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Networking

Networking. Introduction. In this chapter Swing GUI component JEditorPane (view HTML docs) Socket-based communications View networking like file I/O Stream sockets When connection in place, data flows in continuous streams TCP protocol Preferred. Manipulating URLs.

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Networking

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  1. Networking

  2. Introduction • In this chapter • Swing GUI component JEditorPane (view HTML docs) • Socket-based communications • View networking like file I/O • Stream sockets • When connection in place, data flows in continuous streams • TCP protocol • Preferred

  3. Manipulating URLs • Internet has many protocols • HTTP protocol • Basis of World Wide Web • Uses URLs to locate data • Can represent files, directories, complex lookups • Java can manipulate URLs easily • Example Applet • Display list of web sites with JList • Get parameters from HTML file • Change to web site user selects • Applet must be run in Explorer or Communicator (not appletviewer)

  4. 1 <APPLET CODE= "SiteSelector.class" WIDTH=300 HEIGHT=75> 67 sites.put( title, url ); 2 <PARAM NAME="title0" VALUE="Java Home Page"> 3 <PARAM NAME="location0" VALUE="http://java.sun.com/"> 26 sites = new Hashtable(); 60 title = getParameter( "title" + counter ); 63 location = getParameter( "location" + counter ); 66 url = new URL( location ); Manipulating URLs • getParameter( name ) • Returns parameter from HTML document • Hashtable • Stores key/value pairs (method put)

  5. 36 siteChooser.addListSelectionListener( 37 new ListSelectionListener() { 38 public void valueChanged( ListSelectionEvent e ) 39 { 40 Object o = siteChooser.getSelectedValue(); 41 URL newDocument = (URL) sites.get( o ); 42 AppletContext browser = getAppletContext(); 43 browser.showDocument( newDocument ); 44 } 45 } Manipulating URLs • ListSelectionListener • Method valueChanged (event handler) • getSelectedValue • Returns selected item • Hashtable method get( key ) • Returns value as Object reference • Cast to URL reference

  6. 36 siteChooser.addListSelectionListener( 37 new ListSelectionListener() { 38 public void valueChanged( ListSelectionEvent e ) 39 { 40 Object o = siteChooser.getSelectedValue(); 41 URL newDocument = (URL) sites.get( o ); 42 AppletContext browser = getAppletContext(); 43 browser.showDocument( newDocument ); 44 } 45 } Manipulating URLs • getAppletContext • Returns reference to AppletContext object • Represents browser executing applet • showDocument( URL ) • Loads URL

  7. 1 <APPLET CODE= "SiteSelector.class" WIDTH=300 HEIGHT=75> 2 <PARAM NAME="title0" VALUE="Java Home Page"> 3 <PARAM NAME="location0" VALUE="http://java.sun.com/"> 4 <PARAM NAME="title1" VALUE="Deitel"> 5 <PARAM NAME="location1" VALUE="http://www.deitel.com/"> 6 <PARAM NAME="title2" VALUE="Gamelan"> 7 <PARAM NAME="location2" VALUE="http://www.gamelan.com/"> 8 <PARAM NAME="title3" VALUE="JavaWorld"> 9 <PARAM NAME="location3" VALUE="http://www.javaworld.com/"> Hashtables store key/value pairs (more chapter 23). 10 </APPLET> 11 // Fig. 21.1: SiteSelector.java 12 // This program uses a button to load a document from a URL. 13 import java.net.*; 14 import java.util.*; 15 import javax.swing.*; 16 import javax.swing.event.*; 17 import java.awt.*; 18 import java.applet.AppletContext; 19 20 public class SiteSelector extends JApplet { 21 private Hashtable sites; 22 private Vector siteNames; 23 24 public void init() 25 { 26 sites = new Hashtable(); 27 siteNames = new Vector(); 28 29 getSitesFromHTMLParameters(); 1. HTML file with parameters ------------------- 1. import 1.1 Class SiteSelector (extends JApplet) 1.2 Hashtable

