StringBuffer不追加数据-由于线程?

以下是服务器类的代码:

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;

public class UDPServer extends Thread {

     DatagramSocket serverSocket;
     DatagramPacket receivePacket;
     String sentence;
     InetAddress IPAddress;
     StringBuffer data = new StringBuffer();

    public UDPServer() throws SocketException {
        serverSocket = new DatagramSocket(9876);
    }

    public void run()    {
        while (true)  {
             byte[] receiveData = new byte[1024];
             byte[] sendData = new byte[1024];
             receivePacket = new DatagramPacket(receiveData, receiveData.length);

             try {
                 serverSocket.receive(receivePacket);
            } catch (IOException e) {
                 System.out.println("receive....");
                e.printStackTrace();
            }

             sentence = new String(receivePacket.getData());
             IPAddress = receivePacket.getAddress();
             int port = receivePacket.getPort();
             data.append(sentence);
             sendData = data.toString().trim().getBytes();
             DatagramPacket sendPacket = new DatagramPacket(sendData,
             sendData.length, IPAddress, port);
             try {
                serverSocket.send(sendPacket);
            } catch (IOException e) {
                System.out.println("send...");
                e.printStackTrace();
            } 
        }
    }
}

我已经调试过了:变量“句”总是得到正确的数据,但行数据是正确的;什么都不做。它只是没有附加数据。为什么会这样?