/*
 * 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.builder.combined;

import java.net.URL;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;

import org.apache.commons.configuration2.CombinedConfiguration;
import org.apache.commons.configuration2.Configuration;
import org.apache.commons.configuration2.ConfigurationLookup;
import org.apache.commons.configuration2.HierarchicalConfiguration;
import org.apache.commons.configuration2.SystemConfiguration;
import org.apache.commons.configuration2.XMLConfiguration;
import org.apache.commons.configuration2.beanutils.BeanDeclaration;
import org.apache.commons.configuration2.beanutils.BeanHelper;
import org.apache.commons.configuration2.beanutils.CombinedBeanDeclaration;
import org.apache.commons.configuration2.beanutils.XMLBeanDeclaration;
import org.apache.commons.configuration2.builder.BasicBuilderParameters;
import org.apache.commons.configuration2.builder.BasicConfigurationBuilder;
import org.apache.commons.configuration2.builder.BuilderParameters;
import org.apache.commons.configuration2.builder.ConfigurationBuilder;
import org.apache.commons.configuration2.builder.ConfigurationBuilderEvent;
import org.apache.commons.configuration2.builder.FileBasedBuilderParametersImpl;
import org.apache.commons.configuration2.builder.FileBasedBuilderProperties;
import org.apache.commons.configuration2.builder.FileBasedConfigurationBuilder;
import org.apache.commons.configuration2.builder.XMLBuilderParametersImpl;
import org.apache.commons.configuration2.builder.XMLBuilderProperties;
import org.apache.commons.configuration2.event.EventListener;
import org.apache.commons.configuration2.ex.ConfigurationException;
import org.apache.commons.configuration2.interpol.ConfigurationInterpolator;
import org.apache.commons.configuration2.interpol.Lookup;
import org.apache.commons.configuration2.io.FileSystem;
import org.apache.commons.configuration2.resolver.CatalogResolver;
import org.apache.commons.configuration2.tree.DefaultExpressionEngineSymbols;
import org.apache.commons.configuration2.tree.OverrideCombiner;
import org.apache.commons.configuration2.tree.UnionCombiner;
import org.xml.sax.EntityResolver;

/**
 * <p>
 * A specialized {@code ConfigurationBuilder} implementation that creates a {@link CombinedConfiguration} from multiple
 * configuration sources defined by an XML-based <em>configuration definition file</em>.
 * </p>
 * <p>
 * This class provides an easy and flexible means for loading multiple configuration sources and combining the results
 * into a single configuration object. The sources to be loaded are defined in an XML document that can contain certain
 * tags representing the different supported configuration classes. If such a tag is found, a corresponding
 * {@code ConfigurationBuilder} class is instantiated and initialized using the classes of the {@code beanutils} package
 * (namely {@link org.apache.commons.configuration2.beanutils.XMLBeanDeclaration XMLBeanDeclaration} will be used to
 * extract the configuration's initialization parameters, which allows for complex initialization scenarios).
 * </p>
 * <p>
 * It is also possible to add custom tags to the configuration definition file. For this purpose an implementation of
 * {@link CombinedConfigurationBuilderProvider} has to be created which is responsible for the creation of a
 * {@code ConfigurationBuilder} associated with the custom tag. An instance of this class has to be registered at the
 * {@link CombinedBuilderParametersImpl} object which is used to initialize this {@code CombinedConfigurationBuilder}.
 * This provider will then be called when the corresponding custom tag is detected. For many default configuration
 * classes providers are already registered.
 * </p>
 * <p>
 * The configuration definition file has the following basic structure:
 * </p>
 *
 * <pre>
 * &lt;configuration systemProperties="properties file name"&gt;
 *   &lt;header&gt;
 *     &lt;!-- Optional meta information about the combined configuration --&gt;
 *   &lt;/header&gt;
 *   &lt;override&gt;
 *     &lt;!-- Declarations for override configurations --&gt;
 *   &lt;/override&gt;
 *   &lt;additional&gt;
 *     &lt;!-- Declarations for union configurations --&gt;
 *   &lt;/additional&gt;
 * &lt;/configuration&gt;
 * </pre>
 *
 * <p>
 * The name of the root element (here {@code configuration}) is arbitrary. The optional {@code systemProperties}
 * attribute identifies the path to a property file containing properties that should be added to the system properties.
 * If specified on the root element, the system properties are set before the rest of the configuration is processed.
 * </p>
 * <p>
 * There are two sections (both of them are optional) for declaring <em>override</em> and <em>additional</em>
 * configurations. Configurations in the former section are evaluated in the order of their declaration, and properties
 * of configurations declared earlier hide those of configurations declared later. Configurations in the latter section
 * are combined to a union configuration, i.e. all of their properties are added to a large hierarchical configuration.
 * Configuration declarations that occur as direct children of the root element are treated as override declarations.
 * </p>
 * <p>
 * Each configuration declaration consists of a tag whose name is associated with a
 * {@code CombinedConfigurationBuilderProvider}. This can be one of the predefined tags like {@code properties}, or
 * {@code xml}, or a custom tag, for which a configuration builder provider was registered (as described above).
 * Attributes and sub elements with specific initialization parameters can be added. There are some reserved attributes
 * with a special meaning that can be used in every configuration declaration:
 * </p>
 * <table border="1">
 * <caption>Standard attributes for configuration declarations</caption>
 * <tr>
 * <th>Attribute</th>
 * <th>Meaning</th>
 * </tr>
 * <tr>
 * <td valign="top">{@code config-name}</td>
 * <td>Allows specifying a name for this configuration. This name can be used to obtain a reference to the configuration
 * from the resulting combined configuration (see below). It can also be passed to the {@link #getNamedBuilder(String)}
 * method.</td>
 * </tr>
 * <tr>
 * <td valign="top">{@code config-at}</td>
 * <td>With this attribute an optional prefix can be specified for the properties of the corresponding
 * configuration.</td>
 * </tr>
 * <tr>
 * <td valign="top">{@code config-optional}</td>
 * <td>Declares a configuration source as optional. This means that errors that occur when creating the configuration
 * are ignored.</td>
 * </tr>
 * <tr>
 * <td valign="top">{@code config-reload}</td>
 * <td>Many configuration sources support a reloading mechanism. For those sources it is possible to enable reloading by
 * providing this attribute with a value of <strong>true</strong>.</td>
 * </tr>
 * </table>
 * <p>
 * The optional <em>header</em> section can contain some meta data about the created configuration itself. For instance,
 * it is possible to set further properties of the {@code NodeCombiner} objects used for constructing the resulting
 * configuration.
 * </p>
 * <p>
 * The default configuration object returned by this builder is an instance of the {@link CombinedConfiguration} class.
 * This allows for convenient access to the configuration objects maintained by the combined configuration (e.g. for
 * updates of single configuration objects). It has also the advantage that the properties stored in all declared
 * configuration objects are collected and transformed into a single hierarchical structure, which can be accessed using
 * different expression engines. The actual {@code CombinedConfiguration} implementation can be overridden by specifying
 * the class in the <em>config-class</em> attribute of the result element.
 * </p>
 * <p>
 * A custom EntityResolver can be used for all XMLConfigurations by adding
 * </p>
 *
 * <pre>
 * &lt;entity-resolver config-class="EntityResolver fully qualified class name"&gt;
 * </pre>
 *
 * <p>
 * A specific CatalogResolver can be specified for all XMLConfiguration sources by adding
 * </p>
 *
 * <pre>
 * &lt;entity-resolver catalogFiles="comma separated list of catalog files"&gt;
 * </pre>
 *
 * <p>
 * Additional ConfigurationProviders can be added by configuring them in the <em>header</em> section.
 * </p>
 *
 * <pre>
 * &lt;providers&gt;
 *   &lt;provider config-tag="tag name" config-class="provider fully qualified class name"/&gt;
 * &lt;/providers&gt;
 * </pre>
 *
 * <p>
 * Additional variable resolvers can be added by configuring them in the <em>header</em> section.
 * </p>
 *
 * <pre>
 * &lt;lookups&gt;
 *   &lt;lookup config-prefix="prefix" config-class="StrLookup fully qualified class name"/&gt;
 * &lt;/lookups&gt;
 * </pre>
 *
 * <p>
 * All declared override configurations are directly added to the resulting combined configuration. If they are given
 * names (using the {@code config-name} attribute), they can directly be accessed using the
 * {@code getConfiguration(String)} method of {@code CombinedConfiguration}. The additional configurations are
 * altogether added to another combined configuration, which uses a union combiner. Then this union configuration is
 * added to the resulting combined configuration under the name defined by the {@code ADDITIONAL_NAME} constant. The
 * {@link #getNamedBuilder(String)} method can be used to access the {@code ConfigurationBuilder} objects for all
 * configuration sources which have been assigned a name; care has to be taken that these names are unique.
 * </p>
 *
 * @since 1.3
 */
public class CombinedConfigurationBuilder extends BasicConfigurationBuilder<CombinedConfiguration> {
    /**
     * Constant for the name of the additional configuration. If the configuration definition file contains an
     * {@code additional} section, a special union configuration is created and added under this name to the resulting
     * combined configuration.
     */
    public static final String ADDITIONAL_NAME = CombinedConfigurationBuilder.class.getName() + "/ADDITIONAL_CONFIG";

