/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.commons.configuration2;

import java.math.BigDecimal;
import java.math.BigInteger;
import java.time.Duration;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Properties;
import java.util.concurrent.atomic.AtomicReference;
import java.util.stream.Collectors;

import org.apache.commons.configuration2.convert.ConversionHandler;
import org.apache.commons.configuration2.convert.DefaultConversionHandler;
import org.apache.commons.configuration2.convert.DisabledListDelimiterHandler;
import org.apache.commons.configuration2.convert.ListDelimiterHandler;
import org.apache.commons.configuration2.event.BaseEventSource;
import org.apache.commons.configuration2.event.ConfigurationErrorEvent;
import org.apache.commons.configuration2.event.ConfigurationEvent;
import org.apache.commons.configuration2.event.EventListener;
import org.apache.commons.configuration2.ex.ConversionException;
import org.apache.commons.configuration2.interpol.ConfigurationInterpolator;
import org.apache.commons.configuration2.interpol.InterpolatorSpecification;
import org.apache.commons.configuration2.interpol.Lookup;
import org.apache.commons.configuration2.io.ConfigurationLogger;
import org.apache.commons.configuration2.sync.LockMode;
import org.apache.commons.configuration2.sync.NoOpSynchronizer;
import org.apache.commons.configuration2.sync.Synchronizer;
import org.apache.commons.lang3.ArrayUtils;
import org.apache.commons.lang3.ClassUtils;
import org.apache.commons.lang3.ObjectUtils;
import org.apache.commons.lang3.StringUtils;

/**
 * <p>
 * Abstract configuration class. Provides basic functionality but does not store any data.
 * </p>
 * <p>
 * If you want to write your own Configuration class then you should implement only abstract methods from this class. A
 * lot of functionality needed by typical implementations of the {@code Configuration} interface is already provided by
 * this base class. Following is a list of features implemented here:
 * </p>
 * <ul>
 * <li>Data conversion support. The various data types required by the {@code Configuration} interface are already
 * handled by this base class. A concrete sub class only needs to provide a generic {@code getProperty()} method.</li>
 * <li>Support for variable interpolation. Property values containing special variable tokens (like {@code ${var}}) will
 * be replaced by their corresponding values.</li>
 * <li>Optional support for string lists. The values of properties to be added to this configuration are checked whether
 * they contain a list delimiter character. If this is the case and if list splitting is enabled, the string is split
 * and multiple values are added for this property. List splitting is controlled by a {@link ListDelimiterHandler}
 * object which can be set using the {@link #setListDelimiterHandler(ListDelimiterHandler)} method. It is disabled per
 * default. To enable this feature, set a suitable {@code ListDelimiterHandler}, e.g. an instance of
 * {@link org.apache.commons.configuration2.convert.DefaultListDelimiterHandler DefaultListDelimiterHandler} configured
 * with the desired list delimiter character.</li>
 * <li>Allows specifying how missing properties are treated. Per default the get methods returning an object will return
 * <b>null</b> if the searched property key is not found (and no default value is provided). With the
 * {@code setThrowExceptionOnMissing()} method this behavior can be changed to throw an exception when a requested
 * property cannot be found.</li>
 * <li>Basic event support. Whenever this configuration is modified registered event listeners are notified. Refer to
 * the various {@code EVENT_XXX} constants to get an impression about which event types are supported.</li>
 * <li>Support for proper synchronization based on the {@link Synchronizer} interface.</li>
 * </ul>
 * <p>
 * Most methods defined by the {@code Configuration} interface are already implemented in this class. Many method
 * implementations perform basic book-keeping tasks (e.g. firing events, handling synchronization), and then delegate to
 * other (protected) methods executing the actual work. Subclasses override these protected methods to define or adapt
 * behavior. The public entry point methods are final to prevent subclasses from breaking basic functionality.
 * </p>
 */
public abstract class AbstractConfiguration extends BaseEventSource implements Configuration {

    /** The list delimiter handler. */
    private ListDelimiterHandler listDelimiterHandler;

    /** The conversion handler. */
    private ConversionHandler conversionHandler;

    /**
     * Whether the configuration should throw NoSuchElementExceptions or simply return null when a property does not exist.
     * Defaults to return null.
     */
    private boolean throwExceptionOnMissing;

    /** Stores a reference to the object that handles variable interpolation. */
    private AtomicReference<ConfigurationInterpolator> interpolator;

    /** The object responsible for synchronization. */
    private volatile Synchronizer synchronizer;

    /** The object used for dealing with encoded property values. */
    private ConfigurationDecoder configurationDecoder;

    /** Stores the logger. */
    private ConfigurationLogger log;

    /**
     * Creates a new instance of {@code AbstractConfiguration}.
     */
    public AbstractConfiguration() {
        interpolator = new AtomicReference<>();
        initLogger(null);
        installDefaultInterpolator();
        listDelimiterHandler = DisabledListDelimiterHandler.INSTANCE;
        conversionHandler = DefaultConversionHandler.INSTANCE;
    }

    /**
     * Gets the {@code ListDelimiterHandler} used by this instance.
     *
     * @return the {@code ListDelimiterHandler}
     * @since 2.0
     */
    public ListDelimiterHandler getListDelimiterHandler() {
        return listDelimiterHandler;
    }

    /**
     * <p>
     * Sets the {@code ListDelimiterHandler} to be used by this instance. This object is invoked every time when dealing
     * with string properties that may contain a list delimiter and thus have to be split to multiple values. Per default, a
     * {@code ListDelimiterHandler} implementation is set which does not support list splitting. This can be changed for
     * instance by setting a {@link org.apache.commons.configuration2.convert.DefaultListDelimiterHandler
     * DefaultListDelimiterHandler} object.
     * </p>
     * <p>
     * <strong>Warning:</strong> Be careful when changing the list delimiter handler when the configuration has already been
     * loaded/populated. List handling is typically applied already when properties are added to the configuration. If later
     * another handler is set which processes lists differently, results may be unexpected; some operations may even cause
     * exceptions.
     * </p>
     *
     * @param listDelimiterHandler the {@code ListDelimiterHandler} to be used (must not be <b>null</b>)
     * @throws IllegalArgumentException if the {@code ListDelimiterHandler} is <b>null</b>
     * @since 2.0
     */
    public void setListDelimiterHandler(final ListDelimiterHandler listDelimiterHandler) {
        if (listDelimiterHandler == null) {
            throw new IllegalArgumentException("List delimiter handler must not be null!");
        }
        this.listDelimiterHandler = listDelimiterHandler;
    }

    /**
     * Gets the {@code ConversionHandler} used by this instance.
     *
     * @return the {@code ConversionHandler}
     * @since 2.0
     */
    public ConversionHandler getConversionHandler() {
        return conversionHandler;
    }

