AMS的启动过程

基于Android U

AMS的启动是在SystemServer进程中启动的。从SystemServer的main()方法开始。

frameworks/base/services/java/com/android/server/SystemServer.java
public static void main(String[] args) {
    new SystemServer().run();
}

调用了run()方法。

frameworks/base/services/java/com/android/server/SystemServer.java
private void run() {
    ...
    try {
        ...
        // 创建消息Looper
        Looper.prepareMainLooper();
        ...
        // Initialize native services.
        System.loadLibrary("android_servers");  // 1
        ...
        // Initialize the system context.
        // 创建系统的Context
        createSystemContext();
        ...
        // Create the system service manager.
        mSystemServiceManager = new SystemServiceManager(mSystemContext);  // 2
        mSystemServiceManager.setStartInfo(mRuntimeRestart,
                mRuntimeStartElapsedTime, mRuntimeStartUptime);
        mDumper.addDumpable(mSystemServiceManager);

        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
        ...
    } finally {
        ...
    }
    ...
    // Start services.
    try {
        t.traceBegin("StartServices");
        // 启动引导服务
        startBootstrapServices(t);  //3
        // 启动核心服务
        startCoreServices(t);  // 4
        // 启动其他服务
        startOtherServices(t);  // 5
        // 启动apex服务
        startApexServices(t);  // 6
        // Only update the timeout after starting all the services so that we use
        // the default timeout to start system server.
        updateWatchdogTimeout(t);
    } catch (Throwable ex) {
        ...
    } finally {
        ...
    }
    ...
    // Loop forever.
    Looper.loop();
    throw new RuntimeException("Main thread loop unexpectedly exited");
}

注释1处加载了动态库libandroid_servers.so。

注释2处创建SystemServiceManager，它会对系统的服务进行创建、启动和生命周期管理。

注释3处的startBootstrapServices()方法中用SystemServiceManager启动了ActivityManagerService、PowerManagerService、PackageManagerService等服务。

注释4处的startCoreServices()方法中则启动了BatteryService、UsageStatsService、WebViewUpdateService等服务。

注释5处的startOtherServices()方法中启动了AlarmManagerService、VrManagerService、InputManagerService等服务，主要是一些和设备功能相关的服务，是一些非紧要和不需要立即启动的服务。

注释6处的startApexServices()方法会遍历所有已安装的Apex服务，并调用它们的启动方法，使它们在系统启动时自动运行。

frameworks/base/services/java/com/android/server/SystemServer.java
private void startBootstrapServices(@NonNull TimingsTraceAndSlog t) {
    ...
    // Activity manager runs the show.
    t.traceBegin("StartActivityManager");
    // TODO: Might need to move after migration to WM.
    ActivityTaskManagerService atm = mSystemServiceManager.startService(
            ActivityTaskManagerService.Lifecycle.class).getService();
    mActivityManagerService = ActivityManagerService.Lifecycle.startService(
            mSystemServiceManager, atm);  // 1
    mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
    mActivityManagerService.setInstaller(installer);
    mWindowManagerGlobalLock = atm.getGlobalLock();
    t.traceEnd();
    ...
}

调用ActivityManagerService#Lifecycle.startService()。

frameworks/base/services/core/java/com/android/server/am/ActivityManagerService$Lifecycle
public static final class Lifecycle extends SystemService {
    private final ActivityManagerService mService;
    private static ActivityTaskManagerService sAtm;

    public Lifecycle(Context context) {
        super(context);
        mService = new ActivityManagerService(context, sAtm);
    }
    
    public static ActivityManagerService startService(
            SystemServiceManager ssm, ActivityTaskManagerService atm) {
        sAtm = atm;
        return ssm.startService(ActivityManagerService.Lifecycle.class).getService();
    }
}

调用SystemServiceManager的startService()方法。

frameworks/base/services/core/java/com/android/server/SystemServiceManager.java
public <T extends SystemService> T startService(Class<T> serviceClass) {
    try {
        ...
        final T service;
        try {
            Constructor<T> constructor = serviceClass.getConstructor(Context.class);
            service = constructor.newInstance(mContext);  // 1
        } catch (InstantiationException ex) {
            ...
        } catch (IllegalAccessException ex) {
            ...
        } catch (NoSuchMethodException ex) {
            ...
        } catch (InvocationTargetException ex) {
            ...
        }

        startService(service);  // 2
        return service;
    } finally {
        ...
    }
}

public void startService(@NonNull final SystemService service) {
    ...
    // Register it.
    mServices.add(service);  // 3
    ...
    try {
        service.onStart();  // 4
    } catch (RuntimeException ex) {
        ...
    }
    ...
}

注释1处SystemServiceManager中通过反射，调用了ActivityManagerService$Lifecycle的构造方法。

注释2处又调用了startService()。

注释3处将service对象添加到ArrayList类型的mServices中来完成注册。

注释4处调用service.onStart()来启动service对象，即ActivityManagerService$Lifecycle.onStart()。

frameworks/base/services/core/java/com/android/server/am/ActivityManagerService$Lifecycle
public static final class Lifecycle extends SystemService {
    private final ActivityManagerService mService;

    public Lifecycle(Context context) {
        super(context);
        mService = new ActivityManagerService(context, sAtm);  // 1
    }
    
    public void onStart() {
        mService.start();  // 2
    }
    
    public ActivityManagerService getService() {
        return mService;  // 3
    }
}

上面的代码需要结合SystemServiceManager的startService()方法来分析。

注释1处，在Lifecycle的构造方法中创建了AMS实例。当调用SystemService类型的service的onStart()方法时，实际上是调用了注释2处AMS的start()方法。注释3处的Lifecycle的getService()方法返回AMS实例，这样我们就知道SystemServer的startBootstrapServices()方法的注释1处实际得到的就是AMS实例。