Jak korzystać z internetowego serwera czasu, aby uzyskać czas?

Question

Chciałbym uzyskać GMT [Greenwich Mean Time], a także nie chcę polegać na moim systemie czasu na to. Zasadniczo chcę używać serwera synchronizacji czasu, takiego jak in.pool.ntp.org [india] do obliczania GMT, lub może idę w złym kierunku !!!
Jak to zrobić w Javie?

Czy istnieje biblioteka Java, aby uzyskać czas z serwera czasu?

Answer 1

Ten odsyłacz demonstruje klasę języka Java o nazwie NtpMessage.java, którą można wkleić do programu, który pobierze bieżący czas z serwera NTP.

W poniższym linku znajdź sekcję "Załącznik" u dołu i pobierz NtpMessage.java i SntpClient.java i wklej ją do swojej aplikacji java. Wykona całą pracę i przyniesie ci czas.

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

Kopiowanie i wklejanie kodu, jeśli idzie w dół:

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 ""; 
    } 
} 

Answer 2

Oto kod znalazłem somewhr innego .. i używam go.

wykorzystuje bibliotekę apache commons.

// Lista serwerów czasu: http://tf.nist.gov/service/time-servers.html

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); 
    } 
} 

Zwraca wyjściowy czas od time-d.nist.gov: Pn lis 25 06:04:34 IST 2012

Answer 3

sp0d nie jest rację:

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

Więc to po prostu zwraca aktualny czas systemowy, a nie otrzymał jeden. Zobacz TimeInfo man page.
Należy użyć

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

zamiast.
Więc blok kodu będzie:

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);