    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.math.BigDecimal;
    021import java.math.BigInteger;
    022import java.time.Duration;
    023import java.util.ArrayList;
    024import java.util.Arrays;
    025import java.util.Collection;
    026import java.util.Collections;
    027import java.util.Iterator;
    028import java.util.List;
    029import java.util.Map;
    030import java.util.NoSuchElementException;
    031import java.util.Properties;
    032import java.util.concurrent.atomic.AtomicReference;
    033import java.util.stream.Collectors;
    034
    035import org.apache.commons.configuration2.convert.ConversionHandler;
    036import org.apache.commons.configuration2.convert.DefaultConversionHandler;
    037import org.apache.commons.configuration2.convert.DisabledListDelimiterHandler;
    038import org.apache.commons.configuration2.convert.ListDelimiterHandler;
    039import org.apache.commons.configuration2.event.BaseEventSource;
    040import org.apache.commons.configuration2.event.ConfigurationErrorEvent;
    041import org.apache.commons.configuration2.event.ConfigurationEvent;
    042import org.apache.commons.configuration2.event.EventListener;
    043import org.apache.commons.configuration2.ex.ConversionException;
    044import org.apache.commons.configuration2.interpol.ConfigurationInterpolator;
    045import org.apache.commons.configuration2.interpol.InterpolatorSpecification;
    046import org.apache.commons.configuration2.interpol.Lookup;
    047import org.apache.commons.configuration2.io.ConfigurationLogger;
    048import org.apache.commons.configuration2.sync.LockMode;
    049import org.apache.commons.configuration2.sync.NoOpSynchronizer;
    050import org.apache.commons.configuration2.sync.Synchronizer;
    051import org.apache.commons.lang3.ArrayUtils;
    052import org.apache.commons.lang3.ClassUtils;
    053import org.apache.commons.lang3.ObjectUtils;
    054import org.apache.commons.lang3.StringUtils;
    055
    056/**
    057 * <p>
    058 * Abstract configuration class. Provides basic functionality but does not store any data.
    059 * </p>
    060 * <p>
    061 * If you want to write your own Configuration class then you should implement only abstract methods from this class. A
    062 * lot of functionality needed by typical implementations of the {@code Configuration} interface is already provided by
    063 * this base class. Following is a list of features implemented here:
    064 * </p>
    065 * <ul>
    066 * <li>Data conversion support. The various data types required by the {@code Configuration} interface are already
    067 * handled by this base class. A concrete sub class only needs to provide a generic {@code getProperty()} method.</li>
    068 * <li>Support for variable interpolation. Property values containing special variable tokens (like {@code ${var}}) will
    069 * be replaced by their corresponding values.</li>
    070 * <li>Optional support for string lists. The values of properties to be added to this configuration are checked whether
    071 * they contain a list delimiter character. If this is the case and if list splitting is enabled, the string is split
    072 * and multiple values are added for this property. List splitting is controlled by a {@link ListDelimiterHandler}
    073 * object which can be set using the {@link #setListDelimiterHandler(ListDelimiterHandler)} method. It is disabled per
    074 * default. To enable this feature, set a suitable {@code ListDelimiterHandler}, e.g. an instance of
    075 * {@link org.apache.commons.configuration2.convert.DefaultListDelimiterHandler DefaultListDelimiterHandler} configured
    076 * with the desired list delimiter character.</li>
    077 * <li>Allows specifying how missing properties are treated. Per default the get methods returning an object will return
    078 * <b>null</b> if the searched property key is not found (and no default value is provided). With the
    079 * {@code setThrowExceptionOnMissing()} method this behavior can be changed to throw an exception when a requested
    080 * property cannot be found.</li>
    081 * <li>Basic event support. Whenever this configuration is modified registered event listeners are notified. Refer to
    082 * the various {@code EVENT_XXX} constants to get an impression about which event types are supported.</li>
    083 * <li>Support for proper synchronization based on the {@link Synchronizer} interface.</li>
    084 * </ul>
    085 * <p>
    086 * Most methods defined by the {@code Configuration} interface are already implemented in this class. Many method
    087 * implementations perform basic book-keeping tasks (e.g. firing events, handling synchronization), and then delegate to
    088 * other (protected) methods executing the actual work. Subclasses override these protected methods to define or adapt
    089 * behavior. The public entry point methods are final to prevent subclasses from breaking basic functionality.
    090 * </p>
    091 */
    092public abstract class AbstractConfiguration extends BaseEventSource implements Configuration {
    093
    094    /** The list delimiter handler. */
    095    private ListDelimiterHandler listDelimiterHandler;
    096
    097    /** The conversion handler. */
    098    private ConversionHandler conversionHandler;
    099
    100    /**
    101     * Whether the configuration should throw NoSuchElementExceptions or simply return null when a property does not exist.
    102     * Defaults to return null.
    103     */
    104    private boolean throwExceptionOnMissing;
    105
    106    /** Stores a reference to the object that handles variable interpolation. */
    107    private AtomicReference<ConfigurationInterpolator> interpolator;
    108
    109    /** The object responsible for synchronization. */
    110    private volatile Synchronizer synchronizer;
    111
    112    /** The object used for dealing with encoded property values. */
    113    private ConfigurationDecoder configurationDecoder;
    114
    115    /** Stores the logger. */
    116    private ConfigurationLogger log;
    117
    118    /**
    119     * Creates a new instance of {@code AbstractConfiguration}.
    120     */
    121    public AbstractConfiguration() {
    122        interpolator = new AtomicReference<>();
    123        initLogger(null);
    124        installDefaultInterpolator();
    125        listDelimiterHandler = DisabledListDelimiterHandler.INSTANCE;
    126        conversionHandler = DefaultConversionHandler.INSTANCE;
    127    }
    128
    129    /**
    130     * Gets the {@code ListDelimiterHandler} used by this instance.
    131     *
    132     * @return the {@code ListDelimiterHandler}
    133     * @since 2.0
    134     */
    135    public ListDelimiterHandler getListDelimiterHandler() {
    136        return listDelimiterHandler;
    137    }
    138
    139    /**
    140     * <p>
    141     * Sets the {@code ListDelimiterHandler} to be used by this instance. This object is invoked every time when dealing
    142     * with string properties that may contain a list delimiter and thus have to be split to multiple values. Per default, a
    143     * {@code ListDelimiterHandler} implementation is set which does not support list splitting. This can be changed for
    144     * instance by setting a {@link org.apache.commons.configuration2.convert.DefaultListDelimiterHandler
    145     * DefaultListDelimiterHandler} object.
    146     * </p>
    147     * <p>
    148     * <strong>Warning:</strong> Be careful when changing the list delimiter handler when the configuration has already been
    149     * loaded/populated. List handling is typically applied already when properties are added to the configuration. If later
    150     * another handler is set which processes lists differently, results may be unexpected; some operations may even cause
    151     * exceptions.
    152     * </p>
    153     *
    154     * @param listDelimiterHandler the {@code ListDelimiterHandler} to be used (must not be <b>null</b>)
    155     * @throws IllegalArgumentException if the {@code ListDelimiterHandler} is <b>null</b>
    156     * @since 2.0
    157     */
    158    public void setListDelimiterHandler(final ListDelimiterHandler listDelimiterHandler) {
    159        if (listDelimiterHandler == null) {
    160            throw new IllegalArgumentException("List delimiter handler must not be null!");
    161        }
    162        this.listDelimiterHandler = listDelimiterHandler;
    163    }
    164
    165    /**
    166     * Gets the {@code ConversionHandler} used by this instance.
