Web app Architecture

Container:

Container helps servlets.

It instantiate the servlet or make a thread to handle the request

It calls the servlet’s doGet() or doPost() method

It create HTTP request and response object to the servlet.

It manages the life cycle of servlet.

It finds the URL in DD to find the correct servlet for the request.

Delete the request and response object once thread completes

It calls the service method.

Tomcat is the example of Container.

Container provides

Communication support:

It build serversocket,listen on port,create stream etc.

It knows the protocol between itself and webserver.

b.    Lifecycle Management:

Control the life and death of servlet

Loading class, instantiate and intialize servlet.

Making servlet instance eligible for garbage collection.

c.    Multithreading support

It creates new thread for each and every request it receives

d.    Declarative Security

e.    JSP Support

Steps how container handles the request
1.    get the HTTP request from the client
2.    create HTTPServlet request and response.
3.    Finds the correct servelt based on the URL and creates thread for each request.
4.    Container calls the servlet’s Service() method, Depending upon type of request service method calls doGet() or doPost() method.
5.    doGet() method generate the dynamic page and stuff in response object.
6.    The thread completes the container converts response object to HTTP response and send to client and then delete the request and response object which it creates (in step 2).

Webserver do :
It gets the response from the container and converts the response object to an HTTP response.
It uses HTTP to talk to the client browser.
It knows how to forward the dynamic content to the container.

Servlet do:
It add the html to the response object
It has a name that matches the <Servlet-class> element in the DD.

Application server:
It will have/acts as web container and EJB container. J2EE, Weblogic, JBOSS,Websphere

Webserver:
Handles only HTTP request/response. Apache

Container:
It handles both HTTP and servlets and JSP. Tomcat.

How to pass the control to jsp from servlet:
a.    RequestDispatcher
b.    SendRedirect
Examples:
a.    RequestDispatcher view = request.getRequestDispatcher(“result.jsp”);
View.forward(request, response);

b.    response.sendRedirect(“www.google.com”); (it should have string as parameter).