/*
* Copyright 2004 The Apache Software Foundation.
*
* Licensed 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.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.*;
/**
* <p>Implementation of <code>Hashtable</code> that uses <code>WeakReference</code>'s
* to hold its keys thus allowing them to be reclaimed by the garbage collector.
* The associated values are retained using strong references.</p>
*
* <p>This class follows the symantics of <code>Hashtable</code> as closely as
* possible. It therefore does not accept null values or keys.</p>
*
* <p><strong>Note:</strong>
* This is <em>not</em> intended to be a general purpose hash table replacement.
* This implementation is also tuned towards a particular purpose: for use as a replacement
* for <code>Hashtable</code> in <code>LogFactory</code>. This application requires
* good liveliness for <code>get</code> and <code>put</code>. Various tradeoffs
* have been made with this in mind.
* </p>
* <p>
* <strong>Usage:</strong> typical use case is as a drop-in replacement
* for the <code>Hashtable</code> used in <code>LogFactory</code> for J2EE enviroments
* running 1.3+ JVMs. Use of this class <i>in most cases</i> (see below) will
* allow classloaders to be collected by the garbage collector without the need
* to call {@link org.apache.commons.logging.LogFactory#release(ClassLoader) LogFactory.release(ClassLoader)}.
* </p>
*
* <p><code>org.apache.commons.logging.LogFactory</code> checks whether this class
* can be supported by the current JVM, and if so then uses it to store
* references to the <code>LogFactory</code> implementationd it loads
* (rather than using a standard Hashtable instance).
* Having this class used instead of <code>Hashtable</code> solves
* certain issues related to dynamic reloading of applications in J2EE-style
* environments. However this class requires java 1.3 or later (due to its use
* of <code>java.lang.ref.WeakReference</code> and associates).
* And by the way, this extends <code>Hashtable</code> rather than <code>HashMap</code>
* for backwards compatibility reasons. See the documentation
* for method <code>LogFactory.createFactoryStore</code> for more details.</p>
*
* <p>The reason all this is necessary is due to a issue which
* arises during hot deploy in a J2EE-like containers.
* Each component running in the container owns one or more classloaders; when
* the component loads a LogFactory instance via the component classloader
* a reference to it gets stored in the static LogFactory.factories member,
* keyed by the component's classloader so different components don't
* stomp on each other. When the component is later unloaded, the container
* sets the component's classloader to null with the intent that all the
* component's classes get garbage-collected. However there's still a
* reference to the component's classloader from a key in the "global"
* <code>LogFactory</code>'s factories member! If <code>LogFactory.release()</code>
* is called whenever component is unloaded, the classloaders will be correctly
* garbage collected; this <i>should</i> be done by any container that
* bundles commons-logging by default. However, holding the classloader
* references weakly ensures that the classloader will be garbage collected
* without the container performing this step. </p>
*
* <p>
* <strong>Limitations:</strong>
* There is still one (unusual) scenario in which a component will not
* be correctly unloaded without an explicit release. Though weak references
* are used for its keys, it is necessary to use strong references for its values.
* </p>
*
* <p> If the abstract class <code>LogFactory</code> is
* loaded by the container classloader but a subclass of
* <code>LogFactory</code> [LogFactory1] is loaded by the component's
* classloader and an instance stored in the static map associated with the
* base LogFactory class, then there is a strong reference from the LogFactory
* class to the LogFactory1 instance (as normal) and a strong reference from
* the LogFactory1 instance to the component classloader via
* <code>getClass().getClassLoader()</code>. This chain of references will prevent
* collection of the child classloader.</p>
*
* <p>
* Such a situation occurs when the commons-logging.jar is
* loaded by a parent classloader (e.g. a server level classloader in a
* servlet container) and a custom <code>LogFactory</code> implementation is
* loaded by a child classloader (e.g. a web app classloader).</p>
*
* <p>To avoid this scenario, ensure
* that any custom LogFactory subclass is loaded by the same classloader as
* the base <code>LogFactory</code>. Creating custom LogFactory subclasses is,
* however, rare. The standard LogFactoryImpl class should be sufficient
* for most or all users.</p>
*
*
* @author Brian Stansberry
*
* @since 1.1
*/
public final class WeakHashtable extends Hashtable {
/**
* The maximum number of times put() or remove() can be called before
* the map will be purged of all cleared entries.
