ActivityManagerService（以后简称AMS）都有所耳闻。AMS 是 Android 中最核心的服务，主要负责系统中四大组件的启动、切换、调度及应用进程的管理和调度等工作，其职责与操作系统中的进程管理和调度模块相类似，因此它在 Android 中非常重要。先来看AMS的家族图谱
与 PMS 启动 类似，AMS 服务也是在 系统引导服务中启动。
第一坨代码是创建并启动了 AMS ，第二坨是将 AMS 服务添加到SystemServer 中。
public final class SystemServer { private SystemServiceManager mSystemServiceManager; private void startBootstrapServices() { ...... mActivityManagerService = mSystemServiceManager.startService( ActivityManagerService.Lifecycle.class).getService(); mActivityManagerService.setSystemServiceManager(mSystemServiceManager); mActivityManagerService.setInstaller(installer); ...... mActivityManagerService.setSystemProcess(); ...... } } public class ActivityManagerService extends IActivityManager.Stub implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback { public void setSystemProcess() { ...... ServiceManager.addService(Context.ACTIVITY_SERVICE, this, /* allowIsolated= */ true, DUMP_FLAG_PRIORITY_CRITICAL | DUMP_FLAG_PRIORITY_NORMAL | DUMP_FLAG_PROTO); ...... } } public class SystemServiceManager { public SystemService startService(String className) { final Class<SystemService> serviceClass; try { //通过名字找到相应的 class serviceClass = (Class<SystemService>)Class.forName(className); } catch (ClassNotFoundException ex) { Slog.i(TAG, "Starting " + className); throw new RuntimeException("Failed to create service " + className + ": service class not found, usually indicates that the caller should " + "have called PackageManager.hasSystemFeature() to check whether the " + "feature is available on this device before trying to start the " + "services that implement it", ex); } return startService(serviceClass); } public <T extends SystemService> T startService(Class<T> 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 { //创建相应的 class Constructor<T> constructor = serviceClass.getConstructor(Context.class); 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); } 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 { //启动服务 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"); } }Activity 启动我们在熟悉不过了，我们就以 startActivity 为入口来梳理下。
前面没有什么过多代码，顺着走下去发现调用了 AMS 的 startActivity 方法。但是呢，AMS 是运行在系统进程中的，这里就用到了 Binder 进行 IPC 通信。
startActivity(new Intent(this,NewActivity.class)); public class Activity extends ContextThemeWrapper{ private Instrumentation mInstrumentation; public void startActivityForResult(@RequiresPermission Intent intent, int requestCode, @Nullable Bundle options) { ...... Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity( this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode, options); ...... } } public class Instrumentation { public ActivityResult execStartActivity( Context who, IBinder contextThread, IBinder token, Activity target, Intent intent, int requestCode, Bundle options) { ...... int result = ActivityManager.getService() .startActivity(whoThread, who.getBasePackageName(), intent, intent.resolveTypeIfNeeded(who.getContentResolver()), token, target != null ? target.mEmbeddedID : null, requestCode, 0, null, options); ...... } } public class ActivityManager { public static IActivityManager getService() { return IActivityManagerSingleton.get(); } private static final Singleton<IActivityManager> IActivityManagerSingleton = new Singleton<IActivityManager>() { @Override protected IActivityManager create() { final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE); final IActivityManager am = IActivityManager.Stub.asInterface(b); return am; } }; }上面只贴了怎么从用户进程通过 Binder 调用系统进程的相关服务。系统进程服务处理完相应工作后，也通过 Binder 调用用户进程的启动服务，下面在贴出总体的调用链，大伙可以自己在跟一下后续的调用链。
