AMS被SystemServer启动之后，SystemServer会通过AMS调用startHomeActivity启动Launcher. Launcher其实就是个Activity，学习Launcher的启动后，再去看Activity启动，会容易很多。

Launcher的启动分2个阶段：

第一阶段是startHomeActivity，触发activity的进程创建；

第二阶段就是AMS的attachApplication在进程创建后被调用，最终执行realStartActivityLocked会回调Activity的onCreate函数。

class ActivityManagerService{public void systemReady(final Runnable goingCallback, @NonNull TimingsTraceAndSlog t) {...mAtmInternal.resumeTopActivities(false /* scheduleIdle */);...}    
}class ActivityTaskManagerService{@Overridepublic void resumeTopActivities(boolean scheduleIdle) {synchronized (mGlobalLock) {mRootWindowContainer.resumeFocusedTasksTopActivities();if (scheduleIdle) {mTaskSupervisor.scheduleIdle();}}}
}class RootWindowContainer{boolean resumeFocusedTasksTopActivities(Task targetRootTask, ActivityRecord target, ActivityOptions targetOptions,boolean deferPause) {for (int displayNdx = getChildCount() - 1; displayNdx >= 0; --displayNdx) {...result |= resumedOnDisplay[0];if (!resumedOnDisplay[0]) {if (focusedRoot != null) {result |= focusedRoot.resumeTopActivityUncheckedLocked(target, targetOptions);} else if (targetRootTask == null) {result |= resumeHomeActivity(null /* prev */, "no-focusable-task",display.getDefaultTaskDisplayArea());}}}}boolean startHomeOnTaskDisplayArea(int userId, String reason, TaskDisplayArea taskDisplayArea, boolean allowInstrumenting, boolean fromHomeKey) {Intent homeIntent = null;ActivityInfo aInfo = null;if (taskDisplayArea == getDefaultTaskDisplayArea()) {homeIntent = mService.getHomeIntent();aInfo = resolveHomeActivity(userId, homeIntent);} else if (shouldPlaceSecondaryHomeOnDisplayArea(taskDisplayArea)) {Pair<ActivityInfo, Intent> info = resolveSecondaryHomeActivity(userId, taskDisplayArea);aInfo = info.first;homeIntent = info.second;}if (aInfo == null || homeIntent == null) {return false;}if (!canStartHomeOnDisplayArea(aInfo, taskDisplayArea, allowInstrumenting)) {return false;}// Updates the home component of the intent.homeIntent.setComponent(new ComponentName(aInfo.applicationInfo.packageName, aInfo.name));homeIntent.setFlags(homeIntent.getFlags() | FLAG_ACTIVITY_NEW_TASK);// Updates the extra information of the intent.if (fromHomeKey) {homeIntent.putExtra(WindowManagerPolicy.EXTRA_FROM_HOME_KEY, true);if (mWindowManager.getRecentsAnimationController() != null) {mWindowManager.getRecentsAnimationController().cancelAnimationForHomeStart();}}homeIntent.putExtra(WindowManagerPolicy.EXTRA_START_REASON, reason);// Update the reason for ANR debugging to verify if the user activity is the one that// actually launched.final String myReason = reason + ":" + userId + ":" + UserHandle.getUserId(aInfo.applicationInfo.uid) + ":" + taskDisplayArea.getDisplayId();mService.getActivityStartController().startHomeActivity(homeIntent, aInfo, myReason, taskDisplayArea);return true;}
}

在线源码：

1 Launcher

2 ActivityManagerService