最近项目中涉及到一个时间验证的问题，需要根据当前时间来验证业务数据是否过期。所以直接写代码如下：
new java.util.Date().getTime();
结果测试的时候出现了问题，怎么验证都是过期。后来发现是服务器主机时间不对。也就是说如果服务器时间不准确或者被篡改，那么验证这部分会出现问题。所以决定采用获取网络当前时间来代替获取系统当前时间。
搜索了一下，原来获取网络时间有一个协议：Network Time Protocol（NTP： 网络时间协议 ）。
协议有了，那么java有没有相关实现呢。当然也有了。apache的commons-net包下面有ntp的实现。主要的类是：
org.apache.commons.net.ntp.NTPUDPClient 和 org.apache.commons.net.ntp.TimeInfo
看下用法，NTPUDPClient中有方法：
public TimeInfo getTime(InetAddress host, int port) throws IOException
public TimeInfo getTime(InetAddress host) throws IOException
第二个重载方法是用协议规范默认端口：123。
看下具体实现代码：
/***
* Retrieves the time information from the specified server and port and
* returns it. The time is the number of miliiseconds since
* 00:00 (midnight) 1 January 1900 UTC, as specified by RFC 1305.
* This method reads the raw NTP packet and constructs a <i>TimeInfo</i>
* object that allows access to all the fields of the NTP message header.
* <p>
* @param host The address of the server.
* @param port The port of the service.
* @return The time value retrieved from the server.
* @exception IOException If an error occurs while retrieving the time.
***/
public TimeInfo getTime(InetAddress host, int port) throws IOException
{
// if not connected then open to next available UDP port
if (!isOpen())
{
open();
}
NtpV3Packet message = new NtpV3Impl();
message.setMode(NtpV3Packet.MODE_CLIENT);
message.setVersion(_version);
DatagramPacket sendPacket = message.getDatagramPacket();
sendPacket.setAddress(host);
sendPacket.setPort(port);
NtpV3Packet recMessage = new NtpV3Impl();
DatagramPacket receivePacket = recMessage.getDatagramPacket();
/*
* Must minimize the time between getting the current time,
* timestamping the packet, and sending it out which
* introduces an error in the delay time.
* No extraneous logging and initializations here !!!
*/
TimeStamp now = TimeStamp.getCurrentTime();
// Note that if you do not set the transmit time field then originating time
// in server response is all 0's which is "Thu Feb 07 01:28:16 EST 2036".
message.setTransmitTime(now);
_socket_.send(sendPacket);
_socket_.receive(receivePacket);
long returnTime = System.currentTimeMillis();
// create TimeInfo message container but don't pre-compute the details yet
TimeInfo info = new TimeInfo(recMessage, returnTime, false);
return info;
}
大概过程就是想目标网络地址发包来获取网络时间，具体细节由协议来规范。
所以我们还需要来确定网络地址，继续搜索，发现网络上有时间服务器，也叫授时服务器。我们的用智能手机的时间是不是通过这种方式来同步的呢？
找到了这样一些服务器地址：
中国时间网
国外的
NIST Internet Time Servers
代码例子：
public static void main(String[] args) {
try {
NTPUDPClient timeClient = new NTPUDPClient();
String timeServerUrl = "time-a.nist.gov";
InetAddress timeServerAddress = InetAddress.getByName(timeServerUrl);
TimeInfo timeInfo = timeClient.getTime(timeServerAddress);
TimeStamp timeStamp = timeInfo.getMessage().getTransmitTimeStamp();
DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
System.out.println(dateFormat.format(timeStamp.getDate()));
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
输出：2013-04-0211:01:08