            super.beginRead(false);
    440            if (isUpToDate()) {
    441                lockObtained = true;
    442            } else {
    443                // release read lock and try to obtain a write lock
    444                endRead();
    445                beginWrite(false); // this constructs the root node
    446                endWrite();
    447            }
    448        } while (!lockObtained);
    449    }
    450
    451    /**
    452     * {@inheritDoc} This implementation checks whether a combined root node is available. If not, it is constructed now.
    453     */
    454    @Override
    455    protected void beginWrite(final boolean optimize) {
    456        super.beginWrite(true);
    457        if (optimize) {
    458            // just need a lock, don't construct configuration
    459            return;
    460        }
    461
    462        boolean success = false;
    463        try {
    464            if (!isUpToDate()) {
    465                getSubConfigurationParentModel().replaceRoot(constructCombinedNode(), this);
    466                upToDate = true;
    467            }
    468            success = true;
    469        } finally {
    470            if (!success) {
    471                endWrite();
    472            }
    473        }
    474    }
    475
    476    /**
    477     * Clears this configuration. All contained configurations will be removed.
    478     */
    479    @Override
    480    protected void clearInternal() {
    481        unregisterListenerAtChildren();
    482        initChildCollections();
    483        invalidateInternal();
    484    }
    485
    486    /**
    487     * Returns a copy of this object. This implementation performs a deep clone, i.e. all contained configurations will be
    488     * cloned, too. For this to work, all contained configurations must be cloneable. Registered event listeners won't be
    489     * cloned. The clone will use the same node combiner than the original.
    490     *
    491     * @return the copied object
    492     */
    493    @Override
    494    public Object clone() {
    495        beginRead(false);
    496        try {
    497            final CombinedConfiguration copy = (CombinedConfiguration) super.clone();
    498            copy.initChildCollections();
    499            configurations.forEach(cd -> copy.addConfiguration(ConfigurationUtils.cloneConfiguration(cd.getConfiguration()), cd.getName(), cd.getAt()));
    500
    501            return copy;
    502        } finally {
    503            endRead();
    504        }
    505    }
    506
    507    /**
    508     * Creates the root node of this combined configuration.
    509     *
    510     * @return the combined root node
    511     */
    512    private ImmutableNode constructCombinedNode() {
    513        if (getNumberOfConfigurationsInternal() < 1) {
    514            if (getLogger().isDebugEnabled()) {
    515                getLogger().debug("No configurations defined for " + this);
    516            }
    517            return EMPTY_ROOT;
    518        }
    519        final Iterator<ConfigData> it = configurations.iterator();
    520        ImmutableNode node = it.next().getTransformedRoot();
    521        while (it.hasNext()) {
    522            node = nodeCombiner.combine(node, it.next().getTransformedRoot());
    523        }
    524        if (getLogger().isDebugEnabled()) {
    525            final ByteArrayOutputStream os = new ByteArrayOutputStream();
    526            final PrintStream stream = new PrintStream(os);
    527            TreeUtils.printTree(stream, node);
    528            getLogger().debug(os.toString());
    529        }
    530        return node;
    531    }
    532
    533    /**
    534     * Determines the configurations to which the specified node belongs. This is done by inspecting the nodes structures of
    535     * all child configurations.
    536     *
    537     * @param node the node
    538     * @return a set with the owning configurations
    539     */
    540    private Set<Configuration> findSourceConfigurations(final ImmutableNode node) {
    541        final Set<Configuration> result = new HashSet<>();
    542        final FindNodeVisitor<ImmutableNode> visitor = new FindNodeVisitor<>(node);
    543
    544        configurations.forEach(cd -> {
    545            NodeTreeWalker.INSTANCE.walkBFS(cd.getRootNode(), visitor, getModel().getNodeHandler());
    546            if (visitor.isFound()) {
    547                result.add(cd.getConfiguration());
    548                visitor.reset();
    549            }
    550        });
    551
    552        return result;
    553    }
    554
    555    /**
    556     * Gets the configuration at the specified index. The contained configurations are numbered in the order they were
    557     * added to this combined configuration. The index of the first configuration is 0.
