一、系统启动后在SystemServer进程中启动CarServiceHelperService
1、在Android系统之后，系统首先会启动一个名为Zygote的进程，而Zygote进程又会启动SystemServer进程，这里我们先来看SystemServer的main方法。
frameworks/base/services/java/com/android/server/SystemServer.java
public final class SystemServer implements Dumpable {
public static void main(String[] args) {
new SystemServer().run();
}
private void run(){
...代码省略...
// Start services.
try {
t.traceBegin("StartServices");
startBootstrapServices(t);//启动引导服务
startCoreServices(t);//启动核心服务
startOtherServices(t);//启动其他服务
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
t.traceEnd(); // StartServices
}
...代码省略...
}
}
main 方法里启动了 run方法，而在 run 方法中会调用了startOtherServices() 方法。
2、startOtherServices和CarService相关的关键代码如下所示。
public final class SystemServer implements Dumpable {
private static final String CAR_SERVICE_HELPER_SERVICE_CLASS =
"com.android.internal.car.CarServiceHelperService";
private PackageManager mPackageManager;
private void startOtherServices() {
mActivityManagerService.systemReady(() -> {
...代码省略...
if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_AUTOMOTIVE)) {
t.traceBegin("StartCarServiceHelperService");
//如果有车机服务，则开启车机帮助服务
final SystemService cshs = mSystemServiceManager
.startService(CAR_SERVICE_HELPER_SERVICE_CLASS);
if (cshs instanceof Dumpable) {
mDumper.addDumpable((Dumpable) cshs);
}
if (cshs instanceof DevicePolicySafetyChecker) {
dpms.setDevicePolicySafetyChecker((DevicePolicySafetyChecker) cshs);
}
t.traceEnd();
}
...代码省略...
}
}
}
3、SystemServiceManager的startService方法如下所示。
frameworks/base/services/core/java/com/android/server/SystemServiceManager.java
public class SystemServiceManager {
//存储了SystemServiceManager负责启动的各种服务
private final ArrayList<SystemService> mServices = new ArrayList<SystemService>();
public SystemService startService(String className) {
final Class<SystemService> serviceClass = loadClassFromLoader(className,this.getClass().getClassLoader());
return startService(serviceClass);
}
@SuppressWarnings("unchecked")
public <T extends SystemService> T startService(Class<T> serviceClass) {
try {
final String name = serviceClass.getName();
...代码省略...
final T service;
try {
Constructor<T> constructor = serviceClass.getConstructor(Context.class);
service = constructor.newInstance(mContext);
}
...代码省略...
startService(service);
return service;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}
public void startService(@NonNull final SystemService service) {
// Register it.
mServices.add(service);
// Start it.
long time = SystemClock.elapsedRealtime();
try {
service.onStart();
} catch (RuntimeException ex) {
throw new RuntimeException("Failed to start service " + service.getClass().getName()
+ ": onStart threw an exception", ex);
}
warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onStart");
}
}
因为前面我们传入的参数为com.android.internal.car.CarServiceHelperService，所以这里startService方法首先会通过反射创建CarServiceHelperService对象实例，然后将其存储在类型ArrayList的mServices中，紧接着会调用CarServiceHelperService的onStart方法。
4、CarServiceHelperService的onStart方法如下所示。
frameworks/opt/car/services/src/com/android/internal/car/CarServiceHelperService.java
public class CarServiceHelperService extends SystemService
implements Dumpable, DevicePolicySafetyChecker {
@Override
public void onStart() {
EventLog.writeEvent(EventLogTags.CAR_HELPER_START);
IntentFilter filter = new IntentFilter(Intent.ACTION_REBOOT);
filter.addAction(Intent.ACTION_SHUTDOWN);
mContext.registerReceiverForAllUsers(mShutdownEventReceiver, filter, null, null);
mCarWatchdogDaemonHelper.addOnConnectionChangeListener(mConnectionListener);
mCarWatchdogDaemonHelper.connect();
Intent intent = new Intent();
intent.setPackage("com.android.car");
intent.setAction(CAR_SERVICE_INTERFACE);
//通过bindService绑定车机服务CarService
if (!mContext.bindServiceAsUser(intent, mCarServiceConnection, Context.BIND_AUTO_CREATE,
mHandler, UserHandle.SYSTEM)) {
Slogf.wtf(TAG, "cannot start car service");
}
loadNativeLibrary();
}
void loadNativeLibrary() {
System.loadLibrary("car-framework-service-jni");
}
}
packages/services/Car/car-lib/src/com/android/car/internal/common/CommonConstants.java
public final class CommonConstants {
// CarService Constants
public static final String CAR_SERVICE_INTERFACE = "android.car.ICar";
}
CarServiceHelperService的onStart方法首先创建一个Action为android.car.ICar，包名为com.android.car的Intent，然后通过bindService的方式启动该Intent对应的服务，而这个服务正是车机模块才有的CarService服务。
二、CarService启动
1、系统关于CarService服务的声明如下所示。
packages/services/Car/service/AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:androidprv="http://schemas.android.com/apk/prv/res/android"
package="com.android.car"
coreApp="true"
android:sharedUserId="android.uid.system">
<!--...代码省略...-->
<application android:label="@string/app_title"
android:directBootAware="true"
android:allowBackup="false"
android:persistent="true">
<service android:name=".CarService"
android:singleUser="true"
android:exported="true">
<intent-filter>
<action android:name="android.car.ICar"/>
</intent-filter>
</service>
<!--...代码省略...-->
</application>
</manifest>
结合这个配置文件我们可以知道CarServiceHelperService最终所启动的，就是CarService这个服务。
2、CarService的onCreate方法如下所示。
public class CarService extends Service {
private ICarImpl mICarImpl;
@Override
public void onCreate() {
...代码省略...
mICarImpl = new ICarImpl(this,
mVehicle,
SystemInterface.Builder.defaultSystemInterface(this).build(),
mVehicleInterfaceName);
mICarImpl.init();
linkToDeath(mVehicle, mVehicleDeathRecipient);
//将ICarImpl存储到系统服务管理者ServiceManager中
ServiceManager.addService("car_service", mICarImpl);
SystemProperties.set("boot.car_service_created", "1");
super.onCreate();
initTiming.traceEnd(); // "CarService.onCreate"
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
// keep it alive.
return START_STICKY;
}
@Override
public IBinder onBind(Intent intent) {
return mICarImpl;
}
}
CarService的onCreate方法会创建一个关键对象ICarImpl的实例，并将该实例赋值给mICarImpl属性变量，然后会调用该对象的init方法，之后还会将mICarImpl存储到ServiceManager中。另外结合CarService的onBind方法我们可以知道，CarServiceHelperService通过bindService方式开启CarService，CarService会返回mICarImpl对象，通过该Binder对象使二者建立双向跨进程通信。