  8. 34 35 final JList siteChooser = new JList( siteNames ); 36 siteChooser.addListSelectionListener( Get selected site, cast to URL. Display in browser. 37 new ListSelectionListener() { 38 public void valueChanged( ListSelectionEvent e ) 39 { 40 Object o = siteChooser.getSelectedValue(); 41 URL newDocument = (URL) sites.get( o ); 42 AppletContext browser = getAppletContext(); 43 browser.showDocument( newDocument ); 44 } 45 } 46 ); 47 c.add( new JScrollPane( siteChooser ), 48 BorderLayout.CENTER ); 49 } 50 51 private void getSitesFromHTMLParameters() 52 { 53 // look for applet parameters in the HTML document 54 // and add sites to Hashtable 55 String title, location; 56 URL url; 57 int counter = 0; 58 30 31 Container c = getContentPane(); 32 c.add( new JLabel( "Choose a site to browse" ), 33 BorderLayout.NORTH ); 1.3 GUI 1.4 Event handler 1.5 getSelectedValue 1.6 get 1.7 getAppletContext 1.8 showDocument 2. getSitesFromHTML Parameters

  9. 67 sites.put( title, url ); 68 siteNames.addElement( title ); 69 } 70 catch ( MalformedURLException e ) { Add key/value pair to hashtable. 71 e.printStackTrace(); 72 } 73 } 74 else 75 break; 76 77 ++counter; 78 } 79 } 80 } 59 while ( true ) { 60 title = getParameter( "title" + counter ); 61 62 if ( title != null ) { 63 location = getParameter( "location" + counter ); 64 65 try { 66 url = new URL( location ); 2.1 getParameter 2.2 put

  10. Reading a File on a Web Server • Reading files from a web server • JEditorPane (javax.swing) • Can display text and HTML • Applets can only read files from original server

  11. 34 contents.addHyperlinkListener( 35 new HyperlinkListener() { 36 public void hyperlinkUpdate( HyperlinkEvent e ) 58 contents.setPage( location ); 32 contents = new JEditorPane(); 33 contents.setEditable( false ); Reading a File on a Web Server • JEditorPane • Method setPage( locationString ) • Download and display page • Hyperlinks • Text, images, GUI components that access another document on web • Generates HyperLinkEvent, notifies HyperLinkListeners

  12. 36 public void hyperlinkUpdate( HyperlinkEvent e ) 37 { 38 if ( e.getEventType() == 39 HyperlinkEvent.EventType.ACTIVATED ) 40 getThePage( e.getURL().toString() ); 41 } Reading a File on a Web Server • Method getEventType • Inner class EventType has three types • ACTIVATED - clicked • ENTERED - mouse over hyperlink • EXITED - mouse leaves hyperlink • Method getURL - returns URL of link

  13. 68 setCursor( Cursor.getPredefinedCursor( 69 Cursor.DEFAULT_CURSOR ) ); 54 setCursor( Cursor.getPredefinedCursor( 55 Cursor.WAIT_CURSOR ) ); Reading a File on a Web Server • setCursor( Cursor ) (inherited by JFrame) • Sets mouse cursor • static method getPredefinedCursor( int cursorType ) • Returns Cursor • cursorType • Cursor.WAIT_CURSOR • Cursor.DEFAULT_CURSOR

  14. 1 // Fig. 21.2: ReadServerFile.java 2 // This program uses a JEditorPane to display the 3 // contents of a file on a Web server. 4 import java.awt.*; 5 import java.awt.event.*; JFrame inherits setCursor method, which our class can now use. 6 import java.net.*; 7 import java.io.*; 8 import javax.swing.*; 9 import javax.swing.event.*; 10 11public class ReadServerFile extends JFrame { 12 private JTextField enter; 13 private JEditorPane contents; 14 15 public ReadServerFile() 16 { 17 super( "Simple Web Browser" ); 18 19 Container c = getContentPane(); 20 21 enter = new JTextField( "Enter file URL here" ); 22 enter.addActionListener( 23 new ActionListener() { 24 public void actionPerformed( ActionEvent e ) 25 { 26 getThePage( e.getActionCommand() ); 27 } 28 } 29 ); 1. import 1.1 extendsJFrame 1.2 Constructor

