Android中的网络时间同步
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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.javatelephony/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。
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