Many ranchers seem confused about which listeners to declare in web.xml.
The main point is :
Classes implementing interfaces other than HttpSessionBindingListener and HttpSessionActivationListener need to be declared in DD.
A trick to remember which listeners to register in the deployment descriptor: Listeners can be classified in 2 categories:
- Those that are interested in all events of one type (e.g. HttpSessionListener? - interested in ALL HTTP session creation and destruction events, HttpSessionAttributeListener? - interested in all session attribute events - any attribute added / removed / replaced), and all the listeners for ServletContext and ServletRequest. The container creates one instance of each of these listeners.
- Those that are interested only in specific events of one type. There are 2 listeners of this type :
- HttpSessionBindingListener : interested in knowing when it is itself bound / unbound to a HttpSession
- HttpSessionActivationListener : interested in knowing when only the session it is bound to is activated or passivated, so that it can prepare itself for activation / passivation)
We create the instances of these listeners and set them as attributes in specific http sessions.
The first type of listeners are configured in the deployment descriptor - as their methods are ALWAYS called when the event occurs.
The second type of listeners are not configured in the deployment descriptor.
Note: The HFSJ book is in error on this matter, as the errata has confirmed.
ServletsFaq
