Unit 02: Web Technologies (2/2)

Unit 2: Web Technologies (2/2)‫‏‬
 Core
 Web browsers and web servers
 URIs
 HTTP

 Client-Side
 Basic: HTML, CSS
 Advanced: JavaScript, DOM, AJAX, HTML 5, RIA

 Server-Side technologies for Web Applications
 CGI
 PHP
 Java Servlets
 JavaServer Pages (JSPs)‫‏‬

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Enabling Technologies for Web Applications

 Web applications use enabling technologies to make their content
dynamic and to allow their users to affect the business logic.
 While a simple web site requires only a Web server, a network
connection, and client browsers; Web applications include also an
application server to manage the business logic and state.
 The application server can be located on the same machine as the
Web server and can even execute in the same process space as the
Web server.
 The application server is logically a separate architectural element
since it is concerned only with the execution of business logic and
can use a technology completely different from that of the Web
server.

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Session State Management
 HTTP is a stateless protocol, so web servers do not keep track of
each user request nor associate it with the previous request.
 However, many Web applications need to support users’ sessions:
each session represents a single cohesive use of the system,
involving many executable Web pages and a lot of interaction with
the business logic.
 The session state keeps track of a user's progress throughout the
use case session. Without a session state management mechanism,
you would have to continually supply all previous information
entered for each new Web page.
 Ways to store session state:
 Client Session State: URI rewriting, Cookies, Hidden fields in web
forms
 Application Server Session State: In-memory dictionary or map.
 Database Server State.

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Cookies
 A cookie is a piece of data that a Web server ask a Web browser to
hold on to, and to return when the browser makes a subsequent
HTTP request to that server.
 Limited size: between 100 and 4K bytes.
 Initially, a cookie is sent from a server to a browser by adding a line
to the HTTP headers:
 Content-type: text/html
 Set-Cookie: sessionid=12345; path=/; expires Mon, 09-Dec-2002
11:21:00 GMT; secure
 A cookie can have up to six parameters:
 Name (required)‫‏‬
 Value (required)‫‏‬
 Expiration date
 Path
 Domain
 Requires a secure connection
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Common Gateway Interface (CGI)‫‏‬
 It is a standardized interface to allow Web servers to talk to back-
end programs and scripts that can accept input (e.g., from forms)
and generate HTML pages in response
 Typically, these back-ends are scripts written in the Perl (Python or
even some UNIX shell) scripting language.
 By convention, CGI scripts live in a directory called cgi-bin, which is
visible in the URI :
 http://www.enwebats.com/cgi-bin/enwebats.pl
 CGI’s major drawbacks:
 At each HTTP request for a CGI script the Web server spawns a new
process, which is terminated at the end of execution.
 CGI, like HTTP, is stateless. Therefore, terminating the process where
the CGI script is executed after each request prevents the CGI engine
to retain session state between consecutive user requests.

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CGI: Typical Scenario

Environment
:WebBrowser :WebServer
Variables
Submit
HTTP Req
(GET | POST)‫‏‬
update

:CGIScript
get

write read
do_something
(POST Request Body)‫‏‬

read write

X
HTTP Resp.

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CGI: User Input Forwarding
 METHOD=GET
 User input is appended to the requested URI: parameters are
encoded as label/value pairs appended to the URI after a
question mark symbol.
 The Web server initializes a environment variable called
QUERY_STRING with the label/value pairs.
 In most OSs, environment variables are bounded to 256/1024
chars.
 METHOD=POST
 User input – encoded as label/value pairs too – is attached to
the HTTP request using the message body.
 Upon receipt of a POST request, the Web server extracts the
label/value pairs from the message body, and writes them on
the standard input of the CGI script.
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“EnWEBats” Example: The Web Form

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“EnWEBats” Example: The Output

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“EnWEBats” Example: enwebats.html
<html>
....
<form action="/cgi-bin/enwebats.ppl" method="post">
Full Name: <input name="name" size="57">
Address: <input name="address" size="57">
City: <input name="city" size="32">
Postal Code: <input name="postcode" size="5">
Credit Card Number: <input name="cardnum" size="19">
Expires: (MM/YY) <input name="expdate" size="5">
AMEX <input name="cardtype" value="amex" type="radio">
VISA <input name="cardtype" value="visa" type="radio">
Express Mail (+10 euros): <input name="express"
type="checkbox">
<input value="Submit" type="submit"> </p> </form>
.... </html>

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“EnWEBats” Example: HTTP Request

POST /enWebatsPHP/enwebats.php HTTP/1.1
Content-type: application/x-www-form-urlencoded

name=Walter+Kurtz&address=78+Tahoma+Drive&city=
Los+Sauces& postcode=08997&cardnum=888899997777
0000&expdate=09%2F14&cardtype=visa&express=on

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“EnWEBats” Example: enwebats.pl (Perl)‫‏‬
#!c:apachefriendsxamppperlbinperl.exe

