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 * @version $Id: ConstantLookup.java 1790899 2017-04-10 21:56:46Z ggregory $ 049 * @since 1.4 050 * @author <a 051 * href="http://commons.apache.org/configuration/team-list.html">Commons 052 * Configuration team</a> 053 */ 054public class ConstantLookup implements Lookup 055{ 056 /** Constant for the field separator. */ 057 private static final char FIELD_SEPRATOR = '.'; 058 059 /** An internally used cache for already retrieved values. */ 060 private static Map<String, Object> constantCache = new HashMap<>(); 061 062 /** The logger. */ 063 private final Log log = LogFactory.getLog(getClass()); 064 065 /** 066 * Tries to resolve the specified variable. The passed in variable name is 067 * interpreted as the name of a <b>static final</b> member field of a 068 * class. If the value has already been obtained, it can be retrieved from 069 * an internal cache. Otherwise this method will invoke the 070 * {@code resolveField()} method and pass in the name of the class 071 * and the field. 072 * 073 * @param var the name of the variable to be resolved 074 * @return the value of this variable or <b>null</b> if it cannot be 075 * resolved 076 */ 077 @Override 078 public Object lookup(String var) 079 { 080 if (var == null) 081 { 082 return null; 083 } 084 085 Object result; 086 synchronized (constantCache) 087 { 088 result = constantCache.get(var); 089 } 090 if (result != null) 091 { 092 return result; 093 } 094 095 int fieldPos = var.lastIndexOf(FIELD_SEPRATOR); 096 if (fieldPos < 0) 097 { 098 return null; 099 } 100 try 101 { 102 Object value = resolveField(var.substring(0, fieldPos), var 103 .substring(fieldPos + 1)); 104 if (value != null) 105 { 106 synchronized (constantCache) 107 { 108 // In worst case, the value will be fetched multiple times 109 // because of this lax synchronization, but for constant 110 // values this shouldn't be a problem. 111 constantCache.put(var, value); 112 } 113 result = value; 114 } 115 } 116 catch (Exception ex) 117 { 118 log.warn("Could not obtain value for variable " + var, ex); 119 } 120 121 return result; 122 } 123 124 /** 125 * Clears the shared cache with the so far resolved constants. 126 */ 127 public static void clear() 128 { 129 synchronized (constantCache) 130 { 131 constantCache.clear(); 132 } 133 } 134 135 /** 136 * Determines the value of the specified constant member field of a class. 137 * This implementation will call {@code fetchClass()} to obtain the 138 * {@code java.lang.Class} object for the target class. Then it will 139 * use reflection to obtain the field's value. For this to work the field 140 * must be accessable. 141 * 142 * @param className the name of the class 143 * @param fieldName the name of the member field of that class to read 144 * @return the field's value 145 * @throws Exception if an error occurs 146 */ 147 protected Object resolveField(String className, String fieldName) 148 throws Exception 149 { 150 Class<?> clazz = fetchClass(className); 151 Field field = clazz.getField(fieldName); 152 return field.get(null); 153 } 154 155 /** 156 * Loads the class with the specified name. If an application has special 157 * needs regarding the class loaders to be used, it can hook in here. This 158 * implementation delegates to the {@code getClass()} method of 159 * Commons Lang's 160 * <code><a href="http://commons.apache.org/lang/api-release/org/apache/commons/lang/ClassUtils.html"> 161 * ClassUtils</a></code>. 162 * 163 * @param className the name of the class to be loaded 164 * @return the corresponding class object 165 * @throws ClassNotFoundException if the class cannot be loaded 166 */ 167 protected Class<?> fetchClass(String className) throws ClassNotFoundException 168 { 169 return ClassUtils.getClass(className); 170 } 171}