    167     *
    168     * @return the {@code ConversionHandler}
    169     * @since 2.0
    170     */
    171    public ConversionHandler getConversionHandler() {
    172        return conversionHandler;
    173    }
    174
    175    /**
    176     * Sets the {@code ConversionHandler} to be used by this instance. The {@code ConversionHandler} is responsible for
    177     * every kind of data type conversion. It is consulted by all get methods returning results in specific data types. A
    178     * newly created configuration uses a default {@code ConversionHandler} implementation. This can be changed while
    179     * initializing the configuration (e.g. via a builder). Note that access to this property is not synchronized.
    180     *
    181     * @param conversionHandler the {@code ConversionHandler} to be used (must not be <b>null</b>)
    182     * @throws IllegalArgumentException if the {@code ConversionHandler} is <b>null</b>
    183     * @since 2.0
    184     */
    185    public void setConversionHandler(final ConversionHandler conversionHandler) {
    186        if (conversionHandler == null) {
    187            throw new IllegalArgumentException("ConversionHandler must not be null!");
    188        }
    189        this.conversionHandler = conversionHandler;
    190    }
    191
    192    /**
    193     * Allows to set the {@code throwExceptionOnMissing} flag. This flag controls the behavior of property getter methods
    194     * that return objects if the requested property is missing. If the flag is set to <b>false</b> (which is the default
    195     * value), these methods will return <b>null</b>. If set to <b>true</b>, they will throw a
    196     * {@code NoSuchElementException} exception. Note that getter methods for primitive data types are not affected by this
    197     * flag.
    198     *
    199     * @param throwExceptionOnMissing The new value for the property
    200     */
    201    public void setThrowExceptionOnMissing(final boolean throwExceptionOnMissing) {
    202        this.throwExceptionOnMissing = throwExceptionOnMissing;
    203    }
    204
    205    /**
    206     * Returns true if missing values throw Exceptions.
    207     *
    208     * @return true if missing values throw Exceptions
    209     */
    210    public boolean isThrowExceptionOnMissing() {
    211        return throwExceptionOnMissing;
    212    }
    213
    214    /**
    215     * Gets the {@code ConfigurationInterpolator} object that manages the lookup objects for resolving variables.
    216     * Unless a custom interpolator has been set or the instance has been modified, the returned interpolator will
    217     * resolve values from this configuration instance and support the
    218     * {@link ConfigurationInterpolator#getDefaultPrefixLookups() default prefix lookups}.
    219     *
    220     * @return the {@code ConfigurationInterpolator} associated with this configuration
    221     * @since 1.4
    222     * @see ConfigurationInterpolator#getDefaultPrefixLookups()
    223     */
    224    @Override
    225    public ConfigurationInterpolator getInterpolator() {
    226        return interpolator.get();
    227    }
    228
    229    /**
    230     * {@inheritDoc} This implementation sets the passed in object without further modifications. A <b>null</b> argument is
    231     * allowed; this disables interpolation.
    232     *
    233     * @since 2.0
    234     */
    235    @Override
    236    public final void setInterpolator(final ConfigurationInterpolator ci) {
    237        interpolator.set(ci);
    238    }
    239
    240    /**
    241     * {@inheritDoc} This implementation creates a new {@code ConfigurationInterpolator} instance and initializes it with
    242     * the given {@code Lookup} objects. In addition, it adds a specialized default {@code Lookup} object which queries this
    243     * {@code Configuration}.
    244     *
    245     * @since 2.0
    246     */
    247    @Override
    248    public final void installInterpolator(final Map<String, ? extends Lookup> prefixLookups, final Collection<? extends Lookup> defLookups) {
    249        final InterpolatorSpecification spec = new InterpolatorSpecification.Builder().withPrefixLookups(prefixLookups).withDefaultLookups(defLookups)
    250            .withDefaultLookup(new ConfigurationLookup(this)).create();
    251        setInterpolator(ConfigurationInterpolator.fromSpecification(spec));
    252    }
    253
    254    /**
    255     * Registers all {@code Lookup} objects in the given map at the current {@code ConfigurationInterpolator} of this
    256     * configuration. The set of default lookup objects (for variables without a prefix) is not modified by this method. If
    257     * this configuration does not have a {@code ConfigurationInterpolator}, a new instance is created. Note: This method is
    258     * mainly intended to be used for initializing a configuration when it is created by a builder. Normal client code
    259     * should better call {@link #installInterpolator(Map, Collection)} to define the {@code ConfigurationInterpolator} in a
    260     * single step.
    261     *
    262     * @param lookups a map with new {@code Lookup} objects and their prefixes (may be <b>null</b>)
    263     * @since 2.0
    264     */
    265    public void setPrefixLookups(final Map<String, ? extends Lookup> lookups) {
    266        boolean success;
    267        do {
    268            // do this in a loop because the ConfigurationInterpolator
    269            // instance may be changed by another thread
    270            final ConfigurationInterpolator ciOld = getInterpolator();
    271            final ConfigurationInterpolator ciNew = ciOld != null ? ciOld : new ConfigurationInterpolator();
    272            ciNew.registerLookups(lookups);
    273            success = interpolator.compareAndSet(ciOld, ciNew);
    274        } while (!success);
    275    }
    276
    277    /**
    278     * Adds all {@code Lookup} objects in the given collection as default lookups (i.e. lookups without a variable prefix)
    279     * to the {@code ConfigurationInterpolator} object of this configuration. In addition, it adds a specialized default
    280     * {@code Lookup} object which queries this {@code Configuration}. The set of {@code Lookup} objects with prefixes is
    281     * not modified by this method. If this configuration does not have a {@code ConfigurationInterpolator}, a new instance
    282     * is created. Note: This method is mainly intended to be used for initializing a configuration when it is created by a
    283     * builder. Normal client code should better call {@link #installInterpolator(Map, Collection)} to define the
    284     * {@code ConfigurationInterpolator} in a single step.
    285     *
    286     * @param lookups the collection with default {@code Lookup} objects to be added
    287     * @since 2.0
    288     */
    289    public void setDefaultLookups(final Collection<? extends Lookup> lookups) {
    290        boolean success;
    291        do {
    292            final ConfigurationInterpolator ciOld = getInterpolator();
    293            final ConfigurationInterpolator ciNew = ciOld != null ? ciOld : new ConfigurationInterpolator();
    294            Lookup confLookup = findConfigurationLookup(ciNew);
    295            if (confLookup == null) {
    296                confLookup = new ConfigurationLookup(this);
    297            } else {
    298                ciNew.removeDefaultLookup(confLookup);
    299            }
    300            ciNew.addDefaultLookups(lookups);
    301            ciNew.addDefaultLookup(confLookup);
    302            success = interpolator.compareAndSet(ciOld, ciNew);
    303        } while (!success);
    304    }
    305
    306    /**
    307     * Sets the specified {@code ConfigurationInterpolator} as the parent of this configuration's
    308     * {@code ConfigurationInterpolator}. If this configuration does not have a {@code ConfigurationInterpolator}, a new
    309     * instance is created. Note: This method is mainly intended to be used for initializing a configuration when it is
    310     * created by a builder. Normal client code can directly update the {@code ConfigurationInterpolator}.
    311     *
    312     * @param parent the parent {@code ConfigurationInterpolator} to be set
    313     * @since 2.0
    314     */
    315    public void setParentInterpolator(final ConfigurationInterpolator parent) {
    316        boolean success;
    317        do {
    318            final ConfigurationInterpolator ciOld = getInterpolator();
    319            final ConfigurationInterpolator ciNew = ciOld != null ? ciOld : new ConfigurationInterpolator();
    320            ciNew.setParentInterpolator(parent);
    321            success = interpolator.compareAndSet(ciOld, ciNew);
    322        } while (!success);
    323    }
    324
    325    /**
    326     * Sets the {@code ConfigurationDecoder} for this configuration. This object is used by
    327     * {@link #getEncodedString(String)}.
