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     */
017    package org.apache.commons.configuration.tree;
018    
019    import 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     */
050    public 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    }