    /**
     * Sets the {@code ConversionHandler} to be used by this instance. The {@code ConversionHandler} is responsible for
     * every kind of data type conversion. It is consulted by all get methods returning results in specific data types. A
     * newly created configuration uses a default {@code ConversionHandler} implementation. This can be changed while
     * initializing the configuration (e.g. via a builder). Note that access to this property is not synchronized.
     *
     * @param conversionHandler the {@code ConversionHandler} to be used (must not be <b>null</b>)
     * @throws IllegalArgumentException if the {@code ConversionHandler} is <b>null</b>
     * @since 2.0
     */
    public void setConversionHandler(final ConversionHandler conversionHandler) {
        if (conversionHandler == null) {
            throw new IllegalArgumentException("ConversionHandler must not be null!");
        }
        this.conversionHandler = conversionHandler;
    }

    /**
     * Allows to set the {@code throwExceptionOnMissing} flag. This flag controls the behavior of property getter methods
     * that return objects if the requested property is missing. If the flag is set to <b>false</b> (which is the default
     * value), these methods will return <b>null</b>. If set to <b>true</b>, they will throw a
     * {@code NoSuchElementException} exception. Note that getter methods for primitive data types are not affected by this
     * flag.
     *
     * @param throwExceptionOnMissing The new value for the property
     */
    public void setThrowExceptionOnMissing(final boolean throwExceptionOnMissing) {
        this.throwExceptionOnMissing = throwExceptionOnMissing;
    }

    /**
     * Returns true if missing values throw Exceptions.
     *
     * @return true if missing values throw Exceptions
     */
    public boolean isThrowExceptionOnMissing() {
        return throwExceptionOnMissing;
    }

    /**
     * Gets the {@code ConfigurationInterpolator} object that manages the lookup objects for resolving variables.
     * Unless a custom interpolator has been set or the instance has been modified, the returned interpolator will
     * resolve values from this configuration instance and support the
     * {@link ConfigurationInterpolator#getDefaultPrefixLookups() default prefix lookups}.
     *
     * @return the {@code ConfigurationInterpolator} associated with this configuration
     * @since 1.4
     * @see ConfigurationInterpolator#getDefaultPrefixLookups()
     */
    @Override
    public ConfigurationInterpolator getInterpolator() {
        return interpolator.get();
    }

    /**
     * {@inheritDoc} This implementation sets the passed in object without further modifications. A <b>null</b> argument is
     * allowed; this disables interpolation.
     *
     * @since 2.0
     */
    @Override
    public final void setInterpolator(final ConfigurationInterpolator ci) {
        interpolator.set(ci);
    }

    /**
     * {@inheritDoc} This implementation creates a new {@code ConfigurationInterpolator} instance and initializes it with
     * the given {@code Lookup} objects. In addition, it adds a specialized default {@code Lookup} object which queries this
     * {@code Configuration}.
     *
     * @since 2.0
     */
    @Override
    public final void installInterpolator(final Map<String, ? extends Lookup> prefixLookups, final Collection<? extends Lookup> defLookups) {
        final InterpolatorSpecification spec = new InterpolatorSpecification.Builder().withPrefixLookups(prefixLookups).withDefaultLookups(defLookups)
            .withDefaultLookup(new ConfigurationLookup(this)).create();
        setInterpolator(ConfigurationInterpolator.fromSpecification(spec));
    }

    /**
     * Registers all {@code Lookup} objects in the given map at the current {@code ConfigurationInterpolator} of this
     * configuration. The set of default lookup objects (for variables without a prefix) is not modified by this method. If
     * this configuration does not have a {@code ConfigurationInterpolator}, a new instance is created. Note: This method is
     * mainly intended to be used for initializing a configuration when it is created by a builder. Normal client code
     * should better call {@link #installInterpolator(Map, Collection)} to define the {@code ConfigurationInterpolator} in a
     * single step.
     *
     * @param lookups a map with new {@code Lookup} objects and their prefixes (may be <b>null</b>)
     * @since 2.0
     */
    public void setPrefixLookups(final Map<String, ? extends Lookup> lookups) {
        boolean success;
        do {
            // do this in a loop because the ConfigurationInterpolator
            // instance may be changed by another thread
            final ConfigurationInterpolator ciOld = getInterpolator();
            final ConfigurationInterpolator ciNew = ciOld != null ? ciOld : new ConfigurationInterpolator();
            ciNew.registerLookups(lookups);
            success = interpolator.compareAndSet(ciOld, ciNew);
        } while (!success);
    }

    /**
     * Adds all {@code Lookup} objects in the given collection as default lookups (i.e. lookups without a variable prefix)
     * to the {@code ConfigurationInterpolator} object of this configuration. In addition, it adds a specialized default
     * {@code Lookup} object which queries this {@code Configuration}. The set of {@code Lookup} objects with prefixes is
     * not modified by this method. If this configuration does not have a {@code ConfigurationInterpolator}, a new instance
     * is created. Note: This method is mainly intended to be used for initializing a configuration when it is created by a
     * builder. Normal client code should better call {@link #installInterpolator(Map, Collection)} to define the
     * {@code ConfigurationInterpolator} in a single step.
     *
     * @param lookups the collection with default {@code Lookup} objects to be added
     * @since 2.0
     */
    public void setDefaultLookups(final Collection<? extends Lookup> lookups) {
        boolean success;
        do {
            final ConfigurationInterpolator ciOld = getInterpolator();
            final ConfigurationInterpolator ciNew = ciOld != null ? ciOld : new ConfigurationInterpolator();
            Lookup confLookup = findConfigurationLookup(ciNew);
            if (confLookup == null) {
                confLookup = new ConfigurationLookup(this);
            } else {
                ciNew.removeDefaultLookup(confLookup);
            }
            ciNew.addDefaultLookups(lookups);
            ciNew.addDefaultLookup(confLookup);
            success = interpolator.compareAndSet(ciOld, ciNew);
        } while (!success);
    }

    /**
     * Sets the specified {@code ConfigurationInterpolator} as the parent of this configuration's
     * {@code ConfigurationInterpolator}. If this configuration does not have a {@code ConfigurationInterpolator}, a new
     * instance is created. Note: This method is mainly intended to be used for initializing a configuration when it is
     * created by a builder. Normal client code can directly update the {@code ConfigurationInterpolator}.
     *
     * @param parent the parent {@code ConfigurationInterpolator} to be set
     * @since 2.0
     */
    public void setParentInterpolator(final ConfigurationInterpolator parent) {
        boolean success;
        do {
            final ConfigurationInterpolator ciOld = getInterpolator();
            final ConfigurationInterpolator ciNew = ciOld != null ? ciOld : new ConfigurationInterpolator();
            ciNew.setParentInterpolator(parent);
            success = interpolator.compareAndSet(ciOld, ciNew);
        } while (!success);
    }

