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 */ 017package org.apache.commons.text; 018 019import 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 * @param <V> the type of the values supported by the lookup 035 * @since 1.0 036 */ 037public abstract class StrLookup<V> { 038 039 /** 040 * Lookup that always returns null. 041 */ 042 private static final StrLookup<String> NONE_LOOKUP = new MapStrLookup<>(null); 043 044 /** 045 * Lookup based on system properties. 046 */ 047 private static final StrLookup<String> SYSTEM_PROPERTIES_LOOKUP = new SystemPropertiesStrLookup(); 048 049 //----------------------------------------------------------------------- 050 /** 051 * Returns a lookup which always returns null. 052 * 053 * @return a lookup that always returns null, not null 054 */ 055 public static StrLookup<?> noneLookup() { 056 return NONE_LOOKUP; 057 } 058 059 /** 060 * Returns a new lookup which uses a copy of the current 061 * {@link System#getProperties() System properties}. 062 * <p> 063 * If a security manager blocked access to system properties, then null will 064 * be returned from every lookup. 065 * <p> 066 * If a null key is used, this lookup will throw a NullPointerException. 067 * 068 * @return a lookup using system properties, not null 069 */ 070 public static StrLookup<String> systemPropertiesLookup() { 071 return SYSTEM_PROPERTIES_LOOKUP; 072 } 073 074 /** 075 * Returns a lookup which looks up values using a map. 076 * <p> 077 * If the map is null, then null will be returned from every lookup. 078 * The map result object is converted to a string using toString(). 079 * 080 * @param <V> the type of the values supported by the lookup 081 * @param map the map of keys to values, may be null 082 * @return a lookup using the map, not null 083 */ 084 public static <V> StrLookup<V> mapLookup(final Map<String, V> map) { 085 return new MapStrLookup<>(map); 086 } 087 088 //----------------------------------------------------------------------- 089 /** 090 * Constructor. 091 */ 092 protected StrLookup() { 093 super(); 094 } 095 096 /** 097 * Looks up a String key to a String value. 098 * <p> 099 * The internal implementation may use any mechanism to return the value. 100 * The simplest implementation is to use a Map. However, virtually any 101 * implementation is possible. 102 * <p> 103 * For example, it would be possible to implement a lookup that used the 104 * key as a primary key, and looked up the value on demand from the database 105 * Or, a numeric based implementation could be created that treats the key 106 * as an integer, increments the value and return the result as a string - 107 * converting 1 to 2, 15 to 16 etc. 108 * <p> 109 * The {@link #lookup(String)} method always returns a String, regardless of 110 * the underlying data, by converting it as necessary. For example: 111 * <pre> 112 * Map<String, Object> map = new HashMap<String, Object>(); 113 * map.put("number", Integer.valueOf(2)); 114 * assertEquals("2", StrLookup.mapLookup(map).lookup("number")); 115 * </pre> 116 * @param key the key to be looked up, may be null 117 * @return the matching value, null if no match 118 */ 119 public abstract String lookup(String key); 120 121 //----------------------------------------------------------------------- 122 /** 123 * Lookup implementation that uses a Map. 124 */ 125 static class MapStrLookup<V> extends StrLookup<V> { 126 127 /** Map keys are variable names and value. */ 128 private final Map<String, V> map; 129 130 /** 131 * Creates a new instance backed by a Map. 132 * 133 * @param map the map of keys to values, may be null 134 */ 135 MapStrLookup(final Map<String, V> map) { 136 this.map = map; 137 } 138 139 /** 140 * Looks up a String key to a String value using the map. 141 * <p> 142 * If the map is null, then null is returned. 143 * The map result object is converted to a string using toString(). 144 * 145 * @param key the key to be looked up, may be null 146 * @return the matching value, null if no match 147 */ 148 @Override 149 public String lookup(final String key) { 150 if (map == null) { 151 return null; 152 } 153 final Object obj = map.get(key); 154 if (obj == null) { 155 return null; 156 } 157 return obj.toString(); 158 } 159 } 160 161 //----------------------------------------------------------------------- 162 /** 163 * Lookup implementation based on system properties. 164 */ 165 private static class SystemPropertiesStrLookup extends StrLookup<String> { 166 /** 167 * {@inheritDoc} This implementation directly accesses system properties. 168 */ 169 @Override 170 public String lookup(final String key) { 171 if (key.length() > 0) { 172 try { 173 return System.getProperty(key); 174 } catch (final SecurityException scex) { 175 // Squelched. All lookup(String) will return null. 176 return null; 177 } 178 } 179 return null; 180 } 181 } 182}