Suggested readingSuggested readingkarens/ServletsPartIOctober2010Introduction.pdfSuggested readingSuggested reading • Heads First Servlets and JSP Passing the Sun Certified Web Component

Suggested readingSuggested reading

Department of Electronic Engineering 1

Course contentCourse content

• How to write distributed and networked applications• Client server programming – browsers and web servers

• Servlets & JSPs

• Client server programming – specialised clients and serversservers

• Peer to peer programs• JavaScript for client side manipulation

• Ajax





• Heads First Servlets and JSPPassing the Sun Certified Web Component Developer

ExamBryan Basham, Kathy Sierra & Bert BatesO’R ill A t 2004O’Reilly August 2004


How to install TomcatHow to install Tomcat

Look at Resource Lecture Karen Shoopalso in Heads First book

http://www.moreservlets.com/Using-Tomcat-4.htmlp g


Java Servlets and the HTTP Protocoland the HTTP Protocol

• What are servlets and why are they useful?• Basic servlet structure and lifecycley• Handling request data• HTTP request headersq• Generating the HTTP response• Session trackingSession tracking


A general purpose Web Server –e g Apachee.g. Apache

Uses environment variables

Function call

URL & Web Server cgi program

request container e.g Apache

HTTP request

Tomcat container/plug in

response page

HTTP response

Uses java method call on an object page

php container/plug in

p call on an object of type Servlet


php scriptStill HTTP

Calling (or browsing) the servlet/cgi/scriptservlet/cgi/script

How to findIt is a servlet

• e.g. http://localhost:8080/servlet/myhelloservlet.HelloServlethttp://localhost:8080/servlet/myhelloservlet.HelloServlet

• calling servlets/cgis/scripts differs from page browsing because you're executing a method of a servlet class instance not just looking at a page. • i.e. you are executing a program .... that usually returns a page

• The servlets/CGI/script handle form input calculations and• The servlets/CGI/script handle form input, calculations and database queries.• JSP is often used to format the results when using servlets.


g• Note: The above URL is simplified and only indictive

Why are dynamic pages useful?Why are dynamic pages useful?

• The response web page is based on data submitted by the user, or derived from data that changes frequentlyfrequently• Can use information from corporate databases or other

server-side sources• In principle, servlets could be used for requests

other than via HTTP, though this is not common tyet.

• But e.g SIP servlets are becoming (a little) more common


UUses

C ti “ h f t”• Creating an e-commerce “shop front”• programmers use servlets in conjunction with JSP to create

clearer and simpler applications.clearer and simpler applications.

• Providing web interfaces to legacy and data base systemsy• Avoids re-engineering existing system architecture

• Can obtain access through firewalls by “HTTP g ytunnelling”

• c.f. RMI


Data is collected and submitted by using an HTML formusing an HTML form

• Each element of a form can have a name and a value• This includes a submit button

• When the submit button is pressed the name value pairs are• When the submit button is pressed the name value pairs are sent to the web server at the URL named in the action part of the form• If a name is not provided the name value pair is not sent• If a name is not provided the name value pair is not sent• Names and values are just strings of characters

• AJAX allows data to be sent using a JavaScript function• Not dealt with in this part of the course – later• So for the moment data is only submitted using a form

• Or a hyperlink, where the name value pairs are hard coded


The entire HTML file downloaded so that data can be submittedso that data can be submitted

<HTML><HEAD>… <TITLE> </TITLE.</HEAD><BODY><BODY>

<FORM….>

/ O</FORM></BODY><HTML>


<HTML>

ExampleExample

<FORM METHOD=“GET” ACTION=“http://www.eecs.qmul.ac.uk/cgi-bin/cgiprogram”><P> E mail<P> E-mail<INPUT TYPE=“text” NAME=“visitor-email”><P>ComputerP Computer<INPUT TYPE=“radio” NAME=“computer”

value=“Mac”>Macintosh<INPUT TYPE=“radio” NAME=“computer” value=“PC”>Windows<INPUT TYPE=“submit” NAME=“submit” value=“Send Info”>

/FORM


</FORM>

If the method is a GET methodIf the method is a GET methodA domain name that has been

registered using DNS

The URL

http://www.eecs.qmul.ac.uk/cgi-bin/cgiprogramis modified by adding the name value pairs

http://www.elec.qmw.ac.uk/cgi-bin/[email protected]&computer=Mac&submit=Send+InfoIf POST th l i dd d ft th t h dIf POST the name value pairs are added after the request header ……………..later The server

determines the


interpretation of the path field and beyond

How CGI worksHow CGI works

• Browser requests a cgi program by giving the CGI program’s URLBrowser requests a cgi program by giving the CGI program s URL• started by submitting a form or clicking a link

• Web server checks whether URL refers to a CGI program• usually identified by the path (e.g. contains cgi-bin) or • filename extension (e.g. .cgi or .pl)

• Server executes CGI program as a separate process and passes any• Server executes CGI program as a separate process and passes any data included in the URL to the program (or in the POST body)

• Output is returned to the Web server which is passed to the browser

CGIserverURL request data of request


browserCGI program

outputresponse

Request stored in Environment VariablesVariables

• All cgi programs can receive data from web servers via environment variables

• Used to define an environment for a program’s execution• are global with respect to the program• around 20 of them, e.g.

