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.configuration2.interpol; 018 019import java.lang.reflect.Field; 020import java.util.HashMap; 021import java.util.Map; 022 023import org.apache.commons.lang3.ClassUtils; 024import org.apache.commons.logging.Log; 025import org.apache.commons.logging.LogFactory; 026 027/** 028 * <p> 029 * A specialized lookup implementation that allows access to constant fields of 030 * classes. 031 * </p> 032 * <p> 033 * Sometimes it is necessary in a configuration file to refer to a constant 034 * defined in a class. This can be done with this lookup implementation. 035 * Variable names passed in must be of the form 036 * {@code mypackage.MyClass.FIELD}. The {@code lookup()} method 037 * will split the passed in string at the last dot, separating the fully 038 * qualified class name and the name of the constant (i.e. <strong>static final</strong>) 039 * member field. Then the class is loaded and the field's value is obtained 040 * using reflection. 041 * </p> 042 * <p> 043 * Once retrieved values are cached for fast access. This class is thread-safe. 044 * It can be used as a standard (i.e. global) lookup object and serve multiple 045 * clients concurrently. 046 * </p> 047 * 048 * @since 1.4 049 */ 050public class ConstantLookup implements Lookup 051{ 052 /** Constant for the field separator. */ 053 private static final char FIELD_SEPRATOR = '.'; 054 055 /** An internally used cache for already retrieved values. */ 056 private static Map<String, Object> constantCache = new HashMap<>(); 057 058 /** The logger. */ 059 private final Log log = LogFactory.getLog(getClass()); 060 061 /** 062 * Tries to resolve the specified variable. The passed in variable name is 063 * interpreted as the name of a <b>static final</b> member field of a 064 * class. If the value has already been obtained, it can be retrieved from 065 * an internal cache. Otherwise this method will invoke the 066 * {@code resolveField()} method and pass in the name of the class 067 * and the field. 068 * 069 * @param var the name of the variable to be resolved 070 * @return the value of this variable or <b>null</b> if it cannot be 071 * resolved 072 */ 073 @Override 074 public Object lookup(final String var) 075 { 076 if (var == null) 077 { 078 return null; 079 } 080 081 Object result; 082 synchronized (constantCache) 083 { 084 result = constantCache.get(var); 085 } 086 if (result != null) 087 { 088 return result; 089 } 090 091 final int fieldPos = var.lastIndexOf(FIELD_SEPRATOR); 092 if (fieldPos < 0) 093 { 094 return null; 095 } 096 try 097 { 098 final Object value = resolveField(var.substring(0, fieldPos), var 099 .substring(fieldPos + 1)); 100 if (value != null) 101 { 102 synchronized (constantCache) 103 { 104 // In worst case, the value will be fetched multiple times 105 // because of this lax synchronization, but for constant 106 // values this shouldn't be a problem. 107 constantCache.put(var, value); 108 } 109 result = value; 110 } 111 } 112 catch (final Exception ex) 113 { 114 log.warn("Could not obtain value for variable " + var, ex); 115 } 116 117 return result; 118 } 119 120 /** 121 * Clears the shared cache with the so far resolved constants. 122 */ 123 public static void clear() 124 { 125 synchronized (constantCache) 126 { 127 constantCache.clear(); 128 } 129 } 130 131 /** 132 * Determines the value of the specified constant member field of a class. 133 * This implementation will call {@code fetchClass()} to obtain the 134 * {@code java.lang.Class} object for the target class. Then it will 135 * use reflection to obtain the field's value. For this to work the field 136 * must be accessable. 137 * 138 * @param className the name of the class 139 * @param fieldName the name of the member field of that class to read 140 * @return the field's value 141 * @throws Exception if an error occurs 142 */ 143 protected Object resolveField(final String className, final String fieldName) 144 throws Exception 145 { 146 final Class<?> clazz = fetchClass(className); 147 final Field field = clazz.getField(fieldName); 148 return field.get(null); 149 } 150 151 /** 152 * Loads the class with the specified name. If an application has special 153 * needs regarding the class loaders to be used, it can hook in here. This 154 * implementation delegates to the {@code getClass()} method of 155 * Commons Lang's 156 * <code><a href="https://commons.apache.org/lang/api-release/org/apache/commons/lang/ClassUtils.html"> 157 * ClassUtils</a></code>. 158 * 159 * @param className the name of the class to be loaded 160 * @return the corresponding class object 161 * @throws ClassNotFoundException if the class cannot be loaded 162 */ 163 protected Class<?> fetchClass(final String className) throws ClassNotFoundException 164 { 165 return ClassUtils.getClass(className); 166 } 167}