网络时间同步
从NTP服务器获取时间同步客户端：
1 import java.io.IOException; 2 import java.io.InterruptedIOException; 3 import java.net.ConnectException; 4 import java.net.DatagramPacket; 5 import java.net.DatagramSocket; 6 import java.net.InetAddress; 7 import java.net.NoRouteToHostException; 8 import java.net.UnknownHostException; 9 10 public class TestNtp{ 11 12 public static void main(String[] args){ 13 int retry = 2; 14 int port = 123; 15 int timeout = 3000; 16 17 // get the address and NTP address request 18 // 19 InetAddress ipv4Addr = null; 20 try { 21 ipv4Addr = InetAddress.getByName("203.117.180.36");//更多NTP时间服务器参考附注 22 } catch (UnknownHostException e1) { 23 e1.printStackTrace(); 24 } 25 26 int serviceStatus = -1; 27 DatagramSocket socket = null; 28 long responseTime = -1; 29 try { 30 socket = new DatagramSocket(); 31 socket.setSoTimeout(timeout); // will force the 32 // InterruptedIOException 33 34 for (int attempts = 0; attempts <= retry && serviceStatus != 1; attempts++) { 35 try { 36 // Send NTP request 37 // 38 byte[] data = new NtpMessage().toByteArray(); 39 DatagramPacket outgoing = new DatagramPacket(data, data.length, ipv4Addr, port); 40 long sentTime = System.currentTimeMillis(); 41 socket.send(outgoing); 42 43 // Get NTP Response 44 // 45 // byte[] buffer = new byte[512]; 46 DatagramPacket incoming = new DatagramPacket(data, data.length); 47 socket.receive(incoming); 48 responseTime = System.currentTimeMillis() - sentTime; 49 double destinationTimestamp = (System.currentTimeMillis() / 1000.0) + 2208988800.0; 50 //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 51 52 // Validate NTP Response 53 // IOException thrown if packet does not decode as expected. 54 NtpMessage msg = new NtpMessage(incoming.getData()); 55 double localClockOffset = ((msg.receiveTimestamp - msg.originateTimestamp) + (msg.transmitTimestamp - destinationTimestamp)) / 2; 56 57 System.out.println("poll: valid NTP request received the local clock offset is " + localClockOffset + ", responseTime= " + responseTime + "ms"); 58 System.out.println("poll: NTP message : " + msg.toString()); 59 serviceStatus = 1; 60 } catch (InterruptedIOException ex) { 61 // Ignore, no response received. 62 } 63 } 64 } catch (NoRouteToHostException e) { 65 System.out.println("No route to host exception for address: " + ipv4Addr); 66 } catch (ConnectException e) { 67 // Connection refused. Continue to retry. 68 e.fillInStackTrace(); 69 System.out.println("Connection exception for address: " + ipv4Addr); 70 } catch (IOException ex) { 71 ex.fillInStackTrace(); 72 System.out.println("IOException while polling address: " + ipv4Addr); 73 } finally { 74 if (socket != null) 75 socket.close(); 76 } 77 78 // Store response time if available 79 // 80 if (serviceStatus == 1) { 81 System.out.println("responsetime=="+responseTime); 82 } 83 84 85 } 86 }
协议解析模型
1 import java.text.DecimalFormat; 2 import java.text.SimpleDateFormat; 3 import java.util.Date; 4 5 public class NtpMessage { 6 /** *//** 7 * This is a two-bit code warning of an impending leap second to be 8 * inserted/deleted in the last minute of the current day. It''s values may 9 * be as follows: 10 * 11 * Value Meaning ----- ------- 0 no warning 1 last minute has 61 seconds 2 12 * last minute has 59 seconds) 3 alarm condition (clock not synchronized) 13 */ 14 public byte leapIndicator = 0; 15 16 /** *//** 17 * This value indicates the NTP/SNTP version number. The version number is 3 18 * for Version 3 (IPv4 only) and 4 for Version 4 (IPv4, IPv6 and OSI). If 19 * necessary to distinguish between IPv4, IPv6 and OSI, the encapsulating 20 * context must be inspected. 21 */ 22 public byte version = 3; 23 24 /** *//** 25 * This value indicates the mode, with values defined as follows: 26 * 27 * Mode Meaning ---- ------- 0 reserved 1 symmetric active 2 symmetric 28 * passive 3 client 4 server 5 broadcast 6 reserved for NTP control message 29 * 7 reserved for private use 30 * 31 * In unicast and anycast modes, the client sets this field to 3 (client) in 32 * the request and the server sets it to 4 (server) in the reply. In 33 * multicast mode, the server sets this field to 5 (broadcast). 