Java Servlets

What are Servlets?

Servlets are Java programs that run on a web server
It handle client requests.
They extend the capabilities of servers that host applications accessed via request-response programming model.

Servlet Lifecycle

Date Time

This diagram shows the complete servlet lifecycle:

Client request arrives at the servlet container
Servlet class is loaded and instantiated if needed
init() method is called for initialization
service() method routes to appropriate handler (doGet, doPost, etc.)
Request is processed and response generated
Response is sent back to client
On container shutdown, destroy() method is called for cleanup

The lifecycle has three main phases:

Initialization: init() method
Request handling: service(), doGet(), doPost() methods
Destruction: destroy() method

Basic Servlet Structure

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

public class HelloServlet extends HttpServlet {

    // Initialize servlet
    public void init() throws ServletException {
        // Initialization code
    }

    // Handle GET requests
    protected void doGet(HttpServletRequest request,
                        HttpServletResponse response)
            throws ServletException, IOException {

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        out.println("<html><body>");
        out.println("<h1>Hello World Servlet!</h1>");
        out.println("</body></html>");
    }

    // Handle POST requests
    protected void doPost(HttpServletRequest request,
                         HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }

    // Clean up resources
    public void destroy() {
        // Cleanup code
    }
}

Web.xml Configuration (Traditional)

<web-app>
    <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>com.example.HelloServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
</web-app>

Annotation-based Configuration (Servlet 3.0+)

@WebServlet(
    name = "HelloServlet",
    urlPatterns = {"/hello", "/greet"},
    loadOnStartup = 1
)
public class HelloServlet extends HttpServlet {
    // Servlet code
}

Request Handling Example

@WebServlet("/user")
public class UserServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request,
                        HttpServletResponse response)
            throws ServletException, IOException {

        // Get request parameters
        String name = request.getParameter("name");
        String age = request.getParameter("age");

        // Set response type
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        out.println("<h1>User Information</h1>");
        out.println("<p>Name: " + name + "</p>");
        out.println("<p>Age: " + age + "</p>");
    }

    protected void doPost(HttpServletRequest request,
                         HttpServletResponse response)
            throws ServletException, IOException {

        // Handle form submission
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        // Process data (simple authentication example)
        if ("admin".equals(username) && "password".equals(password)) {
            response.sendRedirect("welcome.jsp");
        } else {
            response.sendRedirect("error.jsp");
        }
    }
}

Session Management

@WebServlet("/session")
public class SessionServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request,
                        HttpServletResponse response)
            throws ServletException, IOException {

        // Get or create session
        HttpSession session = request.getSession(true);

        // Set session attributes
        session.setAttribute("user", "JohnDoe");
        session.setAttribute("visitCount", 1);

        // Get session attributes
        Integer count = (Integer) session.getAttribute("visitCount");
        if (count != null) {
            session.setAttribute("visitCount", count + 1);
        }

        response.getWriter().println("Visit count: " +
            session.getAttribute("visitCount"));
    }
}

Servlet Context and Initialization Parameters

@WebServlet(
    urlPatterns = "/config",
    initParams = {
        @WebInitParam(name = "dbUrl", value = "jdbc:mysql://localhost:3306/mydb"),
        @WebInitParam(name = "maxUsers", value = "100")
    }
)
public class ConfigServlet extends HttpServlet {

    private String dbUrl;
    private int maxUsers;

    public void init() throws ServletException {
        // Get init parameters
        dbUrl = getInitParameter("dbUrl");
        maxUsers = Integer.parseInt(getInitParameter("maxUsers"));

        // Get context parameters
        ServletContext context = getServletContext();
        String appName = context.getInitParameter("appName");
    }

    protected void doGet(HttpServletRequest request,
                        HttpServletResponse response)
            throws ServletException, IOException {

        response.getWriter().println("DB URL: " + dbUrl);
        response.getWriter().println("Max Users: " + maxUsers);
    }
}

