building web applications using servlets_java30

8/6/2019 Building Web Applications Using Servlets_java30

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JavaTM Education & Technology Services

Copyright Information Technology Institute 1

Building WebApplications Using

Servlets Technology



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Table of Contents

Chapter 1: Web/HTTP Overview Chapter 2: Servlets Technology

Chapter 3: How to write servlets code?

Chapter 4: Accessing Databases with JDBC Chapter 5: Getting Familiar with HTTP package

Chapter 6: Session Management


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Web/HTTP Overview

Chapter 1


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Overview of HTTP Protocol

Request and Response

System Architectures

Developing Web Applications in Java

Different Web Technologies

Chapter 1 Outline


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HTTP

What is HTTP?

Hyper Text Transfer Protocol. Stateless Protocol.

Basic Scenario.

HTTP components: HTTP Request.

HTTP Response.

Request or Response consists of three main parts: Response or Request Line.

Header information.

The Body.


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HTTP

1.Establishes Tcp/Ip connection.

2.Sends Request.

3.Receives Response.

4.Closes connection.

Client

Server

http://163.121.12.2/webindex

Client Browser

Web Server

URL

Protocol Actual Path URI


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HTTP Request

HTTP Request consists of 3 components:

1.Method-URI-Protocol/Version Methods could be : Get ,Post, Head,Put,Trace, Options

Example: Get /servlet /default.jsp HTTP/1.1

Post / servlet /index.jsp HTTP/1.1


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HTTP Request contd

2.Request Headers :

Indirectly set by the browser and sent immediately after therequest line. The following are examples of request headers:

Accept: text/plain ;text/html (Note : type/* , */*)

Accept-language: en-ar

Accept-Encoding : gzip

Connection : keep- AliveHost : www.iti.gov.eg or localhost:8080

Referer : http://www.mcit.gov.eg/main.htm

User-agent : Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)

Content-length : 33 (in case of POST)

If-Modified-Since: specified dateIf-Unmodified-Since : Sat, 29 Oct 1994 19:43:31 GMT

Cookie: userID=id456578


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HTTP Request contd

3. Entity Body (in case of POST method):

UserName=ITI & Password =iti

Note: If the GET method was used:

There will be no entity body. URL in the address bar of the browser will look something like:

www.sitename.com/MyServlet? userName=ITI&password=iti

ITI

***

Sign in

Username

Password


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HTTP Response

HTTP Response consists of 3 components:

1. Response Line (Protocol/Version Status Code -Description):

Examples:

HTTP/1.1 200 OK

HTTP/1.1 404 error

Status Code Ranges:

100-199 informational, the client must respond with an action

200-299 : request is successful

300-399 : for Redirection , it includes a Locationheaderindicating the new address

400-499 :indicates an error by the client

500-599 : indicates an error by the server


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HTTP Response (contd)

2.Response HeadersServer: Tomcat/5.5

Date: Mon, 3 Jan 2006 13:13:33 GMT

Content-Type: text/html

Last-Modified: Mon, 11 Jan 2005 13:23:42 GMTContent-Length: 112

Content-Encoding = gzip


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HTTP Response (contd)

3.Entity Body

HTTP Response Example

Welcome to servlets and jsp course


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System Architecture

Typically we got 3 logical layers between client browser

and web server :

Presentation

Business logic

The data layer

Handle UI & UserInteraction

Programming logic

Data Base Acess


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Types of Architecture

Three-Tier Architecture: Presentation layer.

Business logic layer.

Data layer.

Two- Tier Architecture: Fat client (Presentation &

Business logic).

Disadvantages.


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System Architectures contd

N-Tier Architecture:

Provides more flexibility in modules development. The business logic layer is divided by function instead of being

physically divided. It is divided into:

User interface.

Presentation logic.

Business logic.

Infrastructure services.

Data layer.


