NTP是仍在使用中的最古老的網路協議之一（在1985年前開始）。NTP最初由德拉瓦州大學的Dave Mills設計，他與一群志願者仍在維護NTP。

身在北郵，當然是使用北郵的NTP伺服器最快，但是在外網發現訪問不到。推薦上海交通大學的NTP服務，在其他網路仍然能夠訪問到。

兩個類，一個用於解析NTP訊息，另一個根據配置檔案使用NTP協議獲取時間。由於是JavaEE專案，檔案路徑使用的ServletActionContext的路徑。如果是一般Java專案寫成配置所在路徑即可。

package monitor.util;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InterruptedIOException;
import java.net.ConnectException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.NoRouteToHostException;
import java.net.UnknownHostException;
import java.text.DecimalFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Properties;

import org.apache.struts2.ServletActionContext;

public class NtpUtil {

	static final String configFile = ServletActionContext.getServletContext()
			.getRealPath(
					File.separator + "WEB-INF" + File.separator + "time.txt");
	static String ntpServer = null;
	static Integer retry = null;
	static Integer port = null;
	static Integer timeout = null;

	/**
	 * 讀取time.txt中的配置，返回標準時間與本地時間的差值，單位秒
	 * 
	 * @return
	 */
	public static Double localClockOffset() {

		if (ntpServer == null) {
			Properties props = new Properties();
			try {
				props.load(new FileInputStream(new File(configFile)));
			} catch (FileNotFoundException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			ntpServer = props.getProperty("server");
			retry = Integer.parseInt(props.getProperty("retry"));
			port = Integer.parseInt(props.getProperty("port"));
			timeout = Integer.parseInt(props.getProperty("timeout"));
		}

		// get the address and NTP address request
		//
		InetAddress ipv4Addr = null;
		try {
			ipv4Addr = InetAddress.getByName(ntpServer);// 更多NTP時間伺服器參考附註
		} catch (UnknownHostException e1) {
			e1.printStackTrace();
		}

		int serviceStatus = -1;
		DatagramSocket socket = null;
		long responseTime = -1;
		double localClockOffset = 0;
		try {
			socket = new DatagramSocket();
			socket.setSoTimeout(timeout); // will force the
			// InterruptedIOException

			for (int attempts = 0; attempts <= retry && serviceStatus != 1; attempts++) {
				try {
					// Send NTP request
					//
					byte[] data = new NtpMessage().toByteArray();
					DatagramPacket outgoing = new DatagramPacket(data,
							data.length, ipv4Addr, port);
					long sentTime = System.currentTimeMillis();
					socket.send(outgoing);

					// Get NTP Response
					//
					// byte[] buffer = new byte[512];
					DatagramPacket incoming = new DatagramPacket(data,
							data.length);
					socket.receive(incoming);
					responseTime = System.currentTimeMillis() - sentTime;
					double destinationTimestamp = (System.currentTimeMillis() / 1000.0) + 2208988800.0;
					// 這裡要加2208988800，是因為獲得到的時間是格林尼治時間，所以要變成東八區的時間，否則會與與北京時間有8小時的時差

					// Validate NTP Response
					// IOException thrown if packet does not decode as expected.
					NtpMessage msg = new NtpMessage(incoming.getData());
					localClockOffset = ((msg.receiveTimestamp - msg.originateTimestamp) + (msg.transmitTimestamp - destinationTimestamp)) / 2;
					// System.out
					// .println("poll: valid NTP request received the local clock offset is "
					// + localClockOffset
					// + ", responseTime= "
					// + responseTime + "ms");
					// System.out.println("poll: NTP message : " +
					// msg.toString());
					serviceStatus = 1;
				} catch (InterruptedIOException ex) {
					// Ignore, no response received.
				}
			}
		} catch (NoRouteToHostException e) {
			System.out.println("No route to host exception for address: "
					+ ipv4Addr);
		} catch (ConnectException e) {
			// Connection refused. Continue to retry.
			e.fillInStackTrace();
			System.out.println("Connection exception for address: " + ipv4Addr);
		} catch (IOException ex) {
			ex.fillInStackTrace();
			System.out
					.println("IOException while polling address: " + ipv4Addr);
		} finally {
			if (socket != null)
				socket.close();
		}

