Java获取NTP网络时间

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