Building an Echo Client in Java

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The following client can be used with the echo service and can easily be adapted to support other services Repeated packets are sent to the echo service, and a timeout is caught to prevent the service from stalling if a packet becomes lost, and the client then waits to receive it Remember that packet loss in an intranet environment is unlikely, but with slow network connections on the Internet it is quite possible
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import javanet*; import javaio*; // 5, Listing 4 public class EchoClient { // UDP port to which service is bound public static final int SERVICE_PORT = 7; // Max size of packet public static final int BUFSIZE = 256; public static void main(String args[]) { if (argslength != 1) {
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Systemerrprintln ("Syntax - java EchoClient hostname"); return; } String hostname = args[0]; // Get an InetAddress for the specified hostname InetAddress addr = null; try { // Resolve the hostname to an InetAddr addr = InetAddressgetByName(hostname); } catch (UnknownHostException uhe) { Systemerrprintln ("Unable to resolve host"); return; } try { // Bind to any free port DatagramSocket socket = new DatagramSocket(); // Set a timeout value of two seconds socketsetSoTimeout (2 * 1000); for (int i = 1 ; i <= 10; i++) { // Copy some data to our packet String message = "Packet number " + i ; char[] cArray = messagetoCharArray(); byte[] sendbuf = new byte[cArraylength]; for (int offset = 0; offset < cArraylength ; offset++) { sendbuf[offset] = (byte) cArray[offset]; } // Create a packet to send to the UDP server DatagramPacket sendPacket = new DatagramPacket(sendbuf, cArraylength, addr, SERVICE_PORT); Systemoutprintln ("Sending packet to " + hostname); // Send the packet socketsend (sendPacket); Systemoutprint ("Waiting for packet "); // Create a small packet for receiving UDP packets byte[] recbuf = new byte[BUFSIZE]; DatagramPacket receivePacket = new DatagramPacket(recbuf,
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BUFSIZE); // Declare a timeout flag boolean timeout = false; // Catch any InterruptedIOException that is thrown // while waiting to receive a UDP packet try { socketreceive (receivePacket); } catch (InterruptedIOException ioe) { timeout = true; } if (!timeout) { Systemoutprintln ("packet received!"); Systemoutprintln ("Details : " + receivePacketgetAddress()); // Obtain a byte input stream to read the // UDP packet ByteArrayInputStream bin = new ByteArrayInputStream ( receivePacketgetData(), 0, receivePacketgetLength() ); // Connect a reader for easier access BufferedReader reader = new BufferedReader ( new InputStreamReader ( bin ) ); // Loop indefinitely for (;;) { String line = readerreadLine(); // Check for end of data if (line == null) break; else Systemoutprintln (line); } } else { Systemoutprintln ("packet lost!"); } // Sleep for a second, to allow user to see packet try { Threadsleep(1000); } catch (InterruptedException ie) { } } } catch (IOException ioe)
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{ Systemerrprintln ("Socket error " + ioe); } } }
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573 Running the Echo Client and Server
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Before clients can send requests, the echo server must be active Otherwise, UDP packets will be sent and then ignored, as there is no program running to read them However, the client will not stall if a response is not sent back; it will send packets again after two seconds of waiting This is important, as servers may be inactive or packets may become lost in transmission To run the echo server, type the following:
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java EchoServer
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To run the echo client (either on the same machine as the server or a different one), type the following:
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java EchoClient hostname
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where hostname (or localhost, if running locally) is the location of the echo service NOTE Remember that on Unix systems, you must have root privileges to bind to a port under 1024 You can change the service port to a higher number, but this must be done in both the client and the server in order to recompile
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