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Using Cookies under Servlets

Below is a clear, structured, and detailed discussion of Using Cookies under Servlets.

Using Cookies under Servlets

Introduction

Cookies are small pieces of information stored on the client side by the web browser. In servlet-based applications, cookies are used to maintain state, store user preferences, and track user activity across multiple HTTP requests. Since HTTP is stateless, cookies provide a mechanism to remember information between requests.

What is a Cookie

A cookie is a name–value pair sent by the server to the client and stored by the browser. On subsequent requests, the browser automatically sends the cookie back to the server.

In servlets, cookies are managed using the javax.servlet.http.Cookie class.

Creating a Cookie in a Servlet

Cookie userCookie = new Cookie("username", "admin");

This creates a cookie object with a name and value.

Adding a Cookie to Response

response.addCookie(userCookie);

Once added, the cookie is sent to the client and stored in the browser.

Reading Cookies from Request

Cookie[] cookies = request.getCookies();

if (cookies != null) {
    for (Cookie c : cookies) {
        if (c.getName().equals("username")) {
            String user = c.getValue();
        }
    }
}

The getCookies() method returns all cookies sent by the client.

Setting Cookie Attributes

Cookie Lifetime

userCookie.setMaxAge(60 * 60);   // 1 hour
  • Value in seconds
  • -1 → cookie exists until browser closes
  • 0 → delete cookie

Cookie Path

userCookie.setPath("/app");

Defines the URL path where the cookie is accessible.

Cookie Domain

userCookie.setDomain("example.com");

Allows cookie sharing across subdomains.

Secure Cookie

userCookie.setSecure(true);

Ensures the cookie is sent only over HTTPS.

HttpOnly Cookie

userCookie.setHttpOnly(true);

Prevents client-side scripts from accessing the cookie.

Deleting a Cookie

Cookie c = new Cookie("username", "");
c.setMaxAge(0);
response.addCookie(c);

This removes the cookie from the client browser.

Uses of Cookies in Servlets

  • User authentication tracking
  • Remembering user preferences
  • Shopping cart identification
  • Session tracking
  • Analytics and logging

Advantages of Cookies

  • Simple to implement
  • Lightweight data storage
  • Supported by all browsers
  • Automatic transfer with requests

Limitations of Cookies

  • Stored on client side (less secure)
  • Size limit (approximately 4 KB)
  • Can be disabled by users
  • Not suitable for sensitive data

Cookies vs Sessions (Conceptual Difference)

AspectCookiesSessions
StorageClient sideServer side
SecurityLess secureMore secure
SizeLimitedLarger
LifetimeControlled by browserControlled by server

Best Practices for Using Cookies

  • Avoid storing sensitive information
  • Use HTTPS with secure cookies
  • Set proper expiration time
  • Use HttpOnly and Secure flags
  • Validate cookie values on server

Conclusion

Cookies provide an effective mechanism for maintaining state in servlet-based web applications. By allowing small pieces of data to be stored on the client side, cookies help track users, remember preferences, and support session management. When used carefully with proper security measures, cookies play a vital role in building responsive and user-friendly web applications using servlets.