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
18 package org.apache.commons.text.lookup;
19
20 import java.util.Objects;
21 import java.util.concurrent.ConcurrentHashMap;
22
23 import org.apache.commons.lang3.ClassUtils;
24 import org.apache.commons.text.StringSubstitutor;
25
26 /**
27 * <p>
28 * Looks up the value of a fully-qualified static final value.
29 * </p>
30 * <p>
31 * Sometimes it is necessary in a configuration file to refer to a constant defined in a class. This can be done with
32 * this lookup implementation. Variable names must be in the format {@code apackage.AClass.AFIELD}. The
33 * {@code lookup(String)} method will split the passed in string at the last dot, separating the fully qualified class
34 * name and the name of the constant (i.e. <b>static final</b>) member field. Then the class is loaded and the field's
35 * value is obtained using reflection.
36 * </p>
37 * <p>
38 * Once retrieved values are cached for fast access. This class is thread-safe. It can be used as a standard (i.e.
39 * global) lookup object and serve multiple clients concurrently.
40 * </p>
41 * <p>
42 * Using a {@link StringLookup} from the {@link StringLookupFactory}:
43 * </p>
44 *
45 * <pre>
46 * StringLookupFactory.INSTANCE.constantStringLookup().lookup("java.awt.event.KeyEvent.VK_ESCAPE");
47 * </pre>
48 * <p>
49 * Using a {@link StringSubstitutor}:
50 * </p>
51 *
52 * <pre>
53 * StringSubstitutor.createInterpolator().replace("... ${const:java.awt.event.KeyEvent.VK_ESCAPE} ..."));
54 * </pre>
55 * <p>
56 * The above examples convert {@code java.awt.event.KeyEvent.VK_ESCAPE} to {@code "27"}.
57 * </p>
58 * <p>
59 * This class was adapted from Apache Commons Configuration.
60 * </p>
61 *
62 * @since 1.5
63 */
64 class ConstantStringLookup extends AbstractStringLookup {
65
66 /** An internally used cache for already retrieved values. */
67 private static final ConcurrentHashMap<String, String> CONSTANT_CACHE = new ConcurrentHashMap<>();
68
69 /** Constant for the field separator. */
70 private static final char FIELD_SEPARATOR = '.';
71
72 /**
73 * Defines the singleton for this class.
74 */
75 static final ConstantStringLookup INSTANCE = new ConstantStringLookup();
76
77 /**
78 * Clears the shared cache with the so far resolved constants.
79 */
80 static void clear() {
81 CONSTANT_CACHE.clear();
82 }
83
84 /**
85 * Loads the class with the specified name. If an application has special needs regarding the class loaders to be
86 * used, it can hook in here. This implementation delegates to the {@code getClass()} method of Commons Lang's
87 * <code><a href="https://commons.apache.org/lang/api-release/org/apache/commons/lang/ClassUtils.html">
88 * ClassUtils</a></code>.
89 *
90 * @param className the name of the class to be loaded
91 * @return The corresponding class object
92 * @throws ClassNotFoundException if the class cannot be loaded
93 */
94 protected Class<?> fetchClass(final String className) throws ClassNotFoundException {
95 return ClassUtils.getClass(className);
96 }
97
98 /**
99 * Tries to resolve the specified variable. The passed in variable name is interpreted as the name of a <b>static
100 * final</b> member field of a class. If the value has already been obtained, it can be retrieved from an internal
101 * cache. Otherwise this method will invoke the {@code resolveField()} method and pass in the name of the class and
102 * the field.
103 *
104 * @param key the name of the variable to be resolved
105 * @return The value of this variable or <b>null</b> if it cannot be resolved
106 */
107 @Override
108 public synchronized String lookup(final String key) {
109 if (key == null) {
110 return null;
111 }
112 String result;
113 result = CONSTANT_CACHE.get(key);
114 if (result != null) {
115 return result;
116 }
117 final int fieldPos = key.lastIndexOf(FIELD_SEPARATOR);
118 if (fieldPos < 0) {
119 return null;
120 }
121 try {
122 final Object value = resolveField(key.substring(0, fieldPos), key.substring(fieldPos + 1));
123 if (value != null) {
124 final String string = Objects.toString(value, null);
125 CONSTANT_CACHE.put(key, string);
126 result = string;
127 }
128 } catch (final Exception ex) {
129 // TODO it would be nice to log
130 return null;
131 }
132 return result;
133 }
134
135 /**
136 * Determines the value of the specified constant member field of a class. This implementation will call
137 * {@code fetchClass()} to obtain the {@link Class} object for the target class. Then it will use
138 * reflection to obtain the field's value. For this to work the field must be accessible.
139 *
140 * @param className the name of the class
141 * @param fieldName the name of the member field of that class to read
142 * @return The field's value
143 * @throws ReflectiveOperationException if an error occurs
144 */
145 protected Object resolveField(final String className, final String fieldName) throws ReflectiveOperationException {
146 final Class<?> clazz = fetchClass(className);
147 if (clazz == null) {
148 return null;
149 }
150 return clazz.getField(fieldName).get(null);
151 }
152 }