Java 实现UDP打洞

首先向各位大哥大姐求一个UDP打洞的例子。
我自己写了一个。可是怎么做都不成功。我把代码发出来，大家帮忙看看有什么问题。在这里先谢谢各位了！
Java code:

//服务器端
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;

public class TestServer
{

public static void main(String[] args) throws IOException
{
DatagramSocket chatUDPSocket = new DatagramSocket(8888);
while(true)
{
byte[] recvBuf = new byte[100];
DatagramPacket recvPacket = new DatagramPacket(recvBuf , recvBuf.length);
chatUDPSocket.receive(recvPacket);
System.out.println(recvPacket.getSocketAddress().toString());
}
}

}



Java code:

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.util.Scanner;

public class TestClient
{

public static void main(String[] args) throws IOException
{
DatagramSocket clienttoclient = new DatagramSocket();
String sendStr = "hello";
byte[] sendBuf = sendStr.getBytes();
InetAddress addr = InetAddress.getByName("125.34.69.243");