Filter Example

@WebFilter("/*")
public class LoggingFilter implements Filter {

    public void init(FilterConfig config) throws ServletException {
        // Initialization code
    }

    public void doFilter(ServletRequest request,
                        ServletResponse response,
                        FilterChain chain)
            throws IOException, ServletException {

        // Pre-processing
        System.out.println("Request received at: " + new Date());
        long startTime = System.currentTimeMillis();

        // Pass request to next filter or servlet
        chain.doFilter(request, response);

        // Post-processing
        long endTime = System.currentTimeMillis();
        System.out.println("Request processed in: " +
                          (endTime - startTime) + "ms");
    }

    public void destroy() {
        // Cleanup code
    }
}

Servlet Architecture

graph TB
    subgraph "Client Side"
        Browser[Web Browser]
    end
    
    subgraph "Web Server"
        subgraph "Servlet Container"
            Filter1[Authentication Filter]
            Filter2[Logging Filter]
            Servlet1[Servlet 1]
            Servlet2[Servlet 2]
            Session[Session Manager]
        end
    end
    
    Browser -->|HTTP Request| Filter2
    Filter2 -->|Request| Filter1
    Filter1 -->|Request| Servlet1
    Servlet1 -->|Response| Filter1
    Filter1 -->|Response| Filter2
    Filter2 -->|HTTP Response| Browser
    
    Servlet1 --> Session
    Servlet2 --> Session

Common Interview Questions with Code Examples

1. How to handle file upload?

@WebServlet("/upload")
@MultipartConfig
public class FileUploadServlet extends HttpServlet {

    protected void doPost(HttpServletRequest request,
                         HttpServletResponse response)
            throws ServletException, IOException {

        // Get uploaded file
        Part filePart = request.getPart("file");
        String fileName = filePart.getSubmittedFileName();

        // Save file
        InputStream fileContent = filePart.getInputStream();
        Files.copy(fileContent, Paths.get("/uploads/" + fileName));

        response.getWriter().println("File uploaded successfully!");
    }
}

2. How to implement redirect and forward?

@WebServlet("/router")
public class RouterServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request,
                        HttpServletResponse response)
            throws ServletException, IOException {

        String action = request.getParameter("action");

        if ("redirect".equals(action)) {
            // Client-side redirect
            response.sendRedirect("https://example.com");
        } else if ("forward".equals(action)) {
            // Server-side forward
            RequestDispatcher dispatcher =
                request.getRequestDispatcher("/targetServlet");
            dispatcher.forward(request, response);
        } else if ("include".equals(action)) {
            // Include another resource
            RequestDispatcher dispatcher =
                request.getRequestDispatcher("/header.jsp");
            dispatcher.include(request, response);
        }
    }
}

3. How to handle cookies?

@WebServlet("/cookie")
public class CookieServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request,
                        HttpServletResponse response)
            throws ServletException, IOException {

        // Create cookie
        Cookie userCookie = new Cookie("username", "JohnDoe");
        userCookie.setMaxAge(3600); // 1 hour
        response.addCookie(userCookie);

        // Read cookies
        Cookie[] cookies = request.getCookies();
        if (cookies != null) {
            for (Cookie cookie : cookies) {
                if ("username".equals(cookie.getName())) {
                    response.getWriter().println(
                        "Welcome back " + cookie.getValue());
                }
            }
        }
    }
}

Key Points to Remember for Interview:

Lifecycle Methods: init(), service(), doGet(), doPost(), destroy()
Thread Safety: Servlets are multithreaded - avoid instance variables
Session vs Request: Session persists across requests, request is per-request
GET vs POST: GET for data retrieval, POST for data modification
Filters: For cross-cutting concerns (logging, security, etc.)
Annotations vs web.xml: Annotations are simpler, web.xml offers more configuration options
Servlet Context: Application-wide scope, shared by all servlets
Response Types: setContentType() is crucial for proper response handling