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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
     
    Consider Paul's rocket mass heater.
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