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.
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
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.
