Unit 5 Servlet

7/30/2019 Unit 5 Servlet

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Java Servlets


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Servlet

A servlet is a web component, managed by acontainer, that generates dynamic content.

It is a server-side program written in Java. After

writing the source code, it has to be compiledinto a class file, like any Java program.

Whenever a HTTP request comes, requesting forthe execution of this Servlet, the class file is

interpreted by the Java Virtual Machine (JVM).

This produces HTML output and is sent back tothe browser in the form of HTTP response.


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Why Servlets?

Web pages with dynamic content

Easy coordination between Servlets tomake Web applications

Containers support many features Sessions, persistence, resource management

(e.g., database connections), security, etc.


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Where are Servlets?

HTTPWeb

Server

File system

Servlet

Server

Static

Dynamic

Ex : Tomcat = Web Server + Servlet Server


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Servlet job

Read explicit data sent by client (form data) Read implicit data sent by client (request headers) Generate the results

Send the explicit data back to client(HTML)

Send the implicit data to client (status codes and response headers)


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When Servlets?

Receive Request for

Servlet S

Is Sloaded?

Is Scurrent? (re)Load S

Forward Request

to S

Servlets die when

Servlet Server dies

Loaded when first used,

or after modifiedno

no

yes

yes


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CGI Scripts

CGIstands for Common GatewayInterface

Client sends a request to server

Server starts a CGI script

Script computes a result for server

and quits

Another client sends a request

client server

clientscript

Server starts the CGI script again

script

Server returns response to client


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Servlets

A servlet is like an applet, but on theserver side

Client sends a request to server

Server starts a servlet

Servlet computes a result for

server and does not quit

Another client sends a request

client server

clientservlet

Server calls the servlet again

Server returns response to client


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Servlets vs. CGI scripts

Running a servlet doesnt require creating aseparate process each time (threads used)

A servlet stays in memory, so it doesnt haveto be reloaded each time

There is only one instance handling multiplerequests, not a separate instance for every

request A variety of different languages were used to

build CGI programs. These included C, C++,and Perl. Where as Servlet are coded in java


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Servlets vs. CGI scripts

CGI suffered serious performanceproblems. It was expensive in terms ofprocessor and memory resources to createa separate process for each client request.

It was also expensive to open and closedatabase connections for each clientrequest.

In addition, the CGI programs were notplatform-independent. Servlet is platform-independent.


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What are the advantages of servlets?

Portability

Portable across operating systems and across webservers.

Power Harness the full power of the core Java APIs: networking

and URL access, multithreading, image manipulation,data compression, JDBC.

Efficiency & Endurance

Memory resident, so invocation highly efficientnoprocess to spawn or interpreter to invoke.


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What are the advantages of servlets?

(Contd)

Safety

Support safe programming since inherit Javas strong typesafety, exception-handling mechanism.

Security

the Java security manager on the server enforces a set ofrestrictions to protect the resources on a server machine

Elegance Code is clean, object-oriented, modular, and simple (i.e..

Session tracking, cookie).

Integration

Tightly integrated with the servertranslate file paths,perform logging, check authorization, and MIME type mapping.


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What are the disadvantages of

servlets?

Developers MUST know JAVA

Web Administrator will need to learn howto install and maintain Java Servlets

Less choice of languages (CGI scripts canbe in any language)

Tedious uses of out.println() statements

Can be remedied by using Java Server Page(JSP)


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Servlet Container

A servlet runs inside a Servlet container.

A Servlet container is the hosting

environment for Java Servlets. A compilerplus run-time hosting environment forServlets.

Ex : Tomcat is a Servlet container whichruns inside a Web server, Apache.


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What is HTTP

Hypertext Transfer Protocol is used to describehow HTML documents are sent over the Internet.

It is stateless protocol HTTP Request Model

A client application, such as web browser opens a socketto the web servers HTTP port (80) Through the connection, the client writes an ASCII text

request line, followed by zeros and more HTTP headers,an empty line, and any data that accompanies therequest

The web server parses the request and locates thespecified resource The server writes a copy of the resource to the socket,

where its read by the client The server closes the connection


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HTTP Request methodsMethod Description

GET A simple request to retrieve identified in the URI

HEAD Same as GET, except server doesnt return the requesteddocument. The server returns only the status line and

headers

POST A request for the server to accept data that will be writtento the clients output stream

PUT A request for the server to store the data in the request asthe new contents of the specified URI

DELETE A request for the server to delete the resource named inthe URI

OPTIONS A request for information about what request methods theserver supports

TRACE A request for the web server to echo the HTTP request andits headers

CONNECT A documented but currently unimplemented methodreserved for use with a tunneling proxy.