• PATH INFO• PATH_INFO• QUERY_STRING stores the name value pairs


Summary: The GET method and CGICGI

• If cgi URL is:• If cgi URL is:http://www.elec.qmw.ac.uk/cgi-bin/cgiprogram

the name value pairs are url encoded and appended to the URL and sentthe name value pairs are url encoded and appended to the URL and sent to the server ashttp://www.eecs.qmul.ac.uk/cgi-bin/cgiprogram?visitor-email=John Bigham@elec qmul ac uk&computer=Mac&[email protected]&computer=Mac&submit=Send+Info

• The server then sets the QUERY STRING environment variable for the Q _process running the cgi program to the name value pairs• the cgi program accesses this variable and parses it


An HTTP GET message requestDoesn’t want to use

i

An HTTP GET message request

persistent connectionsGET /somedir/index.html HTTP/1.1

Host: www.chamelion.elec.qmw.ac.ukqConnection: closeUser-agent: Mozilla/4.0

A Netscape browserg

Accept-language:frextra carriage return and line feed

browser

Prefer to receive a

extra carriage return and line feed


French version if such a version exists

HTTP t f t

If HEAD , then similar to POST, but

HTTP request message formatserver omits requested object in

response

method sp spURL version cr lf

H d

Request line

Header field name value cr lf:

Header lines


……..

cr lffield name


Entity body: form name value pairs if POSTNot used if GET

A response messageServer is going to

close the TCP i

p g

HTTP/1 1 200 OKRequest succeeded and

the info is in the response

connection

HTTP/1.1 200 OKConnection : closeDate: Fri, 17th September 2010 12:01:14 GMT

the info. is in the response

Server: Apache/1.3.0 (Unix)Last-Modified: Thur, 16 September 2010 08:44:01 GMTC t t L th 5993Content-Length: 5993Content-Type: text/html Important for caching

(both client and network)C di i l GET

(d d d )

Conditional GETThe official indicator of type, not the file extension


(data data data…) entity body

F t f400 Bad

RFormat of response message

Status

Request:

version sp spStatus code phrase cr lf

Header l cr lf:

Status line

field name value cr lf:Header lines

……..Set-cookie12345


12345

cr lf


Entity body

What are servlets?What are servlets?

• Servlets and JSPs are Java’s equivalent to CGI and• Servlets and JSPs are Java s equivalent to CGI and ASPs.• Are programs that run on web server acting as middle layerAre programs that run on web server acting as middle layer

between HTTP requests and databases or other applications.

• Used to generate dynamic web in response to client.

S l tRequest

Java

pages Tomc

Web

Serv

ServletResponse

Javacat b

ver

HTTP

Department of Electronic Engineering 22Web Browser

Web Server

With Apache and TomcatWith Apache and Tomcat

ServletHttpRequest

Java

Web

Serve

Tom

HTTP

ServletHttpResponse

b er

HTTPm

cat

Web BrowserWeb Server


Beans, JSP and Servlets

• Although a servlet can be a completely self-contained program, to ease server-side programming, generating content should be split into:into: • The business logic (content generation), which governs the relationship

between input, processing, and output • The presentation logic (content presentation or graphic design rules) which• The presentation logic (content presentation, or graphic design rules), which

determines how information is presented to the user • Typically,

• the servlet handles the HTTP protocol and coordination of which other servlets• the servlet handles the HTTP protocol and coordination of which other servlets & auxiliary class methods to call

• Java classes/ beans the business logic (model),• Java Server Pages the presentation logic (view)


• Java Server Pages the presentation logic (view)

Advantages of servlets over CGI (1)Advantages of servlets over CGI (1)

• Efficient• Servlets run in JVM. Each request is serviced using a thread rather

than a new process (lower overhead)than a new process (lower overhead)• Though some scripting languages e.g. PERL on certain web servers

do this now

• Convenient• Convenient• Provides infrastructure that parses and decodes HTML forms

• Powerful• Can communicate directly with web server• Multiple servlets can share database connections• Simplifies session tracking


• Simplifies session tracking

Advantages of servlets over CGI (2)Advantages of servlets over CGI (2)

• Portable• Written in Java and follow standard API

• Secure• CGI often executed using O/S shells, which can cause

many security breaches• Array checking & exception handling automatic in Java

I i• Inexpensive• Many Java web servers freely available


A general purpose Web Server –e g Apachee.g. Apache

Servlet URL W b S

service() methodcgi

servlet

& requestrequest request

response

Web Server container

servlet

response page

response

phppage


Servlet container, or engine app

Tomcat “plug in”

What servlets doWhat servlets do

R d d t t b th ( li it)• Read any data sent by the user (explicit)• Look up info embedded in HTTP request (implicit)• Generate results• Format results inside a document Format HTML or XML or GIF or Excel..

