java udp客户端无法接收服务器发送的数据包

from java udp programming every thing is working fine,except the server responding to the client is not detected.i.e sending data to server is ok, server received data and respond back to the client.The problem is the data not receiving to client. My local port used is 501 server listening port is 500, and i send data to 500 to server and wait the connection at 501 at client. The code can illustrate this: this is the recieving thread.. Dont know where goes wrong! Note i am useing the online server.from java udp programming every thing is workin