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Flow of execution in the Servlet Environment

1. The browser sends an HTTP request to the Web server forexecuting a Servlet.

2. The Web server hands it over to the servlet container, afterproviding the appropriate execution environment.

3. The Servlet container loads and executes the Servlet (.class fileof the Servlet) by interpreting its contents via the JVM. Theresult is usually some HTML output.

4. This HTML output is sent back to the Web browser via the webserver, as a part of the HTTP response.


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Apache

Apache is a verypopular server 66% of the web sites on the Internet use

Apache

Apache is: Full-featured and extensible

Efficient

Robust

Secure (at least, more secure than otherservers)

Up to date with current standards

Open source

Free


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Ports

A port is a connection between a server and aclient Ports are identified by positive integers

A port is a software notion, not a hardware notion, so

there may be very many of them A service is associated with a specific port

Typical port numbers: 21FTP, File Transfer Protocol

22SSH, Secure Shell

25SMTP, Simple Mail Transfer Protocol 53DNS, Domain Name Service

80HTTP, Hypertext Transfer Protocol

8080HTTP (used for testing HTTP)

7648, 7649CU-SeeMe

These are the ports

of most interest to us


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Tomcat Tomcat is the Servlet Engine than handles

servlet requests for Apache

Tomcat is a helper application for Apache

Its best to think of Tomcat as a servlet container

Its easier to install Tomcat standalone than aspart of Apache

By itself, Tomcat can handle web pages, servlets,

and JSP

Apache and Tomcat are open source (andtherefore free)


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Servlet lifecycle

Servlet initialization -init() method

Happens once in lifecycle

Service client requests-service() method happens once for every client request

Servlet destruction- destroy() method happens once in life cycle


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Servlet Life Cycle


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Servlet Life Cycle

A user enters a Uniform Resource Locator(URL) to a Web browser. The browserthen generates an HTTP request for thisURL. This request is then sent to theappropriate server.

This HTTP request is received by the Webserver. The server maps this request to aparticular servlet.

The servlet is dynamically retrieved andloaded into the address space of theserver.


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Servlet Life Cycle

The server invokes the init( ) method of theservlet. This method is invoked only when theservlet is first loaded into memory. It is possibleto pass initialization parameters to the servlet so

it may configure itself. The server invokes the service( ) method of the

servlet. This method is called to process the HTTPrequest. You will see that it is possible for the

servlet to read data that has been provided in theHTTP request. It may also formulate an HTTPresponse for the client.


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Servlet Life Cycle

The servlet remains in the servers address spaceand is available to process any other HTTPrequests received from clients. The service( )method is called for each HTTP request.

Finally, the server may decide to unload theservlet from its memory. The algorithms by whichthis determination is made are specific to eachserver. The server calls the destroy( ) methodto relinquish any resources such as file handles

that are allocated for the servlet. Important data may be saved to a persistent

store. The memory allocated for the servlet andits objects can then be garbage collected.


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Installation of Apache Tomcat

First install j2sdk1.4.2_13 for access of Java classes & interfaces asthe servlet is written in Java language.

Set the environment variable PATH to c:\j2sdk1.4.2_13\bin;c:\j2sdk1.4.2_13\lib

Next install Tomcat

Create a new SYSTEM environment variable CLASSPATH toc:\Program Files\Apache Group\Tomcat 4.1\common\lib\servlet.jar

Alternatively, you can specify this class file when you compile theservlets.

For example, the following command compiles the first servlet

example: javac HelloServlet.java -classpath "C:\Program Files\Apache Tomcat

4.0\common\lib\servlet.jar"

To execute already provided examples of servlet

In Internet Explorer add the URL http://localhost:8080


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Execution in Tomcat

Create .java file in c:\j2sdk1.4.2_13\bin

Compile it there usingjavac filename.java

Copy the .class file to Tomcat server. C:\ProgramFiles\Apache Group\Tomcat4.1\webapps\examples\WEB-INF\classes\

Start the Tomcat server and minimize the

window.

