Chapter 6: HTTP protocol Servlets and CGI Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

Chapter 6: HTTP protocolServlets and CGI

Rufin Soh

INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

Content

1. HTTPa. HTTP protocol

b. HTTP exchanges

c. HTTP requests and responses

d. HTTP headers

e. HTTP requests format

f. HTTP session examples

g. HTTP responses

h. HTTP responses status codes

2. HTMLa. HTML language

1. HTML concepts

2. HTML examples

3. HTML editors

3. HTML forms

a. HTML formsb. Forms SUBMITc. Forms SUBMIT : GET

methodd. Forms SUBMIT : POST

method

4. HTTP challenges

a. Client-side scripting

b. Server-side scripting

5. Java Servlets

a. What are servlets

b. What is a Servlet used for?

c. Servlet working mode ?

d. The Java API for servlets

e. Servlet example

f. Install your servlet

g. Run your servlet from the

h. browser

6. Java servlet vs CGI

A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

Content

1. HTTP

a. HTTP protocol

b. HTTP exchanges HTTP requests and responses

c. HTTP headers

d. HTTP requests format

e. HTTP session examples

f. HTTP responses

g. HTTP responses status codes


HTTP protocol

HTTP (Hyper Text Transfer Protocol) is a protocol used on the web

for hypermedia systems

It enables data transfer and negociation of the transfered data types.

A web server is a program that receives HTTP request from different

clients (www users) and return requested documents.

World-Wide Web (WWW) is a network made of a set of servers ( that speak

HTTP) and the information that they contains.

Actual HTTP version is 1.1


HTTP exchanges

An HTTP request has the following steps:1. The client makes a TCP connexion on HTTPD port 80.

2. The client transmit a request ( a line os text that ends with a CR LF)

3. The request format is <Méthode> URL

4. The server returns the document specified in the URL

5. The server ends the connexion

Some servers support a keep-alive features that help to keep the

connexion for many transfers for the same document.

The HTTP server is stateless: It does not keep any information on the

clients


HTTP/1.0 enables the transfer of documents in many formats,

following the MIME standard(Multi-purpose Internet Mail Extensions).

An HTTP message may be a client request or a server response.

The request-response format is

A request-response line

many headers

an empty line

the request-response body

A request-response line

Empty line

Headers

Body

HTTP requests and responses


HTTP-Version: Identifies the HTTP version.

The clients are going to indicate here which protocol they are

using.

The default value is HTTP/0.9

Date: Date when the message has been created

Forwarded: Identifies the sender machine. Useful when intermediaries proxies are

used

Mime-Version: The MIME version of the client

Accept: The client will mention the type of documents that he can accept

Accept-Charset: The client specified the supported character encoding.

The default values are US-ASCII and ISO-8859-1.

If-Modified-Since: Specified a date. Useful for the browser to request a document from

the server only if this one has been modified since the date indicated

on the local cached copy.

HTTP headers


User-Agent: Identification of WWW clients

Retry-After: Used by an HTTP server to mention thsat it temporally not availaible

Server: The server identification.

Content-Length: The length in bytes of the body in HTTP message (request or response)

Content-Type: The HTTP message (request or response) body’s MIME encoding type

e.g. text/plain, text/html, text/xml, …

Expires: Timestamp for a document. Used to manage documents caches.

Last-Modified: Date of the latest modification of the document.

HTTP headers (2)


HTTP requests are formatted:

<Method> URL <Method> URL HTTP/1.0

Requests may contains parameters in their URL

GET URL?parameter1+parameter2+…+parameterN

Some of these programs may required options. They are also transmitted in the URL:

GET /cgi-bin/search?mozart HTTP/1.0

HTTP requests format


HTTP session examples

GET method: Ask the server to read a web page. This may be

• <a HREF="http://www.info.uqam.ca/Test/text.html"> Anchor </a>

GET Test/texte.html HTTP/1.0Accept: text/plain , image/*If-Modified-Since: Wed, 10 Sep 1998 14:20:21 GMTReferer: http://www.info.uqa.ca/index.htmlUser-Agent: Mozilla/2.0

empty line

HTTP/1.0 200 OKDate: Wed, 10 Sep 1998 14:20:21 GMTServer: NCSA/1.5.2Mime-Vesion: 1.0Content-Type: text/htmlLast-Modified: Wed, 10 Sep 1998 14:20:23 GMTContent-Length: 139

empty line<html> ……</html>

Connects to the server « www.info.uqam.ca » on the port « 80 »

Request

Response

HTTP server

HTTP server

Brow ser

Brow ser


HEAD method: Ask the server to read the web page header.

