Listener Thread Question

[Tom Scott]

Hi,

Are there any threading issues with Listener class instance variables? Specifically if I try to implement a Listener something like this...

package com.test;
import javax.servlet.http.*;
 

public class MyListener implements HttpSessionListener {
   private int sessionCount = 0;
   
   public void sessionCreated(HttpSessionEvent e) {
      System.out.println("A new session has been created : " + ++sessionCount);
      
   }
 
   public void sessionDestroyed(HttpSessionEvent e) {
      System.out.println("A session has been destroyed : " + --sessionCount);
   }
}
 
Could the container attempt to run these methods in seperate parallel threads and somehow corrupt the sessionCount?
 
Many thanks for any clarification...
Tom

[Mark Garland]

I really don't know - sounds a good question.

Would be interested to know the answer for filters too.

[Ulf Dittmer]

There's only one SessionListener object which gets called for all sessions. So, yes, since sessions can be created in parallel threads, the listener object can be called from parallel threads, and it needs to be programmed in a thread-safe manner.

[Tom Scott]

Hi Ulf,

Many thanks for the reply. I suspect my question isn't too relevent to the exam since I've not had many answers but I would appreciate any further confirmation or maybe a suggestion of how such functionality would usually be achieved. I wondered about synchronizing on the Listener class object...but then I wondered if this would affect performance and if maybe I should create an object specificaly to guard updates to the count...

Thanks in advance..

-Tom

[Tom Scott]

Hmm,

And then I thought all calls to the Listener affect the count. So either we synchronize or we don't - but I'm unclear if we need to or not!

-Tom

[Ulf Dittmer]

Well, as I said, the code needs to be written in a multithread-safe way. That means that operations like "++sessionCount" need to be synchronized to make them atomic. Using the object itself for synchronizing should be no big performance problem, unless you have other -more time-consuming- blocks of code that also need protection.

[Dan Polak]

In my opinion only you need is to synchronize these two methods. Thats all.

[Ulf Dittmer]

Originally posted by Dan Polak:
In my opinion only you need is to synchronize these two methods.

That is sufficient, but not necessary. The real class will have more elaborate method bodies, and synchronizing the complete method may become a performance bottleneck (if there are many session being created or removed), since only access to the sessionCount field needs to be synchronized.