    /** Constant for the name of the configuration bean factory. */
    static final String CONFIG_BEAN_FACTORY_NAME = CombinedConfigurationBuilder.class.getName() + ".CONFIG_BEAN_FACTORY_NAME";

    /** Constant for the reserved name attribute. */
    static final String ATTR_NAME = DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_START + XMLBeanDeclaration.RESERVED_PREFIX + "name"
        + DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_END;

    /** Constant for the name of the at attribute. */
    static final String ATTR_ATNAME = "at";

    /** Constant for the reserved at attribute. */
    static final String ATTR_AT_RES = DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_START + XMLBeanDeclaration.RESERVED_PREFIX + ATTR_ATNAME
        + DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_END;

    /** Constant for the at attribute without the reserved prefix. */
    static final String ATTR_AT = DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_START + ATTR_ATNAME + DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_END;

    /** Constant for the name of the optional attribute. */
    static final String ATTR_OPTIONALNAME = "optional";

    /** Constant for the reserved optional attribute. */
    static final String ATTR_OPTIONAL_RES = DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_START + XMLBeanDeclaration.RESERVED_PREFIX + ATTR_OPTIONALNAME
        + DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_END;

    /** Constant for the optional attribute without the reserved prefix. */
    static final String ATTR_OPTIONAL = DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_START + ATTR_OPTIONALNAME
        + DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_END;

    /** Constant for the forceCreate attribute. */
    static final String ATTR_FORCECREATE = DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_START + XMLBeanDeclaration.RESERVED_PREFIX + "forceCreate"
        + DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_END;

    /** Constant for the reload attribute. */
    static final String ATTR_RELOAD = DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_START + XMLBeanDeclaration.RESERVED_PREFIX + "reload"
        + DefaultExpressionEngineSymbols.DEFAULT_ATTRIBUTE_END;

    /**
     * Constant for the tag attribute for providers.
     */
    static final String KEY_SYSTEM_PROPS = "[@systemProperties]";

    /** Constant for the name of the header section. */
    static final String SEC_HEADER = "header";

    /** Constant for an expression that selects the union configurations. */
    static final String KEY_UNION = "additional";

    /** An array with the names of top level configuration sections. */
    static final String[] CONFIG_SECTIONS = {"additional", "override", SEC_HEADER};

    /**
     * Constant for an expression that selects override configurations in the override section.
     */
    static final String KEY_OVERRIDE = "override";

    /**
     * Constant for the key that points to the list nodes definition of the override combiner.
     */
    static final String KEY_OVERRIDE_LIST = SEC_HEADER + ".combiner.override.list-nodes.node";

    /**
     * Constant for the key that points to the list nodes definition of the additional combiner.
     */
    static final String KEY_ADDITIONAL_LIST = SEC_HEADER + ".combiner.additional.list-nodes.node";

    /**
     * Constant for the key for defining providers in the configuration file.
     */
    static final String KEY_CONFIGURATION_PROVIDERS = SEC_HEADER + ".providers.provider";

    /**
     * Constant for the tag attribute for providers.
     */
    static final String KEY_PROVIDER_KEY = XMLBeanDeclaration.ATTR_PREFIX + "tag]";

    /**
     * Constant for the key for defining variable resolvers
     */
    static final String KEY_CONFIGURATION_LOOKUPS = SEC_HEADER + ".lookups.lookup";

    /**
     * Constant for the key for defining entity resolvers
     */
    static final String KEY_ENTITY_RESOLVER = SEC_HEADER + ".entity-resolver";

    /**
     * Constant for the prefix attribute for lookups.
     */
    static final String KEY_LOOKUP_KEY = XMLBeanDeclaration.ATTR_PREFIX + "prefix]";

    /**
     * Constant for the FileSystem.
     */
    static final String FILE_SYSTEM = SEC_HEADER + ".fileSystem";

    /**
     * Constant for the key of the result declaration. This key can point to a bean declaration, which defines properties of
     * the resulting combined configuration.
     */
    static final String KEY_RESULT = SEC_HEADER + ".result";

    /** Constant for the key of the combiner in the result declaration. */
    static final String KEY_COMBINER = KEY_RESULT + ".nodeCombiner";

    /** Constant for the XML file extension. */
    static final String EXT_XML = "xml";

