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 * https://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 018package org.apache.commons.beanutils; 019 020import java.util.Map; 021import java.util.WeakHashMap; 022 023/** 024 * An instance of this class represents a value that is provided per (thread) 025 * context classloader. 026 * 027 * <p>Occasionally it is necessary to store data in "global" variables 028 * (including uses of the Singleton pattern). In applications which have only 029 * a single classloader such data can simply be stored as "static" members on 030 * some class. When multiple classloaders are involved, however, this approach 031 * can fail; in particular, this doesn't work when the code may be run within a 032 * servlet container or a j2ee container, and the class on which the static 033 * member is defined is loaded via a "shared" classloader that is visible to all 034 * components running within the container. This class provides a mechanism for 035 * associating data with a ClassLoader instance, which ensures that when the 036 * code runs in such a container each component gets its own copy of the 037 * "global" variable rather than unexpectedly sharing a single copy of the 038 * variable with other components that happen to be running in the same 039 * container at the same time (eg servlets or EJBs.)</p> 040 * 041 * <p>This class is strongly patterned after the java.lang.ThreadLocal 042 * class, which performs a similar task in allowing data to be associated 043 * with a particular thread.</p> 044 * 045 * <p>When code that uses this class is run as a "normal" application, ie 046 * not within a container, the effect is identical to just using a static 047 * member variable to store the data, because Thread.getContextClassLoader 048 * always returns the same classloader (the system classloader).</p> 049 * 050 * <p>Expected usage is as follows:<br> 051 * <pre> 052 * public class SomeClass { 053 * private static final ContextClassLoaderLocal<String> global 054 * = new ContextClassLoaderLocal<String>() { 055 * protected String initialValue() { 056 * return new String("Initial value"); 057 * }; 058 * 059 * public void testGlobal() { 060 * String s = global.get(); 061 * System.out.println("global value:" + s); 062 * buf.set("New Value"); 063 * } 064 * </pre> 065 * <p><strong>Note:</strong> This class takes some care to ensure that when 066 * a component which uses this class is "undeployed" by a container the 067 * component-specific classloader and all its associated classes (and their 068 * static variables) are garbage-collected. Unfortunately there is one 069 * scenario in which this does <em>not</em> work correctly and there 070 * is unfortunately no known workaround other than ensuring that the 071 * component (or its container) calls the "unset" method on this class for 072 * each instance of this class when the component is undeployed. The problem 073 * occurs if: 074 * </p> 075 * <ul> 076 * <li>the class containing a static instance of this class was loaded via 077 * a shared classloader, and</li> 078 * <li>the value stored in the instance is an object whose class was loaded 079 * via the component-specific classloader (or any of the objects it refers 080 * to were loaded via that classloader).</li> 081 * </ul> 082 * <p> 083 * The result is that the map managed by this object still contains a strong 084 * reference to the stored object, which contains a strong reference to the 085 * classloader that loaded it, meaning that although the container has 086 * "undeployed" the component the component-specific classloader and all the 087 * related classes and static variables cannot be garbage-collected. This is 088 * not expected to be an issue with the commons-beanutils library as the only 089 * classes which use this class are BeanUtilsBean and ConvertUtilsBean and 090 * there is no obvious reason for a user of the beanutils library to subclass 091 * either of those classes.</p> 092 * 093 * <p><strong>Note:</strong> A WeakHashMap bug in several 1.3 JVMs results in 094 * a memory leak for those JVMs.</p> 095 * 096 * <p><strong>Note:</strong> Of course all of this would be unnecessary if 097 * containers required each component to load the full set of classes it 098 * needs, ie avoided providing classes loaded via a "shared" classloader.</p> 099 * 100 * @param <T> the type of data stored in an instance 101 * @see java.lang.Thread#getContextClassLoader 102 */ 103public class ContextClassLoaderLocal<T> { 104 private final Map<ClassLoader, T> valueByClassLoader = new WeakHashMap<>(); 105 private boolean globalValueInitialized; 106 private T globalValue; 107 108 /** 109 * Construct a context classloader instance 110 */ 111 public ContextClassLoaderLocal() { 112 } 113 114 /** 115 * Gets the instance which provides the functionality for {@link BeanUtils}. 116 * This is a pseudo-singleton - an single instance is provided per (thread) context classloader. 117 * This mechanism provides isolation for web apps deployed in the same container. 118 * @return the object currently associated with the context-classloader of the current thread. 119 */ 120 public synchronized T get() { 121 // synchronizing the whole method is a bit slower 122 // but guarantees no subtle threading problems, and there's no 123 // need to synchronize valueByClassLoader 124 125 // make sure that the map is given a change to purge itself 126 valueByClassLoader.isEmpty(); 127 try { 128 129 final ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader(); 130 if (contextClassLoader != null) { 131 132 T value = valueByClassLoader.get(contextClassLoader); 133 if (value == null 134 && !valueByClassLoader.containsKey(contextClassLoader)) { 135 value = initialValue(); 136 valueByClassLoader.put(contextClassLoader, value); 137 } 138 return value; 139 140 } 141 142 } catch (final SecurityException e) { /* SWALLOW - should we log this? */ } 143 144 // if none or exception, return the globalValue 145 if (!globalValueInitialized) { 146 globalValue = initialValue(); 147 globalValueInitialized = true; 148 }//else already set 149 return globalValue; 150 } 151 152 /** 153 * Returns the initial value for this ContextClassLoaderLocal 154 * variable. This method will be called once per Context ClassLoader for 155 * each ContextClassLoaderLocal, the first time it is accessed 156 * with get or set. If the programmer desires ContextClassLoaderLocal variables 157 * to be initialized to some value other than null, ContextClassLoaderLocal must 158 * be subclassed, and this method overridden. Typically, an anonymous 159 * inner class will be used. Typical implementations of initialValue 160 * will call an appropriate constructor and return the newly constructed 161 * object. 162 * 163 * @return a new Object to be used as an initial value for this ContextClassLoaderLocal 164 */ 165 protected T initialValue() { 166 return null; 167 } 168 169 /** 170 * Sets the value - a value is provided per (thread) context classloader. 171 * This mechanism provides isolation for web apps deployed in the same container. 172 * 173 * @param value the object to be associated with the entrant thread's context classloader 174 */ 175 public synchronized void set(final T value) { 176 // synchronizing the whole method is a bit slower 177 // but guarentees no subtle threading problems 178 179 // make sure that the map is given a change to purge itself 180 valueByClassLoader.isEmpty(); 181 try { 182 183 final ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader(); 184 if (contextClassLoader != null) { 185 valueByClassLoader.put(contextClassLoader, value); 186 return; 187 } 188 189 } catch (final SecurityException e) { /* SWALLOW - should we log this? */ } 190 191 // if in doubt, set the global value 192 globalValue = value; 193 globalValueInitialized = true; 194 } 195 196 /** 197 * Unsets the value associated with the current thread's context classloader 198 */ 199 public synchronized void unset() { 200 try { 201 202 final ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader(); 203 unset(contextClassLoader); 204 205 } catch (final SecurityException e) { /* SWALLOW - should we log this? */ } 206 } 207 208 /** 209 * Unsets the value associated with the given classloader 210 * @param classLoader The classloader to <em>unset</em> for 211 */ 212 public synchronized void unset(final ClassLoader classLoader) { 213 valueByClassLoader.remove(classLoader); 214 } 215}