Servlet Communication

1. Servlet Communication • Other Servlets, HTML pages, objects shared among servlets on same server • Servlets on another server with HTTP request of the other

2. Using Other Server Resourse • Have servlet make an HTTP request (working with URL’s) • Make a request with RequestDispatcher object if resource available on server that is running servlet.

3. Creating an URL URL gamelan = new URL("http://www.gamelan.com/pages/"); URL gamelanGames = new URL(gamelan, "Gamelan.game.html"); URL gamelanNetwork = new URL(gamelan, "Gamelan.net.html");

4. Parsing a URL • getProtocol • Returns the protocol identifier component of the URL. • getHost • Returns the host name component of the URL. • getPort • Returns the port number component of the URL. The getPort method returns an integer that is the port number. If the port is not set, getPort returns -1. • getFile • Returns the filename component of the URL. • getRef • Returns the reference component of the URL.

5. Working with URL’s • try { • URL yahoo = new URL("http://www.yahoo.com/"); • URLConnection yahooConnection = yahoo.openConnection(); • } catch (MalformedURLException e) { // new URL() failed • . . . • } catch (IOException e) { // openConnection() failed • . . . • }

6. import java.net.*; import java.io.*; public class URLReader { public static void main(String[] args) throws Exception { URL yahoo = new URL("http://www.yahoo.com/"); BufferedReader in = new BufferedReader( new InputStreamReader( yahoo.openStream())); String inputLine; while ((inputLine = in.readLine()) != null) System.out.println(inputLine); in.close(); } }

7. import java.net.*; import java.io.*; public class URLConnectionReader { public static void main(String[] args) throws Exception { URL yahoo = new URL("http://www.yahoo.com/"); URLConnection yc = yahoo.openConnection(); BufferedReader in = new BufferedReader( new InputStreamReader( yc.getInputStream())); String inputLine; while ((inputLine = in.readLine()) != null) System.out.println(inputLine); in.close(); } }

8. import java.io.*; • import java.net.*; • public class Reverse { • public static void main(String[] args) throws Exception { • if (args.length != 1) { • System.err.println("Usage: java Reverse " • + "string_to_reverse"); • System.exit(1); • } • String stringToReverse = URLEncoder.encode(args[0]); • URL url = new URL("http://java.sun.com/cgi-bin/backwards"); • URLConnection connection = url.openConnection(); • connection.setDoOutput(true); • PrintWriter out = new PrintWriter(

9. connection.getOutputStream()); out.println("string=" + stringToReverse); out.close(); BufferedReader in = new BufferedReader( new InputStreamReader( connection.getInputStream())); String inputLine; while ((inputLine = in.readLine()) != null) System.out.println(inputLine); in.close(); } }

10. RequestDispatcher • Get a RequestDispatcher for the resource. • Forward the client request to that resource, having it reply to the user's request. • Include the resource's response in the servlet's output.

11. Using Other Server Resources • Use getRequestDispatcher() to return RequestDispatcher object associated with URL of resource • /servlet/myservlet • /servlet/tests/MyServlet.class • /myinfo.html

12. public class BookStoreServlet extends HttpServlet { public void service (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Get the dispatcher; it gets the main page to the user RequestDispatcher dispatcher = getServletContext().getRequestDispatcher( "/examples/applications/bookstore/bookstore.html"); ... } }

13. Prepare for Errors public class BookStoreServlet extends HttpServlet { public void service (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Get the dispatcher; it gets the main page to the user RequestDispatcher dispatcher = ...; if (dispatcher == null) { // No dispatcher means the html file can not be delivered response.sendError(response.SC_NO_CONTENT); } ... } }

14. Forwarding Request • Resource associated with RequestDispatcher given responsibility for replying to request. • If ServletOutputStream on original, Exception thrown. • If already started reply, use inlcude

15. Forwarding a Request public class BookStoreServlet extends HttpServlet { public void service (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ... // Get or start a new session for this user HttpSession session = request.getSession(); // Send the user the bookstore's opening page dispatcher.forward(request, response); ... } }

16. Include • Calling servlet uses associated resource for part of reply. • Called resource cannot set headers • Caller can use output steams before and after call.

17. Include Order Summary public class ReceiptServlet extends HttpServlet { public void doPut(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { // Process a customer's order ... // Thank the customer for the order res.setContentType("text/html"); PrintWriter toClient = res.getWriter(); ... toClient.println("Thank you for your order!"); // Get the request-dispatcher, to send an order-summary to the client RequestDispatcher summary = getServletContext().getRequestDispatcher( "/OrderSummary");

18. // Have the servlet summarize the order; skip summary on error. if (summary != null) try { summary.include(req, res); } catch (IOException e) { } catch (ServletException e) { } toClient.println("Come back soon!"); toClient.println("</html>"); toClient.close(); }

19. Sharing Resources • On same server, single applications share resources • Use ServletContext interfaces methods: setAttribute, getAttribute, removeAttribute • Name as with packages • examples.bookstore.database.BookDBFrontEnd

20. Setting an Attribute • In init() • Check for attribute and set if not found. • public class CatalogServlet extends HttpServlet { • public void init() throws ServletException { • BookDBFrontEnd bookDBFrontEnd = ... • if (bookDBFrontEnd == null) { • getServletContext().setAttribute( • "examples.bookstore.database.BookDBFrontEnd", • BookDBFrontEnd.instance()); • } • } • ... • }

21. Getting an Attribute public class CatalogServlet extends HttpServlet { public void init() throws ServletException { BookDBFrontEnd bookDBFrontEnd = (BookDBFrontEnd)getServletContext().getAttribute( "examples.bookstore.database.BookDBFrontEnd"); if (bookDBFrontEnd == null) { getServletContext().setAttribute( "examples.bookstore.database.BookDBFrontEnd", BookDBFrontEnd.instance()); } } ... }