Unit IV: Java Servlets & Dynamic Web Programming

<web-app>
    <!-- Global Application Parameter (ServletContext) -->
    <context-param>
        <param-name>dbURL</param-name>
        <param-value>jdbc:mysql://localhost:3306/bca_db</param-value>
    </context-param>

    <!-- Servlet Definition & Private Parameter (ServletConfig) -->
    <servlet>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>com.exam.LoginServlet</servlet-class>
        <init-param>
            <param-name>maxAttempts</param-name>
            <param-value>3</param-value>
        </init-param>
    </servlet>
</web-app>

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

public class UserSessionServlet extends HttpServlet {
    protected void doPost(HttpServletRequest req, HttpServletResponse res) 
            throws ServletException, IOException {
        res.setContentType("text/html");
        PrintWriter out = res.getWriter();

        String username = req.getParameter("user");

        // 1. Obtain or create server-side HTTP session
        HttpSession session = req.getSession();
        session.setAttribute("currentUser", username); // Bind object to session
        session.setMaxInactiveInterval(30 * 60);       // Session timeout: 30 minutes

        // 2. Alternatively, create and attach a persistent Cookie
        Cookie userCookie = new Cookie("userToken", username);
        userCookie.setMaxAge(24 * 60 * 60);            // 24 hours persistence
        res.addCookie(userCookie);

        out.println("<h2>Welcome, " + username + "!</h2>");
        out.println("<p>Active Session ID: <code>" + session.getId() + "</code></p>");
        out.println("<p>Creation Time: " + new java.util.Date(session.getCreationTime()) + "</p>");
        out.println("<a href='#dashboard'>Go to Secure Dashboard</a>");
    }
}

<!DOCTYPE html>
<html>
<head>
    <title>Square Calculator</title>
</head>
<body>
    <h2>Calculate Square of a Number</h2>
    <form action="SquareServlet" method="POST">
        Enter a Number: <input type="number" name="num" required />
        <input type="submit" value="Compute Square" />
    </form>
</body>
</html>

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

public class SquareServlet extends HttpServlet {
    protected void doPost(HttpServletRequest req, HttpServletResponse res) 
            throws ServletException, IOException {
        res.setContentType("text/html");
        PrintWriter out = res.getWriter();

        try {
            // Read input parameter from HTTP request
            String numStr = req.getParameter("num");
            double n = Double.parseDouble(numStr);
            double square = n * n;

            // Generate dynamic HTML response
            out.println("<!DOCTYPE html><html><head><title>Result</title></head><body>");
            out.println("<h2>Calculation Result</h2>");
            out.println("<p>Original Number: <b>" + n + "</b></p>");
            out.println("<p>Square: <b>" + square + "</b></p>");
            out.println("<a href='#square'>Calculate Another</a>");
            out.println("</body></html>");
        } catch (NumberFormatException e) {
            out.println("<h3 style='color:red;'>Error: Please enter a valid numerical value!</h3>");
        }
    }
}