Servlet Cookies

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 A Servlet in Java is a server-side program that handles client requests and responses in a web application. Cookies in a servlet are typically used for storing small pieces of data on the client’s browser, which can later be retrieved when needed. A cookie is a small piece of information sent from the server to the client's browser, which stores it and sends it back with subsequent requests.

Below is a basic example of how to work with cookies in a Java Servlet:

Steps:

  1. Set a Cookie (Sending from the server to the client)
  2. Read a Cookie (Retrieving the cookie from the client on subsequent requests)

Example Servlet for Cookies:

1. Setting a Cookie in the Servlet:

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import javax.servlet.annotation.*;

@WebServlet("/SetCookieServlet")
public class SetCookieServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Set the response content type
        response.setContentType("text/html");
        
        // Get the writer to send response
        PrintWriter out = response.getWriter();

        // Create a cookie with a name and value
        Cookie userCookie = new Cookie("username", "john_doe");

        // Set the cookie to expire in 30 minutes (1800 seconds)
        userCookie.setMaxAge(1800);

        // Add the cookie to the response
        response.addCookie(userCookie);

        // Output the response
        out.println("<html><body>");
        out.println("<h2>Cookie has been set!</h2>");
        out.println("<p>Cookie 'username' set with value 'john_doe'.</p>");
        out.println("</body></html>");
    }
}

2. Reading a Cookie in the Servlet:

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import javax.servlet.annotation.*;

@WebServlet("/GetCookieServlet")
public class GetCookieServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Set the response content type
        response.setContentType("text/html");
        
        // Get the writer to send response
        PrintWriter out = response.getWriter();
        
        // Retrieve all cookies from the request
        Cookie[] cookies = request.getCookies();
        
        if (cookies != null) {
            boolean cookieFound = false;
            
            // Loop through the cookies to find the 'username' cookie
            for (Cookie cookie : cookies) {
                if ("username".equals(cookie.getName())) {
                    String username = cookie.getValue();
                    out.println("<html><body>");
                    out.println("<h2>Cookie Found!</h2>");
                    out.println("<p>Welcome back, " + username + "!</p>");
                    out.println("</body></html>");
                    cookieFound = true;
                    break;
                }
            }

            if (!cookieFound) {
                out.println("<html><body>");
                out.println("<h2>No 'username' cookie found.</h2>");
                out.println("</body></html>");
            }
        } else {
            out.println("<html><body>");
            out.println("<h2>No cookies found.</h2>");
            out.println("</body></html>");
        }
    }
}

Key Points:

  • Setting a Cookie: You use response.addCookie(Cookie cookie) to send the cookie from the server to the client. You can also set expiration time using cookie.setMaxAge(int seconds).
  • Reading a Cookie: You retrieve cookies with request.getCookies(), which returns an array of Cookie objects. You can then iterate through them to find a specific cookie.
  • Cookie Expiration: A cookie will expire after the time specified in setMaxAge. If you set it to 0, the cookie is deleted.
  • Cookie Attributes: You can set additional attributes like path, domain, secure flag, and HttpOnly flag.

Notes:

  • Cookies are stored on the client's browser, and they can be viewed or deleted by the user. Therefore, they are not meant to store sensitive or large amounts of data.
  • If the browser does not have cookies enabled, the servlet will not be able to store or retrieve cookies.

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