DatagramSocket socket = new DatagramSocket();
String message = "Hello via UDP!";
byte[] data = message.getBytes();

InetAddress address = InetAddress.getByName("localhost");
DatagramPacket packet = new DatagramPacket(
    data, data.length, address, 9999);

socket.send(packet);
socket.close();

DatagramSocket socket = new DatagramSocket(9999);
byte[] buffer = new byte[1024];
DatagramPacket packet = new DatagramPacket(buffer, buffer.length);

System.out.println("等待 UDP 数据...");
socket.receive(packet); // 阻塞等待

String received = new String(packet.getData(), 0, packet.getLength());
System.out.println("收到：" + received +
    " 来自 " + packet.getAddress());

socket.close();

// IP 地址
InetAddress local = InetAddress.getLocalHost();
System.out.println(local.getHostName());  // 主机名
System.out.println(local.getHostAddress()); // IP

InetAddress remote = InetAddress.getByName("www.baidu.com");
System.out.println(remote.getHostAddress()); // 解析后的 IP