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.configuration2.tree;
18
19 import java.util.List;
20
21 /**
22 * <p>
23 * Definition of an interface for evaluating keys for hierarchical configurations.
24 * </p>
25 * <p>
26 * An <em>expression engine</em> knows how to map a key for a configuration's property to a single or a set of
27 * configuration nodes. Thus it defines the way how properties are addressed in this configuration. Methods of a
28 * configuration that have to handle property keys (e.g. {@code getProperty()} or {@code addProperty()} do not interpret
29 * the passed in keys on their own, but delegate this task to an associated expression engine. This expression engine
30 * will then find out, which configuration nodes are addressed by the key.
31 * </p>
32 * <p>
33 * Separating the task of evaluating property keys from the configuration object has the advantage that multiple
34 * different expression languages (i.e. ways for querying or setting properties) can be supported. Just set a suitable
35 * implementation of this interface as the configuration's expression engine, and you can use the syntax provided by
36 * this implementation.
37 * </p>
38 * <p>
39 * An {@code ExpressionEngine} can deal with nodes of different types. To achieve this, a {@link NodeHandler} for the
40 * desired type must be passed to the methods.
41 * </p>
42 *
43 * @since 1.3
44 */
45 public interface ExpressionEngine {
46 /**
47 * Returns the key of an attribute. The passed in {@code parentKey} must reference the parent node of the attribute. A
48 * concrete implementation must concatenate this parent key with the attribute name to a valid key for this attribute.
49 *
50 * @param parentKey the key to the node owning this attribute
51 * @param attributeName the name of the attribute in question
52 * @return the resulting key referencing this attribute
53 */
54 String attributeKey(String parentKey, String attributeName);
55
56 /**
57 * Determines a "canonical" key for the specified node in the expression language supported by this
58 * implementation. This means that always a unique key if generated pointing to this specific node. For most concrete
59 * implementations, this means that an index is added to the node name to ensure that there are no ambiguities with
60 * child nodes having the same names.
61 *
62 * @param <T> the type of the node to be processed
63 * @param node the node, for which the key must be constructed
64 * @param parentKey the key of this node's parent (can be <b>null</b> for the root node)
65 * @param handler the {@code NodeHandler} for accessing the node
66 * @return the canonical key of this node
67 */
68 <T> String canonicalKey(T node, String parentKey, NodeHandler<T> handler);
69
70 /**
71 * Returns the key for the specified node in the expression language supported by an implementation. This method is
72 * called whenever a property key for a node has to be constructed, e.g. by the
73 * {@link org.apache.commons.configuration2.Configuration#getKeys() getKeys()} method.
74 *
75 * @param <T> the type of the node to be processed
76 * @param node the node, for which the key must be constructed
77 * @param parentKey the key of this node's parent (can be <b>null</b> for the root node)
78 * @param handler the {@code NodeHandler} for accessing the node
79 * @return this node's key
80 */
81 <T> String nodeKey(T node, String parentKey, NodeHandler<T> handler);
82
83 /**
84 * Returns information needed for an add operation. This method gets called when new properties are to be added to a
85 * configuration. An implementation has to interpret the specified key, find the parent node for the new elements, and
86 * provide all information about new nodes to be added.
87 *
88 * @param <T> the type of the node to be processed
89 * @param root the root node
90 * @param key the key for the new property
91 * @param handler the {@code NodeHandler} for accessing the node
92 * @return an object with all information needed for the add operation
93 */
94 <T> NodeAddData<T> prepareAdd(T root, String key, NodeHandler<T> handler);
95
96 /**
97 * Finds the nodes and/or attributes that are matched by the specified key. This is the main method for interpreting
98 * property keys. An implementation must traverse the given root node and its children to find all results that are
99 * matched by the given key. If the key is not correct in the syntax provided by that implementation, it is free to
100 * throw a (runtime) exception indicating this error condition. The passed in {@code NodeHandler} can be used to gather
101 * the required information from the node object.
102 *
103 * @param <T> the type of the node to be processed
104 * @param root the root node of a hierarchy of nodes
105 * @param key the key to be evaluated
106 * @param handler the {@code NodeHandler} for accessing the node
107 * @return a list with the results that are matched by the key (should never be <b>null</b>)
108 */
109 <T> List<QueryResult<T>> query(T root, String key, NodeHandler<T> handler);
110 }