/*

  * 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.configuration;

 import java.io.File;

 import java.net.URL;

 import java.util.ArrayList;

 import java.util.Collections;

 import java.util.HashMap;

 import java.util.Iterator;

 import java.util.List;

 import java.util.Map;

 import org.apache.commons.configuration.beanutils.BeanDeclaration;

 import org.apache.commons.configuration.beanutils.BeanFactory;

 import org.apache.commons.configuration.beanutils.BeanHelper;

 import org.apache.commons.configuration.beanutils.DefaultBeanFactory;

 import org.apache.commons.configuration.beanutils.XMLBeanDeclaration;

 import org.apache.commons.configuration.tree.ConfigurationNode;

 import org.apache.commons.configuration.tree.DefaultExpressionEngine;

 import org.apache.commons.configuration.tree.OverrideCombiner;

 import org.apache.commons.configuration.tree.UnionCombiner;

 import org.apache.commons.logging.LogFactory;

 /**

  * <p>

  * A factory class that creates a composite configuration from 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, the corresponding <code>Configuration</code>

  * class is instantiated and initialized using the classes of the

  * <code>beanutils</code> package (namely

  * <code>{@link org.apache.commons.configuration.beanutils.XMLBeanDeclaration XMLBeanDeclaration}</code>

  * will be used to extract the configuration's initialization parameters, which

  * allows for complex initialization szenarios).

  * </p>

  * <p>

  * It is also possible to add custom tags to the configuration definition file.

  * For this purpose register your own <code>ConfigurationProvider</code>

  * implementation for your tag using the <code>addConfigurationProvider()</code>

  * method. This provider will then be called when the corresponding custom tag

  * is detected. For the default configuration classes providers are already

  * registered.

  * </p>

  * <p>

  * The configuration definition file has the following basic structure:

  * </p>

  * <p>

  *

  * <pre>

  * <configuration>

  *   <header>

  *     <!-- Optional meta information about the composite configuration -->

  *   </header>

  *   <override>

  *     <!-- Declarations for override configurations -->

  *   </override>

  *   <additional>

  *     <!-- Declarations for union configurations -->

  *   </additional>

  * </configuration>

  * </pre>

  *

  * </p>

  * <p>

  * The name of the root element (here <code>configuration</code>) is

  * arbitrary. 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>ConfigurationProvider</code>. This can be one of the

  * predefined tags like <code>properties</code>, or <code>xml</code>, or

  * a custom tag, for which a configuration provider was registered. 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>

  * <p>

  * <table border="1">

  * <tr>

  * <th>Attribute</th>

  * <th>Meaning</th>

  * </tr>

  * <tr>

  * <td valign="top"><code>config-name</code></td>

  * <td>Allows to specify a name for this configuration. This name can be used

  * to obtain a reference to the configuration from the resulting combined

  * configuration (see below).</td>

  * </tr>

  * <tr>

  * <td valign="top"><code>config-at</code></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</code></td>

  * <td>Declares a configuration as optional. This means that errors that occur

  * when creating the configuration are ignored. (However

  * <code>{@link org.apache.commons.configuration.event.ConfigurationErrorListener}</code>s

  * registered at the builder instance will get notified about this error: they

  * receive an event of type <code>EVENT_ERR_LOAD_OPTIONAL</code>. The key

  * property of this event contains the name of the optional configuration source

  * that caused this problem.)</td>

  * </tr>

  * </table>

  * </p>

  * <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</code> objects used for constructing

  * the resulting configuration.

  * </p>

  * <p>

  * The configuration object returned by this builder is an instance of the

  * <code>{@link CombinedConfiguration}</code> class. The return value of the

  * <code>getConfiguration()</code> method can be casted to this type, and the

  * <code>getConfiguration(boolean)</code> method directly declares

  * <code>CombinedConfiguration</code> as return type. 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.

  * </p>

  * <p>

  * All declared override configurations are directly added to the resulting

  * combined configuration. If they are given names (using the

  * <code>config-name</code> attribute), they can directly be accessed using

  * the <code>getConfiguration(String)</code> method of

  * <code>CombinedConfiguration</code>. The additional configurations are

  * alltogether 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</code>

  * constant.

  * </p>

  * <p>

  * Implementation note: This class is not thread-safe. Especially the

  * <code>getConfiguration()</code> methods should be called by a single thread

  * only.

  * </p>

  *

  * @since 1.3

  * @author <a

  * href="http://commons.apache.org/configuration/team-list.html">Commons

  * Configuration team</a>

  * @version $Id: DefaultConfigurationBuilder.java 595043 2007-11-14 20:44:43Z oheger $

  */

 public class DefaultConfigurationBuilder extends XMLConfiguration implements

         ConfigurationBuilder

 {

     /**

      * Constant for the name of the additional configuration. If the

      * configuration definition file contains an <code>additional</code>

      * section, a special union configuration is created and added under this

      * name to the resulting combined configuration.