    /**
     * Sets the {@code ConfigurationDecoder} for this configuration. This object is used by
     * {@link #getEncodedString(String)}.
     *
     * @param configurationDecoder the {@code ConfigurationDecoder}
     * @since 2.0
     */
    public void setConfigurationDecoder(final ConfigurationDecoder configurationDecoder) {
        this.configurationDecoder = configurationDecoder;
    }

    /**
     * Gets the {@code ConfigurationDecoder} used by this instance.
     *
     * @return the {@code ConfigurationDecoder}
     * @since 2.0
     */
    public ConfigurationDecoder getConfigurationDecoder() {
        return configurationDecoder;
    }

    /**
     * Creates a clone of the {@code ConfigurationInterpolator} used by this instance. This method can be called by
     * {@code clone()} implementations of derived classes. Normally, the {@code ConfigurationInterpolator} of a
     * configuration instance must not be shared with other instances because it contains a specific {@code Lookup} object
     * pointing to the owning configuration. This has to be taken into account when cloning a configuration. This method
     * creates a new {@code ConfigurationInterpolator} for this configuration instance which contains all lookup objects
     * from the original {@code ConfigurationInterpolator} except for the configuration specific lookup pointing to the
     * passed in original configuration. This one is replaced by a corresponding {@code Lookup} referring to this
     * configuration.
     *
     * @param orgConfig the original configuration from which this one was cloned
     * @since 2.0
     */
    protected void cloneInterpolator(final AbstractConfiguration orgConfig) {
        interpolator = new AtomicReference<>();
        final ConfigurationInterpolator orgInterpolator = orgConfig.getInterpolator();
        final List<Lookup> defaultLookups = orgInterpolator.getDefaultLookups();
        final Lookup lookup = findConfigurationLookup(orgInterpolator, orgConfig);
        if (lookup != null) {
            defaultLookups.remove(lookup);
        }

        installInterpolator(orgInterpolator.getLookups(), defaultLookups);
    }

    /**
     * Creates a default {@code ConfigurationInterpolator} which is initialized with all default {@code Lookup} objects.
     * This method is called by the constructor. It ensures that default interpolation works for every new configuration
     * instance.
     */
    private void installDefaultInterpolator() {
        installInterpolator(ConfigurationInterpolator.getDefaultPrefixLookups(), null);
    }

    /**
     * Finds a {@code ConfigurationLookup} pointing to this configuration in the default lookups of the specified
     * {@code ConfigurationInterpolator}. This method is called to ensure that there is exactly one default lookup querying
     * this configuration.
     *
     * @param ci the {@code ConfigurationInterpolator} in question
     * @return the found {@code Lookup} object or <b>null</b>
     */
    private Lookup findConfigurationLookup(final ConfigurationInterpolator ci) {
        return findConfigurationLookup(ci, this);
    }

    /**
     * Finds a {@code ConfigurationLookup} pointing to the specified configuration in the default lookups for the specified
     * {@code ConfigurationInterpolator}.
     *
     * @param ci the {@code ConfigurationInterpolator} in question
     * @param targetConf the target configuration of the searched lookup
     * @return the found {@code Lookup} object or <b>null</b>
     */
    private static Lookup findConfigurationLookup(final ConfigurationInterpolator ci, final ImmutableConfiguration targetConf) {
        for (final Lookup l : ci.getDefaultLookups()) {
            if (l instanceof ConfigurationLookup && targetConf == ((ConfigurationLookup) l).getConfiguration()) {
                return l;
            }
        }
        return null;
    }

    /**
     * Gets the logger used by this configuration object.
     *
     * @return the logger
     * @since 2.0
     */
    public ConfigurationLogger getLogger() {
        return log;
    }

    /**
     * Allows setting the logger to be used by this configuration object. This method makes it possible for clients to
     * exactly control logging behavior. Per default a logger is set that will ignore all log messages. Derived classes that
     * want to enable logging should call this method during their initialization with the logger to be used. It is legal to
     * pass a <b>null</b> logger; in this case, logging will be disabled.
     *
     * @param log the new logger
     * @since 2.0
     */
    public void setLogger(final ConfigurationLogger log) {
        initLogger(log);
    }

    /**
     * Adds a special {@link EventListener} object to this configuration that will log all internal errors. This method is
     * intended to be used by certain derived classes, for which it is known that they can fail on property access (e.g.
     * {@code DatabaseConfiguration}).
     *
     * @since 1.4
     */
    public final void addErrorLogListener() {
        addEventListener(ConfigurationErrorEvent.ANY, event -> getLogger().warn("Internal error", event.getCause()));
    }

    /**
     * Gets the object responsible for synchronizing this configuration. All access to this configuration - both read and
     * write access - is controlled by this object. This implementation never returns <b>null</b>. If no
     * {@code Synchronizer} has been set, a {@link NoOpSynchronizer} is returned. So, per default, instances of
     * {@code AbstractConfiguration} are not thread-safe unless a suitable {@code Synchronizer} is set!
     *
     * @return the {@code Synchronizer} used by this instance
     * @since 2.0
     */
    @Override
    public final Synchronizer getSynchronizer() {
        final Synchronizer sync = synchronizer;
        return sync != null ? sync : NoOpSynchronizer.INSTANCE;
    }

    /**
     * Sets the object responsible for synchronizing this configuration. This method has to be called with a suitable
     * {@code Synchronizer} object when initializing this configuration instance in order to make it thread-safe.
     *
     * @param synchronizer the new {@code Synchronizer}; can be <b>null</b>, then this instance uses a
     *        {@link NoOpSynchronizer}
     * @since 2.0
     */
    @Override
    public final void setSynchronizer(final Synchronizer synchronizer) {
        this.synchronizer = synchronizer;
    }

    /**
     * {@inheritDoc} This implementation delegates to {@code beginRead()} or {@code beginWrite()}, depending on the
     * {@code LockMode} argument. Subclasses can override these protected methods to perform additional steps when a
     * configuration is locked.
     *
     * @since 2.0
     * @throws NullPointerException if the argument is <b>null</b>
     */
    @Override
    public final void lock(final LockMode mode) {
        switch (mode) {
        case READ:
            beginRead(false);
            break;
        case WRITE:
            beginWrite(false);
            break;
        default:
            throw new IllegalArgumentException("Unsupported LockMode: " + mode);
        }
    }

    /**
     * {@inheritDoc} This implementation delegates to {@code endRead()} or {@code endWrite()}, depending on the
     * {@code LockMode} argument. Subclasses can override these protected methods to perform additional steps when a
     * configuration's lock is released.
     *
     * @throws NullPointerException if the argument is <b>null</b>
     */
    @Override
    public final void unlock(final LockMode mode) {
        switch (mode) {
        case READ:
            endRead();
            break;
        case WRITE:
            endWrite();
            break;
        default:
            throw new IllegalArgumentException("Unsupported LockMode: " + mode);
        }
    }