    /** Constant for the basic configuration builder class. */
    private static final String BASIC_BUILDER = "org.apache.commons.configuration2.builder.BasicConfigurationBuilder";

    /** Constant for the file-based configuration builder class. */
    private static final String FILE_BUILDER = "org.apache.commons.configuration2.builder.FileBasedConfigurationBuilder";

    /** Constant for the reloading file-based configuration builder class. */
    private static final String RELOADING_BUILDER = "org.apache.commons.configuration2.builder.ReloadingFileBasedConfigurationBuilder";

    /** Constant for the name of the file-based builder parameters class. */
    private static final String FILE_PARAMS = "org.apache.commons.configuration2.builder.FileBasedBuilderParametersImpl";

    /** Constant for the provider for properties files. */
    private static final ConfigurationBuilderProvider PROPERTIES_PROVIDER = new FileExtensionConfigurationBuilderProvider(FILE_BUILDER, RELOADING_BUILDER,
        "org.apache.commons.configuration2.XMLPropertiesConfiguration", "org.apache.commons.configuration2.PropertiesConfiguration", EXT_XML,
        Collections.singletonList(FILE_PARAMS));

    /** Constant for the provider for XML files. */
    private static final ConfigurationBuilderProvider XML_PROVIDER = new BaseConfigurationBuilderProvider(FILE_BUILDER, RELOADING_BUILDER,
        "org.apache.commons.configuration2.XMLConfiguration", Collections.singletonList("org.apache.commons.configuration2.builder.XMLBuilderParametersImpl"));

    /** Constant for the provider for JNDI sources. */
    private static final BaseConfigurationBuilderProvider JNDI_PROVIDER = new BaseConfigurationBuilderProvider(BASIC_BUILDER, null,
        "org.apache.commons.configuration2.JNDIConfiguration",
        Collections.singletonList("org.apache.commons.configuration2.builder.JndiBuilderParametersImpl"));

    /** Constant for the provider for system properties. */
    private static final BaseConfigurationBuilderProvider SYSTEM_PROVIDER = new BaseConfigurationBuilderProvider(BASIC_BUILDER, null,
        "org.apache.commons.configuration2.SystemConfiguration", Collections.singletonList("org.apache.commons.configuration2.builder.BasicBuilderParameters"));

    /** Constant for the provider for ini files. */
    private static final BaseConfigurationBuilderProvider INI_PROVIDER = new BaseConfigurationBuilderProvider(FILE_BUILDER, RELOADING_BUILDER,
        "org.apache.commons.configuration2.INIConfiguration", Collections.singletonList(FILE_PARAMS));

    /** Constant for the provider for environment properties. */
    private static final BaseConfigurationBuilderProvider ENV_PROVIDER = new BaseConfigurationBuilderProvider(BASIC_BUILDER, null,
        "org.apache.commons.configuration2.EnvironmentConfiguration",
        Collections.singletonList("org.apache.commons.configuration2.builder.BasicBuilderParameters"));

    /** Constant for the provider for plist files. */
    private static final BaseConfigurationBuilderProvider PLIST_PROVIDER = new FileExtensionConfigurationBuilderProvider(FILE_BUILDER, RELOADING_BUILDER,
        "org.apache.commons.configuration2.plist.XMLPropertyListConfiguration", "org.apache.commons.configuration2.plist.PropertyListConfiguration", EXT_XML,
        Collections.singletonList(FILE_PARAMS));

    /** Constant for the provider for configuration definition files. */
    private static final BaseConfigurationBuilderProvider COMBINED_PROVIDER = new CombinedConfigurationBuilderProvider();

    /** Constant for the provider for multiple XML configurations. */
    private static final MultiFileConfigurationBuilderProvider MULTI_XML_PROVIDER = new MultiFileConfigurationBuilderProvider(
        "org.apache.commons.configuration2.XMLConfiguration", "org.apache.commons.configuration2.builder.XMLBuilderParametersImpl");

    /** An array with the names of the default tags. */
    private static final String[] DEFAULT_TAGS = {"properties", "xml", "hierarchicalXml", "plist", "ini", "system", "env", "jndi", "configuration",
        "multiFile"};

    /** An array with the providers for the default tags. */
    private static final ConfigurationBuilderProvider[] DEFAULT_PROVIDERS = {PROPERTIES_PROVIDER, XML_PROVIDER, XML_PROVIDER, PLIST_PROVIDER, INI_PROVIDER,
        SYSTEM_PROVIDER, ENV_PROVIDER, JNDI_PROVIDER, COMBINED_PROVIDER, MULTI_XML_PROVIDER};

    /** A map with the default configuration builder providers. */
    private static final Map<String, ConfigurationBuilderProvider> DEFAULT_PROVIDERS_MAP;

    /** The builder for the definition configuration. */
    private ConfigurationBuilder<? extends HierarchicalConfiguration<?>> definitionBuilder;

    /** Stores temporarily the configuration with the builder definitions. */
    private HierarchicalConfiguration<?> definitionConfiguration;

    /** The object with data about configuration sources. */
    private ConfigurationSourceData sourceData;

    /** Stores the current parameters object. */
    private CombinedBuilderParametersImpl currentParameters;

    /** The current XML parameters object. */
    private XMLBuilderParametersImpl currentXMLParameters;

    /** The configuration that is currently constructed. */
    private CombinedConfiguration currentConfiguration;

    /**
     * A {@code ConfigurationInterpolator} to be used as parent for all child configurations to enable cross-source
     * interpolation.
     */
    private ConfigurationInterpolator parentInterpolator;

    /**
     * Creates a new instance of {@code CombinedConfigurationBuilder}. No parameters are set.
     */
    public CombinedConfigurationBuilder() {
        super(CombinedConfiguration.class);
    }

    /**
     *
     * Creates a new instance of {@code CombinedConfigurationBuilder} and sets the specified initialization parameters.
     *
     * @param params a map with initialization parameters
     */
    public CombinedConfigurationBuilder(final Map<String, Object> params) {
        super(CombinedConfiguration.class, params);
    }

    /**
     *
     * Creates a new instance of {@code CombinedConfigurationBuilder} and sets the specified initialization parameters and
     * the <em>allowFailOnInit</em> flag.
     *
     * @param params a map with initialization parameters
     * @param allowFailOnInit the <em>allowFailOnInit</em> flag
     */
    public CombinedConfigurationBuilder(final Map<String, Object> params, final boolean allowFailOnInit) {
        super(CombinedConfiguration.class, params, allowFailOnInit);
    }

