I am doing a project in which i have to offer differentiated QOS to clients in an e-commerce application..
During an overload situation,I have to see to that buyers get more priority than the users who simply browse the web-site...I am using jsp,tomcat server,linux...Someone please give me idea how i have to assign more priority to the buy page.....How to consider that particular url and assign priority to that..How should my code be.....Please help me...

you can't.
One should never rely on thread priorities to do anything.

Just make sure you have enough bandwidth and CPU power to keep the site responsive during expected peak loads.
And that's a hardware and server configuration issue.