The JSP Files (Part 6) - State of Grace

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.

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

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

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

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();

// 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

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

setDomain(someURL) − sets the DOMAIN attribute of the cookie

getDomain() − returns the current value of the DOMAIN attribute

setSecure(flag) − sets the SECURE attribute of the cookie as either true or false

getSecure() − returns the current value of the SECURE attribute

Note that you can only save string values in a cookie with setValue() − which entails a lot of

string−to−number−to−string conversions if you actually want to store a number (as in this example)

The JSP Files (part 6): State Of Grace

And Read

So that takes care of writing a cookie − but how about reading it? Here's the code

<%

// 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;

}

}

%>

Before you can read the cookie, you need to find it on the client's hard drive Since JSP does not currently allow you to directly locate and identify the cookie by name, you need to iterate through all available cookies until you find the one you're looking for In the example above, the "for" loop does just that; if and when it finds the cookie, it sets the "cookieFound" variable to 1 and breaks out of the loop

At this point, the cookie is stored in the Cookie object "cookieCounter" You can then use the getValue() object method to get the current value of the cookie variable, and use it in your script

// if found

if(cookieFound == 1)

{

// get the counter value as string

tempString = cookieCounter.getValue();

// 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);

}

%>

The JSP Files (part 6): State Of Grace

What's In A Name?

Once you've understood these two fundamental techniques, the rest of the code should be simple to decipher

If a cookie is found, the counter variable is incremented, and the setValue() method is used to write a new value to the cookie; this counter variable is then displayed on the page If a cookie is not found, it implies that this is the user's first visit to the page (or a visit made after a previous cookie has expired); a new cookie is set and an appropriate message displayed

Again, since this example deals with numbers rather than strings, innumerable contortions are required to convert the string value in the cookie to a number, increment it, and then convert it back to a string for storage

in the cookie

Here's another example, this one a simple form Enter your name and submit the form − a cookie will be created containing the name you entered When you next visit the page, your name will be automatically filled

in for you

<%

// form.jsp

// declare some variables

Cookie thisCookie = null;

// the cookie you want

String cookieName = "username";

int cookieFound = 0;

String username = "";

// 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++)

{

thisCookie = cookies[i];

if (cookieName.equals(thisCookie.getName()))

{

cookieFound = 1;

break;

}

}

// if found

if(cookieFound == 1)

{

// get the counter value

username = thisCookie.getValue();

}

%>

<html>

<head>

<basefont face="Arial">

</head>

<body>

<form action="login.jsp" method="post">

<table>

<tr>

<td>Your name</td>

<td><input type=text name=username value="<%= username

%>"> <input

type="Submit" value="Click me"></td>

</tr>

</table>

</form>

</body>

</html>

This is the initial login form, "form.jsp" − as you can see, it checks for the presence of a cookie, and uses it to fill in the account username if available

When the form is submitted, "login.jsp" is called to process the data entered into the form; it will also set cookie attributes appropriately

<%

// login.jsp

// get values from form

String username = request.getParameter("username");

// create a new cookie to store the username

Cookie alpha = null;

The JSP Files (part 6): State Of Grace

alpha = new Cookie("username", username);

alpha.setMaxAge(300);

alpha.setPath("/");

// send it to client

response.addCookie(alpha);

%>

<html>

<head>

<basefont face="Arial">

</head>

<body>

Get lost, <b><%= username %></b>!

</body>

</html>

Simple, huh?

The JSP Files (part 6): State Of Grace

Plan B

The cookie−based approach is quite common; many Web sites use it, because it is flexible, simple, and independent of the server−side language (once the cookie has been saved to the client's hard drive, you can read it using JavaScript, or PHP, or JSP, or ) The only problem: it is dependent on the cookie being accepted

by the client

And so, another common approach is the use of a "session" to store specific bits of information when a client visits a Web site; this session data is preserved for the duration of the visit, and is usually destroyed on its conclusion A session can thus be considered a basket of information which contains a host of variable−value pairs; these variable−value pairs exist for the duration of the visit, and can be accessed at any point during it This approach provides an elegant solution to the "stateless" nature of the protocol, and is used on many of today's largest sites to track and maintain information for personal and commercial transactions

Every session created is associated with a unique identification string, or "session ID"; this string is sent to the client, while a temporary entry with the same unique identification number is created on the server, either in a flat file or in a database It now becomes possible to register any number of "session variables" − these are ordinary variables, which can be used to store textual or numeric information, and can be read from, or written

to, throughout the session

The session ID is transmitted to the client either via a cookie, or via the URL GET method The client, in turn, must reference each request with this session ID, so that the server knows which session each client is

associated with and uses the appropriate session variables for each client In case the client doesn't support cookies and the URL method is rejected or not used, session management capabilities and session variables will not be available to the client, and every request will be treated as though it were coming for the first time

Sessions are typically left active for as long as the user's browser is open, or for a pre−defined period Once the user's browser is closed, or the specified time period is exceeded, the session and all variables within it are automatically destroyed