SystemServer进程是zygote孵化进程fork出的第一个进程。
SystemServer的main函数首先做一些虚拟机相关的初始化工作，然后调用初始化函数，该函数首先创建一个线程，启动各种服务线程，包括：电源管理，ams，wms，震动，蓝牙，网络，定位等服务，其中ams：
ams启动过程：
- public void setSystemProcess()
- public final void installSystemProviders()
- public void systemReady(final Runnable goingCallback)
ams的systemReader最后有：
mStackSupervisor.resumeFocusedStackTopActivityLocked();
由于系统刚启动时，队列中中无任何activity对象，那么将调用ActivityStack的：
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options)
// Find the first activity that is not finishing.
final ActivityRecord next = topRunningActivityLocked();
if (next == null) {
...
// Let's just start up the Launcher...
ActivityOptions.abort(options);
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: No more activities go home");
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
// Only resume home if on home display
return isOnHomeDisplay() &&
mStackSupervisor.resumeHomeStackTask(returnTaskType, prev, reason);
}
boolean resumeHomeStackTask(int homeStackTaskType, ActivityRecord prev, String reason) {
if (!mService.mBooting && !mService.mBooted) {
// Not ready yet!
return false;
}
if (homeStackTaskType == RECENTS_ACTIVITY_TYPE) {
mWindowManager.showRecentApps(false /* fromHome */);
return false;
}
if (prev != null) {
prev.task.setTaskToReturnTo(APPLICATION_ACTIVITY_TYPE);
}
mHomeStack.moveHomeStackTaskToTop(homeStackTaskType);
ActivityRecord r = getHomeActivity();
final String myReason = reason + " resumeHomeStackTask";
// Only resume home activity if isn't finishing.
if (r != null && !r.finishing) {
mService.setFocusedActivityLocked(r, myReason);
return resumeFocusedStackTopActivityLocked(mHomeStack, prev, null);
}
return mService.startHomeActivityLocked(mCurrentUser, myReason);
}
然后，在startHomeActivtyLocked()中，发intent：
intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
intent.addCategory(Intent.CATEGORY_HOME);
}
无论哪个应用程序，只要声明自己能够响应该intent，就会被认为是home程序。
