The servlet may be initialized well before its first reference. The web.xml may specify that the servlet be loaded upon startup, for example. Also, the container is free to load the servlet at any time as long as it is before a request is delivered to it.
David & Bear, Thanks you both. Bear got it correct. Still I am not sure, why servlet has to made available ( ie. place in servlet directory and configure ) to the webserver , before the web server comes up.