June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 1 Lecture 10 Object Oriented Programming in Java Advanced Topics Servlets

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 2 Today’s Lecture Trail: Servlets Lesson:overview,Cient Interaction, LifeCycle,... Additional Link ml

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 3 Servlet Architecture Client Browser Web server with servlet engine HTTP Servlet 1: send http request 2: call servlet with request info 5: send HTTP response 4: return information from servlet engine 3: perform logic

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 4 1: send HTTP request Handled by regular html tags examples: –simple –with parameters –from a form post

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 5 2: call servlet with request info Handled by the web server transparent to the developer

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 6 3: perform logic Handled by a custom java class which inherits from the Servlet class

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 7 4: return response from servlet engine Handled by the web server transparent to the developer

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 8 5: final step The browser renders the html page which is returned In this class we will only deal with HTML, but others types could be returned by servlets –I.e.: audio, images,...

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 9 Overall Servlet API base class is HttpServlet APIs to: –initialize servlets –receive http requests –retrieve information on the request –send a response to the browser –maintain session across multiple invocations (remember that HTTP is session-less)

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 10 Base Class: HttpServlet import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWorldServlet extends HttpServlet { … } Example from HelloWorldServlet:

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 11 Initialization of Servlet Example from PhoneServlet.java public class PhoneServlet extends HttpServlet { private Hashtable phones = new Hashtable(); private String dataFilePath = "phonelist"; // Public Methods public void init(ServletConfig conf) throws ServletException { super.init(conf); String param = getInitParameter("phonelist"); if (param != null) { dataFilePath = param; }... readFromFile(); }

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 12 HTTP GET and POST Example from PhoneServlet: public class PhoneServlet extends HttpServlet { public void init(ServletConfig conf) throws ServletException {... } public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {... } public void doPost(HttpServletRequest req, HttpServletResponse res) throws IOException {... } doGet is called when we have HTTP GET doPost is called when we have HTTP POST

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 13 Inside doGet and doPost Three usual steps: –read request data such as input parameters –set response headers length, type –write the response data

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 14 Get Request Data and Parameters Example from PhoneServlet public void doPost (HttpServletRequest req, HttpServletResponse res)throws IOException {... String name, number, opcode; name = req.getParameter("name"); number = req.getParameter("number"); opcode = req.getParameter("opcode");... }

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 15 Set Response Headers Example from PhoneServlet: public void doGet (HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { res.setContentType("text/html"); ServletOutputStream out = res.getOutputStream(); String title = "Phones";... out.println(" Phone Servlet Output "); out.println(" "); out.println(" Phone Records "); out.println(" ");... )

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 16 Create response Example from PhoneServlet: public void doGet (HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { res.setContentType("text/html"); ServletOutputStream out = res.getOutputStream(); String title = "Phones";... out.println(" Phone Servlet Output "); out.println(" "); out.println(" Phone Records "); out.println(" ");... )

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 17 HTTP protocol is session-less –between two requests the web server does not remember the caller protocol is as follows: –browser connects to HTTP server –browser makes a request for a web resource –web server looks for the resource –web server forwards the resource to the browser –web server disconnects the browser

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 18 Sessions with Servlets The servlet engine fills-in the gap and provides a way to keep a session across multiple requests to a web server Mechanism is handled by the servlet engine “on demand” by the servlets The base class is HttpSession

June 1, 2000 Object Oriented Programming in Java (95-707) Advanced Topics 19 HttpSession Example from SessionServlet.java public void doGet (HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { HttpSession session = req.getSession(true);... //Here's the meat Integer ival = (Integer) session.getValue("sessiontest.counter"); if (ival==null) ival = new Integer(1); else ival = new Integer(ival.intValue() + 1); session.putValue("sessiontest.counter", ival);... )