Using Cookies
Using Cookies:
Cookies are usually small text files, given ID tags that are stored on your computer's browser directory or program data subfolders. Cookies are created when you use your browser to visit a website that uses cookies to keep track of your movements within the site, help you resume where you left off, remember your registered login, theme selection, preferences, and other customization functions.The website stores a corresponding file(with same ID tag)to the one they set in your browser and in this file they can track and keep information on your movements within the site and any information you may have voluntarily given while visiting the website, such as email address.
There are two types of cookies: session cookies and persistent cookies. Session cookies are created temporarily in your browser's subfolder while you are visiting a website. Once you leave the site, the session cookie is deleted. On the other hand, persistent cookie files remain in your browser's subfolder and are activated again once you visit the website that created that particular cookie. A persistent cookie remains in the browser's subfolder for the duration period set within the cookie's file.
Now, let’s develop a servlet that illustrates how to use cookies. The servlet is invoked when a form on a Web page is submitted. The example contains three files as summarized here
File Description
AddCookie.html Allows a user to specify a value for the cookie named MyCookie.
AddCookieServlet.java Processes the submission of AddCookie.html.
GetCookiesServlet.java Displays cookie values.
The HTML source code for AddCookie.html is shown in the following listing. This page contains a text field in which a value can be entered. There is also a submit button on the page. When this button is pressed, the value in the text field is sent to AddCookieServlet via an HTTP POST request.
AddCookie.html:
<html>
<body>
<center>
<form name="Form1"
method="post"
action="http://localhost:8080/examples/servlet/AddCookieServlet">
<B>Enter a value for MyCookie:</B>
<input type=textbox name="data" size=25 value="">
<input type=submit value="Submit">
</form>
</body>
</html>
The source code for AddCookieServlet.java is shown in the following listing. It gets the value of the parameter named “data”. It then creates a Cookie object that has the name “MyCookie” and contains the value of the “data” parameter. The cookie is then added to the header of the HTTP response via the addCookie( ) method. A feedback message is then written to the browser.
AddCookieServlet.java:
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class AddCookieServlet extends HttpServlet
{
public void doPost(HttpServletRequest request,HttpServletResponse response)
throws ServletException, IOException
{
// Get parameter from HTTP request.
String data = request.getParameter("data");
// Create cookie.
Cookie cookie = new Cookie("MyCookie", data);
// Add cookie to HTTP response.
response.addCookie(cookie);
// Write output to browser.
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
pw.println("<B>MyCookie has been set to");
pw.println(data);
pw.close();
}
}
The source code for GetCookiesServlet.java is shown in the following listing. It invokes the getCookies( ) method to read any cookies that are included in the HTTP GET request. The names and values of these cookies are then written to the HTTP response. Observe that the getName( ) and getValue( ) methods are called to obtain this information.
GetCookiesServlet.java:
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class GetCookiesServlet extends HttpServlet
{
public void doGet(HttpServletRequest request,HttpServletResponse response)
throws ServletException, IOException
{
// Get cookies from header of HTTP request.
Cookie[] cookies = request.getCookies();
// Display these cookies.
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
pw.println("<B>");
for(int i = 0; i < cookies.length; i++)
{
String name = cookies[i].getName();
String value = cookies[i].getValue();
pw.println("name = " + name +
"; value = " + value);
}
pw.close();
}
}
Compile the servlet and perform these steps:
1. Start Tomcat, if it is not already running.
2. Display AddCookie.htm in a browser.
3. Enter a value for MyCookie.
4. Submit the Web page.
After completing these steps you will observe that a feedback message is displayed by the browser.
Next, request the following URL via the browser:
http://localhost:9999/servlets/GCS
Observe that the name and value of the cookie are displayed in the browser. In this example, an expiration date is not explicitly assigned to the cookie via the setMaxAge( ) method of Cookie. Therefore, the cookie expires when the browser session ends. You can experiment by using setMaxAge( ) and observe that the cookie is then saved to the disk on the client machine.
Labels: Advanced Java ( Unit 3 )
posted by Swapna @ 3:50 PM
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