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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.commons.logging.impl;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

import org.apache.commons.logging.LogFactory;

/**
 * This class is capable of receiving notifications about the undeployment of
 * a webapp, and responds by ensuring that commons-logging releases all
 * memory associated with the undeployed webapp.
 * 

* In general, the WeakHashtable support added in commons-logging release 1.1 * ensures that logging classes do not hold references that prevent an * undeployed webapp's memory from being garbage-collected even when multiple * copies of commons-logging are deployed via multiple classloaders (a * situation that earlier versions had problems with). However there are * some rare cases where the WeakHashtable approach does not work; in these * situations specifying this class as a listener for the web application will * ensure that all references held by commons-logging are fully released. *

* To use this class, configure the webapp deployment descriptor to call * this class on webapp undeploy; the contextDestroyed method will tell * every accessible LogFactory class that the entry in its map for the * current webapp's context classloader should be cleared. * * @version $Id: ServletContextCleaner.java 1432580 2013-01-13 10:41:05Z tn $ * @since 1.1 */ public class ServletContextCleaner implements ServletContextListener { private static final Class[] RELEASE_SIGNATURE = {ClassLoader.class}; /** * Invoked when a webapp is undeployed, this tells the LogFactory * class to release any logging information related to the current * contextClassloader. */ public void contextDestroyed(ServletContextEvent sce) { ClassLoader tccl = Thread.currentThread().getContextClassLoader(); Object[] params = new Object[1]; params[0] = tccl; // Walk up the tree of classloaders, finding all the available // LogFactory classes and releasing any objects associated with // the tccl (ie the webapp). // // When there is only one LogFactory in the classpath, and it // is within the webapp being undeployed then there is no problem; // garbage collection works fine. // // When there are multiple LogFactory classes in the classpath but // parent-first classloading is used everywhere, this loop is really // short. The first instance of LogFactory found will // be the highest in the classpath, and then no more will be found. // This is ok, as with this setup this will be the only LogFactory // holding any data associated with the tccl being released. // // When there are multiple LogFactory classes in the classpath and // child-first classloading is used in any classloader, then multiple // LogFactory instances may hold info about this TCCL; whenever the // webapp makes a call into a class loaded via an ancestor classloader // and that class calls LogFactory the tccl gets registered in // the LogFactory instance that is visible from the ancestor // classloader. However the concrete logging library it points // to is expected to have been loaded via the TCCL, so the // underlying logging lib is only initialised/configured once. // These references from ancestor LogFactory classes down to // TCCL classloaders are held via weak references and so should // be released but there are circumstances where they may not. // Walking up the classloader ancestry ladder releasing // the current tccl at each level tree, though, will definitely // clear any problem references. ClassLoader loader = tccl; while (loader != null) { // Load via the current loader. Note that if the class is not accessible // via this loader, but is accessible via some ancestor then that class // will be returned. try { Class logFactoryClass = loader.loadClass("org.apache.commons.logging.LogFactory"); Method releaseMethod = logFactoryClass.getMethod("release", RELEASE_SIGNATURE); releaseMethod.invoke(null, params); loader = logFactoryClass.getClassLoader().getParent(); } catch(ClassNotFoundException ex) { // Neither the current classloader nor any of its ancestors could find // the LogFactory class, so we can stop now. loader = null; } catch(NoSuchMethodException ex) { // This is not expected; every version of JCL has this method System.err.println("LogFactory instance found which does not support release method!"); loader = null; } catch(IllegalAccessException ex) { // This is not expected; every ancestor class should be accessible System.err.println("LogFactory instance found which is not accessable!"); loader = null; } catch(InvocationTargetException ex) { // This is not expected System.err.println("LogFactory instance release method failed!"); loader = null; } } // Just to be sure, invoke release on the LogFactory that is visible from // this ServletContextCleaner class too. This should already have been caught // by the above loop but just in case... LogFactory.release(tccl); } /** * Invoked when a webapp is deployed. Nothing needs to be done here. */ public void contextInitialized(ServletContextEvent sce) { // do nothing } }





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