//sending data to host and its port
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();

//receiving data from its port
 DatagramSocket dsocket = new DatagramSocket(port);
 // Create a buffer to read datagrams into. If a
 // packet is larger than this buffer, the
 // excess will simply be discarded!
 byte[] buffer = new byte[2048];
 // Create a packet to receive data into the buffer
 DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
 // Now loop forever, waiting to receive packets and printing them.
 while (true) {
   // Wait to receive a datagram
   dsocket.receive(packet);
   // Convert the contents to a string, and display them
   String msg = new String(buffer, 0, packet.getLength());
   System.out.println(packet.getAddress().getHostName() + ": "
       + msg);
   // Reset the length of the packet before reusing it.
   packet.setLength(buffer.length);
 }

// join a Multicast group and send the group salutations
...
String msg = "Hello";
InetAddress group = InetAddress.getByName("228.5.6.7");
MulticastSocket s = new MulticastSocket(6789);
s.joinGroup(group);
DatagramPacket hi = new DatagramPacket(msg.getBytes(), msg.length(),
                            group, 6789);
s.send(hi);
// get their responses!
byte[] buf = new byte[1000];
DatagramPacket recv = new DatagramPacket(buf, buf.length);
s.receive(recv);
...
// OK, I'm done talking - leave the group...
s.leaveGroup(group);

//UDP servers
InetAddress group = InetAddress.getByName("228.5.6.7");  
MulticastSocket s = new MulticastSocket(6789);   
byte[] arb = new byte[1024];  
s.joinGroup(group);//加入该组  
while(true){  
    DatagramPacket datagramPacket =new DatagramPacket(arb,arb.length);  
    s.receive(datagramPacket);  
    System.out.println(arb.length);  
    System.out.println(new String(arb));   
}

//UDP sender
int port = 6789;  
String sendMessage="hello multicast";  
InetAddress inetAddress = InetAddress.getByName("228.5.6.7");  
DatagramPacket datagramPacket = new DatagramPacket(sendMessage.getBytes(), sendMessage.length(), inetAddress, port);  
MulticastSocket multicastSocket = new MulticastSocket(); //it is client, it won't join group
multicastSocket.send(datagramPacket);

public final class QuoteOfTheMomentServer {
 
     private static final int PORT = Integer.parseInt(System.getProperty("port", "7686"));
 
     public static void main(String[] args) throws Exception {
         EventLoopGroup group = new NioEventLoopGroup();
         try {
             Bootstrap b = new Bootstrap();
             b.group(group)
              .channel(NioDatagramChannel.class)
              .option(ChannelOption.SO_BROADCAST, true)
              .handler(new QuoteOfTheMomentServerHandler());
 
             b.bind(PORT).sync().channel().closeFuture().await();
         } finally {
             group.shutdownGracefully();
         }
     }
 }

public final class QuoteOfTheMomentClient {
  
     static final int PORT = Integer.parseInt(System.getProperty("port", "7686"));
  
      public static void main(String[] args) throws Exception {
  
          EventLoopGroup group = new NioEventLoopGroup();
          try {
              Bootstrap b = new Bootstrap();
              b.group(group)
               .channel(NioDatagramChannel.class)
               .option(ChannelOption.SO_BROADCAST, true)
               .handler(new QuoteOfTheMomentClientHandler());
  
              Channel ch = b.bind(0).sync().channel();
  
              // Broadcast the QOTM request to port 8080.
              ch.writeAndFlush(new DatagramPacket(
                      Unpooled.copiedBuffer("QOTM?", CharsetUtil.UTF_8),
                      new InetSocketAddress("255.255.255.255", PORT))).sync();
  
              // QuoteOfTheMomentClientHandler will close the DatagramChannel when a
              // response is received.  If the channel is not closed within 5 seconds,
              // print an error message and quit.
              if (!ch.closeFuture().await(5000)) {
                  System.err.println("QOTM request timed out.");
              }
          } finally {
              group.shutdownGracefully();
          }
      }
  }