    /**
     * Returns the {@code ConfigurationBuilder} which creates the definition configuration.
     *
     * @return the builder for the definition configuration
     * @throws ConfigurationException if an error occurs
     */
    public synchronized ConfigurationBuilder<? extends HierarchicalConfiguration<?>> getDefinitionBuilder() throws ConfigurationException {
        if (definitionBuilder == null) {
            definitionBuilder = setupDefinitionBuilder(getParameters());
            addDefinitionBuilderChangeListener(definitionBuilder);
        }
        return definitionBuilder;
    }

    /**
     * {@inheritDoc} This method is overridden to adapt the return type.
     */
    @Override
    public CombinedConfigurationBuilder configure(final BuilderParameters... params) {
        super.configure(params);
        return this;
    }

    /**
     * <p>
     * Returns the configuration builder with the given name. With this method a builder of a child configuration which was
     * given a name in the configuration definition file can be accessed directly.
     * </p>
     * <p>
     * <strong>Important note:</strong> This method only returns a meaningful result after the result configuration has been
     * created by calling {@code getConfiguration()}. If called before, always an exception is thrown.
     * </p>
     *
     * @param name the name of the builder in question
     * @return the child configuration builder with this name
     * @throws ConfigurationException if information about named builders is not yet available or no builder with this name
     *         exists
     */
    public synchronized ConfigurationBuilder<? extends Configuration> getNamedBuilder(final String name) throws ConfigurationException {
        if (sourceData == null) {
            throw new ConfigurationException("Information about child builders" + " has not been setup yet! Call getConfiguration() first.");
        }
        final ConfigurationBuilder<? extends Configuration> builder = sourceData.getNamedBuilder(name);
        if (builder == null) {
            throw new ConfigurationException("Builder cannot be resolved: " + name);
        }
        return builder;
    }

    /**
     * <p>
     * Returns a set with the names of all child configuration builders. A tag defining a configuration source in the
     * configuration definition file can have the {@code config-name} attribute. If this attribute is present, the
     * corresponding builder is assigned this name and can be directly accessed through the {@link #getNamedBuilder(String)}
     * method. This method returns a collection with all available builder names.
     * </p>
     * <p>
     * <strong>Important note:</strong> This method only returns a meaningful result after the result configuration has been
     * created by calling {@code getConfiguration()}. If called before, always an empty set is returned.
     * </p>
     *
     * @return a set with the names of all builders
     */
    public synchronized Set<String> builderNames() {
        if (sourceData == null) {
            return Collections.emptySet();
        }
        return Collections.unmodifiableSet(sourceData.builderNames());
    }

    /**
     * {@inheritDoc} This implementation resets some specific internal state of this builder.
     */
    @Override
    public synchronized void resetParameters() {
        super.resetParameters();
        definitionBuilder = null;
        definitionConfiguration = null;
        currentParameters = null;
        currentXMLParameters = null;

        if (sourceData != null) {
            sourceData.cleanUp();
            sourceData = null;
        }
    }

    /**
     * Obtains the {@code ConfigurationBuilder} object which provides access to the configuration containing the definition
     * of the combined configuration to create. If a definition builder is defined in the parameters, it is used. Otherwise,
     * we check whether the combined builder parameters object contains a parameters object for the definition builder. If
     * this is the case, a builder for an {@code XMLConfiguration} is created and configured with this object. As a last
     * resort, it is looked for a {@link FileBasedBuilderParametersImpl} object in the properties. If found, also a XML
     * configuration builder is created which loads this file. Note: This method is called from a synchronized block.
     *
     * @param params the current parameters for this builder
     * @return the builder for the definition configuration
     * @throws ConfigurationException if an error occurs
     */
    protected ConfigurationBuilder<? extends HierarchicalConfiguration<?>> setupDefinitionBuilder(final Map<String, Object> params)
        throws ConfigurationException {
        final CombinedBuilderParametersImpl cbParams = CombinedBuilderParametersImpl.fromParameters(params);
        if (cbParams != null) {
            final ConfigurationBuilder<? extends HierarchicalConfiguration<?>> defBuilder = cbParams.getDefinitionBuilder();
            if (defBuilder != null) {
                return defBuilder;
            }

            if (cbParams.getDefinitionBuilderParameters() != null) {
                return createXMLDefinitionBuilder(cbParams.getDefinitionBuilderParameters());
            }
        }

        final BuilderParameters fileParams = FileBasedBuilderParametersImpl.fromParameters(params);
        if (fileParams != null) {
            return createXMLDefinitionBuilder(fileParams);
        }

        throw new ConfigurationException("No builder for configuration definition specified!");
    }

    /**
     * Creates a default builder for the definition configuration and initializes it with a parameters object. This method
     * is called if no definition builder is defined in this builder's parameters. This implementation creates a default
     * file-based builder which produces an {@code XMLConfiguration}; it expects a corresponding file specification. Note:
     * This method is called in a synchronized block.
     *
     * @param builderParams the parameters object for the builder
     * @return the standard builder for the definition configuration
     */
    protected ConfigurationBuilder<? extends HierarchicalConfiguration<?>> createXMLDefinitionBuilder(final BuilderParameters builderParams) {
        return new FileBasedConfigurationBuilder<>(XMLConfiguration.class).configure(builderParams);
    }

    /**
     * Returns the configuration containing the definition of the combined configuration to be created. This method only
     * returns a defined result during construction of the result configuration. The definition configuration is obtained
     * from the definition builder at first access and then stored temporarily to ensure that during result construction
     * always the same configuration instance is used. (Otherwise, it would be possible that the definition builder returns
     * a different instance when queried multiple times.)
     *
     * @return the definition configuration
     * @throws ConfigurationException if an error occurs
     */
    protected HierarchicalConfiguration<?> getDefinitionConfiguration() throws ConfigurationException {
        if (definitionConfiguration == null) {
            definitionConfiguration = getDefinitionBuilder().getConfiguration();
        }
        return definitionConfiguration;
    }

    /**
     * Returns a collection with the builders for all child configuration sources. This method can be used by derived
     * classes providing additional functionality on top of the declared configuration sources. It only returns a defined
     * value during construction of the result configuration instance.
     *
     * @return a collection with the builders for child configuration sources
     */
    protected synchronized Collection<ConfigurationBuilder<? extends Configuration>> getChildBuilders() {
        return sourceData.getChildBuilders();
    }

    /**
     * {@inheritDoc} This implementation evaluates the {@code result} property of the definition configuration. It creates a
     * combined bean declaration with both the properties specified in the definition file and the properties defined as
     * initialization parameters.
     */
    @Override
    protected BeanDeclaration createResultDeclaration(final Map<String, Object> params) throws ConfigurationException {
        final BeanDeclaration paramsDecl = super.createResultDeclaration(params);
        final XMLBeanDeclaration resultDecl = new XMLBeanDeclaration(getDefinitionConfiguration(), KEY_RESULT, true, CombinedConfiguration.class.getName());
        return new CombinedBeanDeclaration(resultDecl, paramsDecl);
    }