34 */ 35 public byte mode = 0; 36 37 /** *//** 38 * This value indicates the stratum level of the local clock, with values 39 * defined as follows: 40 * 41 * Stratum Meaning ---------------------------------------------- 0 42 * unspecified or unavailable 1 primary reference (e.g., radio clock) 2-15 43 * secondary reference (via NTP or SNTP) 16-255 reserved 44 */ 45 public short stratum = 0; 46 47 /** *//** 48 * This value indicates the maximum interval between successive messages, in 49 * seconds to the nearest power of two. The values that can appear in this 50 * field presently range from 4 (16 s) to 14 (16284 s); however, most 51 * applications use only the sub-range 6 (64 s) to 10 (1024 s). 52 */ 53 public byte pollInterval = 0; 54 55 /** *//** 56 * This value indicates the precision of the local clock, in seconds to the 57 * nearest power of two. The values that normally appear in this field 58 * range from -6 for mains-frequency clocks to -20 for microsecond clocks 59 * found in some workstations. 60 */ 61 public byte precision = 0; 62 63 /** *//** 64 * This value indicates the total roundtrip delay to the primary reference 65 * source, in seconds. Note that this variable can take on both positive and 66 * negative values, depending on the relative time and frequency offsets. 67 * The values that normally appear in this field range from negative values 68 * of a few milliseconds to positive values of several hundred milliseconds. 69 */ 70 public double rootDelay = 0; 71 72 /** *//** 73 * This value indicates the nominal error relative to the primary reference 74 * source, in seconds. The values that normally appear in this field range 75 * from 0 to several hundred milliseconds. 76 */ 77 public double rootDispersion = 0; 78 79 /** *//** 80 * This is a 4-byte array identifying the particular reference source. In 81 * the case of NTP Version 3 or Version 4 stratum-0 (unspecified) or 82 * stratum-1 (primary) servers, this is a four-character ASCII string, left 83 * justified and zero padded to 32 bits. In NTP Version 3 secondary servers, 84 * this is the 32-bit IPv4 address of the reference source. In NTP Version 4 85 * secondary servers, this is the low order 32 bits of the latest transmit 86 * timestamp of the reference source. NTP primary (stratum 1) servers should 87 * set this field to a code identifying the external reference source 88 * according to the following list. If the external reference is one of 89 * those listed, the associated code should be used. Codes for sources not 90 * listed can be contrived as appropriate. 91 * 92 * Code External Reference Source ---- ------------------------- LOCL 93 * uncalibrated local clock used as a primary reference for a subnet without 94 * external means of synchronization PPS atomic clock or other 95 * pulse-per-second source individually calibrated to national standards 96 * ACTS NIST dialup modem service USNO USNO modem service PTB PTB (Germany) 97 * modem service TDF Allouis (France) Radio 164 kHz DCF Mainflingen 98 * (Germany) Radio 77.5 kHz MSF Rugby (UK) Radio 60 kHz WWV Ft. Collins (US) 99 * Radio 2.5, 5, 10, 15, 20 MHz WWVB Boulder (US) Radio 60 kHz WWVH Kaui 100 * Hawaii (US) Radio 2.5, 5, 10, 15 MHz CHU Ottawa (Canada) Radio 3330, 101 * 7335, 14670 kHz LORC LORAN-C radionavigation system OMEG OMEGA 102 * radionavigation system GPS Global Positioning Service GOES Geostationary 103 * Orbit Environment Satellite 104 */ 105 public byte[] referenceIdentifier = { 0, 0, 0, 0 }; 106 107 /** *//** 108 * This is the time at which the local clock was last set or corrected, in 109 * seconds since 00:00 1-Jan-1900. 110 */ 111 public double referenceTimestamp = 0; 112 113 /** *//** 114 * This is the time at which the request departed the client for the server, 115 * in seconds since 00:00 1-Jan-1900. 116 */ 117 public double originateTimestamp = 0; 118 119 /** *//** 120 * This is the time at which the request arrived at the server, in seconds 121 * since 00:00 1-Jan-1900. 122 */ 123 public double receiveTimestamp = 0; 124 125 /** *//** 126 * This is the time at which the reply departed the server for the client, 127 * in seconds since 00:00 1-Jan-1900. 128 */ 129 public double transmitTimestamp = 0; 130 131 /** *//** 132 * Constructs a new NtpMessage from an array of bytes. 