1. 240-322 Cli/Serv.: jnet/111 Client/Server Distributed Systems v Objectives –describe basic networking in Java – Web page retrieval and sockets programming 240-322, Semester 1, 2005-2006 11. Java Networking

2. 240-322 Cli/Serv.: jnet/112 Contents 1.Networking Restrictions in Java 2.Basic Networking Classes 3.Sockets (Java Style) 4.Retrieving a Web Page –4 different approaches 5.A Stream Socket Client/Server continued

3. 240-322 Cli/Serv.: jnet/113 6.A Datagram Socket Client/Server 7.Networked Tic-Tac-Toe 8.More Information

4. 240-322 Cli/Serv.: jnet/114 1. Networking Restrictions in Java v Many browsers place networking restrictions on applets –usually communication is limited to being between your machine and the one where the applet came from  There are no restrictions on Java applications or when using appletviewer

5. 240-322 Cli/Serv.: jnet/115 Relaxing Security v Applets can be signed with trusted certificates –a browser can be configured to relax security depending on an applet’s signature –an advanced topic

6. 240-322 Cli/Serv.: jnet/116 2. Basic Networking Classes  The J2SE documentation for java.net lists over 30 classes. A few of the key ones:  InetAddress –the class that represents IP addresses and contains operations for manipulating them u see section 2.1 continued

7. 240-322 Cli/Serv.: jnet/117  URL –used to retrieve the Web page at the given URL  URLConnection –also used to retrieve a Web page –allows extra parameters to be sent to the URL u e.g HTTP request headers continued

8. 240-322 Cli/Serv.: jnet/118  Socket –the client-side socket class for TCP  ServerSocket –the server-side socket class for TCP  DatagramSocket –allows a client or server to send/receive UDP packets

9. 240-322 Cli/Serv.: jnet/119 2.1. Finding an IP Address  Java’s InetAddress class makes the mapping between hostnames and IP addresses much easier than in UNIX.  For details, look at the the documentation for java.net.InetAddress

10. 240-322 Cli/Serv.: jnet/1110 WhatIP.java import java.io.*; import java.net.*; public class WhatIP { public static void main(String args[]) throws IOException { InetAddress addr = InetAddress.getByName(args[0]); System.out.println("Inet address is "+ addr); } }

11. 240-322 Cli/Serv.: jnet/1111 Use Carried out from a DOS prompt on my Windows machine.

12. 240-322 Cli/Serv.: jnet/1112 3. Sockets (Java Style) v 1. Stream Sockets –the client/server connection exists for the entire duration of the request and answer u similar to a telephone call –a connection-oriented service u corresponds to TCP –Java has separate classes for client and server stream sockets (see section 5)

13. 240-322 Cli/Serv.: jnet/1113 v 2. Datagram Sockets –the client/server send messages (packets, datagrams) to each other u similar to the postal service –a connectionless service u corresponds to UDP –Java has classes for datagram sockets and packets (see section 6).

14. 240-322 Cli/Serv.: jnet/1114 3.1. Pinging  ping uses the ICMP protocol's ECHO_REQUEST datagram. The host’s answer is an ICMP ECHO_RESPONSE. ping is a DOS command, not Java.

15. 240-322 Cli/Serv.: jnet/1115  ICMP packets can only be created via a socket of type SOCK_RAW. –but Java only supports SOCK_STREAM (TCP) and SOCK_DGRAM (UDP) sockets  Our solution: use a TCP connection to the daytime service to pretend to ping –the daytime server listens at port 13

16. 240-322 Cli/Serv.: jnet/1116 3.2. DayPing.java JJ2, p.677 import java.io.*; import java.net.*; public class DayPing { public static void main(String args[]) throws Exception { if (args.length != 1) { System.out.println( "Usage: java DayPing "); System.exit(0); } :

17. 240-322 Cli/Serv.: jnet/1117 String machine = args[0]; Socket so = new Socket(machine, 13); // daytime server listens at port 13 BufferedReader br = new BufferedReader( new InputStreamReader( so.getInputStream() ) ); System.out.println( machine + " is alive at " + br.readLine()); so.close(); } // end of main() } // end of DayPing class