    328     *
    329     * @param configurationDecoder the {@code ConfigurationDecoder}
    330     * @since 2.0
    331     */
    332    public void setConfigurationDecoder(final ConfigurationDecoder configurationDecoder) {
    333        this.configurationDecoder = configurationDecoder;
    334    }
    335
    336    /**
    337     * Gets the {@code ConfigurationDecoder} used by this instance.
    338     *
    339     * @return the {@code ConfigurationDecoder}
    340     * @since 2.0
    341     */
    342    public ConfigurationDecoder getConfigurationDecoder() {
    343        return configurationDecoder;
    344    }
    345
    346    /**
    347     * Creates a clone of the {@code ConfigurationInterpolator} used by this instance. This method can be called by
    348     * {@code clone()} implementations of derived classes. Normally, the {@code ConfigurationInterpolator} of a
    349     * configuration instance must not be shared with other instances because it contains a specific {@code Lookup} object
    350     * pointing to the owning configuration. This has to be taken into account when cloning a configuration. This method
    351     * creates a new {@code ConfigurationInterpolator} for this configuration instance which contains all lookup objects
    352     * from the original {@code ConfigurationInterpolator} except for the configuration specific lookup pointing to the
    353     * passed in original configuration. This one is replaced by a corresponding {@code Lookup} referring to this
    354     * configuration.
    355     *
    356     * @param orgConfig the original configuration from which this one was cloned
    357     * @since 2.0
    358     */
    359    protected void cloneInterpolator(final AbstractConfiguration orgConfig) {
    360        interpolator = new AtomicReference<>();
    361        final ConfigurationInterpolator orgInterpolator = orgConfig.getInterpolator();
    362        final List<Lookup> defaultLookups = orgInterpolator.getDefaultLookups();
    363        final Lookup lookup = findConfigurationLookup(orgInterpolator, orgConfig);
    364        if (lookup != null) {
    365            defaultLookups.remove(lookup);
    366        }
    367
    368        installInterpolator(orgInterpolator.getLookups(), defaultLookups);
    369    }
    370
    371    /**
    372     * Creates a default {@code ConfigurationInterpolator} which is initialized with all default {@code Lookup} objects.
    373     * This method is called by the constructor. It ensures that default interpolation works for every new configuration
    374     * instance.
    375     */
    376    private void installDefaultInterpolator() {
    377        installInterpolator(ConfigurationInterpolator.getDefaultPrefixLookups(), null);
    378    }
    379
    380    /**
    381     * Finds a {@code ConfigurationLookup} pointing to this configuration in the default lookups of the specified
    382     * {@code ConfigurationInterpolator}. This method is called to ensure that there is exactly one default lookup querying
    383     * this configuration.
    384     *
    385     * @param ci the {@code ConfigurationInterpolator} in question
    386     * @return the found {@code Lookup} object or <b>null</b>
    387     */
    388    private Lookup findConfigurationLookup(final ConfigurationInterpolator ci) {
    389        return findConfigurationLookup(ci, this);
    390    }
    391
    392    /**
    393     * Finds a {@code ConfigurationLookup} pointing to the specified configuration in the default lookups for the specified
    394     * {@code ConfigurationInterpolator}.
    395     *
    396     * @param ci the {@code ConfigurationInterpolator} in question
    397     * @param targetConf the target configuration of the searched lookup
    398     * @return the found {@code Lookup} object or <b>null</b>
    399     */
    400    private static Lookup findConfigurationLookup(final ConfigurationInterpolator ci, final ImmutableConfiguration targetConf) {
    401        for (final Lookup l : ci.getDefaultLookups()) {
    402            if (l instanceof ConfigurationLookup && targetConf == ((ConfigurationLookup) l).getConfiguration()) {
    403                return l;
    404            }
    405        }
    406        return null;
    407    }
    408
    409    /**
    410     * Gets the logger used by this configuration object.
    411     *
    412     * @return the logger
    413     * @since 2.0
    414     */
    415    public ConfigurationLogger getLogger() {
    416        return log;
    417    }
    418
    419    /**
    420     * Allows setting the logger to be used by this configuration object. This method makes it possible for clients to
    421     * exactly control logging behavior. Per default a logger is set that will ignore all log messages. Derived classes that
    422     * want to enable logging should call this method during their initialization with the logger to be used. It is legal to
    423     * pass a <b>null</b> logger; in this case, logging will be disabled.
    424     *
    425     * @param log the new logger
    426     * @since 2.0
    427     */
    428    public void setLogger(final ConfigurationLogger log) {
    429        initLogger(log);
    430    }
    431
    432    /**
    433     * Adds a special {@link EventListener} object to this configuration that will log all internal errors. This method is
    434     * intended to be used by certain derived classes, for which it is known that they can fail on property access (e.g.
    435     * {@code DatabaseConfiguration}).
    436     *
    437     * @since 1.4
    438     */
    439    public final void addErrorLogListener() {
    440        addEventListener(ConfigurationErrorEvent.ANY, event -> getLogger().warn("Internal error", event.getCause()));
    441    }
    442
    443    /**
    444     * Gets the object responsible for synchronizing this configuration. All access to this configuration - both read and
    445     * write access - is controlled by this object. This implementation never returns <b>null</b>. If no
    446     * {@code Synchronizer} has been set, a {@link NoOpSynchronizer} is returned. So, per default, instances of
    447     * {@code AbstractConfiguration} are not thread-safe unless a suitable {@code Synchronizer} is set!
    448     *
    449     * @return the {@code Synchronizer} used by this instance
    450     * @since 2.0
    451     */
    452    @Override
    453    public final Synchronizer getSynchronizer() {
    454        final Synchronizer sync = synchronizer;
    455        return sync != null ? sync : NoOpSynchronizer.INSTANCE;
    456    }
    457
    458    /**
    459     * Sets the object responsible for synchronizing this configuration. This method has to be called with a suitable
    460     * {@code Synchronizer} object when initializing this configuration instance in order to make it thread-safe.
    461     *
    462     * @param synchronizer the new {@code Synchronizer}; can be <b>null</b>, then this instance uses a
    463     *        {@link NoOpSynchronizer}
    464     * @since 2.0
    465     */
    466    @Override
    467    public final void setSynchronizer(final Synchronizer synchronizer) {
    468        this.synchronizer = synchronizer;
    469    }
    470
    471    /**
    472     * {@inheritDoc} This implementation delegates to {@code beginRead()} or {@code beginWrite()}, depending on the
    473     * {@code LockMode} argument. Subclasses can override these protected methods to perform additional steps when a
    474     * configuration is locked.
    475     *
    476     * @since 2.0
    477     * @throws NullPointerException if the argument is <b>null</b>
    478     */
    479    @Override
    480    public final void lock(final LockMode mode) {
    481        switch (mode) {
    482        case READ:
    483            beginRead(false);
    484            break;
    485        case WRITE:
    486            beginWrite(false);
    487            break;
    488        default:
    489            throw new IllegalArgumentException("Unsupported LockMode: " + mode);
    490        }
    491    }
    492
    493    /**
    494     * {@inheritDoc} This implementation delegates to {@code endRead()} or {@code endWrite()}, depending on the
    495     * {@code LockMode} argument. Subclasses can override these protected methods to perform additional steps when a
    496     * configuration's lock is released.