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Developing Web Applications in java

Web Server architecture: Web Server

ASP

Web container


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Developing Web Applications in java

Application Server

Web Tier

Business

EJB Container

J2EE

DB

Browser

Mobile

Desktop

Legacy

SOAP

HTTP

SOAP

RMI

COBOL

XML


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Web Technologies

CGI (Common Gateway Interface)

Cold Fusion

Server Side Java Script (SSJS)

PHP

Servlet

JSP

ASP

ASP.NET


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Servlets Technology

Chapter 2


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Chapter 2 Outline

Advantages of Servlets

Servlet Container

Servlet Life Cycle

Building and Deploying a Web Application

The Deployment Descriptor

Tomcat Web Server Structure

First Servlet Example


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What is a Servlet?

Client info (host name,form data)

Success or failure

Process results(access database)

Format results and produceHTML

Send page back to client

Browser Servlet

Request

Response


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What is a Servlet? (contd)

A Servlet is a Java program that runs on aserver, that produces dynamic pages

typically in response to client requests.

Servlet main jobs: It reads and process implicit and explicit data sent by the

client

Send the implicit / explicit data to the client with a proper

format Its not restricted to HTTP requests


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Advantages of Servlets

Performance

Portability (supported by many web servers)

Rapid development cycle (rich library , high-level utilities)

Secure

Inexpensive


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Servlet Container

What is a Servlet container ?

Its actually the interface between the servletclass and the client requests.

Is a container that is responsible for

loading , processing and

unloading the servlets.

Managing servlets.


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Servlet Container (contd)

Request

Response

Servlet

HTTP

Request

Response

WebServer

Servlet Container


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Servlet Container contd

Well known servlet containers:

Tomcat JRun

WebSphere

Glassfish

OC4J (Oracle)

StarNine WebStar


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How Servlets works?


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Servlets Life Cycle

Load Initialize

init()

Destroydestroy()

Execute

service()

1 2 3

4


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JavaTM Education & Technology ServicesCopyright Information Technology Institute 29

Building & Deploying a Web Application

A servlet or JSP application must have a folder named

WEB-INF.

The WEB-INF folder must contain the following: classes folder: it holds the servlets after compilation (i.e. the

.class fileof each servlet), and any helper classes or beans.

lib folder: it holds helper classes that are packaged inside a jarfile.

web.xml file: which is the deployment descriptor of the whole webapplication.

The web application may also contain html pages, jsppages, tld files (tag library descriptors), images or anyother resource files.


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What is the deployment descriptor ?

Contains meta-data for a Web Application and

that is used by the container when loading aWeb Application.

It identifies all the elements contained in the webapplication for the servlet container to be able to knowthem

It maps them into URL patterns to be able to callthem as clients

It contains configuration settings specific to thatapplication.


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Deployment Descriptor - Example

TestingTestingServlet

Testing/MyURL

index.htmlindex.htmindex.jsp


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Tomcat Web Server Structure


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First Servlet Example

To develop the first servlet application, you have to go

through the following 6 steps:1. Create the necessary directory structure under tomcats webappsfolder.

2. Write your servlet code (.java).

3. Compile your servlet (produce .class). Remember that we have to

include the servlets and jsp libraries in the class path. This is done bydoing either of the following:

javac -classpath C:\tomcat\common\lib\servlet-api.jar TestingServlet.java

Or permanently referring to the jar files in the classpath environmentvariable of the operating system.

Or put the two jar files in the JDK Installation folder, under ../jre/lib/ext. Or include the two jar files to your project (if you are using an IDE).


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First Servlet Example contd

4. Create the deployment descriptor (web.xml) file:

MyServletNamemyPkg.MyServlet

MyServletName /MyServletURL

5. Run Tomcat.

6. Open your browser (e.g. Internet Explorer) and invoke the servletusing its specific URL (http://domain-name/virtual-directory/servlet-url)Example: http://localhost:8080/iti/MyServletURL
http://domain-name/virtual-directory/servlet-urlhttp://domain-name/virtual-directory/servlet-urlhttp://domain-name/virtual-directory/servlet-urlhttp://domain-name/virtual-directory/servlet-urlhttp://domain-name/virtual-directory/servlet-urlhttp://domain-name/virtual-directory/servlet-urlhttp://domain-name/virtual-directory/servlet-urlhttp://domain-name/virtual-directory/servlet-url


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How to write servlets code?