      */

     public static final String ADDITIONAL_NAME = DefaultConfigurationBuilder.class

             .getName()

             + "/ADDITIONAL_CONFIG";

     /**

      * Constant for the type of error events caused by optional configurations

      * that cannot be loaded.

      */

     public static final int EVENT_ERR_LOAD_OPTIONAL = 51;

     /** Constant for the name of the configuration bean factory. */

     static final String CONFIG_BEAN_FACTORY_NAME = DefaultConfigurationBuilder.class

             .getName()

             + ".CONFIG_BEAN_FACTORY_NAME";

     /** Constant for the reserved name attribute. */

     static final String ATTR_NAME = DefaultExpressionEngine.DEFAULT_ATTRIBUTE_START

             + XMLBeanDeclaration.RESERVED_PREFIX

             + "name"

             + DefaultExpressionEngine.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 = DefaultExpressionEngine.DEFAULT_ATTRIBUTE_START

             + XMLBeanDeclaration.RESERVED_PREFIX

             + ATTR_ATNAME

             + DefaultExpressionEngine.DEFAULT_ATTRIBUTE_END;

     /** Constant for the at attribute without the reserved prefix. */

     static final String ATTR_AT = DefaultExpressionEngine.DEFAULT_ATTRIBUTE_START

             + ATTR_ATNAME + DefaultExpressionEngine.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 = DefaultExpressionEngine.DEFAULT_ATTRIBUTE_START

             + XMLBeanDeclaration.RESERVED_PREFIX

             + ATTR_OPTIONALNAME

             + DefaultExpressionEngine.DEFAULT_ATTRIBUTE_END;

     /** Constant for the optional attribute without the reserved prefix. */

     static final String ATTR_OPTIONAL = DefaultExpressionEngine.DEFAULT_ATTRIBUTE_START

             + ATTR_OPTIONALNAME + DefaultExpressionEngine.DEFAULT_ATTRIBUTE_END;

     /** Constant for the file name attribute. */

     static final String ATTR_FILENAME = DefaultExpressionEngine.DEFAULT_ATTRIBUTE_START

             + "fileName" + DefaultExpressionEngine.DEFAULT_ATTRIBUTE_END;

     /** Constant for the forceCreate attribute. */

     static final String ATTR_FORCECREATE = DefaultExpressionEngine.DEFAULT_ATTRIBUTE_START

             + XMLBeanDeclaration.RESERVED_PREFIX

             + "forceCreate"

             + DefaultExpressionEngine.DEFAULT_ATTRIBUTE_END;

     /** 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 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 provider for properties files. */

     private static final ConfigurationProvider PROPERTIES_PROVIDER = new FileExtensionConfigurationProvider(

             XMLPropertiesConfiguration.class, PropertiesConfiguration.class,

             EXT_XML);

     /** Constant for the provider for XML files. */

     private static final ConfigurationProvider XML_PROVIDER = new XMLConfigurationProvider();

     /** Constant for the provider for JNDI sources. */

     private static final ConfigurationProvider JNDI_PROVIDER = new ConfigurationProvider(

             JNDIConfiguration.class);

     /** Constant for the provider for system properties. */

     private static final ConfigurationProvider SYSTEM_PROVIDER = new ConfigurationProvider(

             SystemConfiguration.class);

     /** Constant for the provider for plist files. */

     private static final ConfigurationProvider PLIST_PROVIDER = new FileExtensionConfigurationProvider(

             "org.apache.commons.configuration.plist.XMLPropertyListConfiguration",

             "org.apache.commons.configuration.plist.PropertyListConfiguration",

             EXT_XML);

     /** Constant for the provider for configuration definition files.*/

     private static final ConfigurationProvider BUILDER_PROVIDER = new ConfigurationBuilderProvider();

     /** An array with the names of the default tags. */

     private static final String[] DEFAULT_TAGS =

     {"properties", "xml", "hierarchicalXml", "jndi", "system", "plist", "configuration"};

     /** An array with the providers for the default tags. */

     private static final ConfigurationProvider[] DEFAULT_PROVIDERS =

     {PROPERTIES_PROVIDER, XML_PROVIDER, XML_PROVIDER, JNDI_PROVIDER,

             SYSTEM_PROVIDER, PLIST_PROVIDER, BUILDER_PROVIDER};

     /**

      * The serial version UID.

      */

     private static final long serialVersionUID = -3113777854714492123L;

     /** Stores the configuration that is currently constructed.*/

     private CombinedConfiguration constructedConfiguration;

     /** Stores a map with the registered configuration providers. */

     private Map providers;

     /** Stores the base path to the configuration sources to load. */

     private String configurationBasePath;

     /**

      * Creates a new instance of <code>DefaultConfigurationBuilder</code>. A

      * configuration definition file is not yet loaded. Use the diverse setter

      * methods provided by file based configurations to specify the

      * configuration definition file.