  15. 34 contents.addHyperlinkListener( Create JEditorPane, register event handler (HyperLinkEvents). 35 new HyperlinkListener() { 36 public void hyperlinkUpdate( HyperlinkEvent e ) Make sure user clicked hyperlink (ACTIVATED). 37 { 38 if ( e.getEventType() == Get the URL of hyperlink, call utility method getThePage. 39 HyperlinkEvent.EventType.ACTIVATED ) 40 getThePage( e.getURL().toString() ); 41 } Change cursor to a waiting state (downloading file). 42 } 43 ); Display new page in JEditorPane. 44 45 c.add( new JScrollPane( contents ), 46 BorderLayout.CENTER ); 47 48 setSize( 400, 300 ); 49 show(); 50 } 51 52 private void getThePage( String location ) 53 { 54 setCursor( Cursor.getPredefinedCursor( 55 Cursor.WAIT_CURSOR ) ); 56 57 try { 58 contents.setPage( location ); 59 enter.setText( location ); 60 } 30 c.add( enter, BorderLayout.NORTH ); 31 32 contents = new JEditorPane(); 33 contents.setEditable( false ); 7800 1.3 JEditorPane 1.4 addHyperLink Listener 2. Method getThePage 2.1 setCursor 2.2 setPage

  16. 67 68 setCursor( Cursor.getPredefinedCursor( 69 Cursor.DEFAULT_CURSOR ) ); 70 } 71 72 public static void main( String args[] ) 73 { 74 ReadServerFile app = new ReadServerFile(); 75 76 app.addWindowListener( 77 new WindowAdapter() { 78 public void windowClosing( WindowEvent e ) 79 { 80 System.exit( 0 ); 81 } 82 } 83 ); 84 } 85 } 61 catch ( IOException io ) { 62 JOptionPane.showMessageDialog( this, 63 "Error retrieving specified URL", 64 "Bad URL", 65 JOptionPane.ERROR_MESSAGE ); 66 } 3. main

  17. Program Output

  18. Program Output

  19. Establishing a Simple Server (Using Stream Sockets) • Creating a Java server • Create ServerSocket object ServerSocket s = new ServerSocket( port, queueLength ); • Register port number and set max number of waiting clients • Binds server to port, waits for clients • Server listens indefinitely (blocks) for clients Socket connection = s.accept() • Connections managed by Socket objects • Returns Socket when connection established

  20. Establishing a Simple Client (Using Stream Sockets) • Creating a Java server • Get I/O objects for communication • OutputStream - sends info to client • InputStream - gets info from client • Server's input stream is client's output stream, and vice versa • Use Socket object to get references to streams • connection.getInputStream() • connection.getOutputStream() • Methods write and read • Send individual bytes

  21. Establishing a Simple Client (Using Stream Sockets) • Creating a Java server • Chaining stream types (sending objects and data, not bytes) ObjectInputStream input = new ObjectInputStream( connection.getInputStream() ) • Processing phase • Server/client communicate using InputStream and OutputStream • Termination • Transmission complete • Method close (of Socket) connection.close() • Networking appears as sequential file I/O • Sockets simplify networking

  22. Establishing a Simple Client (Using Stream Sockets) • Creating a Java client • Create Socket to connect to server Socket connection = new Socket( serverAddress, port ) • If successful, returns Socket • Else throws subclass of IOException • Get stream objects • connection.getInputStream() • connection.getOutputStream() • Use chaining if transmitting objects and data, not pure bytes • ObjectInputStream, ObjectOutputStream

  23. Establishing a Simple Client (Using Stream Sockets) • Creating a Java client • Processing phase • Communicate using stream objects • Termination • Transmission complete • connection.close() (close Socket) • Client must determine when server done • read returns -1 when eof found • ObjectInputStream generates EOFException

