java UDP通信的原理是什么

package test;
 
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
 
/**
 * UDPCLient deom
 * @author <dht925nerd@126.com>
 */
public class UDPClient {
    public static void main(String[] args) throws Exception {
        DatagramSocket clientSocket = new DatagramSocket();
        BufferedReader inFromUser =
                new BufferedReader(
                        new InputStreamReader(System.in)
                );
        //获取本地 IP 地址
        InetAddress IPAddress = InetAddress.getLocalHost();
        byte[] sendData;
        byte[] receiveData = new byte[1024];
        System.out.println("请输入一句英文，服务器会返回其大写形式（输入exit退出）");
        while (true) {
            String sentence = inFromUser.readLine();
            if (sentence.equals("exit")) break;
            sendData = sentence.getBytes();
            //创建发送数据报包，并标注源地址#，目的地址#
            DatagramPacket sendPacket =
                    new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
            //发送数据报包
            clientSocket.send(sendPacket);
            //创建接收数据报包
            DatagramPacket receivePacket =
                    new DatagramPacket(receiveData, receiveData.length);
            //接收服务器的数据报包
            clientSocket.receive(receivePacket);
            String modifiedSentence = new String(receivePacket.getData());
            System.out.println("FROM SERVER: " + modifiedSentence);
        }
        clientSocket.close();
    }
}