18. 240-322 Cli/Serv.: jnet/1118  DayPing.java converts the socket connection into a BufferedReader –this allows readLine() to be used for socket input v Using layered streams to add I/O functionality on top of sockets is a powerful feature of Java. Notes

19. 240-322 Cli/Serv.: jnet/1119 Use v v C> javac DayPing.java v v C> java DayPing calvin calvin is alive at Sat May 7 13:46:06 2005 v v C> java DayPing fourdots Exception in thread "main" java.net.ConnectException: Connection refused: no further information at java.net.PlainSocketImpl.socketConnect (Native Method) at java.net.PlainSocketImpl.doConnect... : at ping.main(DayPing.java:23) All the examples were tested on my Windows machine continued

20. 240-322 Cli/Serv.: jnet/1120 v v C> java DayPing takasilla Exception in thread "main" java.net.UnknownHostException: takasilla at java.net.PlainSocketImpl. connect(Unknown Source) : at ping.main(DayPing.java:23) v v C> java DayPing www.amazon.com Exception in thread "main" java.net.NoRouteToHostException: Operation timed out: connect at java.net.PlainSocketImpl. socketConnect(Native Method) : at ping.main(DayPing.java:23) After a long wait After a wait

21. 240-322 Cli/Serv.: jnet/1121 3.3. Why Doesn't DayPing Work? v The examples that don't work show three different errors: –ConnectException –UnknownHostException –NoRouteToHostException v The "unknown host" exception is caused by an incorrect IP address. continued

22. 240-322 Cli/Serv.: jnet/1122 v The "connection" exception probably means that the server is unavailable (switched off) –this can be confirmed for a TCP service by trying to connect to it with telnet : $ telnet fourdots 13 Trying 172.30.255.4... telnet: Unable to connect to remote host: Connection refused $ continued executed from a different machine (fivedots)

23. 240-322 Cli/Serv.: jnet/1123 v The "no route to host" exception usually means that there is a firewall preventing the connection from being made.  telnet response: $ telnet www.amazon.com 13 Trying 207.171.163.90... telnet: Unable to connect to remote host: Connection timed out $ A long wait, and then... tried on (fivedots)

24. 240-322 Cli/Serv.: jnet/1124 3.4. Firewalls at PSU takasila calvin fivedots Departmental firewall (no external socket creation; URLs allowed) ratree Univ. firewall (no external socket creation; URLs only through the ‘cache’ proxy) cache The Internet University CoE Department (simplified) 8080

25. 240-322 Cli/Serv.: jnet/1125 v Ordinary users (i.e. students) cannot write socket-based programs that communicate outside PSU –this is true in any language (Java, C, etc.)  But programs can retrieve Web pages (and other things) using the Java URL class –the URL request must go through PSU's cache machine

26. 240-322 Cli/Serv.: jnet/1126 URLs and Sockets v From your data communications course, you may recall that the Web protocol, HTTP, is built on top of TCP/IP. v Java's URL class uses stream sockets (TCP/IP) for its implementation –so why does it get past cache ? continued

27. 240-322 Cli/Serv.: jnet/1127  cache accepts socket links to valid Web servers, but also checks that the HTTP GET (or POST) message includes additional valid HTTP headers. –all of this HTTP stuff is carried out by the URL class for us

28. 240-322 Cli/Serv.: jnet/1128 4. Retrieving a Web Page v Five ways of obtaining a Web page: –1. use a socket, and send a GET message to the server  see GetSocketPage.java –2. use a URL object, and read the page via a stream  see GetURLPage.java continued

29. 240-322 Cli/Serv.: jnet/1129 –3. use a URL object, and display the page in a browser  see showPage.java –4. display a URL in an JEditorPane  see ShowURLPage.java –5. use a HTTP URL connection, and send a GET message u see Java’s networking tutorial

30. 240-322 Cli/Serv.: jnet/1130 4.1. Sockets and GET  GetSocketPage.java retrieves the page: http:// /index.html e.g. http://fivedots.coe.psu.ac.th/index.html  It prints the text of the page to stdout. continued

31. 240-322 Cli/Serv.: jnet/1131 v It opens a socket at port 80 for the host, which is the usually place where the Web server is listening.  It sends the HTTP GET message: GET /index.html