		// Store response time if available
		//
		// if (serviceStatus == 1) {
		// System.out.println("responsetime==" + responseTime);
		// }
		return localClockOffset;
	}
}

class NtpMessage {
	/** */
	/**
	 * This is a two-bit code warning of an impending leap second to be
	 * inserted/deleted in the last minute of the current day. It''s values may
	 * be as follows:
	 * 
	 * Value Meaning ----- ------- 0 no warning 1 last minute has 61 seconds 2
	 * last minute has 59 seconds) 3 alarm condition (clock not synchronized)
	 */
	public byte leapIndicator = 0;

	/** */
	/**
	 * This value indicates the NTP/SNTP version number. The version number is 3
	 * for Version 3 (IPv4 only) and 4 for Version 4 (IPv4, IPv6 and OSI). If
	 * necessary to distinguish between IPv4, IPv6 and OSI, the encapsulating
	 * context must be inspected.
	 */
	public byte version = 3;

	/** */
	/**
	 * This value indicates the mode, with values defined as follows:
	 * 
	 * Mode Meaning ---- ------- 0 reserved 1 symmetric active 2 symmetric
	 * passive 3 client 4 server 5 broadcast 6 reserved for NTP control message
	 * 7 reserved for private use
	 * 
	 * In unicast and anycast modes, the client sets this field to 3 (client) in
	 * the request and the server sets it to 4 (server) in the reply. In
	 * multicast mode, the server sets this field to 5 (broadcast).
	 */
	public byte mode = 0;

	/** */
	/**
	 * This value indicates the stratum level of the local clock, with values
	 * defined as follows:
	 * 
	 * Stratum Meaning ---------------------------------------------- 0
	 * unspecified or unavailable 1 primary reference (e.g., radio clock) 2-15
	 * secondary reference (via NTP or SNTP) 16-255 reserved
	 */
	public short stratum = 0;

	/** */
	/**
	 * This value indicates the maximum interval between successive messages, in
	 * seconds to the nearest power of two. The values that can appear in this
	 * field presently range from 4 (16 s) to 14 (16284 s); however, most
	 * applications use only the sub-range 6 (64 s) to 10 (1024 s).
	 */
	public byte pollInterval = 0;

	/** */
	/**
	 * This value indicates the precision of the local clock, in seconds to the
	 * nearest power of two. The values that normally appear in this field range
	 * from -6 for mains-frequency clocks to -20 for microsecond clocks found in
	 * some workstations.
	 */
	public byte precision = 0;

	/** */
	/**
	 * This value indicates the total roundtrip delay to the primary reference
	 * source, in seconds. Note that this variable can take on both positive and
	 * negative values, depending on the relative time and frequency offsets.
	 * The values that normally appear in this field range from negative values
	 * of a few milliseconds to positive values of several hundred milliseconds.
	 */
	public double rootDelay = 0;

	/** */
	/**
	 * This value indicates the nominal error relative to the primary reference
	 * source, in seconds. The values that normally appear in this field range
	 * from 0 to several hundred milliseconds.
	 */
	public double rootDispersion = 0;