    /**
     * Notifies this configuration's {@link Synchronizer} that a read operation is about to start. This method is called by
     * all methods which access this configuration in a read-only mode. Subclasses may override it to perform additional
     * actions before this read operation. The boolean <em>optimize</em> argument can be evaluated by overridden methods in
     * derived classes. Some operations which require a lock do not need a fully initialized configuration object. By
     * setting this flag to <strong>true</strong>, such operations can give a corresponding hint. An overridden
     * implementation of {@code beginRead()} can then decide to skip some initialization steps. All basic operations in this
     * class (and most of the basic {@code Configuration} implementations) call this method with a parameter value of
     * <strong>false</strong>. <strong>In any case the inherited method must be called! Otherwise, proper synchronization is
     * not guaranteed.</strong>
     *
     * @param optimize a flag whether optimization can be performed
     * @since 2.0
     */
    protected void beginRead(final boolean optimize) {
        getSynchronizer().beginRead();
    }

    /**
     * Notifies this configuration's {@link Synchronizer} that a read operation has finished. This method is called by all
     * methods which access this configuration in a read-only manner at the end of their execution. Subclasses may override
     * it to perform additional actions after this read operation. <strong>In any case the inherited method must be called!
     * Otherwise, the read lock will not be released.</strong>
     *
     * @since 2.0
     */
    protected void endRead() {
        getSynchronizer().endRead();
    }

    /**
     * Notifies this configuration's {@link Synchronizer} that an update operation is about to start. This method is called
     * by all methods which modify this configuration. Subclasses may override it to perform additional operations before an
     * update. For a description of the boolean <em>optimize</em> argument refer to the documentation of
     * {@code beginRead()}. <strong>In any case the inherited method must be called! Otherwise, proper synchronization is
     * not guaranteed.</strong>
     *
     * @param optimize a flag whether optimization can be performed
     * @see #beginRead(boolean)
     * @since 2.0
     */
    protected void beginWrite(final boolean optimize) {
        getSynchronizer().beginWrite();
    }

    /**
     * Notifies this configuration's {@link Synchronizer} that an update operation has finished. This method is called by
     * all methods which modify this configuration at the end of their execution. Subclasses may override it to perform
     * additional operations after an update. <strong>In any case the inherited method must be called! Otherwise, the write
     * lock will not be released.</strong>
     *
     * @since 2.0
     */
    protected void endWrite() {
        getSynchronizer().endWrite();
    }

    @Override
    public final void addProperty(final String key, final Object value) {
        beginWrite(false);
        try {
            fireEvent(ConfigurationEvent.ADD_PROPERTY, key, value, true);
            addPropertyInternal(key, value);
            fireEvent(ConfigurationEvent.ADD_PROPERTY, key, value, false);
        } finally {
            endWrite();
        }
    }

    /**
     * Actually adds a property to this configuration. This method is called by {@code addProperty()}. It performs list
     * splitting if necessary and delegates to {@link #addPropertyDirect(String, Object)} for every single property value.
     *
     * @param key the key of the property to be added
     * @param value the new property value
     * @since 2.0
     */
    protected void addPropertyInternal(final String key, final Object value) {
        getListDelimiterHandler().parse(value).forEach(obj -> addPropertyDirect(key, obj));
    }

    /**
     * Adds a key/value pair to the Configuration. Override this method to provide write access to underlying Configuration
     * store.
     *
     * @param key key to use for mapping
     * @param value object to store
     */
    protected abstract void addPropertyDirect(String key, Object value);

    /**
     * interpolate key names to handle ${key} stuff
     *
     * @param base string to interpolate
     *
     * @return returns the key name with the ${key} substituted
     */
    protected String interpolate(final String base) {
        return Objects.toString(interpolate((Object) base), null);
    }

    /**
     * Returns the interpolated value. This implementation delegates to the current {@code ConfigurationInterpolator}. If no
     * {@code ConfigurationInterpolator} is set, the passed in value is returned without changes.
     *
     * @param value the value to interpolate
     * @return the value with variables substituted
     */
    protected Object interpolate(final Object value) {
        final ConfigurationInterpolator ci = getInterpolator();
        return ci != null ? ci.interpolate(value) : value;
    }

    @Override
    public Configuration subset(final String prefix) {
        return new SubsetConfiguration(this, prefix, ".");
    }

    @Override
    public ImmutableConfiguration immutableSubset(final String prefix) {
        return ConfigurationUtils.unmodifiableConfiguration(subset(prefix));
    }

    @Override
    public final void setProperty(final String key, final Object value) {
        beginWrite(false);
        try {
            fireEvent(ConfigurationEvent.SET_PROPERTY, key, value, true);
            setPropertyInternal(key, value);
            fireEvent(ConfigurationEvent.SET_PROPERTY, key, value, false);
        } finally {
            endWrite();
        }
    }

    /**
     * Actually sets the value of a property. This method is called by {@code setProperty()}. It provides a default
     * implementation of this functionality by clearing the specified key and delegating to {@code addProperty()}.
     * Subclasses should override this method if they can provide a more efficient algorithm for setting a property value.
     *
     * @param key the property key
     * @param value the new property value
     * @since 2.0
     */
    protected void setPropertyInternal(final String key, final Object value) {
        setDetailEvents(false);
        try {
            clearProperty(key);
            addProperty(key, value);
        } finally {
            setDetailEvents(true);
        }
    }

    /**
     * Removes the specified property from this configuration. This implementation performs some preparations and then
     * delegates to {@code clearPropertyDirect()}, which will do the real work.
     *
     * @param key the key to be removed
     */
    @Override
    public final void clearProperty(final String key) {
        beginWrite(false);
        try {
            fireEvent(ConfigurationEvent.CLEAR_PROPERTY, key, null, true);
            clearPropertyDirect(key);
            fireEvent(ConfigurationEvent.CLEAR_PROPERTY, key, null, false);
        } finally {
            endWrite();
        }
    }

    /**
     * Removes the specified property from this configuration. This method is called by {@code clearProperty()} after it has
     * done some preparations. It must be overridden in sub classes.
     *
     * @param key the key to be removed
     */
    protected abstract void clearPropertyDirect(String key);

    @Override
    public final void clear() {
        beginWrite(false);
        try {
            fireEvent(ConfigurationEvent.CLEAR, null, null, true);
            clearInternal();
            fireEvent(ConfigurationEvent.CLEAR, null, null, false);
        } finally {
            endWrite();
        }
    }

