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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
*
* https://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.beanutils2;
import java.util.Map;
import java.util.WeakHashMap;
/**
* An instance of this class represents a value that is provided per (thread) context classloader.
*
*
* Occasionally it is necessary to store data in "global" variables (including uses of the Singleton pattern). In applications which have only a single
* classloader such data can simply be stored as "static" members on some class. When multiple class loaders are involved, however, this approach can fail; in
* particular, this doesn't work when the code may be run within a servlet container or a j2ee container, and the class on which the static member is defined is
* loaded via a "shared" classloader that is visible to all components running within the container. This class provides a mechanism for associating data with a
* ClassLoader instance, which ensures that when the code runs in such a container each component gets its own copy of the "global" variable rather than
* unexpectedly sharing a single copy of the variable with other components that happen to be running in the same container at the same time (eg servlets or
* EJBs.)
*
*
*
* This class is strongly patterned after the java.lang.ThreadLocal class, which performs a similar task in allowing data to be associated with a particular
* thread.
*
*
*
* When code that uses this class is run as a "normal" application, ie not within a container, the effect is identical to just using a static member variable to
* store the data, because Thread.getContextClassLoader always returns the same classloader (the system classloader).
*
*
*
* Expected usage is as follows:
*
*
*
*
* public class SomeClass {
* private static final ContextClassLoaderLocal<String> global
* = new ContextClassLoaderLocal<String>() {
* protected String initialValue() {
* return new String("Initial value");
* };
*
* public void testGlobal() {
* String s = global.get();
* System.out.println("global value:" + s);
* buf.set("New Value");
* }
*
*
*
*
* Note: This class takes some care to ensure that when a component which uses this class is "undeployed" by a container the component-specific
* classloader and all its associated classes (and their static variables) are garbage-collected. Unfortunately there is one scenario in which this does
* not work correctly and there is unfortunately no known workaround other than ensuring that the component (or its container) calls the "unset" method
* on this class for each instance of this class when the component is undeployed. The problem occurs if:
*
* - the class containing a static instance of this class was loaded via a shared classloader, and
* - the value stored in the instance is an object whose class was loaded via the component-specific classloader (or any of the objects it refers to were
* loaded via that classloader).
*
*
* The result is that the map managed by this object still contains a strong reference to the stored object, which contains a strong reference to the
* classloader that loaded it, meaning that although the container has "undeployed" the component the component-specific classloader and all the related classes
* and static variables cannot be garbage-collected. This is not expected to be an issue with the commons-beanutils library as the only classes which use this
* class are BeanUtilsBean and ConvertUtilsBean and there is no obvious reason for a user of the BeanUtils library to subclass either of those classes.
*
*
*
* Note: A WeakHashMap bug in several 1.3 JVMs results in a memory leak for those JVMs.
*
*
*
* Note: Of course, all of this would be unnecessary if containers required each component to load the full set of classes it needs, that is,
* avoided providing classes loaded via a "shared" classloader.
*
*
* @param the type of data stored in an instance
* @see Thread#getContextClassLoader
*/
public class ContextClassLoaderLocal {
private final Map valueByClassLoader = new WeakHashMap<>();
private boolean globalValueInitialized;
private T globalValue;
/**
* Constructs a context classloader instance
*/
public ContextClassLoaderLocal() {
}
/**
* Gets the instance which provides the functionality for {@link BeanUtils}. This is a pseudo-singleton - an single instance is provided per (thread)
* context classloader. This mechanism provides isolation for web apps deployed in the same container.
*
* @return the object currently associated with the context-classloader of the current thread.
*/
public synchronized T get() {
// synchronizing the whole method is a bit slower
// but guarantees no subtle threading problems, and there's no
// need to synchronize valueByClassLoader
// make sure that the map is given a change to purge itself
valueByClassLoader.isEmpty();
try {
final ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
if (contextClassLoader != null) {
T value = valueByClassLoader.get(contextClassLoader);
if (value == null && !valueByClassLoader.containsKey(contextClassLoader)) {
value = initialValue();
valueByClassLoader.put(contextClassLoader, value);
}
return value;
}
} catch (final SecurityException e) {
/* SWALLOW - should we log this? */ }
// if none or exception, return the globalValue
if (!globalValueInitialized) {
globalValue = initialValue();
globalValueInitialized = true;
} // else already set
return globalValue;
}
/**
* Returns the initial value for this ContextClassLoaderLocal variable. This method will be called once per Context ClassLoader for each
* ContextClassLoaderLocal, the first time it is accessed with get or set. If the programmer desires ContextClassLoaderLocal variables to be initialized to
* some value other than null, ContextClassLoaderLocal must be subclassed, and this method overridden. Typically, an anonymous inner class will be used.
* Typical implementations of initialValue will call an appropriate constructor and return the newly constructed object.
*
* @return a new Object to be used as an initial value for this ContextClassLoaderLocal
*/
protected T initialValue() {
return null;
}
/**
* Sets the value - a value is provided per (thread) context classloader. This mechanism provides isolation for web apps deployed in the same container.
*
* @param value the object to be associated with the entrant thread's context classloader
*/
public synchronized void set(final T value) {
// synchronizing the whole method is a bit slower
// but guarantees no subtle threading problems
// make sure that the map is given a change to purge itself
valueByClassLoader.isEmpty();
try {
final ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
if (contextClassLoader != null) {
valueByClassLoader.put(contextClassLoader, value);
return;
}
} catch (final SecurityException e) {
/* SWALLOW - should we log this? */ }
// if in doubt, set the global value
globalValue = value;
globalValueInitialized = true;
}
/**
* Unsets the value associated with the current thread's context classloader
*/
public synchronized void unset() {
try {
final ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
unset(contextClassLoader);
} catch (final SecurityException e) {
/* SWALLOW - should we log this? */ }
}
/**
* Unsets the value associated with the given classloader
*
* @param classLoader The classloader to unset for
*/
public synchronized void unset(final ClassLoader classLoader) {
valueByClassLoader.remove(classLoader);
}
}