从Android BatteryService中可以知道，BatteryStatusService（同样的，后面简称BSS）是在AMS中启动的，并读取一个保存电池信息的文件，设置Handler。这里分析BSS与AMS的密切关系，分析进程耗电统计。
深入BSS的启动
AMS启动BSS
// TODO: Move creation of battery stats service outside of activity manager service.
File dataDir = Environment.getDataDirectory();
File systemDir = new File(dataDir, "system");
// /data/system
systemDir.mkdirs();
//AMS创建一个线程给到BSS，BSS可以往线程中发送消息
//mHandler = new MainHandler(mHandlerThread.getLooper());
//见到下一小节对BSS构造函数的说明
mBatteryStatsService = new BatteryStatsService(systemDir, mHandler);
//利用刚创建的BatteryStatsImpl，调用其readLocked()
//1 解析batterystats-daily.xml，记录第三方apk的信息，还不知道这些信息有什么用
// 把信息保存到一个mDailyItems，这是一个DailyItems的数组
//2 解析batterystats.bin
// 得到一大波，系统时间、电池信息、屏幕点亮时间、网络使用时间等等
mBatteryStatsService.getActiveStatistics().readLocked();
//更新信息到磁盘
//具体做法是：发送msg(BatteryStatsHandler.MSG_WRITE_TO_DISK)给BSS：
// BSS - updateExternalStatsSync 更新外部设备耗电信息，如CPU/RADIO/WIFI/BT
// 远程调用，利用Parcel对象返回信息，异步获取，可能会超时
// BSI - writeAsyncLocked 写入磁盘
mBatteryStatsService.scheduleWriteToDisk();
//是不是应该打开debug功能
mOnBattery = DEBUG_POWER ? true
: mBatteryStatsService.getActiveStatistics().getIsOnBattery();
//设置AMS为BSS的回调对象
mBatteryStatsService.getActiveStatistics().setCallback(this);
BSS的构造函数
BatteryStatsService(File systemDir, Handler handler) {
// Our handler here will be accessing the disk, use a different thread than
// what the ActivityManagerService gave us (no I/O on that one!).
// BSS也创建一个工作线程，并把Looper给到BatteryStatsHandler
// 如果使用Handler的post方式，执行一个runable对象，msg.target被设置为callback
final ServiceThread thread = new ServiceThread("batterystats-sync",
Process.THREAD_PRIORITY_DEFAULT, true);
thread.start();//执行HandlerThread的run(): Looper.prepare()/Looper.loop()
//Handler，给定新的Looper，否则使用当前线程的Looper.
mHandler = new BatteryStatsHandler(thread.getLooper());
//上面的内容告诉我们：
//mHandler.post中的Runable是在一个ServiceThread执行的，而不是当前线程。
//这样的逻辑就是：创建一个工作，当有需要的时候，让它在别的线程中执行。
// BatteryStatsImpl expects the ActivityManagerService handler, so pass that one through.
//创建一个BatteryStatsImpl(BSI)，它可以往AMS和BSS发送消息
mStats = new BatteryStatsImpl(systemDir, handler, mHandler, this);
}
BSI的构造函数
public BatteryStatsImpl(File systemDir, Handler handler, ExternalStatsSync externalSync,
PlatformIdleStateCallback cb) {
this(new SystemClocks(), systemDir, handler, externalSync, cb);
}
public BatteryStatsImpl(Clocks clocks, File systemDir, Handler handler,
ExternalStatsSync externalSync, PlatformIdleStateCallback cb) {
//与移动网络和wifi网络相关
init(clocks);
//创建/data/system/batterystats.bin
if (systemDir != null) {
mFile = new JournaledFile(new File(systemDir, "batterystats.bin"),
new File(systemDir, "batterystats.bin.tmp"));
} else {
mFile = null;
}
//创建batterystats-checkin.bin batterystats-daily.xml
mCheckinFile = new AtomicFile(new File(systemDir, "batterystats-checkin.bin"));
mDailyFile = new AtomicFile(new File(systemDir, "batterystats-daily.xml"));
//BSS的Handler
mExternalSync = externalSync;
//BSI的Handler
mHandler = new MyHandler(handler.getLooper());
mStartCount++;
//一大批的StopwatchTimer和LongSamplingCounter，略。。。
//初始化两个变量
mOnBattery = mOnBatteryInternal = false;
//记录系统时间
long uptime = mClocks.uptimeMillis() * 1000;
long realtime = mClocks.elapsedRealtime() * 1000;
initTimes(uptime, realtime);
//ro.build.id属性
mStartPlatformVersion = mEndPlatformVersion = Build.ID;
//各种时间、Trace等初始化
//BSS对象
mPlatformIdleStateCallback = cb;
}
BSS setBatteryState
从另一篇文章Android-BatteryService中可以看到，当底层电池信息发生改变时，会回调BS中设定的listerner，然后BS会通知BSS，通知方法就是通过setBatteryState.