Open Internet Explorer and in the URL enterhttp://localhost:8080/examples/servlet/filename


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Steps to create Servlet

Create and compile the servlet sourcecode.

Start Tomcat.

Start a Web browser and request theservlet.


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Create and compile the servlet source

code

create a file named HelloServletpro.java that contains thefollowing program

import java.io.*;import javax.servlet.*;public class HelloServletpro extends GenericServlet {public void service(ServletRequest request,

ServletResponse response)throws ServletException, IOException {response.setContentType("text/html");PrintWriter pw = response.getWriter();pw.println("Hello welcome to servlet programming!");pw.close();}} Copy the .class file to Tomcat server. C:\Program Files\Apache

Group\Tomcat 4.1\webapps\examples\WEB-INF\classes\


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Explanation of program First, note that it imports thejavax.servlet package.

This package contains the classes and interfaces required to buildservlets.

Next, the program defines HelloServlet as a subclass ofGenericServlet. The GenericServlet class provides functionality that makes it easy to

handle requests and responses. Inside HelloServet, the service( ) method (which is inherited from

GenericServlet) is overridden. This method handles requests from aclient. Notice that the first argument is a ServletRequest object.

This enables the servlet to read data that is provided via the clientrequest.

The second argument is a ServletResponse object. This enables the servlet to formulate a response for the client.

The call to setContentType( ) establishes the MIME type of the HTTPresponse. In this program, the MIME type is text/html. This indicates that the

browser should interpret the content as HTML source code. Next, the getWriter( ) method obtains a PrintWriter.

Anything written to this stream is sent to the client as part of the HTTPresponse.

Then println( ) is used to write some simple HTML source code as theHTTP response.


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Start Tomcat

As explained, to start Tomcat, select StartTomcat in the Start | Programs menu, orrun

startup.bat from the C:\ProgramFiles\Apache Tomcat 4.0\bin\directory.


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Start a Web Browser and Request the

Servlet

Start a Web browser and enter the URLshown here:

http://localhost:8080/examples/servlet/HelloS

ervletpro Alternatively, you may enter the URL

shown here:

http://127.0.0.1:8080/examples/servlet/Hello

Servletpro


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Servlet Architecture

javax.servlet.Servlet interface

javax.servlet.GenericServlet and

javax.servlet.http.HttpServlet class

Sun has provided a Java class calledHttpServlet having methods for

initialization, servicing, and destruction ofServlets. A new Servlet has to be writtenby extending from HttpServlet class.


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The Servlet API

javax.servlet

javax.servlet.http


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Thejavax.servlet Package


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Thejavax.servlet Package


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Servlet Interface


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ServletConfig Interface


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ServletContext Interface


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ServletRequest Interface


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ServletResponse Interface


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SingleThreadModel Interface

only a single thread will execute theservice( ) at a time

First, it can create several instances of the

servlet. Second, it can synchronize access to the

servlet

No constant, no methods declared


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GenericServlet Class

Basic life cycle of servlet

Implements servlet and servletconfiginterface

void log(String s) void log(String s, Throwable e)

a method to append a string to the server logfile


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ServletInputStream Class

Extends InputStream

Implemented by the server

It defines the default constructor.

int readLine(byte[ ] buffer, int offset, int size)throws IOException

Here, bufferis the array into which size bytes areplaced starting at offset. The method returns theactual number of bytes read or 1 if an end-of-stream condition is encountered.


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ServletOutputStream Class

extends OutputStream

implemented by the server

A default constructor is defined.

the print( ) and println( ) methods


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Servlet Exception Classes

javax.servlet defines two exceptions.

The ServletException, which indicatesthat a servlet problem has occurred.

The UnavailableException, whichextends ServletException. It indicatesthat a servlet is unavailable.


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Servlets

A servlet is any class that implements thejavax.servlet.Servlet interface

In practice, most servlets extend thejavax.servlet.http.HttpServlet class

Some servlets extendjavax.servlet.GenericServlet instead

Servlets, like applets, usually lack a mainmethod, but must implement or overridecertain other methods


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Example


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Exampleimport java.io.*;import java.util.*;import javax.servlet.*;public class PostParametersServletextends GenericServlet {public void service(ServletRequest request,ServletResponse response)throws ServletException, IOException {

// Get print writer.

