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SystemServer 启动SensorService
Zygote启动之后，调用SystemServer的main方法（调用run方法）启动系统服务；
代码路径：
./frameworks/base/services/java/com/android/server/SystemServer.java
SystemServer类中提供的run 方法中，在启动service之前，会加载本地动态库System.loadLibrary(“android_servers”)初始化本地 Native service，过程如下：
/*** The main entry point from zygote.*/
public static void main(String[] args) {new SystemServer().run();
}
public final class SystemServer {private static final String TAG = "SystemServer";private void run() {// Initialize native services.System.loadLibrary("android_servers");// Start services.try {Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartServices");startBootstrapServices();startCoreServices();startOtherServices();}
...
}
在 startBootstrapServices(); 的过程的最后 会调用startSensorService(); 如下：
private void startBootstrapServices() {
......// The sensor service needs access to package manager service, app ops// service, and permissions service, therefore we start it after them.startSensorService();}
在SystemServer类定义中，定义了本地方法startSensorService 如下：
public final class SystemServer { private static final String TAG = "SystemServer";//......private static native void startSensorService();//......
}
当SystemServer调用run 方法，会通过JNI调用到本地方法；
JNI 访问native 方法
通过对JNI的了解 System.loadLibrary(“android_servers”); 会去查找libandroid_servers.so 这个库文件；
yujixuan@yujixuan:~/prj/SC20_R06_master_0526/code/frameworks$ grep -rn libandroid_servers ./
./base/services/Android.mk:54:LOCAL_MODULE:= libandroid_servers
通过检索可知，加载该本地库，会调用在 base/services/ 下编译库文件的onload函数；即：
代码路径： ./base/services/core/jni/onload.cpp
extern "C" jint JNI_OnLoad(JavaVM* vm, void* /* reserved */)
{JNIEnv* env = NULL;jint result = -1;if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {ALOGE("GetEnv failed!");return result;}ALOG_ASSERT(env, "Could not retrieve the env!");register_android_server_ActivityManagerService(env);register_android_server_PowerManagerService(env);register_android_server_SerialService(env);register_android_server_InputApplicationHandle(env);register_android_server_InputWindowHandle(env);register_android_server_InputManager(env);register_android_server_LightsService(env);register_android_server_AlarmManagerService(env);register_android_server_UsbDeviceManager(env);register_android_server_UsbMidiDevice(env);register_android_server_UsbHostManager(env);register_android_server_vr_VrManagerService(env);register_android_server_VibratorService(env);register_android_server_SystemServer(env);
//......return JNI_VERSION_1_4;
}
由上可知，在JNI_OnLoad 中会调用添加若干个 server，没有发现sensor相关的server；
实则它是 SystemServer中,下面是register_android_server_SystemServer内容：
代码路径：./base/services/core/jni/com_android_server_SystemServer.cpp
/** JNI registration.*/
static const JNINativeMethod gMethods[] = {/* name, signature, funcPtr */{ "startSensorService", "()V", (void*) android_server_SystemServer_startSensorService },
};int register_android_server_SystemServer(JNIEnv* env)
{return jniRegisterNativeMethods(env, "com/android/server/SystemServer",gMethods, NELEM(gMethods));
}
可以看到 register_android_server_SystemServer 实际上是注册绑定了 sensor相关的 service启动方法（为啥不直接命名为sensor server，目前还不清楚）；
通过method可知，systemServer调用run方法启动 startSensorService，会调用到android_server_SystemServer_startSensorService ； 以下是它的实现：
static void android_server_SystemServer_startSensorService(JNIEnv* /* env */, jobject /* clazz */) {char propBuf[PROPERTY_VALUE_MAX];property_get("system_init.startsensorservice", propBuf, "1");if (strcmp(propBuf, "1") == 0) {// Start the sensor service in a new threadcreateThreadEtc(start_sensor_service, nullptr,"StartSensorThread", PRIORITY_FOREGROUND);}
}
判断属性 “system_init.startsensorservice” 是否存在，如果存在 创建线程"StartSensorThread"（默认被配置了，还找到哪里配置的）
线程内容如下：
static int start_sensor_service(void* /*unused*/) {SensorService::instantiate();return 0;
}
该线程调用SensorService中的instantiate方法，创建了SensorService的实例；
SensorService实现
