socket.receive(packet); //服务器端接收到packet
String str = "welcome"; //准备回发的内容
DatagramPacket packet2 = new DatagramPacket(str.getBytes(),
str.length(), packet.getAddress(), packet.getPort()); //从客户端接收到的packet里读出地址和端口信息,与内容一起封装到packet2里。
socket.send(packet2) //回发