32. 240-322 Cli/Serv.: jnet/1132 Diagram GetSocketPage client host Web server GET /index.html Web page (as text) 80

33. 240-322 Cli/Serv.: jnet/1133 GetSocketPage.java import java.io.*; import java.net.*; public class GetSocketPage { public static void main(String args[]) throws IOException { Socket sock = new Socket(args[0],80); :

34. 240-322 Cli/Serv.: jnet/1134 BufferedReader dis = new BufferedReader( new InputStreamReader( sock.getInputStream() )); PrintStream ps = new PrintStream( sock.getOutputStream() ); ps.println("GET /index.html"); String line; while ((line = dis.readLine()) != null) System.out.println(line); sock.close(); } } // end of GetSocketPage.java

35. 240-322 Cli/Serv.: jnet/1135  GetSocketPage.java converts the socket connection into a BufferedReader for input, and a PrintStream for output –uses readLine() for socket input –uses println() for socket output Notes

36. 240-322 Cli/Serv.: jnet/1136 Use C> javac GetSocketPage.java C> java GetSocketPage fivedots Un title page : C> text of Web page printed to stdout

37. 240-322 Cli/Serv.: jnet/1137 But... Exception in thread "main" java.net.ConnectException: Connection timed out: connect at java.net.PlainSocketImpl. socketConnect(Native Method) : at GetSocketPage.main(GetSocketPage.java:18) C> java GetSocketPage www.amazon.com Exception in thread "main" java.net.ConnectException: Connection timed out: connect at java.net.PlainSocketImpl. socketConnect(Native Method) : at GetSocketPage.main(GetSocketPage.java:18) The firewall around PSU prevents Web server access by sockets. continued

38. 240-322 Cli/Serv.: jnet/1138 v cache rejected the GET message to the external site since the message didn't include additional HTTP headers. v These can be supplied by us, but it's easier to use the URL class.

39. 240-322 Cli/Serv.: jnet/1139 4.2. URL Object  GetURLPage.java avoids the firewall problem with sockets by using a URL object.  A URL object allows a Web page to be retrieved as a stream of text –our program prints the text to stdout.

40. 240-322 Cli/Serv.: jnet/1140 GetURLPage.java import java.net.*; import java.io.*; public class GetURLPage { public static void main(String args[]) { try { URL url = new URL(args[0]); BufferedReader dis = new BufferedReader( new InputStreamReader( url.openStream() )); :

41. 240-322 Cli/Serv.: jnet/1141 String line; while ( (line = dis.readLine()) != null ) System.out.println(line); dis.close(); } catch (Exception e) { System.out.println(e); } } } // end of GetURLPage.java

42. 240-322 Cli/Serv.: jnet/1142  A stream for the URL object is obtained using openStream() –after that the same input stream layering technique is used as in GetSocketPage.java –there is no need to send a GET message Notes

43. 240-322 Cli/Serv.: jnet/1143 Use v v C> javac GetURLPage.java v v C> java GetURLPage http://www.amazon.co.uk java.net.ConnectException: Connection timed out: connect v v C> java -DproxySet=true -DproxyHost=cache.psu.ac.th -DproxyPort=8080 GetURLPage http://www.amazon.com/ Amazon Page : C> typed on one line after a long wait

44. 240-322 Cli/Serv.: jnet/1144  Ordinary users can access outside PSU by using URLs, but they must route their requests via PSU cache machine.

45. 240-322 Cli/Serv.: jnet/1145 Batch Files for Long Commands  GetURLPage.bat contains: @echo off echo Executing GetURLPage... java -DproxySet=true -DproxyHost=cache.psu.ac.th -DproxyPort=8080 GetURLPage %1 v Use: c> GetURLPage http://www.amazon.co.uk : // same output as last slide

46. 240-322 Cli/Serv.: jnet/1146 Proxy Username/Password v A further level of firewall security is to require the user to enter a username and password –called proxy authorization v Java has network support for authorization –it allows a username and password to be sent by a client program to the firewall continued

47. 240-322 Cli/Serv.: jnet/1147 v A good tutorial: –“Jump a Proxy/Firewall and Live to Tell About it” http://www.devx.com/upload/free/features/ javapro/2000/03mar00/te0003/te0003.asp  Slightly modified code is in GetThroughProxy.java at: http://fivedots.coe.psu.ac.th/ Software.coe/Cliserv/ Code%20Examples/Java%20Code/ Basic%20Networking/

48. 240-322 Cli/Serv.: jnet/1148 4.3. Passing a URL to a Browser v If the Java code is in an applet, then the downloaded URL can be displayed in the browser.  showPage.html displays a user-specified Web page in the (Opera) browser, using the ShowPage.java applet.

