If a class or servlet implements SingleThreadModel interface it depends on the server which technology is implemented.

for server there r 2 options. .......1)Multiple Instances

2) Request serialization

if it(server) follows 1st choice it creates multiple instances for each servlet and then processes the incoming requests.

if it follows 2nd choice it creates single instacne for each servlet and then allows only one servlet to be get processed or handle requests. if more than one request came at once it queues up requests or let them(requests) wait until earlier requests processed.

with regards

santh