Chapter 3


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Servlets package structure

javax.servlet package

Servlet Interface

ServletConfig Interface

The ServletContext Interface

The ServletRequest Interface

The ServletResponse interface

The GenericServlet Wrapper class

Chapter 3 Outline


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Servlet Package Structure

el

http

javax.servlet

JSP

tagext


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javax.servlet.*

Some basic interfaces :

Servlet

ServletConfig

ServletContext ServletRequest

ServletResponse

SingleThreadModel

Request Dispatcher


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javax.servlet.* (contd)

Some basic classes :

GenericServlet

ServletInputStream

ServletOutputStream The Exception Classes are:

ServletException

UnavailableException


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Servlet Interface

Servlet : its the basic interface , any servletmust implements it directly or indirectly

It consists of 5 main methods :

init (ServletConfig config) service( ServletRequest , ServletResponse)

destroy()

getServletConfig()

getServletInfo()

l l


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Servlet Example The following code is a typical implementation of the interface:

import javax.servlet.*;

import java.io.*;public class MyServlet implements Servlet

{

public voidinit(ServletConfig config) throws ServletException

{

System.out.println("I am inside the init method");}

public voidservice(ServletRequest request,

ServletResponse response)

throws ServletException, IOException

{

response.setContentType("text/html");

PrintWriter out = response.getWriter();

out.println(
Welcome to Servlets and JSP Course");

System.out.println("I am inside the service method");

}

Write to the response

S l E l d


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Servlet Example contd

public voiddestroy()

{System.out.println(I am inside the destroy method");

}

public String getServletInfo()

{return null;

}

public ServletConfig getServletConfig()

{return null;

}

}

Si l Th dM d l I t f


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SingleThreadModel Interface

The SingleThreadModel interface is used to prevent

simultaneous access to instance variables by multiplethreads.

Only one request thread accesses a single instance of the

servlet at a time.

The server queues the requests and passes them one at atime to a single servlet instance.

Or the server creates a pool of multiple instances, eachhandles one request at a time.(however, unsafe).


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S l tC fi I t f


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ServletConfig Interface

The ServletConfig interface is used by a servlet to obtain

any configuration information written specifically for it in thedeployment descriptor file (web.xml).

It is passed by the servlet container to the init(..) method

as parameter.

Such configuration information is better written in the xml fileinstead of hard coding it inside the servlets code.

Such approach makes it easy to alter such information after theservlets source code has been compiled.

Each servlet is passed its own set of configuration parameters.

S l tC fi I t f td


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ServletConfig Interface contd

Configuration parameters (init params) can be written inside

the web.xml file as follows:

MyServlet

ServletConfigTest

AdminEmail

[email protected]

The whole tag is repeated for each parameter.

S l tC fi I t f td


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ServletConfig Interface contd

The ServletConfig interface has 4 methods:

Enumeration getInitParmeterNames()

String getInitParameter(String paramName)

String getServletName()

ServletContext getServletContext()


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Lab Exercise

Assignment


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Assignment

Use servletConfig to pass variables fromweb.xml to the servlet and display it in theresponse.

Like DB connection Statement. DB user name and password.

DatabaseAddress

163.121.12.25

userName

iTi

password

iTi

ServletContext Interface


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ServletContext Interface

The context is the environment in which the servlets of a webapplication run.

There is only one context per web application (i.e. all theservlets of a web application have references to the samecontext).

The deployment descriptor (web.xml) can include tags (contextparams) that are general for the whole web application.

The ServletContext interface defines a set of methods that

a servlet uses in order to obtain such context parameters.

ServletContext Interface contd


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Moreover, a servlet can also bind an object attribute to thecontext (a pair: name and object value), which will beavailable to any other servlet that is part of the same webapplication.

In order to obtain a reference to the context, the followingmethod of the ServletConfig interface is used:




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ServletContext Interface cont d

Context parameters of a web application (context params)

can be written inside the web.xml file as follows:

DollarExchangeRate5.6

....

The whole tag is repeated for eachparameter. It should be placed at the beginning of the file.



