Java中利用UDP实现，客户端给服务端发送小写字母文本，服务端会将文本转成大写在返回给客户端

在Java中，可以使用DatagramSocket和DatagramPacket类配合UDP（User Datagram Protocol，用户数据报协议）来实现在客户端和服务端之间的简单文本通信，其中服务端将接收到的小写字母转换为大写后再返回。

以下是简单的步骤：

**客户端（Client.java）**:

import java.io.*;
import java.net.*;

public class Client {
    private static final String SERVER_ADDRESS = "localhost";
    private static final int PORT = 9090;

    public static void main(String[] args) throws IOException {
        DatagramSocket clientSocket = new DatagramSocket();
        try {
            String message = "hello world"; // 小写字母文本

            byte[] sendData = message.getBytes("UTF-8");
            InetAddress serverAddress = InetAddress.getByName(SERVER_ADDRESS);
            DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, serverAddress, PORT);

            clientSocket.send(sendPacket);
            
            // 接收服务端响应
            byte[] receiveData = new byte[1024];
            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
            clientSocket.receive(receivePacket);

            String response = new String(receivePacket.getData(), "UTF-8").toUpperCase(); // 转换为大写并打印
            System.out.println("Response from server: " + response);
        } finally {
            clientSocket.close();
        }
    }
}

**服务端（Server.java）**:

import java.io.*;
import java.net.*;

public class Server extends Thread {
    private static final int PORT = 9090;
    private DatagramSocket serverSocket = null;

    @Override
    public void run() {
        try {
            serverSocket = new DatagramSocket(PORT);
            while (true) {
                byte[] buffer = new byte[1024];
                DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
                
                // 接收客户端消息并转换为大写
                serverSocket.receive(packet);
                String message = new String(packet.getData(), "UTF-8").toUpperCase();

                // 构造回复
                byte[] reply = message.getBytes("UTF-8");

                InetAddress clientAddress = packet.getAddress();
                int clientPort = packet.getPort();
                DatagramPacket replyPacket = new DatagramPacket(reply, reply.length, clientAddress, clientPort);
                serverSocket.send(replyPacket);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (serverSocket != null) {
                try {
                    serverSocket.close();
                } catch (IOException ex) {
                    ex.printStackTrace();
                }
            }
        }
    }

    public static void main(String[] args) {
        new Server().start();
    }
}

当客户端运行并发送字符串"hello world"时，服务端会将其转换为"HELLO WORLD"并返回给客户端。