Android中通过NTP服务器获取时间功能源码分析_盼望着

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Android中通过NTP服务器获取时间功能源码分析

(2015-05-07 10:21:13)

标签：

股票

it

1 相关文件：
frameworks\base\services\java\com\android\server\ SystemServer.java
frameworks\base\services\java\com\android\server\ NetworkTimeUpdateService.java
frameworks\base\core\java\android\util\NtpTrustedTime.java
frameworks\base\core\java\android\net\SntpClient.java
frameworks\base\core\res\res\values\config.xml

2 实现原理：
2.1 SystemServer的run中：
ActivityManagerService.self().systemReady(new Runnable() {
            public void run() {
                try {
                    if (networkTimeUpdaterF != null) networkTimeUpdaterF.systemReady();
                } catch (Throwable e) {
                    reportWtf("making Network Time Service ready", e);
                }
...
}
2.2 再来看看NetworkTimeUpdateService中的相关代码：
systemReady

public void systemReady() {

registerForTelephonyIntents();

//注册定时器广播

registerForAlarms();

//注册网络连接消息广播

registerForConnectivityIntents();

//创建用于接收NTP请求事件的HandlerThread

//用于处理：

// EVENT_AUTO_TIME_CHANGED:

// EVENT_POLL_NETWORK_TIME:

// EVENT_NETWORK_CONNECTED:

//三个消息

mThread = new HandlerThread(TAG);

mThread.start();

mHandler = new MyHandler(mThread.getLooper());

// Check the network time on the new thread

//发送请求NTP时间消息

mHandler.obtainMessage(EVENT_POLL_NETWORK_TIME).sendToTarget();

//添加一个用于监听设置中时间改变消息通知

mSettingsObserver = new SettingsObserver(mHandler, EVENT_AUTO_TIME_CHANGED);

mSettingsObserver.observe(mContext);

}

MyHandler

private class MyHandler extends Handler {

public MyHandler(Looper l) {

super(l);

}

@Override

public void handleMessage(Message msg) {

switch (msg.what) {

case EVENT_AUTO_TIME_CHANGED:

case EVENT_POLL_NETWORK_TIME:

case EVENT_NETWORK_CONNECTED:

onPollNetworkTime(msg.what);

break;

}

重点来看看onPollNetworkTime这个函数：

在看这个函数之前先来理解几个相关变量，理解了这几个变量之后，该函数就比较好理解了。
在NetworkTimeUpdateService的构造函数中：

public NetworkTimeUpdateService(Context context) {

mContext = context;

mTime = NtpTrustedTime.getInstance(context);

mAlarmManager = (AlarmManager) mContext.getSystemService(Context.ALARM_SERVICE);

Intent pollIntent = new Intent(ACTION_POLL, null);

mPendingPollIntent = PendingIntent.getBroadcast(mContext, POLL_REQUEST, pollIntent, 0);

mPollingIntervalMs = mContext.getResources().getInteger(

com.android.internal.R.integer.config_ntpPollingInterval);

mPollingIntervalShorterMs = mContext.getResources().getInteger(

com.android.internal.R.integer.config_ntpPollingIntervalShorter);

mTryAgainTimesMax = mContext.getResources().getInteger(

com.android.internal.R.integer.config_ntpRetry);

mTimeErrorThresholdMs = mContext.getResources().getInteger(

com.android.internal.R.integer.config_ntpThreshold);

}

几个关键的变量：
mPollingIntervalMs
当NTP时间获取成功后，再次请求NTP时间的间隔
mPollingIntervalShorterMs
当NTP时间获取失败后，再次请求NTP时间的间隔
mTimeErrorThresholdMs
当NTP时间和系统时间不相同时，要更新系统时间的阀值
这几个变量的值是通过资源文件里读取的，配置的地址为config.xml,来看看相关的内容：

translatable="false" name="config_ntpServer">0.android.pool.ntp.org

name="config_ntpPollingInterval">864000000

name="config_ntpPollingIntervalShorter">5000

name="config_ntpRetry">-1

name="config_ntpThreshold">5000

name="config_ntpTimeout">20000

其实只要看下注释这几个变量的功能就清楚了，可见注释是多么的重要，如果要自己看代码理解的话，可能要花比较多的时间。

好，最后来看下onPollNetworkTime的代码：

private void onPollNetworkTime(int event) {

// If Automatic time is not set, don't bother.

if (!isAutomaticTimeRequested()) return;

final long refTime = SystemClock.elapsedRealtime();

// If NITZ time was received less than mPollingIntervalMs time ago,

// no need to sync to NTP.

if (mNitzTimeSetTime != NOT_SET && refTime - mNitzTimeSetTime < mPollingIntervalMs) {

resetAlarm(mPollingIntervalMs);

return;

}

final long currentTime = System.currentTimeMillis();

if (DBG) Log.d(TAG, "System time = " + currentTime);

