上篇介绍了模拟器的内核goldfish中的battery模块，也介绍了power_supply是如何工作的，一般我自己写驱动也差不多是这个流程，驱动抄着抄着自己就熟悉了，呵呵，记得之前我问过一个前辈，驱动要怎么学习，她说：抄代码先！！！
废话不多扯，这里介绍android jni层是如何处理battery的信息的。
上一篇中讲到在driver中做的事情对我们user有用的就只有2件事情，一是当battery信息发生变化的时候会给用户层发送一个uevent，二是在在文件系统中生成了battery信息的文件，包括电池电量，电池状态灯信息。
这里介绍中间层jni，主要封装读取battery信息的函数，代码位置：android2.3.3/frameworks/base/services/jni/com_android_server_BatteryService.cpp
这里做的事情比较简单：
1. 找到文件系统中我们生成的battery信息的文件路径
2. 找到我们java文件中用到中battery信息相关的类
3. 得到我们java文件中定义的battery信息的变量的fieldID
4. 读取文件系统中相应的battery 信息的文件，通过fieldID给java中的变量赋值，进行battery状态的更新。
1. 找到文件系统中我们生成的battery信息的文件路径
DIR* dir = opendir(POWER_SUPPLY_PATH);
if (dir == NULL) {
LOGE("Could not open %s\n", POWER_SUPPLY_PATH);
return -1;
}
while ((entry = readdir(dir))) {
const char* name = entry->d_name;
// ignore "." and ".."
if (name[0] == '.' && (name[1] == 0 || (name[1] == '.' && name[2] == 0))) {
continue;
}
char buf[20];
// Look for "type" file in each subdirectory
snprintf(path, sizeof(path), "%s/%s/type", POWER_SUPPLY_PATH, name);
int length = readFromFile(path, buf, sizeof(buf));
if (length > 0) {
if (buf[length - 1] == '\n')
buf[length - 1] = 0;
if (strcmp(buf, "Mains") == 0) {
snprintf(path, sizeof(path), "%s/%s/online", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0)
gPaths.acOnlinePath = strdup(path);
}
else if (strcmp(buf, "USB") == 0) {
snprintf(path, sizeof(path), "%s/%s/online", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0)
gPaths.usbOnlinePath = strdup(path);
}
else if (strcmp(buf, "Battery") == 0) {
snprintf(path, sizeof(path), "%s/%s/status", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0)
gPaths.batteryStatusPath = strdup(path);
snprintf(path, sizeof(path), "%s/%s/health", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0)
gPaths.batteryHealthPath = strdup(path);
snprintf(path, sizeof(path), "%s/%s/present", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0)
gPaths.batteryPresentPath = strdup(path);
snprintf(path, sizeof(path), "%s/%s/capacity", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0)
gPaths.batteryCapacityPath = strdup(path);
snprintf(path, sizeof(path), "%s/%s/voltage_now", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0) {
gPaths.batteryVoltagePath = strdup(path);
// voltage_now is in microvolts, not millivolts
gVoltageDivisor = 1000;
} else {
snprintf(path, sizeof(path), "%s/%s/batt_vol", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0)
gPaths.batteryVoltagePath = strdup(path);
}
snprintf(path, sizeof(path), "%s/%s/temp", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0) {
gPaths.batteryTemperaturePath = strdup(path);
} else {
snprintf(path, sizeof(path), "%s/%s/batt_temp", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0)
gPaths.batteryTemperaturePath = strdup(path);
}
snprintf(path, sizeof(path), "%s/%s/technology", POWER_SUPPLY_PATH, name);
if (access(path, R_OK) == 0)
gPaths.batteryTechnologyPath = strdup(path);
}
}
}
closedir(dir);
if (!gPaths.acOnlinePath)
LOGE("acOnlinePath not found");
if (!gPaths.usbOnlinePath)
LOGE("usbOnlinePath not found");
if (!gPaths.batteryStatusPath)
LOGE("batteryStatusPath not found");
if (!gPaths.batteryHealthPath)
LOGE("batteryHealthPath not found");
if (!gPaths.batteryPresentPath)
LOGE("batteryPresentPath not found");
if (!gPaths.batteryCapacityPath)
LOGE("batteryCapacityPath not found");
if (!gPaths.batteryVoltagePath)
LOGE("batteryVoltagePath not found");
if (!gPaths.batteryTemperaturePath)
LOGE("batteryTemperaturePath not found");
if (!gPaths.batteryTechnologyPath)
LOGE("batteryTechnologyPath not found");
2. 找到我们java文件中用到中battery信息相关的类
jclass clazz = env->FindClass("com/android/server/BatteryService");
if (clazz == NULL) {
LOGE("Can't find com/android/server/BatteryService");
return -1;
}
3. 得到我们java文件中定义的battery信息的变量的fieldID
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");
4. 读取文件系统中相应的battery 信息的文件，通过fieldID给java中的变量赋值，进行battery状态的更新。
static void android_server_BatteryService_update(JNIEnv* env, jobject obj)
{
setBooleanField(env, obj, gPaths.acOnlinePath, gFieldIds.mAcOnline);
setBooleanField(env, obj, gPaths.usbOnlinePath, gFieldIds.mUsbOnline);
setBooleanField(env, obj, gPaths.batteryPresentPath, gFieldIds.mBatteryPresent);
setIntField(env, obj, gPaths.batteryCapacityPath, gFieldIds.mBatteryLevel);
setVoltageField(env, obj, gPaths.batteryVoltagePath, gFieldIds.mBatteryVoltage);
setIntField(env, obj, gPaths.batteryTemperaturePath, gFieldIds.mBatteryTemperature);
const int SIZE = 128;
char buf[SIZE];
if (readFromFile(gPaths.batteryStatusPath, buf, SIZE) > 0)
env->SetIntField(obj, gFieldIds.mBatteryStatus, getBatteryStatus(buf));
else
env->SetIntField(obj, gFieldIds.mBatteryStatus,
gConstants.statusUnknown);
if (readFromFile(gPaths.batteryHealthPath, buf, SIZE) > 0)
env->SetIntField(obj, gFieldIds.mBatteryHealth, getBatteryHealth(buf));
if (readFromFile(gPaths.batteryTechnologyPath, buf, SIZE) > 0)
env->SetObjectField(obj, gFieldIds.mBatteryTechnology, env->NewStringUTF(buf));
}
具体代码自行分析，这边我推荐我的一个关于JNI的专栏，那里我写的比较详细，在jni是如何调用java中的成员变量和成员函数的。
http://blog.csdn.net/column/details/jnijni.html
jni层介绍到这边，比较简单。