6. Life of a Servlet Web Browser Web Server Java Servlet Database 1,2 3 4,5 6
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34. import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWorld extends HttpServlet { public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { res.setContentType("text/html"); PrintWriter out = res.getWriter(); out.println("<HTML>"); out.println("<HEAD><TITLE>Hello World</TITLE></HEAD>"); out.println("<BODY>"); out.println("<BIG>Hello World</BIG>"); out.println("</BODY></HTML>"); } } A Java Servlet : Looks like a regular Java program
35. <html> <head> <title>Hello, World JSP Example</title> </head> <body> <h2> Hello, World! The current time in milliseconds is <%= System.currentTimeMillis() %> </h2> </body> </html> A JSP Page : Looks like a regular HTML page. Embedded Java command to print current time.
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41. // Handle an HTTP GET Request public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/plain"); PrintWriter out = response.getWriter(); out.println ("I was born on: "+birthDate); out.close(); } }
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45. import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class Counter extends HttpServlet { // Create an instance variable int count = 0; // Handle an HTTP GET Request public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/plain"); PrintWriter out = response.getWriter(); count++; out.println ("Since loading, this servlet has " + "been accessed "+ count + " times."); out.close(); } } Only one instance of the counter Servlet is created. Each browser request is therefore incrementing the same count variable.
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48. package coreservlets; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class UserIDs extends HttpServlet { private int nextID = 0; public void doGet( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String title = "Your ID"; String docType = … String id = "User-ID-" + nextID; out.println("<H2>" + id + "</H2>"); nextID = nextID + 1; out.println("</BODY></HTML>"); } } This code is problematic. Can result in a race condition, where two users can actually get the same User-ID! For example: User 1 makes request: String id = "User-ID-" + nextID; Gets nextId of 45. Now User 2 makes request, and pre-empts user 1: String id = "User-ID-" + nextID; Gets nextId of 45 (same one!) Admittedly, this case is rare, but it’s especially problematic. Imagine if user Id was tied to credit card number!
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54. import java.io.*; import java.util.*; import javax.servlet.*; import javax.servlet.http.*; public class Death extends HttpServlet { // Handle an HTTP GET Request public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/plain"); PrintWriter out = response.getWriter(); out.println ("I am alive!"); out.close(); } Continued….
55. // This method is called when one stops // the Java Web Server public void destroy() { try { FileWriter fileWriter = new FileWriter ("rip.txt"); Date now = new Date(); String rip = "I was destroyed at: "+now.toString(); fileWriter.write (rip); fileWriter.close(); } catch (IOException e) { e.printStackTrace(); } } }
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58. import java.io.*; import java.util.*; import javax.servlet.*; import javax.servlet.http.*; public class CounterPersist extends HttpServlet { String fileName = "counter.txt"; int count; public void init () { try { FileReader fileReader = new FileReader (fileName); BufferedReader bufferedReader = new BufferedReader (fileReader); String initial = bufferedReader.readLine(); count = Integer.parseInt (initial); } catch (FileNotFoundException e) { count = 0; } catch (IOException e) { count = 0; } catch (NumberFormatException e) { count = 0; } } At Start-up, load the counter from file. In the event of any exception, initialize count to 0. Continued….
59. // Handle an HTTP GET Request public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/plain"); PrintWriter out = response.getWriter(); count++; out.println ("Since loading, this servlet has " +"been accessed "+ count + " times."); out.close(); } Each time the doGet() method is called, increment the count variable. Continued….
60. // At Shutdown, store counter back to file public void destroy() { try { FileWriter fileWriter = new FileWriter (fileName); String countStr = Integer.toString (count); fileWriter.write (countStr); fileWriter.close(); } catch (IOException e) { e.printStackTrace(); } } } When destroy() is called, store new counter variable back to counter.txt. Any problems with this code?
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82. import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class CookieCounter extends HttpServlet { public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { String name, value = null; Cookie cookie; int counter; res.setContentType("text/html"); // Try to extract the counter cookie (if one exists) Cookie[] cookies = req.getCookies(); for (int i=0; i<cookies.length; i++) { cookie = cookies[i]; name = cookie.getName(); if (name.equals("counter")) value = cookie.getValue(); }
83. // If possible, parse the counter value // Otherwise, start over at 0. if (value != null) counter = Integer.parseInt (value); else counter = 0; // Increment the counter counter++; // Create a new counter cookie // Cookie will exist for one year cookie = new Cookie ("counter", Integer.toString(counter)); cookie.setMaxAge (60*60*24*365); res.addCookie (cookie); // Output number of visits PrintWriter out = res.getWriter(); out.println ("<HTML><BODY>"); out.println ("<H1>Number of visits: "+counter); out.println ("</H1>"); out.println ("</BODY></HTML>"); out.close(); } }
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87. for(int i=0; i<3; i++) { // Default maxAge is -1, indicating cookie // applies only to current browsing session. Cookie cookie = new Cookie("Session-Cookie-" + i, "Cookie-Value-S" + i); response.addCookie(cookie); cookie = new Cookie("Persistent-Cookie-" + i, "Cookie-Value-P" + i); // Cookie is valid for a year, regardless of whether // user quits browser, reboots computer, or whatever. cookie.setMaxAge (60*60*24*365); response.addCookie(cookie); } Code Fragment
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