Servlet Request and Response

@WebServlet("/request-demo")
public class RequestDemoServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {

        // ===== Request Parameters =====
        String name   = req.getParameter("name");           // Single value
        String[] tags = req.getParameterValues("tags");     // Multiple values (checkboxes)
        java.util.Map<String, String[]> params = req.getParameterMap(); // All params

        // ===== Request Headers =====
        String userAgent    = req.getHeader("User-Agent");
        String contentType  = req.getContentType();
        String accept       = req.getHeader("Accept");
        java.util.Enumeration<String> headerNames = req.getHeaderNames();

        // ===== Request Info =====
        String method      = req.getMethod();          // GET, POST, etc.
        String uri         = req.getRequestURI();      // /myapp/request-demo
        String url         = req.getRequestURL().toString(); // http://localhost:8080/myapp/request-demo
        String contextPath = req.getContextPath();     // /myapp
        String servletPath = req.getServletPath();     // /request-demo
        String queryString = req.getQueryString();     // name=Alice&age=25
        String remoteAddr  = req.getRemoteAddr();      // Client IP
        String remoteHost  = req.getRemoteHost();      // Client hostname
        int    serverPort  = req.getServerPort();      // 8080

        // ===== Request Attributes (set by other servlets/filters) =====
        req.setAttribute("processedBy", "RequestDemoServlet");
        Object attr = req.getAttribute("processedBy");

        // ===== Session =====
        HttpSession session = req.getSession();        // Get or create session
        HttpSession existing = req.getSession(false);  // Get only if exists

        // ===== Cookies =====
        Cookie[] cookies = req.getCookies();

        // ===== Request Body (for POST) =====
        // req.getInputStream() - binary data
        // req.getReader()      - text data

        resp.setContentType("text/html;charset=UTF-8");
        PrintWriter out = resp.getWriter();
        out.println("<p>Method: " + method + "</p>");
        out.println("<p>URI: " + uri + "</p>");
        out.println("<p>Name param: " + name + "</p>");
        out.println("<p>Remote IP: " + remoteAddr + "</p>");
    }
}

@WebServlet("/response-demo")
public class ResponseDemoServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {

        String format = req.getParameter("format");

        if ("json".equals(format)) {
            // ===== JSON Response =====
            resp.setContentType("application/json;charset=UTF-8");
            resp.setStatus(HttpServletResponse.SC_OK);
            PrintWriter out = resp.getWriter();
            out.println("{\"name\":\"Alice\",\"age\":25,\"role\":\"admin\"}");

        } else if ("file".equals(format)) {
            // ===== File Download =====
            resp.setContentType("application/octet-stream");
            resp.setHeader("Content-Disposition", "attachment; filename=\"data.txt\"");
            resp.getWriter().println("File content here");

        } else {
            // ===== HTML Response =====
            resp.setContentType("text/html;charset=UTF-8");
            resp.setCharacterEncoding("UTF-8");

            // Set response headers
            resp.setHeader("X-Custom-Header", "MyValue");
            resp.addHeader("Cache-Control", "no-cache, no-store, must-revalidate");
            resp.setIntHeader("X-Request-Count", 42);
            resp.setDateHeader("Expires", 0);

            // Set status code
            resp.setStatus(HttpServletResponse.SC_OK); // 200

            PrintWriter out = resp.getWriter();
            out.println("<h2>Response Demo</h2>");
            out.println("<p>Content-Type: " + resp.getContentType() + "</p>");
            out.println("<p>Buffer size: " + resp.getBufferSize() + "</p>");
        }
    }
}

@WebServlet("/api/users")
public class UserApiServlet extends HttpServlet {

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {

        // Read JSON body from request
        StringBuilder sb = new StringBuilder();
        java.io.BufferedReader reader = req.getReader();
        String line;
        while ((line = reader.readLine()) != null) {
            sb.append(line);
        }
        String jsonBody = sb.toString();
        // jsonBody = {"name":"Alice","email":"alice@example.com"}

        // Parse JSON (using a library like Gson or Jackson in real apps)
        // For demo: simple string parsing
        System.out.println("Received JSON: " + jsonBody);

        // Send JSON response
        resp.setContentType("application/json;charset=UTF-8");
        resp.setStatus(HttpServletResponse.SC_CREATED); // 201
        resp.getWriter().println("{\"status\":\"created\",\"message\":\"User created successfully\"}");
    }
}