Listener that flushes the JDK's
cache on web app shutdown. Register this listener in your
guarantee proper release of the web application class loader and its loaded classes.
If the JavaBeans Introspector has been used to analyze application classes,
the system-level Introspector cache will hold a hard reference to those classes.
Consequently, those classes and the web application class loader will not be
garbage-collected on web app shutdown! This listener performs proper cleanup,
to allow for garbage collection to take effect.
Unfortunately, the only way to clean up the Introspector is to flush
the entire cache, as there is no way to specifically determine the
application's classes referenced there. This will remove cached
introspection results for all other applications in the server too.
Note that this listener is not necessary when using Spring's beans
infrastructure within the application, as Spring's own introspection results
cache will immediately flush an analyzed class from the JavaBeans Introspector
cache and only hold a cache within the application's own ClassLoader.
Although Spring itself does not create JDK Introspector leaks, note that this
listener should nevertheless be used in scenarios where the Spring framework classes
themselves reside in a 'common' ClassLoader (such as the system ClassLoader).
In such a scenario, this listener will properly clean up Spring's introspection cache.
Application classes hardly ever need to use the JavaBeans Introspector
directly, so are normally not the cause of Introspector resource leaks.
Rather, many libraries and frameworks do not clean up the Introspector:
e.g. Struts and Quartz.
Note that a single such Introspector leak will cause the entire web
app class loader to not get garbage collected! This has the consequence that
you will see all the application's static class resources (like singletons)
around after web app shutdown, which is not the fault of those classes!
This listener should be registered as the first one in
before any application listeners such as Spring's ContextLoaderListener.
This allows the listener to take full effect at the right time of the lifecycle.