• Set appropriate HTTP response parameters• Send the document back to the client


Typical generic servlet codeTypical generic servlet code

public class AnyServlet extends GenericServlet {

public AnyServlet() {} // DO NOT write constructor –USE DEFAULT

bli id i it(S l tC fi fi ) th

ONLY creates an object.

ONLY becomes a “proper” servlet after init

public void init(ServletConfig config) throwsServletException;

// The method actually called by container when servlet is first created or loadedfirst created or loaded

public void service(ServletRequest req, ServletResponse res)

throws ServletException, IOException;// called by a new thread in the container each time a //request is received


public void destroy();// called before servlet is to be destroyed

}

Servlet is “deployed” in a container which iscontainer, which is…..

• a program that receives (e.g. HTTP) requests to servlets p g ( g ) qfrom a web server application • finds the servlet (and loads, calls constructor & init if not ready)• creates or re-uses a thread which will call the service method ofcreates or re uses a thread which will call the service method of

chosen servlet• creates & passes request and response objects to the chosen servlet • passes the response (e.g. HTTP response) back to the web server p p ( g p )

app; kills servlet thread or recycles into thread pool; and deletes request and response objects

• More generally, manages the life cycle of its servlets• Calls constructor, init, service, destroy• also invokes methods of listening classes that a servlet implements

• It has a “main” and is working “all the time”


It has a main and is working all the time

Also providesAlso provides

• declarative security using settings in Deployment Descriptor

• JSP support


The servlet life cycle

Call constructor

The servlet life cycleat first request or

server start up (depending on web server

configuration)

Called once onlye.g. Can be used

Exists but not ready for

work

init(..)

doGet(..)

doDelete(..)configuration) g

for opening a DB connection

waiting for client requests

request

GET

doOptions(..)

i i

Should be thread safe

service(..) doPost(..)POST

doPut( )

on receiving request

doTrace(..)

doPut(..)

destroy(..)at server shutdowne.g. Can be used for closing a DB


closing a DB connection

can now be g.c.

If HTTPServlet

Mapping names using the DDMapping names using the DD

F h l t i th b <web-app ….><servlet>

<servlet-name> </servlet-name>

For each servlet in the web application

<servlet name>….</servlet name><servlet-class>…</servlet-class>

</servlet>< l t i >

maps internal name to fully qualified class name.

(except without .class)<servlet-mapping>

<servlet-name>….</servlet-name><url-pattern>…</url-pattern > maps internal name to public

URL name</servlet-mapping>………………….</web-app>

URL name

e.g. /makebooking


</web app>Internal name can be “anything”

following XML rules

Lots m

ore g

oes h

ere

A complete servletA complete servlet

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

GenericServlet

HttpServlet

service(..)

service(..)

public class S1 extends HttpServlet{public void doGet(HttpServletRequest req

HttpServletservice(..)

public void doGet(HttpServletRequest req,HttpServletResponse res) throws IOException {

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

out.println(“<html><body>Hello!</body></html>”);}}


}

Why we do not need to write a service method for an HttpServletservice method for an HttpServlet

serviceGenericServlet

Over rides

serviceHttpServlet

Class objects

service in GenericServlet

doGetS doGet

doPostS1

Department of Electronic Engineering 35instance

service(…)

Its DDIts DD<web-app xmlns=“http://java.sun.com/xml/ns/j2ee”

xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance”xsi:schemaLocation=“http//java.sun.com/xml/ns/j2ee”web-app_2.4.xsd”version=“2.4” >

<servlet> <servlet-name>Hello World Servlet</servlet-name><servlet-class>S1</servlet-class>

</servlet><servlet-mapping>

This example is so simple

there is no <servlet-name>Hello World Servlet</servlet-name><url-pattern>/Hello</url-pattern >

</servlet-mapping>

input data


pp g</web-app> http://localhost:8080/Hello

Typical HTTPServlet codeTypical HTTPServlet code

import javax servlet http *;import javax.servlet.http.*;import javax.servlet.*;

public class AnyHttpServlet extends HttpServlet {public class AnyHttpServlet extends HttpServlet {

public void doGet(HttpServletRequest req, HttpServletResponse resp)p p p

throws ServletException, IOException;// called when HTTP request uses GET

public void doPost(HttpServletRequest req, HttpServletResponse resp)

throws ServletException, IOException; // ll d h HTTP t POST


// called when HTTP request uses POST}

Handling the Client Request:HTML Form DataHTML Form Data

• Form data (or query data) is used to transfer information to the server-side program via POST or GET methodsor GET methods.