      */

     public DefaultConfigurationBuilder()

     {

         super();

         providers = new HashMap();

         registerDefaultProviders();

         registerBeanFactory();

         setLogger(LogFactory.getLog(getClass()));

         addErrorLogListener();  // log errors per default

     }

     /**

      * Creates a new instance of <code>DefaultConfigurationBuilder</code> and

      * sets the specified configuration definition file.

      *

      * @param file the configuration definition file

      */

     public DefaultConfigurationBuilder(File file)

     {

         this();

         setFile(file);

     }

     /**

      * Creates a new instance of <code>DefaultConfigurationBuilder</code> and

      * sets the specified configuration definition file.

      *

      * @param fileName the name of the configuration definition file

      * @throws ConfigurationException if an error occurs when the file is loaded

      */

     public DefaultConfigurationBuilder(String fileName)

             throws ConfigurationException

     {

         this();

         setFileName(fileName);

     }

     /**

      * Creates a new instance of <code>DefaultConfigurationBuilder</code> and

      * sets the specified configuration definition file.

      *

      * @param url the URL to the configuration definition file

      * @throws ConfigurationException if an error occurs when the file is loaded

      */

     public DefaultConfigurationBuilder(URL url) throws ConfigurationException

     {

         this();

         setURL(url);

     }

     /**

      * Returns the base path for the configuration sources to load. This path is

      * used to resolve relative paths in the configuration definition file.

      *

      * @return the base path for configuration sources

      */

     public String getConfigurationBasePath()

     {

         return (configurationBasePath != null) ? configurationBasePath

                 : getBasePath();

     }

     /**

      * Sets the base path for the configuration sources to load. Normally a base

      * path need not to be set because it is determined by the location of the

      * configuration definition file to load. All relative pathes in this file

      * are resolved relative to this file. Setting a base path makes sense if

      * such relative pathes should be otherwise resolved, e.g. if the

      * configuration file is loaded from the class path and all sub

      * configurations it refers to are stored in a special config directory.

      *

      * @param configurationBasePath the new base path to set

      */

     public void setConfigurationBasePath(String configurationBasePath)

     {

         this.configurationBasePath = configurationBasePath;

     }

     /**

      * Adds a configuration provider for the specified tag. Whenever this tag is

      * encountered in the configuration definition file this provider will be

      * called to create the configuration object.

      *

      * @param tagName the name of the tag in the configuration definition file

      * @param provider the provider for this tag

      */

     public void addConfigurationProvider(String tagName,

             ConfigurationProvider provider)

     {

         if (tagName == null)

         {

             throw new IllegalArgumentException("Tag name must not be null!");

         }

         if (provider == null)

         {

             throw new IllegalArgumentException("Provider must not be null!");

         }

         providers.put(tagName, provider);

     }

     /**

      * Removes the configuration provider for the specified tag name.

      *

      * @param tagName the tag name

      * @return the removed configuration provider or <b>null</b> if none was

      * registered for that tag

      */

     public ConfigurationProvider removeConfigurationProvider(String tagName)

     {

         return (ConfigurationProvider) providers.remove(tagName);

     }

     /**

      * Returns the configuration provider for the given tag.

      *

      * @param tagName the name of the tag

      * @return the provider that was registered for this tag or <b>null</b> if

      * there is none

      */

     public ConfigurationProvider providerForTag(String tagName)

     {

         return (ConfigurationProvider) providers.get(tagName);

     }

     /**

      * Returns the configuration provided by this builder. Loads and parses the

      * configuration definition file and creates instances for the declared

      * configurations.

      *

      * @return the configuration

      * @throws ConfigurationException if an error occurs

      */

     public Configuration getConfiguration() throws ConfigurationException

     {

         return getConfiguration(true);

     }

     /**

      * Returns the configuration provided by this builder. If the boolean

      * parameter is <b>true</b>, the configuration definition file will be

      * loaded. It will then be parsed, and instances for the declared

      * configurations will be created.

