Servlet
A servlet is a small Java program that runs
within a Web server. Servlets receive and respond to requests from Web clients,
usually across HTTP, the HyperText Transfer Protocol.
What is
web application?
A web application is an
application accessible from the web. A web application is composed of web
components like Servlet, JSP, Filter etc. and other components such as HTML.
The web components typically execute in Web Server and respond to HTTP request.
CGI(Commmon
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.
Disadvantages
of CGI
There are many problems in CGI
technology:
- If
number of clients increases, it takes more time for sending response.
- For
each request, it starts a process and Web server is limited to start
processes.
- It
uses platform dependent language e.g. C, C++, perl.
Advantage
of Servlet
There are many advantages of
Servlet over CGI. The web container creates threads for handling the multiple
requests to the servlet. Threads have a lot of benefits over the Processes such
as they share a common memory area, lighweight, cost of communication between
the threads are low. The basic benefits of servlet are as follows:
- 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 no need
to worry about momory leak, garbage collection etc.
- Secure: because it uses java language..
Servlet Terminology
There are some key points that
must be known by the servlet programmer like server, container, get request,
post request etc. Let's first discuss these points before starting the servlet
technology.
The basic terminology used in servlet are given below:
- HTTP
- HTTP
Request Types
- Difference
between Get and Post method
- Container
- Server
and Difference between web server and application server
- Content
Type
- Introduction
of XML
- Deployment
HTTP
(Hyper Text Transfer Protocol)
- Http
is the protocol that allows web servers and browsers to exchange data over
the web.
- It
is a request response protocol.
- Http
uses reliable TCP connections bydefault on TCP port 80.
- It is stateless means each request is considered as the new request. In other words, server doesn't recognize the user bydefault.
Http
Request Methods
Every request has a header that
tells the status of the client. There are many request methods. Get and Post
requests are mostly used.
The http request methods are:
- GET
- POST
- HEAD
- PUT
- DELETE
- OPTIONS
- TRACE
|
HTTP
Request
|
Description
|
|
GET
|
Asks
to get the resource at the requested URL.
|
|
POST
|
Asks
the server to accept the body info attached. It is like GET request with
extra info sent with the request.
|
|
HEAD
|
Asks
for only the header part of whatever a GET would return. Just like GET but
with no body.
|
|
TRACE
|
Asks
for the loopback of the request message, for testing or troubleshooting.
|
|
PUT
|
Says
to put the enclosed info (the body) at the requested URL.
|
|
DELETE
|
Says
to delete the resource at the requested URL.
|
|
OPTIONS
|
Asks
for a list of the HTTP methods to which the thing at the request URL can
respond
|
What is
the difference between Get and Post?
There are many differences
between the Get and Post request. Let's see these differences:
|
GET
|
POST
|
|
1) In
case of Get request, only limited
amount of datacan be sent because data is sent in header.
|
In
case of post request, large
amount of data can be sent
because data is sent in body.
|
|
2)
Get request is not secured because data is exposed in URL bar.
|
Post
request is secured because data is not exposed in URL
bar.
|
|
3)
Get request can be
bookmarked
|
Post
request cannot be bookmarked
|
|
4)
Get request is idempotent.
It means second request will be ignored until response of first request is
delivered.
|
Post
request is non-idempotent
|
|
5)
Get request is more
efficient and used more
than Post
|
Post
request is less efficient and used less than get.
|
Anatomy
of Get Request
As we know that data
is sent in request header in case of get request. It is the default request
type. Let's see what informations are sent to the server.
Anatomy
of Post Request
As we know, in case
of post request original data is sent in message body. Let's see how
informations are passed to the server in case of post
Container
It provides runtime environment
for JavaEE (j2ee) applications.
It performs many operations
that are given below:
- Life
Cycle Management
- Multithreaded
support
- Object
Pooling
- Security
etc.
Server
It is a running program or
software that provides services.
There are two types of servers:
- Web
Server
- Application
Server
Web Server
Web server contains only web or
servlet container. It can be used for servlet, jsp, struts, jsf etc. It can't
be used for EJB.
Example of Web Servers are: Apache Tomcat and Resin.
Application Server
Application server contains Web
and EJB containers. It can be used for servlet, jsp, struts, jsf, ejb etc.
Example of Application Servers
are:
- JBoss Open-source server from JBoss
community.
- Glassfish provided by Sun Microsystem. Now
acquired by Oracle.
- Weblogic provided by Oracle. It more secured.
- Websphere provided by IBM.
Content
Type
Content Type is also known as
MIME (Multipurpose internet Mail Extension) Type. It is a HTTP header that provides the description about
what are you sending to the browser.
There are many content types:
- text/html
- text/plain
- application/msword
- application/vnd.ms-excel
- application/jar
- application/pdf
- application/octet-stream
- application/x-zip
- images/jpeg
- video/quicktime
etc.
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