    /**
     * {@inheritDoc} This implementation processes the definition configuration in order to
     * <ul>
     * <li>initialize the resulting {@code CombinedConfiguration}</li>
     * <li>determine the builders for all configuration sources</li>
     * <li>populate the resulting {@code CombinedConfiguration}</li>
     * </ul>
     */
    @Override
    protected void initResultInstance(final CombinedConfiguration result) throws ConfigurationException {
        super.initResultInstance(result);

        currentConfiguration = result;
        final HierarchicalConfiguration<?> config = getDefinitionConfiguration();
        if (config.getMaxIndex(KEY_COMBINER) < 0) {
            // No combiner defined => set default
            result.setNodeCombiner(new OverrideCombiner());
        }

        setUpCurrentParameters();
        initNodeCombinerListNodes(result, config, KEY_OVERRIDE_LIST);
        registerConfiguredProviders(config);
        setUpCurrentXMLParameters();
        currentXMLParameters.setFileSystem(initFileSystem(config));
        initSystemProperties(config, getBasePath());
        registerConfiguredLookups(config, result);
        configureEntityResolver(config, currentXMLParameters);
        setUpParentInterpolator(currentConfiguration, config);

        final ConfigurationSourceData data = getSourceData();
        final boolean createBuilders = data.getChildBuilders().isEmpty();
        final List<ConfigurationBuilder<? extends Configuration>> overrideBuilders = data.createAndAddConfigurations(result, data.getOverrideSources(),
            data.overrideBuilders);
        if (createBuilders) {
            data.overrideBuilders.addAll(overrideBuilders);
        }
        if (!data.getUnionSources().isEmpty()) {
            final CombinedConfiguration addConfig = createAdditionalsConfiguration(result);
            result.addConfiguration(addConfig, ADDITIONAL_NAME);
            initNodeCombinerListNodes(addConfig, config, KEY_ADDITIONAL_LIST);
            final List<ConfigurationBuilder<? extends Configuration>> unionBuilders = data.createAndAddConfigurations(addConfig, data.unionDeclarations,
                data.unionBuilders);
            if (createBuilders) {
                data.unionBuilders.addAll(unionBuilders);
            }
        }

        result.isEmpty(); // this sets up the node structure
        currentConfiguration = null;
    }

    /**
     * Creates the {@code CombinedConfiguration} for the configuration sources in the {@code &lt;additional&gt;} section.
     * This method is called when the builder constructs the final configuration. It creates a new
     * {@code CombinedConfiguration} and initializes some properties from the result configuration.
     *
     * @param resultConfig the result configuration (this is the configuration that will be returned by the builder)
     * @return the {@code CombinedConfiguration} for the additional configuration sources
     * @since 1.7
     */
    protected CombinedConfiguration createAdditionalsConfiguration(final CombinedConfiguration resultConfig) {
        final CombinedConfiguration addConfig = new CombinedConfiguration(new UnionCombiner());
        addConfig.setListDelimiterHandler(resultConfig.getListDelimiterHandler());
        return addConfig;
    }

    /**
     * Processes custom {@link Lookup} objects that might be declared in the definition configuration. Each {@code Lookup}
     * object is registered at the definition configuration and at the result configuration. It is also added to all child
     * configurations added to the resulting combined configuration.
     *
     * @param defConfig the definition configuration
     * @param resultConfig the resulting configuration
     * @throws ConfigurationException if an error occurs
     */
    protected void registerConfiguredLookups(final HierarchicalConfiguration<?> defConfig, final Configuration resultConfig) throws ConfigurationException {
        final Map<String, Lookup> lookups = new HashMap<>();

        final List<? extends HierarchicalConfiguration<?>> nodes = defConfig.configurationsAt(KEY_CONFIGURATION_LOOKUPS);
        for (final HierarchicalConfiguration<?> config : nodes) {
            final XMLBeanDeclaration decl = new XMLBeanDeclaration(config);
            final String key = config.getString(KEY_LOOKUP_KEY);
            final Lookup lookup = (Lookup) fetchBeanHelper().createBean(decl);
            lookups.put(key, lookup);
        }

        if (!lookups.isEmpty()) {
            final ConfigurationInterpolator defCI = defConfig.getInterpolator();
            if (defCI != null) {
                defCI.registerLookups(lookups);
            }
            resultConfig.getInterpolator().registerLookups(lookups);
        }
    }

    /**
     * Creates and initializes a default {@code FileSystem} if the definition configuration contains a corresponding
     * declaration. The file system returned by this method is used as default for all file-based child configuration
     * sources.
     *
     * @param config the definition configuration
     * @return the default {@code FileSystem} (may be <b>null</b>)
     * @throws ConfigurationException if an error occurs
     */
    protected FileSystem initFileSystem(final HierarchicalConfiguration<?> config) throws ConfigurationException {
        if (config.getMaxIndex(FILE_SYSTEM) == 0) {
            final XMLBeanDeclaration decl = new XMLBeanDeclaration(config, FILE_SYSTEM);
            return (FileSystem) fetchBeanHelper().createBean(decl);
        }
        return null;
    }

    /**
     * Handles a file with system properties that may be defined in the definition configuration. If such property file is
     * configured, all of its properties are added to the system properties.
     *
     * @param config the definition configuration
     * @param basePath the base path defined for this builder (may be <b>null</b>)
     * @throws ConfigurationException if an error occurs.
     */
    protected void initSystemProperties(final HierarchicalConfiguration<?> config, final String basePath) throws ConfigurationException {
        final String fileName = config.getString(KEY_SYSTEM_PROPS);
        if (fileName != null) {
            try {
                SystemConfiguration.setSystemProperties(basePath, fileName);
            } catch (final Exception ex) {
                throw new ConfigurationException("Error setting system properties from " + fileName, ex);
            }
        }
    }

