    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()</code> or <code>addProperty()</code> 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     * Seperating 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 Oliver Heger
    046     */
    047    public interface ExpressionEngine
    048    {
    049        /**
    050         * Finds the node(s) that is (are) matched by the specified key. This is the
    051         * main method for interpreting property keys. An implementation must
    052         * traverse the given root node and its children to find all nodes that are
    053         * matched by the given key. If the key is not correct in the syntax
    054         * provided by that implementation, it is free to throw a (runtime)
    055         * exception indicating this error condition.
    056         *
    057         * @param root the root node of a hierarchy of configuration nodes
    058         * @param key the key to be evaluated
    059         * @return a list with the nodes that are matched by the key (should never
    060         * be <b>null</b>)
    061         */
    062        List query(ConfigurationNode root, String key);
    063
    064        /**
    065         * Returns the key for the specified node in the expression language
    066         * supported by an implementation. This method is called whenever a property
    067         * key for a node has to be constructed, e.g. by the
    068         * <code>{@link org.apache.commons.configuration.Configuration#getKeys() getKeys()}</code>
    069         * method.
    070         *
    071         * @param node the node, for which the key must be constructed
    072         * @param parentKey the key of this node's parent (can be <b>null</b> for
    073         * the root node)
    074         * @return this node's key
    075         */
    076        String nodeKey(ConfigurationNode node, String parentKey);
    077
    078        /**
    079         * Returns information needed for an add operation. This method gets called
    080         * when new properties are to be added to a configuration. An implementation
    081         * has to interpret the specified key, find the parent node for the new
    082         * elements, and provide all information about new nodes to be added.
    083         *
    084         * @param root the root node
    085         * @param key the key for the new property
    086         * @return an object with all information needed for the add operation
    087         */
    088        NodeAddData prepareAdd(ConfigurationNode root, String key);
    089    }