Difference between Filter and Listener in Servlet (Java EE)

Question

There are Filters and Listeners functionality in Servlet. I want to know exact difference between Filter and Listener.

Answer 1

Filters are used for pre and post process requests. Look at the javax.servlet.Filter in your tomcat/jboss/other container javadoc.

Where as the listeners are like triggers that can be attached to events in your app server (let's use the term container here). With listeners you can track application-level, session-level, life-cycle changes, attribute changes etc. The implemented interfaces are javax.servlet.Listener interface.

Based on @fnt 's responses below, let me try to clarify some more. Listeners are targeted for lifecycle changes, without having to have a client request coming in. So for one client request, there could be many more lifecycle events may happen before the request is disposed of. Example: You want to log all the sessions that timeout. Please note that SesionTimeout is a lifecycle event, which can happen without having the user to do anything. For such a scenario, a listener will be appropriate.

To the question of logging when a request arrives. There is no direct mapping of a new request to an equivalent listener (read lifecycle event) event. And hence for each incoming request if you want to log something, Filter in my opinion is the right thing to use.

This material from Oracle should be able to clarify some more Filters and Listeners

HTH