第一部分 Android Automative的整体架构

image.png

从这幅图中我们可以看出，Android Automative是在原先Android的系统架构上增加了一些与车相关的（图中虚线框中绿色背景的）模块。

包括：

Car App：包括OEM和第三方开发的App
Car API：提供给汽车App特有的接口
Car Service：系统中与车相关的服务
Vehicle Network Service：汽车的网络服务
Vehicle HAL：汽车的硬件抽象层描述

下面我们采取从上到下的顺序对主要模块做一些介绍。

Car App

packages/services/Car/car_product/build/car.mk 这个文件中列出了汽车系统中专有的模块：

BOARD_PLAT_PUBLIC_SEPOLICY_DIR += packages/services/Car/car_product/sepolicy/public   
BOARD_PLAT_PRIVATE_SEPOLICY_DIR += packages/services/Car/car_product/sepolicy/private

# Automotive specific packages
PRODUCT_PACKAGES += \
    CarService \
    CarTrustAgentService \
    CarDialerApp \
    CarRadioApp \
    OverviewApp \
    CarLauncher \
    CarLensPickerApp \
    LocalMediaPlayer \
    CarMediaApp \
    CarMessengerApp \
    CarHvacApp \
    CarMapsPlaceholder \
    CarLatinIME \                                                                                                                                                                                           
    CarSettings \ 
    CarUsbHandler \
    android.car \
    car-frameworks-service \
    com.android.car.procfsinspector \
    libcar-framework-service-jni \
  
# System Server components 
PRODUCT_SYSTEM_SERVER_JARS += car-frameworks-service

# should add to BOOT_JARS only once
ifeq (,$(INCLUDED_ANDROID_CAR_TO_PRODUCT_BOOT_JARS))                                                                                                                                                        
PRODUCT_BOOT_JARS += \
    android.car
INCLUDED_ANDROID_CAR_TO_PRODUCT_BOOT_JARS := yes
endif

这些app源码都位于packages/apps/Car/目录下，OEM厂商可以添加更多的App

Car API

packages/services/Car/car-lib/
开发汽车专有的App自然需要专有的API。这些API对于其他平台（例如手机和平板）通常是没有意义的。所以这些API没有包含在Android Framework SDK中，而是生成在android.car.jar中，源码编译时需要通过在Android.mk中添加LOCAL_JAVA_LIBRARIES += android.car 加入编译，Android Studio需要添加 android.car.jar加入编译，通过INCLUDED_ANDROID_CAR_TO_PRODUCT_BOOT_JARS 加入运行时。
下面这张大图列出了所有的Car API：

car_api.png

从这个图中我们可以看到Car API主要包括：

android.car：包含了与车相关的基本API。例如：车辆后视镜，门，座位，窗口等。
annotation：包含了两个注解。
app
menu：车辆应用菜单相关API。
cluster：仪表盘相关API。
render：渲染相关API。
content
pm：应用包相关API。
diagnostic：包含与汽车诊断相关的API。
hardware：车辆硬件相关API。
cabin：座舱相关API。
hvac：通风空调相关API。（hvac是Heating, ventilation and air conditioning的缩写）
property：属性相关API。
radio：收音机相关API。
input：输入相关API。
media：多媒体相关API。
navigation：导航相关API。
settings：设置相关API。
vms：汽车监测相关API，见下文。

CarService

源码：http://androidxref.com/9.0.0_r3/xref/packages/services/Car/
Android Automative中的Car Service集中在一个App中。可以想象，这个App需要非常高的权限，所以这是一个系统App。其Manifest开头如下：
<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">
android:sharedUserId属性"android.uid.system"使得这个应用具有系统权限
Car Service并非一个服务，而是一系列的服务这些服务都在ICarImpl.java构造函数中列了出来。

public ICarImpl(Context serviceContext, IVehicle vehicle, SystemInterface systemInterface,
            CanBusErrorNotifier errorNotifier, String vehicleInterfaceName) {                                                                                                                               
        mContext = serviceContext;
        mSystemInterface = systemInterface;
        mHal = new VehicleHal(vehicle);
        mVehicleInterfaceName = vehicleInterfaceName;
        mSystemActivityMonitoringService = new SystemActivityMonitoringService(serviceContext);
        mCarPowerManagementService = new CarPowerManagementService(mContext, mHal.getPowerHal(),
                systemInterface);
        mCarPropertyService = new CarPropertyService(serviceContext, mHal.getPropertyHal());
        mCarDrivingStateService = new CarDrivingStateService(serviceContext, mCarPropertyService);
        mCarUXRestrictionsService = new CarUxRestrictionsManagerService(serviceContext,
                mCarDrivingStateService, mCarPropertyService);
        mCarPackageManagerService = new CarPackageManagerService(serviceContext,
                mCarUXRestrictionsService,
                mSystemActivityMonitoringService);
        mCarInputService = new CarInputService(serviceContext, mHal.getInputHal());
        mCarProjectionService = new CarProjectionService(serviceContext, mCarInputService);
        mGarageModeService = new GarageModeService(mContext, mCarPowerManagementService);
        mCarLocationService = new CarLocationService(mContext, mCarPowerManagementService,
                mCarPropertyService);
        mAppFocusService = new AppFocusService(serviceContext, mSystemActivityMonitoringService);
        mCarAudioService = new CarAudioService(serviceContext);
        mCarNightService = new CarNightService(serviceContext, mCarPropertyService);
        mInstrumentClusterService = new InstrumentClusterService(serviceContext,
                mAppFocusService, mCarInputService);
        mSystemStateControllerService = new SystemStateControllerService(serviceContext,
                mCarPowerManagementService, mCarAudioService, this);
        mPerUserCarServiceHelper = new PerUserCarServiceHelper(serviceContext);
        mCarBluetoothService = new CarBluetoothService(serviceContext, mCarPropertyService,
                mPerUserCarServiceHelper, mCarUXRestrictionsService);
        mVmsSubscriberService = new VmsSubscriberService(serviceContext, mHal.getVmsHal());
        mVmsPublisherService = new VmsPublisherService(serviceContext, mHal.getVmsHal());
        mCarDiagnosticService = new CarDiagnosticService(serviceContext, mHal.getDiagnosticHal());
        mCarStorageMonitoringService = new CarStorageMonitoringService(serviceContext,
                systemInterface);
        mCarConfigurationService =
                new CarConfigurationService(serviceContext, new JsonReaderImpl());
        mUserManagerHelper = new CarUserManagerHelper(serviceContext);