  24. Client/Server Interaction with Stream Socket Connections • Upcoming program • Simple client/server chat application • Use stream sockets • Client connects, transmit String objects • do-while loop to get input • Event handler calls sendData method to send output • When either sends TERMINATE, connection terminates • Server waits for next client • GUI:

  25. 60 connection.getInetAddress().getHostName() ); 63 output = new ObjectOutputStream( 64 connection.getOutputStream() ); 65 output.flush(); Client/Server Interaction with Stream Socket Connections • Server • Once connection (Socket) established • getInetAddress() - returns InetAddress object • getHostName() - returns client's host name • Create ObjectOutputStream • flush() • Sends stream header to client • Required by client's ObjectInputStream

  26. 108 private void sendData( String s ) 109 { 111 output.writeObject( "SERVER>>> " + s ); 112 output.flush(); 113 display.append( "\nSERVER>>>" + s ); 81 display.setCaretPosition( 82 display.getText().length() ); Client/Server Interaction with Stream Socket Connections • Send/receive data • Method sendData called by event handler • Buffer flushed after every transmission • Sets cursor position to end of text • Allows JTextArea to scroll with text • Server processes single connection at a time • More likely to have separate threads for each connection

  27. 1 // Fig. 21.3: Server.java 2 // Set up a Server that will receive a connection 3 // from a client, send a string to the client, 4 // and close the connection. 5 import java.io.*; 6 import java.net.*; 7 import java.awt.*; 8 import java.awt.event.*; 9 import javax.swing.*; 10 11 public class Server extends JFrame { 12 private JTextField enter; 13 private JTextArea display; 14 ObjectOutputStream output; 15 ObjectInputStream input; 16 17 public Server() 18 { 19 super( "Server" ); 20 21 Container c = getContentPane(); 22 23 enter = new JTextField(); 24 enter.setEnabled( false ); 1. Constructor 1.1 GUI components

  28. 34 35 display = new JTextArea(); 36 c.add( new JScrollPane( display ), The event handlers calls method sendData, using text in JTextField as argument. 37 BorderLayout.CENTER ); 38 39 setSize( 300, 150 ); 40 show(); 41 } Register port 5000, server will allow 100 clients to wait in queue. 42 43 public void runServer() 44 { 45 ServerSocket server; 46 Socket connection; 47 int counter = 1; 48 49 try { 50 // Step 1: Create a ServerSocket. 51 server = new ServerSocket( 5000, 100 ); 52 25 enter.addActionListener( 26 new ActionListener() { 27 public void actionPerformed( ActionEvent e ) 28 { 29 sendData( e.getActionCommand() ); 30 } 31 } 32 ); 33 c.add( enter, BorderLayout.NORTH ); 1.2 Event handler 1.3 GUI 2. Method runServer 2.1 ServerSocket

  29. 67 connection.getInputStream() ); Infinite loop to wait for connections from clients. 68 display.append( "\nGot I/O streams\n" ); 69 Wait for connection (server.accept() waits indefinitely). 70 // Step 4: Process connection. 71 String message = Display host location. 72 "SERVER>>> Connection successful"; Set up I/O streams, flush buffer (sends stream data). Send confirmation message. 73 output.writeObject( message ); 74 output.flush(); Loop to get input. Set cursor to end of text. 75 enter.setEnabled( true ); 76 77 do { 78 try { 79 message = (String) input.readObject(); 80 display.append( "\n" + message ); 81 display.setCaretPosition( 82 display.getText().length() ); 83 } 53 while ( true ) { 54 // Step 2: Wait for a connection. 55 display.setText( "Waiting for connection\n" ); 56 connection = server.accept(); 57 58 display.append( "Connection " + counter + 59 " received from: " + 60 connection.getInetAddress().getHostName() ); 61 62 // Step 3: Get input and output streams. 63 output = new ObjectOutputStream( 64 connection.getOutputStream() ); 65 output.flush(); 66 input = new ObjectInputStream( 2.2 accept 2.3 getHostName 2.4 Get streams 2.5 writeObject 2.6 Loop

