# Unit 5: Servlets and Java Server Pages (BCS502) ## Servlet Overview and Architecture - **Servlet**: Java class running on a web server that handles requests and generates dynamic responses; runs in a **servlet container** (Tomcat); more efficient than CGI because it uses threads, not new processes. - **Servlet interface** (`javax.servlet.Servlet`): methods `init()`, `service()`, `destroy()`, `getServletConfig()`, `getServletInfo()`. **GenericServlet** and **HttpServlet** are the base classes; HttpServlet provides `doGet()` and `doPost()`. ## Servlet Life Cycle 1. **Loading and instantiation** by the container. 2. **`init(ServletConfig)`** called once. 3. **`service(req,res)`** called for each request, dispatching to `doGet`, `doPost`. 4. **`destroy()`** called once before removal. - Configured in `web.xml` or with `@WebServlet("/path")`. **[Figure: Servlet life cycle]** Load, init, repeated service calls and destroy shown in sequence. ## HTTP GET and POST - `doGet(HttpServletRequest req, HttpServletResponse res)`: parameters in URL, idempotent, bookmarkable, limited length. - `doPost`: parameters in body, secure for sensitive data, no size limit. - Read data with `req.getParameter("name")`; write with `res.getWriter().println()`; set `res.setContentType("text/html")`. - **Redirecting**: `res.sendRedirect(url)` (client makes a new request; URL changes) versus `RequestDispatcher.forward()` (server side, URL unchanged) and `include()`. ## Session Tracking - HTTP is **stateless**, so session tracking maintains state across requests. Techniques: **cookies**, **URL rewriting**, **hidden form fields**, **HttpSession**. - **Cookies**: small text pieces stored in the browser. `Cookie c=new Cookie("user","Ravi"); c.setMaxAge(3600); res.addCookie(c);` read with `req.getCookies()`. Limits: size, can be disabled. - **HttpSession**: server-side object; `HttpSession s=req.getSession(); s.setAttribute("id",5); s.getAttribute("id"); s.invalidate(); setMaxInactiveInterval()`. Session id is passed by cookie `JSESSIONID` or URL rewriting. ## Java Server Pages (JSP) - **JSP**: HTML with embedded Java that is translated into a servlet on first request, compiled and run; easier for presentation than servlets. - Life cycle: translation, compilation, `jspInit()`, `_jspService()`, `jspDestroy()`. - **First JSP example**: `<% out.println("Hello"); %>`. - **Scripting elements**: **declaration** `<%! int x=0; %>`, **scriptlet** `<% ... %>`, **expression** `<%= x %>`, comment `<%-- --%>`. - **Implicit objects**: `request`, `response`, `out`, `session`, `application`, `config`, `pageContext`, `page`, `exception`. - **Directives**: `<%@ page import="java.util.*" errorPage="e.jsp" %>`, `<%@ include file="h.jsp" %>`, `<%@ taglib uri="..." prefix="c" %>`. - **Standard actions**: ``, ``, ``, ``, ``, ``. - **Custom tag libraries**: user-defined tags implemented by tag handler classes and described in a **TLD**; JSTL provides standard tags such as ``, ``. | Servlet | JSP | |---|---| | Java code with HTML in strings | HTML with Java snippets | | Suited for logic and control | Suited for presentation | | Compiled manually | Compiled automatically | ## Likely Exam Questions 1. List the life cycle methods of a servlet. (2 marks) 2. Differentiate between `doGet` and `doPost`. 3. Explain session tracking techniques with cookies and HttpSession. 4. Differentiate between `sendRedirect` and `forward`. (2 marks) 5. Explain JSP implicit objects and scripting elements. 6. Write a JSP page that reads form data and displays it.