    497     *
    498     * @throws NullPointerException if the argument is <b>null</b>
    499     */
    500    @Override
    501    public final void unlock(final LockMode mode) {
    502        switch (mode) {
    503        case READ:
    504            endRead();
    505            break;
    506        case WRITE:
    507            endWrite();
    508            break;
    509        default:
    510            throw new IllegalArgumentException("Unsupported LockMode: " + mode);
    511        }
    512    }
    513
    514    /**
    515     * Notifies this configuration's {@link Synchronizer} that a read operation is about to start. This method is called by
    516     * all methods which access this configuration in a read-only mode. Subclasses may override it to perform additional
    517     * actions before this read operation. The boolean <em>optimize</em> argument can be evaluated by overridden methods in
    518     * derived classes. Some operations which require a lock do not need a fully initialized configuration object. By
    519     * setting this flag to <strong>true</strong>, such operations can give a corresponding hint. An overridden
    520     * implementation of {@code beginRead()} can then decide to skip some initialization steps. All basic operations in this
    521     * class (and most of the basic {@code Configuration} implementations) call this method with a parameter value of
    522     * <strong>false</strong>. <strong>In any case the inherited method must be called! Otherwise, proper synchronization is
    523     * not guaranteed.</strong>
    524     *
    525     * @param optimize a flag whether optimization can be performed
    526     * @since 2.0
    527     */
    528    protected void beginRead(final boolean optimize) {
    529        getSynchronizer().beginRead();
    530    }
    531
    532    /**
    533     * Notifies this configuration's {@link Synchronizer} that a read operation has finished. This method is called by all
    534     * methods which access this configuration in a read-only manner at the end of their execution. Subclasses may override
    535     * it to perform additional actions after this read operation. <strong>In any case the inherited method must be called!
    536     * Otherwise, the read lock will not be released.</strong>
    537     *
    538     * @since 2.0
    539     */
    540    protected void endRead() {
    541        getSynchronizer().endRead();
    542    }
    543
    544    /**
    545     * Notifies this configuration's {@link Synchronizer} that an update operation is about to start. This method is called
    546     * by all methods which modify this configuration. Subclasses may override it to perform additional operations before an
    547     * update. For a description of the boolean <em>optimize</em> argument refer to the documentation of
    548     * {@code beginRead()}. <strong>In any case the inherited method must be called! Otherwise, proper synchronization is
    549     * not guaranteed.</strong>
    550     *
    551     * @param optimize a flag whether optimization can be performed
    552     * @see #beginRead(boolean)
    553     * @since 2.0
    554     */
    555    protected void beginWrite(final boolean optimize) {
    556        getSynchronizer().beginWrite();
    557    }
    558
    559    /**
    560     * Notifies this configuration's {@link Synchronizer} that an update operation has finished. This method is called by
    561     * all methods which modify this configuration at the end of their execution. Subclasses may override it to perform
    562     * additional operations after an update. <strong>In any case the inherited method must be called! Otherwise, the write
    563     * lock will not be released.</strong>
    564     *
    565     * @since 2.0
    566     */
    567    protected void endWrite() {
    568        getSynchronizer().endWrite();
    569    }
    570
    571    @Override
    572    public final void addProperty(final String key, final Object value) {
    573        beginWrite(false);
    574        try {
    575            fireEvent(ConfigurationEvent.ADD_PROPERTY, key, value, true);
    576            addPropertyInternal(key, value);
    577            fireEvent(ConfigurationEvent.ADD_PROPERTY, key, value, false);
    578        } finally {
    579            endWrite();
    580        }
    581    }
    582
    583    /**
    584     * Actually adds a property to this configuration. This method is called by {@code addProperty()}. It performs list
    585     * splitting if necessary and delegates to {@link #addPropertyDirect(String, Object)} for every single property value.
    586     *
    587     * @param key the key of the property to be added
    588     * @param value the new property value
    589     * @since 2.0
    590     */
    591    protected void addPropertyInternal(final String key, final Object value) {
    592        getListDelimiterHandler().parse(value).forEach(obj -> addPropertyDirect(key, obj));
    593    }
    594
    595    /**
    596     * Adds a key/value pair to the Configuration. Override this method to provide write access to underlying Configuration
    597     * store.
    598     *
    599     * @param key key to use for mapping
    600     * @param value object to store
    601     */
    602    protected abstract void addPropertyDirect(String key, Object value);
    603
    604    /**
    605     * interpolate key names to handle ${key} stuff
    606     *
    607     * @param base string to interpolate
    608     *
    609     * @return returns the key name with the ${key} substituted
    610     */
    611    protected String interpolate(final String base) {
    612        final Object result = interpolate((Object) base);
    613        return result == null ? null : result.toString();
    614    }
    615
    616    /**
    617     * Returns the interpolated value. This implementation delegates to the current {@code ConfigurationInterpolator}. If no
    618     * {@code ConfigurationInterpolator} is set, the passed in value is returned without changes.
    619     *
    620     * @param value the value to interpolate
    621     * @return the value with variables substituted
    622     */
    623    protected Object interpolate(final Object value) {
    624        final ConfigurationInterpolator ci = getInterpolator();
    625        return ci != null ? ci.interpolate(value) : value;
    626    }
    627
    628    @Override
    629    public Configuration subset(final String prefix) {
    630        return new SubsetConfiguration(this, prefix, ".");
    631    }
    632
    633    @Override
    634    public ImmutableConfiguration immutableSubset(final String prefix) {
    635        return ConfigurationUtils.unmodifiableConfiguration(subset(prefix));
    636    }
    637
    638    @Override
    639    public final void setProperty(final String key, final Object value) {
    640        beginWrite(false);
    641        try {
    642            fireEvent(ConfigurationEvent.SET_PROPERTY, key, value, true);
    643            setPropertyInternal(key, value);
    644            fireEvent(ConfigurationEvent.SET_PROPERTY, key, value, false);
    645        } finally {
    646            endWrite();
    647        }
    648    }
    649
    650    /**
    651     * Actually sets the value of a property. This method is called by {@code setProperty()}. It provides a default
    652     * implementation of this functionality by clearing the specified key and delegating to {@code addProperty()}.
    653     * Subclasses should override this method if they can provide a more efficient algorithm for setting a property value.
    654     *
    655     * @param key the property key
    656     * @param value the new property value
    657     * @since 2.0
    658     */
    659    protected void setPropertyInternal(final String key, final Object value) {
    660        setDetailEvents(false);
    661        try {
    662            clearProperty(key);
    663            addProperty(key, value);
    664        } finally {
    665            setDetailEvents(true);
    666        }
    667    }
    668
    669    /**
    670     * Removes the specified property from this configuration. This implementation performs some preparations and then
    671     * delegates to {@code clearPropertyDirect()}, which will do the real work.
    672     *
    673     * @param key the key to be removed
    674     */
    675    @Override
    676    public final void clearProperty(final String key) {
    677        beginWrite(false);
    678        try {
    679            fireEvent(ConfigurationEvent.CLEAR_PROPERTY, key, null, true);
    680            clearPropertyDirect(key);
    681            fireEvent(ConfigurationEvent.CLEAR_PROPERTY, key, null, false);
    682        } finally {
    683            endWrite();
    684        }
    685    }
    686
    687    /**
    688     * Removes the specified property from this configuration. This method is called by {@code clearProperty()} after it has
    689     * done some preparations. It must be overridden in sub classes.
    690     *
    691     * @param key the key to be removed
    692     */
    693    protected abstract void clearPropertyDirect(String key);
    694
    695    @Override
    696    public final void clear() {
    697        beginWrite(false);
    698        try {
    699            fireEvent(ConfigurationEvent.CLEAR, null, null, true);
    700            clearInternal();
    701            fireEvent(ConfigurationEvent.CLEAR, null, null, false);
    702        } finally {
    703            endWrite();
    704        }
    705    }
    706
    707    /**
    708     * Clears the whole configuration. This method is called by {@code clear()} after some preparations have been made. This
    709     * base implementation uses the iterator provided by {@code getKeys()} to remove every single property. Subclasses
    710     * should override this method if there is a more efficient way of clearing the configuration.