133 */ 134 public NtpMessage(byte[] array) { 135 // See the packet format diagram in RFC 2030 for details 136 leapIndicator = (byte) ((array[0] >> 6) & 0x3); 137 version = (byte) ((array[0] >> 3) & 0x7); 138 mode = (byte) (array[0] & 0x7); 139 stratum = unsignedByteToShort(array[1]); 140 pollInterval = array[2]; 141 precision = array[3]; 142 143 rootDelay = (array[4] * 256.0) + unsignedByteToShort(array[5]) + (unsignedByteToShort(array[6]) / 256.0) + (unsignedByteToShort(array[7]) / 65536.0); 144 145 rootDispersion = (unsignedByteToShort(array[8]) * 256.0) + unsignedByteToShort(array[9]) + (unsignedByteToShort(array[10]) / 256.0) + (unsignedByteToShort(array[11]) / 65536.0); 146 147 referenceIdentifier[0] = array[12]; 148 referenceIdentifier[1] = array[13]; 149 referenceIdentifier[2] = array[14]; 150 referenceIdentifier[3] = array[15]; 151 152 referenceTimestamp = decodeTimestamp(array, 16); 153 originateTimestamp = decodeTimestamp(array, 24); 154 receiveTimestamp = decodeTimestamp(array, 32); 155 transmitTimestamp = decodeTimestamp(array, 40); 156 } 157 158 /** *//** 159 * Constructs a new NtpMessage 160 */ 161 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) { 162 // ToDo: Validity checking 163 this.leapIndicator = leapIndicator; 164 this.version = version; 165 this.mode = mode; 166 this.stratum = stratum; 167 this.pollInterval = pollInterval; 168 this.precision = precision; 169 this.rootDelay = rootDelay; 170 this.rootDispersion = rootDispersion; 171 this.referenceIdentifier = referenceIdentifier; 172 this.referenceTimestamp = referenceTimestamp; 173 this.originateTimestamp = originateTimestamp; 174 this.receiveTimestamp = receiveTimestamp; 175 this.transmitTimestamp = transmitTimestamp; 176 } 177 178 /** *//** 179 * Constructs a new NtpMessage in client -> server mode, and sets the 180 * transmit timestamp to the current time. 181 */ 182 public NtpMessage() { 183 // Note that all the other member variables are already set with 184 // appropriate default values. 185 this.mode = 3; 186 this.transmitTimestamp = (System.currentTimeMillis() / 1000.0) + 2208988800.0; 187 } 188 189 /** *//** 190 * This method constructs the data bytes of a raw NTP packet. 191 */ 192 public byte[] toByteArray() { 193 // All bytes are automatically set to 0 194 byte[] p = new byte[48]; 195 196 p[0] = (byte) (leapIndicator << 6 | version << 3 | mode); 197 p[1] = (byte) stratum; 198 p[2] = (byte) pollInterval; 199 p[3] = (byte) precision; 200 201 // root delay is a signed 16.16-bit FP, in Java an int is 32-bits 202 int l = (int) (rootDelay * 65536.0); 203 p[4] = (byte) ((l >> 24) & 0xFF); 204 p[5] = (byte) ((l >> 16) & 0xFF); 205 p[6] = (byte) ((l >> 8) & 0xFF); 206 p[7] = (byte) (l & 0xFF); 207 208 // root dispersion is an unsigned 16.16-bit FP, in Java there are no 209 // unsigned primitive types, so we use a long which is 64-bits 210 long ul = (long) (rootDispersion * 65536.0); 211 p[8] = (byte) ((ul >> 24) & 0xFF); 212 p[9] = (byte) ((ul >> 16) & 0xFF); 213 p[10] = (byte) ((ul >> 8) & 0xFF); 214 p[11] = (byte) (ul & 0xFF); 215 216 p[12] = referenceIdentifier[0]; 217 p[13] = referenceIdentifier[1]; 218 p[14] = referenceIdentifier[2]; 219 p[15] = referenceIdentifier[3]; 220 221 encodeTimestamp(p, 16, referenceTimestamp); 222 encodeTimestamp(p, 24, originateTimestamp); 223 encodeTimestamp(p, 32, receiveTimestamp); 224 encodeTimestamp(p, 40, transmitTimestamp); 225 226 return p; 227 } 228 229 /** *//** 230 * Returns a string representation of a NtpMessage 231 */ 232 public String toString() { 233 String precisionStr = new DecimalFormat("0.#E0").format(Math.pow(2, precision)); 234 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); 235 } 236 237 /** *//** 238 * Converts an unsigned byte to a short. By default, Java assumes that a 239 * byte is signed. 240 */ 241 public static short unsignedByteToShort(byte b) { 242 if ((b & 0x80) == 0x80) 243 return (short) (128 + (b & 0x7f)); 244 else 245 return (short) b; 246 } 247 248 /** *//** 249 * Will read 8 bytes of a message beginning at <code>pointer</code> and 250 * return it as a double, according to the NTP 64-bit timestamp format. 