NTP是仍在使用中的最古老的网络协议之一（在1985年前开始）。NTP最初由德拉瓦州大学的Dave Mills设计，他与一群志愿者仍在维护NTP。
常用的NTP服务器参考https://www.douban.com/note/171309770/
身在北邮，当然是使用北邮的NTP服务器最快，但是在外网发现访问不到。推荐上海交通大学的NTP服务，在其他网络仍然能够访问到。
两个类，一个用于解析NTP消息，另一个根据配置文件使用NTP协议获取时间。由于是JavaEE项目，文件路径使用的ServletActionContext的路径。如果是一般Java项目写成配置所在路径即可。
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;
/**
* 读取time.txt中的配置，返回标准时间与本地时间的差值，单位秒
*
* @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时间服务器参考附注
} 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;
// 这里要加2208988800，是因为获得到的时间是格林尼治时间，所以要变成东八区的时间，否则会与与北京时间有8小时的时差
// 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 "";
}
}配置文件很简单：
server = ntp.sjtu.edu.cn
retry = 2
port = 123
timeout = 3000