•E.g. a search engine request

HEAD Test/texte.html HTTP/1.0User-Agent: Mozilla/2.0Referer: http://www.info.uqa.ca/index.html

Empty line

HTTP session examples (3)

Brow ser

HTTP server

HTTP/1.0 200 OKDate: Wed, 10 Sep 1998 14:20:21 GMTServer: NCSA/1.5.2Mime-Vesion: 1.0Content-Type: text/htmlLast-Modified: Wed, 10 Sep 1998 14:20:23 GMT

Brow ser

HTTP server


GET method with:http://www.info.uqam.ca/cgi/research?item1+item2+item3


GET method from a form with:http://www.info.uqam.ca/cgi/saisie?champ1=Val1&champ2=Val2

GET /cgi/saisie?champ1=Val1&champ2=Val2 Accept: */*Accept-language: en, fcUser-Agent: Mozilla/2.0Host: saturne.info.uqam.ca

Ligne blanche

GET cgi/research?item1+item2+item3 HTTP/1.0Accept: */*Accept-language: en, fcUser-Agent: Mozilla/2.0Host: saturne.info.uqam.ca

Ligne blanche


POST method from a form with:• http://www.info.uqam.ca/cgi/saisie• champ1=Val1 and champ2=Val2


POST /cgi/saisieAccept: */*Accept-Language: en, fcUser-Agent: Mozilla/2.0Host: saturne.info.uqam.caContent-type: multipart/form-data; boundary=-----------------404040878888Content-Length: 112

empty line-----------------404040878888Content-Disposition: form-data; name="Champ1"

empty lineVal1-----------------404040878888Content-Disposition: form-data; name="Champ2"

empty line Val2-----------------404040878888


HTTP responses

Response Class Meaning Example

1XX Informational unused

2xx Success 200 OK

3xx Redirection to another server

301 MOVED TEMPORARILY

4xx Client did something wrong

401 UNAUTHORISED

5xx Server did something wrong

500 INTERNAL SERVER ERROR

Every HTTP response is identfied with a code:


2xxOK 200 The request was fulfilled. CREATED 201 success following a POST command. Accepted 202 The request has been accepted for processing, but the processing

has not been completed. Partial Information 203 The returned meta information is not a definitive .No Response 204 Server has received the request but there is no information to send back,

and the client should stay in the same document view.

3xxMoved 301 The data requested has been assigned a new URI, the change is

permanent. Found 302 The data requested actually resides under a different URL.Method 303 Like the found response, this suggests that the client go try another

network address. In this case, a different method may be used too, rather than GET.

Not Modified 304

HTTP responses status codes


HTTP responses status codes (2) 4xxBad request 400 The request had bad syntax or was inherently impossible to be

satisfied. Unauthorized 401 The client should retry the request with a suitable Authorization

header. PaymentRequired 402 The client may retry the request with a suitable ChargeTo

header. Forbidden 403 The request is for something forbidden. Authorization will not

help. Not found 404 The server has not found anything matching the URI given

5xxInternal Error 500 The server encountered an unexpected condition which prevented it

from fulfilling the request. Not implemented 501 The server does not support the facility required. Service temporarily overloaded 502 The server cannot process the request due to a

high load. Gateway timeout 503 This is equivalent to Internal Error 500, but in the case of a server

which is in turn accessing some other service, this indicates that the respose from the other service did not return within a time that the

gateway was prepared to wait.


Content

1. HTTP

2. HTML

a. HTML language

b. HTML concepts

c. HTML examples

d. HTML editors


HyperText Markup Language is:

A marking language used to specified:

o Pages tables and frames, … formato Fonts, colors, … attributes

A language used to describe hypermedia documents:

o Imageso texto hypertext links between documents hosted on different sites

A normal text language. This enables its portability between different machines.

