Servlets are Java platform-independent server-side components that extend the functionality of web servers. Servlets receive HTTP requests and return HTTP responses. The servlet container manages the lifecycle of servlets by calling init(), service(), and destroy() methods. The servlet API includes GenericServlet, HttpServlet, ServletConfig, and ServletContext classes that provide functionality for handling HTTP requests and responses and accessing configuration and context information.

1. SERVLETS

2. Servlet
• Platform- independent Server side
components , written in Java which
extends the standard web server and
provide a general framework for services
build on the Request –Response paradigm
• An Efficient and Powerful technology for
creating dynamic content for the Web and
fundamental part of all Java web
technologies

3. Servlet Container
The servlet container
is a part of a Web server or
application server that
provides the network
services over which
requests and responses
are sent, decodes MIME-
based requests, and formats
MIME-based responses. A
servlet container also
manages servlets through
their lifecycle.

6. Servlet API

7. javax.servlet.GenericServlet
• Signature: public abstract class GenericServlet extends
java.lang.Object implements Servlet, ServletConfig,
java.io.Serializable
• GenericServlet defines a generic, protocol-independent
servlet.
• GenericServlet gives a blueprint and makes writing
servlet easier.
• GenericServlet implements the log method, declared in
the ServletContext interface.
• To write a generic servlet, it is sufficient to override the
abstract service method.

8. javax.servlet.http.HttpServlet
• Signature: public abstract class HttpServlet extends
GenericServlet implements java.io.Serializable
• HttpServlet defines a HTTP protocol specific servlet.
• HttpServlet gives a blueprint for Http servlet and makes
writing them easier.
• HttpServlet extends the GenericServlet and hence
inherits the properties GenericServlet.

9. Servlet Life Cycle
The Web container manages the life cycle of servlet instances
New Destroyed
Running
init( destroy(
) )
...(
service(
)
)
doGet(
) doDelete(
)
doPost( doPut(
) )

10. The init ( ) method
• Called by the Web container when the servlet instance is first created
• The Servlets specification guarantees that no requests will be processed by
this servlet until the init method has completed
• Override the init() method when:
i) You need to create or open any servlet-specific resources that you need
for processing user requests
ii)You need to initialize the state of the servlet
//A simple servlet lifecycle counter
// int count ;
public void init(ServletConfig config) throws ServletException
{
super.init(config);
getServletContext().log("init() called");
count=0;
}

11. The service ( ) method
• Called by the Web container to process a user request
• Dispatches the HTTP requests to doGet(…),
doPost(…), etc. depending on the HTTP request
method (GET, POST, and so on)
– Sends the result as HTTP response
• Usually we do not need to override this method
protected void service(HttpServletRequest request, HttpServletResponse response) throws
ServletException, IOException
{
getServletContext().log("service() called");
count++;
response.getWriter().write("Incrementig the count: Count = "+count);
}

12. The destroy() Method
• Called by the Web container when the servlet instance is being eliminated ( All
threads within the servlets service method have exited (or) Time-out period have
passed )
• The Servlet specification guarantees that all requests will be completely processed
before this method is called
• Override the destroy method when:
– You need to release any servlet-specific resources that you had opened in the
init() method
– You need to persist the state of the servlet
public void destroy() {
getServletContext().log("destroy() called");
}

13. servletConfig
• Object of servletConfig used to pass initialization and
context information to servlets
• getServletConfig ( ) method allows the servlet to retrieve
the object and obtain configuration at anytime
• Init () method stores the servletConfig object in a private
transient instance variable called config

14. ServletContext
• The javax.servlet.ServletContext interface provides a set of methods
that the servlet can use to communicate with the web server.
• The ServletContext object is contained within
javax.servlet.ServletConfig Object which is provided to the servlet
when it is initialized.
• Define URI to name mappings
• Allow Data to be shared between different servlets
• Contains a servlet log
• Access to global servlet config and other application servlet context
are accessible
• It communicates with the server in a non request specific manner

15. Http Request and Response
Structure
• HTTP is a stateless protocol.
• Server does not have the overhead of tracking client
connections.
• HTTP transactions are either a request or response.
• Servlet can overcome the stateless nature of HTTP by
tracking client state using session information stored in
URL, hidden fields or cookies.

16. Http Transactions
A single request or response line
– <HTTP Method>/<document address>HTTP/
<Version No> e.g.
– GET /index.html HTTP/1.1
– Response line contains an HTTP status code.
HTTP/1.1 200 OK
Date:……………………….GMT
Server:
Last-modified:
Content-type: text/html
Content-length:….bytes

17. Http Servlet Methods
• doGet()
• doPost()
• doHead()
• doDelete()
• doOptions()
• doPut()
• doTrace()
• getLastModified()
• service()

18. Http Servlet Request
• getMethod()
– Returns get/post with which request was made.
• getQueryString()
• getRemoteUser()
• getRequestSessionId()
• getSession(boolean)
– If FALSE returns current valid session otherwise
creates a new session.
• isRequestedSessionIdValid()
• getCookies()

19. Http Servlet Response
• addCookie(Cookie)
– Add cookie to response
• encodeUrl(String)
• sendRedirect(String)
• sendError(int)

20. Thread Safety
• Multithreaded = one servlet, multiple
requests simultaneously
• Thread safe – not using class variables
since one copy of these variables is
shared by all threads
• Synchronized code blocks , all threads
wait until they can enter, one at a time
• Servlet 2.4 deprecates SingleThreadModel
interface – could not resolve all potential
threading issues.