*/
private static final int MAX_CHANGES_BEFORE_PURGE = 100;
/**
* The maximum number of times put() or remove() can be called before
* the map will be purged of one cleared entry.
*/
private static final int PARTIAL_PURGE_COUNT = 10;
/* ReferenceQueue we check for gc'd keys */
private ReferenceQueue queue = new ReferenceQueue();
/* Counter used to control how often we purge gc'd entries */
private int changeCount = 0;
/**
* Constructs a WeakHashtable with the Hashtable default
* capacity and load factor.
*/
public WeakHashtable() {}
/**
*@see Hashtable
*/
public boolean containsKey(Object key) {
// purge should not be required
Referenced referenced = new Referenced(key);
return super.containsKey(referenced);
}
/**
*@see Hashtable
*/
public Enumeration elements() {
purge();
return super.elements();
}
/**
*@see Hashtable
*/
public Set entrySet() {
purge();
Set referencedEntries = super.entrySet();
Set unreferencedEntries = new HashSet();
for (Iterator it=referencedEntries.iterator(); it.hasNext();) {
Map.Entry entry = (Map.Entry) it.next();
Referenced referencedKey = (Referenced) entry.getKey();
Object key = referencedKey.getValue();
Object value = entry.getValue();
if (key != null) {
Entry dereferencedEntry = new Entry(key, value);
unreferencedEntries.add(dereferencedEntry);
}
}
return unreferencedEntries;
}
/**
*@see Hashtable
*/
public Object get(Object key) {
// for performance reasons, no purge
Referenced referenceKey = new Referenced(key);
return super.get(referenceKey);
}
/**
*@see Hashtable
*/
public Enumeration keys() {
purge();
final Enumeration enumer = super.keys();
return new Enumeration() {
public boolean hasMoreElements() {
return enumer.hasMoreElements();
}
public Object nextElement() {
Referenced nextReference = (Referenced) enumer.nextElement();
return nextReference.getValue();
}
};
}
/**
*@see Hashtable
*/
public Set keySet() {
purge();
Set referencedKeys = super.keySet();
Set unreferencedKeys = new HashSet();
for (Iterator it=referencedKeys.iterator(); it.hasNext();) {
Referenced referenceKey = (Referenced) it.next();
Object keyValue = referenceKey.getValue();
if (keyValue != null) {
unreferencedKeys.add(keyValue);
}
}
return unreferencedKeys;
}
/**
*@see Hashtable
*/
public Object put(Object key, Object value) {
// check for nulls, ensuring symantics match superclass
if (key == null) {
throw new NullPointerException("Null keys are not allowed");
}
if (value == null) {
throw new NullPointerException("Null values are not allowed");
}
// for performance reasons, only purge every
// MAX_CHANGES_BEFORE_PURGE times
if (changeCount++ > MAX_CHANGES_BEFORE_PURGE) {
purge();
changeCount = 0;
}
// do a partial purge more often
else if ((changeCount % PARTIAL_PURGE_COUNT) == 0) {
purgeOne();
}
Object result = null;
Referenced keyRef = new Referenced(key, queue);
return super.put(keyRef, value);
}
/**
*@see Hashtable
*/
public void putAll(Map t) {
if (t != null) {
Set entrySet = t.entrySet();
for (Iterator it=entrySet.iterator(); it.hasNext();) {
Map.Entry entry = (Map.Entry) it.next();
put(entry.getKey(), entry.getValue());
}
}
}
/**
*@see Hashtable
*/
public Collection values() {
purge();
return super.values();
}
/**
*@see Hashtable
*/
public Object remove(Object key) {
// for performance reasons, only purge every
// MAX_CHANGES_BEFORE_PURGE times
if (changeCount++ > MAX_CHANGES_BEFORE_PURGE) {
purge();
changeCount = 0;
}
// do a partial purge more often
else if ((changeCount % PARTIAL_PURGE_COUNT) == 0) {
purgeOne();
}
return super.remove(new Referenced(key));
}
/**
*@see Hashtable
*/
public boolean isEmpty() {
purge();
return super.isEmpty();
}
/**
*@see Hashtable
*/
public int size() {
purge();
return super.size();
}
/**
*@see Hashtable
*/
public String toString() {
purge();
return super.toString();
}
/**
* @see Hashtable
*/
protected void rehash() {
// purge here to save the effort of rehashing dead entries
purge();
super.rehash();
}
/**
* Purges all entries whose wrapped keys
* have been garbage collected.