    /**
     * Clears the whole configuration. This method is called by {@code clear()} after some preparations have been made. This
     * base implementation uses the iterator provided by {@code getKeys()} to remove every single property. Subclasses
     * should override this method if there is a more efficient way of clearing the configuration.
     */
    protected void clearInternal() {
        setDetailEvents(false);
        boolean useIterator = true;
        try {
            final Iterator<String> it = getKeys();
            while (it.hasNext()) {
                final String key = it.next();
                if (useIterator) {
                    try {
                        it.remove();
                    } catch (final UnsupportedOperationException usoex) {
                        useIterator = false;
                    }
                }

                if (useIterator && containsKey(key)) {
                    useIterator = false;
                }

                if (!useIterator) {
                    // workaround for Iterators that do not remove the
                    // property
                    // on calling remove() or do not support remove() at all
                    clearProperty(key);
                }
            }
        } finally {
            setDetailEvents(true);
        }
    }

    /**
     * {@inheritDoc} This implementation takes care of synchronization and then delegates to {@code getKeysInternal()} for
     * obtaining the actual iterator. Note that depending on a concrete implementation, an iteration may fail if the
     * configuration is updated concurrently.
     */
    @Override
    public final Iterator<String> getKeys() {
        beginRead(false);
        try {
            return getKeysInternal();
        } finally {
            endRead();
        }
    }

    /**
     * {@inheritDoc} This implementation returns keys that either match the prefix or start with the prefix followed by a
     * dot ('.'). So the call {@code getKeys("db");} will find the keys {@code db}, {@code db.user}, or {@code db.password},
     * but not the key {@code dbdriver}.
     */
    @Override
    public final Iterator<String> getKeys(final String prefix) {
        beginRead(false);
        try {
            return getKeysInternal(prefix);
        } finally {
            endRead();
        }
    }

    /**
     * {@inheritDoc} This implementation returns keys that either match the prefix or start with the prefix followed by the delimiter.
     * So the call {@code getKeys("db");} will find the keys {@code db}, {@code db@user}, or {@code db@password},
     * but not the key {@code dbdriver}.
     */
    @Override
    public final Iterator<String> getKeys(final String prefix, final String delimiter) {
        beginRead(false);
        try {
            return getKeysInternal(prefix, delimiter);
        } finally {
            endRead();
        }
    }

    /**
     * Actually creates an iterator for iterating over the keys in this configuration. This method is called by
     * {@code getKeys()}, it has to be defined by concrete subclasses.
     *
     * @return an {@code Iterator} with all property keys in this configuration
     * @since 2.0
     */
    protected abstract Iterator<String> getKeysInternal();

    /**
     * Gets an {@code Iterator} with all property keys starting with the specified prefix. This method is called by
     * {@link #getKeys(String)}. It is fully implemented by delegating to {@code getKeysInternal()} and returning a special
     * iterator which filters for the passed in prefix. Subclasses can override it if they can provide a more efficient way
     * to iterate over specific keys only.
     *
     * @param prefix the prefix for the keys to be taken into account
     * @return an {@code Iterator} returning the filtered keys
     * @since 2.0
     */
    protected Iterator<String> getKeysInternal(final String prefix) {
        return new PrefixedKeysIterator(getKeysInternal(), prefix);
    }

    /**
     * Gets an {@code Iterator} with all property keys starting with the specified prefix and specified delimiter. This method is called by
     * {@link #getKeys(String)}. It is fully implemented by delegating to {@code getKeysInternal()} and returning a special
     * iterator which filters for the passed in prefix. Subclasses can override it if they can provide a more efficient way
     * to iterate over specific keys only.
     *
     * @param prefix the prefix for the keys to be taken into account
     * @param delimiter the prefix delimiter
     * @return an {@code Iterator} returning the filtered keys
     * @since 2.10.0
     */
    protected Iterator<String> getKeysInternal(final String prefix, final String delimiter) {
        return new PrefixedKeysIterator(getKeysInternal(), prefix, delimiter);
    }

    /**
     * {@inheritDoc} This implementation ensures proper synchronization. Subclasses have to define the abstract
     * {@code getPropertyInternal()} method which is called from here.
     */
    @Override
    public final Object getProperty(final String key) {
        beginRead(false);
        try {
            return getPropertyInternal(key);
        } finally {
            endRead();
        }
    }

    /**
     * Actually obtains the value of the specified property. This method is called by {@code getProperty()}. Concrete
     * subclasses must define it to fetch the value of the desired property.
     *
     * @param key the key of the property in question
     * @return the (raw) value of this property
     * @since 2.0
     */
    protected abstract Object getPropertyInternal(String key);

    /**
     * {@inheritDoc} This implementation handles synchronization and delegates to {@code isEmptyInternal()}.
     */
    @Override
    public final boolean isEmpty() {
        beginRead(false);
        try {
            return isEmptyInternal();
        } finally {
            endRead();
        }
    }

    /**
     * Actually checks whether this configuration contains data. This method is called by {@code isEmpty()}. It has to be
     * defined by concrete subclasses.
     *
     * @return <b>true</b> if this configuration contains no data, <b>false</b> otherwise
     * @since 2.0
     */
    protected abstract boolean isEmptyInternal();

    /**
     * {@inheritDoc} This implementation handles synchronization and delegates to {@code sizeInternal()}.
     */
    @Override
    public final int size() {
        beginRead(false);
        try {
            return sizeInternal();
        } finally {
            endRead();
        }
    }

    /**
     * Actually calculates the size of this configuration. This method is called by {@code size()} with a read lock held.
     * The base implementation provided here calculates the size based on the iterator returned by {@code getKeys()}. Sub
     * classes which can determine the size in a more efficient way should override this method.
     *
     * @return the size of this configuration (i.e. the number of keys)
     */
    protected int sizeInternal() {
        int size = 0;
        for (final Iterator<String> keyIt = getKeysInternal(); keyIt.hasNext(); size++) {
            keyIt.next();
        }
        return size;
    }

    /**
     * {@inheritDoc} This implementation handles synchronization and delegates to {@code containsKeyInternal()}.
     */
    @Override
    public final boolean containsKey(final String key) {
        beginRead(false);
        try {
            return containsKeyInternal(key);
        } finally {
            endRead();
        }
    }

    /**
     * Actually checks whether the specified key is contained in this configuration. This method is called by
     * {@code containsKey()}. It has to be defined by concrete subclasses.
     *
     * @param key the key in question
     * @return <b>true</b> if this key is contained in this configuration, <b>false</b> otherwise
     * @since 2.0
     */
    protected abstract boolean containsKeyInternal(String key);

    @Override
    public Properties getProperties(final String key) {
        return getProperties(key, null);
    }

    /**
     * Gets a list of properties associated with the given configuration key.
     *
     * @param key The configuration key.
     * @param defaults Any default values for the returned {@code Properties} object. Ignored if {@code null}.
     *
     * @return The associated properties if key is found.
     *
     * @throws ConversionException is thrown if the key maps to an object that is not a String/List of Strings.
     *
     * @throws IllegalArgumentException if one of the tokens is malformed (does not contain an equals sign).
     */
    public Properties getProperties(final String key, final Properties defaults) {
        /*
         * Grab an array of the tokens for this key.
         */
        final String[] tokens = getStringArray(key);

