Java 如何使用 Internet 时间服务器获取时间？

How to use an Internet time server to get the time?

提问by Rakesh Juyal

I would like to get the GMT [ Greenwich Mean Time ], and also I don't want to rely on my system date time for that. Basically, I want to use time sync server like in.pool.ntp.org[ India ] for GMT calculation, or may be I am going in wrong direction!

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in.pool.ntp.org[ India ] 之类的时间同步服务器进行 GMT 计算，或者可能是我走错了方向！

How to do this in java ?

如何在 java 中做到这一点？

Is there any java library to get time from Time server?

是否有任何 Java 库可以从时间服务器获取时间？

回答by Eric Leschinski

This link demonstrates a java class called NtpMessage.java that you can paste into your program which will fetch the current time from an NTP server.

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。

At the following link, Find the "Attachment" section near the bottom and download NtpMessage.java and SntpClient.java and paste it into your java application. It will do all the work and fetch you the time.

在以下链接中，找到靠近底部的“附件”部分并下载 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节省时间。

http://support.ntp.org/bin/view/Support/JavaSntpClient

http://support.ntp.org/bin/view/Support/JavaSntpClient

Copy and paste of the code if it goes down:

如果出现故障，请复制并粘贴代码：

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 "";
    }
}   

回答by ChAnDu353

Here is a code i found somewhere else.. and i am using it. Uses apache commons library.

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。

List of time servers: NIST Internet Time Service

时间服务器列表：NIST Internet 时间服务

 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);
    }
}

Returns the output Time from time-d.nist.gov: Sun Nov 25 06:04:34 IST 2012

从 返回输出时间time-d.nist.gov：Sun Nov 25 06:04:34 IST 2012

回答by aikikode

sp0d is not quite right:

sp0d 不太正确：

timeInfo.getReturnTime(); // Returns time at which time message packet was received by local machine

So it just returns current system time, not the received one. See TimeInfo man page.
You should use

所以它只返回当前系统时间，而不是接收到的时间。请参见TimeInfo 手册页。
你应该使用

timeInfo.getMessage().getTransmitTimeStamp().getTime();

instead.
So the code block will be:

反而。
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);

回答by 5377037

The server time-a.nist.govdoes not list the time port; you have to use correct server ntp.xs4all.nlfor getting date and time from internet:

服务器time-a.nist.gov不列出时间端口；您必须使用正确的服务器ntp.xs4all.nl才能从 Internet 获取日期和时间：

String TIME_SERVER = "ntp.xs4all.nl";
//... some other code

回答by Teocci

I know this is an old question but I notice that all the answers are not correct or are complicated.

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。

A nice and simple way to implement it is using Apache Commons Net library. This library will provide a NTPUDPClientclass to manage connectionless NTP requests. This class will return a TimeInfoinstance. This object should run the compute method to calculate the offset between your system's time and the NTP server's time. Lets try to implement it here

实现它的一个很好且简单的方法是使用Apache Commons Net 库。这个库将提供一个NTPUDPClient类来管理无连接的 NTP 请求。这个类将返回一个TimeInfo实例。此对象应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

1. Add the Apache Commons Net libraryto your project.

1. 将Apache Commons Net 库添加到您的项目中。

<dependency>
  <groupId>commons-net</groupId>
  <artifactId>commons-net</artifactId>
  <version>3.6</version>
</dependency>

1. Create a new instance of the NTPUDPClientclass.
2. Setup the default timeout
3. Get the InetAddressof the NTP Server.
4. Call the NTPUDPClient.getTime()method to retrieve a TimeInfoinstance with the time information from the specified server.
5. Call the computeDetails() method to compute and validate details of the NTP message packet.
6. Finally, get a NTP timestamp object based on a Java time by using this code TimeStamp.getNtpTime(currentTime + offset).getTime().

2. 创建类的新实例NTPUDPClient。
3. 设置默认超时
4. 获取InetAddressNTP 服务器的。
5. 调用该NTPUDPClient.getTime()方法以TimeInfo从指定服务器检索具有时间信息的实例。
6. 调用computeDetails() 方法来计算和验证NTP 消息包的详细信息。
7. 最后，使用此代码获取基于 Java 时间的 NTP 时间戳对象TimeStamp.getNtpTime(currentTime + offset).getTime()。

Here we have a basic implementation:

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;
  }
}

You will get something like that:

你会得到这样的东西：

System time:    dfaa2c15.2083126e  Thu, Nov 29 2018 18:12:53.127
Atomic time:    dfaa2c15.210624dd  Thu, Nov 29 2018 18:12:53.129