*/
private void purge() {
synchronized (queue) {
WeakKey key;
while ((key = (WeakKey) queue.poll()) != null) {
super.remove(key.getReferenced());
}
}
}
/**
* Purges one entry whose wrapped key
* has been garbage collected.
*/
private void purgeOne() {
synchronized (queue) {
WeakKey key = (WeakKey) queue.poll();
if (key != null) {
super.remove(key.getReferenced());
}
}
}
/** Entry implementation */
private final static class Entry implements Map.Entry {
private final Object key;
private final Object value;
private Entry(Object key, Object value) {
this.key = key;
this.value = value;
}
public boolean equals(Object o) {
boolean result = false;
if (o != null && o instanceof Map.Entry) {
Map.Entry entry = (Map.Entry) o;
result = (getKey()==null ?
entry.getKey() == null :
getKey().equals(entry.getKey()))
&&
(getValue()==null ?
entry.getValue() == null :
getValue().equals(entry.getValue()));
}
return result;
}
public int hashCode() {
return (getKey()==null ? 0 : getKey().hashCode()) ^
(getValue()==null ? 0 : getValue().hashCode());
}
public Object setValue(Object value) {
throw new UnsupportedOperationException("Entry.setValue is not supported.");
}
public Object getValue() {
return value;
}
public Object getKey() {
return key;
}
}
/** Wrapper giving correct symantics for equals and hashcode */
private final static class Referenced {
private final WeakReference reference;
private final int hashCode;
/**
*
* @throws NullPointerException if referant is <code>null</code>
*/
private Referenced(Object referant) {
reference = new WeakReference(referant);
// Calc a permanent hashCode so calls to Hashtable.remove()
// work if the WeakReference has been cleared
hashCode = referant.hashCode();
}
/**
*
* @throws NullPointerException if key is <code>null</code>
*/
private Referenced(Object key, ReferenceQueue queue) {
reference = new WeakKey(key, queue, this);
// Calc a permanent hashCode so calls to Hashtable.remove()
// work if the WeakReference has been cleared
hashCode = key.hashCode();
}
public int hashCode() {
return hashCode;
}
private Object getValue() {
return reference.get();
}
public boolean equals(Object o) {
boolean result = false;
if (o instanceof Referenced) {
Referenced otherKey = (Referenced) o;
Object thisKeyValue = getValue();
Object otherKeyValue = otherKey.getValue();
if (thisKeyValue == null) {
result = (otherKeyValue == null);
// Since our hashcode was calculated from the original
// non-null referant, the above check breaks the
// hashcode/equals contract, as two cleared Referenced
// objects could test equal but have different hashcodes.
// We can reduce (not eliminate) the chance of this
// happening by comparing hashcodes.
if (result == true) {
result = (this.hashCode() == otherKey.hashCode());
}
// In any case, as our c'tor does not allow null referants
// and Hashtable does not do equality checks between
// existing keys, normal hashtable operations should never
// result in an equals comparison between null referants
}
else
{
result = thisKeyValue.equals(otherKeyValue);
}
}
return result;
}
}
/**
* WeakReference subclass that holds a hard reference to an
* associated <code>value</code> and also makes accessible
* the Referenced object holding it.
*/
private final static class WeakKey extends WeakReference {
private final Referenced referenced;
private WeakKey(Object key,
ReferenceQueue queue,
Referenced referenced) {
super(key, queue);
this.referenced = referenced;
}
private Referenced getReferenced() {
return referenced;
}
}
}