basicservlet_1[1]

8/8/2019 BasicServlet_1[1]

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Servlet Basics


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What is A Servlet?

Servlets are pieces of Java source code that addfunctionality to a web server in a manner similar to theway applets add functionality to a browser.

Servlets are server-side software components, writtenin Java and run inside a server .

Servlets are loaded and executed within the JavaVirtual Machine (JVM) of the server, in much the same

way that applets are loaded and executed within theJRE of the Web client.

Works on request /response model


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Servlet


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Multitiered J2EE applications


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J2EE Components

The J2EE specification defines the following J2EE

components:

Application clients and applets are components that run onthe client.

Java Servlet and JavaServer Pages (JSP ) technology

components are web components that run on the server.

Enterprise JavaBeans (EJB ) components (enterprise beans)

are business components that run on the server.


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Where are Servlet and JSP?


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J2EE Container


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First Servlet Code

public class HelloServlet extends HttpServlet {

public void doGet(HttpServletRequest request,

HttpServletResponse response)

{

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

out.println("Hello World!");

}

...

}


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CGI versus Servlet


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CGI versus Servlet

Main Process

Java Servlet-based Web Server

JVM

Servlet1Servlet1

Servlet2Servlet2

Requ

est forServlet 1 Thread

Request for

Servlet 1

ThreadRequest for

Servlet 2

Thread


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CGI versus Servlet


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Advantages of Servlet

No CGI limitations

Abundant third-party tools and Web servers

supporting Servlet

Access to entire family of Java APIs

Reliable, better performance and scalability

Platform and server independent

Secure


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HTTP Basics

HTTP is a simple, stateless protocol .

A client makes a request and the web serverresponds.

For example:GET /intro.html HTTP/1.0

the client sends an HTTP command, a method, thattells the server the type of action .

also specifies the address of a document (a URL) andthe version of the HTTP protocol it is using.


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Http Basics cont..

After the client sends the request, the server

processes it and sends back a response.

For example :

HTTP/1.0 200 OK

the response specifies the version of theHTT

Pprotocol the server is using, a status code, and a

description of the status code.


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HTTP GET and POST

The most common client requests

HTTP GET & HTTP POST

GET requests:

User entered information is appended to the URL ina query string

Can only send limited amount of data

POST

requests: User entered information is sent as data (notappended to URL)

Can send any amount of data


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Demo


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GET Request


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POST Request


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Servlet Request and Response Model


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What are Servlets?

Servlets are programs that run on a Web server .

Act as a middle layer between a request coming from

a Web browser or otherHTTP client and databases or

applications on the HTTP server.


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What Servlet does?

Servlets job is to:

Read any data sent by the user.

Look up any other information about the request that is

embedded in the HTTP request. Generate the results.

Format the results inside a document.

Set the appropriate HTTP response parameters.

Send the document back to the client.


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Servlet Request and Response

model


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Requests and Responses

What is a request?

Information that is sent from client to a server

* Who made the request

* What user-entered data is sent* Which HTTP headers are sent

What is a response?

Information that is sent to client from a server

* Text(html, plain) or binary(image) data

* HTTP headers, cookies, etc


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Servlet API


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Servlet API

Servlets use classes and interfaces from twopackages :

Generic (protocol-independent) servlet classes:

javax.servlet.

HTTP specific servlet classes:

javax.servlet.http.


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Servlet Classes and Interfaces


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Servlet

Servlet does not have a main() method.

Each time the server dispatches a request to a servlet

it invokes the servlets service() method.


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Servlet API

Servlet Interface (javax.servlet.Servlet)

This interface has the servlet life cycle methods .

GenericServlet Class(javax.servlet.GenericServlet)

A protocol independent servlet should subclass

this servlet & should override the service()

method.

HttpServlet Class(javax.servlet.http.HttpServlet)

An abstract class that implements the service()method to reflect the HTTPness of the servlet .

The subclass should not override the service

method .


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Calling service() method


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Calling doGet() / doPost()


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Writing Hello World

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

public class HelloWorld extends HttpServlet {

public void doGet(HttpServletRequest req, HttpServletResponseres)throws ServletException, IOException {

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

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

}


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Servlet Life Cycle


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Servlet Life Cycle


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First Client Request

When the first client request comes :

Container creates the servlet instance .

Invokes its init() method .

Later, each user request results in a thread that calls theservice() method of the previously created instance.

The service method then calls doGet(), doPost(), or anotherdoXxxmethod depending on the type ofHTTP request itreceived.

Finally, when the server decides to unload a servlet, it first callsthe servlets destroy() method.


