ContextClassLoaderLocal

/*
 * 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.beanutils2;

import java.util.Map;
import java.util.WeakHashMap;

/**
 * An instance of this class represents a value that is provided per (thread) context classloader.
 *
 * <p>
 * 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.)
 * </p>
 *
 * <p>
 * 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.
 * </p>
 *
 * <p>
 * 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).
 * </p>
 *
 * <p>
 * Expected usage is as follows:
 * </p>
 *
 * <pre>
 * <code>
 *  public class SomeClass {
 *    private static final ContextClassLoaderLocal&lt;String&gt; global
 *      = new ContextClassLoaderLocal&lt;String&gt;() {
 *          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");
 *    }
 * </code>
 * </pre>
 *
 * <p>
 * <strong>Note:</strong> 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
 * <em>not</em> 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:
 * <ul>
 * <li>the class containing a static instance of this class was loaded via a shared classloader, and</li>
 * <li>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).</li>
 * </ul>
 * <p>
 * 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.
 * </p>
 *
 * <p>
 * <strong>Note:</strong> A WeakHashMap bug in several 1.3 JVMs results in a memory leak for those JVMs.
 * </p>
 *
 * <p>
 * <strong>Note:</strong> 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.
 * </p>
 *
 * @param <T> the type of data stored in an instance
 * @see Thread#getContextClassLoader
 */
public class ContextClassLoaderLocal<T> {
    private final Map<ClassLoader, T> 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 <em>unset</em> for
     */
    public synchronized void unset(final ClassLoader classLoader) {
        valueByClassLoader.remove(classLoader);
    }
}