      *

      * @param load a flag whether the configuration definition file should be

      * loaded; a value of <b>false</b> would make sense if the file has already

      * been created or its content was manipulated using some of the property

      * accessor methods

      * @return the configuration

      * @throws ConfigurationException if an error occurs

      */

     public CombinedConfiguration getConfiguration(boolean load)

             throws ConfigurationException

     {

         if (load)

         {

             load();

         }

         CombinedConfiguration result = createResultConfiguration();

         constructedConfiguration = result;

         List overrides = fetchTopLevelOverrideConfigs();

         overrides.addAll(fetchChildConfigs(KEY_OVERRIDE));

         initCombinedConfiguration(result, overrides, KEY_OVERRIDE_LIST);

         List additionals = fetchChildConfigs(KEY_UNION);

         if (!additionals.isEmpty())

         {

             CombinedConfiguration addConfig = new CombinedConfiguration(

                     new UnionCombiner());

             result.addConfiguration(addConfig, ADDITIONAL_NAME);

             initCombinedConfiguration(addConfig, additionals,

                     KEY_ADDITIONAL_LIST);

         }

         return result;

     }

     /**

      * Creates the resulting combined configuration. This method is called by

      * <code>getConfiguration()</code>. It checks whether the

      * <code>header</code> section of the configuration definition file

      * contains a <code>result</code> element. If this is the case, it will be

      * used to initialize the properties of the newly created configuration

      * object.

      *

      * @return the resulting configuration object

      * @throws ConfigurationException if an error occurs

      */

     protected CombinedConfiguration createResultConfiguration()

             throws ConfigurationException

     {

         XMLBeanDeclaration decl = new XMLBeanDeclaration(this, KEY_RESULT, true);

         CombinedConfiguration result = (CombinedConfiguration) BeanHelper

                 .createBean(decl, CombinedConfiguration.class);

         if (getMaxIndex(KEY_COMBINER) < 0)

         {

             // No combiner defined => set default

             result.setNodeCombiner(new OverrideCombiner());

         }

         return result;

     }

     /**

      * Initializes a combined configuration for the configurations of a specific

      * section. This method is called for the override and for the additional

      * section (if it exists).

      *

      * @param config the configuration to be initialized

      * @param containedConfigs the list with the declaratinos of the contained

      * configurations

      * @param keyListNodes a list with the declaration of list nodes

      * @throws ConfigurationException if an error occurs

      */

     protected void initCombinedConfiguration(CombinedConfiguration config,

             List containedConfigs, String keyListNodes) throws ConfigurationException

     {

         List listNodes = getList(keyListNodes);

         for (Iterator it = listNodes.iterator(); it.hasNext();)

         {

             config.getNodeCombiner().addListNode((String) it.next());

         }

         for (Iterator it = containedConfigs.iterator(); it.hasNext();)

         {

             HierarchicalConfiguration conf = (HierarchicalConfiguration) it

                     .next();

             ConfigurationDeclaration decl = new ConfigurationDeclaration(this,

                     conf);

             AbstractConfiguration newConf = createConfigurationAt(decl);

             if (newConf != null)

             {

                 config.addConfiguration(newConf, decl.getConfiguration()

                         .getString(ATTR_NAME), decl.getAt());

             }

         }

     }

     /**

      * Registers the default configuration providers supported by this class.

      * This method will be called during initialization. It registers

      * configuration providers for the tags that are supported by default.

      */

     protected void registerDefaultProviders()

     {

         for (int i = 0; i < DEFAULT_TAGS.length; i++)

         {

             addConfigurationProvider(DEFAULT_TAGS[i], DEFAULT_PROVIDERS[i]);

         }

     }

     /**

      * Performs interpolation. This method will not only take this configuration

      * instance into account (which is the one that loaded the configuration

      * definition file), but also the so far constructed combined configuration.

      * So variables can be used that point to properties that are defined in

      * configuration sources loaded by this builder.

      *

      * @param value the value to be interpolated

      * @return the interpolated value

      */

     protected Object interpolate(Object value)

     {

         Object result = super.interpolate(value);

         if (constructedConfiguration != null)

         {

             result = constructedConfiguration.interpolate(result);

         }

         return result;

     }

     protected void fireError(int type, String propName, Object propValue,

             Throwable ex)

     {

         // This method is only overridden to fix a mysterious MethodNotFound

         // error in the test cases when run under a JDK 1.3.

         super.fireError(type, propName, propValue, ex);

     }

     /**

      * Creates a configuration object from the specified configuration

      * declaration.

      *

      * @param decl the configuration declaration

      * @return the new configuration object

      * @throws ConfigurationException if an error occurs

      */

     private AbstractConfiguration createConfigurationAt(

             ConfigurationDeclaration decl) throws ConfigurationException

     {

         try

         {

             return (AbstractConfiguration) BeanHelper.createBean(decl);

         }

         catch (Exception ex)

         {

             // redirect to configuration exceptions

             throw new ConfigurationException(ex);

         }

     }

     /**

      * Returns a list with <code>SubnodeConfiguration</code> objects for the

      * child nodes of the specified configuration node.

      *

      * @param node the start node

      * @return a list with subnode configurations for the node's children

      */

     private List fetchChildConfigs(ConfigurationNode node)

     {

         List children = node.getChildren();

         List result = new ArrayList(children.size());

         for (Iterator it = children.iterator(); it.hasNext();)

         {

             result.add(createSubnodeConfiguration((Node) it.next()));

         }

         return result;

     }

     /**

      * Returns a list with <code>SubnodeConfiguration</code> objects for the

      * child nodes of the node specified by the given key.