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ServletContext Interface cont d

Commonly used methods:

Enumeration getInitParameterNames() String getInitParameter(String paramName)

Enumeration getAttributeNames()

Object getAttribute(String attName)

void setAttribute(String attName, Object value)

void removeAttribute(String attName)

void log(String msg)

Example


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Example

public class servletContext implements Servlet {ServletConfig sConf;public void init(ServletConfig config) throws

ServletException {sConf = config;}public void service(HttpServletRequest

request,HttpServletResponse response)throwsServletException, IOException{ServletContext servletContext = sConf.getServletContext();

PrintWriter out = response.getWriter();String str= servletContext.

getInitParameter(AdminEmail);out.println(AdminEmail is: " + str);

}..}

Example


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Example

public class servletContext implements Servlet {

ServletConfig sConf;

public void init(ServletConfig config) throwsServletException {

sConf = config;

}

public void service(ServletRequestrequest,ServletResponse response)throwsServletException, IOException

{

ServletContext servletContext = sConf.getServletContext();


servletContext.setAttribute(MyPassword,iti);

}

..}

SetterServlet

Example


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Example

public class servletContext implements Servlet {ServletConfig sConf;public void init(ServletConfig config) throws

ServletException {sConf = config;}public void service(ServletRequest

request,HttpServletResponse response)throwsServletException, IOException{ServletContext servletContext = sConf.getServletContext();

PrintWriter out = response.getWriter();String str= servletContext.getAttribute(MyPassword);

out.println(MyPassword is: " + str);}..}

GetterServlet

Config & Context


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Config & Context

Web Application 1

The Servlet Container

ServletContext

Servlet1

ServletConfig1

Servlet2

ServletConfig2

Web Application 2

ServletContext

Servlet1

ServletConfig1

Servlet2

ServletConfig2


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Lab Exercise

Assignment


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Assignment

Sharing info between servlets: A servlet willassign a username and password attributes ,and another servlet will display it

Your servlets should read the initializedparameters given to them by

tag in web.xml

Country

Egypt

ServletRequest Interface


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ServletRequest Interface

The ServletRequest defines an object that is used to hold

information coming from the user's request, includingparameter name-value pairs, attributes, and an input stream.

Commonly used methods:

Enumeration getParameterNames() String getParameter(String paramName)

String[] getParameterValues(String paramName)

String getRemoteHost()

Sending Request Parameters to a Servlet


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Sending Request Parameters to a Servlet

The tag in HTML is used as follows:

Sending parameters in the request


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Sending Request Parameters to a Servlet cont d

In order for the servlet to get the parameters from the request,the following code is written in the service(..) method:

public void service(ServletRequest request,ServletResponse response)


{

PrintWriter out = response.getWriter();String str = request.getParameter(MyName");

out.println(The user input is: + str);

}

ServletResponse Interface


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ServletResponse Interface

The ServletResponse interface defines an object thatcarries the servers response to the user (i.e. back to theclients browser).

Commonly used methods: PrintWriter getWriter()

setContentType(String mimeType) setContentLength(int len)

ServletResponse Example


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ServletResponse Example

Sending response in Excel Sheet Format:

public void service(ServletRequest request,

ServletResponse response)throws ServletException, IOException

{

response.setContentType("application/vnd.ms-excel");


out.println("\t jan \t feb \t march \t total");out.println("salary \t100 \t200 \t300 \t=sum(B2:D2)");

}

GenericServlet Wrapper Class


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JavaTM

Education & Technology ServicesCopyright Information Technology Institute 66

GenericServlet Wrapper Class

The GenericServlet class is a convenient class thatimplements the Servlet interface.

It overrides the required 5 methods, and hence makes yourcode simpler (i.e. you extend the class and only overridethe necessary methods).


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JavaTM


Accessing Databaseswith JDBC

Chapter 4

Chapter 4 Outline


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JavaTM


Review on JDBC

Steps to Work with a Database

Sample Applications of Database Operations

Online SQL Tool

Chapter 4 Outline

Review on JDBC


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Review on JDBC

JDBC API: set of Java interfaces for connecting to anyDBMS and executing SQL statements. Packages are:

java.sql package: basic functionalities.

javax.sql package: advanced functionalities.

JDBC Driver: Vendor-specific implementation of JDBCinterfaces. Every DBMS has JDBC driver(s).

Connection String: A special DBMS-specific string that isused to connect to the database, for example:

MS Access:jdbc:odbc:DataSourceName

Oracle (thin):jdbc:oracle:thin:@host:port:sid



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1. Load (register) the JDBC driver.