CarTool

packages/services/Car/tools
略

EVS

packages/services/Car/evs/
略

ODB2

packages/services/Car/obd2-lib/
略

Vehicle Network Service

略

HAL

略

第二部分 CarService启动流程

SystemServer启动CarService

SystemServer里会启动CarServiceHelperService，如下：

            if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_AUTOMOTIVE)) {                
            traceBeginAndSlog("StartCarServiceHelperService");
                mSystemServiceManager.startService(CarServiceHelperService.class);
                traceEnd();
            }

而CarServiceHelperService.onStart()会bind到CarService，如下：

        Intent intent = new Intent();
        intent.setPackage("com.android.car");
        intent.setAction(CAR_SERVICE_INTERFACE);
        if (!getContext().bindServiceAsUser(intent, mCarServiceConnection, Context.BIND_AUTO_CREATE,
                UserHandle.SYSTEM)) { 
            Slog.wtf(TAG, "cannot start car service");
        }

注意：这里用的是bindService方式，mCarServiceConnection是ServiceConnection类对象，它能监听目标Service的状态，当目标Service所在进程异常退出时，会导致其onServiceDisconnected()方法被调用

CarService启动其他service

CarService.onCreate()里会创建ICarImpl的实例，并调用ICarImpl.init()方法；

        Log.i(CarLog.TAG_SERVICE, "Service onCreate"); 
        mCanBusErrorNotifier = new CanBusErrorNotifier(this /* context */);
        mVehicle = getVehicle();
 
        if (mVehicle == null) { 
            throw new IllegalStateException("Vehicle HAL service is not available.");
        } 
        try { 
            mVehicleInterfaceName = mVehicle.interfaceDescriptor();
        } catch (RemoteException e) { 
            throw new IllegalStateException("Unable to get Vehicle HAL interface descriptor", e);
        } 
 
        Log.i(CarLog.TAG_SERVICE, "Connected to " + mVehicleInterfaceName);
 
        mICarImpl = new ICarImpl(this, mVehicle, SystemInterface.getDefault(this),
                mCanBusErrorNotifier);
        mICarImpl.init();
        SystemProperties.set("boot.car_service_created", "1");
 
        linkToDeath(mVehicle, mVehicleDeathRecipient);
 
        super.onCreate();

而ICarImpl.init()里会启动一大堆Service，其中就包括InstrumentClusterService，如下

        // Be careful with order. Service depending on other service should be inited later. 
        List<CarServiceBase> allServices = new ArrayList<>(Arrays.asList(
                mSystemActivityMonitoringService,
                mCarPowerManagementService,
                mCarSensorService,
                mCarPackageManagerService,
                mCarInputService,
                mGarageModeService,
                mCarInfoService,
                mAppFocusService,
                mCarAudioService,
                mCarCabinService,
                mCarHvacService,
                mCarRadioService,
                mCarNightService,
                mInstrumentClusterService,
                mCarProjectionService,
                mSystemStateControllerService,
                mCarVendorExtensionService,
                mCarBluetoothService,
                mCarDiagnosticService,
                mPerUserCarServiceHelper
        ));

注意服务启动顺序，依赖于其他服务的service靠后初始化。

Car App

通过Car.createCar 获得car，再通过car.getCarManager获得service
例如，CarAudioManager carAudioManager = car.getCarManager(Car.AUDIO_SERVICE);
app通过各种CarXXManager获取服务然后注册Listener或者调用接口。

system.img

android.car编译并加入PRODUCT_BOOT_JARS,存储路径/system/framework/android.car.jar
CarService编译后路径为 /system/priv-app/CarService
其他app编译后路径,例如/system/priv-app/CarLauncher，/system/priv-app/CarMediaApp

参考:

https://yq.aliyun.com/articles/610105?utm_content=m_1000006763
https://www.jianshu.com/p/dacc21e2b32e
https://www.jianshu.com/p/2c3659800fbc makefile PRODUCT_BOOT_JARS 处理流程及实例