        /*
         * Each token is of the form 'key=value'.
         */
        final Properties props = defaults == null ? new Properties() : new Properties(defaults);
        for (final String token : tokens) {
            final int equalSign = token.indexOf('=');
            if (equalSign > 0) {
                final String pkey = token.substring(0, equalSign).trim();
                final String pvalue = token.substring(equalSign + 1).trim();
                props.put(pkey, pvalue);
            } else if (tokens.length == 1 && StringUtils.isEmpty(key)) {
                // Semantically equivalent to an empty Properties
                // object.
                break;
            } else {
                throw new IllegalArgumentException('\'' + token + "' does not contain an equals sign");
            }
        }
        return props;
    }

    @Override
    public boolean getBoolean(final String key) {
        final Boolean b = convert(Boolean.class, key, null, true);
        return checkNonNullValue(key, b).booleanValue();
    }

    @Override
    public boolean getBoolean(final String key, final boolean defaultValue) {
        return getBoolean(key, Boolean.valueOf(defaultValue)).booleanValue();
    }

    /**
     * Obtains the value of the specified key and tries to convert it into a {@code Boolean} object. If the property has no
     * value, the passed in default value will be used.
     *
     * @param key the key of the property
     * @param defaultValue the default value
     * @return the value of this key converted to a {@code Boolean}
     * @throws ConversionException if the value cannot be converted to a {@code Boolean}
     */
    @Override
    public Boolean getBoolean(final String key, final Boolean defaultValue) {
        return convert(Boolean.class, key, defaultValue, false);
    }

    @Override
    public byte getByte(final String key) {
        final Byte b = convert(Byte.class, key, null, true);
        return checkNonNullValue(key, b).byteValue();
    }

    @Override
    public byte getByte(final String key, final byte defaultValue) {
        return getByte(key, Byte.valueOf(defaultValue)).byteValue();
    }

    @Override
    public Byte getByte(final String key, final Byte defaultValue) {
        return convert(Byte.class, key, defaultValue, false);
    }

    @Override
    public double getDouble(final String key) {
        final Double d = convert(Double.class, key, null, true);
        return checkNonNullValue(key, d).doubleValue();
    }

    @Override
    public double getDouble(final String key, final double defaultValue) {
        return getDouble(key, Double.valueOf(defaultValue)).doubleValue();
    }

    @Override
    public Double getDouble(final String key, final Double defaultValue) {
        return convert(Double.class, key, defaultValue, false);
    }

    @Override
    public Duration getDuration(final String key) {
        return checkNonNullValue(key, convert(Duration.class, key, null, true));
    }

    @Override
    public Duration getDuration(final String key, final Duration defaultValue) {
        return convert(Duration.class, key, defaultValue, false);
    }

    @Override
    public float getFloat(final String key) {
        final Float f = convert(Float.class, key, null, true);
        return checkNonNullValue(key, f).floatValue();
    }

    @Override
    public float getFloat(final String key, final float defaultValue) {
        return getFloat(key, Float.valueOf(defaultValue)).floatValue();
    }

    @Override
    public Float getFloat(final String key, final Float defaultValue) {
        return convert(Float.class, key, defaultValue, false);
    }

    @Override
    public int getInt(final String key) {
        final Integer i = convert(Integer.class, key, null, true);
        return checkNonNullValue(key, i).intValue();
    }

    @Override
    public int getInt(final String key, final int defaultValue) {
        return getInteger(key, Integer.valueOf(defaultValue)).intValue();
    }

    @Override
    public Integer getInteger(final String key, final Integer defaultValue) {
        return convert(Integer.class, key, defaultValue, false);
    }

    @Override
    public long getLong(final String key) {
        final Long l = convert(Long.class, key, null, true);
        return checkNonNullValue(key, l).longValue();
    }

    @Override
    public long getLong(final String key, final long defaultValue) {
        return getLong(key, Long.valueOf(defaultValue)).longValue();
    }

    @Override
    public Long getLong(final String key, final Long defaultValue) {
        return convert(Long.class, key, defaultValue, false);
    }

    @Override
    public short getShort(final String key) {
        final Short s = convert(Short.class, key, null, true);
        return checkNonNullValue(key, s).shortValue();
    }

    @Override
    public short getShort(final String key, final short defaultValue) {
        return getShort(key, Short.valueOf(defaultValue)).shortValue();
    }

    @Override
    public Short getShort(final String key, final Short defaultValue) {
        return convert(Short.class, key, defaultValue, false);
    }

    /**
     * {@inheritDoc}
     *
     * @see #setThrowExceptionOnMissing(boolean)
     */
    @Override
    public BigDecimal getBigDecimal(final String key) {
        return convert(BigDecimal.class, key, null, true);
    }

    @Override
    public BigDecimal getBigDecimal(final String key, final BigDecimal defaultValue) {
        return convert(BigDecimal.class, key, defaultValue, false);
    }

    /**
     * {@inheritDoc}
     *
     * @see #setThrowExceptionOnMissing(boolean)
     */
    @Override
    public BigInteger getBigInteger(final String key) {
        return convert(BigInteger.class, key, null, true);
    }

    @Override
    public BigInteger getBigInteger(final String key, final BigInteger defaultValue) {
        return convert(BigInteger.class, key, defaultValue, false);
    }

    /**
     * {@inheritDoc}
     *
     * @see #setThrowExceptionOnMissing(boolean)
     */
    @Override
    public String getString(final String key) {
        return convert(String.class, key, null, true);
    }

    @Override
    public String getString(final String key, final String defaultValue) {
        final String result = convert(String.class, key, null, false);
        return result != null ? result : interpolate(defaultValue);
    }

    /**
     * {@inheritDoc} This implementation delegates to {@link #getString(String)} in order to obtain the value of the passed
     * in key. This value is passed to the decoder. Because {@code getString()} is used behind the scenes all standard
     * features like handling of missing keys and interpolation work as expected.
     */
    @Override
    public String getEncodedString(final String key, final ConfigurationDecoder decoder) {
        if (decoder == null) {
            throw new IllegalArgumentException("ConfigurationDecoder must not be null!");
        }

        final String value = getString(key);
        return value != null ? decoder.decode(value) : null;
    }

    /**
     * {@inheritDoc} This implementation makes use of the {@code ConfigurationDecoder} set for this configuration. If no
     * such object has been set, an {@code IllegalStateException} exception is thrown.
     *
     * @throws IllegalStateException if no {@code ConfigurationDecoder} is set
     * @see #setConfigurationDecoder(ConfigurationDecoder)
     */
    @Override
    public String getEncodedString(final String key) {
        final ConfigurationDecoder decoder = getConfigurationDecoder();
        if (decoder == null) {
            throw new IllegalStateException("No default ConfigurationDecoder defined!");
        }
        return getEncodedString(key, decoder);
    }