	/** */
	/**
	 * This is a 4-byte array identifying the particular reference source. In
	 * the case of NTP Version 3 or Version 4 stratum-0 (unspecified) or
	 * stratum-1 (primary) servers, this is a four-character ASCII string, left
	 * justified and zero padded to 32 bits. In NTP Version 3 secondary servers,
	 * this is the 32-bit IPv4 address of the reference source. In NTP Version 4
	 * secondary servers, this is the low order 32 bits of the latest transmit
	 * timestamp of the reference source. NTP primary (stratum 1) servers should
	 * set this field to a code identifying the external reference source
	 * according to the following list. If the external reference is one of
	 * those listed, the associated code should be used. Codes for sources not
	 * listed can be contrived as appropriate.
	 * 
	 * Code External Reference Source ---- ------------------------- LOCL
	 * uncalibrated local clock used as a primary reference for a subnet without
	 * external means of synchronization PPS atomic clock or other
	 * pulse-per-second source individually calibrated to national standards
	 * ACTS NIST dialup modem service USNO USNO modem service PTB PTB (Germany)
	 * modem service TDF Allouis (France) Radio 164 kHz DCF Mainflingen
	 * (Germany) Radio 77.5 kHz MSF Rugby (UK) Radio 60 kHz WWV Ft. Collins (US)
	 * Radio 2.5, 5, 10, 15, 20 MHz WWVB Boulder (US) Radio 60 kHz WWVH Kaui
	 * Hawaii (US) Radio 2.5, 5, 10, 15 MHz CHU Ottawa (Canada) Radio 3330,
	 * 7335, 14670 kHz LORC LORAN-C radionavigation system OMEG OMEGA
	 * radionavigation system GPS Global Positioning Service GOES Geostationary
	 * Orbit Environment Satellite
	 */
	public byte[] referenceIdentifier = { 0, 0, 0, 0 };

	/** */
	/**
	 * This is the time at which the local clock was last set or corrected, in
	 * seconds since 00:00 1-Jan-1900.
	 */
	public double referenceTimestamp = 0;

	/** */
	/**
	 * This is the time at which the request departed the client for the server,
	 * in seconds since 00:00 1-Jan-1900.
	 */
	public double originateTimestamp = 0;

	/** */
	/**
	 * This is the time at which the request arrived at the server, in seconds
	 * since 00:00 1-Jan-1900.
	 */
	public double receiveTimestamp = 0;

	/** */
	/**
	 * This is the time at which the reply departed the server for the client,
	 * in seconds since 00:00 1-Jan-1900.
	 */
	public double transmitTimestamp = 0;

	/** */
	/**
	 * Constructs a new NtpMessage from an array of bytes.
	 */
	public NtpMessage(byte[] array) {
		// See the packet format diagram in RFC 2030 for details
		leapIndicator = (byte) ((array[0] >> 6) & 0x3);
		version = (byte) ((array[0] >> 3) & 0x7);
		mode = (byte) (array[0] & 0x7);
		stratum = unsignedByteToShort(array[1]);
		pollInterval = array[2];
		precision = array[3];

		rootDelay = (array[4] * 256.0) + unsignedByteToShort(array[5])
				+ (unsignedByteToShort(array[6]) / 256.0)
				+ (unsignedByteToShort(array[7]) / 65536.0);

		rootDispersion = (unsignedByteToShort(array[8]) * 256.0)
				+ unsignedByteToShort(array[9])
				+ (unsignedByteToShort(array[10]) / 256.0)
				+ (unsignedByteToShort(array[11]) / 65536.0);

		referenceIdentifier[0] = array[12];
		referenceIdentifier[1] = array[13];
		referenceIdentifier[2] = array[14];
		referenceIdentifier[3] = array[15];

		referenceTimestamp = decodeTimestamp(array, 16);
		originateTimestamp = decodeTimestamp(array, 24);
		receiveTimestamp = decodeTimestamp(array, 32);
		transmitTimestamp = decodeTimestamp(array, 40);
	}