    711     */
    712    protected void clearInternal() {
    713        setDetailEvents(false);
    714        boolean useIterator = true;
    715        try {
    716            final Iterator<String> it = getKeys();
    717            while (it.hasNext()) {
    718                final String key = it.next();
    719                if (useIterator) {
    720                    try {
    721                        it.remove();
    722                    } catch (final UnsupportedOperationException usoex) {
    723                        useIterator = false;
    724                    }
    725                }
    726
    727                if (useIterator && containsKey(key)) {
    728                    useIterator = false;
    729                }
    730
    731                if (!useIterator) {
    732                    // workaround for Iterators that do not remove the
    733                    // property
    734                    // on calling remove() or do not support remove() at all
    735                    clearProperty(key);
    736                }
    737            }
    738        } finally {
    739            setDetailEvents(true);
    740        }
    741    }
    742
    743    /**
    744     * {@inheritDoc} This implementation takes care of synchronization and then delegates to {@code getKeysInternal()} for
    745     * obtaining the actual iterator. Note that depending on a concrete implementation, an iteration may fail if the
    746     * configuration is updated concurrently.
    747     */
    748    @Override
    749    public final Iterator<String> getKeys() {
    750        beginRead(false);
    751        try {
    752            return getKeysInternal();
    753        } finally {
    754            endRead();
    755        }
    756    }
    757
    758    /**
    759     * {@inheritDoc} This implementation returns keys that either match the prefix or start with the prefix followed by a
    760     * dot ('.'). So the call {@code getKeys("db");} will find the keys {@code db}, {@code db.user}, or {@code db.password},
    761     * but not the key {@code dbdriver}.
    762     */
    763    @Override
    764    public final Iterator<String> getKeys(final String prefix) {
    765        beginRead(false);
    766        try {
    767            return getKeysInternal(prefix);
    768        } finally {
    769            endRead();
    770        }
    771    }
    772
    773    /**
    774     * Actually creates an iterator for iterating over the keys in this configuration. This method is called by
    775     * {@code getKeys()}, it has to be defined by concrete subclasses.
    776     *
    777     * @return an {@code Iterator} with all property keys in this configuration
    778     * @since 2.0
    779     */
    780    protected abstract Iterator<String> getKeysInternal();
    781
    782    /**
    783     * Gets an {@code Iterator} with all property keys starting with the specified prefix. This method is called by
    784     * {@link #getKeys(String)}. It is fully implemented by delegating to {@code getKeysInternal()} and returning a special
    785     * iterator which filters for the passed in prefix. Subclasses can override it if they can provide a more efficient way
    786     * to iterate over specific keys only.
    787     *
    788     * @param prefix the prefix for the keys to be taken into account
    789     * @return an {@code Iterator} returning the filtered keys
    790     * @since 2.0
    791     */
    792    protected Iterator<String> getKeysInternal(final String prefix) {
    793        return new PrefixedKeysIterator(getKeysInternal(), prefix);
    794    }
    795
    796    /**
    797     * {@inheritDoc} This implementation ensures proper synchronization. Subclasses have to define the abstract
    798     * {@code getPropertyInternal()} method which is called from here.
    799     */
    800    @Override
    801    public final Object getProperty(final String key) {
    802        beginRead(false);
    803        try {
    804            return getPropertyInternal(key);
    805        } finally {
    806            endRead();
    807        }
    808    }
    809
    810    /**
    811     * Actually obtains the value of the specified property. This method is called by {@code getProperty()}. Concrete
    812     * subclasses must define it to fetch the value of the desired property.
    813     *
    814     * @param key the key of the property in question
    815     * @return the (raw) value of this property
    816     * @since 2.0
    817     */
    818    protected abstract Object getPropertyInternal(String key);
    819
    820    /**
    821     * {@inheritDoc} This implementation handles synchronization and delegates to {@code isEmptyInternal()}.
    822     */
    823    @Override
    824    public final boolean isEmpty() {
    825        beginRead(false);
    826        try {
    827            return isEmptyInternal();
    828        } finally {
    829            endRead();
    830        }
    831    }
    832
    833    /**
    834     * Actually checks whether this configuration contains data. This method is called by {@code isEmpty()}. It has to be
    835     * defined by concrete subclasses.
    836     *
    837     * @return <b>true</b> if this configuration contains no data, <b>false</b> otherwise
    838     * @since 2.0
    839     */
    840    protected abstract boolean isEmptyInternal();
    841
    842    /**
    843     * {@inheritDoc} This implementation handles synchronization and delegates to {@code sizeInternal()}.
    844     */
    845    @Override
    846    public final int size() {
    847        beginRead(false);
    848        try {
    849            return sizeInternal();
    850        } finally {
    851            endRead();
    852        }
    853    }
    854
    855    /**
    856     * Actually calculates the size of this configuration. This method is called by {@code size()} with a read lock held.
    857     * The base implementation provided here calculates the size based on the iterator returned by {@code getKeys()}. Sub
    858     * classes which can determine the size in a more efficient way should override this method.
    859     *
    860     * @return the size of this configuration (i.e. the number of keys)
    861     */
    862    protected int sizeInternal() {
    863        int size = 0;
    864        for (final Iterator<String> keyIt = getKeysInternal(); keyIt.hasNext(); size++) {
    865            keyIt.next();
    866        }
    867        return size;
    868    }
    869
    870    /**
    871     * {@inheritDoc} This implementation handles synchronization and delegates to {@code containsKeyInternal()}.
    872     */
    873    @Override
    874    public final boolean containsKey(final String key) {
    875        beginRead(false);
    876        try {
    877            return containsKeyInternal(key);
    878        } finally {
    879            endRead();
    880        }
    881    }
    882
    883    /**
    884     * Actually checks whether the specified key is contained in this configuration. This method is called by
    885     * {@code containsKey()}. It has to be defined by concrete subclasses.
    886     *
    887     * @param key the key in question
    888     * @return <b>true</b> if this key is contained in this configuration, <b>false</b> otherwise
    889     * @since 2.0
    890     */
    891    protected abstract boolean containsKeyInternal(String key);
    892
    893    @Override
    894    public Properties getProperties(final String key) {
    895        return getProperties(key, null);
    896    }
    897
    898    /**
    899     * Get a list of properties associated with the given configuration key.
    900     *
    901     * @param key The configuration key.
    902     * @param defaults Any default values for the returned {@code Properties} object. Ignored if {@code null}.
    903     *
    904     * @return The associated properties if key is found.
    905     *
    906     * @throws ConversionException is thrown if the key maps to an object that is not a String/List of Strings.
    907     *
    908     * @throws IllegalArgumentException if one of the tokens is malformed (does not contain an equals sign).
    909     */
    910    public Properties getProperties(final String key, final Properties defaults) {
    911        /*
    912         * Grab an array of the tokens for this key.
    913         */
    914        final String[] tokens = getStringArray(key);
    915
    916        /*
    917         * Each token is of the form 'key=value'.
    918         */
    919        final Properties props = defaults == null ? new Properties() : new Properties(defaults);
    920        for (final String token : tokens) {
    921            final int equalSign = token.indexOf('=');
    922            if (equalSign > 0) {
    923                final String pkey = token.substring(0, equalSign).trim();
    924                final String pvalue = token.substring(equalSign + 1).trim();
    925                props.put(pkey, pvalue);
    926            } else if (tokens.length == 1 && StringUtils.isEmpty(key)) {
    927                // Semantically equivalent to an empty Properties
    928                // object.
    929                break;
    930            } else {
    931                throw new IllegalArgumentException('\'' + token + "' does not contain an equals sign");
    932            }
    933        }
    934        return props;
    935    }
    936
    937    @Override
    938    public boolean getBoolean(final String key) {
    939        final Boolean b = convert(Boolean.class, key, null, true);
    940        return checkNonNullValue(key, b).booleanValue();
    941    }
    942
    943    @Override
    944    public boolean getBoolean(final String key, final boolean defaultValue) {
    945        return getBoolean(key, Boolean.valueOf(defaultValue)).booleanValue();
    946    }
    947
    948    /**
    949     * Obtains the value of the specified key and tries to convert it into a {@code Boolean} object. If the property has no
    950     * value, the passed in default value will be used.
