Android中的网络时间同步

在 Android的系统设置中，有自动同步网络时间的选项。因为Broncho A1移植到froyo版本之后，我们发现时间同步选项无效了。所以我花了一点时间去研究 Android的网络时间同步的流程。研究的结果让我感到惊讶，Android的网络时间同步居然与SNTP协议无关，甚至与TCP/IP协议也毫无关系。

从设置的应用程序中可以了解到，自动同步网络时间的选项只是修改了Settings.System.AUTO_TIME这个设置：

private void setAutoState(boolean isEnabled, boolean autotimeStatus) {
       if (isEnabled == false) {
           mAutoPref.setChecked(autotimeStatus);
           mAutoPref.setEnabled(isEnabled);
       }
       else {
           Settings.System.putInt(getContentResolver(),
              Settings.System.AUTO_TIME, autotimeStatus ? 1 : 0);
       }
       mTimePref.setEnabled(!autotimeStatus);
       mDatePref.setEnabled(!autotimeStatus);
       mTimeZone.setEnabled(!autotimeStatus);
    }

谁会用这个设置呢？然后从代码中查找Settings.System.AUTO_TIME，主要有下面两处：

telephony/java/com/android/internal/telephony/gsm/GsmServiceStateTracker.java
telephony/java/com/android/internal/telephony/cdma/CdmaServiceStateTracker.java

GSM和CDMA的实现应该是类似的，这里只是看看GSM：

1. reference-ril/reference-ril.c处理主动上报消息。

    if (strStartsWith(s, "%CTZV:")) {
        /* TI specific -- NITZ time */
        char *response;
 
        line = p = strdup(s);
        at_tok_start(&p);
 
        err = at_tok_nextstr(&p, &response);
 
        free(line);
        if (err != 0) {
            LOGE("invalid NITZ line %s\n", s);
        } else {
            RIL_onUnsolicitedResponse (
                RIL_UNSOL_NITZ_TIME_RECEIVED,
                response, strlen(response));
        }
}

这里是处理模组主动上报的消息，如果是时间和时区消息，则调用RIL_onUnsolicitedResponse。
 
2. RIL_onUnsolicitedResponse会把消息发送给RIL的客户端。

ret = sendResponse(p, client_id);

时间和时区信息的格式在RIL_UNSOL_NITZ_TIME_RECEIVED消息的定义处有说明：

“data” is const char * pointing to NITZ time string in the form “yy/mm/dd,hh:mm:ss(+/-)tz,dt”

3. RIL客户端处理RIL_UNSOL_NITZ_TIME_RECEIVED消息(telephony/java/com/android/internal/telephony/RIL.java: processUnsolicited)

case RIL_UNSOL_NITZ_TIME_RECEIVED:
                if (RILJ_LOGD) unsljLogRet(response, ret);
 
                // has bonus long containing milliseconds since boot that the NITZ
                // time was received
                long nitzReceiveTime = p.readLong();
 
                Object[] result = new Object[2];
 
                result[0] = ret;
                result[1] = Long.valueOf(nitzReceiveTime);
 
                if (mNITZTimeRegistrant != null) {
 
                    mNITZTimeRegistrant
                        .notifyRegistrant(new AsyncResult (null, result, null));
                } else {
                    // in case NITZ time registrant isnt registered yet
                    mLastNITZTimeInfo = result;
                }

是GsmServiceStateTracker向RIL注册的，所以事件会由GsmServiceStateTracker来处理。

4. GsmServiceStateTracker 处理EVENT_NITZ_TIME事件：

            case EVENT_NITZ_TIME:
                ar = (AsyncResult) msg.obj;
                String nitzString = (String)((Object[])ar.result)[0];
                long nitzReceiveTime = ((Long)((Object[])ar.result)[1]).longValue();
                setTimeFromNITZString(nitzString, nitzReceiveTime);
                break;

这里nitzString是时间字符串，由setTimeFromNITZString负责解析。

private void setTimeFromNITZString (String nitz, long nitzReceiveTime) {
            String[] nitzSubs = nitz.split("[/:,+-]");
 
            int year = 2000 + Integer.parseInt(nitzSubs[0]);
            c.set(Calendar.YEAR, year);
 
            // month is 0 based!
            int month = Integer.parseInt(nitzSubs[1]) - 1;
            c.set(Calendar.MONTH, month);
 
            int date = Integer.parseInt(nitzSubs[2]);
            c.set(Calendar.DATE, date);
 
            int hour = Integer.parseInt(nitzSubs[3]);
            c.set(Calendar.HOUR, hour);
 
            int minute = Integer.parseInt(nitzSubs[4]);
            c.set(Calendar.MINUTE, minute);

如果在系统设置中，用户选择了自动同步网络时间，才会去设置系统时间。

           if (getAutoTime()) {
               setAndBroadcastNetworkSetTimeZone(zone.getID());
           }
           if (getAutoTime()) {
setAndBroadcastNetworkSetTime(c.getTimeInMillis());
           }

关于NITZ在WIKI上有说明：
NITZ, or Network Identity and Time Zone[1], is a mechanism for provisioning local time and date, as well as network provider identity information to mobile devices via a wireless network[2]. NITZ has been part of the official GSM standard since phase 2+ release 96[3]. NITZ is often used to automatically update the system clock of mobile phones.

由于NITZ的实现是可选的，如果运营商不支持它，Android手机就无法使用此功能了。此时用最好用SNTP来代替，否则用户会感到迷惑。但Android目前好像并没有这样做，我只找到两处地方调用SntpClient，但它们都没有去设置系统时间。