Servlets: GET/POST Requests, Redirecting, Session Tracking, and Cookies

Handling HTTP GET Requests

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    String name = req.getParameter("name"); // e.g., /greet?name=Alice
    resp.setContentType("text/html");
    PrintWriter out = resp.getWriter();
    out.println("<h2>Hello, " + name + "!</h2>");
}

Handling HTTP POST Requests

protected void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    String username = req.getParameter("username");
    String password = req.getParameter("password");

    resp.setContentType("text/html");
    PrintWriter out = resp.getWriter();
    out.println("<p>Login received for: " + username + "</p>");
}

GET vs POST in Servlets

Redirecting Requests to Other Resources

sendRedirect() — Client-side redirect

resp.sendRedirect("welcome.jsp"); // browser makes a NEW request; URL changes

RequestDispatcher — Server-side forward/include

RequestDispatcher rd = req.getRequestDispatcher("welcome.jsp");
rd.forward(req, resp);   // hands off control internally; URL stays the same
// rd.include(req, resp); // includes another resource's output, then continues

Session Tracking

HTTP is stateless — each request is independent. Session tracking techniques let a server recognize a user across multiple requests.

Cookies

// Creating and sending a cookie
Cookie userCookie = new Cookie("username", "alice");
userCookie.setMaxAge(60 * 60 * 24); // 1 day, in seconds
resp.addCookie(userCookie);

// Reading cookies on a later request
Cookie[] cookies = req.getCookies();
if (cookies != null) {
    for (Cookie c : cookies) {
        if (c.getName().equals("username")) {
            System.out.println("Welcome back, " + c.getValue());
        }
    }
}

Session Tracking with HttpSession

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    HttpSession session = req.getSession(); // creates one if it doesn't exist

    Integer visitCount = (Integer) session.getAttribute("visitCount");
    if (visitCount == null) {
        visitCount = 1;
    } else {
        visitCount++;
    }
    session.setAttribute("visitCount", visitCount);

    resp.setContentType("text/html");
    resp.getWriter().println("<p>Visit count: " + visitCount + "</p>");
}

Key HttpSession Methods

Key Takeaway: doGet and doPost handle the two main HTTP methods differently — GET via query parameters, POST via the request body. Use sendRedirect for a full client round-trip and RequestDispatcher.forward for an internal handoff. HttpSession is the standard way to track a user across the stateless HTTP protocol, typically backed by a cookie holding the session ID.