Begin Servlet

8/22/2019 Begin Servlet

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Servlet: Introduction


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New methods added in HTTPServlet10

HTTPServlet9

GenericServlet8Servlet Class hierarchy7

Servlet instance6

Servlet Life Cycle5

Servlet interface4

JSDK3

Servlet Request Response Mechanism2

Definition1

Contents


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Deploying20

Packaging19

Writing web.xml18

web.xml17

Brief introduction to XML16

Deployment Descriptors15

HTML file with a link to servlet14

Steps to deploy a web application13

A Simple Servlet12

Request and Response object11

Contents


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Getting single parameters30

HttpServletResponse Interface29

ServletResponse Interface28

HttpServletRequest Interface27

ServletRequest26

Request and Response hierarchy25

Execution24Deployment23

Steps for packaging22

Packaging and deploying the servlet21

Contents


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SingleThreadModel Interface34

Servlet variable initialization33Servlet to get multiple param32

Getting multiple parameters31

Contents


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Know

What a servlet is and its lifecycle

The Request Response Mechanism

Servlet classes

HttpServletRequest and HttpServletResponse

classes


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Be Able To

Write a simple servlet


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Definition

A servlet is a server side program writtenin Java that resides and executes in an

application server

It enables the delivery ofdynamic content


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Servlet Request Response Mechanism

Servlet

Database

Client Web Server

Application ServerRequest

Response

If request is for static pagereturn the page as response

ElsePass the request to application server


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JSDK

Java Servlet Development Kit (JSDK)contains the class library that is requiredto create servlets and JSP.

The 2 packages that are required to createservlets are javax.servlet andjavax.servlet.http.

A java class is a servlet if it directly orindirectly implements the Servletinterface.


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Servlet interface

public void init(ServletConfigconfig) throws ServletException

public void service(ServletRequestreq, ServletResponse res) throwsServletException,java.io.IOException

public void destroy()

public ServletConfiggetServletConfig()

public String getServletInfo()

Life cycle methods


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Servlet instanceServletInstantiation

New instancecreated on firstrequest

Initialized Servlet init() is called

Application Server

Request servicingServlet

service() is called

DestroyedServlet

destroy() is called

Initialization

Begin service

Destruction

Servlet Life Cycle

First request

Subsequent requests


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Servlet instance

Only one instance of a particular servlet

class is created. The same instance is

used for all the requests.

When the container shuts down, this

instance is destroyed.


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Servlet instance

The container runs multiple threads on the

service() method to process multiple

requests.

Only a single instance of servlet of each

type ensures minimum number of servlet

objects created and destroyed on the

server adds to scalability


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Servlet Class hierarchy

javax.servlet.GenericServlet

javax.servlet.http.HttpServlet

javax.servlet.Servlet


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GenericServlet GenericServlet class provides basic

implementation of the Servlet interface.

This is an abstract class. It implements all the

methods except service() method.


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GenericServlet Other methods included are:

public void log(String message)

public void init() throwsServletException

The implementation ofinit(ServletConfig config)callsinit()method.


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HTTPServlet

HTTPServlet inherits from theGenericServlet and provides a HTTP

specific implementation of the Servlet

interface.

The service(ServletRequest req,

ServletResponse res) method is

implemented.


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HTTPServlet

Since this implementation specific toHTTP protocol, there are methods thatspecific to each HTTP methods like GET,

POST, TRACE etc. It is an abstract class and is subclassed to

create an HTTP servlet suitable for a Website.


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New methods added in HTTPServlet

protected voidservice(HttpServletRequest req,HttpServletResponse res) throwsServletException, java.io.IOException

protected void doXXX(HttpServletRequestreq, HttpServletResponse res)

XXX is GET, POST, TRACE, PUT, DELETE

A subclass of HttpServlet must override

at least one method, usually one ofdoGet,doPost, doDelete, doPut, doHead

init and destroy to manage resources thatare held for the life of the servlet.

Our focus


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Request and Response object

Servlet A

Request forservlet A

Response

Client

Web Server

Application server

HTTP

requestpacket

HttpServletRequest Object

HttpServletResponse Object

Web Container

HTTPresponse

packet


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A Simple Servlet

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

publicclass GreetingServlet extendsHttpServlet{String welcomeMsg

=Welcome to simple servlet;java.util.Date currDate;

greet.war


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public void doGet(HttpServletRequestrequest,HttpServletResponse response)throws ServletException, IOException{

currDate=new java.util.Date();PrintWriter out = response.getWriter();response.setContentType("text/html");

out.println("Greetings Servlet );

out.println("+

welcomeMsg+
+currDate+");}}


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Steps to deploy a web application

Write servlets. Compile them.

Write html and jsp files.

Write deployment descriptor web.xml. Package the application

Deploy the application


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HTML file with a link to servlet

Greet

Simple ExampleClick to invoke the servletGreetings

index.html

We will map the servlet and the url shortly


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Deployment Descriptor

Deployment descriptors is an xml file that helps inmanaging the configuration of an application.

This is the file using which an application can

communicate with the container and vice versa.

For a web application, the deployment descriptor file

name is web.xml.

The Java Servlet specifies a document type

definition for deployment descriptor which isavailable at

http://java.sun.com/j2ee/dtds/web_app_2_2.dtd


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Brief introduction to XML

XML stands for EXtensible Markup Language.XML is a markup language much like HTML.

XML was designed to describe data. XML was

created to structure, store and to send

information.

XML is a cross-platform, software and hardware

independent tool for transmitting information.

XML tags are not predefined. You must defineyour own tags.

