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。
java
class ActivityManagerService{
public void systemReady(final Runnable goingCallback, @NonNull TimingsTraceAndSlog t) {
...
mAtmInternal.resumeTopActivities(false /* scheduleIdle */);
...
}
}
class ActivityTaskManagerService{
@Override
public 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