PrintWriter pw = response.getWriter();// Get enumeration of parameter names.Enumeration e = request.getParameterNames();

// Display parameter names and values.while(e.hasMoreElements()) {String pname = (String)e.nextElement();pw.print(pname + " = ");String pvalue = request.getParameter(pname);pw.println(pvalue);}pw.close();}}

PostParametersServlet.java
http://c/j2sdk1.4.2_13/bin/PostParametersServlet.javahttp://c/j2sdk1.4.2_13/bin/PostParametersServlet.java


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How to Run

Compile the servlet and perform thesesteps to test this example:

1. Start Tomcat (if it is not already

running). 2. Display the Web page in a browser.

3. Enter an employee name and phonenumber in the text fields.

4. Submit the Web page.


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javax.servlet.http Package


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HttpServletRequest


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HttpServletResponse


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HttpSession

All of these methods throw an IllegalStateException ifthe session has already been invalidated


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HttpSessionBindingListener

Is implemented by objects that need to be

notified when they are bound to or unbound froman HTTP session.

void valueBound(HttpSessionBindingEvent e)

void valueUnbound(HttpSessionBindingEvent e)

Here, e is the event object that describes thebinding.


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Cookie Class

A cookie is stored on a client and contains stateinformation.

Cookies are valuable for tracking user activities A servlet can write a cookie to a users machine via the

addCookie( ) method of the HttpServletResponseinterface

Cookie data includes The name of the cookie The value of the cookie The expiration date of the cookie The domain and path of the cookie

Constructor Cookie(String name, String value)


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Cookie Class


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HttpServlet Class

It extends GenericServlet class


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HttpSessionEvent Class

It encapsulates session events

It extents EventObject and is generatedwhen a change occurs to the session

HttpSessionEvent(HttpSession session) session is the source of the event

HttpSession getSession( )

It returns the session in which the eventoccurred.


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HttpSessionBindingEvent Class

It extends HttpSessionEvent HttpSessionBindingEvent(HttpSession

session, String name)

HttpSessionBindingEvent(HttpSessionsession, String name, Object val) Here, session is the source of the event and

name is the name associated with the objectthat is being bound or unbound. If an attribute

is being bound or unbound, its value is passedin val.


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methods

String getName( ) obtains the name that is being bound or

unbound

HttpSession getSession( ) obtains the session to which the listener is

being bound or unbound

Object getValue( ) obtains the value of the attribute that is being

bound or unbound


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Servlets

A servlet is any class that implements thejavax.servlet.Servlet interface

In practice, most servlets extend thejavax.servlet.http.HttpServlet class

Some servlets extendjavax.servlet.GenericServlet instead

Servlets, like applets, usually lack a mainmethod, but must implement or overridecertain other methods


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Important servlet methods, I When a servlet is first started up, its init(ServletConfig config)method iscalled

init should perform any necessary initializations init is called only once, and does not need to be thread-safe

(syncronization not required)

Every servlet request results in a call to

service(ServletRequest request, ServletResponse response) service calls another method depending on the type of service

requested Usually you would override the called methods of interest, not service

itself service handles multiple simultaneous requests, so it and the methods

it calls must be thread safe (syncronization required)

When the servlet is shut down, destroy()is called destroy is called only once, but must be thread safe (because other

threads may still be running)


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HTTP requests

When a request is submitted from a Web page, itis almost always a GET or a POST request

The HTTP tag has an attribute action will bethe URL,method whose value can be "get" or"post"

The "get" action results in the form informationbeing put after a ? in the URL

Example:http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-

8&q=servlets

The& separates the various parameters

Only a limited amount of information can be sent thisway

"put" can send large amounts of information


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Important servlet methods, II

The service method dispatches the following kinds of requests:DELETE, GET, HEAD, OPTIONS, POST, PUT, and TRACE

A GET request is dispatched to the doGet(HttpServletRequestrequest, HttpServletResponse response)method

A POST request is dispatched to the doPost(HttpServletRequestrequest, HttpServletResponse response) method

These are the two methods you will usually override

doGet and doPost typically do the same thing, so usually you do

the real work in one, and have the other just call it

public void doGet(HttpServletRequest request,

HttpServletResponse response) {

doPost(request, response);

}


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HTTP requests & responses with respect to Servlets

The browsers HTTP request is sent to the server. The servlets doGet() method receives this request as a

parameter.

