Servlets by Kamalakar Dandu

8/2/2019 Servlets by Kamalakar Dandu

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Objectives

Understand the Servlet Architecture.

Compare Servlet & Other Similartechnologies.

Understand the Servlet Life Cycle. Handle Request & Response.

Generate Dynamic HTML.

Handle Sessions.


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Common Gateway Interface (CGI)

ColdFusion (Allaire)

Active Server Pages (ASP)

Server-Side JavaScript (SSJS) Personal Home Page tools (PHP)

Java Servlets

JavaServer Pages

Evolution of Dynamic Content Tech.


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Main Process

CGI-Based Web Server

Request for CGI 1

Request for CGI 2

Request for CGI 3

Child Process for CGI 1



The CGI Life Cycle


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Java Servlet A Java program that

extends the functionality of a Web server, generating dynamic

content and interacting with Web clients using a request

response paradigm.

An extensible Web technology that uses template data,

custom elements, scripting languages, and server-side

Java objects to return dynamic content to a client.

Typically the template data is HTML or XML elements.

The client is often a Web browser.

J2EE 1.2 Architecture


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Advantages of Servlets

Java objects which extend thefunctionality of a HTTP server(a big plus). Dynamic customization of content Platform and server independent No CGI limitations

better performance Easier to write than CGI code

no need to parse headers and get info(all that is done by the HttpServlet class)

No Networking restrictions (like applets) Do not create a new process (or Servlet for

every request) Servlets can persist over time


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Server Extensions

Enhance or Change the base functionality

of the server, allowing the server to handle tasks

that were once relegated to external CGI

programs.


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The Power of Servlets

Portability

Power

Efficiency and Endurance

Safety

Elegance

Integration

Extensibility and Flexibility


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The Servlet Life Cycle A web server

communicates with

a Servlet through asimple interface,

javax.servlet.Servlet. This interface

consists of threemain methods:

init()

service()

destroy()and two ancillary

methods:

getServletConfig()

getServletInfo()
http://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.htmlhttp://java.sun.com/products/servlet/2.1/api/javax.servlet.Servlet.html


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Servlet Life Cycle (contd)

Request for Servlet1



Main Process

JVM

Servlet1

Servlet2

Thread

Thread

Thread

Java Servlet-based Web Server

Servlet initialized once; reused until destroyed andgarbage collected.


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Life Cycle Of A Servlet (contd)

A servlet is constructed and initialized. It then services zeroor more requests until the service that it extends shuts down

Initialized only once, stays resident in memory while

servicing requests

Multiple threads of execute service(), one for each client

connection

The servlet interface defines the life cycle methods :

init()

service()

destroy()


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init() Method

Called by server immediately after servlet isinstantiated

Called only once Servlet creates and initializes the resources

needed for handling requests

public void init(ServletConfig config) throwsServletException;

ServletException - thrown when servlet cannot

initialize necessary resources


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service() Method

It is the heart of the servlet.

It handles all requests sent by a client.

Each request message from a client results in a single

call to the servlet's service() method. It reads the request and produces the response message

public void service(ServletRequest req,

ServletResponse res) throws ServletException,IOException;


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destroy() Method

This signifies the end of a servlets life.

Its also a good place to save any persistent information

that will be used the next time the servlet is loaded.

It is called to allow the servlet to clean up any

resources.

Public void destroy();


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service()

destroy()

init()

Servlet loaded on demand (first time

user accesses the servlet). Once loaded theservlet is in memory

Unloaded on closing server;

manual unloading

Client requestsarrive

each client request creates a

new thread with service


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Servlet Interface

Servlets Classes / Interfaces are available in packages

(in java servlet development kit)

javax.servlet.http

javax.servlet


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Servlet Interface (contd)


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KEY : implemented by subclass

Server GenericServletsubclass

service()request

response

A generic servlet handling a request

GenericServlet Class


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HEAD requestresponse

KEY : implemented by subclass

Web ServerHttpServletsubclass

service()

GET requestresponse

A Http Servlet Handling a Request

POST requestresponse

doGet()

doPost()

doHead()

HTTP request

Implementation of doGet() doPost() is toreturn an error to the calling client ifservlet does not override these methods.


