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SERVLET

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Where are we?

1. Intro toJava, Course

2. Java lang. basics

3. Arrays

5. Encapsulation

4. ObjectClasses 7. Inheritance

8. Polymorphism

10. JDBC

6. ExceptionHandling

Introduction Object-oriented design Advanced topics

Newbie Programmers Developers ProfessionalsDesigners

JAVA

9. Abstract classesand Interfaces

13. Servlets

14. JSP

Server-side coding

12. Networking

11.Streams

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ConceptsFeatures

javax.servlet.Servlet

javax.servlet.http.HttpServletExample: get and send a page to clientExample: post

Image generatingSession tracking Cookie, HttpSession, URL-rewriting

Practical examples

Outline

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Todays buzzwords

Applets Java programs capable of running within a

web browser

Servlets Java code running on server side

Session tracking Keeping context information during a session

of browsing related web pages

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What is java servlet ?A servlet is a small Java program that runs within a Web server. Servletsreceive and respond to requests from Web clients, usually across HTTP,the HyperText Transfer Protocol. Servlet is an opposite of applet as aserver-side applet. Applet is an application running on client while servletis running on server.

Client Server

Request

Response

Servlet

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What is Java Servlets?

Servlet module run inside request/response-oriented server

BrowserJava-enabledWeb Server Servlet Database

HTML form

Server

Servlet

= BrowserApplet

Servlets are to Servers what Applets are to Browsers

Substitute for CGI scriptseasier to write and fast to run

Not part of the core Java Framework

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Example use of servlet

Processing data POST over HTTPs using HTML form as purchaseorder or credit card dataAllowing collaborative between people such as on-lineconferencing

Many servlet can chain together among Web servers to reduceload on one Web server.

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Why Use Servlets ?

One of the reasons that Java becamepopular is Applets.

but there are problems with Applets Browser compatibility Download bandwidth

Server-side Java the code is executed on the server side not the

client side a dynamically loaded module that services

requests from a Web server

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Servlets (contd.)

vs. Common Gateway Interface (CGI) create new process for each request

most platform independent CGI language - Perl start a new instance of interpreter for every request

CGI runs in a completely separate process from theWeb server

vs. Server-Side JavaScript only available on certain web servers

vs. Active Server Pages (ASP) only available on certain web servers

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CGI Communication (traditional approach)

Web server

Web browser

1. request

CGI profile file2 CGI Application(write result to CGI output file

and terminate)

3. execute CGI programand wait f or CGI to terminate

CGI output file4

5CGI terminated

1. Web browser requset a response from CGI program.2. Web server create CGI profile file that was contain information about CGI

variable, server and CGI output file.

3. Web server starts CGI application and wait for its termination.4. CGI program runs and writes the result to CGI output file and then terminates.5. Web server reads from CGI output file and send back to Web browser.

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Servlet Communication

Web browser request servlet byspecified URL ashttp://www.host.com/serlet/servletNa

meWeb server call service() method inServletLoader class which willdynamically load a specified servletname from a special directory callservlet.

Client

HTML

Form

Java-Enabled

Web Server

call

Servlet

Servlet

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Servlet vs CGI

Servlet run as light

weight thread in

process.

CGI run as heavy

weight process.

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Advantage of servlet over CGI

PlatForm IndependenceServlets can run on any platform. PERL also can be moved fromplatform to platform while CGI such as C are not portable.Performance

Servlets only needs be loaded once, while CGI programs needs tobe load for every request so that servlet should performs fasterthan CGISecurityWhile CGI can do many insecure things, java provided security in

language level.

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Three Tier Applications.

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Java Servlet Development

Kit installation

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Requirement for running java servlet

Java Servlet APIA set of java classes. This is a Java Servlet DevelopmentKit(JSDK) that can be download from http://java.sun.com JSDK allows most standard Web server such as Netscape servers,IIS, Apache and others to load servletsJava-enabled Web serverThere are many web server that support java virtual machinesuch as Java Web server by SUN, WebSite Pro V2.0 by OReillyand the latest version of World Wide Web Consortiums free

jigsaw Web Server.

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Servlet Lifecycle

Server loads Servlets- run init method

Servlets Accept Request fromClients and return Data back

- run service method

Server removes Servlets- run destroy method

No Concurrency Issue Server runs init only once, not per request

service must be thread-safe - multiple service method at a timeif that is impossible, servlet must

implement SingleThreadModel interface

Server reloads Servlets

- run init method

destroy must be thread-safe - when server runs destroy, other threads

might be accessing shared resources

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A Typical Servlet Lifecycle

from Servlet Essential 1.3.5 by Stefan Zeigerhttp://www.novocode.com/doc/servlet-essentials/

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Servlet class Example

Interface ServletDefine the standard way in which a networkserver will access a servlet. All servletclasses must (fundamentally) implementthis interface.

GenericServletDefines a generic, protocol-independent servlet. For servlet that isnot intended to use with Web server. Towrite an HTTP servlet for use on theWeb, extend HttpServlet instead HttpServletFor Servlet that is intended to use withWeb server. This class adds HTTP-specific to work with a Web servercontext

Servlet class

GenericServletclass

HttpServletclass

http://localhost/var/www/apps/conversion/java/servlet_doc/javax/servlet/http/HttpServlet.htmlhttp://localhost/var/www/apps/conversion/java/servlet_doc/javax/servlet/http/HttpServlet.html

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Servlet Structure

All servlet classes and interfaces that form the APIare found in the javax.servlet package. All servlets, no matter what type of server theyare destined to be used with, implement the

javax.servlet.Servlet interface. This interface definesthe basic functionality that a servlet must possess

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ServletConfig

When the servlet first starts up, the systemcan pass specific initialization informationto the servlet for possible processing via

the init ...) method.Parameters are passed in using akey/value pairing, and two methods areprovided to access this data: getInitParameter ...) and getInitParameterName ).

