in struts applications all the requests will be handled by 
the ActionServlet for that struts-config.xml is passed as a 
init-param in web.xml

"org.apache.action.ActionServlet" is responsible for 
handling all HTTP Requests. ActionServlet is responsible 
for configuring your application using struts configuration 
file.
     After that it calls RequestProcessor which is 
responsible for processing content type,mapping,forward and 
many other methods.Its duty is to process request i.e fetch 
appropiate action from Struts configuration file as per the 
request and assigning bean to appropiate ActinForm.
     One can customize the way request are handled in 
Struts by customizing RequestProcessor Class.
And finally our code begins ..... and controller and model 
come into play to give resulting view.