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.lang3.text;
018
019 import java.util.Map;
020
021 /**
022 * Lookup a String key to a String value.
023 * <p>
024 * This class represents the simplest form of a string to string map.
025 * It has a benefit over a map in that it can create the result on
026 * demand based on the key.
027 * <p>
028 * This class comes complete with various factory methods.
029 * If these do not suffice, you can subclass and implement your own matcher.
030 * <p>
031 * For example, it would be possible to implement a lookup that used the
032 * key as a primary key, and looked up the value on demand from the database
033 *
034 * @since 2.2
035 * @version $Id: StrLookup.java 1153484 2011-08-03 13:39:42Z ggregory $
036 */
037 public abstract class StrLookup<V> {
038
039 /**
040 * Lookup that always returns null.
041 */
042 private static final StrLookup<String> NONE_LOOKUP;
043 /**
044 * Lookup that uses System properties.
045 */
046 private static final StrLookup<String> SYSTEM_PROPERTIES_LOOKUP;
047 static {
048 NONE_LOOKUP = new MapStrLookup<String>(null);
049 StrLookup<String> lookup = null;
050 try {
051 final Map<?, ?> propMap = System.getProperties();
052 @SuppressWarnings("unchecked") // System property keys and values are always Strings
053 final Map<String, String> properties = (Map<String, String>) propMap;
054 lookup = new MapStrLookup<String>(properties);
055 } catch (SecurityException ex) {
056 lookup = NONE_LOOKUP;
057 }
058 SYSTEM_PROPERTIES_LOOKUP = lookup;
059 }
060
061 //-----------------------------------------------------------------------
062 /**
063 * Returns a lookup which always returns null.
064 *
065 * @return a lookup that always returns null, not null
066 */
067 public static StrLookup<?> noneLookup() {
068 return NONE_LOOKUP;
069 }
070
071 /**
072 * Returns a lookup which uses {@link System#getProperties() System properties}
073 * to lookup the key to value.
074 * <p>
075 * If a security manager blocked access to system properties, then null will
076 * be returned from every lookup.
077 * <p>
078 * If a null key is used, this lookup will throw a NullPointerException.
079 *
080 * @return a lookup using system properties, not null
081 */
082 public static StrLookup<String> systemPropertiesLookup() {
083 return SYSTEM_PROPERTIES_LOOKUP;
084 }
085
086 /**
087 * Returns a lookup which looks up values using a map.
088 * <p>
089 * If the map is null, then null will be returned from every lookup.
090 * The map result object is converted to a string using toString().
091 *
092 * @param <V> the type of the values supported by the lookup
093 * @param map the map of keys to values, may be null
094 * @return a lookup using the map, not null
095 */
096 public static <V> StrLookup<V> mapLookup(Map<String, V> map) {
097 return new MapStrLookup<V>(map);
098 }
099
100 //-----------------------------------------------------------------------
101 /**
102 * Constructor.
103 */
104 protected StrLookup() {
105 super();
106 }
107
108 /**
109 * Looks up a String key to a String value.
110 * <p>
111 * The internal implementation may use any mechanism to return the value.
112 * The simplest implementation is to use a Map. However, virtually any
113 * implementation is possible.
114 * <p>
115 * For example, it would be possible to implement a lookup that used the
116 * key as a primary key, and looked up the value on demand from the database
117 * Or, a numeric based implementation could be created that treats the key
118 * as an integer, increments the value and return the result as a string -
119 * converting 1 to 2, 15 to 16 etc.
120 * <p>
121 * The {@link #lookup(String)} method always returns a String, regardless of
122 * the underlying data, by converting it as necessary. For example:
123 * <pre>
124 * Map<String, Object> map = new HashMap<String, Object>();
125 * map.put("number", Integer.valueOf(2));
126 * assertEquals("2", StrLookup.mapLookup(map).lookup("number"));
127 * </pre>
128 * @param key the key to be looked up, may be null
129 * @return the matching value, null if no match
130 */
131 public abstract String lookup(String key);
132
133 //-----------------------------------------------------------------------
134 /**
135 * Lookup implementation that uses a Map.
136 */
137 static class MapStrLookup<V> extends StrLookup<V> {
138
139 /** Map keys are variable names and value. */
140 private final Map<String, V> map;
141
142 /**
143 * Creates a new instance backed by a Map.
144 *
145 * @param map the map of keys to values, may be null
146 */
147 MapStrLookup(Map<String, V> map) {
148 this.map = map;
149 }
150
151 /**
152 * Looks up a String key to a String value using the map.
153 * <p>
154 * If the map is null, then null is returned.
155 * The map result object is converted to a string using toString().
156 *
157 * @param key the key to be looked up, may be null
158 * @return the matching value, null if no match
159 */
160 @Override
161 public String lookup(String key) {
162 if (map == null) {
163 return null;
164 }
165 Object obj = map.get(key);
166 if (obj == null) {
167 return null;
168 }
169 return obj.toString();
170 }
171 }
172 }