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ServletContext

allows the servlet to interrogate the serverabout various pieces of environmental dataExample :

Many servers, particularly web servers, refer tofiles and resources as virtual files. However, thismakes opening up files difficult, as the virtualfile or alias needs to be mapped onto a real file.The ServletContext interface, for example, providesa method to convert an alias and real path filenames.

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ServletContext

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GenericServletThe base class on which the majority ofservlets are based is the GenericServlet class.

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Interface Servlet class

void init( ServletConfig ) Initialized the servlet when it is loaded into server (system).

abstract void service( ServletRequest, ServletResponse ) This is an abstract class so that it must be implemented by a subclass ofGenericServlet class. A method that the servlet processeseach client request

destroy() This method is called when the server remove a servlet from memory.

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ServletRequest

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ServletResponse

The most important method of thisinterface is the getOutputStream ) method.This returns the ServletOutputStream, whichcan be used to provide any otherpreferred output stream class from the java.io package.

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GenericServlet : Example

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GenericServlet

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Web-Based Servlet

The developers at Java recognized thefact that the majority of servlets would beemployed in the field of the Internet, withparticular emphasis on the world wideweb. The servlet API offers you a directalternative to using CGI and Microsoft'sActive Server Pages (ASP) to implementserver-side solutions. To make codingservlets for the web relatively easy, aspecial HttpServlet class was developed.This provided methods to access themore common header fields found in theHTTP protocol.

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HttpServlet

The HttpServlet, based on the GenericServlet class.It provides an improved interface fordealing with HTTP-specific clientrequests.In addition to the service ...) method that isused to deal with all requests, sevenadditional methods exist for processing

requests; doGet ...), doPost ...), doHead ...),doPut ...), doTrace ...), doOptions ...) , anddoDelete ...).

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Servlet Architecture Overview - HTTP servlets

IS-A

Servletinterface

Servlet

Client

GET

POSTServlet

doGet(ServletRequest req, ServletResponse res);ServletRequest

ServletResponse

ServletResponnse

ServletRequestdoPost(ServletRequest req, ServletResponse res);

parameters, protocol, remote host,ServerInputStream(binary data etc..)

mime type to reply,reply data,

ServletOutputStream

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HttpServlet Interface

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ServletRequest Interface

This interface return information about parameter sent withthe request.

int getContentLength()

String getContentType() String getProtocol()String getScheme() String getServerName()

int getServerPort() String getRemoteAddr()

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HttpServletRequest :

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ServletRequest Interface (continued)

String getRemoteHost() String getRealPath( String path) ServletInputStream getInputStream() String getParameter( String name) String[] getParameterValues( String name) Enumeration getParameterNames() Object getAttribute( String name)

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ServletResponse Interface

This interface defined method for sending information back toWeb browser.void setContentLength( int len)void setContentType( String type)

ServletOutputStream getOutputStream()

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HttpServletRequest Interface

The method of HttpServletRequest interface inherits the methodin ServletRequest interface. GET, POST, and HEAD in HTTP arealso supported.

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Servlets (contd.)

HttpServletRequest information sent from the client

getParameter() getParameterValues() getParameterNames()

will discuss in detail later in FormPost HttpServletResponse

information sent to the client getWriter()

return an instance of PrintWriter you can use print and println with a PrintWriter see SimpleServlet

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HttpServletResponse Interface

String getMethod()String getServletPath()String getPathInfo()String getPathTranslated()String getQueryString()

String getRemoteUser()String getAuthTypeString getHeader(String name)String getIntHeader(String name)

long getDateHeader()Enumeration getHeaderNames()

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Writing the Servlet

Implement the javax.servlet.Servletinterface

HTTP: extends javax.servlet.http.HttpServlet

class

public class SurveyServlet extends HttpServlet {

/* typical servlet code, with no threading concerns* in the service method. */

...}

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Interacting with Clients(1)- HTTP servlets

doGet

doPost

doPut

doDelete

ClientGet

Post

PutDelete

HttpServletRequest - argument & HTTP header data String[] getParameterValues(String name) - get user parameter

For GET method- String getQueryString() For POST, PUT, DELETE method

- BufferedReader getReader() - text data - ServletInputStream getInputStream() - binary data

methods

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Example of an HTTP Servlet -GET/HEAD methods

public class SimpleServlet extends HttpServlet {public void doGet (HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {// set header field first

res.setContentType("text/html"); // then get the writer and write the response dataPrintWriter out = res.getWriter(); out.println ( SimpleServlet );out.println ( SimpleServlet Output );out.println ( );out.close();

}public String getServletInfo() {

return " A simple servlet ";} }

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Example (2) use ofHttpServlet

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Accessing web-based servlets

Assume the servlets' class file hasbeen placed in the server's servletsdirectory. The servlet can be called

by including /servlet/ before theservlet name in the URL.For example, if you set the classname of the sample servlet to TestServlet, then it could be accessedusing http:///servlet/TestServlet