how to send messages to http client

[roberto]

I need to send a message (an html page.. or a simple alert) to each users connected to the system (also in a cluster env).
For example because the sys admin needs to comunicate that the system need to be offline from 10:00 AM to 3:00PM..
So i need to send this message to all live http session ..
There is a way to do it ?
The better solution is an web container independent way... of course..
Thank you

[ragavgomatam]

Try DWR (Direct Web Remoting) ...It is open source & container independent...It has some cool features like reverse Ajax, through which you can send messages to clients connected to your server