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.configuration.tree;
018
019import java.util.List;
020
021/**
022 * <p>
023 * Definition of an interface for evaluating keys for hierarchical
024 * configurations.
025 * </p>
026 * <p>
027 * An <em>expression engine</em> knows how to map a key for a configuration's
028 * property to a single or a set of configuration nodes. Thus it defines the way
029 * how properties are addressed in this configuration. Methods of a
030 * configuration that have to handle property key (e.g.
031 * {@code getProperty()} or {@code addProperty()} do not interpret
032 * the passed in keys on their own, but delegate this task to an associated
033 * expression engine. This expression engine will then find out, which
034 * configuration nodes are addressed by the key.
035 * </p>
036 * <p>
037 * Separating the task of evaluating property keys from the configuration object
038 * has the advantage that many different expression languages (i.e. ways for
039 * querying or setting properties) can be supported. Just set a suitable
040 * implementation of this interface as the configuration's expression engine,
041 * and you can use the syntax provided by this implementation.
042 * </p>
043 *
044 * @since 1.3
045 * @author <a
046 * href="http://commons.apache.org/configuration/team-list.html">Commons
047 * Configuration team</a>
048 * @version $Id: ExpressionEngine.java 1206474 2011-11-26 16:14:09Z oheger $
049 */
050public interface ExpressionEngine
051{
052    /**
053     * Finds the node(s) that is (are) matched by the specified key. This is the
054     * main method for interpreting property keys. An implementation must
055     * traverse the given root node and its children to find all nodes that are
056     * matched by the given key. If the key is not correct in the syntax
057     * provided by that implementation, it is free to throw a (runtime)
058     * exception indicating this error condition.
059     *
060     * @param root the root node of a hierarchy of configuration nodes
061     * @param key the key to be evaluated
062     * @return a list with the nodes that are matched by the key (should never
063     * be <b>null</b>)
064     */
065    List<ConfigurationNode> query(ConfigurationNode root, String key);
066
067    /**
068     * Returns the key for the specified node in the expression language
069     * supported by an implementation. This method is called whenever a property
070     * key for a node has to be constructed, e.g. by the
071     * {@link org.apache.commons.configuration.Configuration#getKeys() getKeys()}
072     * method.
073     *
074     * @param node the node, for which the key must be constructed
075     * @param parentKey the key of this node's parent (can be <b>null</b> for
076     * the root node)
077     * @return this node's key
078     */
079    String nodeKey(ConfigurationNode node, String parentKey);
080
081    /**
082     * Returns information needed for an add operation. This method gets called
083     * when new properties are to be added to a configuration. An implementation
084     * has to interpret the specified key, find the parent node for the new
085     * elements, and provide all information about new nodes to be added.
086     *
087     * @param root the root node
088     * @param key the key for the new property
089     * @return an object with all information needed for the add operation
090     */
091    NodeAddData prepareAdd(ConfigurationNode root, String key);
092}