• Servlets have built-in features to parse this data• no need to extract attribute-value pairs AKA key-value pairsno need to extract attribute-value pairs• no need for URL-decoding (much nicer than CGI!).

• ServletRequest methods:

AKA key value pairs

q• String getParameter(String);• Enumeration getParameters();St i [] tP t V l (St i )


• String [] getParameterValues(String);

A Small FormA Small Form


Obtain single parametersSmallForm htmlSmallForm.html

<html><title>Sending Form Information to a Servlet</title>g<body>

<form action="http://localhost:8080/servlet/elem004 ProcessSmallForm"action="http://localhost:8080/servlet/elem004.ProcessSmallForm" method="post">

Please input your login: <br> NB. Can use absolute<input type="text" name="Login"><input type="submit" value="Login">

</form>

NB. Can use absolute or relative URLs or pre-configured names

</form></body></html>


Extract fromProcessSmallForm javaProcessSmallForm.java

public void doPost( ) {public void doPost(…) { …// obtain the login from the Request objectString loginName = req.getParameter("Login");

out.println("<html><head>");out.println("<title>Form Processed!</title></head>\n");out.println("<h1>Hello, ");

if(loginName != null)out.println(loginName);

elset i tl (" t ")out.println("mystery person");

out.println(“</h1></body></html>");out.close();


()} NB Almost but not complete –

need PrintWriter and need to setContentType

After form is processedAfter form is processed


Simple exampleSimple example

CounterServlet dproduces a page

displaying the number of requestsnumber of requests the servlet has processed since it pwas first created.


CounterServlet’s dutiesCounterServlet’s duties

• Must keep track of how many requests it has serviced, so …• when created must load previous value• when destroyed must save current value

• For each request, increment the counter.• Generate some HTML to display the results.• Will be in a file called CounterServlet.java


countVal is an instance variablecountVal is an instance variable

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

It is important that this is an int not e.g. a doubleimport java.io. ;

public class CounterServlet extends HttpServlet {i t tV l C it h d h

g

int countVal;

public void init() …….

Can use it here and here

public void doGet(HttpServletRequest req, HttpServletResponse resp) {..}


HttpServletResponse resp) {..}

CounterServlet java (1)CounterServlet.java (1)

public void init() throws ServletException {public void init() throws ServletException {

try { // to read in value of counter

BufferedReader countFile = Text open("count txt");BufferedReader countFile = Text.open( count.txt );

String valString = countFile.readLine();if(valString != null)

countVal = Integer parseInt(valString);countVal = Integer.parseInt(valString);else

countVal = -9999;System.out.println("Counter Servlet initialised, ");System out println(" countVal is " + countVal);System.out.println( countVal is + countVal);countFile.close();

}catch (..) { // ..etc }

}


}


public void doGet(HttpServletRequest req HttpServletResponse res)public void doGet(HttpServletRequest req, HttpServletResponse res)throws ServletException, IOException {// just refer to doPost(..)doPost( req, res );

}}

public void doPost(HttpServletRequest req, HttpServletResponse res)throws ServletException, IOException {

res.setContentType("text/html");

// obtain output stream (ie what will be displayed in browser)PrintWriter out = new PrintWriter(res getOutputStream());

Note default is plain text

PrintWriter out = new PrintWriter(res.getOutputStream());

out.println("<html>");out.println("<head><title>Request Counter Servlet</title>");out println("</head><body>");


out.println( </head><body> );


countVal++; // counter incremented if new sessioncountVal++; // counter incremented if new session

out.println("<b>This servlet has processed " + countVal+ " request(s)</b><br>");

out.println("</body></html>");out.close();

}

public void destroy() { // overrides GenericServlet.destroy(..)try { // save countVal back to file

PrintWriter countFile = Text.create("count.txt");countFile println(countVal);countFile.println(countVal);System.err.println("Request counter written to file");countFile.close();

}catch( ) { // etc }


catch(..) { // ..etc }}

Servlet initialisation & Servlet Configuration objectConfiguration object

NB. Need to prevent race conditions (or use single

th d d l d t d)• Only one servlet instance is created • each request serviced by a separate thread in container

• Prior to initialisation the ServletConfig object created by the container

thread model - deprecated)

container• One ServletConfig object per servlet• Container uses it to pass deploy time information to the servlet

• Facts you do not want to hard code into the servlet, e.g. DB name• The names are specified in the DD