    558     *
    559     * @param index the index
    560     * @return the configuration at this index
    561     */
    562    public Configuration getConfiguration(final int index) {
    563        beginRead(true);
    564        try {
    565            final ConfigData cd = configurations.get(index);
    566            return cd.getConfiguration();
    567        } finally {
    568            endRead();
    569        }
    570    }
    571
    572    /**
    573     * Gets the configuration with the given name. This can be <strong>null</strong> if no such configuration exists.
    574     *
    575     * @param name the name of the configuration
    576     * @return the configuration with this name
    577     */
    578    public Configuration getConfiguration(final String name) {
    579        beginRead(true);
    580        try {
    581            return namedConfigurations.get(name);
    582        } finally {
    583            endRead();
    584        }
    585    }
    586
    587    /**
    588     * Gets a List of the names of all the configurations that have been added in the order they were added. A NULL value
    589     * will be present in the list for each configuration that was added without a name.
    590     *
    591     * @return A List of all the configuration names.
    592     * @since 1.7
    593     */
    594    public List<String> getConfigurationNameList() {
    595        beginRead(true);
    596        try {
    597            return configurations.stream().map(ConfigData::getName).collect(Collectors.toList());
    598        } finally {
    599            endRead();
    600        }
    601    }
    602
    603    /**
    604     * Gets a set with the names of all configurations contained in this combined configuration. Of course here are only
    605     * these configurations listed, for which a name was specified when they were added.
    606     *
    607     * @return a set with the names of the contained configurations (never <strong>null</strong>)
    608     */
    609    public Set<String> getConfigurationNames() {
    610        beginRead(true);
    611        try {
    612            return namedConfigurations.keySet();
    613        } finally {
    614            endRead();
    615        }
    616    }
    617
    618    /**
    619     * Gets a List of all the configurations that have been added.
    620     *
    621     * @return A List of all the configurations.
    622     * @since 1.7
    623     */
    624    public List<Configuration> getConfigurations() {
    625        beginRead(true);
    626        try {
    627            return configurations.stream().map(ConfigData::getConfiguration).collect(Collectors.toList());
    628        } finally {
    629            endRead();
    630        }
    631    }
    632
    633    /**
    634     * Gets the {@code ExpressionEngine} for converting flat child configurations to hierarchical ones.
    635     *
    636     * @return the conversion expression engine
    637     * @since 1.6
    638     */
    639    public ExpressionEngine getConversionExpressionEngine() {
    640        beginRead(true);
    641        try {
    642            return conversionExpressionEngine;
    643        } finally {
    644            endRead();
    645        }
    646    }
    647
    648    /**
    649     * Gets the node combiner that is used for creating the combined node structure.
    650     *
    651     * @return the node combiner
    652     */
    653    public NodeCombiner getNodeCombiner() {
    654        beginRead(true);
    655        try {
    656            return nodeCombiner;
    657        } finally {
    658            endRead();
    659        }
    660    }
    661
    662    /**
    663     * Gets the number of configurations that are contained in this combined configuration.
    664     *
    665     * @return the number of contained configurations
    666     */
    667    public int getNumberOfConfigurations() {
    668        beginRead(true);
    669        try {
    670            return getNumberOfConfigurationsInternal();
    671        } finally {
    672            endRead();
    673        }
    674    }
    675
    676    /**
    677     * Gets the number of child configurations in this combined configuration. The internal list of child configurations
    678     * is accessed without synchronization.
