上一篇http://www.2cto.com/kf/201203/121809.html中我们提到了jni，若大家对android中jni是怎么运作的，可以先参考我之前关于android jni的一个专栏：
http://blog.csdn.net/column/details/jnijni.html
这里有讲到jni和java是怎么互调的，好，这里我们来介绍下battery service在android中是如何工作的，我们先理清楚android framework层总的来说是做什么呢？
其实就是对底层进行封装成API，然后使用SDK开发的人员可以根据这些API做应用层的开发，所以说，我们这边的目的就是，让用户知道什么时候电池信息发生了变化，什么信息发生了变化，还有就是实时可以让用户查询到电池的信息。
1. 用户知道什么时候电池信息发生了变化
2. 什么信息发生了变化
3. 实时可以让用户查询到电池的信息
下面就先针对这3个问题分析下代码
这部分之前有提到，我们driver层有注册uevent，当电池信息发生变化的时候会向用户层发送一个uevent，OK，我们来找用户在哪边去抓这个uevent的，找啊找原来在hal层：
hardware/libhardware_legacy/uevent/uevent.c
[cpp] view plaincopyprint?int uevent_init()
{
struct sockaddr_nl addr;
int sz = 64*1024;
int s;
memset(&addr, 0, sizeof(addr));
addr.nl_family = AF_NETLINK;
addr.nl_pid = getpid();
addr.nl_groups = 0xffffffff;
<span style="color:#ff0000;">s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT);</span>
if(s < 0)
return 0;
setsockopt(s, SOL_SOCKET, SO_RCVBUFFORCE, &sz, sizeof(sz));
if(bind(s, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
close(s);
return 0;
}
fd = s;
return (fd > 0);
}
int uevent_init()
{
struct sockaddr_nl addr;
int sz = 64*1024;
int s;
memset(&addr, 0, sizeof(addr));
addr.nl_family = AF_NETLINK;
addr.nl_pid = getpid();
addr.nl_groups = 0xffffffff;
<span style="color:#ff0000;">s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT);</span>
if(s < 0)
return 0;
setsockopt(s, SOL_SOCKET, SO_RCVBUFFORCE, &sz, sizeof(sz));
if(bind(s, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
close(s);
return 0;
}
fd = s;
return (fd > 0);
}
也就是说在android启动的时候这里的uevent_init也会被调用到，在这里最关心的就是上面红色部分的代码，这里注册了一个socket来抓取kobject_uevent，判断环境变量是否发生变化，这里uevent被frameworks的封装在jni中：
frameworks/base/core/jni/android_os_UEventObserver.cpp
[cpp] view plaincopyprint?static void
android_os_UEventObserver_native_setup(JNIEnv *env, jclass clazz)
{
if (!uevent_init()) {
jniThrowException(env, "java/lang/RuntimeException",
"Unable to open socket for UEventObserver");
}
}
static int
android_os_UEventObserver_next_event(JNIEnv *env, jclass clazz, jbyteArray jbuffer)
{
int buf_sz = env->GetArrayLength(jbuffer);
char *buffer = (char*)env->GetByteArrayElements(jbuffer, NULL);
int length = uevent_next_event(buffer, buf_sz - 1);
env->ReleaseByteArrayElements(jbuffer, (jbyte*)buffer, 0);
return length;
}
static JNINativeMethod gMethods[] = {
{"native_setup", "()V",(void *)android_os_UEventObserver_native_setup},
{"next_event","([B)I", (void *)android_os_UEventObserver_next_event},
};
static void
android_os_UEventObserver_native_setup(JNIEnv *env, jclass clazz)
{
if (!uevent_init()) {
jniThrowException(env, "java/lang/RuntimeException",
"Unable to open socket for UEventObserver");
}
}
static int
android_os_UEventObserver_next_event(JNIEnv *env, jclass clazz, jbyteArray jbuffer)
{
int buf_sz = env->GetArrayLength(jbuffer);
char *buffer = (char*)env->GetByteArrayElements(jbuffer, NULL);
int length = uevent_next_event(buffer, buf_sz - 1);
env->ReleaseByteArrayElements(jbuffer, (jbyte*)buffer, 0);
return length;
}
static JNINativeMethod gMethods[] = {
{"native_setup", "()V",(void *)android_os_UEventObserver_native_setup},
{"next_event","([B)I", (void *)android_os_UEventObserver_next_event},
};
这里提供了2个方法，一个是setup，在setup中调用了刚才hal中的init函数，还有就是一个next_event函数，这个函数会去遍历所有的uevent事件是否发生了变化。
这边顺带讲一下android framework中对uevent的java代码的封装，因为在我们的battery service中会用到。
