服務端：

/**
 * Created by cuboo on 2016/10/12.
 */
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;
import java.net.UnknownHostException;

public class udpserver {
    private static DatagramSocket socket;
    private DatagramPacket packet;
    private static int port = 8001;
    public static void main(String[] args) {
        udpserver server = new udpserver();
        try {
            socket = new DatagramSocket(port);
            while (true){

                DatagramPacket packet = server.receiveMessage(socket);
                server.sendMessage(socket,packet);
            }


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

    }
    public void sendMessage(DatagramSocket socket,DatagramPacket receive_packet){
        try {
            byte[] datas = "hello this is udpserver echo".getBytes();
            //receive_packet接受到的客服端的ip地址和埠
            packet = new DatagramPacket(datas,datas.length,receive_packet.getAddress(),receive_packet.getPort());
            socket.send(packet);

        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (SocketException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public DatagramPacket receiveMessage(DatagramSocket socket){
        try {
            byte[] data = new byte[100];
            packet = new DatagramPacket(data,data.length);
            socket.receive(packet);
            System.out.println(new String(packet.getData(),0,packet.getLength()));

        } catch (IOException e) {
            e.printStackTrace();
        }
        //返回接受到的客服端的ip地址和埠
        return packet;
    }
}
 

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

/**
 * Created by cuboo on 2016/10/12.
 */
public class udpclient {
    private static DatagramSocket socket;
    private DatagramPacket packet;
    private int port = 8001;

    public static void main(String agrs[]){
        udpclient client = new udpclient();
        try {
            socket = new DatagramSocket();
            while (true){
                client.sendMessage();
                client.receivMessage();
            }
        } catch (SocketException e) {
            e.printStackTrace();
        }


    }
    public void sendMessage(){
        try {
            byte[] datas = "hello this is client echo".getBytes();
            packet = new DatagramPacket(datas,datas.length, InetAddress.getLocalHost(),port);
            socket.send(packet);

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

    public void receivMessage(){
        try {
            byte[] data = new byte[100];
            packet = new DatagramPacket(data,data.length);
            socket.receive(packet);
            System.out.println(new String(packet.getData(),0,packet.getLength()));

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