Java獲取網絡時間

轉載自 http://brokendreams.iteye.com/blog/1840603

獲取網絡時間有一個協議：Network Time Protocol（NTP： 網絡時間協議 ）。 

        協議有了，那麼java有沒有相關實現呢。當然也有了。apache的commons-net包下面有ntp的實現。主要的類是： 

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

和 

public TimeInfo getTime(InetAddress host) throws IOException

第二個重載方法是用協議規範默認端口：123。 

看下具體實現代碼： 

/***
     * 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;
    }

       大概過程就是想目標網絡地址發包來獲取網絡時間，具體細節由協議來規範。 
所以我們還需要來確定網絡地址，繼續搜索，發現網絡上有時間服務器，也叫授時服務器。我們的用智能手機的時間是不是通過這種方式來同步的呢？ 
       找到了這樣一些服務器地址： 
中國時間網 
       國外的 
NIST Internet Time Servers 

代碼例子： 

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();
    }
}

輸出：

2013-04-02 11:01:08

可調整本機時間，然後觀察輸出是否正確。