The JSP Files (Part 6) - State of Grace

The JSP Files (part 6): State Of Grace
By Vikram Vaswani and Harish Kamath
This article copyright Melonfire 2000−2002. All rights reserved.
Table of Contents
A Perfect State.....................................................................................................................................................1
Wasted, Dude!.....................................................................................................................................................2
A Few Ground Rules..........................................................................................................................................3
Learning To Write..............................................................................................................................................4
...And Read..........................................................................................................................................................8
What's In A Name?...........................................................................................................................................10
Plan B.................................................................................................................................................................13
Session Dissection..............................................................................................................................................14
Access Denied....................................................................................................................................................16
The JSP Files (part 6): State Of Grace
i
A Perfect State
Over the past few weeks, you've learned a great deal about the various control structures and objects available
in JSP. You've see how to retrieve information posted in an online form, and connect your JSP document to a
database for dynamic content generation.
This week in The JSP Files, we're going to tackle yet another very interesting topic − the problem of
maintaining "state" on a Web site. We'll be looking at two common solutions to this problem − cookies and
server−based sessions − and using simple examples to illustrate the JSP constructs available to help you
identify and track client requests on your Web site.
You'll also learn more than you want to know about what exactly "maintaining state" actually means, the
advantages and disadvantages of each of the approaches just described...and, if we're doing our job right, get a
laugh or two out of the whole exercise.
A Perfect State 1
Wasted, Dude!
It's one of the things geeks say to each other when they want to impress the young women in earshot: "HTTP
is a stateless protocol, and the Internet is a stateless development environment". In simple language, all this
means is that the HyperText Transfer Protocol, which is the backbone of the Web, is unable to retain a

memory of the identity of each client that connects to a Web site, and therefore treats each request for a Web
page as a unique and independent connection, with no relationship whatsoever to the connections that
preceded it − very similar to the behaviour of some of today's more adventurous teenagers, who get drunk
every night, wake up the next morning with no memory at all of what happened, and go out again in the
evening to do the same thing all over again...
Now, so long as you're aimlessly surfing from one site to another, this works without a problem. But what if
you've decided to buy a few discs from CDNow.com? In a "stateless environment", it would be very difficult
to keep track of all the items you've shortlisted for purchase, as the stateless nature of the HTTP protocol
would make it impossible to keep track of the items selected.
Consequently, what is required is a method that makes it possible to "maintain state", something that allows
client connections to be tracked and connection−specific data to be maintained. And thus came about
"cookies", which allowed Web sites to store client−specific information in a file on the client system, and
access the information in the file whenever required. So, in the shopping cart example above, the items
selected would be added to the cookie, and would be retrieved and presented to the customer in a consolidated
list during the billing process.
Why are they called "cookies"? The PR agency handling the account was obviously asleep at the wheel.
Wasted, Dude! 2
A Few Ground Rules
Since cookies are used to record information about your activities on a particular site, they can only be read by
the site that created them. For example, Yahoo and Deja.com store your username in a cookie on your hard
drive and use this information to automatically fill in log−in forms the next time you visit their Web sites. It's
kinda like going to a chic restaurant, and having the maitre'd call you by name (something which hasn't
happened to us of late!)
Before getting into the nitty−gritty of cookie technology, a few ground rules are in order:
1. A single domain cannot set more than twenty cookies. A single cookie cannot exceed 4 KB in size. The
maximum number of cookies that may be set is 300.
2. The most common method of transmitting a cookie to a client is via the "Set−Cookie" HTTP header.
3. A cookie usually possesses five types of attributes.
The first of these is a NAME=VALUE pair, used to store information such as a username, email address or
credit−card number. The NAME is a string used to identify the cookie, while the VALUE is the data to be