2. Obtain a connection.

3. Execute query statements.

4. Process results if any (ResultSet).

Steps to Work with a Database contd


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Steps to Work with a Database cont d

First Step: Put the driver (if jar file) in WEB-INF/lib for deploying (and in

CLASSPATH environment variable for development). The driver could also be put in the common/lib folder (will be common

among all web applications).

Register an object of the driver, for example:

DriverManager.registerDriver(

new sun.jdbc.odbc.JdbcOdbcDriver());

Second Step: Obtain the connection through the Connection interface, for example:

Connection con = DriverManager.getConnection("jdbc:odbc:mydsn",userName",password");



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Steps to o t a atabase co t d

Third Step: Create a statement and execute queries using either

executeUpdate(..) or executeQuery(..), for example:Statement s = connection.createStatement();

ResultSet rs= s.executeQuery(select * from Emp);

Fourth Step: After obtaining the ResultSet, extract data through the getter methods,

for example:while(rs.next())

{

out.println(rs.getInt(1) + : + rs.getString( FNAME

));

}

Note: You can get information about the retrieved ResultSetby using the method: getMetaData()



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p

Do not forget to close the connection.

By default, only one ResultSet object per Statement

object can be open at the same time.

If the defaultResultSet

object is obtained, then columns

within each row should be read in left-to-right order, andeach column should be read only once.

Inserting Records into the Database


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g

You can insert records using the following SQL statement:insert into Users (FirstName, LastName, UserName, Password)

values(JETS, ITI, jets, iti)

Searching for Records in a Database


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JavaTM


g

You can write an SQL statement that searches for a certainname in the database:

select FirstName, LastName, UserName, Password FROM Users WHEREFirstName LIKE '%keyword% OR LastName LIKE '%keyword%'

Online SQL Tool


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Q

You can now create an online SQL tool that executes any queryon the database and display the result in a text area.


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Lab Exercise

Submit User Details to a Servlet


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JavaTM


Registration : ( add a user to the database)

Write an HTML page that has text input fields for the users data (e.g.name, phone, email, etc), along with a submit button.

Upon pressing the submit button, the page should send the data to aservlet.

The servlet should then get the parameters from the request and add

them to the data base(i.e. show that it were add or not in the responsethat is returned back to the user).

Login :

Write an HTML page that has two text input fields for the users name

and mail, along with a submit button.

Upon pressing the submit button, the page should send the data to aservlet.

Checking the username and password against the database.

Submit User Details to a Servlet


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Display: Bonus

develop a servlet that views data of the products from the database (id,in a well formatted HTML table.

Search: Bonus develop a search servlet for products.

Response in Different Formats


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Develop different servlets that send response in different mime

types (e.g. excel sheet, word document, etc).

Hint: Search on tshe internet for names of famous mime types.


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Getting Familiar with HTTPPackage

Chapter 5

javax.servlet.http.*


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HTTPServlet GenericServlet

HTTPServletRequest ServletRequest

HTTPServletResponse ServletResponse

javax.servlet.http.* (contd)


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Some basic interfaces HttpServletRequest HttpServletResponse HttpSession

Classes Cookie HttpServlet

The HTTPServlet Class


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It extends from javax.servlet.GenericServlet

It contains extra methods like :

doPost() doPut()

doGet () doDelete()

doOptions() doTrace()

The HTTPServlet Class (contd)


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Request

Response

service()

Server HttpServlet

doGet()

doPost()

HttpServlet Example


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The following code demonstrates the use of the doGet(..) method:

public void doGet(HttpServletRequest request,

HttpServletResponse response)


{

response.setContentType(text/html);


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

out.println("The servlet has received a GET request." +

"Now, click on the button below.");

out.println("
");

out.println("");

out.println("");

out.println("");


}

HttpServlet Example contd


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The following code demonstrates the use of the doPost(..) method:

public void doPost(HttpServletRequest request,



{

response.setContentType("text/html");



out.println("The POST method");

out.println("");

out.println("");

out.println("The servlet has received a POST request.Thank you.");

out.println("");

out.println("");

