There is only one
servlet instance per registered servlet. Normally, when concurrent requests are handled by the web server, threads are spawned which executes the service() method of the servlet. Instance variables are not
thread safe, but local variables within the service() method are. The request object is also "localized" per thread.
According to the Servlet specs, when a servlet implements the SingleThreadModel interface (which is an empty "tag" interface) and concurrent requests are handled by the web server, the container is supposed to create a new servlet instance per request. It is possible that the container will pool these servlet instances. it is also possible that the container will serialize, i.e. queue, the requests per instance. It is possible that the container implementation is a combination of these. The upshot of all this is that service() is now thread-safe.