5.11 Sending Datagrams

Now that we've thoroughly covered the possibilities of networking with sockets and streams, let's examine how low-level networking can be done using datagrams and packets. Examples Example 5-11 and Example 5-12 show how you can implement simple network communication using datagrams. Datagram communication is sometimes called UDP, for Unreliable Datagram Protocol. Sending datagrams is fast, but the trade-off is that they are not guaranteed to reach their destination. In addition, multiple datagrams are not guaranteed to travel to their destination by the same route or to arrive at their destination in the order in which they were sent. Datagrams are useful when you want low-overhead communication of noncritical data and when a stream model of communication is not necessary. For example, you might implement a multiuser chat server for a local area network using datagrams.

To send and receive datagrams, you use the DatagramPacket and DatagramSocket classes. These objects are created and initialized differently, depending on whether they send or receive datagrams. Example 5-11 shows how to send a datagram; Example 5-12 shows how to receive a datagram and how to find who sent it.

To send a datagram, you first create a DatagramPacket, specifying the data to be sent, the length of the data, the host to send it to, and the port on that host where it is to be sent. You then use the send( ) method of a DatagramSocket to send the packet. The DatagramSocket is a generic one, created with no arguments. It can be reused to send any packet to any address and port.

Example 5-11. UDPSend.java

package je3.net; import java.io.*; import java.net.*; /**  * This class sends the specified text or file as a datagram to the   * specified port of the specified host.  **/ public class UDPSend {     public static final String usage =          "Usage: java UDPSend <hostname> <port> <msg>...\n" +         "   or: java UDPSend <hostname> <port> -f <file>";     public static void main(String args[  ]) {         try {              // Check the number of arguments             if (args.length < 3)                  throw new IllegalArgumentException("Wrong number of args");                          // Parse the arguments             String host = args[0];             int port = Integer.parseInt(args[1]);                          // Figure out the message to send.               // If the third argument is -f, then send the contents of the file             // specified as the fourth argument.  Otherwise, concatenate the              // third and all remaining arguments and send that.             byte[  ] message;             if (args[2].equals("-f")) {                 File f = new File(args[3]);                 int len = (int)f.length( );    // figure out how big the file is                 message = new byte[len];      // create a buffer big enough                 FileInputStream in = new FileInputStream(f);                 int bytes_read = 0, n;                 do {                          // loop until we've read it all                     n = in.read(message, bytes_read, len-bytes_read);                     bytes_read += n;                 } while((bytes_read < len)&& (n != -1));             }             else { // Otherwise, just combine all the remaining arguments.                 String msg = args[2];                   for (int i = 3; i < args.length; i++) msg += " " + args[i];                 // Convert the message to bytes using UTF-8 encoding                 message = msg.getBytes("UTF-8");             }                          // Get the internet address of the specified host             InetAddress address = InetAddress.getByName(host);                          // Initialize a datagram packet with data and address             DatagramPacket packet = new DatagramPacket(message, message.length,                                                        address, port);                          // Create a datagram socket, send the packet through it, close it.             DatagramSocket dsocket = new DatagramSocket( );             dsocket.send(packet);             dsocket.close( );         }         catch (Exception e) {             System.err.println(e);             System.err.println(usage);         }     } }