Launcher启动前
我们知道init进程是Android系统中用户空间的第一个进程，它创建zygote(孵化器)和属性服务等。接着在Android系统中，DVM(Dalvik虚拟机)、应用程序进程以及运行系统的关键服务的SystemServer进程都是由Zygote进程来创建的，我们也将它称为孵化器。它通过fock(复制进程)的形式来创建应用程序进程和SystemServer进程，由于Zygote进程在启动时会创建DVM，因此通过fock而创建的应用程序进程和SystemServer进程可以在内部获取一个DVM的实例拷贝。 当SystemServer启动时它会启动Binder线程池，这样就可以与其他进程进行通信；它同时也创建了SystemServiceManager用于对系统的服务进行创建、启动、和生命周期的管理；SystemServiceManager.startService(Class
SystemServer.java中：
private void run() {
// Start services.
...........此处省略
try {
traceBeginAndSlog("StartServices");
startBootstrapServices();
startCoreServices();
startOtherServices();
SystemServerInitThreadPool.shutdown();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
traceEnd();
}
...........此处省略
SystemServiceManager.java中：
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 {
Constructor constructor = serviceClass.getConstructor(Context.class);
service = constructor.newInstance(mContext);
} ...//省略部分catch
startService(service);
return service;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}
Launcher概述
Android系统启动的最后一步是启动一个Home应用程序，这个应用程序用来显示系统中已经安装的应用程序，这个Home应用程序就叫做Launcher。应用程序Launcher在启动过程中会PackageManagerService返回系统中已经安装的应用程序的信息，并将这些信息封装成一个快捷图标列表显示在系统屏幕上，这样用户可以通过点击这些快捷图标来启动相应的应用程序。
Launcher的启动流程
我们知道SystemServer进程再启动的过程中会启动PackageManagerService，PackageManagerService启动后会将系统中的应用程序安装完成。在此之前已经启动的ActivityManagerService会将Launcher启动起来，Launcher的入口为ActivityManagerService的systemReady函数：
com.android.server.SystemServer.java
private void startBootstrapServices() {
...............
// Activity manager runs the show.
traceBeginAndSlog("StartActivityManager");
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);
traceEnd();
.....................
}
private void startOtherServices() {
...................
mActivityManagerService.systemReady(() -> {
Slog.i(TAG, "Making services ready");
traceBeginAndSlog("StartActivityManagerReadyPhase");
mSystemServiceManager.startBootPhase(
SystemService.PHASE_ACTIVITY_MANAGER_READY)
...................
})
在startOtherServices函数中，会调用ActivityManagerService的systemReady函数：
com.android.server.am.ActivityManagerService
public void systemReady(final Runnable goingCallback) {
...
synchronized (this) {
...
mStackSupervisor.resumeFocusedStackTopActivityLocked();
mUserController.sendUserSwitchBroadcastsLocked(-1, currentUserId);
}
}
systemReady函数中调用了ActivityStackSupervisor的resumeFocusedStackTopActivityLocked函数：
com.android.server.am.ActivityStackSupervisor
boolean resumeFocusedStackTopActivityLocked(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
if (!readyToResume()) {
return false;
}
if (targetStack != null && isFocusedStack(targetStack)) {
return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
if (r == null || !r.isState(RESUMED)) {
mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
} else if (r.isState(RESUMED)) {
// Kick off any lingering app transitions form the MoveTaskToFront operation.
mFocusedStack.executeAppTransition(targetOptions);
}
return false;
}
targetStack.resumeTopActivityUncheckedLocked(target, targetOptions)中的targetStack(ActivityStack)对象是用来描述Activity堆栈的，resumeTopActivityUncheckedLocked函数如下所示:
com.android.server.am.ActivityStack
@GuardedBy("mService")
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
if (mStackSupervisor.inResumeTopActivity) {
// Don't even start recursing.
return false;
}
boolean result = false;
try {
// Protect against recursion.
mStackSupervisor.inResumeTopActivity = true;
result = resumeTopActivityInnerLocked(prev, options);
// When resuming the top activity, it may be necessary to pause the top activity (for
// example, returning to the lock screen. We suppress the normal pause logic in
// {@link #resumeTopActivityUncheckedLocked}, since the top activity is resumed at the
// end. We call the {@link ActivityStackSupervisor#checkReadyForSleepLocked} again here
// to ensure any necessary pause logic occurs. In the case where the Activity will be
// shown regardless of the lock screen, the call to
// {@link ActivityStackSupervisor#checkReadyForSleepLocked} is skipped.
final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
if (next == null || !next.canTurnScreenOn()) {
checkReadyForSleep();
}
} finally {
mStackSupervisor.inResumeTopActivity = false;
}
return result;
}
在该函数中如果ActivityStackSupervisor的inResumeTopActivity递归控制 Tag为true 设为false，终止递归。接着调用了resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options)函数，resumeTopActivityInnerLocked函数的代码很长，我们截取我们要分析的关键的一句：调用ActivityStackSupervisor的resumeHomeStackTask函数，代码如下所示：
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
....
if (!hasRunningActivity) {
// There are no activities left in the stack, let's look somewhere else.
return resumeTopActivityInNextFocusableStack(prev, options, "noMoreActivities");
}
...