      *

      * @param key the key (must define exactly one node)

      * @return a list with subnode configurations for the node's children

      */

     private List fetchChildConfigs(String key)

     {

         List nodes = fetchNodeList(key);

         if (nodes.size() > 0)

         {

             return fetchChildConfigs((ConfigurationNode) nodes.get(0));

         }

         else

         {

             return Collections.EMPTY_LIST;

         }

     }

     /**

      * Finds the override configurations that are defined as top level elements

      * in the configuration definition file. This method will fetch the child

      * elements of the root node and remove the nodes that represent other

      * configuration sections. The remaining nodes are treated as definitions

      * for override configurations.

      *

      * @return a list with subnode configurations for the top level override

      * configurations

      */

     private List fetchTopLevelOverrideConfigs()

     {

         List configs = fetchChildConfigs(getRootNode());

         for (Iterator it = configs.iterator(); it.hasNext();)

         {

             String nodeName = ((SubnodeConfiguration) it.next()).getRootNode()

                     .getName();

             for (int i = 0; i < CONFIG_SECTIONS.length; i++)

             {

                 if (CONFIG_SECTIONS[i].equals(nodeName))

                 {

                     it.remove();

                     break;

                 }

             }

         }

         return configs;

     }

     /**

      * Registers the bean factory used by this class if necessary. This method

      * is called by the constructor to ensure that the required bean factory is

      * available.

      */

     private void registerBeanFactory()

     {

         synchronized (getClass())

         {

             if (!BeanHelper.registeredFactoryNames().contains(

                     CONFIG_BEAN_FACTORY_NAME))

             {

                 BeanHelper.registerBeanFactory(CONFIG_BEAN_FACTORY_NAME,

                         new ConfigurationBeanFactory());

             }

         }

     }

     /**

      * <p>

      * A base class for creating and initializing configuration sources.

      * </p>

      * <p>

      * Concrete sub classes of this base class are responsible for creating

      * specific <code>Configuration</code> objects for the tags in the

      * configuration definition file. The configuration factory will parse the

      * definition file and try to find a matching

      * <code>ConfigurationProvider</code> for each encountered tag. This

      * provider is then asked to create a corresponding

      * <code>Configuration</code> object. It is up to a concrete

      * implementation how this object is created and initialized.

      * </p>

      * <p>

      * Note that at the moment only configuration classes derived from

      * <code>{@link AbstractConfiguration}</code> are supported.

      * </p>

      */

     public static class ConfigurationProvider extends DefaultBeanFactory

     {

         /** Stores the class of the configuration to be created. */

         private Class configurationClass;

         /** Stores the name of the configuration class to be created.*/

         private String configurationClassName;

         /**

          * Creates a new uninitialized instance of

          * <code>ConfigurationProvider</code>.

          */

         public ConfigurationProvider()

         {

             this((Class) null);

         }

         /**

          * Creates a new instance of <code>ConfigurationProvider</code> and

          * sets the class of the configuration created by this provider.

          *

          * @param configClass the configuration class

          */

         public ConfigurationProvider(Class configClass)

         {

             setConfigurationClass(configClass);

         }

         /**

          * Creates a new instance of <code>ConfigurationProvider</code> and

          * sets the name of the class of the configuration created by this

          * provider.

          *

          * @param configClassName the name of the configuration class

          * @since 1.4

          */

         public ConfigurationProvider(String configClassName)

         {

             setConfigurationClassName(configClassName);

         }

         /**

          * Returns the class of the configuration returned by this provider.

          *

          * @return the class of the provided configuration

          */

         public Class getConfigurationClass()

         {

             return configurationClass;

         }

         /**

          * Sets the class of the configuration returned by this provider.

          *

          * @param configurationClass the configuration class

          */

         public void setConfigurationClass(Class configurationClass)

         {

             this.configurationClass = configurationClass;

         }

         /**

          * Returns the name of the configuration class returned by this

          * provider.

          *

          * @return the configuration class name

          * @since 1.4

          */

         public String getConfigurationClassName()

         {

             return configurationClassName;

         }

         /**

          * Sets the name of the configuration class returned by this provider.

          *

          * @param configurationClassName the name of the configuration class

          * @since 1.4

          */

         public void setConfigurationClassName(String configurationClassName)

         {

             this.configurationClassName = configurationClassName;

         }

         /**

          * Returns the configuration. This method is called to fetch the

          * configuration from the provider. This implementation will call the

          * inherited <code>{@link

          * org.apache.commons.configuration.beanutils.DefaultBeanFactory#createBean(Class, BeanDeclaration, Object)

          * createBean()}</code> method to create a new instance of the

          * configuration class.

