Servlet container is nothing but the environment which manages the life-cycle(creating-initializatin-destuction) of your servlet. It also route the requests to appropriate servlet for procession as per servlet mapping in web.xml.
what is the middleware tech that are userd to execute the web applications.
I think you have misunderstood the concept of middleware technology. Middleware technology does not used to execute web application but it is used to connect discreate application components.