stored in the cookie. For example,
clarkkent=superman
The EXPIRES attribute defines the date on which the cookie is automatically removed from the system. The
date must be in the format "weekday, dd−mon−yy hh:mm:ss GMT". For example,
expires="Sun, 31−Dec−2030 17:51:06 GMT"
Cookies without a specifically defined expiry date remain active for so long as the browser remains open, and
are destroyed once the browser is closed. You can delete an existing cookie be setting this attribute to a date in
the past.
The PATH attribute is used to set the top−level directory on the Web server from which cookies can be
accessed. In most cases, this is set to
path=/
to ensure that the cookie can be accessed by each and every document on the server.
The DOMAIN attribute is used to specify the domain which the cookie is linked to, and the SECURE attribute
indicates that a cookie should only be set if there exists a secure protocol between the browser and the server.
4. Of all the five attributes, the first is the only one that is not optional.
5. Every good browser offers users the option to disable cookies. If a user decides to exercise his or her right
to do so, your cookies will not be stored, and any attempt to access them will fail. Users who do this are
usually career criminals or tax evaders.
A Few Ground Rules 3
Learning To Write...
Now, there are innumerable ways to go about creating and reading cookies on a client browser − you can use
Javascript, you can use PHP, you can use any of the wonderful programming languages out there. However,
our concern here is with JSP − so let's take a look at an example which demonstrates how to read and write a
cookie.
This is a simple hit counter which creates a cookie the first time the user visits the Web page, and then
increments the counter on each subsequent visit.
<%
// counter.jsp
// declare some variables
Cookie cookieCounter = null;

// the cookie you want
String cookieName = "counter";
int cookieFound = 0;
// a few more useful variables
String tempString;
int count=0;
// get an array of all cookies available on client
Cookie[] cookies = request.getCookies();
// iterate through array looking for your cookie
for(int i=0; i<cookies.length; i++)
{
cookieCounter = cookies[i];
if (cookieName.equals(cookieCounter.getName()))
{
cookieFound = 1;
break;
}
}
// if found
if(cookieFound == 1)
{
// get the counter value as string
tempString = cookieCounter.getValue();
Learning To Write... 4
// convert it to a number
count = Integer.parseInt(tempString);
// increment it
count++;
// back to a string
tempString = Integer.toString(count);

// store it in the cookie for future use
cookieCounter.setValue(tempString);
// set some other attributes
cookieCounter.setMaxAge(300);
cookieCounter.setPath("/");
// send cookie to client
response.addCookie(cookieCounter);
}
// if not found
else
{
// create a new cookie with counter 0
Cookie alpha = null;
alpha = new Cookie("counter", "0");
alpha.setMaxAge(300);
alpha.setPath("/");
response.addCookie(alpha);
}
%>
<html>
<head>
<basefont face="Arial">
</head>
<body>
<%
// display appropriate message
if (count > 0)
{
out.println("You have visited this page " + count + " time(s)!
Don't you

have anything else to do, you bum?! ");
}
else
{
The JSP Files (part 6): State Of Grace
Learning To Write... 5
out.println("Welcome, stranger!");
}
%>
</body>
</html>
Sure, it looks a little complicated − but it won't once we break it down for you.
The first thing you need to know is how to create a cookie on the client − this is accomplished with the
following code:
<%
Cookie alpha = null;
alpha = new Cookie("counter", "0");
alpha.setMaxAge(300);
alpha.setPath("/");
response.addCookie(alpha);
%>
The first two lines create a new instance of a Cookie object − "alpha". The cookie variable "counter" is then
initialized and set to the string "0". Next, the setMaxAge() and setPath() methods of the Cookie object are
used to set the expiry date (in seconds) and the cookie's availability, respectively. Finally, a call to the
Response object's addCookie() method takes care of actually transmitting the cookie to the client.
As already mentioned, the only attribute which is not optional is the NAME=VALUE pair. If you'd like your
cookie to remain available even after the user closes the browser, you should explicitly set an expiry date; if
not, the cookie will be destroyed once the browser is closed.
The Cookie object also comes with a couple of other interesting methods.
setValue(someString) − sets the value of the cookie to someString

getValue() − returns the current value of the cookie
setPath(someURL) − sets the PATH attribute of a cookie to someURL
getPath() − returns the current value of the PATH attribute
setMaxAge(someSeconds) − sets the EXPIRES attribute of the cookie, in seconds
getMaxAge() − returns the current value of the EXPIRES attribute
The JSP Files (part 6): State Of Grace
Learning To Write... 6