    /**
     * Creates and initializes a default {@code EntityResolver} if the definition configuration contains a corresponding
     * declaration.
     *
     * @param config the definition configuration
     * @param xmlParams the (already partly initialized) object with XML parameters; here the new resolver is to be stored
     * @throws ConfigurationException if an error occurs
     */
    protected void configureEntityResolver(final HierarchicalConfiguration<?> config, final XMLBuilderParametersImpl xmlParams) throws ConfigurationException {
        if (config.getMaxIndex(KEY_ENTITY_RESOLVER) == 0) {
            final XMLBeanDeclaration decl = new XMLBeanDeclaration(config, KEY_ENTITY_RESOLVER, true);
            final EntityResolver resolver = (EntityResolver) fetchBeanHelper().createBean(decl, CatalogResolver.class);
            final FileSystem fileSystem = xmlParams.getFileHandler().getFileSystem();
            if (fileSystem != null) {
                BeanHelper.setProperty(resolver, "fileSystem", fileSystem);
            }
            final String basePath = xmlParams.getFileHandler().getBasePath();
            if (basePath != null) {
                BeanHelper.setProperty(resolver, "baseDir", basePath);
            }
            final ConfigurationInterpolator ci = new ConfigurationInterpolator();
            ci.registerLookups(fetchPrefixLookups());
            BeanHelper.setProperty(resolver, "interpolator", ci);

            xmlParams.setEntityResolver(resolver);
        }
    }

    /**
     * Returns the {@code ConfigurationBuilderProvider} for the given tag. This method is called during creation of the
     * result configuration. (It is not allowed to call it at another point of time; result is then unpredictable!) It
     * supports all default providers and custom providers added through the parameters object as well.
     *
     * @param tagName the name of the tag
     * @return the provider that was registered for this tag or <b>null</b> if there is none
     */
    protected ConfigurationBuilderProvider providerForTag(final String tagName) {
        return currentParameters.providerForTag(tagName);
    }

    /**
     * Initializes a parameters object for a child builder. This combined configuration builder has a bunch of properties
     * which may be inherited by child configurations, e.g. the base path, the file system, etc. While processing the
     * builders for child configurations, this method is called for each parameters object for a child builder. It
     * initializes some properties of the passed in parameters objects which are derived from this parent builder.
     *
     * @param params the parameters object to be initialized
     */
    protected void initChildBuilderParameters(final BuilderParameters params) {
        initDefaultChildParameters(params);

        if (params instanceof BasicBuilderParameters) {
            initChildBasicParameters((BasicBuilderParameters) params);
        }
        if (params instanceof XMLBuilderProperties<?>) {
            initChildXMLParameters((XMLBuilderProperties<?>) params);
        }
        if (params instanceof FileBasedBuilderProperties<?>) {
            initChildFileBasedParameters((FileBasedBuilderProperties<?>) params);
        }
        if (params instanceof CombinedBuilderParametersImpl) {
            initChildCombinedParameters((CombinedBuilderParametersImpl) params);
        }
    }

    /**
     * Initializes the event listeners of the specified builder from this object. This method is used to inherit all
     * listeners from a parent builder.
     *
     * @param dest the destination builder object which is to be initialized
     */
    void initChildEventListeners(final BasicConfigurationBuilder<? extends Configuration> dest) {
        copyEventListeners(dest);
    }

    /**
     * Returns the configuration object that is currently constructed. This method can be called during construction of the
     * result configuration. It is intended for internal usage, e.g. some specialized builder providers need access to this
     * configuration to perform advanced initialization.
     *
     * @return the configuration that us currently under construction
     */
    CombinedConfiguration getConfigurationUnderConstruction() {
        return currentConfiguration;
    }

    /**
     * Initializes a bean using the current {@code BeanHelper}. This is needed by builder providers when the configuration
     * objects for sub builders are constructed.
     *
     * @param bean the bean to be initialized
     * @param decl the {@code BeanDeclaration}
     */
    void initBean(final Object bean, final BeanDeclaration decl) {
        fetchBeanHelper().initBean(bean, decl);
    }

    /**
     * Initializes the current parameters object. This object has either been passed at builder configuration time or it is
     * newly created. In any case, it is manipulated during result creation.
     */
    private void setUpCurrentParameters() {
        currentParameters = CombinedBuilderParametersImpl.fromParameters(getParameters(), true);
        currentParameters.registerMissingProviders(DEFAULT_PROVIDERS_MAP);
    }

    /**
     * Sets up an XML parameters object which is used to store properties related to XML and file-based configurations
     * during creation of the result configuration. The properties stored in this object can be inherited to child
     * configurations.
     *
     * @throws ConfigurationException if an error occurs
     */
    private void setUpCurrentXMLParameters() throws ConfigurationException {
        currentXMLParameters = new XMLBuilderParametersImpl();
        initDefaultBasePath();
    }

    /**
     * Sets up a parent {@code ConfigurationInterpolator} object. This object has a default {@link Lookup} querying the
     * resulting combined configuration. Thus interpolation works globally across all configuration sources.
     *
     * @param resultConfig the result configuration
     * @param defConfig the definition configuration
     */
    private void setUpParentInterpolator(final Configuration resultConfig, final Configuration defConfig) {
        parentInterpolator = new ConfigurationInterpolator();
        parentInterpolator.addDefaultLookup(new ConfigurationLookup(resultConfig));
        final ConfigurationInterpolator defInterpolator = defConfig.getInterpolator();
        if (defInterpolator != null) {
            defInterpolator.setParentInterpolator(parentInterpolator);
        }
    }

    /**
     * Initializes the default base path for all file-based child configuration sources. The base path can be explicitly
     * defined in the parameters of this builder. Otherwise, if the definition builder is a file-based builder, it is
     * obtained from there.
     *
     * @throws ConfigurationException if an error occurs
     */
    private void initDefaultBasePath() throws ConfigurationException {
        assert currentParameters != null : "Current parameters undefined!";
        if (currentParameters.getBasePath() != null) {
            currentXMLParameters.setBasePath(currentParameters.getBasePath());
        } else {
            final ConfigurationBuilder<? extends HierarchicalConfiguration<?>> defBuilder = getDefinitionBuilder();
            if (defBuilder instanceof FileBasedConfigurationBuilder) {
                @SuppressWarnings("rawtypes")
                final FileBasedConfigurationBuilder fileBuilder = (FileBasedConfigurationBuilder) defBuilder;
                final URL url = fileBuilder.getFileHandler().getURL();
                currentXMLParameters.setBasePath(url != null ? url.toExternalForm() : fileBuilder.getFileHandler().getBasePath());
            }
        }
    }

    /**
     * Executes the {@link org.apache.commons.configuration2.builder.DefaultParametersManager DefaultParametersManager}
     * stored in the current parameters on the passed in parameters object. If default handlers have been registered for
     * this type of parameters, an initialization is now performed. This method is called before the parameters object is
     * initialized from the configuration definition file. So default values can be overridden later with concrete property
     * definitions.
     *
     * @param params the parameters to be initialized
     * @throws org.apache.commons.configuration2.ex.ConfigurationRuntimeException if an error occurs when copying properties
     */
    private void initDefaultChildParameters(final BuilderParameters params) {
        currentParameters.getChildDefaultParametersManager().initializeParameters(params);
    }

    /**
     * Initializes basic builder parameters for a child configuration with default settings set for this builder. This
     * implementation ensures that all {@code Lookup} objects are propagated to child configurations and interpolation is
     * setup correctly.
     *
     * @param params the parameters object
     */
    private void initChildBasicParameters(final BasicBuilderParameters params) {
        params.setPrefixLookups(fetchPrefixLookups());
        params.setParentInterpolator(parentInterpolator);
        if (currentParameters.isInheritSettings()) {
            params.inheritFrom(getParameters());
        }
    }

