How do I create a simple Datagram server?

To create a server with UDP, do the following:

1. Create a DatagramSocket attached to a port.

   int port = 1234;
   DatagramSocket socket = new DatagramSocket(port);
   

2. Allocate space to hold the incoming packet, and create an instance of DatagramPacket to hold the incoming data.

   byte[] buffer = new byte[1024];
   DatagramPacket packet =
          new DatagramPacket(buffer, buffer.length);
   

3. Block until a packet is received, then extract the information you need from the packet.

   // Block on receive()
   socket.receive(packet);
   
   // Find out where packet came from
   // so we can reply to the same host/port
   InetAddress remoteHost = packet.getAddress();
   int         remotePort = packet.getPort();
   
   // Extract the packet data
   byte[]      data       = packet.getData();
   

The server can now process the data it has received from the client, and issue an appropriate reply in response to the client's request.

The full source for a UDP echo server and client can be downloaded from the following links:

• http://www.junun.org/Examples/EchoServer.java
• http://www.junun.org/Examples/EchoClient.java