// Get the NTP time

if (mLastNtpFetchTime == NOT_SET || refTime >= mLastNtpFetchTime + mPollingIntervalMs

|| event == EVENT_AUTO_TIME_CHANGED) {

if (DBG) Log.d(TAG, "Before Ntp fetch");

// force refresh NTP cache when outdated

//如果没有获取过NTP时间或者系统时间距离最后一次获取NTP时间超过了mPollingIntervalMs，就去请求NTP时间

if (mTime.getCacheAge() >= mPollingIntervalMs) {

mTime.forceRefresh();

}

// only update when NTP time is fresh

if (mTime.getCacheAge() < mPollingIntervalMs) {

final long ntp = mTime.currentTimeMillis();

mTryAgainCounter = 0;

// If the clock is more than N seconds off or this is the first time it's been

// fetched since boot, set the current time.

if (Math.abs(ntp - currentTime) > mTimeErrorThresholdMs

|| mLastNtpFetchTime == NOT_SET) {

// Set the system time

if (DBG && mLastNtpFetchTime == NOT_SET

&& Math.abs(ntp - currentTime) <= mTimeErrorThresholdMs) {

Log.d(TAG, "For initial setup, rtc = " + currentTime);

}

if (DBG) Log.d(TAG, "Ntp time to be set = " + ntp);

// Make sure we don't overflow, since it's going to be converted to an int

if (ntp / 1000 < Integer.MAX_VALUE) {

SystemClock.setCurrentTimeMillis(ntp);

}

} else {

if (DBG) Log.d(TAG, "Ntp time is close enough = " + ntp);

}

mLastNtpFetchTime = SystemClock.elapsedRealtime();

} else {

// Try again shortly

mTryAgainCounter++;

if (mTryAgainTimesMax < 0 || mTryAgainCounter <= mTryAgainTimesMax) {

resetAlarm(mPollingIntervalShorterMs);

} else {

// Try much later

mTryAgainCounter = 0;

resetAlarm(mPollingIntervalMs);

}

return;

}

resetAlarm(mPollingIntervalMs);

}

改了下，个人感觉比原来代码更容易理解了：

private void onPollNetworkTime(int event) {

// If Automatic time is not set, don't bother.

if (!isAutomaticTimeRequested()) return;

final long refTime = SystemClock.elapsedRealtime();

// If NITZ time was received less than mPollingIntervalMs time ago,

// no need to sync to NTP.

if (mNitzTimeSetTime != NOT_SET && refTime - mNitzTimeSetTime < mPollingIntervalMs) {

resetAlarm(mPollingIntervalMs);

return;

}

final long currentTime = System.currentTimeMillis();

if (DBG) Log.d(TAG, "System time = " + currentTime);

// Get the NTP time

if (mLastNtpFetchTime == NOT_SET || refTime >= mLastNtpFetchTime + mPollingIntervalMs

|| event == EVENT_AUTO_TIME_CHANGED) {

if (DBG) Log.d(TAG, "Before Ntp fetch");

if (mTime.getCacheAge() >= mPollingIntervalMs && !mTime.forceRefresh()) {

mTryAgainCounter++;

if (mTryAgainTimesMax < 0 || mTryAgainCounter <= mTryAgainTimesMax) {

resetAlarm(mPollingIntervalShorterMs);

} else {

// Try much later

mTryAgainCounter = 0;

resetAlarm(mPollingIntervalMs);

}

return;

}

final long ntp = mTime.currentTimeMillis();

mTryAgainCounter = 0;

// If the clock is more than N seconds off or this is the first time it's been

// fetched since boot, set the current time.

if (Math.abs(ntp - currentTime) > mTimeErrorThresholdMs

|| mLastNtpFetchTime == NOT_SET) {

// Set the system time

if (DBG && mLastNtpFetchTime == NOT_SET

&& Math.abs(ntp - currentTime) <= mTimeErrorThresholdMs) {

Log.d(TAG, "For initial setup, rtc = " + currentTime);

}

if (DBG) Log.d(TAG, "Ntp time to be set = " + ntp);

// Make sure we don't overflow, since it's going to be converted to an int

if (ntp / 1000 < Integer.MAX_VALUE) {

SystemClock.setCurrentTimeMillis(ntp);

}

} else {

if (DBG) Log.d(TAG, "Ntp time is close enough = " + ntp);

}

mLastNtpFetchTime = SystemClock.elapsedRealtime();

resetAlarm(mPollingIntervalMs);

}

哪个代码更清晰，大家仁者见仁，智者见智，各取所好。

当然，我要声明一下，虽然我很有信心我改的代码没有问题，但是该代码我没有经过测试的，所以不要随便替换。

多年的写代码的经验告诉我，自信要有，但是不要自负，大脑是的优势在于创造，而机器的优势在于精确。

所以，在实际的工作中，写完代码之后，写测试用例测试下吧！

(完)

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