}
接着又调用了resumeTopActivityInNextFocusableStack函数，代码如下所示：
private boolean resumeTopActivityInNextFocusableStack(ActivityRecord prev,
ActivityOptions options, String reason) {
if (adjustFocusToNextFocusableStack(reason)) {
// Try to move focus to the next visible stack with a running activity if this
// stack is not covering the entire screen or is on a secondary display (with no home
// stack).
return mStackSupervisor.resumeFocusedStackTopActivityLocked(
mStackSupervisor.getFocusedStack(), prev, null);
}
// Let's just start up the Launcher...
ActivityOptions.abort(options);
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityInNextFocusableStack: " + reason + ", go home");
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
// Only resume home if on home display
return isOnHomeDisplay() &&
mStackSupervisor.resumeHomeStackTask(prev, reason);
}
接着看继续调用了resumeHomeStackTask(prev, reason)函数：
boolean resumeHomeStackTask(ActivityRecord prev, String reason) {
if (!mService.mBooting && !mService.mBooted) {
// Not ready yet!
return false;
}
mHomeStack.moveHomeStackTaskToTop();
ActivityRecord r = getHomeActivity();
final String myReason = reason + " resumeHomeStackTask";
// Only resume home activity if isn't finishing.
if (r != null && !r.finishing) {
moveFocusableActivityStackToFrontLocked(r, myReason);
return resumeFocusedStackTopActivityLocked(mHomeStack, prev, null);
}
return mService.startHomeActivityLocked(mCurrentUser, myReason);
}
我们看到它最后调用了ActivityManagerService的startHomeActivityLocked(mCurrentUser, myReason)函数：
boolean startHomeActivityLocked(int userId, String reason) {
if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
&& mTopAction == null) { //tag1
// We are running in factory test mode, but unable to find
// the factory test app, so just sit around displaying the
// error message and don't try to start anything.
return false;
}
Intent intent = getHomeIntent();//tag2
ActivityInfo aInfo = resolveActivityInfo(intent, STOCK_PM_FLAGS, userId);
if (aInfo != null) {
intent.setComponent(new ComponentName(aInfo.applicationInfo.packageName, aInfo.name));
// Don't do this if the home app is currently being
// instrumented.
aInfo = new ActivityInfo(aInfo);
aInfo.applicationInfo = getAppInfoForUser(aInfo.applicationInfo, userId);
ProcessRecord app = getProcessRecordLocked(aInfo.processName,
aInfo.applicationInfo.uid, true);
if (app == null || app.instr == null) { //tag3
intent.setFlags(intent.getFlags() | FLAG_ACTIVITY_NEW_TASK);
final int resolvedUserId = UserHandle.getUserId(aInfo.applicationInfo.uid);
// For ANR debugging to verify if the user activity is the one that actually
// launched.
final String myReason = reason + ":" + userId + ":" + resolvedUserId;
mActivityStartController.startHomeActivity(intent, aInfo, myReason);//tag4
}
} else {
Slog.wtf(TAG, "No home screen found for " + intent, new Throwable());
}
return true;
}
在tag1地方的mFactoryTest代表系统的运行模式，系统的运行模式分为非工厂模式、低级工厂模式、高级工厂模式。mTopAction则用来描述第一个被启动Activity组件的Action，它的值为Intent.ACTION_MAIN。所以当它为null并且是低级工程模式时什么也不启动。
tag2地方调用了getHomeIntent函数代码如下：
Intent getHomeIntent() {
Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null);
intent.setComponent(mTopComponent);
intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
intent.addCategory(Intent.CATEGORY_HOME);
}
return intent;
}
getHomeIntent函数中创建了Intent，并将mTopAction和mTopData传入。mTopAction的值为Intent.ACTION_MAIN，并且如果系统运行模式不是低级工厂模式则将intent的Category设置为Intent.CATEGORY_HOME。我们再回到ActivityManagerService的startHomeActivityLocked函数，假设系统的运行模式不是低级工厂模式，在注释3处判断符合Action为Intent.ACTION_MAIN，Category为Intent.CATEGORY_HOME的应用程序是否已经启动，如果没启动则调用注释4的方法启动该应用程序。
这个被启动的应用程序就是Launcher，因为Launcher的Manifest文件中的intent-filter标签匹配了Action为Intent.ACTION_MAIN，Category为Intent.CATEGORY_HOME。Launcher的Manifest文件如下所示。
/Volumes/android/WORKING_DIRECTORY/packages/apps/Launcher3/AndroidManifest.xml
....
这样，应用程序Launcher就会被启动起来，并执行它的onCreate函数。