          *

          * @param decl the bean declaration with initialization parameters for

          * the configuration

          * @return the new configuration object

          * @throws Exception if an error occurs

          */

         public AbstractConfiguration getConfiguration(

                 ConfigurationDeclaration decl) throws Exception

         {

             return (AbstractConfiguration) createBean(fetchConfigurationClass(),

                     decl, null);

         }

         /**

          * Returns an uninitialized configuration of the represented type. This

          * method will be called for optional configurations when the

          * <code>getConfiguration()</code> method caused an error and the

          * <code>forceCreate</code> attribute is set. A concrete sub class can

          * here try to create an uninitialized, empty configuration, which may

          * be possible if the error was created during initialization. This base

          * implementation just returns <b>null</b>.

          *

          * @param decl the bean declaration with initialization parameters for

          * the configuration

          * @return the new configuration object

          * @throws Exception if an error occurs

          * @since 1.4

          */

         public AbstractConfiguration getEmptyConfiguration(

                 ConfigurationDeclaration decl) throws Exception

         {

             return null;

         }

         /**

          * Returns the configuration class supported by this provider. If a

          * class object was set, it is returned. Otherwise the method tries to

          * resolve the class name.

          *

          * @return the class of the configuration to be created

          * @since 1.4

          */

         protected synchronized Class fetchConfigurationClass() throws Exception

         {

             if (getConfigurationClass() == null)

             {

                 setConfigurationClass(loadClass(getConfigurationClassName()));

             }

             return getConfigurationClass();

         }

         /**

          * Loads the class with the specified name dynamically. If the class's

          * name is <b>null</b>, <b>null</b> will also be returned.

          *

          * @param className the name of the class to be loaded

          * @return the class object

          * @throws ClassNotFoundException if class loading fails

          * @since 1.4

          */

         protected Class loadClass(String className)

                 throws ClassNotFoundException

         {

             return (className != null) ? Class.forName(className, true,

                     getClass().getClassLoader()) : null;

         }

     }

     /**

      * <p>

      * A specialized <code>BeanDeclaration</code> implementation that

      * represents the declaration of a configuration source.

      * </p>

      * <p>

      * Instances of this class are able to extract all information about a

      * configuration source from the configuration definition file. The

      * declaration of a configuration source is very similar to a bean

      * declaration processed by <code>XMLBeanDeclaration</code>. There are

      * very few differences, e.g. some reserved attributes like

      * <code>optional</code> and <code>at</code> and the fact that a bean

      * factory is never needed.

      * </p>

      */

     public static class ConfigurationDeclaration extends XMLBeanDeclaration

     {

         /** Stores a reference to the associated configuration builder. */

         private DefaultConfigurationBuilder configurationBuilder;

         /**

          * Creates a new instance of <code>ConfigurationDeclaration</code> and

          * initializes it.

          *

          * @param builder the associated configuration builder

          * @param config the configuration this declaration is based onto

          */

         public ConfigurationDeclaration(DefaultConfigurationBuilder builder,

                 HierarchicalConfiguration config)

         {

             super(config);

             configurationBuilder = builder;

         }

         /**

          * Returns the associated configuration builder.

          *

          * @return the configuration builder

          */

         public DefaultConfigurationBuilder getConfigurationBuilder()

         {

             return configurationBuilder;

         }

         /**

          * Returns the value of the <code>at</code> attribute.

          *

          * @return the value of the <code>at</code> attribute (can be <b>null</b>)

          */

         public String getAt()

         {

             String result = this.getConfiguration().getString(ATTR_AT_RES);

             return (result == null) ? this.getConfiguration().getString(ATTR_AT)

                     : result;

         }

         /**

          * Returns a flag whether this is an optional configuration.

          *

          * @return a flag if this declaration points to an optional

          * configuration

          */

         public boolean isOptional()

         {

             Boolean value = this.getConfiguration().getBoolean(ATTR_OPTIONAL_RES,

                     null);

             if (value == null)

             {

                 value = this.getConfiguration().getBoolean(ATTR_OPTIONAL,

                         Boolean.FALSE);

             }

             return value.booleanValue();

         }

         /**

          * Returns a flag whether this configuration should always be created

          * and added to the resulting combined configuration. This flag is

          * evaluated only for optional configurations whose normal creation has

          * caused an error. If for such a configuration the

          * <code>forceCreate</code> attribute is set and the corresponding

          * configuration provider supports this mode, an empty configuration

          * will be created and added to the resulting combined configuration.

          *

          * @return the value of the <code>forceCreate</code> attribute

          * @since 1.4

          */

         public boolean isForceCreate()

         {

             return this.getConfiguration().getBoolean(ATTR_FORCECREATE, false);

         }

         /**

          * Returns the name of the bean factory. For configuration source

          * declarations always a reserved factory is used. This factory's name

          * is returned by this implementation.