A tagged language.o Tag effects are applied to the content between the opening tag and

the closing one

HTML language


HTML is a markup language: a language that just consist of adding tags

to simple ASCII text without any formatting. Most of the tags are “twins”. <html> . . . </html>

o Tells you the beginning of an HTML document <head> . . . </head>

o Tells you the section of informations that is not going to be displayed <title> . . . </title>

o Indicates the title of the document (not to be displayed) <body> . . . </body>

o Indicates the section of informations that are going to bre displayed In general, a tag has a name (mandatory), with optional sets of attributes

whith an associated value for some of them.

o E.g. <TagName Attribute_1_Name="Attribute_1_Value"

Attribute_2_Name="Attribute_2_Value" Attribute_1_Name >

HTML concepts


<html> <head> <TITLE>A minimal example</TITLE> </head> <body> This the text of my document </body> </html>

HTML examples

<HTML><HEAD><TITLE>How to call a servlet?</TITLE></HEAD><BODY BGCOLOR="white" TEXT="black" LINK="blue" VLINK="purple" ALINK="yellow"><H1>How to call a servlet?</H1><H2>Form</H2><P>This form uses the P<SMALL>OST</SMALL> method:<BR><FORM ACTION="http://localhost:8080/servlet/query" METHOD="POST"><INPUT NAME="query"><INPUT TYPE="SUBMIT" VALUE="Post"></FORM><H2>Hyperlink</H2><P>This hyperlink uses the G<SMALL>ET</SMALL> method:<BR><A HREF="http://localhost:8080/servlet/query?java">http://localhost:8080/servlet/query?java</A></BODY></HTML>


A Minimal HTML example is

A more complete HTML example is

An HTML editor is a tool that you may use to build web pages, by

generating HTML code from an edited text:

FrontPage

Netscape Composer

PageMill

HotMetal Pro

Amaya

Macromedia DreamWeaver 3 or 4

Hypertext Builder 7

HTML editors


Very simple

Most recommended

Content

1. HTTP

2. HTML3. HTML forms

a. HTML formsb. Forms SUBMITc. Forms SUBMIT : GET methodd. Forms SUBMIT : POST method


An HTML form is a set of elements used to ritrieve informations from the user.

Once filled, the form can be submitted to a processing program, that runs on the

server.

The <FORM> tag may have an attribute “METHOD” that can take the values GET

or POST. The difference between the two resides in how the form data are going to

be sent to the server indicated in the “ACTION” attribute.

DESCRIPTION:

<FORM NAME = “name” ACTION="URL" METHOD="Action">

ACTION : Provides the URL address where the request with the form content will be

sent. If the field ACTION is absent, the request will be sent to the current server.

METHOD : HTTP command used to communicate with the HTTP server. There are

two methods, GET and POST.

HTML forms


Nearly all tha form elements contain a NAME = “xxx” and VALUE = “yyy”

attributes that are used to send the form content to the server mentionned in

the ACTION = “servletURL” attribute.

Every form MUST contain a SUBMIT element. If many SUBMIT buttons are

used in the same form, you can distinguish them with the NAME = “xxx”

attribute which will be sent in the request to the server..

An HTTP may call programs via CGI or servlets interfaces:

HTML forms

Netscape

Form

HTML

http server

Programor Script (3)

CGIHTML

Data

(1)

(2) servlet(4)

(5)


<FORM METHOD="POST" ACTION="http://www.labunix.uqam.ca/cgi-bin/post.cgi"> Nom <INPUT NAME="Nom"><P> Prénom <INPUT NAME="Prenom"> <OL> <LI> <INPUT TYPE="checkbox" CHECKED NAME="etat" VALUE="Marie">Marié <LI> <INPUT TYPE="checkbox" NAME= " langue" VALUE="francais"> Canadien </OL> Volontaire <UL> <LI> <INPUT TYPE="radio" NAME= " info" VALUE="Oui" CHECKED> oui

<li> <INPUT TYPE="radio" NAME= "info" VALUE="Non"> Non <LI> <INPUT TYPE="radio" NAME= "info" VALUE="Ne sait pas"> Ne sait pas </UL> <INPUT TYPE="submit" VALUE="Ok"> <INPUT TYPE="reset" VALUE="Annuler"> </FORM>

Forms examples (2)


During the transmission of the form content to the server mentionned in the

ACTION attribute, the browser encodes this content:

All the spaces are replaced by ‘+’ signs

Non alpha-numeric characters are replaced by a %xx sequence, where xx

corresponds to the hexadecimal representation of the character ASCII code,

Carriage return are encoded with %0a for textarea..