One of the uses of XML is to exchange data.


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web.xml Servlet components that are used in web

application along with their fully qualified class

names

URLs for servlets

Welcome file name

Initialization parameters if any

Session Configuration

Application Lifecycle Listener classes

Filter Definitions and Filter Mappings Error Pages

Environment Variable Mappings

Mime type mapping

Our focus in this chapter


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Writing web.xml

Greet

Greet

simple

GreetingServlet

Optional. IDEs use it for listing webapplication name

entifier for this servlet in the web appliaction

Fully qualified name for the servlet


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Greet

/greet.do

Append a / for tomcat

URL that is going to used to access thiscomponent

specify the identifier for whichcomponent url is going to be mapped

index.htmldefault.jsp

Optional because index.html is welcome file by defaul

index.html is not found, container will look for default.jsp.


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Packaging

Packaging application means placing thefiles in the appropriate placeholders

(directory).


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Packaging

Servlet specification lays out the rules of how

enterprise applications should be packaged.

This is necessary so that the container (any j2ee

container) knows where to find the files (theservlet class files, html class files etc.)

Since a web application is composed of many

files, the specification also tells us how to

archive the files and deploy it as single file

application.


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Deploying

Deploying means uploading theapplication on the server.

If there is any problem with the packaging,

the application will not be uploaded. There are many J2EE IDEs that help in

developing and packaging the application.


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Deploying

Most of the application servers support hotdeployment.

Most of the application servers support

start and stop of applications that aredeployed.


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Packaging and deploying the servlet

Web Application 1

Servlets

JSPs/HTML

JavaClasses

DeploymentDescriptor

(web.xml)

Web Application 2

Servlets

JSPs/HTML

Java

Classes

DeploymentDescriptor

(web.xml)

Components forming the web application.

xxx.war


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Steps for packaging

Create a folder.

Let us name the folder as slide1ex1.

Place htmls and jsps in folder

Place the index.html inside slide1ex1

Create a folder called WEB-INF.Created! Note the case is important here.

Place web.xml inside the folder

Placed!

Create a folder called classes inside WEB-INFand all the java classes created inside it.

Created and placed GreetingServlet.class.


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Deployment

As per j2ee specification, a jar file should becreated with the extension .war. This war filehas to be then deployed depending on theapplication server.

Jar Command to create war

path/simple:> jar cvf simple.war*.*

For tomcat, just copy the simple folder into thefolder called webapps.


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Deployment

Start the tomcat server. Type in the following url in the address bar http://localhost:8080/slide1ex1

/index.html Or simply

http://localhost:8080/slide1ex1

Server nameDefault protocol where tomcat ruprotocolFoldername


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According to j2eespecs, war file has to

be created. Fortomcat we dont doso. So, does this

mean that tomcatdoes not strictly

follow j2ee specs?

Tomcat followsj2ee specs! It just

gives you a simpleway to deploy yourapplication.

Another option todeploy would be toplace the war file


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Request and Response hierarchy

HttpServletResponse

ServletResponse

HttpServletRequest

ServletRequest


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ServletRequest

BufferedReader getReader()

ServletInputStream

getInputStream() int getContentLength()

Enumeration getParameterNames()

String getParameter(String name) String[] getParameterValues(Stringname)

Example ahead

Allows you to read raw bytes

or characters from the strea


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HttpServletRequestInterface

String getMethod()

String getHeader(String name)

Enumeration getHeaderNames() String getQueryString()

Cookies[] getCookies()

HttpSession getSession()

HTTPprotocolspecificmethods

Later


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ServletResponse Interface

void setContentType(String type)

void setContentLength(int len)

ServletOutputStream

getOutputStream() PrintWriter getWriter()

Allows us to writeinto the responsestream

We have seenthis !


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HttpServletResponseInterface

void addCookies(Cookie c)

String addHeader(String name,Stringvalue)

void sendRedirect(String url)

void sendError(int ecode)


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Getting single parameters

In the doGet() method we can get the value enteredby the user in the html form using:String booktitle=request.getParameter(name);

Locate Books

Default is GET

book.war


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Question?

What will happen if your form has a post

method, and the servlet that the form calls has

only doGet() method?

Getting multiple parameters


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Getting multiple parameters

Select Colors:
Red

Green
Blue


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Servlet to get multiple param

public class Colors extendsjavax.servlet.http.HttpServlet{public void doPost(javax.servlet.http.HttpServletRequestreq,javax.servlet.http.HttpServletResponseres)throws java.io.IOException,

javax.servlet.ServletException{java.io.PrintWriter out=res.getWriter();out.println("");


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Stringcolors[]=req.getParameterValues("color");

out.println("");for(int i=0;i


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Question?

What happens ifyou dont select

any of thecheckboxes?


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Servlet variable initialization

Initialization of servlet variables is generallydone in init() method.

In the normal class, initialization happens in theconstructor. But since the container calls the

constructor in this case, having parameterizedconstructor has no meaning.

The initialization values are passed to servletthrough DD and are available only from init()

method onwards. Therefore, the initialization is done in init()

method.Generally one never writes a constructor for a servlet!!

Si l Th dM d l I t f


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SingleThreadModel Interface In the servlet model that we have seen so far, a

single servlet processes multiple requestssimultaneously.

This means that the doGet and doPost methodsmust be careful to synchronize access to fields and

other shared data, since multiple threads may betrying to access the data simultaneously.

On the other hand, you can have your servletimplement the SingleThreadModel interface, as

below.public class YourServlet extendsHttpServlet implementsSingleThreadModel {...}

And say goodbye

to Scalability

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