}

HttpServletRequest Interface


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Common methods: Enumeration getHeaderNames()

String getHeader(String headerName)

Enumeration getHeaders(String headerNames)

String getQueryString()

HttpSession getSession(boolean create)

Cookie[] getCookies()

Remember, methods inherited from ServletRequest: Enumeration getParameterNames()

String getParameter(String paramName) String[] getParameterValues(String paramName)

HttpServletResponse Interface


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Common methods: setStatus(int statusCode)

sendRedirect(String newLocation)

sendError(int statusCode, String msg)

addCookie(Cookie cookie)

setHeader(String headerName, String value)

setIntHeader(String headerName, int value) setDateHeader(String headerName, long milliseconds)

Remember, methods inherited from ServletResponse:

PrintWriter getWriter() setContentType(String mimeType)

setContentLength(int len)

Status Code


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You have to set the status code and response headersbefore writing any document content to the response that

will be sent back to the client.

For the parameter of setStatus(..) method, you can

either use explicit numbers of your own, or use the static

constant values defined in the HttpServletResponseinterface (e.g. for status code 404 use SC_NOT_FOUND).


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Java

TM


Lab Exercise

Assignment


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Java

TM


Display Headers of the request sent to yourservlet

Assignment


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Java

TM


Registration : add a user the database. Continue the login assignment of the last chapter by

checking the username and password against thedatabase instead of hard coding the check insidethe servlet. User the request dispatchers forward method to direct the

user to a success page or failure page. Display :develop a servlet that views data employees data

from the database (id, name, email, salary, etc) in a wellformatted HTML table:

Provide the ability to add a new employee, edit, or delete anexisting one.

Search: develop a search servlet ( Bouns )

The sendRedirect Method


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The sendRedirect(..) method is used to redirect the

browsers client to go to another page other than the one

originally requested.

This is useful in several situations, for example: If the requested page has been migrated to another server or url.

Deciding which page the user will be transferred to after successfullogin. Checking if the browser enables the use of cookies.

The location parameter can either be the relative URL of a

page or its fully qualified URL.

The sendRedirect Method contd


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The sendRedirect(..) method makes a round trip to the

client and then back to the server.

A brand new request object is created, and the previous one islost.

Any data written to the response before calling it will be lost.

If the calling servlet wishes to maintain some information fromthe old request and propagate it to the new request, then youcan pass the information as a query string appended to the

destination URL:

response.sendRedirect("MySecondServlet?userName="+ userName + "&password=" + password);

sendRedirect Example


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The following code demonstrates the use of sendRedirect(..) method:

public void doGet(HttpServletRequest request,



{

.

.

out.println();

out.println("
Username: ");

out.println("
Password: ");

out.println("
");

out.println("");

.

.

}

sendRedirect Example contd


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public void doPost(HttpServletRequest request,



{

.

.

String userName = request.getParameter("userName");

String password = request.getParameter("password");

if(userName!=null && password!=null &&

userName.equals(user") && password.equals(iti"))

{

response.sendRedirect("WelcomeServletURL");

}else { out.println(Login Failed, please try again.); }

.

.

}

Sending Special Characters


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Some characters have special meanings to HTML, forexample .

To be able to display such characters in your response,you have to encode them as follows: < is replaced by: &lt

> is replaced by: &gt

White spaces are replaced by: &nbsp

& is replaced by: &amp

is replaced by: &quot

Populating HTML Elements with Values


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Explicit values example:

Using variables example:

out.println("");

ORout.println("");

out.println("");

Response Buffer


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For committing a response, a servlet sends the string outputonly once when it finishes processing or when the buffer is full.

Enabling/Increasing the size of the response buffer is useful inenhancing the servlets performance in case of large response

content.

It is ON by default and the default size is 8192 characters

You can can specify the buffer size of the response, using the

method: response.setBufferSize(..)

Note: Call this method before writing any output to theresponse.

RequestDispatcher Interface


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The RequestDispatcher interface has 2 methods only:

include(ServletRequest request,


forward(ServletRequest request,



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RequestDispatcher - forward


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The forward(..) method passes the processing of an

HTTP request from the current servlet to another servlet or

JSP (i.e. gives up the control for another servlet or jsp).