    /**
     * Initializes a parameters object for a file-based configuration with properties already set for this parent builder.
     * This method handles properties like a default file system or a base path.
     *
     * @param params the parameters object
     */
    private void initChildFileBasedParameters(final FileBasedBuilderProperties<?> params) {
        params.setBasePath(getBasePath());
        params.setFileSystem(currentXMLParameters.getFileHandler().getFileSystem());
    }

    /**
     * Initializes a parameters object for an XML configuration with properties already set for this parent builder.
     *
     * @param params the parameters object
     */
    private void initChildXMLParameters(final XMLBuilderProperties<?> params) {
        params.setEntityResolver(currentXMLParameters.getEntityResolver());
    }

    /**
     * Initializes a parameters object for a combined configuration builder with properties already set for this parent
     * builder. This implementation deals only with a subset of properties. Other properties are already handled by the
     * specialized builder provider.
     *
     * @param params the parameters object
     */
    private void initChildCombinedParameters(final CombinedBuilderParametersImpl params) {
        params.registerMissingProviders(currentParameters);
        params.setBasePath(getBasePath());
    }

    /**
     * Obtains the data object for the configuration sources and the corresponding builders. This object is created on first
     * access and reset when the definition builder sends a change event. This method is called in a synchronized block.
     *
     * @return the object with information about configuration sources
     * @throws ConfigurationException if an error occurs
     */
    private ConfigurationSourceData getSourceData() throws ConfigurationException {
        if (sourceData == null) {
            if (currentParameters == null) {
                setUpCurrentParameters();
                setUpCurrentXMLParameters();
            }
            sourceData = createSourceData();
        }
        return sourceData;
    }

    /**
     * Creates the data object for configuration sources and the corresponding builders.
     *
     * @return the newly created data object
     * @throws ConfigurationException if an error occurs
     */
    private ConfigurationSourceData createSourceData() throws ConfigurationException {
        final ConfigurationSourceData result = new ConfigurationSourceData();
        result.initFromDefinitionConfiguration(getDefinitionConfiguration());
        return result;
    }

    /**
     * Returns the current base path of this configuration builder. This is used for instance by all file-based child
     * configurations.
     *
     * @return the base path
     */
    private String getBasePath() {
        return currentXMLParameters.getFileHandler().getBasePath();
    }

    /**
     * Registers providers defined in the configuration.
     *
     * @param defConfig the definition configuration
     */
    private void registerConfiguredProviders(final HierarchicalConfiguration<?> defConfig) {
        final List<? extends HierarchicalConfiguration<?>> nodes = defConfig.configurationsAt(KEY_CONFIGURATION_PROVIDERS);
        for (final HierarchicalConfiguration<?> config : nodes) {
            final XMLBeanDeclaration decl = new XMLBeanDeclaration(config);
            final String key = config.getString(KEY_PROVIDER_KEY);
            currentParameters.registerProvider(key, (ConfigurationBuilderProvider) fetchBeanHelper().createBean(decl));
        }
    }

    /**
     * Adds a listener at the given definition builder which resets this builder when a reset of the definition builder
     * happens. This way it is ensured that this builder produces a new combined configuration when its definition
     * configuration changes.
     *
     * @param defBuilder the definition builder
     */
    private void addDefinitionBuilderChangeListener(final ConfigurationBuilder<? extends HierarchicalConfiguration<?>> defBuilder) {
        defBuilder.addEventListener(ConfigurationBuilderEvent.RESET, event -> {
            synchronized (CombinedConfigurationBuilder.this) {
                reset();
                definitionBuilder = defBuilder;
            }
        });
    }

    /**
     * Returns a map with the current prefix lookup objects. This map is obtained from the {@code ConfigurationInterpolator}
     * of the configuration under construction.
     *
     * @return the map with current prefix lookups (may be <b>null</b>)
     */
    private Map<String, ? extends Lookup> fetchPrefixLookups() {
        final CombinedConfiguration cc = getConfigurationUnderConstruction();
        return cc != null ? cc.getInterpolator().getLookups() : null;
    }

    /**
     * Creates {@code ConfigurationDeclaration} objects for the specified configurations.
     *
     * @param configs the list with configurations
     * @return a collection with corresponding declarations
     */
    private Collection<ConfigurationDeclaration> createDeclarations(final Collection<? extends HierarchicalConfiguration<?>> configs) {
        final Collection<ConfigurationDeclaration> declarations = new ArrayList<>(configs.size());
        for (final HierarchicalConfiguration<?> c : configs) {
            declarations.add(new ConfigurationDeclaration(this, c));
        }
        return declarations;
    }

    /**
     * Initializes the list nodes of the node combiner for the given combined configuration. This information can be set in
     * the header section of the configuration definition file for both the override and the union combiners.
     *
     * @param cc the combined configuration to initialize
     * @param defConfig the definition configuration
     * @param key the key for the list nodes
     */
    private static void initNodeCombinerListNodes(final CombinedConfiguration cc, final HierarchicalConfiguration<?> defConfig, final String key) {
        final List<Object> listNodes = defConfig.getList(key);
        for (final Object listNode : listNodes) {
            cc.getNodeCombiner().addListNode((String) listNode);
        }
    }

    /**
     * Creates the map with the default configuration builder providers.
     *
     * @return the map with default providers
     */
    private static Map<String, ConfigurationBuilderProvider> createDefaultProviders() {
        final Map<String, ConfigurationBuilderProvider> providers = new HashMap<>();
        for (int i = 0; i < DEFAULT_TAGS.length; i++) {
            providers.put(DEFAULT_TAGS[i], DEFAULT_PROVIDERS[i]);
        }
        return providers;
    }

    static {
        DEFAULT_PROVIDERS_MAP = createDefaultProviders();
    }

    /**
     * A data class for storing information about all configuration sources defined for a combined builder.
     */
    private class ConfigurationSourceData {
        /** A list with data for all builders for override configurations. */
        private final List<ConfigurationDeclaration> overrideDeclarations;

        /** A list with data for all builders for union configurations. */
        private final List<ConfigurationDeclaration> unionDeclarations;

        /** A list with the builders for override configurations. */
        private final List<ConfigurationBuilder<? extends Configuration>> overrideBuilders;

        /** A list with the builders for union configurations. */
        private final List<ConfigurationBuilder<? extends Configuration>> unionBuilders;

        /** A map for direct access to a builder by its name. */
        private final Map<String, ConfigurationBuilder<? extends Configuration>> namedBuilders;

        /** A collection with all child builders. */
        private final Collection<ConfigurationBuilder<? extends Configuration>> allBuilders;

        /** A listener for reacting on changes of sub builders. */
        private final EventListener<ConfigurationBuilderEvent> changeListener;