An HTML document may content many forms. This may help to optimize the use of

many SUBMIT buttons: any form content is transmitted with its SUBMIT button.

The ACTION attribute generally contains the servlet or CGI program URL or an

email address (mailto:[email protected]).

Forms SUBMIT


With the GET method, browser sends the form’s information after the CGI (or servlet) URL mentionned in the ACTION attribute, with a “?” between them.

o E.g. http://YourServer.com/cgi-bin/YourScript?Form1Object=value1&...

You can see these values in the brower URL textfield, when the server sends back the response.

o Adv: It may be used to save a bookmark of the response so you may reactivate the script later with the same data without fillig the form. You can include the link in a document

o DisAdv : All the form informations are visible and this may give a very big URL.

The servlet or CGI program will retrive the rigth part of “?” in the environment variables.

Forms SUBMIT : GET method


With the POST method, data are sent to the server (the CGI script or your

servlet) in a separate way, in the body of the HTTP request.

o Advantages: suitable when you have many data to transmit, informations

transmitted are not visible.

o Disadvantages: You can’t save a bookmark on the response, if the servlet or the

script application is moved the request can not be redirected.

The difference between GET and POST is very important mainly for the

programmer of the servlet or script which is supposed to receive the form data.

When using an existing servlet or script, you need to know which method is used

to send data.

Forms SUBMIT : POST method


Content

1. HTTP

2. HTML3. HTML forms

4. HTTP challenges

a. Client-side scripting

b. Server-side scripting


Client-side scripting

Java applets are a great solution to create highly interactive Web sites. Not

dynamic web pages.

There are other solutions such as JavaScript, Shockwave, ActiveX or the

old reliable forms: those are client side solution .

Admittedly forms are less fashionable than say applets but many popular

sites make extensive use of them. For example, search engines like

Infoseek or online shops like Amazon.com rely extensively on forms.

Forms are implemented through a combination of HTML and server-side

scripting.

In this section, we will see how to write server-side scripts with Java

servlet


Server-side scripting

A Web server simply fetches files upon request from browsers. For

example, when a browser requests "index.html", the server sends the

corresponding file over the network.

For more complex requests like forms, the server calls specific programs.

These programs are known as scripts because, historically, most of them

were written with scripting languages like Perl.

The scripts collect data from the forms and dynamically build Web pages

in response. These pages do not exist on the server hard-disk, they are

built dynamically in response to browser requests -- hence the name

dynamic Web pages.

Server-side scripting is still very popular for several reasons:

o it is totally independent of the browser since everything takes place on the

server;

o complex requests may execute faster on the server;

o it can be made safer since the programs run under direct control of the

server administrator.


Server-side scripting

The original standard for server-side scripts is CGI or Common Gateway Interface.

CGI:

Advantages simple and widely supported.

Disadvantages: it is not very efficient because the server launches one copy of the

script per request, the overhead can be significant for popular sites.

Vendors have developed proprietary alternatives such ISAPI (Microsoft) and

NSAPI (Netscape) to address the performance issue.

JavaSoft recently proposed the Java servlets as a standard efficient alternative to

CGI.

The servlet API is a Java extension. Java extensions are optional standard

packages. Being standard packages, they are endorsed by JavaSoft and widely

supported in the industry.

Being optional, they are not available on every platform unlike the Core API

(java.io, java.awt, etc.). Indeed it does not make sense to support servlets

anywhere but on servers. To differentiate Java extensions from the Core API, the

package names start with javax (mind the x).


Content

1. HTTP2. HTML3. HTML forms4. HTTP challenges5. Java Servlets

a. What are servlets b. What is a Servlet used for?c. Servlet working mode ?d. The Java API for servlets e. Servlet example f. Install your servletg. Run your servlet from theh. browser

6. Java servlet vs CGI


What are servlets

Servlets are for server, what Applets are for browser

They are “normal” java programs that uses the Java servlet API

which contains the packages:

Javax.servlet.*

Javax.servlet.http.*

They are Included in the archives jsdk.jar (Java Servlet Development

Kit).

