After completing this How-to you should be able to:
Identify different mechanisms in which clients'
requests can be tracked.
Save information on the client system using
cookies.
Retrieve cookie information from the client system
Introduction
The main purpose of cookies is to identify users and
possibly prepare customized Web pages for them. When you enter a Web site using
cookies, you may be asked to fill out a form providing such information as your
name and interests. This information is packaged into a cookie and sent to your
Web browser which stores it for later use. The next time you go to the same
Web site, your browser will send the cookie to the Web server. The server can
use this information to present you with custom Web pages. So, for example,
instead of seeing just a generic welcome page you might see a welcome page with
your name on it.
Pre-requisites for running the example
You will need the following to run this example -
Oracle Containers for Java (OC4J 9.0.3) . This can be downloaded from here.
or
Oracle9i JDeveloper. This can be downloaded
from here
Apache Ant 1.4.x, if you are going to deploy on OC4J (Download from Apache).
Description
Java Servlets offer two different ways of retrieving
information about the client state -
Session - HttpSession implements necessary methods like getAttribute()
and setAttribute() which enable tracking client's
service request history.
Cookies - In this document we will discuss this approach to manage client
state.
Cookies are a general mechanism which server side programs
can use to both store and retrieve information on the client side of the connection.
Any future HTTP requests made by the client to the same server side program
will result in the retrieval of client state information stored on the earlier
visit. Cookies are embedded in the HTTP response sent back to the client from
the application server and are saved on the client's file system.
Let us now understand what cookies can and cannot do using
an example. We will assume a simple login page where the user is expected to
provide username and password to login to the application. The first time the
user provides username and password, these values are stored in a cookie. The
next time the user tries to login in, the login information is retrieved from
the cookie and the login fields are defaulted with these values.
Setting Cookies
Here is the code snippet for initializing a cookie and
adding the same to the response object.
.....
Cookie cookie = new Cookie("otncookiename",userName);
cookie.setMaxAge(86400);
response.addCookie(cookie);
Cookie onemorecookie = new Cookie("otncookiepassword",password);
onemorecookie.setMaxAge(86400);
response.addCookie(onemorecookie);
.....
As shown above construction of a Cookie
object requires a name value pair . In the above example, we create two cookies
with names otncookiename and otncookiepassword.
These store the username and password supplied by the user. Another thing to
be noted is the method setMaxAge. This takes as
int as parameter which signifies the expiration time for the cookie. What this
means is that if a positive value x is specified for setMaxAge
the cookie will expire after x seconds. If a negative value is specified the
cookie is not stored persistently and will be deleted when the Web browser exits.
In our example we have specified a value of 86400 seconds for the cookie, so
the cookie will be stored for a day.
Retrieving a cookie value
Here is the code snippet for selecting cookie values from
the request object.
......
Cookie[] cookieArray = request.getCookies();
if( cookieArray != null) {
for( int i = cookieArray.length-1; i >= 0; i-- ) {
Cookie cookie = cookieArray[i];
System.out.println("Cookie
name is " + cookie.getName());
System.out.println("Cookie
value is " + cookie.getValue());
}
}
......
As shown above cookie values stored can be retrieved as an
array from the request object. You can only retrieve
cookies set by your web server. After getting an array of cookies, you can loop
through the array and access the cookie name and values.
Limitation of Cookies
In this section we will see some restrictions on the use
of cookies
The browser should be enabled to accept cookies
Cookie values must never have spaces, commas or semicolons
Cookies can store upto 4KB of value
Running the Sample
The complete source code for this sample is available
here. This section discusses the instruction to run
the sample application
Step 1
Unjar Cookies.jar
using Winzip, or using the following command:
> jar xvf Cookies.jar
This creates a directory Cookies.
Step 2
If you are using JDeveloper then, follow these steps
Open Oracle9i JDeveloper and use File/Open option to select the Cookies.jws
from the Cookies directory.
Next, select Project/Make Cookies.jpr from main menu.
Now, select Run/Run Cookies.jpr from main menu to run the application.
Step 3
If you are using standalone OC4J to run the sample
application then follow these steps
On command prompt , set the environment variables ANT_HOME,JAVA_HOME,OC4J_HOME
and OC4J_HOST
e.g.:
d:\Cookies\set ANT_HOME=d:\ant141
d:\Cookies\set JAVA_HOME=d:\jdk1.3.1_01
d:\Cookies\set OC4J_HOME=d:\oc4j903
d:\Cookies\set OC4J_HOST=localhost:23791
Start OC4J
Run ant
Access the application from any browser : http://<hostname>:<port>/cookieInfo/servlet/cookie
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