java实现SNTP协议（附完整源码）

以下是Java实现SNTP协议的完整源码，包括网络通信、数据解析和时间计算等功能：

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

public class SNTPClient {

    private static final int NTP_PACKET_SIZE = 48;
    private static final int NTP_PORT = 123;
    private static final int TIMEOUT = 10000;

    private static final byte[] NTP_PACKET = new byte[NTP_PACKET_SIZE];

    static {
        NTP_PACKET[0] = 0b11100011; // LI, Version, Mode
        NTP_PACKET[1] = 0; // Stratum level of the local clock
        NTP_PACKET[2] = 6; // Polling Interval
        NTP_PACKET[3] = 0xEC; // Peer Clock Precision

        // 8 bytes of zero for Root Delay & Root Dispersion
        for (int i = 4; i < 12; i++) {
            NTP_PACKET[i] = 0;
        }

        // 4 bytes of arbitrary data
        for (int i = 12; i < NTP_PACKET_SIZE; i++) {
            NTP_PACKET[i] = (byte) (Math.random() * 256);
        }
    }

    public static void main(String[] args) {
        try {
            InetAddress address = InetAddress.getByName(args[0]);

            DatagramPacket requestPacket = new DatagramPacket(NTP_PACKET, NTP_PACKET.length, address, NTP_PORT);
            DatagramSocket socket = new DatagramSocket();
            socket.setSoTimeout(TIMEOUT);

            socket.send(requestPacket);

            DatagramPacket responsePacket = new DatagramPacket(new byte[NTP_PACKET_SIZE], NTP_PACKET_SIZE);
            socket.receive(responsePacket);

            long localTime = System.currentTimeMillis();

            long receiveTime = readTimestamp(responsePacket.getData(), 32);
            long transmitTime = readTimestamp(responsePacket.getData(), 40);

            long roundTripTime = localTime - receiveTime;
            long clockOffset = (receiveTime - transmitTime + roundTripTime) / 2;

            System.out.println("Local Time: " + localTime);
            System.out.println("Server Time: " + (transmitTime + clockOffset));
            System.out.println("Round Trip Time: " + roundTripTime);
            System.out.println("Clock Offset: " + clockOffset);

            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static long readTimestamp(byte[] data, int offset) {
        long seconds = 0;
        long fraction = 0;

        for (int i = 0; i < 4; i++) {
            seconds = (seconds << 8) | (data[offset + i] & 0xFF);
        }

        for (int i = 4; i < 8; i++) {
            fraction = (fraction << 8) | (data[offset + i] & 0xFF);
        }

        return (seconds * 1000) + ((fraction * 1000L) / 0x100000000L);
    }

}

使用时，可以在命令行中输入以下命令：

java SNTPClient <服务器地址>

例如：

java SNTPClient time.google.com

程序会向指定的SNTP服务器发送请求，计算本地时间与服务器时间之间的差值，并输出结果。需要注意的是，SNTP协议使用UDP协议进行通信，因此可能会存在丢包或延迟较大的情况。为了避免程序长时间等待，可以通过设置socket的超时时间来限制等待时间，如上述代码中的TIMEOUT变量。