        /**
         * Creates a new instance of {@code ConfigurationSourceData}.
         */
        public ConfigurationSourceData() {
            overrideDeclarations = new ArrayList<>();
            unionDeclarations = new ArrayList<>();
            overrideBuilders = new ArrayList<>();
            unionBuilders = new ArrayList<>();
            namedBuilders = new HashMap<>();
            allBuilders = new LinkedList<>();
            changeListener = createBuilderChangeListener();
        }

        /**
         * Initializes this object from the specified definition configuration.
         *
         * @param config the definition configuration
         * @throws ConfigurationException if an error occurs
         */
        public void initFromDefinitionConfiguration(final HierarchicalConfiguration<?> config) throws ConfigurationException {
            overrideDeclarations.addAll(createDeclarations(fetchTopLevelOverrideConfigs(config)));
            overrideDeclarations.addAll(createDeclarations(config.childConfigurationsAt(KEY_OVERRIDE)));
            unionDeclarations.addAll(createDeclarations(config.childConfigurationsAt(KEY_UNION)));
        }

        /**
         * Processes the declaration of configuration builder providers, creates the corresponding builder if necessary, obtains
         * configurations, and adds them to the specified result configuration.
         *
         * @param ccResult the result configuration
         * @param srcDecl the collection with the declarations of configuration sources to process
         * @return a list with configuration builders
         * @throws ConfigurationException if an error occurs
         */
        public List<ConfigurationBuilder<? extends Configuration>> createAndAddConfigurations(final CombinedConfiguration ccResult,
            final List<ConfigurationDeclaration> srcDecl, final List<ConfigurationBuilder<? extends Configuration>> builders) throws ConfigurationException {
            final boolean createBuilders = builders.isEmpty();
            final List<ConfigurationBuilder<? extends Configuration>> newBuilders;
            if (createBuilders) {
                newBuilders = new ArrayList<>(srcDecl.size());
            } else {
                newBuilders = builders;
            }

            for (int i = 0; i < srcDecl.size(); i++) {
                final ConfigurationBuilder<? extends Configuration> b;
                if (createBuilders) {
                    b = createConfigurationBuilder(srcDecl.get(i));
                    newBuilders.add(b);
                } else {
                    b = builders.get(i);
                }
                addChildConfiguration(ccResult, srcDecl.get(i), b);
            }

            return newBuilders;
        }

        /**
         * Frees resources used by this object and performs clean up. This method is called when the owning builder is reset.
         */
        public void cleanUp() {
            for (final ConfigurationBuilder<?> b : getChildBuilders()) {
                b.removeEventListener(ConfigurationBuilderEvent.RESET, changeListener);
            }
            namedBuilders.clear();
        }

        /**
         * Returns a collection containing the builders for all child configuration sources.
         *
         * @return the child configuration builders
         */
        public Collection<ConfigurationBuilder<? extends Configuration>> getChildBuilders() {
            return allBuilders;
        }

        /**
         * Returns a collection with all configuration source declarations defined in the override section.
         *
         * @return the override configuration builders
         */
        public List<ConfigurationDeclaration> getOverrideSources() {
            return overrideDeclarations;
        }

        /**
         * Returns a collection with all configuration source declarations defined in the union section.
         *
         * @return the union configuration builders
         */
        public List<ConfigurationDeclaration> getUnionSources() {
            return unionDeclarations;
        }

        /**
         * Returns the {@code ConfigurationBuilder} with the given name. If no such builder is defined in the definition
         * configuration, result is <b>null</b>.
         *
         * @param name the name of the builder in question
         * @return the builder with this name or <b>null</b>
         */
        public ConfigurationBuilder<? extends Configuration> getNamedBuilder(final String name) {
            return namedBuilders.get(name);
        }

        /**
         * Returns a set with the names of all known named builders.
         *
         * @return the names of the available sub builders
         */
        public Set<String> builderNames() {
            return namedBuilders.keySet();
        }

        /**
         * Creates a configuration builder based on a source declaration in the definition configuration.
         *
         * @param decl the current {@code ConfigurationDeclaration}
         * @return the newly created builder
         * @throws ConfigurationException if an error occurs
         */
        private ConfigurationBuilder<? extends Configuration> createConfigurationBuilder(final ConfigurationDeclaration decl) throws ConfigurationException {
            final ConfigurationBuilderProvider provider = providerForTag(decl.getConfiguration().getRootElementName());
            if (provider == null) {
                throw new ConfigurationException("Unsupported configuration source: " + decl.getConfiguration().getRootElementName());
            }

            final ConfigurationBuilder<? extends Configuration> builder = provider.getConfigurationBuilder(decl);
            if (decl.getName() != null) {
                namedBuilders.put(decl.getName(), builder);
            }
            allBuilders.add(builder);
            builder.addEventListener(ConfigurationBuilderEvent.RESET, changeListener);
            return builder;
        }

        /**
         * Creates a new configuration using the specified builder and adds it to the resulting combined configuration.
         *
         * @param ccResult the resulting combined configuration
         * @param decl the current {@code ConfigurationDeclaration}
         * @param builder the configuration builder
         * @throws ConfigurationException if an error occurs
         */
        private void addChildConfiguration(final CombinedConfiguration ccResult, final ConfigurationDeclaration decl,
            final ConfigurationBuilder<? extends Configuration> builder) throws ConfigurationException {
            try {
                ccResult.addConfiguration(builder.getConfiguration(), decl.getName(), decl.getAt());
            } catch (final ConfigurationException cex) {
                // ignore exceptions for optional configurations
                if (!decl.isOptional()) {
                    throw cex;
                }
            }
        }

        /**
         * Creates a listener for builder change events. This listener is registered at all builders for child configurations.
         */
        private EventListener<ConfigurationBuilderEvent> createBuilderChangeListener() {
            return event -> resetResult();
        }

        /**
         * Finds the override configurations that are defined as top level elements in the configuration definition file. This
         * method fetches the child elements of the root node and removes the nodes that represent other configuration sections.
         * The remaining nodes are treated as definitions for override configurations.
         *
         * @param config the definition configuration
         * @return a list with sub configurations for the top level override configurations
         */
        private List<? extends HierarchicalConfiguration<?>> fetchTopLevelOverrideConfigs(final HierarchicalConfiguration<?> config) {

            final List<? extends HierarchicalConfiguration<?>> configs = config.childConfigurationsAt(null);
            for (final Iterator<? extends HierarchicalConfiguration<?>> it = configs.iterator(); it.hasNext();) {
                final String nodeName = it.next().getRootElementName();
                for (final String element : CONFIG_SECTIONS) {
                    if (element.equals(nodeName)) {
                        it.remove();
                        break;
                    }
                }
            }
            return configs;
        }
    }
}