• Parameters are set in a server-specific manner, e.g.• in Tomcat in a file called web.xml• in Resin in a file call resin config• in Resin in a file call resin.config

• Parameters do not change while servlet is deployed and running• Like constants• If change need to redeploy


Example of init parameters in a DD (web xml for tomcat)(web.xml for tomcat)

<servlet> <servlet-name>Hello World Servlet</servlet-name><servlet-class>S1</servlet-class>

<i it ><init-param> <param-name>lecturersEmail</param-name>< param-value >john@elec qmul ac uk</ param-value >< param value >[email protected]</ param value >

</ init-param ></servlet> Container reads these and gives to


gServletConfig object

Getting a parameter value from the ServletConfig objectthe ServletConfig object

out.println(getServletConfig().getInitParameter(“lecturersEmail”)g g() g ( )

Returns the servlet’s ServletConfig objectServletConfig object

All servlets have this method


You usually write an init method with no parameterswith no parameters

public void init() throws ServletException {

……………… YOU HAVE SEEN THIS

}• This is called when the system calls the inbuiltThis is called when the system calls the inbuilt

init method with one parameter


..but there is an old version of the init methodinit method

public void init(ServletConfig config) throws ServletException {

super.init(config); …………………………..}

• if you write this method must call super.init(…) firsty p ( )

• as you need to ensure the inbuilt init method with the corresponding parameter is invoked

• Was used as a way of getting out hands on the ServletConfig y g g gobject

• Not used much now as can get the ServletConfig object• by using getServletConfig().


by using getServletConfig().

Extracting initialisation variablesExtracting initialisation variables

• The ServletConfig object contains parameter’s• The ServletConfig object contains parameter s from the server’s configuration file • config object retrieved using getServletConfig()co g objec e eved us g getSe etCo g()

method

• Parameters are read from the configuration file and are stored by init method for later use • are extracted from config in a portable way by

S i i (S i )String getInitParameter(String)


Typically you would writeTypically you would write….

public void init() throws ServletException {String initPath= g

getServletConfig().getInitParameter(“count_file”);try { // to read in value of counter

BufferedReader countFile =Text.open(initPath);p ( );………

}


}

• In DD<servlet>

< l t >Th C ti S l t</ l t ><servlet-name>The Counting Servlet</servlet-name><servlet-class> CounterServlet </servlet-class>

<init-param> <param-name>count_file</param-name>

l / l< param-value >count.txt</ param-value ></ init-param ></servlet>


</servlet>

Location of servletsLocation of servlets

• Depends on the web server you’re using but for Tomcat and Resin, servlets should be put in:

/WEB-INF/classes



This is %CATALINA HOME%


This is %CATALINA_HOME%



All your web applications go in here


All your web applications go in here

Inside webappsInside webapps

• Here are two web applications• Usually many morey y



Inside the Chaining2005 web applicationapplication

Servlets go in the classes folder in

here


HTML & JSP files go here

What there is in the ITLWhat there is in the ITL

• You do not see webapps! It is shared by all students.

• You create a directory inside webapps. • I have called mine johnb – my user name

Path in webapps

• To invoke the downloading of an html file held in johnb directory

e.g. http://tomcat_stu.dcs.qmul.ac.uk/ johnb/Currency.htmlMust contain servlet

because of way Apache is configured

• To invoke a servlet

http://tomcat_stu.dcs.qmul.ac.uk/ johnb/servlet/CC

Y dd l f ld h l d j fil i j h b d di h i i b l

Apache is configured

Apache

• You can add a classes folder, html and jsp files in johnb and edit the existing web.xml

NB You cannot create subdirectories each with their own web.xml!

<servlet-mapping>

<servlet-name>My Servlet</servlet-name>


y

<url-pattern> /servlet/CC </url-pattern >

</servlet-mapping>

input htmlinput.html

It i l thi b f <html><head><title> Input book isbn </title> </head><body>

It is only this because of what I have in the

web,xml filey

<form method="POST" action=

"http://localhost:8080/Chaining2005/CH">

<input type="text" name="isbn" value=""><input type="submit" >

The name I have given for the

servletp yp</form></body></html> The web app


Inside WEB INFInside WEB-INF


The DD

N t f ll Note: full name starts from classes following package

structure


Inside classesInside classes


Inside chainInside chain


Inside classes/chain/webInside classes/chain/web


Inside classes/chain/modelInside classes/chain/model


Keep your servlet code in a similarly structured foldersimilarly structured folder


Where we keep the java source for compilationcompilation

h d f h

The corresponding folder

The code for the Chaining2005 web

app


The corresponding folder

Copy into tomcat class di tdirectory

Compile creates the directory structure

This has same structure as classes seen before but

with source code

Department of Electronic Engineering 76aide memoir

Remember - same structure



Compiling from Chaining2005 DLMDLM

javac -classpath C:/jakarta-tomcat-5.0.28/jakarta-tomcat-5.0.28/common/lib/servlet-api.jar;classes;.

