如何使用 Internet 时间服务器获取时间？
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【中文标题】如何使用 Internet 时间服务器获取时间？【英文标题】：How to use an Internet time server to get the time? 【发布时间】：2011-05-25 10:33:07 【问题描述】：我想获得 GMT [格林威治标准时间]，而且我不想依赖我的系统日期时间。基本上，我想使用像 in.pool.ntp.org [ India ] 这样的时间同步服务器进行 GMT 计算，或者我可能走错了方向！
如何在java中做到这一点？
是否有任何 java 库可以从时间服务器获取时间？
【问题讨论】：
【参考方案1】：sp0d 不太对：
timeInfo.getReturnTime(); // Returns time at which time message packet was received by local machine
所以它只返回当前系统时间，而不是接收到的。见TimeInfo man page。 你应该使用
timeInfo.getMessage().getTransmitTimeStamp().getTime();
相反。 所以代码块将是：
String TIME_SERVER = "time-a.nist.gov";
NTPUDPClient timeClient = new NTPUDPClient();
InetAddress inetAddress = InetAddress.getByName(TIME_SERVER);
TimeInfo timeInfo = timeClient.getTime(inetAddress);
long returnTime = timeInfo.getMessage().getTransmitTimeStamp().getTime();
Date time = new Date(returnTime);
【讨论】：
我不同意这个“所以它只是返回当前系统时间”。不是当前系统时间……而是“本地机器接收到消息包的远程系统时间”！ 在我之前所说的上下文中，“本地机器接收消息包的时间”比“传输时间戳”更准确......或者我错过了什么？ ! 我知道这有点老了，但答案似乎是两者都不正确，但两者都用于通过 computeDetails() 和 getOffset() 计算本地偏移量，即：javadocexamples.com/java_source/examples/ntp/…【参考方案2】：这是我在其他地方找到的代码.. 我正在使用它。使用 apache 公共库。
时间服务器列表：NIST Internet Time Service
import java.net.InetAddress;
import java.util.Date;
import org.apache.commons.net.ntp.NTPUDPClient;
import org.apache.commons.net.ntp.TimeInfo;
public class TimeLookup
public static void main() throws Exception
String TIME_SERVER = "time-a.nist.gov";
NTPUDPClient timeClient = new NTPUDPClient();
InetAddress inetAddress = InetAddress.getByName(TIME_SERVER);
TimeInfo timeInfo = timeClient.getTime(inetAddress);
long returnTime = timeInfo.getReturnTime();
Date time = new Date(returnTime);
System.out.println("Time from " + TIME_SERVER + ": " + time);
返回输出
时间来自time-d.nist.gov：Sun Nov 25 06:04:34 IST 2012
【讨论】：
【参考方案3】：我知道这是一个老问题，但我注意到所有答案都不正确或很复杂。
使用Apache Commons Net library 是一种很好且简单的实现方式。这个库将提供一个NTPUDPClient 类来管理无连接的 NTP 请求。此类将返回一个TimeInfo 实例。该对象应该运行计算方法来计算系统时间和 NTP 服务器时间之间的偏移量。让我们尝试在这里实现它
-
将Apache Commons Net library 添加到您的项目中。
<dependency>
<groupId>commons-net</groupId>
<artifactId>commons-net</artifactId>
<version>3.6</version>
</dependency>
-
创建
NTPUDPClient 类的新实例。
设置默认超时时间
获取 NTP 服务器的InetAddress。
调用NTPUDPClient.getTime() 方法从指定服务器检索具有时间信息的TimeInfo 实例。
调用 computeDetails() 方法来计算和验证 NTP 消息包的详细信息。
最后，使用这段代码TimeStamp.getNtpTime(currentTime + offset).getTime()，得到一个基于Java时间的NTP时间戳对象。
这里我们有一个基本的实现：
import java.net.InetAddress;
import java.util.Date;
import org.apache.commons.net.ntp.NTPUDPClient;
import org.apache.commons.net.ntp.TimeInfo;
public class NTPClient
private static final String SERVER_NAME = "pool.ntp.org";
private volatile TimeInfo timeInfo;
private volatile Long offset;
public static void main() throws Exception
NTPUDPClient client = new NTPUDPClient();
// We want to timeout if a response takes longer than 10 seconds
client.setDefaultTimeout(10_000);
InetAddress inetAddress = InetAddress.getByName(SERVER_NAME);
TimeInfo timeInfo = client.getTime(inetAddress);
timeInfo.computeDetails();
if (timeInfo.getOffset() != null)
this.timeInfo = timeInfo;
this.offset = timeInfo.getOffset();
// This system NTP time
TimeStamp systemNtpTime = TimeStamp.getCurrentTime();
System.out.println("System time:\t" + systemNtpTime + " " + systemNtpTime.toDateString());
// Calculate the remote server NTP time
long currentTime = System.currentTimeMillis();
TimeStamp atomicNtpTime = TimeStamp.getNtpTime(currentTime + offset).getTime()
System.out.println("Atomic time:\t" + atomicNtpTime + " " + atomicNtpTime.toDateString());
public boolean isComputed()
