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Web Design and Development

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Web Design & Development ­ CS506
VU
Lesson 25
Web Application Development
Introduction
Because of the wide spread use of internet, web based applications are becoming vital part of IT
infrastructure of large organizations. For example web based employee performance management systems
are used by organizations for weekly or monthly reviews of employees. On the other hand online course
registration and examination systems can allow students to study while staying at their homes.
Web Applications
In general a web application is a piece of code running at the server which facilitates a remote user
connected to web server through HTTP protocol. HTTP protocol follows stateless Request-Response
communication model. Client (usually a web-browser) sends
Figure 1: A typical web application
A web server is software which provides users, access to the services that are present on the internet. These
servers can provide support for many protocols used over internet or intranet like HTTP, FTP, telnet etc
HTTP Basics
A protocol defines the method and way of communication between two parties. For example when we talk
to our teacher we use a certain way which is different from the way that we adopt with our fiends or
parents. Similarly there are many different protocols used by computers to communicate with each other
depending on applications.
Figure 2: HTTP communication model
-HTTP is as request-response oriented protocol.
-It is a stateless protocol since there is no built-in state management between successive requests. We
will discuss state later.
Parts of an HTTP request
-Request Method: It tells the server the type of action that a client wants to perform
-URI: Uniform Resource Indictor specifies the address of required document or resource
-Header Fields: Optional headers can be used by client to tell server extra
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Figure 3: HTTP request example
-Body: Contains data sent by client to the server
-Other request headers like FROM (email of the person responsible for request) and VIA (used by
gateways and proxies to show intermediate sites the request passes) can also be used.
Request Parameters
Request can also contain addition information in form of request parameters
1. In URL as query string e.g. http://www.gmail.com/register?name=ali&state=punjab
2. As part of request body (see Figure 3)
Parts of HTTP response
Result Code: A numeric status code and its description. -Header Fields: Servers
use these fields to tell client about server information like configurations and
software etc. -Body: Data sent by server as part of response that is finally seen
by the user.
Figure 4: HTTP response example
HTTP Response Codes
An HTTP Response code tell the client about the state of the response i.e. whether it's a valid
response or some error has occurred etc. HTTP Response codes fall into five general categories
 100-199
Codes in the 100s are informational, indicating that the client should respond with some
other action.
100: Continue with partial request.
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200-299
Values in the 200s signify that the request was successful.
200: Means every thing is fine.
300-399
Values in the 300s are used for files that have moved and usually include a Location header
indicating the new address.
300: Document requested can be found several places; they'll be listed in the returned
document.
400-499
Values in the 400s indicate an error by the client.
404: Indicates that the requested resource is not available.
401: Indicates that the request requires HTTP authentication.
403: Indicates that access to the requested resource has been denied.
500-599
Codes in the 500s signify an error by the server.
503: Indicates that the HTTP server is temporarily overloaded and unable to handle the
request.
404: indicates that the
requested resource is
not available
401: Indicates that
request requires HTTP
authentication
Server Side Programming
Web server pages can be either
static pages or dynamic pages.
A static web page is a simple
HTML (Hyper Text Transfer
Language) file. When a client
requests an HTML page the
server simple sends back response
with the required page.
Figure
5:
Static
web
page
request
and
response
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An example of static web page is given
below While in case of dynamic web
pages server executes an application
which generates HTML web pages
according to specific requests coming
from client. These dynamically generated
web pages are sent back to client with the
response.
Figure 6: Dynamic web page request and response
Why build Pages Dynamically?
We need to create dynamic web
pages when the content of site
changes frequently and client
specific response is required.
Some of the scenarios are listed
below
-The web page is based on
data submitted by the
user. e.g. results page
from search engines and
order confirmation pages
at on line stores. ]
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The Web page is derived from data that changes frequently. e.g. a weather report or news headlines page.
The Web page uses information from databases or other server-side resources.
e.g. an e-commerce site could use a servlet to build a Web page that lists the current price and
availability of each item that is for sale.
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-Using technologies for developing web pages that include dynamic content.
-Developing web based applications which can produce web pages that contain information that is
connection-dependent or time-dependent.
Dynamic Web Content Technologies Evolution
Dynamic web content development technologies have evolved through time in speed, security, ease of use
and complexity. Initially C based CGI programs were on the server Then template based technologies like
ASP and PHP were then introduced which allowed ease of use for designing complex web pages. Sun Java
introduced Servlets and JSP that provided more speed and security as well as better tools for web page
creation.
Figure 7: Dynamic web content technologies evolution
Layers and Web Application
Normally web applications are partitioned into logical layers. Each layer performs a specific functionality
which should not be mixed with other layers. Layers are isolated from each other to reduce coupling
between them but they provide interfaces to communicate with each other.
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Figure 8: Simplified view of a web application and its layers
-Presentation Layer:
Provides a user interface for client to interact with application. This is the only part of application
visible to client.
-Business Layer
The business or service layer implements the actual business logic or functionality of the application.
For example in case of online shopping systems this layer handles transaction management.
-Data Layer
This layer consists of objects that represent real-world business objects such as an Order,
OrderLineItem, Product, and so on.
There are several Java technologies available for web application development which includes Java
Servlets, JavaServer Pages, JavaServer Faces etc.
Figure 9: Java web application technologies (presentation/web tier)
References:
Java, A Practical Guide by Umair Javed
Java tutorial by Sun: http://java.sun.com/docs/books/tutorial/
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