In most cases, it's the other way round : a servlet calls a JSP.
The MVC (Model View Controller) model 2 states that the Controller should be a servlet where the view (or presentation) should be dealt with by a JSP.
How does a servlet call a JSP ?
Well, first of all,
you should know that Tomcat does the first calling. If you have a
Java web application, this application should have a web.xml file. In this web.xml file you should put the name of the servlet and the URL to which it responds.
ANd then the application gets on its way : Tomcat calls the servlet and the servlet calls its first JSP. And this could well be a login.jsp.
The communication between the servlet and JSP is done by Java Beans.
When you ask a user to log in, Tomcat calls the servlet, the servlet calls the login.jsp. When the user submits his data (username and pasword) via a submit button, the data he/she has filled in in the FORM is transmitted to the servlet. And with this data, the servlet can decide whether the user can be granted access.