    679     *
    680     * @return the number of child configurations
    681     */
    682    private int getNumberOfConfigurationsInternal() {
    683        return configurations.size();
    684    }
    685
    686    /**
    687     * Gets the configuration source, in which the specified key is defined. This method will determine the configuration
    688     * node that is identified by the given key. The following constellations are possible:
    689     * <ul>
    690     * <li>If no node object is found for this key, <strong>null</strong> is returned.</li>
    691     * <li>If the key maps to multiple nodes belonging to different configuration sources, a
    692     * {@code IllegalArgumentException} is thrown (in this case no unique source can be determined).</li>
    693     * <li>If exactly one node is found for the key, the (child) configuration object, to which the node belongs is
    694     * determined and returned.</li>
    695     * <li>For keys that have been added directly to this combined configuration and that do not belong to the namespaces
    696     * defined by existing child configurations this configuration will be returned.</li>
    697     * </ul>
    698     *
    699     * @param key the key of a configuration property
    700     * @return the configuration, to which this property belongs or <strong>null</strong> if the key cannot be resolved
    701     * @throws IllegalArgumentException if the key maps to multiple properties and the source cannot be determined, or if
    702     *         the key is <strong>null</strong>
    703     * @since 1.5
    704     */
    705    public Configuration getSource(final String key) {
    706        if (key == null) {
    707            throw new IllegalArgumentException("Key must not be null!");
    708        }
    709
    710        final Set<Configuration> sources = getSources(key);
    711        if (sources.isEmpty()) {
    712            return null;
    713        }
    714        final Iterator<Configuration> iterator = sources.iterator();
    715        final Configuration source = iterator.next();
    716        if (iterator.hasNext()) {
    717            throw new IllegalArgumentException("The key " + key + " is defined by multiple sources!");
    718        }
    719        return source;
    720    }
    721
    722    /**
    723     * Gets a set with the configuration sources, in which the specified key is defined. This method determines the
    724     * configuration nodes that are identified by the given key. It then determines the configuration sources to which these
    725     * nodes belong and adds them to the result set. Note the following points:
    726     * <ul>
    727     * <li>If no node object is found for this key, an empty set is returned.</li>
    728     * <li>For keys that have been added directly to this combined configuration and that do not belong to the namespaces
    729     * defined by existing child configurations this combined configuration is contained in the result set.</li>
    730     * </ul>
    731     *
    732     * @param key the key of a configuration property
    733     * @return a set with the configuration sources, which contain this property
    734     * @since 2.0
    735     */
    736    public Set<Configuration> getSources(final String key) {
    737        beginRead(false);
    738        try {
    739            final List<QueryResult<ImmutableNode>> results = fetchNodeList(key);
    740            final Set<Configuration> sources = new HashSet<>();
    741
    742            results.forEach(result -> {
    743                final Set<Configuration> resultSources = findSourceConfigurations(result.getNode());
    744                if (resultSources.isEmpty()) {
    745                    // key must be defined in combined configuration
    746                    sources.add(this);
    747                } else {
    748                    sources.addAll(resultSources);
    749                }
    750            });
    751
    752            return sources;
    753        } finally {
    754            endRead();
    755        }
    756    }
    757
    758    /**
    759     * Initializes internal data structures for storing information about child configurations.
    760     */
    761    private void initChildCollections() {
    762        configurations = new ArrayList<>();
    763        namedConfigurations = new HashMap<>();
    764    }
    765
    766    /**
    767     * Invalidates this combined configuration. This means that the next time a property is accessed the combined node
    768     * structure must be re-constructed. Invalidation of a combined configuration also means that an event of type
    769     * {@code EVENT_COMBINED_INVALIDATE} is fired. Note that while other events most times appear twice (once before and
    770     * once after an update), this event is only fired once (after update).
    771     */
    772    public void invalidate() {
    773        beginWrite(true);
    774        try {
    775            invalidateInternal();
    776        } finally {
    777            endWrite();
    778        }
    779    }
    780
    781    /**
    782     * Marks this configuration as invalid. This means that the next access re-creates the root node. An invalidate event is
    783     * also fired. Note: This implementation expects that an exclusive (write) lock is held on this instance.
    784     */
    785    private void invalidateInternal() {
    786        upToDate = false;
    787        fireEvent(COMBINED_INVALIDATE, null, null, false);
    788    }
    789
    790    /**
    791     * Returns a flag whether this configuration has been invalidated. This means that the combined nodes structure has to
    792     * be rebuilt before the configuration can be accessed.
    793     *
    794     * @return a flag whether this configuration is invalid
    795     */
    796    private boolean isUpToDate() {
    797        return upToDate;
    798    }
    799
    800    /**
    801     * Event listener call back for configuration update events. This method is called whenever one of the contained
    802     * configurations was modified. It invalidates this combined configuration.
    803     *
    804     * @param event the update event
    805     */
    806    @Override
    807    public void onEvent(final ConfigurationEvent event) {
    808        if (event.isBeforeUpdate()) {
    809            invalidate();
    810        }
    811    }
    812
    813    /**
    814     * Registers this combined configuration as listener at the given child configuration.
