The difference between HttpSessionAttributeListener and HttpSessionBindingListener

[Maria Tan]

Hello:
I am confusing about these two listeners. I am just wondering if you may help me.
Thanks in advance
Maria

[Kyle Tang]

HttpSessionBindingListener is implemented by session attributes.
HttpSessionAttributeListener instance is created by Web-Container, based on <listern> element in web.xml. For each <listern-class>, one listern instance is created.
Now you can see, if your session has 3 attributes, each of them implements HttpSessionBindingListener, you got 3 ***Binding listeners.
But if you only define one <listener-class> in web.xml, you only get one ***Attribute listener.

another important difference, HttpSessionAttributeListener monitors ALL Sessions in the web-app, whenver any session has an attribute added/removed/replace, the listener got notified.
HttpSessionBingListener only cares about itself. If itself is added into or removed from a session, it is notified.

[Maria Tan]

Thanks a lot. It's clear.
Maria

[Anthony Villanueva]

Some examples

[Thambi Rajah]

I have a question regarding HttpSessionBindingListener and HttpSessionAttributeListener.
Say the definition of classes MyBindingListener , TestServlet and snippet of web.xml are as follows:

  
public class MyBindingListener implements HttpSessionBindingListener 
{
public void valueBound(HttpSessionBindingEvent arg0) 
{
 //Some code
}
public void valueUnbound(HttpSessionBindingEvent arg0) 
{
//Some code
}
}
----------------------
public class TestServlet extends HttpServlet 
{
public void doGet(HttpServletRequest request,HttpServletResponse response)
{
HttpSession session = request.getSession(true); 
MyBindingListener countObject = new MyBindingListener();
session.setAttribute("countObject", countObject);
                //Some code
}
}
----------------------

<web-app>
<listener>
<listener-class> MySessionAttributeListener<listener-class>
<listener>
</web-app>
----------------------

where class MySessionAttributeListener implements HttpSessionAttributeListener interface.
My qustion is:
When we access the TestServlet from a browser, which listener classes will be notified by
session.setAttribute("countObject", countObject) statement?
(A) MySessionAttributeListener
(B) MyBindingListener
(C) Both listeners(MySessionAttributeListener and MyBindingListener)
Thanks in advance
Thambi

[Kyle Tang]

both

[Daniel Roach]

Originally posted by Kyle Tang:
both

Kylie
why is it both and not just the class specified in web.xml?
cheers
Dan

[Daniel Roach]

oops kyle, sorry about misspelling your name
why is it both and not just the class specified in web.xml?
cheers

[Kyle Tang]

that is by definition of these 2 listeners. there is no reason why only one of them get notified, and the other get blocked. your web-container will follow what the spec says. if the attribute that is added to a session implements HttpSessionBindingListener, it will get notified. In the mean time, if there is a HttpSessionAttributeListener defined in web.xml, that listener instance will also get notified.
please refer to my earlier reply to the original post.