-d classes src/chain/web/AddNameServlet.java

javac -classpath C:/jakarta-tomcat-5.0.28/jakarta-tomcat-j p j j5.0.28/common/lib/servlet-api.jar:classes:.

-d classes src/chain/model/BookBean.java


AddNameServlet.java – package namename


Do this whenever you change your recompile!recompile!

And start again!


g

Running a web serverRunning a web server

• To access Tomcat please refer to the instructions on student intranet under• Student Homepage → Computing → HOW TO... →

Use the Tomcat Java Web Server

If i l t d l d R i if• If using own laptop you can download Resin if you prefer from

http://www caucho com/download/http://www.caucho.com/download/

• Follow instructions for installation, set up and how to run the web server


to run the web server.

Extracting unknown parameters and multiple valuesand multiple values

d hNB. Case sensitive

• String getParameter(String) used when parameter name is known• returns null if unknown parameterreturns null if unknown parameter• returns "" (empty string) if parameter has no value

• Else use Enumeration getParameters() to obtain parameters

• Then String[] getParameterValues(String) t bt i f l f hto obtain an array of values for each one• returns null if unknown parameter• returns a single string (“”) if parameter has no values


etu s a s g e st g ( ) pa a ete as o va ues

A Big FormA Big Form


BigForm htmlBigForm.html<form action="http://localhost:8080/servlet/elem004.ProcessBigForm"

method="post">method= post >Please enter: <br><br>

Your login: <input type="text" name="Login"> <br><br>

Single value parameters

Your favourite colour:<input type="radio" name="Colour" value="blue">Blue<input type="radio" name="Colour" value="red">Red<input type="radio" name="Colour" value="green">Green <br><br><input type= radio name= Colour value= green >Green <br><br>

Which of these courses you are taking: <br><input type="checkbox" name="Course" value="elem001">ELEM001 <br><input type="checkbox" name="Course" value="elem002">ELEM002 <br><input type= checkbox name= Course value= elem002 >ELEM002 <br><input type="checkbox" name="Course" value="elem003">ELEM003 <br><input type="checkbox" name="Course" value="elem004">ELEM004 <br>

<input type="submit" value="Send to Servlet">


<input type= submit value= Send to Servlet ></form> Multiple value parameter

ProcessBigForm java (1)ProcessBigForm.java (1)

out.println("<table border=1>");

// obtain all the form’s parameters from the request objectEnumeration paramNames = req.getParameterNames();Enumeration paramNames req.getParameterNames();

while(paramNames.hasMoreElements()) {String paramName = (String) paramNames.nextElement();

//obtain values for this parameter and check how many there areString [] paramValues = req.getParameterValues(paramName);

if(paramValues.length == 1) { // a single valueif(paramValues.length 1) { // a single value

String paramVal = req.getParameter(paramName);out.println("<tr><td>" + paramName +"</td><td>"

+ paramVal + "</td></tr>");


+ paramVal + </td></tr> );}

ProcessBigForm java (2)ProcessBigForm.java (2)

else { // if several values print a list in the table

out.println("<tr><td>" + paramName +"</td><td><ul>");

for(int i = 0; i < paramValues.length; i++)out.println("<li>" + paramValues[i]);

out.println("</ul></td></tr>");out.println( </ul></td></tr> );}

}out.println("</table>");out.println( </table> );


After Big Form is processedAfter Big Form is processed


NB. getParameterNames() returns them in no particular order

HTTP Request Headers (1)HTTP Request Headers (1)

• As with form data, HTTP request headers can be extracted from the HTTPServletRequest object

• Usually use String getHeader(String)NB. not case sensitive

• Some common headers have their own methods• getContentLength()• getContentType()• getContentType()

• Can also access all headers using• Enumeration getHeaderNames()


g ()

HTTP Request Headers (2)HTTP Request Headers (2)

• Information on the main request line also has its own methods• getMethod()• getRequestURI()• getProtocol()g ()

• Servlet to extract all information:


ShowRequestHeaders outputShowRequestHeaders output


Generating the Server ResponseGenerating the Server Response

• Response typically consists of• a status line (containing version, status code + message)• some response headers• a blank line

Version

MessageStatus-code

• the document

• e.g., HTTP/1.1 200 OKContent Type: text/plain

Versionresponse header(s)

Content-Type: text/plain

Hello World!blank line

Department of Electronic Engineering 92document (here just one line of text)

HTTPServletResponseHTTPServletResponse

• Servlets can perform a variety of tasks by manipulating the status line and response headers, for example• tell user that a password is required

i di f h d d (i df h l)• indicate type of attached document (image, pdf, html)• forward user to other sites• etc• etc.


