public class UdpServer {
  public static void main(String[] args) {
    // 实现步骤1：创建DatagramSokcet
    try {
      DatagramSocket mSocket = new DatagramSocket(9999);
      // 实现步骤2：创建DatagramPacket
      byte[] data = new byte[1024];
      DatagramPacket mPacket = new DatagramPacket(data, data.length);
      // 实现步骤3：接收 数据
      mSocket.receive(mPacket);
      // 实现步骤4：处理数据
      String result = new String(data, 0, mPacket.getLength());
      System.out.println(result);
      /**** 回复客户端 ****************/
      byte[] response = "我是UDP服务端，已经回到你的请求".getBytes();
      mPacket.setData(response);
      mSocket.send(mPacket);
      mSocket.close();
    } catch (SocketException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (IOException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
  }
}

public class UdpClient {
  private static String sendData = "我是UDP客户端，请求连接服务端";
  public static void main(String[] args) {
    try {
      // 步骤1：指定服务器的信息
      InetAddress mAddress = InetAddress.getByName("localhost");
      int port = 9999;
      byte[] data = sendData.getBytes();
      // 步骤2：创建DatagramPacket
      DatagramPacket mPacket = new DatagramPacket(data, data.length,
          mAddress, port);
      // 步骤3：创建DatagramSocket
      DatagramSocket mSocket = new DatagramSocket();
      // 步骤4：向服务端发送数据
      mSocket.send(mPacket);
      /***** 下面接收服务器返回数据 ***************************/
      // 实现步骤3：接收 数据
      mSocket.receive(mPacket);
      // 实现步骤4：处理数据
      String response = new String(data, 0, mPacket.getLength());
      System.out.println(response);
      // 关闭资源
      mSocket.close();
    } catch (UnknownHostException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (SocketException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (IOException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
  }
}