    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
    029 * in a flat configuration object (like {@link BaseConfiguration}) because then
    030 * their structure is lost. A prominent example are XML documents.
    031 * </p>
    032 * <p>
    033 * This interface extends the basic {@link ImmutableConfiguration} interface by
    034 * structured access to configuration properties. An {@link ExpressionEngine} is
    035 * used to evaluate complex property keys and to map them to nodes of a
    036 * tree-like structure.
    037 * </p>
    038 *
    039 * @since 2.0
    040 */
    041public interface ImmutableHierarchicalConfiguration extends ImmutableConfiguration
    042{
    043    /**
    044     * Returns the expression engine used by this configuration. This method
    045     * will never return <b>null</b>; if no specific expression engine was set,
    046     * the default expression engine will be returned.
    047     *
    048     * @return the current expression engine
    049     */
    050    ExpressionEngine getExpressionEngine();
    051
    052    /**
    053     * Returns the maximum defined index for the given key. This is useful if
    054     * there are multiple values for this key. They can then be addressed
    055     * separately by specifying indices from 0 to the return value of this
    056     * method.
    057     *
    058     * @param key the key to be checked
    059     * @return the maximum defined index for this key
    060     */
    061    int getMaxIndex(String key);
    062
    063    /**
    064     * Returns the name of the root element of this configuration. This
    065     * information may be of use in some cases, e.g. for sub configurations
    066     * created using the {@code immutableConfigurationsAt()} method. The exact
    067     * meaning of the string returned by this method is specific to a concrete
    068     * implementation. For instance, an XML configuration might return the name
    069     * of the document element.
    070     *
    071     * @return the name of the root element of this configuration
    072     */
    073    String getRootElementName();
    074
    075    /**
    076     * <p>
    077     * Returns an immutable hierarchical configuration object that wraps the
    078     * configuration node specified by the given key. This method provides an
    079     * easy means of accessing sub trees of a hierarchical configuration. In the
    080     * returned configuration the sub tree can directly be accessed, it becomes
    081     * the root node of this configuration. Because of this the passed in key
    082     * must select exactly one configuration node; otherwise an
    083     * {@code IllegalArgumentException} will be thrown.
    084     * </p>
    085     * <p>
    086     * The difference between this method and the
    087     * {@link #immutableSubset(String)} method is that
    088     * {@code immutableSubset()} supports arbitrary subsets of configuration nodes
    089     * while {@code immutableConfigurationAt()} only returns a single sub tree.
    090     * Please refer to the documentation of the
    091     * {@code SubnodeConfiguration} class to obtain further information
    092     * about subnode configurations and when they should be used.
    093     * </p>
    094     *
    095     * @param key the key that selects the sub tree
    096     * @param supportUpdates a flag whether the returned subnode configuration
    097     * should be able to handle updates of its parent
    098     * @return a hierarchical configuration that contains this sub tree
    099     */
    100    ImmutableHierarchicalConfiguration immutableConfigurationAt(String key,
    101            boolean supportUpdates);
    102
    103    /**
    104     * Returns an immutable hierarchical configuration for the node specified by
    105     * the given key. This is a short form for {@code immutableConfigurationAt(key,
    106     * <b>false</b>)}.
    107     *
    108     * @param key the key that selects the sub tree
    109     * @return a hierarchical configuration that contains this sub tree
    110     */
    111    ImmutableHierarchicalConfiguration immutableConfigurationAt(String key);
    112
    113    /**
    114     * Returns a list of immutable configurations for all configuration nodes selected
    115     * by the given key. This method will evaluate the passed in key (using the
    116     * current {@code ExpressionEngine}) and then create an immutable subnode
    117     * configuration for each returned node (like
    118     * {@link #immutableConfigurationAt(String)}}). This is especially
    119     * useful when dealing with list-like structures. As an example consider the
    120     * configuration that contains data about database tables and their fields.
    121     * If you need access to all fields of a certain table, you can simply do
    122     *
    123     * <pre>
    124     * List&lt;ImmutableHierarchicalConfiguration&gt; fields =
    125     *   config.immutableConfigurationsAt("tables.table(0).fields.field");
    126     * for(Iterator&lt;ImmutableHierarchicalConfiguration&gt; it = fields.iterator();
    127     *   it.hasNext();)
    128     * {
    129     *     ImmutableHierarchicalConfiguration sub = it.next();
    130     *     // now the children and attributes of the field node can be
    131     *     // directly accessed
    132     *     String fieldName = sub.getString("name");
    133     *     String fieldType = sub.getString("type");
    134     *     ...
    135     * </pre>
    136     *
    137     * @param key the key for selecting the desired nodes
    138     * @return a list with immutable hierarchical configuration objects; each
    139     * configuration represents one of the nodes selected by the passed in key
    140     */
    141    List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(String key);
    142
    143    /**
    144     * Returns a list of immutable configurations for all direct child elements
    145     * of the node selected by the given key. With this method it is possible to
    146     * inspect the content of a hierarchical structure; all children of a given
    147     * node can be queried without having to know their exact names. If the
    148     * passed in key does not point to a single node, an empty list is returned.
    149     * This is also the result if the node referred to by the key does not have
    150     * child elements.
    151     *
    152     * @param key the key for selecting the desired parent node
    153     * @return a collection with immutable configurations for all child nodes of
    154     *         the selected parent node
    155     */
    156    List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(
    157            String key);
    158}