HTTP Status CodesHTTP Status Codes

• returned by the server to the client software to indicate the outcome of a request, e.g.

• 200 - OKThe request sent by the client was successful. 301 - Moved PermanentlyThe resource has permanently moved to a different URI.

• 303 See Other• 303 - See OtherThe requested response is at a different URI and should be accessed using a GET command at the given URI.

• 400 - Bad RequestqThe syntax of the request was not understood by the server.

• 403 - ForbiddenThe server has refused to fulfill the request.


Setting the status codeSetting the status code

• The servlet only needs to set the status code since the version is determined by the server and the message is associated with the status codemessage is associated with the status code.

• Usual method is simply to call response.setStatus(int)response.setStatus(int)

• If your response includes a special status code anda document you must call setStatus before returning any content via the PrintWriter. • This is because the document itself may not be buffered

but sent in pieces e g a large image file


but sent in pieces, e.g. a large image file.

Special status codesSpecial status codes

• Status codes are integers but it’s better to use the constants defined in HTTPServletResponse class

e g HTTPS l tR SC NOT FOUND• e.g., HTTPServletResponse.SC_NOT_FOUND

• Two common codes have special methodsStatus code 404

public void sendError(int sc, String message)

• this sets the status code plus a short messagepublic void sendRedirect(String url)

• generates a 301 response along with a Location header giving the URL of the new document that the browser should now request


SettingStatusCodes htmlSettingStatusCodes.html


Key part of formKey part of form

<input type="submit" name="non-file" value="Fetch a non-existent file">

<input type="submit" name="redirect" value="Redirect to homepage">

<input type="submit" name="censored" value="Fetch censored content">

Only used to generate cases, in practice we find out that the


practice we find out that the case has arisen by looking at

file system, etc.

Extract fromSetStatusCode javaSetStatusCode.java

bli id d P t(Htt S l tR t Htt S l tR )

Somehow know that file does not exist, so..

public void doPost(HttpServletRequest req, HttpServletResponse res)throws ServletException, IOException {

// generate response according to submit button was pressedif( eq getPa amete ("non file") ! n ll)

fudge404 USED WHEN NOT FOUND

if(req.getParameter("non-file") != null)res.sendError(res.SC_NOT_FOUND,

"The file you requested does not exist!");

if(req getParameter("redirect") ! null) Explain later

FORMATTED & RENDERED BY BROWSER

if(req.getParameter("redirect") != null) res.sendRedirect(res.encodeURL("http://localhost:8080/"));

if(req.getParameter("censored") != null) res setStatus(res SC FORBIDDEN); i b li

p

res.setStatus(res.SC_FORBIDDEN);

res.setContentType("text/html");PrintWriter out = res.getWriter();out println(“<html>”);

Send content only after status code set;

May exist but client not allowed to read it, so..


out.println(“<html>”); code set;

RESPONSE OF SEVLET MAY NOT BE BUFFERED AND ORDER

MATTERS

After “redirect” button pressedAfter “redirect” button pressed


res.sendRedirect("http://localhost:8080/");;

• When a servlet decides request should go to a different URl• Calls sendRedirect(..) on response objectCalls sendRedirect(..) on response object

• Container sends response “302” status code with a location header containing the new URL as valueB d d l ti h d d k• Browser reads response and location header, and makes new request using the URL (might see this in the browser)

• “Original” request now goes to the new URL• Which could refer to another servlet (not in this example)

• MUST be called BEFORE any writing to the response object


object

• 301 moved permanently• 302 moved temporarilyp y

• Most browsers treat them the same


Request DispatchRequest Dispatch

• Servlet wants request to go to a different servlet or JSP in the web app

l ll• Servlet callsRequestDispatcher

view=request getRequestDispatcher(“display jsp”);view=request.getRequestDispatcher( display.jsp );view.forward(request, response);• Now the JSP is invoked as if it were invoked directly• Now the JSP is invoked as if it were invoked directly

by the client• The client will only see the URL of the original servlet


Setting Response HeadersSetting Response Headers

• Response headers are are set using the methodres.setHeader(String header, String value);

• Examples include:• Allow, Content-Encoding (e.g., gzip), Content-Language (e g en en-us)Content-Language (e.g., en, en-us), Content-Length, Content-Type, Date, Expires, Last-Modified, Location, Refresh, Set-Cookie, WWW-Authenticate


Support methodsSupport methods

• Setting some headers is so common that special support methods exist:setDateHeader(String header,long msecs)setIntHeader(String header, int value)

( i )setContentType(String type)setContentLength(int length)addCookie(String cookie)addCookie(String cookie)sendRedirect(String encodedURL)


Setting the Refresh headerSetting the Refresh header

• Indicates how soon (in seconds) the browser should ask for an updated page

res setHeader(“Refresh” “30”);res.setHeader(“Refresh”, “30”);

• specifies next update (not continual) so need to set header each time

• sending status code 204 (NO_CONTENT) stops browser reloading further

• 204 - No ContentThe request was successful but does not require the return of an entity-body.