          *

          * @return the name of the bean factory

          */

         public String getBeanFactoryName()

         {

             return CONFIG_BEAN_FACTORY_NAME;

         }

         /**

          * Returns the bean's class name. This implementation will always return

          * <b>null</b>.

          *

          * @return the name of the bean's class

          */

         public String getBeanClassName()

         {

             return null;

         }

         /**

          * Checks whether the given node is reserved. This method will take

          * further reserved attributes into account

          *

          * @param nd the node

          * @return a flag whether this node is reserved

          */

         protected boolean isReservedNode(ConfigurationNode nd)

         {

             if (super.isReservedNode(nd))

             {

                 return true;

             }

             return nd.isAttribute()

                     && ((ATTR_ATNAME.equals(nd.getName()) && nd.getParentNode()

                             .getAttributeCount(RESERVED_PREFIX + ATTR_ATNAME) == 0) || (ATTR_OPTIONALNAME

                             .equals(nd.getName()) && nd.getParentNode()

                             .getAttributeCount(RESERVED_PREFIX + ATTR_OPTIONALNAME) == 0));

         }

         /**

          * Performs interpolation. This implementation will delegate

          * interpolation to the configuration builder, which takes care that the

          * currently constructed configuration is taken into account, too.

          *

          * @param value the value to be interpolated

          * @return the interpolated value

          */

         protected Object interpolate(Object value)

         {

             return getConfigurationBuilder().interpolate(value);

         }

     }

     /**

      * A specialized <code>BeanFactory</code> implementation that handles

      * configuration declarations. This class will retrieve the correct

      * configuration provider and delegate the task of creating the

      * configuration to this object.

      */

     static class ConfigurationBeanFactory implements BeanFactory

     {

         /**

          * Creates an instance of a bean class. This implementation expects that

          * the passed in bean declaration is a declaration for a configuration.

          * It will determine the responsible configuration provider and delegate

          * the call to this instance. If creation of the configuration fails

          * and the <code>optional</code> attribute is set, the exception will

          * be ignored. If the <code>forceCreate</code> attribute is set, too,

          * the provider is asked to create an empty configuration. A return

          * value of <b>null</b> means that no configuration could be created.

          *

          * @param beanClass the bean class (will be ignored)

          * @param data the declaration

          * @param param an additional parameter (will be ignored)

          * @return the newly created configuration

          * @throws Exception if an error occurs

          */

         public Object createBean(Class beanClass, BeanDeclaration data,

                 Object param) throws Exception

         {

             ConfigurationDeclaration decl = (ConfigurationDeclaration) data;

             String tagName = decl.getNode().getName();

             ConfigurationProvider provider = decl.getConfigurationBuilder()

                     .providerForTag(tagName);

             if (provider == null)

             {

                 throw new ConfigurationRuntimeException(

                         "No ConfigurationProvider registered for tag "

                                 + tagName);

             }

             try

             {

                 return provider.getConfiguration(decl);

             }

             catch (Exception ex)

             {

                 // If this is an optional configuration, ignore the exception

                 if (!decl.isOptional())

                 {

                     throw ex;

                 }

                 else

                 {

                     // Notify registered error listeners

                     decl.getConfigurationBuilder().fireError(

                             EVENT_ERR_LOAD_OPTIONAL,

                             decl.getConfiguration().getString(ATTR_NAME), null,

                             ex);

                     if (decl.isForceCreate())

                     {

                         try

                         {

                             return provider.getEmptyConfiguration(decl);

                         }

                         catch (Exception ex2)

                         {

                             // Ignore exception, return null in this case

                             ;

                         }

                     }

                     return null;

                 }

             }

         }

         /**

          * Returns the default class for this bean factory.

          *

          * @return the default class

          */

         public Class getDefaultBeanClass()

         {

             // Here some valid class must be returned, otherwise BeanHelper

             // will complain that the bean's class cannot be determined

             return Configuration.class;

         }

     }

     /**

      * A specialized provider implementation that deals with file based

      * configurations. Ensures that the base path is correctly set and that the

      * load() method gets called.

      */

     public static class FileConfigurationProvider extends ConfigurationProvider

     {

         /**

          * Creates a new instance of <code>FileConfigurationProvider</code>.

          */

         public FileConfigurationProvider()

         {

             super();

         }

         /**

          * Creates a new instance of <code>FileConfigurationProvider</code>

          * and sets the configuration class.

          *

          * @param configClass the class for the configurations to be created

          */

         public FileConfigurationProvider(Class configClass)

         {

             super(configClass);

         }

         /**

          * Creates a new instance of <code>FileConfigurationProvider</code>

          * and sets the configuration class name.