    951     *
    952     * @param key the key of the property
    953     * @param defaultValue the default value
    954     * @return the value of this key converted to a {@code Boolean}
    955     * @throws ConversionException if the value cannot be converted to a {@code Boolean}
    956     */
    957    @Override
    958    public Boolean getBoolean(final String key, final Boolean defaultValue) {
    959        return convert(Boolean.class, key, defaultValue, false);
    960    }
    961
    962    @Override
    963    public byte getByte(final String key) {
    964        final Byte b = convert(Byte.class, key, null, true);
    965        return checkNonNullValue(key, b).byteValue();
    966    }
    967
    968    @Override
    969    public byte getByte(final String key, final byte defaultValue) {
    970        return getByte(key, Byte.valueOf(defaultValue)).byteValue();
    971    }
    972
    973    @Override
    974    public Byte getByte(final String key, final Byte defaultValue) {
    975        return convert(Byte.class, key, defaultValue, false);
    976    }
    977
    978    @Override
    979    public double getDouble(final String key) {
    980        final Double d = convert(Double.class, key, null, true);
    981        return checkNonNullValue(key, d).doubleValue();
    982    }
    983
    984    @Override
    985    public double getDouble(final String key, final double defaultValue) {
    986        return getDouble(key, Double.valueOf(defaultValue)).doubleValue();
    987    }
    988
    989    @Override
    990    public Double getDouble(final String key, final Double defaultValue) {
    991        return convert(Double.class, key, defaultValue, false);
    992    }
    993
    994    @Override
    995    public Duration getDuration(final String key) {
    996        return checkNonNullValue(key, convert(Duration.class, key, null, true));
    997    }
    998
    999    @Override
    1000    public Duration getDuration(final String key, final Duration defaultValue) {
    1001        return convert(Duration.class, key, defaultValue, false);
    1002    }
    1003
    1004    @Override
    1005    public float getFloat(final String key) {
    1006        final Float f = convert(Float.class, key, null, true);
    1007        return checkNonNullValue(key, f).floatValue();
    1008    }
    1009
    1010    @Override
    1011    public float getFloat(final String key, final float defaultValue) {
    1012        return getFloat(key, Float.valueOf(defaultValue)).floatValue();
    1013    }
    1014
    1015    @Override
    1016    public Float getFloat(final String key, final Float defaultValue) {
    1017        return convert(Float.class, key, defaultValue, false);
    1018    }
    1019
    1020    @Override
    1021    public int getInt(final String key) {
    1022        final Integer i = convert(Integer.class, key, null, true);
    1023        return checkNonNullValue(key, i).intValue();
    1024    }
    1025
    1026    @Override
    1027    public int getInt(final String key, final int defaultValue) {
    1028        return getInteger(key, Integer.valueOf(defaultValue)).intValue();
    1029    }
    1030
    1031    @Override
    1032    public Integer getInteger(final String key, final Integer defaultValue) {
    1033        return convert(Integer.class, key, defaultValue, false);
    1034    }
    1035
    1036    @Override
    1037    public long getLong(final String key) {
    1038        final Long l = convert(Long.class, key, null, true);
    1039        return checkNonNullValue(key, l).longValue();
    1040    }
    1041
    1042    @Override
    1043    public long getLong(final String key, final long defaultValue) {
    1044        return getLong(key, Long.valueOf(defaultValue)).longValue();
    1045    }
    1046
    1047    @Override
    1048    public Long getLong(final String key, final Long defaultValue) {
    1049        return convert(Long.class, key, defaultValue, false);
    1050    }
    1051
    1052    @Override
    1053    public short getShort(final String key) {
    1054        final Short s = convert(Short.class, key, null, true);
    1055        return checkNonNullValue(key, s).shortValue();
    1056    }
    1057
    1058    @Override
    1059    public short getShort(final String key, final short defaultValue) {
    1060        return getShort(key, Short.valueOf(defaultValue)).shortValue();
    1061    }
    1062
    1063    @Override
    1064    public Short getShort(final String key, final Short defaultValue) {
    1065        return convert(Short.class, key, defaultValue, false);
    1066    }
    1067
    1068    /**
    1069     * {@inheritDoc}
    1070     *
    1071     * @see #setThrowExceptionOnMissing(boolean)
    1072     */
    1073    @Override
    1074    public BigDecimal getBigDecimal(final String key) {
    1075        return convert(BigDecimal.class, key, null, true);
    1076    }
    1077
    1078    @Override
    1079    public BigDecimal getBigDecimal(final String key, final BigDecimal defaultValue) {
    1080        return convert(BigDecimal.class, key, defaultValue, false);
    1081    }
    1082
    1083    /**
    1084     * {@inheritDoc}
    1085     *
    1086     * @see #setThrowExceptionOnMissing(boolean)
    1087     */
    1088    @Override
    1089    public BigInteger getBigInteger(final String key) {
    1090        return convert(BigInteger.class, key, null, true);
    1091    }
    1092
    1093    @Override
    1094    public BigInteger getBigInteger(final String key, final BigInteger defaultValue) {
    1095        return convert(BigInteger.class, key, defaultValue, false);
    1096    }
    1097
    1098    /**
    1099     * {@inheritDoc}
    1100     *
    1101     * @see #setThrowExceptionOnMissing(boolean)
    1102     */
    1103    @Override
    1104    public String getString(final String key) {
    1105        return convert(String.class, key, null, true);
    1106    }
    1107
    1108    @Override
    1109    public String getString(final String key, final String defaultValue) {
    1110        final String result = convert(String.class, key, null, false);
    1111        return result != null ? result : interpolate(defaultValue);
    1112    }
    1113
    1114    /**
    1115     * {@inheritDoc} This implementation delegates to {@link #getString(String)} in order to obtain the value of the passed
    1116     * in key. This value is passed to the decoder. Because {@code getString()} is used behind the scenes all standard
    1117     * features like handling of missing keys and interpolation work as expected.
    1118     */
    1119    @Override
    1120    public String getEncodedString(final String key, final ConfigurationDecoder decoder) {
    1121        if (decoder == null) {
    1122            throw new IllegalArgumentException("ConfigurationDecoder must not be null!");
    1123        }
    1124
    1125        final String value = getString(key);
    1126        return value != null ? decoder.decode(value) : null;
    1127    }
    1128
    1129    /**
    1130     * {@inheritDoc} This implementation makes use of the {@code ConfigurationDecoder} set for this configuration. If no
    1131     * such object has been set, an {@code IllegalStateException} exception is thrown.
    1132     *
    1133     * @throws IllegalStateException if no {@code ConfigurationDecoder} is set
    1134     * @see #setConfigurationDecoder(ConfigurationDecoder)
    1135     */
    1136    @Override
    1137    public String getEncodedString(final String key) {
    1138        final ConfigurationDecoder decoder = getConfigurationDecoder();
    1139        if (decoder == null) {
    1140            throw new IllegalStateException("No default ConfigurationDecoder defined!");
    1141        }
    1142        return getEncodedString(key, decoder);
    1143    }
    1144
    1145    /**
    1146     * Get an array of strings associated with the given configuration key. If the key doesn't map to an existing object, an
    1147     * empty array is returned. When a property is added to a configuration, it is checked whether it contains multiple
    1148     * values. This is obvious if the added object is a list or an array. For strings the association
    1149     * {@link ListDelimiterHandler} is consulted to find out whether the string can be split into multiple values.
    1150     *
    1151     * @param key The configuration key.
    1152     * @return The associated string array if key is found.
    1153     *
    1154     * @throws ConversionException is thrown if the key maps to an object that is not a String/List of Strings.
