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.ArrayList;
20 import java.util.Collections;
21 import java.util.Iterator;
22 import java.util.LinkedList;
23 import java.util.List;
24
25 import org.apache.commons.lang3.builder.ToStringBuilder;
26
27 /**
28 * <p>
29 * A class for selecting a specific node based on a key or a set of keys.
30 * </p>
31 * <p>
32 * An instance of this class is initialized with the key of a node. It is also possible to concatenate multiple keys -
33 * for example if a sub key is to be constructed from another sub key. {@code NodeSelector} provides the {@code select()}
34 * method which evaluates the wrapped keys on a specified root node and returns the resulting unique target node. The
35 * class expects that the key(s) stored in an instance select exactly one target node. If this is not the case, result
36 * is <strong>null</strong> indicating that the selection criteria are not sufficient.
37 * </p>
38 * <p>
39 * Implementation node: Instances of this class are immutable. They can be shared between arbitrary components.
40 * </p>
41 *
42 * @since 2.0
43 */
44 public class NodeSelector {
45 /** Stores the wrapped keys. */
46 private final List<String> nodeKeys;
47
48 /**
49 * Creates a new instance of {@code NodeSelector} and initializes it with the list of keys to be used as selection
50 * criteria.
51 *
52 * @param keys the keys for selecting nodes
53 */
54 private NodeSelector(final List<String> keys) {
55 nodeKeys = keys;
56 }
57
58 /**
59 * Creates a new instance of {@code NodeSelector} and initializes it with the key to the target node.
60 *
61 * @param key the key
62 */
63 public NodeSelector(final String key) {
64 this(Collections.singletonList(key));
65 }
66
67 /**
68 * Compares this object with another one. Two instances of {@code NodeSelector} are considered equal if they have the
69 * same keys as selection criteria.
70 *
71 * @param obj the object to be compared
72 * @return a flag whether these objects are equal
73 */
74 @Override
75 public boolean equals(final Object obj) {
76 if (this == obj) {
77 return true;
78 }
79 if (!(obj instanceof NodeSelector)) {
80 return false;
81 }
82
83 final NodeSelector c = (NodeSelector) obj;
84 return nodeKeys.equals(c.nodeKeys);
85 }
86
87 /**
88 * Executes a query for a given key and filters the results for nodes only.
89 *
90 * @param root the root node for the query
91 * @param resolver the {@code NodeKeyResolver}
92 * @param handler the {@code NodeHandler}
93 * @param key the key
94 * @param nodes here the results are stored
95 */
96 private void getFilteredResults(final ImmutableNode root, final NodeKeyResolver<ImmutableNode> resolver, final NodeHandler<ImmutableNode> handler,
97 final String key, final List<ImmutableNode> nodes) {
98 final List<QueryResult<ImmutableNode>> results = resolver.resolveKey(root, key, handler);
99 results.forEach(result -> {
100 if (!result.isAttributeResult()) {
101 nodes.add(result.getNode());
102 }
103 });
104 }
105
106 /**
107 * Returns a hash code for this object.
108 *
109 * @return a hash code
110 */
111 @Override
112 public int hashCode() {
113 return nodeKeys.hashCode();
114 }
115
116 /**
117 * Applies this {@code NodeSelector} on the specified root node. This method applies the selection criteria stored in
118 * this object and tries to determine a single target node. If this is successful, the target node is returned.
119 * Otherwise, result is <strong>null</strong>.
120 *
121 * @param root the root node on which to apply this selector
122 * @param resolver the {@code NodeKeyResolver}
123 * @param handler the {@code NodeHandler}
124 * @return the selected target node or <strong>null</strong>
125 */
126 public ImmutableNode select(final ImmutableNode root, final NodeKeyResolver<ImmutableNode> resolver, final NodeHandler<ImmutableNode> handler) {
127 List<ImmutableNode> nodes = new LinkedList<>();
128 final Iterator<String> itKeys = nodeKeys.iterator();
129 getFilteredResults(root, resolver, handler, itKeys.next(), nodes);
130
131 while (itKeys.hasNext()) {
132 final String currentKey = itKeys.next();
133 final List<ImmutableNode> currentResults = new LinkedList<>();
134 nodes.forEach(currentRoot -> getFilteredResults(currentRoot, resolver, handler, currentKey, currentResults));
135 nodes = currentResults;
136 }
137
138 return nodes.size() == 1 ? nodes.get(0) : null;
139 }
140
141 /**
142 * Creates a sub {@code NodeSelector} object which uses the key(s) of this selector plus the specified key as selection
143 * criteria. This is useful when another selection is to be performed on the results of a first selector.
144 *
145 * @param subKey the additional key for the sub selector
146 * @return the sub {@code NodeSelector} instance
147 */
148 public NodeSelector subSelector(final String subKey) {
149 final List<String> keys = new ArrayList<>(nodeKeys.size() + 1);
150 keys.addAll(nodeKeys);
151 keys.add(subKey);
152 return new NodeSelector(keys);
153 }
154
155 /**
156 * Returns a string representation for this object. This string contains the keys to be used as selection criteria.
157 *
158 * @return a string for this object
159 */
160 @Override
161 public String toString() {
162 return new ToStringBuilder(this).append("keys", nodeKeys).toString();
163 }
164 }