          *

          * @param configClassName the name of the configuration to be created

          * @since 1.4

          */

         public FileConfigurationProvider(String configClassName)

         {

             super(configClassName);

         }

         /**

          * Creates the configuration. After that <code>load()</code> will be

          * called. If this configuration is marked as optional, exceptions will

          * be ignored.

          *

          * @param decl the declaration

          * @return the new configuration

          * @throws Exception if an error occurs

          */

         public AbstractConfiguration getConfiguration(

                 ConfigurationDeclaration decl) throws Exception

         {

             AbstractConfiguration result = getEmptyConfiguration(decl);

             ((FileConfiguration) result).load();

             return result;

         }

         /**

          * Returns an uninitialized file configuration. This method will be

          * called for optional configurations when the

          * <code>getConfiguration()</code> method caused an error and the

          * <code>forceCreate</code> attribute is set. It will create the

          * configuration of the represented type, but the <code>load()</code>

          * method won't be called. This way non-existing configuration files can

          * be handled gracefully: If loading a the file fails, an empty

          * configuration will be created that is already configured with the

          * correct file name.

          *

          * @param decl the bean declaration with initialization parameters for

          * the configuration

          * @return the new configuration object

          * @throws Exception if an error occurs

          * @since 1.4

          */

         public AbstractConfiguration getEmptyConfiguration(

                 ConfigurationDeclaration decl) throws Exception

         {

             return super.getConfiguration(decl);

         }

         /**

          * Initializes the bean instance. Ensures that the file configuration's

          * base path will be initialized with the base path of the factory so

          * that relative path names can be correctly resolved.

          *

          * @param bean the bean to be initialized

          * @param data the declaration

          * @throws Exception if an error occurs

          */

         protected void initBeanInstance(Object bean, BeanDeclaration data)

                 throws Exception

         {

             FileConfiguration config = (FileConfiguration) bean;

             config.setBasePath(((ConfigurationDeclaration) data)

                     .getConfigurationBuilder().getConfigurationBasePath());

             super.initBeanInstance(bean, data);

         }

     }

     /**

      * A specialized configuration provider for XML configurations. This

      * implementation acts like a <code>FileConfigurationProvider</code>, but

      * it will copy all entity IDs that have been registered for the

      * configuration builder to the new XML configuration before it is loaded.

      */

     static class XMLConfigurationProvider extends FileConfigurationProvider

     {

         /**

          * Creates a new instance of <code>XMLConfigurationProvider</code>.

          */

         public XMLConfigurationProvider()

         {

             super(XMLConfiguration.class);

         }

         /**

          * Returns a new empty configuration instance. This implementation

          * performs some additional initialization specific to XML

          * configurations.

          *

          * @param decl the configuration declaration

          * @return the new configuration

          * @throws Exception if an error occurs

          */

         public AbstractConfiguration getEmptyConfiguration(

                 ConfigurationDeclaration decl) throws Exception

         {

             XMLConfiguration config = (XMLConfiguration) super

                     .getEmptyConfiguration(decl);

             // copy the registered entities

             DefaultConfigurationBuilder builder = decl

                     .getConfigurationBuilder();

             config.getRegisteredEntities().putAll(

                     builder.getRegisteredEntities());

             return config;

         }

     }

     /**

      * A specialized configuration provider for file based configurations that

      * can handle configuration sources whose concrete type depends on the

      * extension of the file to be loaded. One example is the

      * <code>properties</code> tag: if the file ends with ".xml" a

      * XMLPropertiesConfiguration object must be created, otherwise a

      * PropertiesConfiguration object.

      */

     static class FileExtensionConfigurationProvider extends

             FileConfigurationProvider

     {

         /**

          * Stores the class to be created when the file extension matches.

          */

         private Class matchingClass;

         /**

          * Stores the name of the class to be created when the file extension

          * matches.

          */

         private String matchingClassName;

         /**

          * Stores the class to be created when the file extension does not

          * match.

          */

         private Class defaultClass;

         /**

          * Stores the name of the class to be created when the file extension

          * does not match.

          */

         private String defaultClassName;

         /** Stores the file extension to be checked against. */

         private String fileExtension;

         /**

          * Creates a new instance of

          * <code>FileExtensionConfigurationProvider</code> and initializes it.

          *

          * @param matchingClass the class to be created when the file extension

          * matches

          * @param defaultClass the class to be created when the file extension

          * does not match

          * @param extension the file extension to be checked agains

          */

         public FileExtensionConfigurationProvider(Class matchingClass,

                 Class defaultClass, String extension)

         {

             this.matchingClass = matchingClass;

             this.defaultClass = defaultClass;

             fileExtension = extension;

         }

         /**

          * Creates a new instance of

          * <code>FileExtensionConfigurationProvider</code> and initializes it

          * with the names of the classes to be created.

          *

          * @param matchingClassName the name of the class to be created when the

          * file extension matches