Android14 AMS启动流程

本文均采用Android 13代码进行讲解，学习可以使用以下地址：Search

一、AMS启动流程

AMS的启动是在SyetemServer进程中启动的，从SyetemServer的main方法开始进入：

1.SystemServer.java

frameworks/base/services/java/com/android/server/SystemServer.java

main(String[] args)

/**
 * The main entry point from zygote.
 */
public static void main(String[] args) {
    new SystemServer().run();
}

main方法中只调用了SystemServer的run方法，如下所示。

run()

从下面的注释中可以看到，官方把系统服务分为了三种类型，分别是引导服务、核心服务和其他服务，其中其他服务是一些非紧要和一些不需要立即启动的服务。系统服务总共大约有80多个，我们主要来查看引导服务AMS是如何启动的。

private void run() {
      		 // Initialize native services.
         	//1.加载了动态库libandroid_servers.so
           System.loadLibrary("android_servers");
       ......
	         // Create the system service manager.
        	 //2.创建SystemServiceManager，它会对系统的服务进行创建、启动和生命周期管理。
	         mSystemServiceManager = new SystemServiceManager(mSystemContext);
	         mSystemServiceManager.setStartInfo(mRuntimeRestart,
	                 mRuntimeStartElapsedTime, mRuntimeStartUptime);
	         mDumper.addDumpable(mSystemServiceManager);
	     ......   
			    	// Start services.
        try {
           t.traceBegin("StartServices");
           //用SystemServiceManager启动了ActivityManagerService、PowerManagerService、PackageManagerService等服务。
			 		 startBootstrapServices(t);
           //启动了BatteryService、UsageStatsService和WebViewUpdateService。
           startCoreServices(t);
        	 //启动了CameraService、AlarmManagerService、VrManagerService等服务。这些服务的父类均为SystemService。
           startOtherServices(t);
        	 startApexServices(t);
       } catch (Throwable ex) {
           Slog.e("System", "******************************************");
           Slog.e("System", "************ Failure starting system services", ex);
           throw ex;
       } finally {
           t.traceEnd(); // StartServices
       }
       ...
   }

startBootstrapServices(@NonNull TimingsTraceAndSlog t)

调用了ActivityManagerService.Lifecycle.startService方法

private void startBootstrapServices(@NonNull TimingsTraceAndSlog t) {
    t.traceBegin("startBootstrapServices");
  	//尽早启动watchdog，这样我们就可以使系统服务器崩溃
    // 如果我们在早期启动时死锁
    t.traceBegin("StartWatchdog");
    final Watchdog watchdog = Watchdog.getInstance();
    watchdog.start();
    mDumper.addDumpable(watchdog);	
    .....
    // 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();
    //调用了ActivityManagerService.Lifecycle.startService方法,跟进代码看最终调用哪个方法
    mActivityManagerService = ActivityManagerService.Lifecycle.startService(
            mSystemServiceManager, atm);
    mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
    mActivityManagerService.setInstaller(installer);
    mWindowManagerGlobalLock = atm.getGlobalLock();
    t.traceEnd();
......

2.ActivityManagerService.java

/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

Lifecycle.startService(SystemServiceManager ssm,ActivityTaskManagerService atm)

最终调用SystemServiceManager的startService方法，参数就是ActivityManagerService.Lifecycle.class，继续跟进SystemServiceManager，最终调用Lifecycle的getService方法，这个方法会返回AMS类型的mService对象，这样AMS实例就会被创建并且返回。

//Lifecycle继承自SystemService
public static final class Lifecycle extends SystemService {
	  public Lifecycle(Context context) {
	      super(context);
      	//当通过反射来创建Lifecycle实例时，会调用此方法创建AMS实例
	      mService = new ActivityManagerService(context, sAtm);
	  }
    public static ActivityManagerService startService(
            SystemServiceManager ssm, ActivityTaskManagerService atm) {
        sAtm = atm;
      	//调用SystemServiceManager的startService方法，参数就是ActivityManagerService.Lifecycle.class,最终调用Lifecycle的getService方法，这个方法会返回AMS类型的mService对象，这样AMS实例就会被创建并且返回。
        return ssm.startService(ActivityManagerService.Lifecycle.class).getService();
				@Override
		public void onStart(){
				Service.start;
		}
  	...
  	//这个方法会返回AMS类型的mService对象，这样AMS实例就会被创建并且返回。
     public ActivityManagerService getService() {
         return mService;
     }
}

3.SystemServiceManager.java

/frameworks/base/services/core/java/com/android/server/SystemServiceManager.java

startService(Class serviceClass)

首先会获取传进来的Lifecycle的构造器constructor，然后调用constructor的newInstance方法来创建Lifecycle类型的service对象，调用重载的startService，最后返回service

public  T startService(Class serviceClass) {
    try {
        final String name = serviceClass.getName();
        Slog.i(TAG, "Starting " + name);
        Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartService " + name);

        // Create the service.
        if (!SystemService.class.isAssignableFrom(serviceClass)) {
            throw new RuntimeException("Failed to create " + name
                    + ": service must extend " + SystemService.class.getName());
        }
        final T service;
        try {
        	  //1.获取传进来的Lifecycle的构造器constructor
            Constructor constructor = serviceClass.getConstructor(Context.class);
            //调用constructor的newInstance方法来创建Lifecycle类型的service对象
						service = constructor.newInstance(mContext);
        } catch (InstantiationException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service could not be instantiated", ex);
        } catch (IllegalAccessException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service must have a public constructor with a Context argument", ex);
        } catch (NoSuchMethodException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service must have a public constructor with a Context argument", ex);
        } catch (InvocationTargetException ex) {
            throw new RuntimeException("Failed to create service " + name
                    + ": service constructor threw an exception", ex);
        }
      	//调用重载的startService
        startService(service);
      	//返回service
        return service;
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
    }
}

startService(@NonNull final SystemService service)

首先会将上面创建的service添加到ArrayList类型的mServices对象中来完成注册，然后调用service的onStart方法来启动service

public void startService(@NonNull final SystemService service) {
    // Check if already started
    String className = service.getClass().getName();
    if (mServiceClassnames.contains(className)) {
        Slog.i(TAG, "Not starting an already started service " + className);
        return;
    }
    mServiceClassnames.add(className);

    //1.将上面创建的service添加到ArrayList类型的mServices对象中来完成注册
    mServices.add(service);

    // Start it.
    long time = SystemClock.elapsedRealtime();
    try {
      	//2.调用service的onStart方法来启动service
        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");
}

4.流程总结：

总的流程主要涉及SystemServer、ActivityManagerService、SystemServiceManager三个类，SystemServer的run方法会调用去启动引导服务startBootstrapServices，其中AMS也在里面，startBootstrapServices调用了startService方法,跟进代码看最终调用SystemServiceManager里面的startService，中间会有一系列的调用，但最后会通过Lifecycle创建AMS实例，然后通过Lifecycle里面的getService()，这个方法会返回AMS类型的mService对象。