Similarly, when the server needs to send back an HTTP response,it populates the corresponding attributes or calls methods in the

other parameter HttpServletResponse, which is used to send backinformation to the browser in the form of an HTTP response.


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Hello World Example A (.java)

import java.io.*;import javax.servlet.*;import javax.servlet.http.*;

public class HelloServlet extends HttpServlet{public void doGet(HttpServletRequest request, HttpServletResponse response) throws

IOException, ServletException

{response.setContentType("text/html");PrintWriter out = response.getWriter();out.println("");out.println("");out.println("");out.println("Hello World!");

out.println("");out.println("");out.println("Hello World!");out.println("");out.println("");}}

http://localhost:8080/examples/servlet/HelloServlet
http://localhost:8080/examples/servlet/HelloServlethttp://localhost:8080/examples/servlet/HelloServlet


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The superclass

public class HelloServlet extends HttpServlet {

Every class must extend GenericServlet or a subclass ofGenericServlet

GenericServletis protocol independent, so you couldwrite a servlet to process any protocol

In practice, you almost always want to respond to anHTTP request, so you extendHttpServlet

A subclass ofHttpServlet must override at least onemethod, usually one doGet, doPost, doPut, doDelete, initand destroy, or getServletInfo


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The doGet method

public void doGet(HttpServletRequest request,HttpServletResponse response)

throws ServletException, IOException {

This method services a GET request

The method uses request to get the information that was

sent to it The method does not return a value; instead, it uses

response to get an I/O stream, and outputs its response

Since the method does I/O, it can throw an IOException

Any other type of exception should be encapsulated as a

ServletException The doPost method works exactlythe same way


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Parameters to doGet

Input is from the HttpServletRequestparameter

Output is via the HttpServletResponseobject,which we have named response

I/O in Java is very flexible but also quitecomplex, so this object acts as an assistant


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Using the HttpServletResponse

The second parameter to doGet(or doPost) isHttpServletResponse response

Everything sent via the Web has a MIME type

The first thing we mustdo with responseis set the

MIME type of our reply:response.setContentType("text/html");

This tells the client to interpret the page as HTML

Because we will be outputting character data, we

need a PrintWriter, handily provided for us by thegetWritermethod ofresponse:PrintWriter out = response.getWriter();

Now were ready to create the actual page to be

returned


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Using the PrintWriter

From here on, its just a matter of using ourPrintWriter, named out, to produce the Web page

First we create a header string:String docType =

"\n";

This line is technically required by the HTML spec

Browsers mostly dont care, but HTML validators docare

Then use the println method ofout one or moretimesout.println(docType +

"\n" +

" ... ");


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Input to a servlet

A GET request supplies parameters in the formURL?name=value&name=value&name=value Actual spaces in the parameter values are

encoded by+ signs Other special characters are encoded in hex;

for example, an ampersand is represented by%26

Parameter names can occur more than once, withdifferent values

A POST request supplies parameters in the same

syntax, only it is in the body section of therequest and is therefore harder for the user tosee


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Getting the parameters

Input parameters are retrieved via messages tothe HttpServletRequest object request Most of the interesting methods are inherited from the

superinterface ServletRequest

public Enumeration getParameterNames() Returns an Enumeration of the parameter names

If no parameters, returns an empty Enumeration

public String getParameter(String name) Returns the value of the parameter name as a String

If the parameter doesnt exist, returns null Ifname has multiple values, only the first is returned

public String[] getParameterValues(name) Returns an array of values of the parameter name

If the parameter doesnt exist, returns null


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Example of input parameters

public void doGet(HttpServletRequest request,HttpServletResponse response) {

...

