UDP协议：
UDP协议传输数据是不可靠的，在java中，java.util.DatagramSocket负责接收和发送UDP数据报，java.util.DatagramPacket表示UDP数据表。每个
DatagramSocket与一个本底地址绑定，每个DatagramSocket可以把UDP数据报发送给任意一个远程DatagramSocket，也可以接收来自任意一个远程的
DatagramSocket的UDP数据报。在UDP数据报中包含了目的地址信息，DatagramSocket根据该信息把数据报发送到目的地

UDP服务端

public class EchoServer{
    
    private int port = 8089;
    private DatagramSocket socket;
    
    public EchoServer()throws IOException{
        socket = new DatagramSocket(port);
        System.out.println("服务端启动");
    }
    
    public String ehco(String msg){
        return "ehco:"+msg;
    }
    
    public void service(){
        while(true){
            try {
                //创建一个数据报对象
                DatagramPacket packet = new DatagramPacket(new byte[512], 512);
                socket.receive(packet);//接收来自任意一个EhcoClient的数据报
                String msg = new String(packet.getData(), 0, packet.getLength());
                System.out.println(packet.getAddress()+":"+packet.getPort()+">"+msg);
                packet.setData(ehco(msg).getBytes());//给EchoClient发送数据报
                socket.send(packet);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
    
    public static void main(String[] args) throws IOException {
        new EchoServer().service();
    }
    
}

UDP客户端

public class EchoClient {

    private String remoteHost = "localhost";
    private int remotePort = 8089;
    private DatagramSocket socket;
    
    public EchoClient() throws SocketException{
        socket = new DatagramSocket();//与本地任意的端口绑定
    }
    
    public void talk() throws IOException{
        try{
            //获取服务端ip地址
            InetAddress remoteIP = InetAddress.getByName(remoteHost);
            
            BufferedReader localReader = new BufferedReader(new InputStreamReader(System.in));
            String msg = null;
            while((msg = localReader.readLine()) != null){
                byte[] outputData = msg.getBytes();
                DatagramPacket outputPacket = new DatagramPacket(
                        outputData, outputData.length,remoteIP,remotePort);
                
                socket.send(outputPacket);//发送数据报
                
                DatagramPacket inputPacket = new DatagramPacket(new byte[512], 512);
                socket.receive(inputPacket);//接收EchoServer的数据报
                System.out.println(new String(inputPacket.getData(),0,inputPacket.getLength()));
                if(msg.equals("bye")){
                    break;
                }
            }
        }catch (Exception e) {
            e.printStackTrace();
        }finally{
            socket.close();
        }
    }
    
    public static void main(String[] args) throws SocketException, IOException {
        new EchoClient().talk();
    }
    
}