    /**
     * Gets an array of strings associated with the given configuration key. If the key doesn't map to an existing object, an
     * empty array is returned. When a property is added to a configuration, it is checked whether it contains multiple
     * values. This is obvious if the added object is a list or an array. For strings the association
     * {@link ListDelimiterHandler} is consulted to find out whether the string can be split into multiple values.
     *
     * @param key The configuration key.
     * @return The associated string array if key is found.
     *
     * @throws ConversionException is thrown if the key maps to an object that is not a String/List of Strings.
     * @see #setListDelimiterHandler(ListDelimiterHandler)
     */
    @Override
    public String[] getStringArray(final String key) {
        final String[] result = (String[]) getArray(String.class, key);
        return result == null ? ArrayUtils.EMPTY_STRING_ARRAY : result;
    }

    /**
     * {@inheritDoc}
     *
     * @see #getStringArray(String)
     */
    @Override
    public List<Object> getList(final String key) {
        return getList(key, new ArrayList<>());
    }

    @Override
    public List<Object> getList(final String key, final List<?> defaultValue) {
        final Object value = getProperty(key);
        final List<Object> list;

        if (value instanceof String) {
            list = new ArrayList<>(1);
            list.add(interpolate((String) value));
        } else if (value instanceof List) {
            list = new ArrayList<>();
            final List<?> l = (List<?>) value;

            // add the interpolated elements in the new list
            l.forEach(elem -> list.add(interpolate(elem)));
        } else if (value == null) {
            // This is okay because we just return this list to the caller
            @SuppressWarnings("unchecked")
            final List<Object> resultList = (List<Object>) defaultValue;
            list = resultList;
        } else if (value.getClass().isArray()) {
            return Arrays.asList((Object[]) value);
        } else if (isScalarValue(value)) {
            return Collections.singletonList((Object) value.toString());
        } else {
            throw new ConversionException('\'' + key + "' doesn't map to a List object: " + value + ", a " + value.getClass().getName());
        }
        return list;
    }

    @Override
    public <T> T get(final Class<T> cls, final String key) {
        return convert(cls, key, null, true);
    }

    /**
     * {@inheritDoc} This implementation delegates to the {@link ConversionHandler} to perform the actual type conversion.
     */
    @Override
    public <T> T get(final Class<T> cls, final String key, final T defaultValue) {
        return convert(cls, key, defaultValue, false);
    }

    @Override
    public Object getArray(final Class<?> cls, final String key) {
        return getArray(cls, key, null);
    }

    /**
     * {@inheritDoc} This implementation delegates to the {@link ConversionHandler} to perform the actual type conversion.
     * If this results in a <b>null</b> result (because the property is undefined), the default value is returned. It is
     * checked whether the default value is an array with the correct component type. If not, an exception is thrown.
     *
     * @throws IllegalArgumentException if the default value is not a compatible array
     */
    @Override
    public Object getArray(final Class<?> cls, final String key, final Object defaultValue) {
        return convertToArray(cls, key, defaultValue);
    }

    @Override
    public <T> List<T> getList(final Class<T> cls, final String key) {
        return getList(cls, key, null);
    }

    /**
     * {@inheritDoc} This implementation delegates to the generic {@code getCollection()}. As target collection a newly
     * created {@code ArrayList} is passed in.
     */
    @Override
    public <T> List<T> getList(final Class<T> cls, final String key, final List<T> defaultValue) {
        final List<T> result = new ArrayList<>();
        if (getCollection(cls, key, result, defaultValue) == null) {
            return null;
        }
        return result;
    }

    @Override
    public <T> Collection<T> getCollection(final Class<T> cls, final String key, final Collection<T> target) {
        return getCollection(cls, key, target, null);
    }

    /**
     * {@inheritDoc} This implementation delegates to the {@link ConversionHandler} to perform the actual conversion. If no
     * target collection is provided, an {@code ArrayList} is created.
     */
    @Override
    public <T> Collection<T> getCollection(final Class<T> cls, final String key, final Collection<T> target, final Collection<T> defaultValue) {
        final Object src = getProperty(key);
        if (src == null) {
            return handleDefaultCollection(target, defaultValue);
        }

        final Collection<T> targetCol = target != null ? target : new ArrayList<>();
        getConversionHandler().toCollection(src, cls, getInterpolator(), targetCol);
        return targetCol;
    }

    /**
     * Checks whether the specified object is a scalar value. This method is called by {@code getList()} and
     * {@code getStringArray()} if the property requested is not a string, a list, or an array. If it returns <b>true</b>,
     * the calling method transforms the value to a string and returns a list or an array with this single element. This
     * implementation returns <b>true</b> if the value is of a wrapper type for a primitive type.
     *
     * @param value the value to be checked
     * @return a flag whether the value is a scalar
     * @since 1.7
     */
    protected boolean isScalarValue(final Object value) {
        return ClassUtils.wrapperToPrimitive(value.getClass()) != null;
    }

    /**
     * Copies the content of the specified configuration into this configuration. If the specified configuration contains a
     * key that is also present in this configuration, the value of this key will be replaced by the new value.
     * <em>Note:</em> This method won't work well when copying hierarchical configurations because it is not able to copy
     * information about the properties' structure (i.e. the parent-child-relationships will get lost). So when dealing with
     * hierarchical configuration objects their {@link BaseHierarchicalConfiguration#clone() clone()} methods should be
     * used.
     *
     * @param c the configuration to copy (can be <b>null</b>, then this operation will have no effect)
     * @since 1.5
     */
    public void copy(final Configuration c) {
        if (c != null) {
            c.lock(LockMode.READ);
            try {
                c.getKeys().forEachRemaining(key -> setProperty(key, encodeForCopy(c.getProperty(key))));
            } finally {
                c.unlock(LockMode.READ);
            }
        }
    }

    /**
     * Appends the content of the specified configuration to this configuration. The values of all properties contained in
     * the specified configuration will be appended to this configuration. So if a property is already present in this
     * configuration, its new value will be a union of the values in both configurations. <em>Note:</em> This method won't
     * work well when appending hierarchical configurations because it is not able to copy information about the properties'
     * structure (i.e. the parent-child-relationships will get lost). So when dealing with hierarchical configuration
     * objects their {@link BaseHierarchicalConfiguration#clone() clone()} methods should be used.
     *
     * @param c the configuration to be appended (can be <b>null</b>, then this operation will have no effect)
     * @since 1.5
     */
    public void append(final Configuration c) {
        if (c != null) {
            c.lock(LockMode.READ);
            try {
                c.getKeys().forEachRemaining(key -> addProperty(key, encodeForCopy(c.getProperty(key))));
            } finally {
                c.unlock(LockMode.READ);
            }
        }
    }