• can also be used to redirect to another page


res.setHeader(“Refresh”, “5; URL=http:/host/path”);

Does not mean reload every 5 seconds

RefreshClock javaRefreshClock.java


extractextract

res.setContentType("text/html");res.setHeader("Refresh", "1");

PrintWriter out = res.getWriter();

t i tl ("<ht l><h d>")out.println("<html><head>");out.println("<title>Telling the Time</title>");out.println("</head><body><h1>");

……

out println(new Date());


out.println(new Date());

Setting the Content Type headerSetting the Content-Type header

• Content-Type tells browser what sort of document is being sent.S f ’ l d l• So far we’ve only used only text/html

• Generally of form maintype/subtype e.g., t t/ l i t t/ht l t t/• text/plain, text/html, text/css

• image/gif, image/png, image/jpeg, image/tiff• application/pdf, application/mswordpp p , pp• video/mpeg, video/quicktime

• Must be set before writing to the OutputStream.


The browser does not render this like html

Is any content type acceptable?Is any content-type acceptable?

May need to check which types browser supports• use String req.getHeader(“Accept”) and check

that output string contains the format you wish to send

String types = req.getHeader(”Accept”);

if(Utilities.contains(types, “image/jpeg”)) {res.setContentType(“image/jpeg”);// send a jpeg file

} } else {

res.setContentType(“image/gif”);// send a gif file

}


}

Creating an Excel spreadsheetCreating an Excel spreadsheet

• As well as sending back html, servlets can dynamically create and send back more specialised content.

• The following example shows a servlet that l d h i f d icreates an Excel spreadsheet using form data input

by the user to format and fill the cells.


MakeTable htmlMakeTable.html


Extract from CreateSpreadSheet javaCreateSpreadSheet.java

tC t tT (" li ti / d l")res.setContentType("application/vnd.ms-excel");PrintWriter out = res.getWriter();

out.println("<table>");fo (int 0 < o s ++) {for(int r = 0; r < rows; r++) {

out.println("<tr>");for(int c = 0; c < cols; c++) {

out.println("<td bgcolor=\"" + colour + "\" >" + chars charAt(next++) + "</td>");+ chars.charAt(next++) + "</td>");

next %= mod;}out.println("</tr>");

}}out.println("</table>");out.close();


Output from CreateSpreadSheetOutput from CreateSpreadSheet


ServletContext object is created for a web appfor a web app

• One per web app• A web app normally has several servlets (& JSPs)

• Used to access web app parameters that need to be seen by all servlets (& JSPs) in the application• Held as parameters in the ServletContext object• A misnomer as relates not to a servlet but the set of

servlets and JSPsin the web appservlets and JSPsin the web app


The DD of the web app specifies the context parametersthe context parameters

<web-app …>……<servlet>

<servlet-name>… </servlet-name><servlet-class>… </servlet-class><init-param><param-name>….</param-name>p p p

<param-value>….</param-value> </init-param></servlet> … + other servlets

Note: Not inside any servletServletContext object created

<context-param><param-name>HOD_Email</param-name><param-value>[email protected]</param-value>

and set up when web app

is deployedparam value [email protected] /param value

</context-param>..</web-app>


</web app>These are parameter name value pairs:

both are strings

To access parameters in servlet codecode …

ServletContext ctx = getServletContext()out.println(ctx.getInitParameter(“HOD Email”)p ( g ( _ )); Note: Same name for get

method as when accessing ServletConfig

• Context parameters generally more commonly used

accessing ServletConfig object

Context parameters generally more commonly used than Config• Typical use (of former) a DB lookup name


Can access ServletContext()Can access ServletContext()….

• directlygetServletContext().getInitParameter(….)g () g ( )• from ServletConfiggetServletConfig().getServletContext().getInitParamter(…..)g g() g () g ( )

Latter useful if in a method of an auxiliary class, e.g. a bean,Latter useful if in a method of an auxiliary class, e.g. a bean, and only the ServletConfig object has been passed as a parameter


ServletContext also has AttributesServletContext also has Attributes

• Parameters are name value pairs, where both name and value are strings

• Attributes are name value pairs where the name is a string, but the value is an object (that may not be

i )a String)• Accessed by getAttribute(String)

S t b tAtt ib t (St i Obj t)• Set by setAttribute(String, Object)

Documents

Suggested readingSuggested readingkarens/ServletsPartIOctober2010Introduction.pdfSuggested readingSuggested reading • Heads First Servlets and JSP Passing the Sun Certified Web Component