49. 240-322 Cli/Serv.: jnet/1149 Usage continued The dialog box appears in front of the browser window.

50. 240-322 Cli/Serv.: jnet/1150 Loaded Page:

51. 240-322 Cli/Serv.: jnet/1151 showPage.html Web Page Loading Applet Web Page Loading Applet Applet is placed here... Web Page Loading Applet Web Page Loading Applet Applet is placed here...

52. 240-322 Cli/Serv.: jnet/1152 showPage.java import java.net.*; import javax.swing.*; import java.applet.AppletContext; import javax.swing.JOptionPane; public class ShowPage extends JApplet { public void init() { try { String urlString = JOptionPane.showInputDialog( "Enter a URL:"); URL url = new URL(urlString); :

53. 240-322 Cli/Serv.: jnet/1153 AppletContext browser = getAppletContext(); browser.showDocument(url); } catch (Exception e) { System.out.println(e); } } // end of init() } // end of ShowPage AppletContext browser = getAppletContext(); browser.showDocument(url); } catch (Exception e) { System.out.println(e); } } // end of init() } // end of ShowPage

54. 240-322 Cli/Serv.: jnet/1154 Notes  The download only works because the browser is set up to work through cache. v There are no Java security restrictions on a applet passing a URL to a browser to be displayed. continued

55. 240-322 Cli/Serv.: jnet/1155  A URL is downloaded with showDocument(), which must be called in the applet’s environment (i.e. the browser): –AppletContext browser = getAppletContext(); browser.showDocument(url);  The URL is automatically loaded into the browser, replacing showPage.html. continued

56. 240-322 Cli/Serv.: jnet/1156  There is a two-argument version of showDocument() that can load URLs into frames.  D&D contains a more complex example ( SiteSelector.java, ch. 21) which allows the user to select a URL from a list.

57. 240-322 Cli/Serv.: jnet/1157 4.4. Displaying a URL with JEditorPane  The Swing GUI includes JEditorPane which allows 'structured' text to be displayed and edited. –offers basic support for plain text, RTF, and HTML –see javax.swing.JEditorPane continued

58. 240-322 Cli/Serv.: jnet/1158  A JEditorPane can be filled by calling setPage() with a URL string argument.  JEditorPane also has support for capturing hyperlink clicks as events. continued

59. 240-322 Cli/Serv.: jnet/1159 v With these features, it is possible to write a basic Web browser!  ShowURLPage.java has a text field for the user to enter a URL –the resulting downloaded page is displayed in a JEditorPane –if a link is clicked inside the pane, then its page will be loaded automatically continued

60. 240-322 Cli/Serv.: jnet/1160 I clicked on “School of Advanced...”

61. 240-322 Cli/Serv.: jnet/1161 ShowURLPage’s Event Model enter contents X hyperlinkUpdate() {...} GUI Code actionPerformed() {...} hyperlink events action events ^ ^

62. 240-322 Cli/Serv.: jnet/1162 ShowURLPage.java import java.awt.*; import java.awt.event.*; import java.net.*; import java.io.*; import javax.swing.*; import javax.swing.event.*; public class ShowURLPage extends JFrame { private JTextField enter; private JEditorPane contents; :

63. 240-322 Cli/Serv.: jnet/1163 public ShowURLPage() { super("Simple Web Browser"); Container c = getContentPane(); enter = new JTextField("Enter file URL here"); enter.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e ) { getPage(e.getActionCommand());} }); c.add(enter, BorderLayout.NORTH); :

64. 240-322 Cli/Serv.: jnet/1164 contents = new JEditorPane(); contents.setEditable(false); contents.addHyperlinkListener( new HyperlinkListener() { public void hyperlinkUpdate( HyperlinkEvent e) { if(e.getEventType() == HyperlinkEvent.EventType.ACTIVATED) getPage(e.getURL().toString()); } }); :