继续分析SensorService instantiate的创建，首先是看SensorService类的定义：
路径：./frameworks/native/services/sensorservice/SensorService.h
class SensorService :public BinderService<SensorService>,public BnSensorServer,protected Thread
{// nested class/struct for internal useclass SensorEventConnection;public:void cleanupConnection(SensorEventConnection* connection);status_t enable(const sp<SensorEventConnection>& connection, int handle,nsecs_t samplingPeriodNs, nsecs_t maxBatchReportLatencyNs, int reservedFlags,const String16& opPackageName);status_t disable(const sp<SensorEventConnection>& connection, int handle);status_t setEventRate(const sp<SensorEventConnection>& connection, int handle, nsecs_t ns,const String16& opPackageName);status_t flushSensor(const sp<SensorEventConnection>& connection,const String16& opPackageName);
private:friend class BinderService<SensorService>;// nested class/struct for internal useclass SensorRecord;class SensorEventAckReceiver;struct SensorRegistrationInfostatic char const* getServiceName() ANDROID_API { return "sensorservice"; }SensorService() ANDROID_API;virtual ~SensorService();virtual void onFirstRef();// Thread interfacevirtual bool threadLoop();// ISensorServer interfacevirtual Vector<Sensor> getSensorList(const String16& opPackageName);virtual status_t dump(int fd, const Vector<String16>& args);String8 getSensorName(int handle) const;bool isVirtualSensor(int handle) const;sp<SensorInterface> getSensorInterfaceFromHandle(int handle) const;bool isWakeUpSensor(int type) const;void recordLastValueLocked(sensors_event_t const* buffer, size_t count);static void sortEventBuffer(sensors_event_t* buffer, size_t count);const Sensor& registerSensor(SensorInterface* sensor,bool isDebug = false, bool isVirtual = false);const Sensor& registerVirtualSensor(SensorInterface* sensor, bool isDebug = false);
};
删了一些相对无关的方法及属性后，还有如上内容，没有instantiate方法；
通过类的定义可以发现SensorService 继承了BinderService；BnSensorServer，Thread这三个class； instantiate 是被定义在它的父类BinderService中了；
以下是BinderService的实现：
代码路径：./frameworks/native/include/binder/BinderService.hclass BinderService
{
public:static status_t publish(bool allowIsolated = false) {sp<IServiceManager> sm(defaultServiceManager());return sm->addService(String16(SERVICE::getServiceName()),new SERVICE(), allowIsolated);}static void publishAndJoinThreadPool(bool allowIsolated = false) {publish(allowIsolated);joinThreadPool();}static void instantiate() { publish(); }
......
};
publish()中
sp sm(defaultServiceManager());
可知SensorService::instantiate()在这一过程创建了SensorService并通过addService将自己新创建的SensorService服务添加到Android服务列表里了。
SensorService有一条继承关系如下：
SensorService : public BnSensorServer
BnSensorServer : public BnInterface<ISensorServer>
BnInterface : public BBinder
BBinder : public IBinder
IBinder : public virtual RefBase
在Android引用计数系统里，当RefBase的子类对象被第一次强引用时自动调用其onFirstRef方法，所以当第一次使用SensorService，onFirstRef方法将被被自动回调，onFirstRef方法里会创建SensorDevice 用来与HAL层进行交互，后面内容会做分析。
note：Android系统智能指针的设计思路（轻量级指针、强指针、弱指针） 虚继承
void SensorService::onFirstRef() {ALOGD("nuSensorService starting...");SensorDevice& dev(SensorDevice::getInstance());
//......
时序图：
根据以上过程，可绘制绘制出简要的uml时序图，如下：
总结过程：
系统初始化进程加载，启动SystemServer，Zygote中会执行SystemServier main方法，导致其run方法被调用；加载本地库文件， System.loadLibrary("android_servers"); 获取本地方法；被加载到的JNI库文件导致JNI_Onload函数被调用；调用本地jni文件注册本地方法jniRegisterNativeMethods 数组；完成Java 到 C++ 函数绑定，使Java能否访问到C库中的函数；启动startBootstrapServices();最后调用native方法 native void startSensorService();JNI文件com_android_server_SystemServer.cpp，绑定的函数数组，由java的startSensorService方法绑定到android_server_SystemServer_startSensorService函数；C++函数中，start_sensor_service被调用；调用SensorService的inistantiate函数（继承父类BinderService得到的）；调用publish创建一个Serivces,通过sm->addService 来添加到android中去； sm->addService( String16(SERVICE::getServiceName()), new SERVICE(), allowIsolated); 其中sm是ServiceManager的引用：sp sm(defaultServiceManager());
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