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Servlet Life Cycle


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Servlet Life Cycle methods

init() Executed once when the servlet is first loaded.

Not called for each request.

Perform any one time set-up code in this method.

service()

Called in a new thread by server for each request. Dispatches to doGet, doPost, etc.

doGet(), doPost(), doXxx()

Handles GET, POST, etc. requests.

Override these to provide desired behavior.

destroy()

Called when server deletes servlet instance.

Perform any clean-up, and save data to be read by the next init().


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service() method


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doGet() / doPost()


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First Servlet

import javax.servlet.*;

import javax.servlet.http.*;

import java.io.*;

public class HelloServlet extends HttpServlet {

public void doGet(HttpServletRequest request,

HttpServletResponse response)

throws ServletException, IOException {

response.setContentType("text/html");

PrintWriter out = response.getWriter();

out.println("First Servlet");out.println(Wel-Come!");

}

}


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Calling service method()

Calls the servlets service() method passing the

request and response objects as arguments .

Based on the Http method, service() method calls the

doGet()/doPost() method passing the request and

response objects as arguments .


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Sending response to client

Servlet uses the response object to write out to client

through the container .

The client gets the response .


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Servlet Request


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Servlet Request

Contains data passed from client to servlet .

All servlet requests implement ServletRequest interface which

defines methods for accessing Client sent parameters , Object-

valued attributes etc..


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Getting Client Sent

Parameters

A request can come with any number of parameters

Parameters are sent from HTML forms:

GET: as a query string, appended to a URL POST: as encoded POST data, not appeared in the URL

getParameter( paramName )

Returns the value of paramName

Returns null if no such parameter is present

Works identically for GET and POST requests


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HTTPServletRequest

Contains data passed from HTTP client to HTTP servlet

Created by servlet container and passed to servlet as aparameter ofdoGet() ordoPost() methods

HttpServletRequest is an extension ofServletRequestand provides additional methods for accessing

HTTP request URL

Context, servlet, query information

Misc.HTT

P Request header information Authentication type & User security information

Cookies

Session


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URI - Uniform Resource Identifier

Identifies the resource on host machine and

access method for that resource.

E.g. http://www.yahoo.com/index.html

3 parts

Scheme or protocol.

DNS name of the host.

File name on the host.


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HTTP Request URL

Contains the following parts

http://[host]:[port]/[request path]?[query

string]


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HTTP Request URL:

[request path]

http://[host]:[port]/[request path]?[querystring]

[request path] is made of

Context: /

Servlet name: /

Examples

http://localhost:7001/hello1/greeting

http://localhost:7001/hello1/greeting.jsp


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HTTP Request URL:

[query string]

http://[host]:[port]/[request path]?[query string]

[query string] are composed of a set of parameters andvalues that are user entered

Two ways query strings are generated A query string can explicitly appear in a web page

ClickHere

String bookId = request.getParameter( Add );

A query string is appended to a URL when a form witha GETHTTP method is submitted

http://localhost/hello1/greeting?username=Monica+Clinton

String userName=request.getParameter(username)


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Context, Path, Query,

ParameterMethods

String getContextPath()

String getQueryString()

String getParameter(String paramName)

String[] getParameterValues(String paramName)

Enumeration getParameterNames()


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HTTP Request

Client sends a request

Request method, URI, and protocol version.

Request headers

request modifiers, client information.

A blank line.

Optional body contents.


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HTTP Request Headers

HTTP requests include headers which provide extra informationabout the request

Example ofHTTP 1.1 Request:

host : localhost:8080

user-agent : Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;SV1; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; InfoPath.1;.NET CLR 3.0.04506.648)

referrer:http://localhost:8080/ServletProject/HandleRequestDataForm.html


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HTTP HeaderMethods

String getHeader(java.lang.String name)

value of the specified request header as String

java.util.Enumeration getHeaders(java.lang.String name)

values of the specified request header java.util.Enumeration getHeaderNames()

names of request headers

int getIntHeader(java.lang.String name)

value of the specified request header as an int


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Servlet Response


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What is Servlet Response?

Contains data passed from servlet to client

All servlet responses implement ServletResponseinterface

Retrieve an output stream

Indicate content type

Indicate whether to buffer output

Set localization information

HttpServletResponse extends ServletResponse

HTTP response status code

Cookies


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Responses


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Http Response

Server responds back with

Status line including message protocol version and astatus code.

Several message headers

server information, entity meta-information. Optional body contents.


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Header in HTTP Response


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Why HTTP Response Headers?

