public class SendMes extends Thread {
      public void run() {// 覆写Thread类中的run方法
            try {
                  while (true) {
                        DatagramSocket socket = new DatagramSocket(2425);
                        byte[] buffer = new byte[65508];
                        DatagramPacket packet = new DatagramPacket(buffer, 0,
                        buffer.length);
                        socket.receive(packet);
                        socket.close();
                        String s = new String(packet.getData(), 0, packet.getLength());
                        String[] strs = s.split(":");
                        System.out.println(strs[4]);
                        if (strs.length > 4 && strs[4].equals("6291457")) {
                              // System.out.println("用户上线了");
                              long IPMSG_SENDMSG = 0x00000020;
                              String address = packet.getAddress().toString().substring(1);
                              String HOST = "系统管理员";
                              String SENDER = "系统管理员";
                              String MSG_CONTENT = "您好，您有一封邮件未处理";
                              DatagramSocket socket1 = new DatagramSocket();
                              byte[] buffer1 = ("1:" + new Date().getTime() + ":"
                              + SENDER + ":" + HOST + ":" + IPMSG_SENDMSG + ":" + MSG_CONTENT)
                              .getBytes("gbk");
                              // 版本号：报文编号：发送者：ip：命令：内容
                              DatagramPacket packet1 = new DatagramPacket(buffer1,
                              buffer1.length, packet.getAddress(), 2425);
                              socket1.send(packet1); // 发送报文
                              socket1.close();
                              System.out.println("send message is ok.");
                        }
                        if (strs.length > 4 && strs[4].equals("6291458")) {
                              System.out.println("用户下线了");
                        }
                  }
            } catch (SocketException e) {
                  e.printStackTrace();
            } catch (IOException e) {
                  e.printStackTrace();
            }
      }
}

发现局域网内飞秋用户上线，自动发送自定义聊天内容！