import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;

public class UdpClient {
    public static void main(String[] args) {
        try {
            DatagramSocket ds = new DatagramSocket();
            String strInfo = "d:/Input.txt";//将要转到服务端的信息
            DatagramPacket dp = new DatagramPacket(strInfo.getBytes(), strInfo
                    .length(), InetAddress.getByName("127.0.0.1"), 3000);
            ds.send(dp);//传送并接接信息
            byte[] len = new byte[1024];//创建一个数据缓冲区大小了1024
            dp = new DatagramPacket(len, 1024);//将得到的数据放到dp中
            ds.receive(dp);//接收数据
            String n = new String(dp.getData(), 0, dp.getLength());//将接收到的数据放到字符串中
            System.out.println("你好！" + n);//输出
            ds.close();
        } catch (SocketException e) {

            e.printStackTrace();
        } catch (IOException e) {

            e.printStackTrace();
        }
    }

}

import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;

public class UdpServer {

    public static void main(String[] args) {
        DatagramSocket ds = null;
        DatagramPacket dp = null;
        String m = null;
        try {
            ds = new DatagramSocket(3000);//创建服务器端
            byte[] len = new byte[1024];//创建一个缓冲区
            dp = new DatagramPacket(len, 1024);//将数据加到dp中
            ds.receive(dp);//接收dp
            String n = new String(dp.getData(), 0, dp.getLength());//将接收到的数据放到n中
            FileReader fr = new FileReader(n);//读取文件
            BufferedReader dr = new BufferedReader(fr);
            InetAddress ia = dp.getAddress();//得到客户端的地址
            int iport = dp.getPort();//客户端的端口
            StringBuffer sb = new StringBuffer();
            m = dr.readLine();
            while (m != null) {
                sb.append(m);//将读取的文件放到sb中
                sb.append("\n");//在每一行结束的时候加一个换行
                m = dr.readLine();
            }
            m = sb.toString();//将sb转为String类型的
            dp = new DatagramPacket(m.getBytes(), m.length(), ia, iport);//数据在转回到客户端
            ds.send(dp);//发送
            ds.close();
        } catch (SocketException e) {
            e.printStackTrace();
        } catch (IOException e) {

            m = "文件不存在   \n ";
            InetAddress ia = dp.getAddress();
            int iport = dp.getPort();
            dp = new DatagramPacket(m.getBytes(), m.length(), ia, iport);
            try {
                ds.send(dp);
            } catch (IOException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
            // e.printStackTrace();
        }

    }

}