在做系统对时的时候，需要使用到ntp来获取时间。
可以使用common-net包来获取ntp服务器的时间（即可以向那些标准时间服务器对时，也可以向自己设置好的ntp服务器进行对时）。
使用java获取ntp的时间（t1,t2,t3,t4）。下面是官网上给出的关于使用common-net关于ntp部分的使用例子。
如果要与指定服务器A对时（非NTP时间服务器），下面的代码也无需修改，只需要修改服务器A的ntp的配置文件，将服务器A设置为时钟服务器，并且不与其他时间服务器对时（设置为与自己对时），然后其他服务器就可以直接使用下面的代码，或者是ntpdate，ntpd都可以。
很详细，很有帮助。
1 public class test {
2
3 private static final NumberFormat numberFormat = new java.text.DecimalFormat("0.00");
4 public static String ServerIP = "202.108.6.95";
5
6 public static final void main(String[] args) throws IOException
7 {
8
9 NTPUDPClient client = new NTPUDPClient();
10 // We want to timeout if a response takes longer than 10 seconds
11 client.setDefaultTimeout(10000);
12 try {
13 client.open();
14 InetAddress hostAddr = InetAddress.getByName(ServerIP);
15 System.out.println(" > " + hostAddr.getHostName() + "/" + hostAddr.getHostAddress());
16 TimeInfo info = client.getTime(hostAddr);
17 processResponse(info);
18 } catch (SocketException e) {
19 e.printStackTrace();
20 }
21 client.close();
22 }
23
24
25 public static void processResponse(TimeInfo info)
26 {
27 NtpV3Packet message = info.getMessage();
28 int stratum = message.getStratum();
29 String refType;
30 if (stratum <= 0)
31 refType = "(Unspecified or Unavailable)";
32 else if (stratum == 1)
33 refType = "(Primary Reference; e.g., GPS)"; // GPS, radio clock, etc.
34 else
35 refType = "(Secondary Reference; e.g. via NTP or SNTP)";
36 // stratum should be 0..15...
37 System.out.println(" Stratum: " + stratum + " " + refType);
38 int version = message.getVersion();
39 int li = message.getLeapIndicator();
40 System.out.println(" leap=" + li + ", version="
41 + version + ", precision=" + message.getPrecision());
42 System.out.println(" mode: " + message.getModeName() + " (" + message.getMode() + ")");
43 int poll = message.getPoll();
44 // poll value typically btwn MINPOLL (4) and MAXPOLL (14)
45 System.out.println(" poll: " + (poll <= 0 ? 1 : (int) Math.pow(2, poll))
46 + " seconds" + " (2 ** " + poll + ")");
47 double disp = message.getRootDispersionInMillisDouble();
48 System.out.println(" rootdelay=" + numberFormat.format(message.getRootDelayInMillisDouble())
49 + ", rootdispersion(ms): " + numberFormat.format(disp));
50 int refId = message.getReferenceId();
51 String refAddr = NtpUtils.getHostAddress(refId);
52 String refName = null;
53 if (refId != 0) {
54 if (refAddr.equals("127.127.1.0")) {
55 refName = "LOCAL"; // This is the ref address for the Local Clock
56 } else if (stratum >= 2) {
57 // If reference id has 127.127 prefix then it uses its own reference clock
58 // defined in the form 127.127.clock-type.unit-num (e.g. 127.127.8.0 mode 5
59 // for GENERIC DCF77 AM; see refclock.htm from the NTP software distribution.
60 if (!refAddr.startsWith("127.127")) {
61 try {
62 InetAddress addr = InetAddress.getByName(refAddr);
63 String name = addr.getHostName();
64 if (name != null && !name.equals(refAddr))
65 refName = name;
66 } catch (UnknownHostException e) {
67 // some stratum-2 servers sync to ref clock device but fudge stratum level higher... (e.g. 2)
68 // ref not valid host maybe it's a reference clock name?
69 // otherwise just show the ref IP address.
70 refName = NtpUtils.getReferenceClock(message);
71 }
72 }
73 } else if (version >= 3 && (stratum == 0 || stratum == 1)) {
74 refName = NtpUtils.getReferenceClock(message);
75 // refname usually have at least 3 characters (e.g. GPS, WWV, LCL, etc.)
76 }
77 // otherwise give up on naming the beast...
78 }
79 if (refName != null && refName.length() > 1)
80 refAddr += " (" + refName + ")";
81 System.out.println(" Reference Identifier:\t" + refAddr);
82 TimeStamp refNtpTime = message.getReferenceTimeStamp();
83 System.out.println(" Reference Timestamp:\t" + refNtpTime + " " + refNtpTime.toDateString());
84 // Originate Time is time request sent by client (t1)
85 TimeStamp origNtpTime = message.getOriginateTimeStamp();
86 System.out.println(" Originate Timestamp:\t" + origNtpTime + " " + origNtpTime.toDateString());
87 long destTime = info.getReturnTime();
88 // Receive Time is time request received by server (t2)
89 TimeStamp rcvNtpTime = message.getReceiveTimeStamp();
90 System.out.println(" Receive Timestamp:\t" + rcvNtpTime + " " + rcvNtpTime.toDateString());
91 // Transmit time is time reply sent by server (t3)
92 TimeStamp xmitNtpTime = message.getTransmitTimeStamp();
93 System.out.println(" Transmit Timestamp:\t" + xmitNtpTime + " " + xmitNtpTime.toDateString());
94 // Destination time is time reply received by client (t4)
95 TimeStamp destNtpTime = TimeStamp.getNtpTime(destTime);
96 System.out.println(" Destination Timestamp:\t" + destNtpTime + " " + destNtpTime.toDateString());
97 info.computeDetails(); // compute offset/delay if not already done
98 Long offsetValue = info.getOffset();
99 Long delayValue = info.getDelay();
100 String delay = (delayValue == null) ? "N/A" : delayValue.toString();
101 String offset = (offsetValue == null) ? "N/A" : offsetValue.toString();
102 System.out.println(" Roundtrip delay(ms)=" + delay
103 + ", clock offset(ms)=" + offset); // offset in ms
104 }
105
106 }