65. 240-322 Cli/Serv.: jnet/1165 c.add( new JScrollPane(contents), BorderLayout.CENTER); setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); setSize(400, 300); setVisible(true); } // end of ShowURLPage()

66. 240-322 Cli/Serv.: jnet/1166 private void getPage(String location) { setCursor(Cursor.getPredefinedCursor( Cursor.WAIT_CURSOR)); try { contents.setPage(location); enter.setText(location); } catch (IOException io) { JOptionPane.showMessageDialog(this, "Error retrieving specified URL", "Bad URL", JOptionPane.ERROR_MESSAGE); } setCursor( Cursor.getPredefinedCursor( Cursor.DEFAULT_CURSOR) ); } // end of getPage()

67. 240-322 Cli/Serv.: jnet/1167 public static void main(String args[]) { new ShowURLPage(); } } // end of ShowURLPage class

68. 240-322 Cli/Serv.: jnet/1168 Use C> javac ShowURLPage.java C> java ShowURLPage // A request for http://www.cs.ait.ac.th/~ad // causes a “Bad URL” dialog box to appear C> java -DproxySet=true -DproxyHost=cache.psu.ac.th -DproxyPort=8080 ShowURLPage Typed all on one line. The window shown on the left of slide 60 appears.

69. 240-322 Cli/Serv.: jnet/1169 Notes  The JEditorPane must be set uneditable so that hyperlinks events can be captured.  Hyperlink events include: ENTERED, EXITED, and ACTIVATED. –see javax.swing.event.HyperlinkEvent continued

70. 240-322 Cli/Serv.: jnet/1170  getPage() calls setPage() to display a URL in the JEditorPane –the call is surrounded by changes to the cursor icon so things look busy  getPage() is called in two possible ways: –as a response to pressing enter in the textfield –as a response to a link being clicked

71. 240-322 Cli/Serv.: jnet/1171 5. A Stream Socket Client/Server AlienClient.java AlienServer1.java socketServer socket socket for client comms. 4444 the new socket is made with accept() Initial Internet connection input and output streams input and output streams

72. 240-322 Cli/Serv.: jnet/1172 v The server creates a new socket when a client contacts it –client/server communication via the new socket –this frees up the main server socket to receive connections from other clients –AlienServer1.java is an iterative server u it can only process one client at a time continued

73. 240-322 Cli/Serv.: jnet/1173  Our client and server both run on localhost.  AlienServer1 actions: –wait for a client connection –after a connection, wait for a message from the client –respond depending on the content of the message –close the client connection, and loop, waiting for another client continued

74. 240-322 Cli/Serv.: jnet/1174 Diagram for Client Processing AlienClient.java AlienServer1.java 1. send message 4. send answer 2. analyse message 3. print message locally 5. close socket

75. 240-322 Cli/Serv.: jnet/1175 AlienServer1.java import java.io.*; import java.net.*; public class AlienServer1 { public static void main(String args[]) throws IOException { Socket sock; BufferedReader in; PrintStream out; ServerSocket servsock = new ServerSocket(4444); :

76. 240-322 Cli/Serv.: jnet/1176 while (true) { // wait for the next client connection sock = servsock.accept(); // Get I/O streams from the socket out = new PrintStream( sock.getOutputStream() ); in = new BufferedReader( new InputStreamReader( sock.getInputStream() ) ); :

77. 240-322 Cli/Serv.: jnet/1177 // get client message and respond String msg = in.readLine(); System.out.println(" Received: " + msg); if (msg.indexOf("hello") > -1) { System.out.println(" Friendly contact made"); out.println("Welcome friend"); } else { System.out.println(" Probably an alien"); out.println("ET go home"); } out.flush(); // Close this connection, // (not the overall server socket) sock.close(); } } // of main() }// end of AlienServer1

78. 240-322 Cli/Serv.: jnet/1178 The client: AlienClient.java  AlienClient.java ’s actions: –open a connection with the server –send a message taken from the command line –receive and print the server’s response –close