    /**
     * Returns a configuration with the same content as this configuration, but with all variables replaced by their actual
     * values. This method tries to clone the configuration and then perform interpolation on all properties. So property
     * values of the form {@code ${var}} will be resolved as far as possible (if a variable cannot be resolved, it remains
     * unchanged). This operation is useful if the content of a configuration is to be exported or processed by an external
     * component that does not support variable interpolation.
     *
     * @return a configuration with all variables interpolated
     * @throws org.apache.commons.configuration2.ex.ConfigurationRuntimeException if this configuration cannot be cloned
     * @since 1.5
     */
    public Configuration interpolatedConfiguration() {
        // first clone this configuration
        final AbstractConfiguration c = (AbstractConfiguration) ConfigurationUtils.cloneConfiguration(this);

        // now perform interpolation
        c.setListDelimiterHandler(new DisabledListDelimiterHandler());
        getKeys().forEachRemaining(key -> c.setProperty(key, getList(key)));
        c.setListDelimiterHandler(getListDelimiterHandler());
        return c;
    }

    /**
     * Initializes the logger. Supports <b>null</b> input. This method can be called by derived classes in order to enable
     * logging.
     *
     * @param log the logger
     * @since 2.0
     */
    protected final void initLogger(final ConfigurationLogger log) {
        this.log = log != null ? log : ConfigurationLogger.newDummyLogger();
    }

    /**
     * Encodes a property value so that it can be added to this configuration. This method deals with list delimiters. The
     * passed in object has to be escaped so that an add operation yields the same result. If it is a list, all of its
     * values have to be escaped.
     *
     * @param value the value to be encoded
     * @return the encoded value
     */
    private Object encodeForCopy(final Object value) {
        if (value instanceof Collection) {
            return encodeListForCopy((Collection<?>) value);
        }
        return getListDelimiterHandler().escape(value, ListDelimiterHandler.NOOP_TRANSFORMER);
    }

    /**
     * Encodes a list with property values so that it can be added to this configuration. This method calls
     * {@code encodeForCopy()} for all list elements.
     *
     * @param values the list to be encoded
     * @return a list with encoded elements
     */
    private Object encodeListForCopy(final Collection<?> values) {
        return values.stream().map(this::encodeForCopy).collect(Collectors.toList());
    }

    /**
     * Obtains the property value for the specified key and converts it to the given target class.
     *
     * @param <T> the target type of the conversion
     * @param cls the target class
     * @param key the key of the desired property
     * @param defaultValue a default value
     * @return the converted value of this property
     * @throws ConversionException if the conversion cannot be performed
     */
    private <T> T getAndConvertProperty(final Class<T> cls, final String key, final T defaultValue) {
        final Object value = getProperty(key);
        try {
            return ObjectUtils.defaultIfNull(getConversionHandler().to(value, cls, getInterpolator()), defaultValue);
        } catch (final ConversionException cex) {
            // improve error message
            throw new ConversionException(String.format("Key '%s' cannot be converted to class %s. Value is: '%s'.", key, cls.getName(), String.valueOf(value)),
                cex.getCause());
        }
    }

    /**
     * Helper method for obtaining a property value with a type conversion.
     *
     * @param <T> the target type of the conversion
     * @param cls the target class
     * @param key the key of the desired property
     * @param defValue a default value
     * @param throwOnMissing a flag whether an exception should be thrown for a missing value
     * @return the converted value
     */
    private <T> T convert(final Class<T> cls, final String key, final T defValue, final boolean throwOnMissing) {
        if (cls.isArray()) {
            return cls.cast(convertToArray(cls.getComponentType(), key, defValue));
        }

        final T result = getAndConvertProperty(cls, key, defValue);
        if (result == null) {
            if (throwOnMissing && isThrowExceptionOnMissing()) {
                throwMissingPropertyException(key);
            }
            return defValue;
        }

        return result;
    }

    /**
     * Performs a conversion to an array result class. This implementation delegates to the {@link ConversionHandler} to
     * perform the actual type conversion. If this results in a <b>null</b> result (because the property is undefined), the
     * default value is returned. It is checked whether the default value is an array with the correct component type. If
     * not, an exception is thrown.
     *
     * @param cls the component class of the array
     * @param key the configuration key
     * @param defaultValue an optional default value
     * @return the converted array
     * @throws IllegalArgumentException if the default value is not a compatible array
     */
    private Object convertToArray(final Class<?> cls, final String key, final Object defaultValue) {
        checkDefaultValueArray(cls, defaultValue);
        return ObjectUtils.defaultIfNull(getConversionHandler().toArray(getProperty(key), cls, getInterpolator()), defaultValue);
    }

    /**
     * Checks an object provided as default value for the {@code getArray()} method. Throws an exception if this is not an
     * array with the correct component type.
     *
     * @param cls the component class for the array
     * @param defaultValue the default value object to be checked
     * @throws IllegalArgumentException if this is not a valid default object
     */
    private static void checkDefaultValueArray(final Class<?> cls, final Object defaultValue) {
        if (defaultValue != null && (!defaultValue.getClass().isArray() || !cls.isAssignableFrom(defaultValue.getClass().getComponentType()))) {
            throw new IllegalArgumentException(
                "The type of the default value (" + defaultValue.getClass() + ")" + " is not an array of the specified class (" + cls + ")");
        }
    }

    /**
     * Handles the default collection for a collection conversion. This method fills the target collection with the content
     * of the default collection. Both collections may be <b>null</b>.
     *
     * @param target the target collection
     * @param defaultValue the default collection
     * @return the initialized target collection
     */
    private static <T> Collection<T> handleDefaultCollection(final Collection<T> target, final Collection<T> defaultValue) {
        if (defaultValue == null) {
            return null;
        }

        final Collection<T> result;
        if (target == null) {
            result = new ArrayList<>(defaultValue);
        } else {
            target.addAll(defaultValue);
            result = target;
        }
        return result;
    }

    /**
     * Checks whether the specified value is <b>null</b> and throws an exception in this case. This method is used by
     * conversion methods returning primitive Java types. Here values to be returned must not be <b>null</b>.
     *
     * @param <T> the type of the object to be checked
     * @param key the key which caused the problem
     * @param value the value to be checked
     * @return the passed in value for chaining this method call
     * @throws NoSuchElementException if the value is <b>null</b>
     */
    private static <T> T checkNonNullValue(final String key, final T value) {
        if (value == null) {
            throwMissingPropertyException(key);
        }
        return value;
    }

    /**
     * Helper method for throwing an exception for a key that does not map to an existing object.
     *
     * @param key the key (to be part of the error message)
     */
    private static void throwMissingPropertyException(final String key) {
        throw new NoSuchElementException(String.format("Key '%s' does not map to an existing object!", key));
    }
}