BS的onStart方法设置listerner - updateBatteryWarningLevelLocked(false) BS的onBootPhase - 监听Settings.Global.LOW_POWER_MODE_TRIGGER_LEVEL的变化 - 一旦有变化执行updateBatteryWarningLevelLocked(true)
@Override
public void setBatteryState(final int status, final int health, final int plugType,
final int level, final int temp, final int volt, final int chargeUAh) {
//非阻塞处理电池状态
mHandler.post(new Runnable() {
@Override
public void run() {
synchronized (mStats) {
//充电状态没有发生切换
mStats.setBatteryStateLocked(status, health, plugType, level, temp, volt,
chargeUAh);
return;
}
}
//先同步一下外部耗电设备，WIFI/BT/MODEM（MODEM：电话、短信、移动网络）
updateExternalStatsSync("battery-state", BatteryStatsImpl.ExternalStatsSync.UPDATE_ALL);
//充电状态已经切换
synchronized (mStats) {
mStats.setBatteryStateLocked(status, health, plugType, level, temp, volt,
chargeUAh);
}
}
});
}
setBatteryStateLocked
public void setBatteryStateLocked(int status, int health, int plugType, int level,
int temp, int volt, int chargeUAh) {
final boolean onBattery = plugType == BATTERY_PLUGGED_NONE;
final long uptime = mClocks.uptimeMillis();//非休眠时长
final long elapsedRealtime = mClocks.elapsedRealtime();//启动时长
if (!mHaveBatteryLevel) {
//mHaveBatteryLevel默认为false，表示首次得到电池信息
mHaveBatteryLevel = true;
// We start out assuming that the device is plugged in (not
// on battery). If our first report is now that we are indeed
// plugged in, then twiddle our state to correctly reflect that
// since we won't be going through the full setOnBattery().
if (onBattery == mOnBattery) {//充电状态没有切换
if (onBattery) {//电池供电
mHistoryCur.states &= ~HistoryItem.STATE_BATTERY_PLUGGED_FLAG;
} else {//充电中
mHistoryCur.states |= HistoryItem.STATE_BATTERY_PLUGGED_FLAG;
}
}
// Always start out assuming charging, that will be updated later.
mHistoryCur.states2 |= HistoryItem.STATE2_CHARGING_FLAG;
mHistoryCur.batteryStatus = (byte)status;
mHistoryCur.batteryLevel = (byte)level;
mHistoryCur.batteryChargeUAh = chargeUAh;
mMaxChargeStepLevel = mMinDischargeStepLevel =
mLastChargeStepLevel = mLastDischargeStepLevel = level;
mLastChargingStateLevel = level;
} else if (mCurrentBatteryLevel != level || mOnBattery != onBattery) {
//电量发生变化或者充电状态切换
//记录日常电池信息
recordDailyStatsIfNeededLocked(level >= 100 && onBattery);
}
int oldStatus = mHistoryCur.batteryStatus;
//下面记录一条历史信息
if (onBattery) {
mDischargeCurrentLevel = level;
if (!mRecordingHistory) {
mRecordingHistory = true;
startRecordingHistory(elapsedRealtime, uptime, true);
}
} else if (level < 96) {
if (!mRecordingHistory) {
mRecordingHistory = true;
startRecordingHistory(elapsedRealtime, uptime, true);
}
}
mCurrentBatteryLevel = level;
if (mDischargePlugLevel < 0) {
mDischargePlugLevel = level;
}
if (onBattery != mOnBattery) {//如果状态切换
mHistoryCur.batteryLevel = (byte)level;
mHistoryCur.batteryStatus = (byte)status;
mHistoryCur.batteryHealth = (byte)health;
mHistoryCur.batteryPlugType = (byte)plugType;
mHistoryCur.batteryTemperature = (short)temp;
mHistoryCur.batteryVoltage = (char)volt;
//电池的实时电量，当变小时，说明电池在放电
//这里就是统计放电
if (chargeUAh < mHistoryCur.batteryChargeUAh) {
// Only record discharges
final long chargeDiff = mHistoryCur.batteryChargeUAh - chargeUAh;
mDischargeCounter.addCountLocked(chargeDiff);
mDischargeScreenOffCounter.addCountLocked(chargeDiff);
}
//更新电量
mHistoryCur.batteryChargeUAh = chargeUAh;
//设置电池信息
setOnBatteryLocked(elapsedRealtime, uptime, onBattery, oldStatus, level, chargeUAh);
} else {//状态切换
//记录变化信息
boolean changed = false;
//略
if (changed) {
//添加记录
//如果在电量统计的间隔中，休眠时间超过20mS，则记录一个tmp信息
//记录一个实时信息
//暂时还不知道tmp信息有什么用，至少知道它标志为非CPU活动，没有wakelock。
addHistoryRecordLocked(elapsedRealtime, uptime);
}
}
}
先熟悉到这里吧，具体到AMS/PMS中再回来分析各种相关函数。