79. 240-322 Cli/Serv.: jnet/1179 AlienClient.java import java.io.*; import java.net.*; public class AlienClient { public static void main(String args[]) throws IOException { Socket sock = new Socket("localhost", 4444); // Get I/O streams from the socket BufferedReader br = new BufferedReader( new InputStreamReader( sock.getInputStream()) ); PrintStream ps = new PrintStream( sock.getOutputStream() ); :

80. 240-322 Cli/Serv.: jnet/1180 // Send a message from the command line ps.println(args[0]); ps.flush(); // Read server’s response String line = br.readLine(); System.out.println( "Got this from server:\n " + line); sock.close(); } // end of main() } // end of AlienClient class

81. 240-322 Cli/Serv.: jnet/1181 Use Client: Server: I used two separate DOS windows on the same machine.

82. 240-322 Cli/Serv.: jnet/1182 Telnet Client (from fivedots) Server: I typed this

83. 240-322 Cli/Serv.: jnet/1183 A More Complex Example v D&D gives a larger stream socket client/server example (ch. 21): –object input and output streams are used –the sequence of communication is more complex u the client maintains a long-lived link with the server until it sends a “TERMINATE” string

84. 240-322 Cli/Serv.: jnet/1184 5.1. A Concurrent Server  AlienServer2.java uses threads to handle multiple clients concurrently. v Three main advantages: –the server code is simpler since server processing is in a separate class called Worker –clients do not have to wait –the server is more scaleable

85. 240-322 Cli/Serv.: jnet/1185 AlienServer2 Visually AlienServer2 AlienClient clients4444 Worker threads each Worker thread can deal with multiple messages from its client

86. 240-322 Cli/Serv.: jnet/1186 5.2. Java Threads (Briefly) v There are two ways to create a thread: –extend the Thread class –write a class to implement the Runnable interface  We use the first approach since Worker does not need to extend any other classes. continued

87. 240-322 Cli/Serv.: jnet/1187  A class extended from Thread must define the run() method: class Worker extends Thread { public void run() {... } } class Worker extends Thread { public void run() {... } }  A threaded object is started by calling start() : Worker w = new Worker(...): w.start(); Worker w = new Worker(...): w.start();  The thread will call run() itself from within start().

88. 240-322 Cli/Serv.: jnet/1188 5.3. AlienServer2.java import java.io.*; import java.net.*; public class Worker extends Thread { private Socket sock; private int id; public Worker(Socket s, int c) { sock = s; id = c; } :

89. 240-322 Cli/Serv.: jnet/1189 public void run() { // Get I/O streams from the socket try { PrintStream out = new PrintStream( sock.getOutputStream() ); BufferedReader in = new BufferedReader( new InputStreamReader( sock.getInputStream() )); :

90. 240-322 Cli/Serv.: jnet/1190 // get client messages and respond String msg; while ((msg = in.readLine()) != null) { System.out.println(" Worker "+id+ " received: "+ msg); if (msg.indexOf("hello") > -1) out.println("Welcome friend") else out.println("ET go home"); out.flush(); } : The worker contains much the same client processing code as the iterative server.

91. 240-322 Cli/Serv.: jnet/1191 // Close this connection, // (not the overall server socket) System.out.println(" Worker " + id + " finished"); sock.close(); } catch(IOException ioe) {System.out.println(ioe); } } // end of run() } // end of Worker class

92. 240-322 Cli/Serv.: jnet/1192 public class AlienServer2 { public static void main(String args[]) throws IOException { Socket sock; ServerSocket servsock = new ServerSocket(4444, 6); while (true) { // wait for the next client connection sock = servsock.accept(); new Worker(sock).start(); } } // of main() } // of AlienServer2 class

93. 240-322 Cli/Serv.: jnet/1193 Use Server:

94. 240-322 Cli/Serv.: jnet/1194 Client on fivedots

95. 240-322 Cli/Serv.: jnet/1195 Two Local Clients (one closed)

96. 240-322 Cli/Serv.: jnet/1196 6. A Datagram Socket Client/Server UDPClientUDPServer Datagram socket 5000 Datagram socket any port msg packets echo

97. 240-322 Cli/Serv.: jnet/1197 6.1. Server Event Model display X GUI Code ^ ^ packet 2) echo back to client waitForPackets() {...} packet 1) receive from client