It is up to the new servlet or JSP to write to the responseobject and then commit it to the client.

Any output written to the response object by the originalservlet will be lost (i.e. dont write to the response before

forwarding to another servlet).

The original servlet can perform some initial tasks on therequest object before forwarding it (e.g. put additionalattributes on it, which will be used by the new servlet or jsp).

Using RequestDispatcher


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There are two ways to get a dispatcher for the desireddestination servlet or page. Either from the request, or from

the context.

From the ServletRequest: getRequestDispatcher(String path)

//relative path or fully qualified path

From the ServletContext: getRequestDispatcher(String fullPath)

// fully qualified path within the context, beginning with /

getNamedDispatcher(String servletName)

// the servlet name in the web.xml file

Using RequestDispatcher - Example


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The following code demonstrates the use of the RequestDispather:

RequestDispatcher rd =

request.getRequestDispatcher("SecondServletURL");

rd.include(request, response);

OR:

rd.forward(request, response);

Note: If you use include or forwardfrom a doPost method, then thedoPost method of the second servlet will be invoked. If you use themfrom a doGet method, then the doGet method of the second servlet will

be invoked. Therefore, it is advisable to implement both doGet anddoPost in the destination servlet (write the code in one of the two

methods, and call it from the other method).

Page Transfer through RequestDispatcher


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Include headers and footers to your pagePrintWriter out = response.getWriter();

// add Header

out.println(This the Header");

// real processingrequest.setAttribute("username",ITI");

RequestDispatcher rd =request.getRequestDispatcher("DispatcherServlet2");

rd.include(request, response);

// back to add footer

out.println(This the Footer");


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Lab Exercise

Login and Error Servlets


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Update a login page (either an html page or a dynamic pagegenerated by the doGet(..) method of a servlet).

Upon submitting the login form, the user should be redirectedto a welcome servlet (in case of successful login).

If the login data is incorrect,display a colored error messageon the login page.

Including Header and Footer


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Create header page and a footer page and then include themin your servlet (at the beginning and end). Bonus

Forwarding to Another Servlet


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Create a servlet that forwards to another servlet.

Try to make the first servlet write to the response beforeforwarding it to the second servlet, and then see whathappens.


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Session Management

Chapter 6

Chapter 6 Outline


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Session Management Techniques

URL Rewriting

Hidden Fields

Cookies

Session Objects


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Session Management Techniques


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The following techniques can be used separately orcooperatively in order to maintain the clients information

throughout the entire session:

URL rewriting.

Hidden fields.

Cookies.

Session objects.

URL Rewriting

URL rewriting is simply appending token(s) to the URL of


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URL rewriting is simply appending token(s) to the URL ofthe next page or servlet. It starts by a question mark

Tokens are strings in the form of parameter=value

Tokens are separated by an ampersand (&)

It is an easy way but has the following concerns: Supports up to 2000 characters only.

The parameters are exposed in the URL.

You will need to encode special characters (e.g. spaces to + and

special characters to %XX).

Example:http://www.site.com/mySerevlet?param1=value1&param2=value2

Hidden Fields


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Forms can include hidden fields among the normal input fields.

Therefore, information can be stored in hidden fields and thenretrieved from the request in the next page or servlet.

The concern is that the user can view the values of the hiddenfields by viewing the html source of the page from within thebrowser.

Example:out.print("


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A cookie is a small piece of information (name and value) thatis passed back and forth in the HTTP request and response.

The cookie sent by a servlet to the client will be passed back tothe server when the client requests another page from thesame application.

Cookies are represented as a class in the HTTP package.

To create a cookie you write such code:Cookie c1 = new Cookie("myCookieName", myValue");

Niether the name nor the value can contain commas,semicolons, white space, nor begin with a $ character.

Cookies contd

T dd ki t th


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To add a cookie to the response:

response.addCookie(c1);

To retrieve a cookie from the request:

Cookie[] cookies = request.getCookies();if(cookies != null)

{ for (int i=0; i


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Cookies must be added to the response before writing anyother response elements.

Advantages of cookies: The values of the cookies are not directly visible.

You don't need to use a form tag.

Cookies are not a serious security threat.