Give forwarding location

Specify cookies

Supply the page modification date

Instruct the browser to reload the page after adesignated interval

Give the file size so that persistent HTTP connectionscan be used

Designate the type of document being generated .

Methods for Setting Arbitrary


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Methods for Setting Arbitrary

Response Headers

public void setHeader( String headerName, StringheaderValue)

public void setDateHeader( String name, long millisecs)

public void setIntHeader( String name, int headerValue)

addHeader, addDateHeader, addIntHeader

setContentType

setContentLength

addCookie

sendRedirect


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Body in HttpResponse


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Writing a Response Body

A servlet almost always returns a response body

Response body could either be a PrintWriteror a

ServletOutputStream

PrintWriter

Using response.getWriter()

For character-based output

ServletOutputStream Using response.getOutputStream()

For binary (image) data


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Web Application Deployment


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Web Application

A Web Application is ,

a collection of servlets,JSP,HTML documents, imagesand other web resources that are setup in such a wayas to be portably deployed across any servlet enabled

web server.

Each Web Application consists of

Configuration file web.xml Static files and JSP

Class files.


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Tomcat Structure


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Apache Tomcat Server

Tomcat can handle Web pages, Servlets, and JSPs

Includes a servlet container used for testing and

deploying servlets .

Tomcat is open source (free)


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Configuring Tomcat

By editing server.xml (conf/server.xml), we canchange the port where Tomcat will listen forHTTPrequests.

By default the port is 8080.


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Web Application : Static parts


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Web Application : WEB-INF


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WEB-INFDirectory

web.xml :

Web application deployment descriptor that contains meta-data forthe web application.

used by the container while loading the web application.

File is always located in the WEB-INF directory of the webapplication.


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web.xml and WEB-INF

classes :

A directory that contains server-side classes: servlets,

utility classes, and JavaBeans components

lib :A directory that contains JAR archives of libraries (tag libraries

and any utility libraries called by server-side classes)

The WEB-INF directory is kept hidden from the users of the Webapplication.


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WEB-INF : Forbidden

Steps for creating a web


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Steps for creating a web

application Create a directory called MyWebapp in the /webapps folder of your

Tomcat

Create a sub-directory WEB-INF inside MyWebApp.

Inside WEB-INF create a web.xml file as shown below :

Create a classes sub-directory to place the serverside classes .


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JAR and WAR

Java Archive file is a convenient method for

consolidating a set of Java classes into one

compressed portable file.

Any JAR file in the WEB-INF/lib directory of a web

application is made for use by the code in the same

web app.


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WAR

Web Application Resource files are similar to JAR files but are

used to consolidate the entire web application into one

compressed portable file.


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ServletConfig Interface


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init() method

init() method is used to perform servlet initialization .

There are two versions of init:

one that takes no arguments one that takes a ServletConfig object as an argument

The first version of init looks like this:

public void init() throws ServletException {// Initialization code...

}


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init(ServletConfig config)

The second version of init is used when the servletneeds to read server-specific settings before it cancomplete the initialization.

The second version of init looks like this:

public void init(ServletConfig config)throws ServletException {super.init(config); //this is necessary// Initialization code...

}


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Which init to use ?

If the init(ServletConfig config) is used then it isimportant to have super.init(config) as its first line. You can use this.servletconfig = config to store the

ServletConfig object.

The no-arg init() method is a convenience methodprovided by the API.

You can use getServletConfig() method to get theServletConfig object.


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init method

ServletConfig parameter used in second version providedconfiguration information to the servlet.

super.init(config) call passes the config object to theGenericServlet

The GenericServlet class use the config parameter to implementthe ServletConfig Interface .

Thus a servlet can invoke ServletConfig methods.


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init method contd..

GenericServlet class also supports no-arg init() method .

When the first version , no-argument init() method isimplemented:

ServletConfig and GenericServlet handling is taken care inbackground .

Web server still calls a servlets init(ServletConfig config)method at initialization time .

GenericServlet then passes the call to no-arg overridden init()method


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ServletConfig methods

ServletConfig methods String getInitParameter(String name)

Enumeration getInitParameterNames()

ServletContext getServletContext()

String getServletName()

Assigning Servlet Initialization


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Assigning Servlet Initialization

Parameters

InitTest

moreservlets.InitServlet

firstName

abc

[email protected]

Reading Servlet Initialization


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Reading Servlet Initialization

Parameters

public class InitServlet extends HttpServlet {

private String firstName, emailAddress;

public void init() {

ServletConfig config = getServletConfig();firstName =config.getInitParameter("firstName");

emailAddress =config.getInitParameter("emailAddress");

}

public void doGet() { }}


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ServletContext class

S l tC t t


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ServletContext

Context init parameters are available to entire web application.