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Generic servlet vs httpservlet

init

destroy

service

HttpServlet

doGet(...)

doPost(...)

extends

Generic Servlet

doOptions()

doDelete()

doTrace()doPut()


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What does the Server Receive

server client

http://www.mcp.com/index.html

GET /index.html HTTP/1.0

Searching for www.mcp.comFound www.mcp.com

Server receives only oneof the HTTP requestmethods (Get/Post)

Servlet programmers must provideimplementations for these requests

Web server recieves


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Web Server

HttpServletsubclass

service()

doGet()

doPost()

doHead()

Web server recievesGET /servlet/myHelloServlet

Web server uses logic:

myHelloServlet is loaded. If not , then load the servlet.

call service method (already implemented in HttpServlet)(it checks the request type and correspondingly calls the

doGet() or doPost() method )

GET/servlet/

myHelloServlet

HTTP/1.0

Check request typeit is GET

hence call doGet()


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javax.servlet

ServletInputStream

ServletOutputStream

GenericServlet

InputStream

OutputStream

java.io

java.lang

Serializable

SingleThreadModel

ServletResponse

ServletRequest

ServletContext

ServletConfig

Servlet

InterfaceABSTRACT CLASSKEY

Object

Class

ServletExceptionException UnavailableException

extends implements


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ServletInputStream ServletOutputStream

GenericServlet

ServletResponseServletRequest

Serializable

ServletConfig

Servlet

InterfaceABSTRACT CLASSClass implements

service(req, res)javax.servlet.*


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ServletContext / ServletConfig

ServletConfig used to get initialization parameters

getServletContext()

String getInitParameter(String)

Enumeration getInitParameterNames()

ServletContext useful for logging & finding out about the

other servlets

Servlet getServlet(String)

Enumeration getServlets()

Enumeration getServletNames() void log(String)

void log(Exception, String)


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GenericServlet Serializable

InterfaceABSTRACT CLASSClass implements

HTTPServlet

extends

javax.servlet.http.*


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Servlet Architecture

HTTPServlet

Servlet

Interface class extends

Methods formanaging a servlet and

communicating with

clients


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Servlet

GET request

response

ServletRequest

ServletResponse

Encapsulates communicationfrom client to server

Encapsulates communicationfrom server to client


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package codecamp;

import javax.servlet.*;

import javax.servlet.http.*;

import java.io.*;Public class ServletGet extends HttpServlet {

public void doGet(HttpServletRequest request,

HttpServletResponse

response) throws ServletException,

IOException {

response.setContentType("text/html");

PrintWriter out = response.getWriter();

out.println("FirstServlet");

out.println("Hello Code Camp!

");

}

}

A Simple Code Example


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Example

Write the following html file: mydisplay.html calling servlets

Click on the button call servletGet and note that theearlier servlet is called.

Eg: mydisplay.htmlNOTE:User clicked on button, and the browser thengenerated the GET method based on action settings.

Generating GET by clicking a


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Generating GET by clicking a

button on browser

server client

http://localhost//mydisplay.html

GET /mydisplay.html HTTP/1.0

Searching for localhostFound localhost

call servletGet

GET /servlet/servletGet HTTP/1.0


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Example

Modify only the following line in html file:mydisplay.html

Click on the button call servletGet and note whathappens.

NOTE:User clicked on button, and the browser then

generated the POST method based on action settings.

NOTE: the servlet servletGet did not contain a doPost

method and hence got the error. Rename doGet() to

doPost() and it would then work.

T f i i f f b t S l t


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Transferring info from browser to Servlets

Generally in web pages, users fill up a formwith fields.

Browser communicates with server (servlets)through GET and POST methods

GET requests : Getting information Appended to the URL in a query string ( limited amount of data)

.../servlet/ViewCourse?name=servlets_codecamp Can be bookmarked and emailed

POST requests: Sending information Send any amount of data .../servlet/PlaceOrderProcess Can Not be bookmarked and emailed. Use POST for things like placing an order.


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Common Requests: GET and POST

GET Getting information

Appended to the URL in a query string ( limitedamount of data)

.../servlet/ViewCourse?name=servlets_codecamp Can be bookmarked and emailed

POST requests:

Sending information

Send any amount of data

.../servlet/PlaceOrderProcess

Can Not be bookmarked and emailed.

Use POST for things like placing an order.


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Example

Write a html with three text fields for name, id and location,

with a submit button. Set the form method as GET, and

action as servlet/servletGetParam

Eg: servlet_get_HTML.html

Write a servlet with doGet() method. In the doGet() method

read the parameters and then send it back to the browser for

display

Eg: servletGetParam.java


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Example

Write a html with three text fields for name, id and location,with a submit button. Set the form method as POST, and

action as servlet/servletGetParam

Eg: servlet_post_HTML.html

Write a servlet with doPost() method. In the doPost() method

read the parameters and then send it back to the browser fordisplay

Eg: servletPost.java


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Interface ServletRequest

information about the client name of the host and the IP address of client

parameters sent from the client browser

scheme used by the browser to communicate

to servlet (eg. http; https; ftp;..) protocol used by client (eg. ftp)

access to the ServletInputStream (whichallows servlet to read binary data from the

client ) information about server

name of the server

port used by the server


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ServletRequest

Subclass ServletRequest to provide protocol specificinformation

Example: HTTPServletRequest (extends ServletRequest)supports methods to retrieve more protocol specificheaders (getMethod() to identify the type of request)

j l t S l tR t


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javax.servlet.ServletRequest

