What's the purpose of servlet listeners in java

Question

how many types of servletlisteners are there in java

Answer 1

Servlet Listener is used for listening to events in a web containers, such as when you create a session, or place an attribute in an session or if you passivate and activate in another container, to subscribe to these events you can configure listener in web.xml , for example HttpSessionListener 

Answer 2

Got to love Google :)