Service method in called by the servlet container to process a request from the browser.
When user calls a servlet, it's service method is executed by servlet container to process the user request.
So, service method is called by Servlet container. You can't call it yourself.
Can you explain me your question in detail?
You can read more about servlet at http://www.roseindia.net/servlets/LifeCycleOfServlet.shtml