98. 240-322 Cli/Serv.: jnet/1198 6.2. UDPServer.java import java.io.*; import java.net.*; import java.awt.*; import java.awt.event.*; import javax.swing.*; public class UDPServer extends JFrame { private JTextArea display; :

99. 240-322 Cli/Serv.: jnet/1199 public UDPServer() { super("UDP Server"); display = new JTextArea(); getContentPane().add( new JScrollPane( display), BorderLayout.CENTER ); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); setSize(400, 300); show(); DatagramSocket socket = null; try { socket = new DatagramSocket( 5000 ); } catch(SocketException se) { se.printStackTrace(); System.exit(1); } waitForPackets(socket); } // end of UDPServer()

100. 240-322 Cli/Serv.: jnet/11100 private void waitForPackets(DatagramSocket socket) { DatagramPacket sendPacket, receivePacket; while (true) { try { // set up packet data structure byte data[] = new byte[100]; receivePacket = new DatagramPacket(data, data.length); // wait for packet from client socket.receive(receivePacket); :

101. 240-322 Cli/Serv.: jnet/11101 // process client packet display.append("\nPacket received:" + "\nFrom host: "+receivePacket.getAddress()+ "\nHost port: " + receivePacket.getPort() + "\nLength: " + receivePacket.getLength() + "\nContaining:\n\t" + new String(receivePacket.getData(), 0, receivePacket.getLength())); :

102. 240-322 Cli/Serv.: jnet/11102 // echo packet info back to client display.append( "\n\nEcho data to client..."); sendPacket = new DatagramPacket( receivePacket.getData(), receivePacket.getLength(), receivePacket.getAddress(), receivePacket.getPort() ); socket.send( sendPacket ); display.append("Packet sent\n"); display.setCaretPosition( display.getText().length() ); } :

103. 240-322 Cli/Serv.: jnet/11103 catch(IOException io) { display.append(io.toString() + "\n"); io.printStackTrace(); } } } // end of waitForPackets{} public static void main( String args[] ) { new UDPServer(); } } // end of UDPServer

104. 240-322 Cli/Serv.: jnet/11104 Server Notes v The server creates one datagram socket at port 5000: socket = new DatagramSocket(5000); v Datagram socket creation must be inside a try/catch block. continued

105. 240-322 Cli/Serv.: jnet/11105  The server loops inside waitForPackets() –creates an empty packet (100 bytes long) –blocks inside receive()  When receive() returns, the packet contains data and: –the Internet address of the sender –the port number of the sender –the actual length of the data continued

106. 240-322 Cli/Serv.: jnet/11106 v The packet information can be accessed via methods. e.g.: receivePacket.getPort(); receivePacket.getData();  receive() must be in a try/catch block. continued

107. 240-322 Cli/Serv.: jnet/11107 v The server echoes the datagram back to the client at the address and port obtained from the client’s message: sendPacket = new DatagramPacket( receivePacket.getData(), receivePacket.getLength(), receivePacket.getAddress(), receivePacket.getPort() ); socket.send(sendPacket); sendPacket = new DatagramPacket( receivePacket.getData(), receivePacket.getLength(), receivePacket.getAddress(), receivePacket.getPort() ); socket.send(sendPacket);  send() must be in a try/catch block.

108. 240-322 Cli/Serv.: jnet/11108 6.3. Client Event Model enter display X GUI Code actionPerformed() {...} action events ^ ^ packet 1) send to server waitForPackets() {...} packet 2) receive echo from sender

109. 240-322 Cli/Serv.: jnet/11109 6.4. UDPClient.java import java.io.*; import java.net.*; import java.awt.*; import java.awt.event.*; import javax.swing.*; public class UDPClient extends JFrame implements ActionListener { private JTextField enter; private JTextArea display; private DatagramSocket socket; :

110. 240-322 Cli/Serv.: jnet/11110 public UDPClient() { super("UDP Client"); enter = new JTextField("Type message here"); enter.addActionListener(this); getContentPane().add(enter,BorderLayout.NORTH); display = new JTextArea(); getContentPane().add(new JScrollPane(display), BorderLayout.CENTER); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); setSize(400, 300); setVisible(true); :