    1155     * @see #setListDelimiterHandler(ListDelimiterHandler)
    1156     */
    1157    @Override
    1158    public String[] getStringArray(final String key) {
    1159        final String[] result = (String[]) getArray(String.class, key);
    1160        return result == null ? ArrayUtils.EMPTY_STRING_ARRAY : result;
    1161    }
    1162
    1163    /**
    1164     * {@inheritDoc}
    1165     *
    1166     * @see #getStringArray(String)
    1167     */
    1168    @Override
    1169    public List<Object> getList(final String key) {
    1170        return getList(key, new ArrayList<>());
    1171    }
    1172
    1173    @Override
    1174    public List<Object> getList(final String key, final List<?> defaultValue) {
    1175        final Object value = getProperty(key);
    1176        final List<Object> list;
    1177
    1178        if (value instanceof String) {
    1179            list = new ArrayList<>(1);
    1180            list.add(interpolate((String) value));
    1181        } else if (value instanceof List) {
    1182            list = new ArrayList<>();
    1183            final List<?> l = (List<?>) value;
    1184
    1185            // add the interpolated elements in the new list
    1186            l.forEach(elem -> list.add(interpolate(elem)));
    1187        } else if (value == null) {
    1188            // This is okay because we just return this list to the caller
    1189            @SuppressWarnings("unchecked")
    1190            final List<Object> resultList = (List<Object>) defaultValue;
    1191            list = resultList;
    1192        } else if (value.getClass().isArray()) {
    1193            return Arrays.asList((Object[]) value);
    1194        } else if (isScalarValue(value)) {
    1195            return Collections.singletonList((Object) value.toString());
    1196        } else {
    1197            throw new ConversionException('\'' + key + "' doesn't map to a List object: " + value + ", a " + value.getClass().getName());
    1198        }
    1199        return list;
    1200    }
    1201
    1202    @Override
    1203    public <T> T get(final Class<T> cls, final String key) {
    1204        return convert(cls, key, null, true);
    1205    }
    1206
    1207    /**
    1208     * {@inheritDoc} This implementation delegates to the {@link ConversionHandler} to perform the actual type conversion.
    1209     */
    1210    @Override
    1211    public <T> T get(final Class<T> cls, final String key, final T defaultValue) {
    1212        return convert(cls, key, defaultValue, false);
    1213    }
    1214
    1215    @Override
    1216    public Object getArray(final Class<?> cls, final String key) {
    1217        return getArray(cls, key, null);
    1218    }
    1219
    1220    /**
    1221     * {@inheritDoc} This implementation delegates to the {@link ConversionHandler} to perform the actual type conversion.
    1222     * If this results in a <b>null</b> result (because the property is undefined), the default value is returned. It is
    1223     * checked whether the default value is an array with the correct component type. If not, an exception is thrown.
    1224     *
    1225     * @throws IllegalArgumentException if the default value is not a compatible array
    1226     */
    1227    @Override
    1228    public Object getArray(final Class<?> cls, final String key, final Object defaultValue) {
    1229        return convertToArray(cls, key, defaultValue);
    1230    }
    1231
    1232    @Override
    1233    public <T> List<T> getList(final Class<T> cls, final String key) {
    1234        return getList(cls, key, null);
    1235    }
    1236
    1237    /**
    1238     * {@inheritDoc} This implementation delegates to the generic {@code getCollection()}. As target collection a newly
    1239     * created {@code ArrayList} is passed in.
    1240     */
    1241    @Override
    1242    public <T> List<T> getList(final Class<T> cls, final String key, final List<T> defaultValue) {
    1243        final List<T> result = new ArrayList<>();
    1244        if (getCollection(cls, key, result, defaultValue) == null) {
    1245            return null;
    1246        }
    1247        return result;
    1248    }
    1249
    1250    @Override
    1251    public <T> Collection<T> getCollection(final Class<T> cls, final String key, final Collection<T> target) {
    1252        return getCollection(cls, key, target, null);
    1253    }
    1254
    1255    /**
    1256     * {@inheritDoc} This implementation delegates to the {@link ConversionHandler} to perform the actual conversion. If no
    1257     * target collection is provided, an {@code ArrayList} is created.
    1258     */
    1259    @Override
    1260    public <T> Collection<T> getCollection(final Class<T> cls, final String key, final Collection<T> target, final Collection<T> defaultValue) {
    1261        final Object src = getProperty(key);
    1262        if (src == null) {
    1263            return handleDefaultCollection(target, defaultValue);
    1264        }
    1265
    1266        final Collection<T> targetCol = target != null ? target : new ArrayList<>();
    1267        getConversionHandler().toCollection(src, cls, getInterpolator(), targetCol);
    1268        return targetCol;
    1269    }
    1270
    1271    /**
    1272     * Checks whether the specified object is a scalar value. This method is called by {@code getList()} and
    1273     * {@code getStringArray()} if the property requested is not a string, a list, or an array. If it returns <b>true</b>,
    1274     * the calling method transforms the value to a string and returns a list or an array with this single element. This
    1275     * implementation returns <b>true</b> if the value is of a wrapper type for a primitive type.
    1276     *
    1277     * @param value the value to be checked
    1278     * @return a flag whether the value is a scalar
    1279     * @since 1.7
    1280     */
    1281    protected boolean isScalarValue(final Object value) {
    1282        return ClassUtils.wrapperToPrimitive(value.getClass()) != null;
    1283    }
    1284
    1285    /**
    1286     * Copies the content of the specified configuration into this configuration. If the specified configuration contains a
    1287     * key that is also present in this configuration, the value of this key will be replaced by the new value.
    1288     * <em>Note:</em> This method won't work well when copying hierarchical configurations because it is not able to copy
    1289     * information about the properties' structure (i.e. the parent-child-relationships will get lost). So when dealing with
    1290     * hierarchical configuration objects their {@link BaseHierarchicalConfiguration#clone() clone()} methods should be
    1291     * used.
    1292     *
    1293     * @param c the configuration to copy (can be <b>null</b>, then this operation will have no effect)
    1294     * @since 1.5
    1295     */
    1296    public void copy(final Configuration c) {
    1297        if (c != null) {
    1298            c.lock(LockMode.READ);
    1299            try {
    1300                c.getKeys().forEachRemaining(key -> setProperty(key, encodeForCopy(c.getProperty(key))));
    1301            } finally {
    1302                c.unlock(LockMode.READ);
    1303            }
    1304        }
    1305    }
    1306
    1307    /**
    1308     * Appends the content of the specified configuration to this configuration. The values of all properties contained in
    1309     * the specified configuration will be appended to this configuration. So if a property is already present in this
    1310     * configuration, its new value will be a union of the values in both configurations. <em>Note:</em> This method won't
    1311     * work well when appending hierarchical configurations because it is not able to copy information about the properties'
    1312     * structure (i.e. the parent-child-relationships will get lost). So when dealing with hierarchical configuration
    1313     * objects their {@link BaseHierarchicalConfiguration#clone() clone()} methods should be used.
    1314     *
    1315     * @param c the configuration to be appended (can be <b>null</b>, then this operation will have no effect)
    1316     * @since 1.5
    1317     */
    1318    public void append(final Configuration c) {
    1319        if (c != null) {
    1320            c.lock(LockMode.READ);
    1321            try {
    1322                c.getKeys().forEachRemaining(key -> addProperty(key, encodeForCopy(c.getProperty(key))));
    1323            } finally {
    1324                c.unlock(LockMode.READ);
    1325            }
    1326        }
    1327    }
    1328
    1329    /**
    1330     * Returns a configuration with the same content as this configuration, but with all variables replaced by their actual
    1331     * values. This method tries to clone the configuration and then perform interpolation on all properties. So property
    1332     * values of the form {@code ${var}} will be resolved as far as possible (if a variable cannot be resolved, it remains
    1333     * unchanged). This operation is useful if the content of a configuration is to be exported or processed by an external
    1334     * component that does not support variable interpolation.