251 */ 252 public static double decodeTimestamp(byte[] array, int pointer) { 253 double r = 0.0; 254 255 for (int i = 0; i < 8; i++) { 256 r += unsignedByteToShort(array[pointer + i]) * Math.pow(2, (3 - i) * 8); 257 } 258 259 return r; 260 } 261 262 /** *//** 263 * Encodes a timestamp in the specified position in the message 264 */ 265 public static void encodeTimestamp(byte[] array, int pointer, double timestamp) { 266 // Converts a double into a 64-bit fixed point 267 for (int i = 0; i < 8; i++) { 268 // 2^24, 2^16, 2^8, .. 2^-32 269 double base = Math.pow(2, (3 - i) * 8); 270 271 // Capture byte value 272 array[pointer + i] = (byte) (timestamp / base); 273 274 // Subtract captured value from remaining total 275 timestamp = timestamp - (double) (unsignedByteToShort(array[pointer + i]) * base); 276 } 277 278 // From RFC 2030: It is advisable to fill the non-significant 279 // low order bits of the timestamp with a random, unbiased 280 // bitstring, both to avoid systematic roundoff errors and as 281 // a means of loop detection and replay detection. 282 array[7] = (byte) (Math.random() * 255.0); 283 } 284 285 /** *//** 286 * Returns a timestamp (number of seconds since 00:00 1-Jan-1900) as a 287 * formatted date/time string. 288 */ 289 public static String timestampToString(double timestamp) { 290 if (timestamp == 0) 291 return "0"; 292 293 // timestamp is relative to 1900, utc is used by Java and is relative 294 // to 1970 295 double utc = timestamp - (2208988800.0); 296 297 // milliseconds 298 long ms = (long) (utc * 1000.0); 299 300 // date/time 301 String date = new SimpleDateFormat("dd-MMM-yyyy HH:mm:ss").format(new Date(ms)); 302 303 // fraction 304 double fraction = timestamp - ((long) timestamp); 305 String fractionSting = new DecimalFormat(".000000").format(fraction); 306 307 return date + fractionSting; 308 } 309 310 /** *//** 311 * Returns a string representation of a reference identifier according to 312 * the rules set out in RFC 2030. 313 */ 314 public static String referenceIdentifierToString(byte[] ref, short stratum, byte version) { 315 // From the RFC 2030: 316 // In the case of NTP Version 3 or Version 4 stratum-0 (unspecified) 317 // or stratum-1 (primary) servers, this is a four-character ASCII 318 // string, left justified and zero padded to 32 bits. 319 if (stratum == 0 || stratum == 1) { 320 return new String(ref); 321 } 322 323 // In NTP Version 3 secondary servers, this is the 32-bit IPv4 324 // address of the reference source. 325 else if (version == 3) { 326 return unsignedByteToShort(ref[0]) + "." + unsignedByteToShort(ref[1]) + "." + unsignedByteToShort(ref[2]) + "." + unsignedByteToShort(ref[3]); 327 } 328 329 // In NTP Version 4 secondary servers, this is the low order 32 bits 330 // of the latest transmit timestamp of the reference source. 331 else if (version == 4) { 332 return "" + ((unsignedByteToShort(ref[0]) / 256.0) + (unsignedByteToShort(ref[1]) / 65536.0) + (unsignedByteToShort(ref[2]) / 16777216.0) + (unsignedByteToShort(ref[3]) / 4294967296.0)); 333 } 334 335 return ""; 336 } 337 }
结果：
poll: valid NTP request received the local clock offset is 3606.92320227623, responseTime= 265ms
poll: NTP message : Leap indicator: 0 Version: 3 Mode: 4 Stratum: 1 Poll: 0 Precision: -18 (3.8E-6 seconds) Root delay: 0.00 ms Root dispersion: 0.00 ms Reference identifier: ACTS Reference timestamp: 26-三月-2009 20:50:23.508540 Originate timestamp: 26-三月-2009 19:51:10.031000 Receive timestamp: 26-三月-2009 20:51:17.086693 Transmit timestamp: 26-三月-2009 20:51:17.086712
responsetime==265
注意看红色部分，这是本地时间，我故意将本地时间调慢了一小时。
附注1：中国大概能用的NTP时间服务器
server 133.100.11.8 prefer
server 210.72.145.44
server 203.117.180.36 //程序中所用的
server 131.107.1.10
server time.asia.apple.com
server 64.236.96.53
server 130.149.17.21
server 66.92.68.246
server www.freebsd.org
server 18.145.0.30
server clock.via.net
server 137.92.140.80
server 133.100.9.2
server 128.118.46.3
server ntp.nasa.gov
server 129.7.1.66
server ntp-sop.inria.frserver 210.72.145.44(国家授时中心服务器IP地址)
ntpdate 131.107.1.10
ntpdate -s time.asia.apple.com
附注2：NTP概念简介
Network Time Protocol（NTP）是用来使计算机时间同步化的一种协议，它可以使计算机对其服务器或时钟源（如石英钟，GPS等等)做同步化，它可以提供高精准度的时间校正（LAN上与标准间差小于1毫秒，WAN上几十毫秒），且可介由加密确认的方式来防止恶毒的协议攻击。
