电池的信息，电压，温度，充电状态等等，都是由BatteryService来提供的。BatteryService是跑在system_process当中，在系统初始化的时候启动，如下
在SystemServer.java中可以看到启动BatteryService的代码：
Log.i(TAG, "Starting Battery Service."); BatteryService battery = new BatteryService(context); ServiceManager.addService("battery", battery);
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1. 数据来源 BatteryService通过JNI（com_android_server_BatteryService.cpp）读取数据。BatteryService通过JNI注册的不仅有函数，还有变量。 如下：//##############在BatteryService.java中声明的变量################
private boolean mAcOnline; private boolean mUsbOnline; private int mBatteryStatus; private int mBatteryHealth; private boolean mBatteryPresent; private int mBatteryLevel; private int mBatteryVoltage; private int mBatteryTemperature; private String mBatteryTechnology;
//在BatteryService.java中声明的变量，在com_android_server_BatteryService.cpp中共用，即在com_android_server_BatteryService.cpp中其实操作的也是BatteryService.java中声明的变量
gFieldIds.mAcOnline = env->GetFieldID(clazz, "mAcOnline", "Z"); gFieldIds.mUsbOnline = env->GetFieldID(clazz, "mUsbOnline", "Z"); gFieldIds.mBatteryStatus = env->GetFieldID(clazz, "mBatteryStatus", "I"); gFieldIds.mBatteryHealth = env->GetFieldID(clazz, "mBatteryHealth", "I"); gFieldIds.mBatteryPresent = env->GetFieldID(clazz, "mBatteryPresent", "Z"); gFieldIds.mBatteryLevel = env->GetFieldID(clazz, "mBatteryLevel", "I"); gFieldIds.mBatteryTechnology = env->GetFieldID(clazz, "mBatteryTechnology", "Ljava/lang/String;"); gFieldIds.mBatteryVoltage = env->GetFieldID(clazz, "mBatteryVoltage", "I"); gFieldIds.mBatteryTemperature = env->GetFieldID(clazz, "mBatteryTemperature", "I");
//上面这些变量的值，对应是从下面的文件中读取的，一只文件存储一个数值。
#define AC_ONLINE_PATH "/sys/class/power_supply/ac/online" #define USB_ONLINE_PATH "/sys/class/power_supply/usb/online" #define BATTERY_STATUS_PATH "/sys/class/power_supply/battery/status" #define BATTERY_HEALTH_PATH "/sys/class/power_supply/battery/health" #define BATTERY_PRESENT_PATH "/sys/class/power_supply/battery/present" #define BATTERY_CAPACITY_PATH "/sys/class/power_supply/battery/capacity" #define BATTERY_VOLTAGE_PATH "/sys/class/power_supply/battery/batt_vol" #define BATTERY_TEMPERATURE_PATH "/sys/class/power_supply/battery/batt_temp" #define BATTERY_TECHNOLOGY_PATH "/sys/class/power_supply/battery/technology"
Android是运行在Linux内核上面的，/sys/class/power_supply亦是Linux内核下面的目录。至于这些文件时怎么生成的，则是由Platform来控制的。
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2. 数据传送 电池的这些信息是通过何种方式，被其他应用所获得的。可以想到的有两种方式，第一种，应用主动从BatteryService获得数据；第二种，BatteryService主动把数据传送给所关心的应用程序。BatteryService采用的是第二种方式，所有的电池的信息数据是通过Intent传送出去的。在BatteryService.java中，Code如下：
Intent intent = new Intent(Intent.ACTION_BATTERY_CHANGED); intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY); intent.putExtra("status", mBatteryStatus); intent.putExtra("health", mBatteryHealth); intent.putExtra("present", mBatteryPresent); intent.putExtra("level", mBatteryLevel); intent.putExtra("scale", BATTERY_SCALE); intent.putExtra("icon-small", icon); intent.putExtra("plugged", mPlugType); intent.putExtra("voltage", mBatteryVoltage); intent.putExtra("temperature", mBatteryTemperature); intent.putExtra("technology", mBatteryTechnology); ActivityManagerNative.broadcastStickyIntent(intent, null); -----------------------------------------------------------------------------------------------------------------------------------
3. 数据接收 应用如果想要接收到BatteryService发送出来的电池信息，则需要注册一个Intent为Intent.ACTION_BATTERY_CHANGED的BroadcastReceiver。注册方法如下：
IntentFilter mIntentFilter = new IntentFilter(); mIntentFilter.addAction(Intent.ACTION_BATTERY_CHANGED); registerReceiver(mIntentReceiver, mIntentFilter); private BroadcastReceiver mIntentReceiver = new BroadcastReceiver() { @Override public void onReceive(Context context, Intent intent) { // TODO Auto-generated method stub String action = intent.getAction(); if (action.equals(Intent.ACTION_BATTERY_CHANGED)) { int nVoltage = intent.getIntExtra("voltage", 0); if(nVoltage!=0){ mVoltage.setText("V: " + nVoltage + "mV - Success..."); } else{ mVoltage.setText("V: " + nVoltage + "mV - fail..."); } } } }; -----------------------------------------------------------------------------------------------------------------------------------
4. 数据更新 电池的信息会随着时间不停变化，自然地，就需要考虑如何实时的更新电池的数据信息。在BatteryService启动的时候，会同时通过UEventObserver启动一个onUEvent Thread。每一个Process最多只能有一个onUEvent Thread，即使这个Process中有多个UEventObserver的实例。当在一个Process中，第一次Call startObserving()方法后，这个UEvent thread就启动了。
而一旦这个UEvent thread启动之后，就不会停止。 //在BatteryService.java中 mUEventObserver.startObserving("SUBSYSTEM=power_supply"); private UEventObserver mUEventObserver = new UEventObserver() { @Override public void onUEvent(UEventObserver.UEvent event) { update(); } }; 在UEvent thread中会不停调用 update()方法，来更新电池的信息数据。
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