...

out.println("Hello");String names[] =

request.getParameterValues("name");

if (names != null)

for (int i = 0; i < names.length; i++)

out.println(" " + names[i]);

out.println("Done");

}


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Java review: Data from Strings

All parameter values are retrieved as Strings

Frequently these Strings represent numbers, and youwant the numeric value

int n = new Integer(param).intValue();

double d = new Double(param).doubleValue(); byte b = new Byte(param).byteValue();

Similarly for short, float, and long

These can all throw a NumberFormatException,which is a subclass ofRuntimeException

boolean p = new Boolean(param).booleanValue(); char c =param.charAt(0);


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form.html Example

Introductions

If you don't mind me asking, what is your name?

form.html
http://localhost/var/www/apps/conversion/tmp/material/php%20material/Downloads/form.htmlhttp://localhost/var/www/apps/conversion/tmp/material/php%20material/Downloads/form.html


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Hello.java Example

import java.io.*;import javax.servlet.*;import javax.servlet.http.*;

public class Hello extends HttpServlet {

public void doGet(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

res.setContentType("text/html");PrintWriter out = res.getWriter();

String name = req.getParameter("name");

out.println("");out.println("Hello, " + name + "");out.println("");out.println("Hello, " + name);out.println("");

}}

Handling HTTP Get Request


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g q

Servlet Example 1 (.html file)

Servlet Example Using an Input Form

Forms Example Using Servlets

Enter your email ID:

form2.html

Handling HTTP Get Request
http://localhost/var/www/apps/conversion/tmp/material/php%20material/Downloads/form2.htmlhttp://localhost/var/www/apps/conversion/tmp/material/php%20material/Downloads/form2.html


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g q

Servlet Example 1 (.java file)

import java.io.*;import java.net.*;import javax.servlet.*;import javax.servlet.http.*;

public class EmailServlet extends HttpServlet{public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException,IOException

{ String email;email = request.getParameter(email);response.setContentType(text/html); PrintWriter out = response.getWriter();

out.println(); out.println();out.println(Servlet Example);

out.println();out.println();out.println(

The email ID you have entered is :+email+
);out.println();out.println(); out.close();
}}

EmailServlet.java
http://c/j2sdk1.4.2_13/bin/EmailServlet.javahttp://c/j2sdk1.4.2_13/bin/EmailServlet.java


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Servlet Example 2 (.java file)import java.io.*;

import java.net.*;import javax.servlet.*;import javax.servlet.http.*;

public class CurrencyConvertor extends HttpServlet{public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException

{response.setContentType(text/html);PrintWriter out = response.getWriter();

out.println();out.println();out.println(Dollars to Rupees Conversion Chart);out.println();out.println();out.println(Currency Conversion Chart);out.println();out.println();out.println(Dollars);out.println(Rupees);out.println();for (int dollars=1; dollars


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Servlet Example 3 (.html file)

Color:

RedGreenBlue

Form3.html

Handling HTTP Post Request
http://localhost/var/www/apps/conversion/tmp/material/php%20material/Downloads/form3.htmlhttp://localhost/var/www/apps/conversion/tmp/material/php%20material/Downloads/form3.html


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Servlet Example 3 (.java file)

import java.io.*;import javax.servlet.*;import javax.servlet.http.*;public class ColorPostServlet extends HttpServlet {public void doPost(HttpServletRequest request,HttpServletResponse response)throws ServletException, IOException {

String color = request.getParameter("color");response.setContentType("text/html");PrintWriter pw = response.getWriter();pw.println("The selected color is: ");pw.println(color);pw.close();

}}

ColorPostServlet.java
http://c/j2sdk1.4.2_13/bin/ColorPostServlet.javahttp://c/j2sdk1.4.2_13/bin/ColorPostServlet.java


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Using Cookies


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Cookie

What is a cookie?

A cookie is a piece of textual information

Send by the Web server to the client

browser Every time the browser visits the Web site

again, the cookie is sent unchanged backto the server


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Cookie

A cookie is a string upto 4K characters that you tell theWeb browser to store on a visitors hard drive.

Cookies give you a way to store information about the sitevisitor that can retrieve each time the visitor returns to the

site so long as the visitor uses the same Web browser andcomputer system.

Each Web browser type stores all cookie data in a single fileunique to the browser.

Without cookies there is no way of determining whathappened the last time the visitor came to the site or evenif the visitor has ever been to your site before.


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Benefits of Cookies

Identification of a user E-commerce

Poll

Customizing a site Portals

Personal Views

Avoiding login

Useful for sites that dont require high security


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Drawbacks of Cookies

User can deny cookies in his Web browser

Might be a privacy thread

E.g. search engines remember user specificsearch topics

Handling cookies using Servlets


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(cookie.html)

Enter a value for MyCookie:

http://c/j2sdk1.4.2_13/bin/AddCookieServlet.java


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(AddCookieServlet.java)import java.io.*;import javax.servlet.*;import javax.servlet.http.*;

public class AddCookieServlet extends HttpServlet {public void doPost(HttpServletRequest request,HttpServletResponse response)throws ServletException, IOException {

// Get parameter from HTTP request.