ServletRequestString getContentType()ServletInputStream getInputStream()String getParameter(String)Enumeration getParameterNames()

String getProtocol()BufferedReader getReader()String getRemoteAddr()String getServerName()int getServerPort()

String getScheme()String getRealPath()String getCharacterEncoding()Object getAttribute()

class Name

public

abstract

methods


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interface HTTPServlet

Information from clientadditional path info (sent along with request)

eg /servlet/myservlet/dict/defn.txt

/dict/defn.txt is the additional info to the servlet

myservlet within your servlet use getPathInfo() (null if no such

data is available)

what was requested (file/ servlet/)

HTTP headers sent by the client

Connection, User-Agent, host, Accept, Accept-language, Accept-Charset


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HTTP Headers

General : Information not related to Client, Server or

HTTP protocol.

Request : Preferred Document formats and server

parameters.

Response : Information about the server.

Entity : Information on the data that is being sent

between the client and server.

j l t htt Htt S l tR t


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javax.servlet.http.HttpServletRequest

HTTPServletRequest extends ServletRequest

Enumeration getHeaderNames()long getDateHeader(String)String getPathInfo()

String getQueryString()String getRemoteUser()String getAuthType()Cookie[ ]getCookies()String getRequestURI()String getServletPath()String getSession()boolean isRequestedSessionIdFromCookie()boolean isRequestedSessionIdFromUrl()

Class Namepublic

abstract

methods


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Example

Write a servlet that implements the doGet() method. Capture the

information encapsulated in the request object (in ServletRequest

and HTTPServletRequest), and send this info back to the client.

Eg: helloservletGet.java


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ServletResponse Interface

Allows the servlet to reply to the client

Allows the servlet to set the content length and MIME

type

Provides a handle to ServletOutputStream (or Writer)to send data to the client

Subclass HTTPServletResponse supports methods to

set more protocol specific headers


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javax.servlet.ServletResponse

ServletResponse

void setContentLength(int)void setContentType(String);ServletOutputStream getOutputStream()PrintWriter getWriter()String getCharacterEncoding(String)

Class Name

publicabstract

methods

javax servlet http


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javax.servlet.http.

HttpServletResponse

HTTPServletResponse

void addCookie(http.Cookie)boolean containsHeader(String);String encodeURL(String)String encodeRedirectURL(String)String encodeUri(String)String encodeRedirectUri(String)void setHeader(String,String)void setIntHeader(String,int)

void setDateHeader(String,long)void setStatus(int,String)void sendError(int,String)void sendRedirect(String)

Class Name

public

abstract

methods

R di i


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Redirecting

Redirection can be set using the following methods in theHttpServletResponse class

setStatus(status code)

status codes available in the HttpServletResponse class as

static variables setHeader(Location, give new site location here)

the new location must be an absolute url path

setHeader(Refresh, 3) will tell the browser for 3 seconds

before refreshing the page setHeader(Refresh, 3; URL=..) will tell the browser for 3seconds before going to the new location specified in thecommand.


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Generating HTML

Hardcoded in the program (servletGet.java)

Using an HTML generator (require additional

software; html generation classes)

Using an HTML generator creatively

Eg : ServletGet.java


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Redirecting a Request

Servlet can use status codes and headers to inform a

client to use another URL. It is useful when

document (html file) has moved.

load balancing is required. (one machine candistribute the load to many machines)


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Abstract class HTTPServlet

HTTPServlet extends GenericServlets implementsSerializable

void doGet(HttpServletRequest, HttpServletResponse)long getLastModified(HttpServletRequest);

void doPost(HttpServletRequest, HttpServletResponse)void doPut(HttpServletRequest, HttpServletResponse)void doDelete(HttpServletRequest, HttpServletResponse)void doOptions(HttpServletRequest, HttpServletResponse)void doTrace(HttpServletRequest, HttpServletResponse)

void service(HttpServletRequest, HttpServletResponse)public void service(ServletRequest, ServletResponse)

class Name

protected

methods

All methods throws ServletException and IOException

The service method supports HTTP 1.0protocol. This method dispatches each requestto the method designed to handle it.


