前一段时间分析的一个小过程，把它记下来：
我们都知道android电量的一些功能很多在service中，低电量也是如此，在BatteryService.java中我们可以从jni层（当然jni层的电量也是从下层kernel接收而来，这里就不做过多分析，有兴趣的可以去深入了解一下）获得机器的电量mBatteryLevel,我们也知道当手机电量低的时候它会有警告有些还发出声音提醒你，并且太低时会自动关机！ 下面简单介绍一下其流程：
在BatteryService.java中，当我们从jni获得当前电量之后，可以在 update()中做一个判断:  final boolean sendBatteryLow = !plugged
                && mBatteryStatus != BatteryManager.BATTERY_STATUS_UNKNOWN
                && mBatteryLevel <= mLowBatteryWarningLevel
                && (oldPlugged || mLastBatteryLevel > mLowBatteryWarningLevel);

           
如果当前电量小于警告电量（在config.xml中 15）则弹出电量低提示，或者电量为0（当然这个有误差也可能是5%时就自动关机）时自动关机。
在下面会有判断是否低电量：
if (sendBatteryLow) {
                mSentLowBatteryBroadcast = true;
                statusIntent.setAction(Intent.ACTION_BATTERY_LOW);
                mContext.sendBroadcast(statusIntent);       } 如果低电量的话就发送一个广播出去。 这段代码是电量太低而自动关机： private final void shutdownIfNoPower() {
     
        if (mBatteryLevel == 0 && !isPowered() && ActivityManagerNative.isSystemReady()) {
            Intent intent = new Intent(Intent.ACTION_REQUEST_SHUTDOWN);
            intent.putExtra(Intent.EXTRA_KEY_CONFIRM, false);
            intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            mContext.startActivity(intent);
        }
    }

而在StatusBarPolicy.java中会有接收广播：private BroadcastReceiver mIntentReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            String action = intent.getAction();                。。。。。。。。。
里面会有判断当接受到ACTION_BATTERY_LOW时：
else if (action.equals(Intent.ACTION_BATTERY_LOW)) {
                onBatteryLow(intent);
            } 在StatusBarPolicy.java中会有onBatteryLow(intent)方法来处理： private void onBatteryLow(Intent intent) {
        if (SHOW_LOW_BATTERY_WARNING) {
            if (false) {
                Slog.d(TAG, "mPhoneState=" + mPhoneState
                      + " mLowBatteryDialog=" + mLowBatteryDialog
                      + " mBatteryShowLowOnEndCall=" + mBatteryShowLowOnEndCall);
            }

            if (SHOW_BATTERY_WARNINGS_IN_CALL || mPhoneState == TelephonyManager.CALL_STATE_IDLE) {
                showLowBatteryWarning();
            } else {
                mBatteryShowLowOnEndCall = true;
            }
        }
    } 然后就是弹出低电提醒的Dialog了： private void showLowBatteryWarning() {
        closeLastBatteryView();

        // Show exact battery level.
        CharSequence levelText = mContext.getString(
                    R.string.battery_low_percent_format, mBatteryLevel);

        if (mBatteryLevelTextView != null) {
            mBatteryLevelTextView.setText(levelText);
        } else {
            View v = View.inflate(mContext, R.layout.battery_low, null);
            mBatteryLevelTextView=(TextView)v.findViewById(R.id.level_percent);

            mBatteryLevelTextView.setText(levelText);

            AlertDialog.Builder b = new AlertDialog.Builder(mContext);
                b.setCancelable(true);
                b.setTitle(R.string.battery_low_title);
                b.setView(v);
                b.setIcon(android.R.drawable.ic_dialog_alert);
                b.setPositiveButton(android.R.string.ok, null);

                final Intent intent = new Intent(Intent.ACTION_POWER_USAGE_SUMMARY);
                intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK
                        | Intent.FLAG_ACTIVITY_MULTIPLE_TASK
                        | Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS
                        | Intent.FLAG_ACTIVITY_NO_HISTORY);
                if (intent.resolveActivity(mContext.getPackageManager()) != null) {
                    b.setNegativeButton(R.string.battery_low_why,
                            new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int which) {
                            mContext.startActivity(intent);
                            if (mLowBatteryDialog != null) {
                                mLowBatteryDialog.dismiss();
                            }
                        }
                    });
                }

            AlertDialog d = b.create();
            d.setOnDismissListener(mLowBatteryListener);
            d.getWindow().setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);
            d.show();
            mLowBatteryDialog = d;
        }
        //waring voiced
        final ContentResolver cr = mContext.getContentResolver();
        if (Settings.System.getInt(cr,
                Settings.System.POWER_SOUNDS_ENABLED, 1) == 1)
        {
            final String soundPath = Settings.System.getString(cr,
                Settings.System.LOW_BATTERY_SOUND);
            if (soundPath != null) {
                final Uri soundUri = Uri.parse("file://" + soundPath);
                if (soundUri != null) {
                    final Ringtone sfx = RingtoneManager.getRingtone(mContext, soundUri);
                    if (sfx != null) {
                        sfx.setStreamType(AudioManager.STREAM_SYSTEM);
                        sfx.play();
                    }
                }
            }
        }
    }    //waring voiced这行下面就是低电量提醒的声音.    大概过程就是这样了！