1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 18 19 package org.apache.commons.logging.impl; 20 21 import java.lang.reflect.InvocationTargetException; 22 import java.lang.reflect.Method; 23 24 import javax.servlet.ServletContextEvent; 25 import javax.servlet.ServletContextListener; 26 27 import org.apache.commons.logging.LogFactory; 28 29 30 /** 31 * This class is capable of receiving notifications about the undeployment of 32 * a webapp, and responds by ensuring that commons-logging releases all 33 * memory associated with the undeployed webapp. 34 * <p> 35 * In general, the WeakHashtable support added in commons-logging release 1.1 36 * ensures that logging classes do not hold references that prevent an 37 * undeployed webapp's memory from being garbage-collected even when multiple 38 * copies of commons-logging are deployed via multiple classloaders (a 39 * situation that earlier versions had problems with). However there are 40 * some rare cases where the WeakHashtable approach does not work; in these 41 * situations specifying this class as a listener for the web application will 42 * ensure that all references held by commons-logging are fully released. 43 * <p> 44 * To use this class, configure the webapp deployment descriptor to call 45 * this class on webapp undeploy; the contextDestroyed method will tell 46 * every accessable LogFactory class that the entry in its map for the 47 * current webapp's context classloader should be cleared. 48 * 49 * @since 1.1 50 */ 51 52 public class ServletContextCleaner implements ServletContextListener { 53 54 private Class[] RELEASE_SIGNATURE = {ClassLoader.class}; 55 56 /** 57 * Invoked when a webapp is undeployed, this tells the LogFactory 58 * class to release any logging information related to the current 59 * contextClassloader. 60 */ 61 public void contextDestroyed(ServletContextEvent sce) { 62 ClassLoader tccl = Thread.currentThread().getContextClassLoader(); 63 64 Object[] params = new Object[1]; 65 params[0] = tccl; 66 67 // Walk up the tree of classloaders, finding all the available 68 // LogFactory classes and releasing any objects associated with 69 // the tccl (ie the webapp). 70 // 71 // When there is only one LogFactory in the classpath, and it 72 // is within the webapp being undeployed then there is no problem; 73 // garbage collection works fine. 74 // 75 // When there are multiple LogFactory classes in the classpath but 76 // parent-first classloading is used everywhere, this loop is really 77 // short. The first instance of LogFactory found will 78 // be the highest in the classpath, and then no more will be found. 79 // This is ok, as with this setup this will be the only LogFactory 80 // holding any data associated with the tccl being released. 81 // 82 // When there are multiple LogFactory classes in the classpath and 83 // child-first classloading is used in any classloader, then multiple 84 // LogFactory instances may hold info about this TCCL; whenever the 85 // webapp makes a call into a class loaded via an ancestor classloader 86 // and that class calls LogFactory the tccl gets registered in 87 // the LogFactory instance that is visible from the ancestor 88 // classloader. However the concrete logging library it points 89 // to is expected to have been loaded via the TCCL, so the 90 // underlying logging lib is only initialised/configured once. 91 // These references from ancestor LogFactory classes down to 92 // TCCL classloaders are held via weak references and so should 93 // be released but there are circumstances where they may not. 94 // Walking up the classloader ancestry ladder releasing 95 // the current tccl at each level tree, though, will definitely 96 // clear any problem references. 97 ClassLoader loader = tccl; 98 while (loader != null) { 99 // Load via the current loader. Note that if the class is not accessable 100 // via this loader, but is accessable via some ancestor then that class 101 // will be returned. 102 try { 103 Class logFactoryClass = loader.loadClass("org.apache.commons.logging.LogFactory"); 104 Method releaseMethod = logFactoryClass.getMethod("release", RELEASE_SIGNATURE); 105 releaseMethod.invoke(null, params); 106 loader = logFactoryClass.getClassLoader().getParent(); 107 } catch(ClassNotFoundException ex) { 108 // Neither the current classloader nor any of its ancestors could find 109 // the LogFactory class, so we can stop now. 110 loader = null; 111 } catch(NoSuchMethodException ex) { 112 // This is not expected; every version of JCL has this method 113 System.err.println("LogFactory instance found which does not support release method!"); 114 loader = null; 115 } catch(IllegalAccessException ex) { 116 // This is not expected; every ancestor class should be accessable 117 System.err.println("LogFactory instance found which is not accessable!"); 118 loader = null; 119 } catch(InvocationTargetException ex) { 120 // This is not expected 121 System.err.println("LogFactory instance release method failed!"); 122 loader = null; 123 } 124 } 125 126 // Just to be sure, invoke release on the LogFactory that is visible from 127 // this ServletContextCleaner class too. This should already have been caught 128 // by the above loop but just in case... 129 LogFactory.release(tccl); 130 } 131 132 /** 133 * Invoked when a webapp is deployed. Nothing needs to be done here. 134 */ 135 public void contextInitialized(ServletContextEvent sce) { 136 // do nothing 137 } 138 }