Servlet Listener (web application listener)

• During the lifetime of a typical web application, a number of events take place.

  • request objects are created and destroyed (response send).

  • session objects are created & destroyed (session invalidated or time out).

  • contexts(web apps) objects are created & destroyed (3 triggers of application's end of life).

  • request or session or context attributes are added, removed, or modified etc.

• The Servlet API provides a number of listener interfaces that one can implement in order to react to these events.

  • Event Listener interface's sub interfaces

    1. ServletRequestListener

       • Interface for receiving notification events about requests coming into and going out of scope of a web application.

    2. HttpSessionListener

       • Interface for receiving notification events about HttpSession lifecycle changes.

    3. ServletContextListener

       • Interface for receiving notification events about ServletContext lifecycle changes.

• Three ways of registering a web listener created by programmer.

  • The implementation class must be either declared in the deployment descriptor of the web application

  • Annotated with WebListener annotations,

  • Registered via one of the addListener methods defined on ServletContext.

Steps for creating a web listener

1. Create a class, implementing from Listener i/f.

2. Register it with WC through annotations or xml tags.

   • @WebListener annotation(class level).

   • XML tags in web.xml.

         <listener>
         <listener-class>F.Q cls name of listener</listener-class>
         </listener>