Developing Event Listeners 

Use

A listener can be used to monitor and react to events in a servlet's life cycle by defining methods that get invoked when life cycle events occur. In this way, you can keep track of events such as application context initialization or destruction, session creation, and so on, and can react accordingly to them.

Application event listeners are classes that implement one or more of the servlet event listener interfaces. Servlet event listeners support notification for state changes in the ServletContext and HttpSession objects, specifically:

●      Servlet context listeners are used to manage resources or state held at Virtual Machine level for the application.

●      HTTP session listeners are used to manage state or resources associated with a series of requests made in a Web application from the same client or user.

Procedure

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       1.      In the Project Explorer, select the Dynamic Web Project.

       2.      In the context menu, choose New → Listener.

       3.      In the New Listener wizard, enter the settings as required.

Choose all types of life cycle events you want to handle. Each event type is associated with one of the servlet event listener interfaces. The following table summarizes events and their corresponding listeners.

Event types and their corresponding listeners

Scope of Event	Event	Event Type	Event Interface
ServletContext	Initialization or destruction of a servlet context	ServletContextEvent	ServletContextListener
ServletContext	Context attribute(s) added, removed or replaced	ServletContextAttributeEvent	ServletContextAttributeListener
Session	Session created or destroyed	HttpSessionEvent	HttpSessionListener
Session	Session attribute(s) added, removed or replaced	HttpSessionBindingEvent	HttpSessionAttributeListener
Session	Session activated or passivated	HttpSessionEvent	HttpSessionActivationListener
Session	The current object is bound or unbound to a session	HttpSessionBindingEvent	HttpSessionBindingListener
Servlet Request	Request comes in and out of scope in the Web component	ServletRequestEvent	ServletRequestListener
Servlet Request	Request attribute(s) added, removed or replaced	ServletRequestAttributeEvent	ServletRequestAttributeListener

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       1.      Choose Finish.

Result

The skeleton of the listener class is created. You can now implement the class body as necessary.

The listener is automatically added to the web.xml of the Dynamic Web Project.

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