001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *     http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.configuration2;
018
019import java.util.List;
020
021import org.apache.commons.configuration2.tree.ExpressionEngine;
022
023/**
024 * <p>
025 * An interface for immutable hierarchical configurations.
026 * </p>
027 * <p>
028 * There are some sources of configuration data that cannot be stored very well in a flat configuration object (like
029 * {@link BaseConfiguration}) because then their structure is lost. A prominent example are XML documents.
030 * </p>
031 * <p>
032 * This interface extends the basic {@link ImmutableConfiguration} interface by structured access to configuration
033 * properties. An {@link ExpressionEngine} is used to evaluate complex property keys and to map them to nodes of a
034 * tree-like structure.
035 * </p>
036 *
037 * @since 2.0
038 */
039public interface ImmutableHierarchicalConfiguration extends ImmutableConfiguration {
040    /**
041     * Gets the expression engine used by this configuration. This method will never return <b>null</b>; if no specific
042     * expression engine was set, the default expression engine will be returned.
043     *
044     * @return the current expression engine
045     */
046    ExpressionEngine getExpressionEngine();
047
048    /**
049     * Gets the maximum defined index for the given key. This is useful if there are multiple values for this key. They
050     * can then be addressed separately by specifying indices from 0 to the return value of this method.
051     *
052     * @param key the key to be checked
053     * @return the maximum defined index for this key
054     */
055    int getMaxIndex(String key);
056
057    /**
058     * Gets the name of the root element of this configuration. This information may be of use in some cases, e.g. for
059     * sub configurations created using the {@code immutableConfigurationsAt()} method. The exact meaning of the string
060     * returned by this method is specific to a concrete implementation. For instance, an XML configuration might return the
061     * name of the document element.
062     *
063     * @return the name of the root element of this configuration
064     */
065    String getRootElementName();
066
067    /**
068     * Returns a list of immutable configurations for all direct child elements of the node selected by the given key. With
069     * this method it is possible to inspect the content of a hierarchical structure; all children of a given node can be
070     * queried without having to know their exact names. If the passed in key does not point to a single node, an empty list
071     * is returned. This is also the result if the node referred to by the key does not have child elements.
072     *
073     * @param key the key for selecting the desired parent node
074     * @return a collection with immutable configurations for all child nodes of the selected parent node
075     */
076    List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(String key);
077
078    /**
079     * Returns an immutable hierarchical configuration for the node specified by the given key. This is a short form for
080     * {@code immutableConfigurationAt(key,
081     * <b>false</b>)}.
082     *
083     * @param key the key that selects the sub tree
084     * @return a hierarchical configuration that contains this sub tree
085     */
086    ImmutableHierarchicalConfiguration immutableConfigurationAt(String key);
087
088    /**
089     * <p>
090     * Returns an immutable hierarchical configuration object that wraps the configuration node specified by the given key.
091     * This method provides an easy means of accessing sub trees of a hierarchical configuration. In the returned
092     * configuration the sub tree can directly be accessed, it becomes the root node of this configuration. Because of this
093     * the passed in key must select exactly one configuration node; otherwise an {@code IllegalArgumentException} will be
094     * thrown.
095     * </p>
096     * <p>
097     * The difference between this method and the {@link #immutableSubset(String)} method is that {@code immutableSubset()}
098     * supports arbitrary subsets of configuration nodes while {@code immutableConfigurationAt()} only returns a single sub
099     * tree. Please refer to the documentation of the {@code SubnodeConfiguration} class to obtain further information about
100     * subnode configurations and when they should be used.
101     * </p>
102     *
103     * @param key the key that selects the sub tree
104     * @param supportUpdates a flag whether the returned subnode configuration should be able to handle updates of its
105     *        parent
106     * @return a hierarchical configuration that contains this sub tree
107     */
108    ImmutableHierarchicalConfiguration immutableConfigurationAt(String key, boolean supportUpdates);
109
110    /**
111     * Returns a list of immutable configurations for all configuration nodes selected by the given key. This method will
112     * evaluate the passed in key (using the current {@code ExpressionEngine}) and then create an immutable subnode
113     * configuration for each returned node (like {@link #immutableConfigurationAt(String)}}). This is especially useful
114     * when dealing with list-like structures. As an example consider the configuration that contains data about database
115     * tables and their fields. If you need access to all fields of a certain table, you can simply do
116     *
117     * <pre>
118     * List&lt;ImmutableHierarchicalConfiguration&gt; fields =
119     *   config.immutableConfigurationsAt("tables.table(0).fields.field");
120     * for(Iterator&lt;ImmutableHierarchicalConfiguration&gt; it = fields.iterator();
121     *   it.hasNext();)
122     * {
123     *     ImmutableHierarchicalConfiguration sub = it.next();
124     *     // now the children and attributes of the field node can be
125     *     // directly accessed
126     *     String fieldName = sub.getString("name");
127     *     String fieldType = sub.getString("type");
128     *     ...
129     * </pre>
130     *
131     * @param key the key for selecting the desired nodes
132     * @return a list with immutable hierarchical configuration objects; each configuration represents one of the nodes
133     *         selected by the passed in key
134     */
135    List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(String key);
136}