UDP通讯是单向还是双向

Question

UDP编程中，为什么服务器不能向客户端发送消息呢，客户端也不能接收服务器发送的消息?
服务器代码：
import java.net.*;

public class Server2 {

public static void main(String[] args) {



try {
    //向客户端发送消息
    byte[] b="我是服务器".getBytes();
    InetAddress ia= InetAddress.getByName("localhost");
    DatagramPacket dp=new DatagramPacket(b,b.length,ia,9999);
    DatagramSocket ds =new DatagramSocket();
    ds.send(dp);
    //接收从客户端发送的数据
    byte[] b2=new byte[1024];
    DatagramPacket dp2=new DatagramPacket(b2,b2.length);
    DatagramSocket ds2 =new DatagramSocket(9438);
    ds2.receive(dp2);
    //打印客户端发送的数据
    System.out.println(new String(b2,0,b2.length));
    ds.close();
    ds2.close();
} catch (Exception e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}
}

}

客户端代码：
import java.net.*;
public class Client2 {

public static void main(String[] args) {


        try {
            //向服务器发送数据
            byte b[]="我是客户端".getBytes();
            InetAddress ia = InetAddress.getByName("localhost");
            DatagramPacket dp=new DatagramPacket(b,b.length,ia,9438);
            DatagramSocket ds=new DatagramSocket();
            ds.send(dp);
            //接收从服务器发送的消息
            byte b2[]=new byte [1024];
            DatagramPacket dp2=new DatagramPacket(b2,b2.length);
            DatagramSocket ds2=new DatagramSocket(9999);
            ds2.receive(dp2);
            //打印服务器发送的数据
            System.out.println("呵呵");
            System.out.println(new String(b2,0,b2.length));
            ds.close();
            ds2.close();

        } catch (Exception e) {

            e.printStackTrace();
        }

}

}

Answer 1

http://blog.csdn.net/zlqqhs/article/details/8759042
参考