$size_of_form_info = $ENV{'CONTENT_LENGTH'};
read (STDIN, $form_info, $size_of_form_info);

print "Content-type: text/htmlnn";
print "<html> n";
print "<head> n";
print "<title>Subscription OK</title> n";
print "</head> n";
print "<body bgcolor=white> n";
print "<h3>Your subscription have been processed
successfully:</h3> n";
print "<BR> n";

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“EnWEBats” Example: enwebats.pl (cont.)‫‏‬
print "<table border=0> n";
# Split up each pair of key=value pairs
foreach $pair (split (/&/, $form_info)) {
# For each pair, split into $key and $value
variables
($key, $value) = split (/=/, $pair);

# Get rid of the pesky %xx encodings
$key =~ s/%([dA-Fa-f][dA-Fa-f])/pack ("C", hex
($1))/eg;
$value =~ s/%([dA-Fa-f][dA-Fa-f])/pack ("C",
hex ($1))/eg;
$value =~ s/+/ /g;
print "<tr><td>$key: </td><td>$value</td></tr>
n";}
print "</table> n";
print "</body> n";
print "</html> n";

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CGI: Some Environment Variables
CONTENT_LENGTH="156"
CONTENT_TYPE="application/x-www-form-urlencoded"
DOCUMENT_ROOT="C:/apachefriends/xampp/htdocs"
GATEWAY_INTERFACE="CGI/1.1“
HTTP_ACCEPT="text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text
/plain;q=0.8,image/png,*/*;q=0.5"
HTTP_ACCEPT_CHARSET="ISO-8859-1,utf-8;q=0.7,*;q=0.7"
HTTP_ACCEPT_ENCODING="gzip,deflate"
HTTP_ACCEPT_LANGUAGE="ca,en-us;q=0.7,en;q=0.3"
HTTP_CONNECTION="keep-alive"
HTTP_HOST="localhost" HTTP_KEEP_ALIVE="300"
HTTP_REFERER="http://localhost/dsbw/enwebats.html"
HTTP_USER_AGENT="Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7b)
Gecko/20040421"
QUERY_STRING=""
REMOTE_ADDR="127.0.0.1"
REMOTE_PORT="1743“
REQUEST_METHOD="POST"

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PHP: PHP: Hypertext Preprocessor
 Scripting language that is executed on the Web server (e.g.
Apache: mod_php)‫‏‬
 PHP files are basically HTML documents with PHP code
embedded through special tags.
 PHP syntax resembles C’s or Perl’s. Version 5 is more object
oriented.
 Works well with major DBMSs
 Is one of the mainstays of the LAMP platform: Linux + Apache
+ MySQL + PHP
 Very popular, with a huge community of developers and tool
providers.

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Exemple “EnWEBats”: enwebats.php
<?php function print_row($item, $key) {
echo "<tr><td>$key: </td><td>$item</td></tr>n";
}?>
<html>
<head>
<title>Subscription OK</title>
</head>
<body bgcolor=white>
<h3>Your subscription have been processed
successfully:</h3>
<BR>
<BR>
<table border=0 width=50%>
<?php array_walk($_POST, 'print_row'); ?>
</table>
</body>
</html>

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The J2EE Platform

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Java Servlets
 A Servlet is an object that receives a request and generates a
response based on that request.
 A Web container is essentially the component of a Web server that
interacts with the servlets. The Web container is responsible for
 Managing the lifecycle of servlets.
 Mapping a URI to a particular servlet.
 Ensuring that the URI requester has the correct access rights.
 Servlets’ strong points:
 A servlet is initialized only once. Each new request is serviced in its
own separate thread.
 The Servlet API easies common tasks like to access request
parameters, create and manage session objects, etc.
 It’s Java, 57up1d! (cross-platform, object-oriented, EJB, JDBC, RMI, …)‫‏‬

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Java Servlets: Request Scenario

:WebBrowser :WebServer :Servlet
Container

:Servlet
init
Submit
Envia Dades req. http
(POST) req. http
(POST) service
doPost

resp. http
resp. http

destroy

X
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Java Servlets: API (partial)‫‏‬

javax.servlet

<<interface>> <<interface>> <<interface>>
Servlet ServletResponse ServletRequest

init(config: ServletConfig)‫‏‬ getWriter(): getParameter(name:
service(req:ServletRequest, PrintWriter String):String
res: ServletResponse)‫‏‬ getOutputStream(): getParameterNames():
destroy()‫‏‬ ServletOutputStream Enumeration

GenericServlet

javax.servlet.http
HttpServlet
service(req: HttpServletRequest,
res: HttpServletResponse)‫‏‬
doGet (req: HttpServletRequest, <<interface>> <<interface>>
res: HttpServletResponse)‫‏‬ HttpServletResponse HttpServletRequest
doPost (req: HttpServletRequest,
res: HttpServletResponse)‫‏‬

