org.apache.commons.logging.impl

Class WeakHashtable


public final class WeakHashtable
extends Hashtable

Implementation of Hashtable that uses WeakReference's to hold its keys thus allowing them to be reclaimed by the garbage collector. The associated values are retained using strong references.

This class follows the symantics of Hashtable as closely as possible. It therefore does not accept null values or keys.

Note: This is not intended to be a general purpose hash table replacement. This implementation is also tuned towards a particular purpose: for use as a replacement for Hashtable in LogFactory. This application requires good liveliness for get and put. Various tradeoffs have been made with this in mind.

Usage: typical use case is as a drop-in replacement for the Hashtable used in LogFactory for J2EE enviroments running 1.3+ JVMs. Use of this class in most cases (see below) will allow classloaders to be collected by the garbage collector without the need to call LogFactory.release(ClassLoader).

org.apache.commons.logging.LogFactory checks whether this class can be supported by the current JVM, and if so then uses it to store references to the LogFactory implementationd it loads (rather than using a standard Hashtable instance). Having this class used instead of Hashtable 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 java.lang.ref.WeakReference and associates). And by the way, this extends Hashtable rather than HashMap for backwards compatibility reasons. See the documentation for method LogFactory.createFactoryStore for more details.

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" LogFactory's factories member! If LogFactory.release() is called whenever component is unloaded, the classloaders will be correctly garbage collected; this should 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.

Limitations: 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.

If the abstract class LogFactory is loaded by the container classloader but a subclass of LogFactory [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 getClass().getClassLoader(). This chain of references will prevent collection of the child classloader.

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 LogFactory implementation is loaded by a child classloader (e.g. a web app classloader).

To avoid this scenario, ensure that any custom LogFactory subclass is loaded by the same classloader as the base LogFactory. Creating custom LogFactory subclasses is, however, rare. The standard LogFactoryImpl class should be sufficient for most or all users.

Author:
Brian Stansberry
Since:
1.1

Nested Class Summary

private static class
WeakHashtable.Entry
Entry implementation
private static class
WeakHashtable.Referenced
Wrapper giving correct symantics for equals and hashcode
private static class
WeakHashtable.WeakKey
WeakReference subclass that holds a hard reference to an associated value and also makes accessible the Referenced object holding it.

Field Summary

private static int
MAX_CHANGES_BEFORE_PURGE
The maximum number of times put() or remove() can be called before the map will be purged of all cleared entries.
private static int
PARTIAL_PURGE_COUNT
The maximum number of times put() or remove() can be called before the map will be purged of one cleared entry.
private int
changeCount
private ReferenceQueue
queue

Constructor Summary

WeakHashtable()
Constructs a WeakHashtable with the Hashtable default capacity and load factor.

Method Summary

boolean
containsKey(Object key)
Enumeration
elements()
Set
entrySet()
Object
get(Object key)
boolean
isEmpty()
Set
keySet()
Enumeration
keys()
private void
purge()
Purges all entries whose wrapped keys have been garbage collected.
private void
purgeOne()
Purges one entry whose wrapped key has been garbage collected.
Object
put(Object key, Object value)
void
putAll(Map t)
protected void
rehash()
Object
remove(Object key)
int
size()
String
toString()
Collection
values()

Field Details

MAX_CHANGES_BEFORE_PURGE

private static final int MAX_CHANGES_BEFORE_PURGE
The maximum number of times put() or remove() can be called before the map will be purged of all cleared entries.
Field Value:
100

PARTIAL_PURGE_COUNT

private static final int PARTIAL_PURGE_COUNT
The maximum number of times put() or remove() can be called before the map will be purged of one cleared entry.
Field Value:
10

changeCount

private int changeCount

queue

private ReferenceQueue queue

Constructor Details

WeakHashtable

public WeakHashtable()
Constructs a WeakHashtable with the Hashtable default capacity and load factor.

Method Details

containsKey

public boolean containsKey(Object key)
See Also:
Hashtable

elements

public Enumeration elements()
See Also:
Hashtable

entrySet

public Set entrySet()
See Also:
Hashtable

get

public Object get(Object key)
See Also:
Hashtable

isEmpty

public boolean isEmpty()
See Also:
Hashtable

keySet

public Set keySet()
See Also:
Hashtable

keys

public Enumeration keys()
See Also:
Hashtable

purge

private void purge()
Purges all entries whose wrapped keys have been garbage collected.

purgeOne

private void purgeOne()
Purges one entry whose wrapped key has been garbage collected.

put

public Object put(Object key,
                  Object value)
See Also:
Hashtable

putAll

public void putAll(Map t)
See Also:
Hashtable

rehash

protected void rehash()
See Also:
Hashtable

remove

public Object remove(Object key)
See Also:
Hashtable

size

public int size()
See Also:
Hashtable

toString

public String toString()
See Also:
Hashtable

values

public Collection values()
See Also:
Hashtable

Copyright 2002-2004 The Apache Software Foundation.