  30. 100 catch ( EOFException eof ) { Loop until TERMINATE string sent. 101 System.out.println( "Client terminated connection" ); 102 } 103 catch ( IOException io ) { Close connection and I/O streams. 104 io.printStackTrace(); 105 } 106 } 107 Send String object to client. Flush buffer to ensure it is sent, and update server's display. 108 private void sendData( String s ) 109 { 110 try { 111 output.writeObject( "SERVER>>> " + s ); 112 output.flush(); 113 display.append( "\nSERVER>>>" + s ); 114 } 84 catch ( ClassNotFoundException cnfex ) { 85 display.append( 86 "\nUnknown object type received" ); 87 } 88 } while ( !message.equals( "CLIENT>>> TERMINATE" ) ); 89 90 // Step 5: Close connection. 91 display.append( "\nUser terminated connection" ); 92 enter.setEnabled( false ); 93 output.close(); 94 input.close(); 95 connection.close(); 96 97 ++counter; 98 } 99 } 2.6 Loop 2.7 close 3. Method sendData 3.1 writeObject 3.2 flush

  31. 133 134 app.runServer(); 135 } 136 } 115 catch ( IOException cnfex ) { 116 display.append( 117 "\nError writing object" ); 118 } 119 } 120 121 public static void main( String args[] ) 122 { 123 Server app = new Server(); 124 125 app.addWindowListener( 126 new WindowAdapter() { 127 public void windowClosing( WindowEvent e ) 128 { 129 System.exit( 0 ); 130 } 131 } 132 ); 4. main

  32. Client/Server Interaction with Stream Socket Connections • Client • Similar to server • Creates same GUI • Loops to wait for input • Sends output through event handler and sendData

  33. 50 client = new Socket( 51 InetAddress.getByName( "127.0.0.1" ), 5000 ); Client/Server Interaction with Stream Socket Connections • Create Socket with two arguments • Internet address of server and port number • static method getByName (Class InetAddress) • Returns InetAddress object • Takes String • Could have taken "localhost" or called static method getLocalHost • Create I/O objects as before • Output of server is input of client

  34. 1 // Fig. 21.4: Client.java 2 // Set up a Client that will read information sent 3 // from a Server and display the information. 4 import java.io.*; 5 import java.net.*; 6 import java.awt.*; 7 import java.awt.event.*; 8 import javax.swing.*; 9 10 public class Client extends JFrame { 11 private JTextField enter; 12 private JTextArea display; 13 ObjectOutputStream output; 14 ObjectInputStream input; 15 String message = ""; 16 17 public Client() 18 { 19 super( "Client" ); 20 21 Container c = getContentPane(); 22 23 enter = new JTextField(); 24 enter.setEnabled( false ); 1. Constructor 1.1 GUI

  35. 34 35 display = new JTextArea(); 36 c.add( new JScrollPane( display ), Client gets text from JTextField and calls sendData. 37 BorderLayout.CENTER ); 38 39 setSize( 300, 150 ); 40 show(); Create Socket to server, make connection. 41 } 42 43 public void runClient() 44 { 45 Socket client; 46 47 try { 48 // Step 1: Create a Socket to make connection. 49 display.setText( "Attempting connection\n" ); 50 client = new Socket( 51 InetAddress.getByName( "127.0.0.1" ), 5000 ); 52 53 54 55 25 enter.addActionListener( 26 new ActionListener() { 27 public void actionPerformed( ActionEvent e ) 28 { 29 sendData( e.getActionCommand() ); 30 } 31 } 32 ); 33 c.add( enter, BorderLayout.NORTH ); 1.2 Event handler 1.3 GUI 2. Method runClient 2.1 Socket