return timeInfo != null && offset != null;
你会得到类似的东西：
System time: dfaa2c15.2083126e Thu, Nov 29 2018 18:12:53.127
Atomic time: dfaa2c15.210624dd Thu, Nov 29 2018 18:12:53.129
【讨论】：
timeInfo.getMessage().getTransmitTimeStamp().getTime() 解决方案有什么问题？测试了这两种解决方案（为此在我的电脑上特别更改了时间）。无论系统设置如何，两者都显示正确的 UTC 时间。那么为什么你的解决方案更好呢？
@LexSav 将 .getTime() 替换为 ;
@LexSav 我的解决方案基于 RFC-1305 中定义的模式，而该解决方案只是从timeInfo 中提取NtpV3Packet 以直接获取传输timestamp。在我们的例子中，我们使用offset 将我们当前的系统时间调整为提供 NTP 服务器的数据。【参考方案4】：
此链接演示了一个名为 NtpMessage.java 的 java 类，您可以将其粘贴到您的程序中，该程序将从 NTP 服务器获取当前时间。
在以下链接中，找到底部附近的“附件”部分，下载 NtpMessage.java 和 SntpClient.java 并将其粘贴到您的 java 应用程序中。它会完成所有工作并为您争取时间。
http://support.ntp.org/bin/view/Support/JavaSntpClient
如果出现问题，复制并粘贴代码：
import java.text.DecimalFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
/**
* This class represents a NTP message, as specified in RFC 2030. The message
* format is compatible with all versions of NTP and SNTP.
*
* This class does not support the optional authentication protocol, and
* ignores the key ID and message digest fields.
*
* For convenience, this class exposes message values as native Java types, not
* the NTP-specified data formats. For example, timestamps are
* stored as doubles (as opposed to the NTP unsigned 64-bit fixed point
* format).
*
* However, the contructor NtpMessage(byte[]) and the method toByteArray()
* allow the import and export of the raw NTP message format.
*
*
* Usage example
*
* // Send message
* DatagramSocket socket = new DatagramSocket();
* InetAddress address = InetAddress.getByName("ntp.cais.rnp.br");
* byte[] buf = new NtpMessage().toByteArray();
* DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 123);
* socket.send(packet);
*
* // Get response
* socket.receive(packet);
* System.out.println(msg.toString());
*
*
* This code is copyright (c) Adam Buckley 2004
*
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License as published by the Free
* Software Foundation; either version 2 of the License, or (at your option)
* any later version. A html version of the GNU General Public License can be
* seen at http://www.gnu.org/licenses/gpl.html
*
* This program is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
* more details.
*
*
* Comments for member variables are taken from RFC2030 by David Mills,
* University of Delaware.
*
* Number format conversion code in NtpMessage(byte[] array) and toByteArray()
* inspired by http://www.pps.jussieu.fr/~jch/enseignement/reseaux/
* NTPMessage.java which is copyright (c) 2003 by Juliusz Chroboczek
*
* @author Adam Buckley
*/
public 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 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 + "\n" +
"Version: " + version + "\n" +
"Mode: " + mode + "\n" +
"Stratum: " + stratum + "\n" +
"Poll: " + pollInterval + "\n" +
"Precision: " + precision + " (" + precisionStr + " seconds)\n" +
"Root delay: " + new DecimalFormat("0.00").format(rootDelay*1000) + " ms\n" +
"Root dispersion: " + new DecimalFormat("0.00").format(rootDispersion*1000) + " ms\n" +
"Reference identifier: " + referenceIdentifierToString(referenceIdentifier, stratum, version) + "\n" +
"Reference timestamp: " + timestampToString(referenceTimestamp) + "\n" +
"Originate timestamp: " + timestampToString(originateTimestamp) + "\n" +
"Receive timestamp: " + timestampToString(receiveTimestamp) + "\n" +
"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 "";
【讨论】：
【参考方案5】：服务器time-a.nist.gov没有列出时间端口；您必须使用正确的服务器 ntp.xs4all.nl 从互联网获取日期和时间：
String TIME_SERVER = "ntp.xs4all.nl";
//... some other code
【讨论】：
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