Polling for other Computers on a LAN in Java
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Could anyone post some code that exemplifies this? I'm not sure how to find the broadcast address in Java.
Here is a similar program I have used before (client version). The server was written in C++, but all it did was create a UDP socket (java DatagramSocket) and every time someone sent it a packet (java DatagramPacket )it just got the address of the sender and echoed "Server name = whatever" back at them.
Here is the client source (includes a stupid GUI, written hastily, don't blame me [smile] )
Here is a simple echo server I wrote for some reason ( I think it was to test UDP through a firewall or some stupid reason ). Note the port is different.
You can try that (it may need a tiny but of work, this was old, throw away code).
As for your question as to how to get the broadcast address, this is the way I seem to have chosen:
Im sure there are easier ways, like this probably:
Here is the client source (includes a stupid GUI, written hastily, don't blame me [smile] )
import java.io.IOException;import java.net.InetAddress;import java.net.*;import java.net.UnknownHostException;import javax.swing.*;import java.awt.*;import java.awt.event.*;public class ServerQuery{ private static final int QUERY_PORT = 1234;//13457; InetAddress address = null; DatagramSocket socket = null; //DatagramPacket packet; JTextArea messageArea; ServerQuery() { //DatagramPacket packet; JFrame frame = new JFrame("ServerQuery"); messageArea = new JTextArea("Servers:\n"); messageArea.setSize(300,300); messageArea.setEditable( false ); frame.setSize(300,300); Container pane = frame.getContentPane(); pane.setLayout( new BorderLayout() ); pane.add( new JScrollPane(messageArea), BorderLayout.CENTER ); JButton button = new JButton("refresh"); button.addActionListener( new ActionListener() { public void actionPerformed( ActionEvent e ) { refresh(); } } ); pane.add( button, BorderLayout.SOUTH ); frame.setVisible( true ); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); try { //address = InetAddress.getByAddress(ip); socket = new DatagramSocket(); socket.setBroadcast( true ); refresh(); } catch( SecurityException e ) { print( e.toString() ); } catch( IOException e ) { print( e.toString() ); } } public static void main( String[] args ) { ServerQuery query = new ServerQuery(); query.run(); } public void print( String msg ) { messageArea.setText( messageArea.getText() + '\n' + msg ); } private void refresh() { messageArea.setText("Servers:\n"); try { byte []ip = new byte[4]; for( int i = 0 ; i < 4; ++i ) { ip = (byte)255; } address = InetAddress.getByAddress(ip); String meaningless = "hi server how are you??? im grand thanks just tell us your ip, no?"; meaningless += '\0'; DatagramPacket packet = new DatagramPacket( meaningless.getBytes(), meaningless.length() ); packet.setPort( QUERY_PORT ); packet.setAddress( address ); socket.send( packet ); } catch( UnknownHostException e ) { print( e.toString() ); } catch( SecurityException e ) { print( e.toString() ); } catch( IOException e ) { print( e.toString() ); } } public void run() { while(true) { try { byte [] bytes = new byte[1024]; DatagramPacket packet = new DatagramPacket(bytes,bytes.length); socket.receive( packet ); int indexOfNul = 0; while( bytes[indexOfNul] != 0 ) { indexOfNul++; } print( new String( packet.getData(),0,indexOfNul ) ); print( "Server at address : " + packet.getAddress().toString() ); } catch( IOException e ) { print( e.toString() ); } } }}Here is a simple echo server I wrote for some reason ( I think it was to test UDP through a firewall or some stupid reason ). Note the port is different.
import java.net.*;class Echo{ private static final int SIZE = 1024; public static void main( String [] args ) { System.out.println("UDP echo server running..."); try { byte [] data = new byte[SIZE]; DatagramPacket packet = new DatagramPacket(data,SIZE); DatagramSocket socket = new DatagramSocket(1234);//24567); while(true) { try { socket.receive( packet ); String string = new String( packet.getData(),0 , packet.getLength() ); System.out.println( java.util.Calendar.getInstance().getTime().toString() + ": Echoing message from " + packet.getAddress().toString() + ':' + packet.getPort() + " " + string ); packet.setAddress( packet.getAddress() ); socket.send( packet ); } catch( Exception e ) { System.out.println(e); } } } catch( Exception e ) { System.out.println(e); } }}You can try that (it may need a tiny but of work, this was old, throw away code).
As for your question as to how to get the broadcast address, this is the way I seem to have chosen:
byte []ip = new byte[4];for( int i = 0 ; i < 4; ++i ){ ip = (byte)255;}address = InetAddress.getByAddress(ip);Im sure there are easier ways, like this probably:
address = InetAddress.getByName("255.255.255.255");
You can mask the address like so:
InetAddress address = InetAddress.getByName("192.168.0.1");byte[] addrBytes= address.getAddress();addrBytes[3]=(byte)255;InetAddress newAddress = InetAddress.getByAddress(addrBytes);This topic is closed to new replies.
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