  36. 67 // Step 3: Process connection. 68 enter.setEnabled( true ); 69 70 do { 71 try { Code similar to server. Create I/O objects, flush buffer, loop to wait for and process input. 72 message = (String) input.readObject(); 73 display.append( "\n" + message ); 74 display.setCaretPosition( 75 display.getText().length() ); 76 } 77 catch ( ClassNotFoundException cnfex ) { 78 display.append( 79 "\nUnknown object type received" ); 80 } 81 } while ( !message.equals( "SERVER>>> TERMINATE" ) ); 82 56 display.append( "Connected to: " + 57 client.getInetAddress().getHostName() ); 58 59 // Step 2: Get the input and output streams. 60 output = new ObjectOutputStream( 61 client.getOutputStream() ); 62 output.flush(); 63 input = new ObjectInputStream( 64 client.getInputStream() ); 65 display.append( "\nGot I/O streams\n" ); 66 2.2 I/O streams 2.3 flush 2.4 Loop

  37. 100 message = s; 101 output.writeObject( "CLIENT>>> " + s ); 102 output.flush(); 103 display.append( "\nCLIENT>>>" + s ); 104 } 105 catch ( IOException cnfex ) { 106 display.append( 107 "\nError writing object" ); 108 } 109 } 110 83 // Step 4: Close connection. 84 display.append( "Closing connection.\n" ); 85 input.close(); 86 output.close(); 87 client.close(); 88 } 89 catch ( EOFException eof ) { 90 System.out.println( "Server terminated connection" ); 91 } 92 catch ( IOException e ) { 93 e.printStackTrace(); 94 } 95 } 96 97 private void sendData( String s ) 98 { 99 try { 2.5 close 3. Method sendData

  38. 111 public static void main( String args[] ) 112 { 113 Client app = new Client(); 114 115 app.addWindowListener( 116 new WindowAdapter() { 117 public void windowClosing( WindowEvent e ) 118 { 119 System.exit( 0 ); 120 } 121 } 122 ); 123 124 app.runClient(); 125 } 126 } 127 4. main Program Output

  39. Program Output

  40. Connectionless Client/Server Interaction with Datagrams • Connection-oriented interaction • Like phone call • Have connection to other end • Connection maintained for duration of call, even if not talking • Connectionless interaction • Like sending postal mail • Use datagrams, packets of data • If large packet, break into smaller pieces • Send separately • May arrive out of order or not arrive at all • Duplicates may arrive

  41. 40 byte data[] = new byte[ 100 ]; 41 receivePacket = 42 new DatagramPacket( data, data.length ); 45 socket.receive( receivePacket ); 27 socket = new DatagramSocket( 5000 ); Connectionless Client/Server Interaction with Datagrams • DatagramSocket( port ) • Binds server to port • DatagramPacket( byteArray, byteArray.length ) • Create DatagramPacket to store received packet • byteArray stores data • Method receive( packetToStore ) • Blocks until packet arrives, stores in packetToStore

  42. 48 display.append( "\nPacket received:" + 49 "\nFrom host: " + receivePacket.getAddress() + 50 "\nHost port: " + receivePacket.getPort() + 51 "\nLength: " + receivePacket.getLength() + 52 "\nContaining:\n\t" + 53 new String( receivePacket.getData(), 0, 54 receivePacket.getLength() ) ); Connectionless Client/Server Interaction with Datagrams • getAddress - returns InetAddress • getPort, getLength - return integers • getData - returns byte array • Used in String constructor to create string

  43. 58 sendPacket = 59 new DatagramPacket( receivePacket.getData(), 60 receivePacket.getLength(), 61 receivePacket.getAddress(), 62 receivePacket.getPort() ); 63 socket.send( sendPacket ); Connectionless Client/Server Interaction with Datagrams • Echo packet back to client • DatagramPacket( byteArray, length, InetAddress, port ) • Method send( packetToSend )