Concerns about cookies:

The user has the choice to set the browser to refuse cookies.

Privacy: if another user has access to the PC, then he may be able toview the values of stored cookies on the hard disk. Therefore, makesure not to write critical information in the cookie. (Anyway, theres no

security without physical security).

Cookies contd

You can detect whether a browser enables cookies or not and


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You can detect whether a browser enables cookies or not, anddisplay a message that asks the user to enable cookies.

To detect this, you can do the scenario of adding a cookie tothe response of the first servlet, then redirecting to a secondservlet that checks the existence of the cookie in the request.

To redirect the clients browser to another page you can use

either of the following: response.sendRedirect(..)

Note: in both cases you have to use a relative URL, not a fullyqualified one.

Cookies contd

A cookie can either be stored on session level or on


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A cookie can either be stored on session level or onpersistence level.

A session level cookie is a cookie that is stored in thebrowsers memory and deleted when the user quits the

browser.

A persistent cookie is stored on the clients hard disk. To

make your cookie persistent, use the methodsetMaxAge(int seconds) before adding it to the

response.

Example: c1.setMaxAge(60*60*24*7); // One week

Session Objects

Th b d f l i i bj


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The best and most powerful was is to use session objects(defined by the HTTPSession interface).

For each user, the servlet container can create anHttpSession object that is associated with that user only.

A session object acts like a Hashtable into which you canstore any number of key/object pairs.

It is accessible from other servlets or jsp pages in the same

application (per user).

Session Objects Basic Scenario


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Servlet1

Servlet2

1234

Browser

1

2 4

3

Session Objects Basic Scenario contd

The basic scenario of creating and then using a session object is


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The basic scenario of creating and then using a session object isas follows:

1. A session object is created by Servlet1 (using the getSession(..)method), where a unique session identifier is generated for it (e.g.jsessionid=1234).

2. Servlet1 implicitly sends the session identifier to the client browser.

3. When the client browser requests another resource from the sameapplication, such as Servlet2, the session identifier is implicitly sentback to the server and passed to Servlet2 in the request object.

4. For Servlet2 to access the session object of this particular client, it usesthe getSession(..) method which automatically retrieves thesession identifier from the request and gains access to the specificsession object associated with that session identifier.

Session Objects - Example

In doPost( ) method of the LoginServlet:


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In doPost(..) method of the LoginServlet:// After user enters correct login information

HttpSession session = request.getSession(true);session.setAttribute("loggedIn", new String("true"));

response.sendRedirect("ContentServletURL");

In doGet(..) method of the ContentServlet:

HttpSession session = request.getSession(false);if (session == null)

response.sendRedirect(LoginServletUrl);

else

{

String loggedIn = (String) session.getAttribute("loggedIn");

if (!loggedIn.equals("true"))

response.sendRedirect (loginServletUrl);

}

HttpSession

Common methods:


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Common methods: Enumeration getAttributeNames()

Object getAttribute(String key) void setAttribute(String key, Object value)

void removeAttribute (String key) // e.g. log out

long getCreationTime()

String getId()

long getLastAccessedTime()

void setMaxInactiveInterval(int seconds)

int getMaxInactiveInterval()


void invalidate() boolean isNew()

Invalidating a Session

A session can become invalid in 3 cases:


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A session can become invalid in 3 cases:

The default session timeout has elapsed (configured in the servletcontainer for all web applications).

The maximum inactive interval time of the session has elapsed (specifiedby calling the method setMaxInactiveInterval(..)).

The programmer decides to invalidate a session (by calling theinvalidate() method).


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1. Testing if browser enables cookies

Implement the cookie testing scenario by developing a


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Implement the cookie testing scenario by developing aservlet that adds a cookie to a response and then

redirecting to another servlet that attempts to retrieve thecookie.

If the user has disabled the use of cookies, send him an

error message asking him to enable cookies. Bonus

2. User login

Keep user s login information on the session and access it


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Keep user s login information on the session and access it

in another page

References & Recommended Reading


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Core Servlets and JSP.

Java for the Web with Servlets, JSP, and EJB.

Manning JSTL in Action.

Sun presentations.

Oracle presentations.

SCJWD study guide.

building web applications using servlets_java30

Documents