111. 240-322 Cli/Serv.: jnet/11111 try { socket = new DatagramSocket(); } catch(SocketException se) { se.printStackTrace(); System.exit(1); } waitForPackets(); } // end of UDPClient()

112. 240-322 Cli/Serv.: jnet/11112 private void waitForPackets() { DatagramPacket receivePacket; while (true) { try { // set up packet data structure byte data[] = new byte[100]; receivePacket = new DatagramPacket(data, data.length); // wait for packet from server socket.receive(receivePacket); :

113. 240-322 Cli/Serv.: jnet/11113 // process packet from server display.append("\nPacket received:" + "\nFrom host: "+receivePacket.getAddress()+ "\nHost port: " + receivePacket.getPort() + "\nLength: " + receivePacket.getLength() + "\nContaining:\n\t" + new String( receivePacket.getData(), 0, receivePacket.getLength())); display.setCaretPosition( display.getText().length() ); } catch(IOException ex) { display.append(ex.toString()+"\n" ); ex.printStackTrace(); } } } // end of waitForPackets()

114. 240-322 Cli/Serv.: jnet/11114 public void actionPerformed(ActionEvent e) { DatagramPacket sendPacket; try { display.append( "\nSending packet containing: " + e.getActionCommand() + "\n" ); String s = e.getActionCommand(); byte data[] = s.getBytes(); sendPacket = new DatagramPacket( data, data.length, InetAddress.getLocalHost(), 5000); socket.send(sendPacket); :

115. 240-322 Cli/Serv.: jnet/11115 display.append("Packet sent\n"); display.setCaretPosition( display.getText().length() ); } catch (IOException ex) { display.append(ex.toString()+"\n"); ex.printStackTrace(); } } // end of actionPerformed() public static void main( String args[] ) { new UDPClient(); } } // end of UDPClient

116. 240-322 Cli/Serv.: jnet/11116 Client Notes  The client gets the data for each packet from the user entering a string into a TextField.  The client creates a datagram socket with no specified port number: socket = new DatagramSocket(); continued

117. 240-322 Cli/Serv.: jnet/11117  In actionPerformed(), the user’s input is converted into a packet, and sent to the server: sendPacket = new DatagramPacket( data, data.length(), InetAddress.getLocalHost(), 5000); socket.send(sendPacket); v The code assumes that the server is on the same machine as the client, and attached to port 5000. continued

118. 240-322 Cli/Serv.: jnet/11118  The client receives the echoed reply in waitForPackets() : –the loop blocks inside receive() until a packet arrives from the server –the packet details are displayed in the JTextArea  The blocked waitForPackets() does not stop the user from entering more strings for the client to send out –the GUI always runs in a separate thread

119. 240-322 Cli/Serv.: jnet/11119 6.5. Use continued 1. message sent to server 3. Echoed message received from the server 2. Client message received and echoed Tested by starting the client and server in two separate DOS windows.

120. 240-322 Cli/Serv.: jnet/11120 Another message sent and echoed back:

121. 240-322 Cli/Serv.: jnet/11121 7. Networked Tic-Tac-Toe D&D Section 21.8, p.1031 thread 5000 synchronized validMove() thread Client: Player ‘X’ Server data thread Client: Player ‘O’ data

122. 240-322 Cli/Serv.: jnet/11122 Output p.1041-1042

123. 240-322 Cli/Serv.: jnet/11123 8. More Information v Chapter 21, Deitel & Deitel (D&D) http://java.coe.psu.ac.th/ForMember/ Books.html#Network  Killer Game Programming in Java Chapter 29, Network Basics  Killer Game Programming in Java Chapter 29, Network Basics http://fivedots.coe.psu.ac.th/~ad/jg/ch18/

124. 240-322 Cli/Serv.: jnet/11124 v Core Java 2, Vol II - Advanced Features Cay S. Horstmann & Gary Cornell Sun Microsystems Press, 2001, chapter 3 –multithreaded server, sending e-mail, URLConnection, HTTP POST –http://java.coe.psu.ac.th/ForMember/ Books.html#Network

125. 240-322 Cli/Serv.: jnet/11125 v Aj. Somchai's Java Site –Network and Distributed Programming Books http://java.coe.psu.ac.th/ForMember/ Books.html#Network