Servlets are interpreted in a JVM (Java Virtual Machine) running in a

compatible web server (e.g. tomcat)


What is a Servlet used for ?

The same as CGI, servlets are used to extend web server

functionalities, processing some operations demanded in HTTP

requests and including the results in the responses.

Actually, servlets are widely used with HTTP for dynamically

publishing HTML web pages. But Servlets are not specific to Web

servers. The API is flexible and supports most Internet servers,

e.g. mail server (with SMTP, POP), FTP server …

Of course JavaSoft recognized that most programmers would

write servlets for HTTP servers, i.e. Web servers, and it has

standardized an HTTP-specific version of servlets, HttpServlet.

In this section, we will concentrate on HTTP-specific servlets.


Servlet working mode ?1. The client fills an HTML form2. SUBMIT it: sends it to the server3. The server receives it and redirect it to the servlet using the ACTION tag

<FORM NAME=”MyForm” ACTION=”http://www.WebServer.com/servlet/Search” METHOD=”POST”>Since the servlet is a java program, it behaves exactly like any program with an access to all the java functionalities. So it can communicate with external resources such as files (read and write), databases and other applications (even remotely via RMI, CORBA, …) and after the processing it may use resulting data to dynamically build a web page to send back.

4. The servlet sends back the HTML page to the user.


The Java API for servlets

Javax.servlet.*

Javax.servlet.http.*

Included in the archives jsdk.jar (Java Servlet Development Kit).

A servlet is Java class that implements the javax.servlet.Servlet interface. This interface defines only five methods:

service() is the heart of servlets. The server calls service() to execute requests. service() accepts a ServletRequest and a ServletResponse objects as parameters. ServletRequest encapsulates the client request while ServletResponse exposes methods to return information to the client;

init() is the place to initialize the servlet. init() accepts a ServletConfig object as parameter. ServletConfig maintains the server configuration;

getServletConfig() must return the ServletConfig object passed to init(); destroy() cleans up the servlet; getServletInfo() returns a string with copyright information.

The “Java Servlet” API provides a number of methods that a servlet may (instead MUST) implement:


The Java API for servlets (2)

Init(): Yhis method will run oonce when the servlet program is loaded

by the server. This method may be personalized (by rewritting the

default code), e.g when you want to initialize a database connexion

with JDBC, ….

Destroy(): Also executed once, when the server stops and drops the

servlet. Generally the default method is enough but you may also use

it to disconnect to your database, by rewritting the method.

Services doGet(), doPost(): Those methods are called by the service()

method. They differe from the previous methods in that they are called

for any client request. The appropriate doXXX() method must be

invoked according to the HTTP method asked in the request through

the METHOD=”GET” or METHOD=”POST” attribute. Those methods

are where all the processing code are going to be included.


Servlet life cycle

1. The servlet is automatically loaded when the web server starts, or after the first

client request.

2. The server creates one instance of the servlet.

3. The server calls the servlet’s init() method.

4. If a client request reaches the servlet (the server remember it if this request has

been the originator of the servlet loading)

5. The server creates a Request Object specific to this request.

6. The server creates a Response Object specific to this request.

7. The server calls the servlet’s service() method to transmit the Request and

Response Objects.

8. The service() method receives informations on the request to process from

Request and Response Objects, and sends back response to the client after

processing using Request and Response objects’ methods. The service()

methods may call doGet() or doPost() or even new methods from your own..

9. For new cleint request, resume from line 4.

10. Once the servlet is no more in use, the web server invokes the destroy() method.


Servlet example

HttpServlet. HttpServlet is the class that implements the Servlet interface

but provides additional methods specific to Web servers.

One typically use of HttpServlet is with HTML forms.

To illustrate HttpServlet, we will see a simple servlet, Query, that lookups a

word in a dictionary and returns its definition in a dynamic Web page.

Servlets execute three steps to process a request:

1. analyze the request, e.g. for a form, collects the value of the various fields;

2. execute the request, e.g. query a database;

3. create a response, e.g. format the result of the query in a Web page.