	/** */
	/**
	 * Constructs a new NtpMessage
	 */
	public NtpMessage(byte leapIndicator, byte version, byte mode,
			short stratum, byte pollInterval, byte precision, double rootDelay,
			double rootDispersion, byte[] referenceIdentifier,
			double referenceTimestamp, double originateTimestamp,
			double receiveTimestamp, double transmitTimestamp) {
		// ToDo: Validity checking
		this.leapIndicator = leapIndicator;
		this.version = version;
		this.mode = mode;
		this.stratum = stratum;
		this.pollInterval = pollInterval;
		this.precision = precision;
		this.rootDelay = rootDelay;
		this.rootDispersion = rootDispersion;
		this.referenceIdentifier = referenceIdentifier;
		this.referenceTimestamp = referenceTimestamp;
		this.originateTimestamp = originateTimestamp;
		this.receiveTimestamp = receiveTimestamp;
		this.transmitTimestamp = transmitTimestamp;
	}

	/** */
	/**
	 * Constructs a new NtpMessage in client -> server mode, and sets the
	 * transmit timestamp to the current time.
	 */
	public NtpMessage() {
		// Note that all the other member variables are already set with
		// appropriate default values.
		this.mode = 3;
		this.transmitTimestamp = (System.currentTimeMillis() / 1000.0) + 2208988800.0;
	}

	/** */
	/**
	 * This method constructs the data bytes of a raw NTP packet.
	 */
	public byte[] toByteArray() {
		// All bytes are automatically set to 0
		byte[] p = new byte[48];

		p[0] = (byte) (leapIndicator << 6 | version << 3 | mode);
		p[1] = (byte) stratum;
		p[2] = (byte) pollInterval;
		p[3] = (byte) precision;

		// root delay is a signed 16.16-bit FP, in Java an int is 32-bits
		int l = (int) (rootDelay * 65536.0);
		p[4] = (byte) ((l >> 24) & 0xFF);
		p[5] = (byte) ((l >> 16) & 0xFF);
		p[6] = (byte) ((l >> 8) & 0xFF);
		p[7] = (byte) (l & 0xFF);

		// root dispersion is an unsigned 16.16-bit FP, in Java there are no
		// unsigned primitive types, so we use a long which is 64-bits
		long ul = (long) (rootDispersion * 65536.0);
		p[8] = (byte) ((ul >> 24) & 0xFF);
		p[9] = (byte) ((ul >> 16) & 0xFF);
		p[10] = (byte) ((ul >> 8) & 0xFF);
		p[11] = (byte) (ul & 0xFF);

		p[12] = referenceIdentifier[0];
		p[13] = referenceIdentifier[1];
		p[14] = referenceIdentifier[2];
		p[15] = referenceIdentifier[3];

		encodeTimestamp(p, 16, referenceTimestamp);
		encodeTimestamp(p, 24, originateTimestamp);
		encodeTimestamp(p, 32, receiveTimestamp);
		encodeTimestamp(p, 40, transmitTimestamp);

		return p;
	}

	/** */
	/**
	 * Returns a string representation of a NtpMessage
	 */
	public String toString() {
		String precisionStr = new DecimalFormat("0.#E0").format(Math.pow(2,
				precision));
		return "Leap indicator: "
				+ leapIndicator
				+ " "
				+ "Version: "
				+ version
				+ " "
				+ "Mode: "
				+ mode
				+ " "
				+ "Stratum: "
				+ stratum
				+ " "
				+ "Poll: "
				+ pollInterval
				+ " "
				+ "Precision: "
				+ precision
				+ " ("
				+ precisionStr
				+ " seconds) "
				+ "Root delay: "
				+ new DecimalFormat("0.00").format(rootDelay * 1000)
				+ " ms "
				+ "Root dispersion: "
				+ new DecimalFormat("0.00").format(rootDispersion * 1000)
				+ " ms "
				+ "Reference identifier: "
				+ referenceIdentifierToString(referenceIdentifier, stratum,
						version) + " " + "Reference timestamp: "
				+ timestampToString(referenceTimestamp) + " "
				+ "Originate timestamp: "
				+ timestampToString(originateTimestamp) + " "
				+ "Receive timestamp:   " + timestampToString(receiveTimestamp)
				+ " " + "Transmit timestamp: "
				+ timestampToString(transmitTimestamp);
	}

