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 {@link org.apache.commons.logging.LogFactory#release(ClassLoader) 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. | 