Servlet example (your web page and form)

POST

As explained previously, POST is only available with forms.

HTMLcode for a simple POST form:

The <FORM> tag takes two parameters:

ACTION is the URL of the servlet;

METHOD forces the method to POST (it defaults to GET).

The first <INPUT> tag is a text field, the second one is a button.

<HTML><HEAD><TITLE>How to call a servlet?</TITLE></HEAD><BODY BGCOLOR="white" TEXT="black" LINK="blue" VLINK="purple" ALINK="yellow"><H1>How to call a servlet?</H1><H2>Form</H2><P>This form uses the P<SMALL>OST</SMALL> method:<BR><FORM ACTION="http://localhost:8080/servlet/query" METHOD="POST"><INPUT NAME="query"><INPUT TYPE="SUBMIT" VALUE="Post"></FORM><H2>Hyperlink</H2><P>This hyperlink uses the G<SMALL>ET</SMALL> method:<BR><A HREF="http://localhost:8080/servlet/query?java">INE 4481 example for GET</A></BODY></HTML>


Servlet example (submit form - doPost())

POST

In the servlet, the form data is routed to the doPost() method. My Query servlet implements doPost() as :

As you can see, the servlet retrieves the form fields through the HttpServletRequest::getParameter() method. It uses the name of the <INPUT> tag to identify the parameters.

Servlet code follows

protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException{ String query = req.getParameter("query"); String definition = getDefinition(query); doResponse(query,definition,resp);}

H ow to call a servlet?

Form

This form uses the P O ST method:

Post

H yperlink

This hyperlink uses the G ET method: http://localhost:8080/servlet/query?java


Servlet example (submit form - doGet())

GET

The GET method also works with forms but I find it more useful with hyperlinks.

This is an example of a hyperlink that jumps to a dynamic page: <A HREF="http://localhost:8080/servlet/query?java">INE 4481 example for GET </A>the beginning of the the URL (before ?) points to the servlet. The parameter appears after the question mark.

In the servlet, the request is routed to doGet():

Servlet code follows

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException{ String query = req.getQueryString(); String definition = getDefinition(query); doResponse(query,definition,resp);}

H ow to call a servlet?

Form

This form uses the P O ST method:

Post

H yperlink

This hyperlink uses the G ET method: http://localhost:8080/servlet/query?java


Servlet example (submit form - doGet())

GET

Again the servlet retrieves the parameters from the HttpServletRequest

object. In this example, it uses HttpServletRequest::getQueryString().

getQueryString() works fine if there is just one parameter. To work with

more parameters, it is necessary to use named parameters like POST does.

An URL with named parameters is of the form:

http://localhost:8080/servlet/query?detail=full&frame=no.

Parameter names and values are grouped in pairs separated by ampersand

&. This URL has two parameters: detail and frame and their values are full

and no respectively. A servlet retrieves named parameters through

HttpServletRequest::getParameter() as always.


Servlet example (execute the request)

After analyzing the request, the servlet must prepare a response.

Typically the servlet connects to a database or reads files on the server.

Another popular option is to create an email but you are only limited by your

imagination.

Processing the request may involve low-level access to the hardware.

Here, my Query servlet checks the word against its dictionary:

protected String getDefinition(String word){ String definition = dictionary.getProperty(word.toLowerCase()); if(null == definition) return "I have never heard of it."; else return definition;}


Servlet example (Create a response)

protected void doResponse(String query, String answer,HttpServletResponse resp) throws IOException{ // first prepare the response as a Web page // StringBuffer is convenient to know the length of the page StringBuffer buffer = new StringBuffer(); buffer.append("<HTML>\n"); buffer.append("<HEAD>\n"); buffer.append("<TITLE>Query</TITLE>\n"); buffer.append("</HEAD>\n"); buffer.append("<BODY BGCOLOR=\"white\" TEXT=\"black\" LINK=\"blue\" VLINK=\"purple\" ALINK=\"yellow\">\n"); buffer.append("<H1>Query</H1>\n"); buffer.append("<P>Query has this to say about <EM>"); buffer.append(word); buffer.append("</EM>:\n<P><BLOCKQUOTE>"); buffer.append(definition); buffer.append("</BLOCKQUOTE>\n"); buffer.append("<HR WIDTH=\"20%\" ALIGN=\"LEFT\">\n"); buffer.append("<P>Query by <A HREF=\"http://www.geocities.com/srufin\"><I>INE 4481</I> sprl</A>.\n"); buffer.append("<BR>Visit <A HREF=\"http://www.javacats.com/\">Digital Cat</A> for Java resources.\n"); buffer.append("</BODY>\n"); buffer.append("</HTML>");