  44. 1 // Fig. 21.5: Server.java 2 // Set up a Server that will receive packets from a 3 // client and send packets to a client. 4 import java.io.*; 5 import java.net.*; 6 import java.awt.*; 7 import java.awt.event.*; 8 import javax.swing.*; 9 Create new DatagramSocket, binds server to port. 10 public class Server extends JFrame { 11 private JTextArea display; 12 13 private DatagramPacket sendPacket, receivePacket; 14 private DatagramSocket socket; 15 16 public Server() 17 { 18 super( "Server" ); 19 20 display = new JTextArea(); 21 getContentPane().add( new JScrollPane( display), 22 BorderLayout.CENTER ); 23 setSize( 400, 300 ); 24 show(); 25 26 try { 27 socket = new DatagramSocket( 5000 ); 28 } 1. Declarations 1.1 DatagramSocket

  45. 34 35 public void waitForPackets() Loop to wait for packets. 36 { Create new DatagramPacket to receive info. 37 while ( true ) { 38 try { 39 // set up packet 40 byte data[] = new byte[ 100 ]; Gather and display packet data. Convert byte array to a String. 41 receivePacket = 42 new DatagramPacket( data, data.length ); 43 Create packet back to client. 44 // wait for packet 45 socket.receive( receivePacket ); 46 47 // process packet 48 display.append( "\nPacket received:" + 49 "\nFrom host: " + receivePacket.getAddress() + 50 "\nHost port: " + receivePacket.getPort() + 51 "\nLength: " + receivePacket.getLength() + 52 "\nContaining:\n\t" + 53 new String( receivePacket.getData(), 0, 54 receivePacket.getLength() ) ); 55 56 // echo information from packet back to client 57 display.append( "\n\nEcho data to client..."); 58 sendPacket = 59 new DatagramPacket( receivePacket.getData(), 29 catch( SocketException se ) { 30 se.printStackTrace(); 31 System.exit( 1 ); 32 } 33 } 2. Method waitForPackets 2.1 receivePacket 2.2 receive 2.3 Process packet 2.4 Echo packet

  46. 67 } 68 catch( IOException io ) { Send packet back to client. 69 display.append( io.toString() + "\n" ); 70 io.printStackTrace(); 71 } 72 } 73 } 74 75 public static void main( String args[] ) 76 { 77 Server app = new Server(); 78 79 app.addWindowListener( 80 new WindowAdapter() { 81 public void windowClosing( WindowEvent e ) 82 { 83 System.exit( 0 ); 84 } 85 } 86 ); 87 88 app.waitForPackets(); 89 } 90 } 60 receivePacket.getLength(), 61 receivePacket.getAddress(), 62 receivePacket.getPort() ); 63 socket.send( sendPacket ); 64 display.append( "Packet sent\n" ); 65 display.setCaretPosition( 66 display.getText().length() ); 2.5 send 3. main

  47. Program Output

  48. 31 socket = new DatagramSocket(); Connectionless Client/Server Interaction with Datagrams • Client • Similar to server • Has JTextField • Sends packets with event handler for JTextField • Convert String to byteArray • String.getBytes() • Have loop to receive echoed packets from server • Constructor needs no arguments • Uses next available port • Server gets client's port number as part of DatagramPacket

  49. 1 // Fig. 21.6: Client.java 2 // Set up a Client that will send packets to a 3 // server and receive packets from a server. 4 import java.io.*; 5 import java.net.*; 6 import java.awt.*; 7 import java.awt.event.*; 8 import javax.swing.*; 9 10 public class Client extends JFrame implements ActionListener { 11 private JTextField enter; 12 private JTextArea display; 13 14 private DatagramPacket sendPacket, receivePacket; 15 private DatagramSocket socket; 16 17 public Client() 18 { 19 super( "Client" ); 20 21 enter = new JTextField( "Type message here" ); 22 enter.addActionListener( this ); 23 getContentPane().add( enter, BorderLayout.NORTH ); 24 display = new JTextArea(); 25 getContentPane().add( new JScrollPane( display ), 26 BorderLayout.CENTER ); 27 setSize( 400, 300 ); 28 show(); 29 1. Constructor 1.1 GUI 1.2 Register event handler

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