This is a copy of my answer on stackoverflow.
When a JSP is requested for the first time or when the webapp starts up, the servlet container will compile it into a class extending
HttpServlet and use it during the webapp's lifetime. You can find the generated source code in the server's work directory. In for example Tomcat, it's the
Servlet is an Java application programming interface (API) running on the server machine which can intercept on the requests made by the client and can generate/send a response accordingly. A well known example is the
HttpServlet which provides methods to hook on HTTP requests using the popular HTTP methods such as
POST. You can configure
HttpServlets to listen on a certain HTTP URL pattern, which is configureable in
web.xml, or more recently with Java EE 6, with
When a Servlet is requested for the first time or when the webapp starts up, the servlet container will create an instance of it and keep it in memory during webapp's lifetime. The same instance will be reused for every incoming request whose URL matches the servlet's URL pattern. You can access the request data by
HttpServletRequest and handle the response by
HttpServletResponse. Both objects are available as method arguments inside any of the overridden methods of
HttpServlet, such as
JSF is a component based MVC framework which is built on top of the Servlet API and provides components in flavor of taglibs which can be used in JSP or any other Java based view technology such as Facelets. Facelets is much more suited to JSF than JSP. It namely provides great templating capabilities such as composite components, while JSP basically only offers the
<jsp:include> for templating, so that you're forced to create custom components with raw Java code (which is a bit opaque and a lot of tedious work in JSF) when you want to replace a repeated group of components by a single component. If you can, I recommend to drop JSP and go for Facelets when you want to develop with JSF.
As being a MVC (Model-View-Controller) framework, JSF provides the
FacesServlet as the sole request-response Controller. It takes all the standard and tedious HTTP request/response work from your hands, such as gathering user input, validating/converting them, putting them in model objects, invoking actions and rendering the response. This way you end up with basically a JSP or Facelets (XHTML) page for View and a Javabean class as Model. The JSF components are been used to bind the view with the model (such as your ASP.NET web control does) and the
FacesServlet uses the JSF component tree to do all the work.