org.apache.commons.logging.impl
Class WeakHashtable

java.lang.Object
  java.util.Dictionary
      java.util.Hashtable
          org.apache.commons.logging.impl.WeakHashtable

All Implemented Interfaces:

java.lang.Cloneable, java.util.Map, java.io.Serializable

public final class WeakHashtable

extends java.util.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.

Since:

1.1

Author:

Brian Stansberry

See Also:

Serialized Form

Constructor Summary

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

Method Summary

boolean	containsKey(java.lang.Object key)
java.util.Enumeration	elements()
java.util.Set	entrySet()
java.lang.Object	get(java.lang.Object key)
boolean	isEmpty()
java.util.Enumeration	keys()
java.util.Set	keySet()
java.lang.Object	put(java.lang.Object key, java.lang.Object value)
void	putAll(java.util.Map t)
protected void	rehash()
java.lang.Object	remove(java.lang.Object key)
int	size()
java.lang.String	toString()
java.util.Collection	values()

Methods inherited from class java.util.Hashtable

clear, clone, contains, containsValue, equals, hashCode

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

WeakHashtable

public WeakHashtable()

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

Method Detail

containsKey

public boolean containsKey(java.lang.Object key)

See Also:

Hashtable

elements

public java.util.Enumeration elements()

See Also:

Hashtable

entrySet

public java.util.Set entrySet()

See Also:

Hashtable

get

public java.lang.Object get(java.lang.Object key)

See Also:

Hashtable

keys

public java.util.Enumeration keys()

See Also:

Hashtable

keySet

public java.util.Set keySet()

See Also:

Hashtable

put

public java.lang.Object put(java.lang.Object key,
                            java.lang.Object value)

See Also:

Hashtable

putAll

public void putAll(java.util.Map t)

See Also:

Hashtable

values

public java.util.Collection values()

See Also:

Hashtable

remove

public java.lang.Object remove(java.lang.Object key)

See Also:

Hashtable

isEmpty

public boolean isEmpty()

See Also:

Hashtable

size

public int size()

See Also:

Hashtable

toString

public java.lang.String toString()

See Also:

Hashtable

rehash

protected void rehash()

See Also:

Hashtable