// next pass the response to the client resp.setContentType("text/html"); resp.setContentLength(buffer.length()); resp.getOutputStream().print(buffer.toString());}


Servlet example (Create a response)

After executing the request, the servlet must return a response to the client.

To communicate with the client, the servlet uses the HttpServletResponse object, a

specialized version of ServletResponse.

Two cases are possible: either an error occurred or some result is available. If an error

occurred, the servlet can either throw an exception or return an error through

HttpServletResponse::sendError().

If there is no error, the servlet will dynamically create a Web page. To return a Web page,

the servlet must first specify the MIME content-type of the response with

ServletResponse::setContentLength(). Web pages are of type text/html but servlets can

return data in any format notably including GIF (image/gif) or JPEG (image/jpeg) images.

Additionally, a servlet may set a content-length with ServletResponse::setContentLength().

Setting length is optional and should be done only if it is not too costly. Setting a wrong

length is dangerous as the server may raise an exception. Finally the servlet writes its

response in the stream returned by ServletResponse::getOutputStream().

My Query servlet constructs a Web page in memory (which is convenient to compute the

length), set the content-type and length and send the page to the Web browser.


Servlet example (overview)

/** * Query is a simple querying mechanism that illustrates how to write * HTTP servlets. * * @author [email protected] * @version 20 sept 2001 */

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

public class Query extends HttpServlet{

private Properties dictionary = new Properties();public void init(ServletConfig config) throws ServletException { }protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException { }protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException { }protected String getDefinition(String word) { }protected void doResponse(String word, String definition,HttpServletResponse resp)throws

IOException { }public String getServletInto(){ }

}

JAVA servlet API

Servlet creation


Install the servlet

Once the sevlet is implemented, you will need to install it on an adequate server. e.g. Apache seb server + the ApacheJServ module Oracle Application server + Jservlet Cartridge Apache tomcat webserver Etc…

On this server, you also need a Java Running Environment (JRE 1.1.X, 1.2) and the JSDK 2.0 or

2.1. The actual installation may be different from one srever to another. But as for CGI_BIN, your

servlets are generally installed and declared in a personal “sandbox”. It is actually a space that

may be submitted to many access restrictions (files, network, …) depending on the server

administrator configuration. A servlet without restrictions could stop a web hosting server with a

simple “system.exit()” ! command. On the client side there is nothing to install, you just need your browser and the web server

URL. You may use the “ServletRunner” provided with the JSDK when implementing your servlet, to

simulate a web server (cf JBUILDER 4) Important: follow up the webserver user guide to identify the proper sandbox and check if there

is no futher configuration to do.


Run your servlet from the browser

There are many ways of running your servlet:

You may call directly by typing the complete servlet URL in your browser

You may indicate the servlet in the form tag (inside <FORM> …. </FORM>), with

ACTION attribute, using the GET and POST methods to send the form content

(entered by the user) to the servlet.

You may indicate the servlet in the <SERVLET> tag of an HTML document (like for

applet)

You may use a link like in our example


Java servlet vs CGI

Servlets have four distinct advantages over CGI and other APIs:

o familiarity: servlets are written in Java, the programming language you've come to

know and love. Servlets have access to all the Java packages and the JavaBeans on

the market;

o portability: servlets are portable across servers, delivering the Java promise of write

once, run anywhere. Netscape, Apache and IIS, arguably the three most popular Web

servers, are supported by Javasoft. Most other vendors have announced plans to

support the servlet API in their products;

o safety: servlets are written in Java and they benefit from the safety features inherent to

Java, like memory management;

o performance: servlets are more efficient than CGI.


Documents

Chapter 6: HTTP protocol Servlets and CGI Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES