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：（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 TimeInfo     * object that allows access to all the fields of the NTP message header.     * 
     * @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： 
 
       国外的 
 

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