Root URl of the servlet

I want to get the root url of my web application from one of the servlet.

If I deploy my application in "www.mydomain.com" I want to get the root url like "http://www.mydomain.com".

Same thing if I deploy it in local tomcat server with 8080 port it should give http://localhost:8080/myapp

Can anyone tell me how to get the root URL of my web application from servlet?

public class MyServlet extends HttpServlet {

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        String rootURL="";
        //Code to get the URL where this servlet is deployed

    }
}