Servlet API

tags: 03-web-and-architecture

Definitions

Servlet: Java class handling HTTP requests and generating responses. Extends HttpServlet, implements request processing lifecycle.

HttpServletRequest: Object containing HTTP request data (headers, parameters, body, session). Provides methods accessing request information.

HttpServletResponse: Object for sending HTTP response (status, headers, body). Provides output stream/writer for response generation.

Filter: Component intercepting requests/responses before reaching servlet. Used for logging, authentication, security, compression.

FilterChain: Chain of filters executed sequentially. Each filter decides whether to pass control to next filter or halt processing.

Listener: Component responding to servlet lifecycle events (initialization, request arrival, session creation). Implements ServletContextListener, HttpSessionListener, etc.

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Q&A

What is the servlet lifecycle?

Servlet lifecycle: (1) init() called once when servlet loaded, (2) doGet()/doPost()/etc. called for each request, (3) destroy() called when servlet unloaded. init() performs one-time initialization (database connections, resource loading). doXxx() methods handle actual requests. destroy() cleanup. Spring’s DispatcherServlet follows this lifecycle.

How do filters differ from servlets?

Servlets handle requests and generate responses directly. Filters intercept requests before reaching servlet and responses before returning to client. Multiple filters chain sequentially; each can modify request/response or halt processing. Filters used for cross-cutting concerns (logging, security, compression) without modifying servlet code.

What can you do with HttpServletRequest?

Access request information: getMethod() returns HTTP method, getParameterNames() returns parameter names, getHeader(name) returns header value, getSession() returns session, getCookies() returns cookies, getInputStream() reads request body. Use for extracting form data, headers, session information needed for processing.

How do you set response headers and status codes?

Use HttpServletResponse methods: setStatus(int code) sets HTTP status (200, 404, 500), setHeader(name, value) sets header, addCookie(Cookie) adds cookie. For content type: setContentType("application/json"). Headers must be set before writing response body. Setting headers after body started raises exception.

What is servlet context and how is it used?

ServletContext represents web application. Provides application-wide functionality: store shared objects, get deployment parameters, log messages. Access via getServletContext() in servlet/filter. Use for accessing resources, logging, storing application-level state shared across servlets.

How do request dispatchers work?

RequestDispatcher allows forwarding requests internally: request.getRequestDispatcher("/page.jsp").forward(request, response) transfers control without URL change. Alternative: response.sendRedirect("/page.jsp") sends browser redirect (URL changes). Forward retains request/response objects; redirect creates new request. Use forward for internal processing, redirect for external navigation.

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Code Examples

Example - Simple Servlet:

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    @Override
    public void init() throws ServletException {
        System.out.println("HelloServlet initialized");
    }
    
    @Override
    protected void doGet(HttpServletRequest request, 
                        HttpServletResponse response) 
            throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<h1>Hello, World!</h1>");
        out.close();
    }
    
    @Override
    public void destroy() {
        System.out.println("HelloServlet destroyed");
    }
}

Example - Request Parameters:

@WebServlet("/process")
public class ProcessServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, 
                         HttpServletResponse response) 
            throws ServletException, IOException {
        String name = request.getParameter("name");
        String email = request.getParameter("email");
        
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("Name: " + name);
        out.println("Email: " + email);
    }
}

Example - Filter Implementation:

@WebFilter("/*")
public class LoggingFilter implements Filter {
    @Override
    public void doFilter(ServletRequest request, 
                        ServletResponse response, 
                        FilterChain chain) 
            throws IOException, ServletException {
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        System.out.println("Request: " + httpRequest.getMethod() + 
                         " " + httpRequest.getRequestURI());
        
        chain.doFilter(request, response);  // Continue chain
        
        System.out.println("Response completed");
    }
}

Example - Session Listener:

@WebListener
public class SessionListener implements HttpSessionListener {
    @Override
    public void sessionCreated(HttpSessionEvent event) {
        System.out.println("Session created: " + 
                         event.getSession().getId());
    }
    
    @Override
    public void sessionDestroyed(HttpSessionEvent event) {
        System.out.println("Session destroyed: " + 
                         event.getSession().getId());
    }
}

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Key Points

• Servlet Container: Manages servlet lifecycle, handles HTTP requests, manages threads. Tomcat, Jetty are containers.
• Thread Safety: Servlets are singletons; same instance handles multiple concurrent requests. Ensure thread-safety in servlet code.
• Request/Response Scope: HttpServletRequest/Response available only during request processing; don’t store references for later use.
• Spring Handles Servlets: Spring Boot abstracts away servlet details via DispatcherServlet; most developers never directly implement servlets.
• Filter Ordering: Filter execution order determined by filter registration order; declare in web.xml or use @Order annotation for ordering.