String data = request.getParameter("data");// Create cookie.Cookie cookie = new Cookie("MyCookie", data);

// Add cookie to HTTP response.response.addCookie(cookie);

// Write output to browser.response.setContentType("text/html");PrintWriter pw = response.getWriter();pw.println("MyCookie has been set to");

pw.println(data);pw.close();}}

http://c/j2sdk1.4.2_13/bin/AddCookieServlet.javahttp://c/j2sdk1.4.2_13/bin/AddCookieServlet.javahttp://c/j2sdk1.4.2_13/bin/GetCookiesServlet.java


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(GetCookiesServlet.java)import java.io.*;import javax.servlet.*;import javax.servlet.http.*;public class GetCookiesServlet extends HttpServlet {public void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException, IOException {

// Get cookies from header of HTTP request.Cookie[] cookies = request.getCookies();

// Display these cookies.response.setContentType("text/html");PrintWriter pw = response.getWriter();pw.println("");for(int i = 0; i < cookies.length; i++) {String name = cookies[i].getName();String value = cookies[i].getValue();pw.println("name = " + name +"; value = " + value);

}pw.close();}}
http://c/j2sdk1.4.2_13/bin/GetCookiesServlet.javahttp://c/j2sdk1.4.2_13/bin/GetCookiesServlet.java


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101

Session Tracking

S i T ki


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102

Session Tracking

HTTP is a stateless protocol. Each request isindependent of the previous one.

However, in some applications, it is necessary to

save state information so that information can becollected from several interactions between abrowser and a server. Sessions provide such amechanism.

Session state is shared among all the servletsthat are associated with a particular client.

P d


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103

Procedure

A session can be created via the getSession( )method of HttpServletRequest. AnHttpSession object is returned.

This object can store a set of bindings thatassociate names with objects. The setAttribute( )

getAttribute( )

getAttributeNames( )

removeAttribute( ) methods ofHttpSession managethese bindings.

D S l j

When you first request this servlet, the browserdisplays one line with the current date and timeinformation. On subsequent invocations, two linesare displayed The first line shows the date and


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DateServlet.javaimport java.io.*;import java.util.*;import javax.servlet.*;import javax.servlet.http.*;

public class DateServlet extends HttpServlet{

public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException

{// Get the HttpSession object.HttpSession hs = request.getSession(true);

// Get writer.response.setContentType("text/html");PrintWriter pw = response.getWriter();pw.print("");

// Display date/time of last access.Date date = (Date)hs.getAttribute("date");

if(date != null){

pw.print("Last access: " + date + "
");}

// Display current date/time.date = new Date();hs.setAttribute("date", date);pw.println("Current date: " + date);

}

}

are displayed. The first line shows the date andtime when the servlet was last accessed. Thesecond line shows the current date and time.

C:\j2sdk1.4.2_13\bin\DateServlet.java

E l i i f D S l j
http://c/j2sdk1.4.2_13/bin/DateServlet.javahttp://c/j2sdk1.4.2_13/bin/DateServlet.javahttp://c/j2sdk1.4.2_13/bin/DateServlet.javahttp://c/j2sdk1.4.2_13/bin/DateServlet.javahttp://c/j2sdk1.4.2_13/bin/DateServlet.javahttp://c/j2sdk1.4.2_13/bin/DateServlet.javahttp://c/j2sdk1.4.2_13/bin/DateServlet.javahttp://c/j2sdk1.4.2_13/bin/DateServlet.java


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Explaination of DateServlet.java

The getSession( ) method gets the current session. A newsession is created if one does not already exist.

The getAttribute( ) method is called to obtain the objectthat is bound to the name date.

That object is a Date object that encapsulates the date andtime when this page was last accessed. (Of course, there isno such binding when the page is first accessed.)

A Date object encapsulating the current date and time isthen created. The setAttribute( ) method is called to bindthe name date to this object.

Unit 5 Servlet

Documents

Transcript of Unit 5 Servlet