frameworks/base/core/java/android/os/UEventObserver.java
[cpp] view plaincopyprint?private static native void native_setup();
private static native int next_event(byte[] buffer);
private static native void native_setup();
private static native int next_event(byte[] buffer);
在这里先是注册了刚才jni中的2个方法，这里首先开启了一个线程：run方法如下：
[cpp] view plaincopyprint?public void run() {
native_setup();
byte[] buffer = new byte[1024];
int len;
while (true) {
len = next_event(buffer);
if (len > 0) {
String bufferStr = new String(buffer, 0, len);// easier to search a String
synchronized (mObservers) {
for (int i = 0; i < mObservers.size(); i += 2) {
if (bufferStr.indexOf((String)mObservers.get(i)) != -1) {
((UEventObserver)mObservers.get(i+1))
.onUEvent(new UEvent(bufferStr));
}
}
}
}
}
}
public void run() {
native_setup();
byte[] buffer = new byte[1024];
int len;
while (true) {
len = next_event(buffer);
if (len > 0) {
String bufferStr = new String(buffer, 0, len);// easier to search a String
synchronized (mObservers) {
for (int i = 0; i < mObservers.size(); i += 2) {
if (bufferStr.indexOf((String)mObservers.get(i)) != -1) {
((UEventObserver)mObservers.get(i+1))
.onUEvent(new UEvent(bufferStr));
}
}
}
}
}
}
这里做的也就是先初始化uevent就是注册socket监听，然后轮询uevent 的环境变量，最后定义了一个抽象类，让用到的程序复写该类达到自己的目的。
[cpp] view plaincopyprint?/**
* Subclasses of UEventObserver should override this method to handle
* UEvents.
*/
public abstract void onUEvent(UEvent event);
/**
* Subclasses of UEventObserver should override this method to handle
* UEvents.
*/
public abstract void onUEvent(UEvent event);
下面我们来看battery service中的代码：
首先是注册UEvent，开启线程，并设置要监听的环境变量，
[java] view plaincopyprint?public BatteryService(Context context) {
mContext = context;
mBatteryStats = BatteryStatsService.getService();
mLowBatteryWarningLevel = mContext.getResources().getInteger(
com.android.internal.R.integer.config_lowBatteryWarningLevel);
mLowBatteryCloseWarningLevel = mContext.getResources().getInteger(
com.android.internal.R.integer.config_lowBatteryCloseWarningLevel);
span style="color:#ff0000;">mUEventObserver.startObserving("SUBSYSTEM=power_supply");</span>
// set initial status
update();
}
public BatteryService(Context context) {
mContext = context;
mBatteryStats = BatteryStatsService.getService();
mLowBatteryWarningLevel = mContext.getResources().getInteger(
com.android.internal.R.integer.config_lowBatteryWarningLevel);
mLowBatteryCloseWarningLevel = mContext.getResources().getInteger(
com.android.internal.R.integer.config_lowBatteryCloseWarningLevel);
<span style="color:#ff0000;">mUEventObserver.startObserving("SUBSYSTEM=power_supply");</span>
// set initial status
update();
}
之后就会去执行update方法，在update方法中就是用到了我们jni中的update方法，之前讲过这个方法就是去更新BatteryService中的battery信息的成员变量：
[java] view plaincopyprint?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;
private boolean mBatteryLevelCritical;
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;
private boolean mBatteryLevelCritical;到这里，就知道了用户如何知道，如何去更新battery信息的数据了。
开发人员也可以注册BatteryService这个系统service来得到BatteryService类，然后直接呼叫成员函数来得到实时的battery的信息。
-------------------------------------------------------------------------------
OK battery 在framework中就介绍到这边。具体的给上层封装了哪些API，哪些ACTION，大家可以仔细查看frameworksbase/services/java/com/android/server/BatteryService.java这个代码。用户要的东西都在这边。
摘自 zhangjie201412的专栏