Any servlet in the application has access to the context init

parameter.

There is one ServletContext object per web application .

S tti t


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Setting parameters

web.xml element: context-param

support-email

[email protected]

Invoking context parameter from a servlet :

ServletContext context=getServletContext();

out.println(context.getInitParameter(support-email ));


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Including, Forwarding, Redirecting

to other Web resources

Including Forwarding Redirecting


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Including, Forwarding, Redirecting

to other Web resources

You do not want to deal with the response rather want another

web component to handle the request ?

There are two options to follow :

Redirect the request to a completely different URL.

Dispatch the request to some other component in the web

application .


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Redirecting the Request

A client types a URL into the browser and a servlet is located :

Suppose the servlet decides to send the request to a completely

different URL .

It calls sendRedirect(NewURL) on the response . the client gets the response and makes a new request for the

NewURL .

The specific web component is called ,the request gets fulfilled

and the response is send back to the client.

Redirect makes the browser do the work.


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Redirecting a Request

Programming model for directing a request

public void sendRedirect(String url)

if(doTheWork){

//handle the request

} else {

response.sendRedirect(http://www.oreilly.com);

}

You can use Relative URLs You cant do a sendRedirect() after writing to the

response


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Relative URLs

Two waysOriginal :

http://www.wickedlysmart.com/myApp/cool/bar.doIf in Servlet (without forward slash(/ )):

sendRedirect(foo/stuff.html);

Container builds the full URL as :

http://www.wickedlysmart.com/myApp/cool/foo/stuff.html

OR

If in Servlet (with forward slash(/)):sendRedirect(/foo/stuff.html);

Container builds the full URL as :

http://www.wickedlysmart.com/myApp/foo/stuff.html

U i R tDi t h


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Using RequestDispatcher

A client types a URL into the browser and a servlet is located :

Suppose the servlet decides to send the request to a completelydifferent URL .

It calls :RequestDispatcher view =

request.getRequestDispatcher(abc.jsp);

view.forward(request,response);

abc.jsp takes over the response .

The client gets the response in usual way , the user does not knowthat the JSP generated the response .


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RequestDispatcher

Request dispatch does the work on the server side i.e.

transparent to the browser .

The method getRequestDispatcher(String path) in class

ServletRequest helps dispatching the request

public RequestDispatcher getRequestDispatcher(String path)


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RequestDispatcherMethods

void forward(ServletRequest request,

ServletResponse

response)

void include(ServletRequest request,

ServletResponse

response)

Forward with RequestDispatcher


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Example

public void doPost(HttpServletRequest req, HttpServletResponseres)

{

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

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

if( userid != null && password != null &&password.equals(users.get(userid)) )

{

req.setAttribute("userid", userid);

ServletContext ct = getServletContext();

RequestDispatcher rd =

ct.getRequestDispatcher("/servlet/AccountServlet");rd.forward(req, res);

return;

}

else


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RequestDispatcher Example

Cont..

{

RequestDispatcher rd =

req.getRequestDispatcher("../login.html");

rd.forward(req, res);return;

}

}


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Including another Web

Resource

When to Include another Web


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When to Include another Web

resource?

When it is useful to add static or dynamic

contents already created by another web

resource

Adding a banner content or copyright information inthe response returned from a Web component


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Example: Including a Servlet

RequestDispatcher dispatcher

getServletContext().getRequestDispatcher("/banner");

if (dispatcher != null)

dispatcher.include(request, response);

}


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Scope Objects

S Obj t


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Scope Objects

Enables sharing information among collaborating web

components via attributes maintained in Scope objects

Attributes are name/object pairs

Attributes maintained in the Scope objects are

accessed with

getAttribute() & setAttribute()

3 Scope objects are defined

Web context, session, request

S Obj t Cl


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Scope Objects : Class

Web context (ServletConext)

javax.servlet.ServletContext

Session

javax.servlet.http.HttpSession

Request

subtype of javax.servlet.ServletRequest:

javax.servlet.http.HttpServletRequest


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Methods for Scope Objects

Object getAttribute(String name)

void setAttribute(String name, Object value)

Enumeration getAttributeNames();

Example: Getting Attribute Value


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Example: Getting Attribute Value

from ServletContext

ServletContext context=getServletContext();

Context.setAttribute(ContextAttribute , SomeVal);

String val=

(String)context.getAttribute("ContextAttribute");

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