Please take the time to CarefullyChooseOneForum. This one is about threads and synchronization, which has little to do with servlet session tracking. I'll move it to a more appropriate forum.
Also, please also give your posts meaningful titles - something like "servlet session tracking" would have been much more indicative of what it's about. You can edit your post (including the title) by clicking the little paper-and-pencil icon.
Millions saw the apple fall, but Newton asked why.
If you understand, say "understand". If you don't understand, say "don't understand". But if you understand and say "don't understand". How do I understand that you understand? Understand!