	/** */
	/**
	 * Converts an unsigned byte to a short. By default, Java assumes that a
	 * byte is signed.
	 */
	public static short unsignedByteToShort(byte b) {
		if ((b & 0x80) == 0x80)
			return (short) (128 + (b & 0x7f));
		else
			return (short) b;
	}

	/** */
	/**
	 * Will read 8 bytes of a message beginning at <code>pointer</code> and
	 * return it as a double, according to the NTP 64-bit timestamp format.
	 */
	public static double decodeTimestamp(byte[] array, int pointer) {
		double r = 0.0;

		for (int i = 0; i < 8; i++) {
			r += unsignedByteToShort(array[pointer + i])
					* Math.pow(2, (3 - i) * 8);
		}

		return r;
	}

	/** */
	/**
	 * Encodes a timestamp in the specified position in the message
	 */
	public static void encodeTimestamp(byte[] array, int pointer,
			double timestamp) {
		// Converts a double into a 64-bit fixed point
		for (int i = 0; i < 8; i++) {
			// 2^24, 2^16, 2^8, .. 2^-32
			double base = Math.pow(2, (3 - i) * 8);

			// Capture byte value
			array[pointer + i] = (byte) (timestamp / base);

			// Subtract captured value from remaining total
			timestamp = timestamp
					- (double) (unsignedByteToShort(array[pointer + i]) * base);
		}

		// From RFC 2030: It is advisable to fill the non-significant
		// low order bits of the timestamp with a random, unbiased
		// bitstring, both to avoid systematic roundoff errors and as
		// a means of loop detection and replay detection.
		array[7] = (byte) (Math.random() * 255.0);
	}

	/** */
	/**
	 * Returns a timestamp (number of seconds since 00:00 1-Jan-1900) as a
	 * formatted date/time string.
	 */
	public static String timestampToString(double timestamp) {
		if (timestamp == 0)
			return "0";

		// timestamp is relative to 1900, utc is used by Java and is relative
		// to 1970
		double utc = timestamp - (2208988800.0);

		// milliseconds
		long ms = (long) (utc * 1000.0);

		// date/time
		String date = new SimpleDateFormat("dd-MMM-yyyy HH:mm:ss")
				.format(new Date(ms));

		// fraction
		double fraction = timestamp - ((long) timestamp);
		String fractionSting = new DecimalFormat(".000000").format(fraction);

		return date + fractionSting;
	}

	/** */
	/**
	 * Returns a string representation of a reference identifier according to
	 * the rules set out in RFC 2030.
	 */
	public static String referenceIdentifierToString(byte[] ref, short stratum,
			byte version) {
		// From the RFC 2030:
		// In the case of NTP Version 3 or Version 4 stratum-0 (unspecified)
		// or stratum-1 (primary) servers, this is a four-character ASCII
		// string, left justified and zero padded to 32 bits.
		if (stratum == 0 || stratum == 1) {
			return new String(ref);
		}

		// In NTP Version 3 secondary servers, this is the 32-bit IPv4
		// address of the reference source.
		else if (version == 3) {
			return unsignedByteToShort(ref[0]) + "."
					+ unsignedByteToShort(ref[1]) + "."
					+ unsignedByteToShort(ref[2]) + "."
					+ unsignedByteToShort(ref[3]);
		}

		// In NTP Version 4 secondary servers, this is the low order 32 bits
		// of the latest transmit timestamp of the reference source.
		else if (version == 4) {
			return ""
					+ ((unsignedByteToShort(ref[0]) / 256.0)
							+ (unsignedByteToShort(ref[1]) / 65536.0)
							+ (unsignedByteToShort(ref[2]) / 16777216.0) + (unsignedByteToShort(ref[3]) / 4294967296.0));
		}
		return "";
	}
}
 

server = ntp.sjtu.edu.cn
retry = 2
port = 123
timeout = 3000