    815     *
    816     * @param configuration the child configuration
    817     */
    818    private void registerListenerAt(final Configuration configuration) {
    819        if (configuration instanceof EventSource) {
    820            ((EventSource) configuration).addEventListener(ConfigurationEvent.ANY, this);
    821        }
    822    }
    823
    824    /**
    825     * Removes the specified configuration from this combined configuration.
    826     *
    827     * @param config the configuration to be removed
    828     * @return a flag whether this configuration was found and could be removed
    829     */
    830    public boolean removeConfiguration(final Configuration config) {
    831        for (int index = 0; index < getNumberOfConfigurations(); index++) {
    832            if (configurations.get(index).getConfiguration() == config) {
    833                removeConfigurationAt(index);
    834                return true;
    835            }
    836        }
    837
    838        return false;
    839    }
    840
    841    /**
    842     * Removes the configuration with the specified name.
    843     *
    844     * @param name the name of the configuration to be removed
    845     * @return the removed configuration (<strong>null</strong> if this configuration was not found)
    846     */
    847    public Configuration removeConfiguration(final String name) {
    848        final Configuration conf = getConfiguration(name);
    849        if (conf != null) {
    850            removeConfiguration(conf);
    851        }
    852        return conf;
    853    }
    854
    855    /**
    856     * Removes the configuration at the specified index.
    857     *
    858     * @param index the index
    859     * @return the removed configuration
    860     */
    861    public Configuration removeConfigurationAt(final int index) {
    862        final ConfigData cd = configurations.remove(index);
    863        if (cd.getName() != null) {
    864            namedConfigurations.remove(cd.getName());
    865        }
    866        unregisterListenerAt(cd.getConfiguration());
    867        invalidateInternal();
    868        return cd.getConfiguration();
    869    }
    870
    871    /**
    872     * Sets the {@code ExpressionEngine} for converting flat child configurations to hierarchical ones. When constructing
    873     * the root node for this combined configuration the properties of all child configurations must be combined to a single
    874     * hierarchical node structure. In this process, non hierarchical configurations are converted to hierarchical ones
    875     * first. This can be problematic if a child configuration contains keys that are no compatible with the default
    876     * expression engine used by hierarchical configurations. Therefore it is possible to specify a specific expression
    877     * engine to be used for this purpose.
    878     *
    879     * @param conversionExpressionEngine the conversion expression engine
    880     * @see ConfigurationUtils#convertToHierarchical(Configuration, ExpressionEngine)
    881     * @since 1.6
    882     */
    883    public void setConversionExpressionEngine(final ExpressionEngine conversionExpressionEngine) {
    884        beginWrite(true);
    885        try {
    886            this.conversionExpressionEngine = conversionExpressionEngine;
    887        } finally {
    888            endWrite();
    889        }
    890    }
    891
    892    /**
    893     * Sets the node combiner. This object will be used when the combined node structure is to be constructed. It must not
    894     * be <strong>null</strong>, otherwise an {@code IllegalArgumentException} exception is thrown. Changing the node combiner causes
    895     * an invalidation of this combined configuration, so that the new combiner immediately takes effect.
    896     *
    897     * @param nodeCombiner the node combiner
    898     */
    899    public void setNodeCombiner(final NodeCombiner nodeCombiner) {
    900        if (nodeCombiner == null) {
    901            throw new IllegalArgumentException("Node combiner must not be null!");
    902        }
    903
    904        beginWrite(true);
    905        try {
    906            this.nodeCombiner = nodeCombiner;
    907            invalidateInternal();
    908        } finally {
    909            endWrite();
    910        }
    911    }
    912
    913    /**
    914     * Removes this combined configuration as listener from the given child configuration.
    915     *
    916     * @param configuration the child configuration
    917     */
    918    private void unregisterListenerAt(final Configuration configuration) {
    919        if (configuration instanceof EventSource) {
    920            ((EventSource) configuration).removeEventListener(ConfigurationEvent.ANY, this);
    921        }
    922    }
    923
    924    /**
    925     * Removes this combined configuration as listener from all child configurations. This method is called on a clear()
    926     * operation.
    927     */
    928    private void unregisterListenerAtChildren() {
    929        if (configurations != null) {
    930            configurations.forEach(child -> unregisterListenerAt(child.getConfiguration()));
    931        }
    932    }
    933}