求助！关于PortUnreachableException的问题

[zhangchao19890805]

我学j2se，学到网络时编了两个程序，作用是一个程序通过UDP向另一个程序发送一个long类型的整数，最开始我用throws Exception抛异常，命令行运行，没有任何问题。后来我用try catch 抓异常，在命令行用javac编译时报出如下异常
已捕捉到异常：java.net.PortUnreachableException
}catch( PortUnreachableException e){
一下是代码
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.SocketException;
import java.net.PortUnreachableException;
import java.io.ByteArrayInputStream;
import java.io.DataInputStream;
import java.io.IOException;
import java.io.EOFException;
import java.nio.channels.IllegalBlockingModeException;
class ReceiveLong
{
public static void main(String[] args)
{
long b = - 1;
byte[] buf = new byte[1024];
ByteArrayInputStream bais = null;

DataInputStream dis = null;


DatagramPacket dp = null;
DatagramSocket ds = null;
dp = new DatagramPacket(buf, buf.length);

try
{
ds = new DatagramSocket(12345);
}catch (SocketException e)
{
e.printStackTrace();
}catch (SecurityException e)
{
e.printStackTrace();
}


try{
ds.receive(dp);
}catch (IOException e){
e.printStackTrace();
}catch (PortUnreachableException e){ //************************这里报错
e.printStackTrace();
}catch (IllegalBlockingModeException e){
e.printStackTrace();
}
bais = new ByteArrayInputStream(buf);
dis = new DataInputStream(bais);
try{
b = dis.readLong();
}catch (EOFException e){
e.printStackTrace();
}catch (IOException e){
e.printStackTrace();
}
System.out.println(b);

System.out.println("Hello World!");
}
}


import java.net.DatagramSocket;
import java.net.DatagramPacket;
import java.net.InetSocketAddress;
import java.io.ByteArrayOutputStream;
import java.io.DataOutputStream;

class SendLong {
public static void main(String[] args)throws Exception{
long a = 10000L;
ByteArrayOutputStream baos = null;
DataOutputStream dos = null;
DatagramPacket dp = null;
DatagramSocket ds = null;
baos = new ByteArrayOutputStream();
dos = new DataOutputStream (baos);
dos.writeLong(a);
byte[] buf = baos.toByteArray();
dp = new DatagramPacket(buf, buf.length, new InetSocketAddress("127.0.0.1", 12345));
ds = new DatagramSocket(23456);
ds.send(dp);
}
}
SendLong向Receive发送一个long类型的整数，异常是由Receive报出 位置代码中已经标出，奇怪的是若把这个catch语句注释掉就没有任何问题，我用的是java1.6update11 请那位高手能帮帮忙吗