The JSP Files (part 6): State Of Grace

Now that you’ve mastered the basics, it’s time to bring out the big iron. This week, The JSP Files explores the various techniques available to “maintain state” on a JSP-based Web site. Learn about the Cookie and Session objects, find out how to build a cookie-based hit counter, and read about a simple yet effective way of protecting sensitive Web pages with the Session object.

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.{mospagebreak title=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.{mospagebreak title=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.{mospagebreak title=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 { 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

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).{mospagebreak title=…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); } %>
{mospagebreak title=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; 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?{mospagebreak title=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.{mospagebreak title=Session Dissection} Creating a JSP session is much simpler than writing a cookie. To demonstrate this, here’s the session equivalent of the cookie-based counter you saw a few pages back.

<html> <head> </head> <body> <% // get the value of the session variable Integer visits = (Integer)session.getValue("counter"); // if null if (visits == null) { // set it to 0 and print a welcome message visits = new Integer(0); session.putValue("counter", visits); out.println("Welcome, stranger!"); } else { // else increment and write the new value visits = new Integer(visits.intValue() + 1); session.putValue("counter", visits); out.println("You have visited this page " + visits + " time(s)! Don't you have anything else to do, you bum?! "); } %> </body> </html>

There isn’t much you have to do to create a session – simply use the putValue() method of the Session object to create one or more session variable, and JSP will automatically create a session and register the variables. You can then use the getValue() method to retrieve the values of the session variables automatically.

An important point to be noted here is that it is necessary to typecast the session variable while using getValue() – in the example above, we’ve specifically stated the type of the variable in parentheses before assigning it to a regular JSP variable. Since JSP allows you to bind objects to the session, you can bind an Integer object and thereby bypass some of the string-to-number conversion routines in the equvalent cookie example.

With this information in mind, the example above becomes much simpler to read. An “if” statement is used to take care of the two possible alternatives: a first-time visitor (no prior session) or a returning visitor (pre-existing session). Depending on whether or not the “counter” variable exists, appropriate action is taken.

The Session object also comes with a bunch of other interesting methods - here are some of them:

getId() – returns a string containing the unique session ID

setMaxInactiveInterval(someSeconds) – keeps the session active for someSeconds duration after the last client request

invalidate() – destroy the session

getAttribute() and setAttribute() – try these is getValue() and putValue() don’t work

getCreationTime() – returns the time at which this session was created, in seconds, as an offset from midnight January 1 1970{mospagebreak title=Access Denied} Here’s another simple example which demonstrates some of the methods above, and also illustrates how JSP sessions can be used to protect Web pages with sensitive information.

This example presents a form (“start.html”) asking for your name, and takes you to a new page (“login.jsp”) once you submit the form. “login.jsp” creates a session to store the name you entered, and offers a link to “rootshell.jsp”, which is the sensitive file to be protected.

So long as the session is active, any attempt to access the page “rootshell.jsp” will succeed. On the flip side, if a session is not active, any attempt to access “rootshell.jsp” by bypassing the initial form will fail, and the user will be redirected to “start.html”.

This is a relatively primitive example, but serves to demonstrate one of the more common uses of session variables.

All the redirection in this example is accomplished using the Response object (you remember this, don’t you?)

<html> <head> <basefont face="Arial"> </head> <body> <!-- start.html --> <form action="login.jsp" method="post"> <table> <tr> <td>Your name</td> <td><input type=text name=username> <input type="Submit" value="Click me"></td> </tr> </table> </form> </body> </html>

Once the form is submitted, “login.jsp” takes over.

<html> <head> <basefont face="Arial" </head> <body> <% // get the form variable String username = request.getParameter("username"); // create a session session.putValue("username", username); // set a timeout period session.setMaxInactiveInterval(300); // display a link to the protected file out.println("Thank you for using this service."); out.println("Click <a href=rootshell.jsp>here</a> for root access"); %> </body> </html>

And here’s the top-secret page.

<html> <head> <basefont face="Arial"> </head> <body> <% // rootshell.jsp // get the username from the session String username = (String)session.getValue("username"); // if null, security breach! if (username == null) { response.setHeader("Location", "start.html"); } else { // display the protected page %> Welcome to your root shell, <b><%= username %></b>! <p> Your session ID is <% out.println( session.getId() ); %> <p> This session will expire in <% out.println( session.getMaxInactiveInterval() ); %> seconds. <% } %> </body> </html>

To test this, first log in and find your way to “rootshell.jsp” – you should have no trouble accessing it. Then close the browser, start it up again, and try to get to “rootshell.jsp” without going through the login process; you should be automatically redirected to the login page.

And that’s about it. You should now have a pretty clear idea of how JSP attempts to solve the “stateless protocol” problem, together with some understanding of how to create and use both client-side cookies and server-side sessions. Go practice!
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