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Exemple “EnWEBats”: Enwebats.java
public class Enwebats extends javax.servlet.http.HttpServlet
{
public void doPost(javax.servlet.http.HttpServletRequest req,
javax.servlet.http.HttpServletResponse res)‫‏‬
throws javax.servlet.ServletException, java.io.IOException {
res.setContentType("text/html");
java.io.PrintWriter output = res.getWriter();
output.println("<html>");
output.println("<title>Subscription OK</title>");
output.println("<body bgcolor=white>");
output.println("<h3>Your subscription have been processed successfully:</h3>");
output.println("<table border=0>");
java.util.Enumeration paramNames = req.getParameterNames();
while (paramNames.hasMoreElements()) {
String name = (String) paramNames.nextElement();
String value = req.getParameter(name);
output.println("<tr><td>"+name+": </td><td>"+value+"</td></tr>"); }
output.println("</table></body></html>");
output.close(); }}

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JavaServer Pages (JSP)‫‏‬
 JSPs allows creating easily Web content that has both static and
dynamic components.
 JSPs project all the dynamic capabilities of Java Servlet technology
but provides a more natural approach to creating static content.
 The main features of JSPs technology are:
 A language for developing JSP pages, which are text-based documents
that describe how to process a request and construct a response
 Constructs for accessing server-side objects
 Mechanisms for defining extensions to the JSP language
 A JSP page is a text-based document that contains two types of
text:
 static template data, which can be expressed in any text-based format
(HTML, XML, …)‫‏‬
 JSP elements, which construct dynamic content.

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JSP and Java Servlets
 When a Web container is requested a JSP it may happen that:
1. The JSP had not been requested yet: The web container
compiles and initializes a servlet to service the request.
2. The JSP had already been requested: The Web container
forwards the request to the corresponding running servlet.
 Therefore, JSPs have all the benefits of using Java Servlets
and, at the same time, allows separating more clearly the
application logic from the appearance of the page
 That, in turn, allows
 using any Web edition tool or suite.
 fast development and testing

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JSP Elements (1/2)‫‏‬
 JSP Directives: Instructions that are processed by the JSP engine when
the page is compiled to a servlet. Directives are used to set page-level
instructions, insert data from external files, and specify custom tag
libraries. Example:
<%@ page language=="java" imports=="java.util.*" %>

 JSP Declaratives: Definitions of global variables and methods.
<%! int time = Calendar.getInstance().get(Calendar.AM_PM); %>

 JSP Scriptlets: Blocks of Java code embedded within a JSP page.
Scriptlet code is inserted verbatim into the servlet generated from the
page, and is defined between <% and %>.
 JSP Expressions: Expressions that are evaluated inside the
corresponding servlet to be inserted into the data returned by the web
server. Example:
<h2> List of Members for the year <%= clock.getYear() %> </h2>

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JSP elements (2/2)‫‏‬
 JSP Tags (or Actions): elements that encapsulate common
tasks.
 Some are standard, which allow forwarding requests to other
JSPs or servlets (<jsp:forward>), instantiate JavaBeans
components (<jsp:useBean>), etc. Example:
<jsp:useBean id=="clock" class=="jspCalendar" />

 Some are custom, that is, they are defined by Java developers
using the JSP Tag Extension API. Developers write a Java class
that implements one of the Tag interfaces and provide a tag
library XML description file that specifies the tags and the java
classes that implement the tags.

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“EnWEBats” example: enwebats.jsp
<%@ page language="java" import="java.util.*" %>
<html>
<head>
</head>
<h3>Your subscription have been processed successfully:</h3>
<BR>
<BR>
<% Enumeration paramNames = request.getParameterNames();
while (paramNames.hasMoreElements())‫‏‬
{
String name = (String)nomsParams.nextElement();
String value = request.getParameterValues(name)[0];%>
<tr>
<td><%= name %>:</td><td><%= value %></td>
</tr>
<% } %>
</table>
</body>
</html>

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JSP Standard Tag Library (JSTL)‫‏‬

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Exemple “EnWEBats”: enwebats.jsp amb JSTL

<%@ taglib uri='http://java.sun.com/jstl/core' prefix='c'
%>
<html>
<head>
</head>
<h3>Your subscription have been processed
successfully:</h3>
<BR>
<BR>
<c:forEach var='parameter' items='${paramValues}'>
<tr>
<td><c:out value='${parameter.key}'/>:</td>
<td><c:out value='${parameter.value[0]}'/></td>
</tr>
</c:forEach>
</table>
</body>
</html>

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References
 SHKLAR, Leon et al. Web Application Architecture: Principles,
Protocols and Practices, Second Edition. John Wiley & Sons,
2009.
 hoohoo.ncsa.uiuc.edu/cgi/

 www.php.net

 java.sun.com/javaee/5/docs/tutorial/doc/

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Unit 02: Web Technologies (2/2)

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