Thread Safety question

What mechanism determines a servlet's thread safety? Is it the container or the implementation of the servlet class?

Thanks again!

An implementation of servlet classes.

If you're not sure how to create a thread safe servlet, follow this basic rule of thumb:
Don't create instance variables.

You could still run into concurrency issues if your servlets or JSPs try to access a session or context scope object at the same time but instance variables are the leading cause of threading problems in servlet applications.