    1335     *
    1336     * @return a configuration with all variables interpolated
    1337     * @throws org.apache.commons.configuration2.ex.ConfigurationRuntimeException if this configuration cannot be cloned
    1338     * @since 1.5
    1339     */
    1340    public Configuration interpolatedConfiguration() {
    1341        // first clone this configuration
    1342        final AbstractConfiguration c = (AbstractConfiguration) ConfigurationUtils.cloneConfiguration(this);
    1343
    1344        // now perform interpolation
    1345        c.setListDelimiterHandler(new DisabledListDelimiterHandler());
    1346        getKeys().forEachRemaining(key -> c.setProperty(key, getList(key)));
    1347        c.setListDelimiterHandler(getListDelimiterHandler());
    1348        return c;
    1349    }
    1350
    1351    /**
    1352     * Initializes the logger. Supports <b>null</b> input. This method can be called by derived classes in order to enable
    1353     * logging.
    1354     *
    1355     * @param log the logger
    1356     * @since 2.0
    1357     */
    1358    protected final void initLogger(final ConfigurationLogger log) {
    1359        this.log = log != null ? log : ConfigurationLogger.newDummyLogger();
    1360    }
    1361
    1362    /**
    1363     * Encodes a property value so that it can be added to this configuration. This method deals with list delimiters. The
    1364     * passed in object has to be escaped so that an add operation yields the same result. If it is a list, all of its
    1365     * values have to be escaped.
    1366     *
    1367     * @param value the value to be encoded
    1368     * @return the encoded value
    1369     */
    1370    private Object encodeForCopy(final Object value) {
    1371        if (value instanceof Collection) {
    1372            return encodeListForCopy((Collection<?>) value);
    1373        }
    1374        return getListDelimiterHandler().escape(value, ListDelimiterHandler.NOOP_TRANSFORMER);
    1375    }
    1376
    1377    /**
    1378     * Encodes a list with property values so that it can be added to this configuration. This method calls
    1379     * {@code encodeForCopy()} for all list elements.
    1380     *
    1381     * @param values the list to be encoded
    1382     * @return a list with encoded elements
    1383     */
    1384    private Object encodeListForCopy(final Collection<?> values) {
    1385        return values.stream().map(this::encodeForCopy).collect(Collectors.toList());
    1386    }
    1387
    1388    /**
    1389     * Obtains the property value for the specified key and converts it to the given target class.
    1390     *
    1391     * @param <T> the target type of the conversion
    1392     * @param cls the target class
    1393     * @param key the key of the desired property
    1394     * @param defaultValue a default value
    1395     * @return the converted value of this property
    1396     * @throws ConversionException if the conversion cannot be performed
    1397     */
    1398    private <T> T getAndConvertProperty(final Class<T> cls, final String key, final T defaultValue) {
    1399        final Object value = getProperty(key);
    1400        try {
    1401            return ObjectUtils.defaultIfNull(getConversionHandler().to(value, cls, getInterpolator()), defaultValue);
    1402        } catch (final ConversionException cex) {
    1403            // improve error message
    1404            throw new ConversionException(String.format("Key '%s' cannot be converted to class %s. Value is: '%s'.", key, cls.getName(), String.valueOf(value)),
    1405                cex.getCause());
    1406        }
    1407    }
    1408
    1409    /**
    1410     * Helper method for obtaining a property value with a type conversion.
    1411     *
    1412     * @param <T> the target type of the conversion
    1413     * @param cls the target class
    1414     * @param key the key of the desired property
    1415     * @param defValue a default value
    1416     * @param throwOnMissing a flag whether an exception should be thrown for a missing value
    1417     * @return the converted value
    1418     */
    1419    private <T> T convert(final Class<T> cls, final String key, final T defValue, final boolean throwOnMissing) {
    1420        if (cls.isArray()) {
    1421            return cls.cast(convertToArray(cls.getComponentType(), key, defValue));
    1422        }
    1423
    1424        final T result = getAndConvertProperty(cls, key, defValue);
    1425        if (result == null) {
    1426            if (throwOnMissing && isThrowExceptionOnMissing()) {
    1427                throwMissingPropertyException(key);
    1428            }
    1429            return defValue;
    1430        }
    1431
    1432        return result;
    1433    }
    1434
    1435    /**
    1436     * Performs a conversion to an array result class. This implementation delegates to the {@link ConversionHandler} to
    1437     * perform the actual type conversion. If this results in a <b>null</b> result (because the property is undefined), the
    1438     * default value is returned. It is checked whether the default value is an array with the correct component type. If
    1439     * not, an exception is thrown.
    1440     *
    1441     * @param cls the component class of the array
    1442     * @param key the configuration key
    1443     * @param defaultValue an optional default value
    1444     * @return the converted array
    1445     * @throws IllegalArgumentException if the default value is not a compatible array
    1446     */
    1447    private Object convertToArray(final Class<?> cls, final String key, final Object defaultValue) {
    1448        checkDefaultValueArray(cls, defaultValue);
    1449        return ObjectUtils.defaultIfNull(getConversionHandler().toArray(getProperty(key), cls, getInterpolator()), defaultValue);
    1450    }
    1451
    1452    /**
    1453     * Checks an object provided as default value for the {@code getArray()} method. Throws an exception if this is not an
    1454     * array with the correct component type.
    1455     *
    1456     * @param cls the component class for the array
    1457     * @param defaultValue the default value object to be checked
    1458     * @throws IllegalArgumentException if this is not a valid default object
    1459     */
    1460    private static void checkDefaultValueArray(final Class<?> cls, final Object defaultValue) {
    1461        if (defaultValue != null && (!defaultValue.getClass().isArray() || !cls.isAssignableFrom(defaultValue.getClass().getComponentType()))) {
    1462            throw new IllegalArgumentException(
    1463                "The type of the default value (" + defaultValue.getClass() + ")" + " is not an array of the specified class (" + cls + ")");
    1464        }
    1465    }
    1466
    1467    /**
    1468     * Handles the default collection for a collection conversion. This method fills the target collection with the content
    1469     * of the default collection. Both collections may be <b>null</b>.
    1470     *
    1471     * @param target the target collection
    1472     * @param defaultValue the default collection
    1473     * @return the initialized target collection
    1474     */
    1475    private static <T> Collection<T> handleDefaultCollection(final Collection<T> target, final Collection<T> defaultValue) {
    1476        if (defaultValue == null) {
    1477            return null;
    1478        }
    1479
    1480        final Collection<T> result;
    1481        if (target == null) {
    1482            result = new ArrayList<>(defaultValue);
    1483        } else {
    1484            target.addAll(defaultValue);
    1485            result = target;
    1486        }
    1487        return result;
    1488    }
    1489
    1490    /**
    1491     * Checks whether the specified value is <b>null</b> and throws an exception in this case. This method is used by
    1492     * conversion methods returning primitive Java types. Here values to be returned must not be <b>null</b>.
    1493     *
    1494     * @param <T> the type of the object to be checked
    1495     * @param key the key which caused the problem
    1496     * @param value the value to be checked
    1497     * @return the passed in value for chaining this method call
    1498     * @throws NoSuchElementException if the value is <b>null</b>
    1499     */
    1500    private static <T> T checkNonNullValue(final String key, final T value) {
    1501        if (value == null) {
    1502            throwMissingPropertyException(key);
    1503        }
    1504        return value;
    1505    }
    1506
    1507    /**
    1508     * Helper method for throwing an exception for a key that does not map to an existing object.
    1509     *
    1510     * @param key the key (to be part of the error message)
    1511     */
    1512    private static void throwMissingPropertyException(final String key) {
    1513        throw new NoSuchElementException(String.format("Key '%s' does not map to an existing object!", key));
    1514    }
    1515}