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 }