java. Unique identifier for each visitor. How better generated?

I need to generate a unique identifier (uniqueID) for each visitor my site. Which algorithm is better use for this? Have any ideas?

Update: small condition: do not u开发者_StackOverflow社区se Session Id.

The easiest would be to store a UUID in the Session.

How about just using the web container's Session ID?

See HttpSession.getId()

I usually use microsec time, but if two users somehow get on at the exact same time i add a random number to the end of it. (still a possibility for repeats, but very unlikely)