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:

Serializable, Cloneable, Map

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 semantics 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 environments 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 implementation 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

Version:

$Id: WeakHashtable.java 1435077 2013-01-18 10:51:35Z tn $

See Also:

Serialized Form

WeakHashtable

public WeakHashtable()

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

containsKey

public boolean containsKey(Object key)

Specified by:

containsKey in interface Map

Overrides:

containsKey in class Hashtable

See Also:

Hashtable

elements

public Enumeration elements()

Overrides:

elements in class Hashtable

See Also:

Hashtable

entrySet

public Set entrySet()

Specified by:

entrySet in interface Map

Overrides:

entrySet in class Hashtable

See Also:

Hashtable

get

public Object get(Object key)

Specified by:

get in interface Map

Overrides:

get in class Hashtable

See Also:

Hashtable

keys

public Enumeration keys()

Overrides:

keys in class Hashtable

See Also:

Hashtable

keySet

public Set keySet()

Specified by:

keySet in interface Map

Overrides:

keySet in class Hashtable

See Also:

Hashtable

put

public Object put(Object key,
                  Object value)

Specified by:

put in interface Map

Overrides:

put in class Hashtable

See Also:

Hashtable

putAll

public void putAll(Map t)

Specified by:

putAll in interface Map

Overrides:

putAll in class Hashtable

See Also:

Hashtable

values

public Collection values()

Specified by:

values in interface Map

Overrides:

values in class Hashtable

See Also:

Hashtable

remove

public Object remove(Object key)

Specified by:

remove in interface Map

Overrides:

remove in class Hashtable

See Also:

Hashtable

isEmpty

public boolean isEmpty()

Specified by:

isEmpty in interface Map

Overrides:

isEmpty in class Hashtable

See Also:

Hashtable

size

public int size()

Specified by:

size in interface Map

Overrides:

size in class Hashtable

See Also:

Hashtable

toString

public String toString()

Overrides:

toString in class Hashtable

See Also:

Hashtable

rehash

protected void rehash()

Overrides:

rehash in class Hashtable

See Also:

Hashtable