session synchronisation

What is session synchronisation?
How can we implement this feature in our application?
How useful is it in the web applications?
Is there any performance issues?

The only point in synchronizing access to a session is when you may have multiple requests from the same user (same sessionID) at one time.
This can happen more easily than you might think IF you have created a page that pulls lots of resources from servlets - for example with a frameset, or with servlet generated graphics, etc. where one request might modify the data used by others.
You would implement it just like any other synchronization.
Otherwise, each user has a unique session and there is no need to synchronize.
Synchronizing does of course have overhead, but it is small compared to all of the other delays in servlets, especially in Java 1.4 - thats the LAST place to look for performance increases.
Bill