Servlet in java , java servlet , servlet servlet and CGI, API

Submitted by
K.Priyadarsini
N.Pandimeena
V.Sarmila
IIM.SC(CS&IT)
Nadar saraswathi college of arts & science ,
Theni.

SERVLET:
 Servlets provide a component-based, platform-
independent method for building Web based applications,
without the performance limitations of CGI programs.
Servlets have access to the entire family of Java APIs,
including the JDBC API to access enterprise databases.
 Java Servlets are programs that run on a Web or
Application server and act as a middle layer between a
requests coming from a Web browser or other HTTP
client and databases or applications on the HTTP server.

Java Servlets often serve the same purpose as
programs implemented using the Common Gateway
Interface (CGI). But Servlets offer several advantages in
comparison with the CGI.
 Performance is significantly better.
 Servlets execute within the address space of a Web
server. It is not necessary to create a separate process to
handle each client request.
 Servlets are platform independent because they are
written in Java.
 Java security manager on the server enforces a set of
restrictions to protect the resources on a server machine.
So servlets are trusted.

Servlet interface between client and server:

CGI(Common Gateway Interface):
CGI technology enables the web server to call
an external program and pass HTTP request information to
the external program to process the request. For each
request, it starts a new process.

 Better performance:
Because it creates a thread for each request not
process.
 Portability:
Because it uses java language.
 Robust:
Servlets are managed by JVM so we don't need
to worry about memory leak, garbage collection etc.
 Secure:
Because it uses java language..

Website:
 Website is a collection of related web pages that may
contain text, images, audio and video. The first page of a
website is called home page. Each website has specific
internet address (URL) that you need to enter in your
browser to access a website.
 Website is hosted on one or more servers and can be
accessed by visiting its homepage using a computer
network. A website is managed by its owner that can be an
individual, company or an organization.

HTTP:
 The Hypertext Transfer Protocol (HTTP) is
application-level protocol for collaborative, distributed,
hypermedia information systems. It is the data
communication protocol used to establish communication
between client and server.
 HTTP is TCP/IP based communication protocol,
which is used to deliver the data like image files, query
results, HTML files etc on the World Wide Web (WWW)
with the default port is TCP 80. It provides the
standardized way for computers to communicate with each
other.

The Basic Architecture of HTTP:

Servlet API:
 The javax.servlet and javax.servlet.http packages
represent interfaces and classes for servlet API.
 The javax.servlet package contains many interfaces
and classes that are used by the servlet or web container.
These are not specific to any protocol.
 The javax.servlet.http package contains interfaces
and classes that are responsible for http requests only.

javax.servlet:
The javax.servlet package contains a number of
classes and interfaces that describe and define the contracts
between a servlet class and the runtime environment
provided for an instance of such a class by a conforming
servlet container.
javax.servlet.http:
The javax.servlet.http package contains a
number of classes and interfaces that describe and define the
contracts between a servlet class running under the HTTP
protocol and the runtime environment provided for an
instance of such a class by a conforming servlet container.

Classes in javax.servlet package:
Generic Servlet
Servlet Input Stream
Servlet Output Stream
Servlet Request Wrapper
Servlet Response Wrapper
Servlet Request Event
Servlet Context Event
Servlet Request Attribute Event
Servlet Context Attribute Event
Servlet Exception
Unavailable Exception

Classes in javax.servlet.http package:
Http Servlet
Cookie
Http Servlet Request Wrapper
Http Servlet Response Wrapper
Http Session Event
Http Session Binding Event
Http Utils (deprecated now)

Http Servlet class:
 The Http Servlet class extends the Generic Servlet
class and implements Serializable interface. It provides
http specific methods such as doGet , doPost , doHead ,
doTrace etc.

Life Cycle of a Servlet (Servlet Life Cycle):
 The web container maintains the life cycle of a servlet
instance. Let's see the life cycle of the servlet:
Servlet class is loaded.
Servlet instance is created.
init method is invoked.
service method is invoked.
destroy method is invoked.

Diagram of servlet life cycle:

 As displayed in the above diagram, there are three
states of a servlet:
New
Ready
End
 The servlet is in new state if servlet instance is
created. After invoking the init() method, Servlet comes in
the ready state. In the ready state, servlet performs all the
tasks. When the web container invokes the destroy()
method, it shifts to the end state.

Written inside the public service method:
The public service method converts the Servlet
Request object into the Http Servlet Request type and Servlet
Response object into the Http Servlet Response type.
Then, calls the service method passing these
objects.

PROGRAM:
public void service(Servlet Request req , Servlet Response res)
throws Servlet Exception, IOException
{
Http Servlet Request request;
Http Servlet Response response;
try
{
request = (Http Servlet Request)req;
response = (Http Servlet Response)res;
}
catch(Class Cast Exception e)
{
throw new Servlet Exception("non HTTP request or response");
}
service(request, response);
}

Written inside the protected service method:
The protected service method checks the type
of request, if request type is get, it calls doGet method, if
request type is post, it calls doPost method, so on.
Let's see the internal code:

PROGRAM:
protected void service(HttpServletRequest req, HttpServlet
Response resp)
throws ServletException, IOException
{
String method = req . getMethod();
if(method . equals("GET"))
{
long lastModified = getLastModified(req);
if(lastModified == -1L)
{
}
....
}
}

Servlet in java , java servlet , servlet servlet and CGI, API

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