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Overriding Methods

Servlet writer who wants to handle the GET and the

HEAD (HTTP protocol) must override the doGet()method in the servlet.

Similarly override doPost() method to handle POST request

doPut() method to handle PUT requests

doDelete() method to handle DELETE requests


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Notes

Servlets are fundamentally multithreaded, hence can

run multiple service() methods.

Therefore the code for service method must be thread

safe. If you do not want a multithreaded server then one

must implement the SingleThreadedModel interface.


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Example

Write a counter servlet that counts the number of clients

served.

Eg: MyCounterServ.java

Modify the above servlet so that if many users are accessing

the variable count, it should be synchronized.

Eg: MyCounterServSyn.java

E l


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Example

Write a servlet that informs the client that the site has moved,

and allow let the browser go to the new url automatically after

9 seconds.

Eg: RelocateServlet.java

R t Di t h


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Request Dispatcher

The basic use of a RequestDispatcher:

One can effectively use the RequestDispatcher to call a JSP

from a servlet or a servlet from another servlet.

RequestDispatcher rd =

getServletContext().getRequestDispatcher("/welcome.jsp");

rd.include(request, response);

S di M l i di C


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Sending Multimedia Content

To send an image that is available with the server to the

client

open a stream to the client browser

open the local image file

read block of bytes/ or byte by byte of image

send information to server by writing to the stream

E l


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Example

Write a serlvet that sends an image to the browser.

Image file must be on the server system.

Eg: servletImage.java

Problem: If we write 2 images to theapplication, it will read only the first one.

Hence, one can send only one image at atime.

Example


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Example

Write a serlvet that sends 2 tags to the

client. The client would load one image after another and

display both of them.

Eg: servletImageHTML.java

Solution: send the html tag , and the

browser will open connection and get the informationfrom server. This way any number of images can be

passed.

Note: All the data to be sent is stored in a

byteArrayOutputStream and sent all at once. This isbuffering but could slow down when lot of information is

being sent to the client.

User Authentication


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User Authentication

Specify in the resource list

servlet resource/ and what type of authentication

once set, then the browser gives a dialog box and

then sends the name and password to the servlet

Eg: secureServlet.java

Session Tracking


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Session Tracking

Mechanism to maintain a state about a series of

requests from the same user (request originating from

same browser)

Sessions are shared across servlets

HttpSession Object

Can Store (name, value) Pairs.

Persistence and disk swapping through Object

Serialization.

Works across protocols (HTTP/HTTPS).

Session validity, Creation time etc.

Session Tracking (contd)


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Session Tracking (contd)

procedures in session tracking get a HttpSession object for a user

HttpSession ss = new HttpSession(true)

true means that if it a new session create a new

session object, otherwise get session id. store/get data from the HttpSession object

void ss.putValue(String name, Object val)

Object ss.getValue(String name)

void ss.removeValue(String name) Invalidate the session

ss.invalidate();

Count Example using Session


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Count Example using Session

Write a servlet that uses session api to track the number of

times a user has visited the site.

Eg: SessionDemo.java

W f S i T ki


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Ways of Session-Tracking

Hidden Form Fields

Persistent Cookies

Servlet API


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Security Issues

The Servlet Sandbox

Access Control Lists (ACLs)


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Servlet Sandbox

Itis an area where Servlets are given restricted

authority on the server.

They may not have access to the file system ornetwork, or they may have been granted a more

trusted status.

It is up to the web server administrator to decide

which servlets are granted this status.

A C t l Li t


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Access Control Lists

An ACL is a list of users who are allowed to perform a

specific function in the server.

The list specifies:

What kind of access is allowed

What object the access applies to

Which users are granted access


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Servlet Environment Inter-Servlet Communication

How to call a method of another Servlet

Servlet Environment (contd)


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Servlet Environment (contd)

Communication with Active Server ResourcesHow to call another Servlet (or any other kind of

active resource) to process a request

Accessing Passive Server Resources

How to access a resource in the server's document

tree

Accessing Servlet Resources

How to access resources which belong to a Servlet

Sharing Data Between Servlets

How to share data between Servlets

Invoking a Servlet From an Applet


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g pp

HTTP Request thru DataOutput created by

URLCorrection object

Formatted results thru DataInputStream object

created by URLConnection objectSQL Query

ResultSet Object

Exercise (contd )


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Exercise (contd..)

20. Due to the implementation of the application on theAWT framework it is required to install the JRE software

& the required application components on each machine

which needs to communicate with the application. A better

alternative would be to implement the same in a http basedserver architecture,(repeat exercises 1 to 5 here)[20]


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ThankQ

Servlets by Kamalakar Dandu

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