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 018package org.apache.commons.logging.impl; 019 020import java.lang.reflect.InvocationTargetException; 021import java.lang.reflect.Method; 022 023import javax.servlet.ServletContextEvent; 024import javax.servlet.ServletContextListener; 025 026import org.apache.commons.logging.LogFactory; 027 028/** 029 * This class is capable of receiving notifications about the undeployment of 030 * a webapp, and responds by ensuring that commons-logging releases all 031 * memory associated with the undeployed webapp. 032 * <p> 033 * In general, the WeakHashtable support added in commons-logging release 1.1 034 * ensures that logging classes do not hold references that prevent an 035 * undeployed webapp's memory from being garbage-collected even when multiple 036 * copies of commons-logging are deployed via multiple class loaders (a 037 * situation that earlier versions had problems with). However there are 038 * some rare cases where the WeakHashtable approach does not work; in these 039 * situations specifying this class as a listener for the web application will 040 * ensure that all references held by commons-logging are fully released. 041 * <p> 042 * To use this class, configure the webapp deployment descriptor to call 043 * this class on webapp undeploy; the contextDestroyed method will tell 044 * every accessible LogFactory class that the entry in its map for the 045 * current webapp's context class loader should be cleared. 046 * 047 * @since 1.1 048 */ 049public class ServletContextCleaner implements ServletContextListener { 050 051 private static final Class<?>[] RELEASE_SIGNATURE = { ClassLoader.class }; 052 053 /** 054 * Invoked when a webapp is undeployed, this tells the LogFactory 055 * class to release any logging information related to the current 056 * contextClassloader. 057 */ 058 @Override 059 public void contextDestroyed(final ServletContextEvent sce) { 060 final ClassLoader tccl = Thread.currentThread().getContextClassLoader(); 061 062 final Object[] params = new Object[1]; 063 params[0] = tccl; 064 065 // Walk up the tree of class loaders, finding all the available 066 // LogFactory classes and releasing any objects associated with 067 // the tccl (ie the webapp). 068 // 069 // When there is only one LogFactory in the classpath, and it 070 // is within the webapp being undeployed then there is no problem; 071 // garbage collection works fine. 072 // 073 // When there are multiple LogFactory classes in the classpath but 074 // parent-first classloading is used everywhere, this loop is really 075 // short. The first instance of LogFactory found will 076 // be the highest in the classpath, and then no more will be found. 077 // This is ok, as with this setup this will be the only LogFactory 078 // holding any data associated with the tccl being released. 079 // 080 // When there are multiple LogFactory classes in the classpath and 081 // child-first classloading is used in any class loader, then multiple 082 // LogFactory instances may hold info about this TCCL; whenever the 083 // webapp makes a call into a class loaded via an ancestor class loader 084 // and that class calls LogFactory the tccl gets registered in 085 // the LogFactory instance that is visible from the ancestor 086 // class loader. However the concrete logging library it points 087 // to is expected to have been loaded via the TCCL, so the 088 // underlying logging lib is only initialized/configured once. 089 // These references from ancestor LogFactory classes down to 090 // TCCL class loaders are held via weak references and so should 091 // be released but there are circumstances where they may not. 092 // Walking up the class loader ancestry ladder releasing 093 // the current tccl at each level tree, though, will definitely 094 // clear any problem references. 095 ClassLoader loader = tccl; 096 while (loader != null) { 097 // Load via the current loader. Note that if the class is not accessible 098 // via this loader, but is accessible via some ancestor then that class 099 // will be returned. 100 try { 101 @SuppressWarnings("unchecked") 102 final Class<LogFactory> logFactoryClass = (Class<LogFactory>) loader.loadClass("org.apache.commons.logging.LogFactory"); 103 final Method releaseMethod = logFactoryClass.getMethod("release", RELEASE_SIGNATURE); 104 releaseMethod.invoke(null, params); 105 loader = logFactoryClass.getClassLoader().getParent(); 106 } catch (final ClassNotFoundException ex) { 107 // Neither the current class loader nor any of its ancestors could find 108 // the LogFactory class, so we can stop now. 109 loader = null; 110 } catch (final NoSuchMethodException ex) { 111 // This is not expected; every version of JCL has this method 112 System.err.println("LogFactory instance found which does not support release method!"); 113 loader = null; 114 } catch (final IllegalAccessException ex) { 115 // This is not expected; every ancestor class should be accessible 116 System.err.println("LogFactory instance found which is not accessible!"); 117 loader = null; 118 } catch (final InvocationTargetException ex) { 119 // This is not expected 120 System.err.println("LogFactory instance release method failed!"); 121 loader = null; 122 } 123 } 124 125 // Just to be sure, invoke release on the LogFactory that is visible from 126 // this ServletContextCleaner class too. This should already have been caught 127 // by the above loop but just in case... 128 LogFactory.release(tccl); 129 } 130 131 /** 132 * Invoked when a webapp is deployed. Nothing needs to be done here. 133 */ 134 @Override 135 public void contextInitialized(final ServletContextEvent sce) { 136 // do nothing 137 } 138}