1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.configuration.tree;
18
19 import java.util.List;
20
21 /**
22 * <p>
23 * Definition of an interface for evaluating keys for hierarchical
24 * configurations.
25 * </p>
26 * <p>
27 * An <em>expression engine</em> knows how to map a key for a configuration's
28 * property to a single or a set of configuration nodes. Thus it defines the way
29 * how properties are addressed in this configuration. Methods of a
30 * configuration that have to handle property key (e.g.
31 * <code>getProperty()</code> or <code>addProperty()</code> do not interpret
32 * the passed in keys on their own, but delegate this task to an associated
33 * expression engine. This expression engine will then find out, which
34 * configuration nodes are addressed by the key.
35 * </p>
36 * <p>
37 * Seperating the task of evaluating property keys from the configuration object
38 * has the advantage that many different expression languages (i.e. ways for
39 * querying or setting properties) can be supported. Just set a suitable
40 * implementation of this interface as the configuration's expression engine,
41 * and you can use the syntax provided by this implementation.
42 * </p>
43 *
44 * @since 1.3
45 * @author Oliver Heger
46 */
47 public interface ExpressionEngine
48 {
49 /**
50 * Finds the node(s) that is (are) matched by the specified key. This is the
51 * main method for interpreting property keys. An implementation must
52 * traverse the given root node and its children to find all nodes that are
53 * matched by the given key. If the key is not correct in the syntax
54 * provided by that implementation, it is free to throw a (runtime)
55 * exception indicating this error condition.
56 *
57 * @param root the root node of a hierarchy of configuration nodes
58 * @param key the key to be evaluated
59 * @return a list with the nodes that are matched by the key (should never
60 * be <b>null</b>)
61 */
62 List query(ConfigurationNode root, String key);
63
64 /**
65 * Returns the key for the specified node in the expression language
66 * supported by an implementation. This method is called whenever a property
67 * key for a node has to be constructed, e.g. by the
68 * <code>{@link org.apache.commons.configuration.Configuration#getKeys() getKeys()}</code>
69 * method.
70 *
71 * @param node the node, for which the key must be constructed
72 * @param parentKey the key of this node's parent (can be <b>null</b> for
73 * the root node)
74 * @return this node's key
75 */
76 String nodeKey(ConfigurationNode node, String parentKey);
77
78 /**
79 * Returns information needed for an add operation. This method gets called
80 * when new properties are to be added to a configuration. An implementation
81 * has to interpret the specified key, find the parent node for the new
82 * elements, and provide all information about new nodes to be added.
83 *
84 * @param root the root node
85 * @param key the key for the new property
86 * @return an object with all information needed for the add operation
87 */
88 NodeAddData prepareAdd(ConfigurationNode root, String key);
89 }