Android(安卓)Launcher启动流程

Launcher App 由SystemServer启动，而SystemServer由Zygote进程孵化出来的。

Zygote是孵化器，所有其他Dalvik/ART虚拟机进程都是用过zygote孵化(fock)出来的
SystemServer进程是Android系统的核心之一，大部分Android提供的服务都在该进程中，主要包括ActivityManagerService(AMS)、WindowManagerService(WMS)、PackageManagerSerice(PMS)等。

来看zygote-> SystemServer 入口：
位于frameworks/base/services/java/com/android/server/SystemServer.java

public static void main(String[] args) {         new SystemServer().run();}

可以看到，这里main方法调用了run方法

private void run() {     //...// Start services.    traceBeginAndSlog("StartServices");    startBootstrapServices(); //启动引导服务    startCoreServices(); //启动核心服务    startOtherServices(); //启动其他服务    //...}

来看startOtherServices，主要来看SystemServer的systemReady方法，这是Launcher启动的唯一入口。这个方法的作用是只有AMS启动完毕后，才会去回调Runnable中的代码。

private void startOtherServices(){     //...mActivityManagerService.systemReady(() -> {     //只有ActivityManagerService启动完之后，才会去启动回调里的这些服务    Slog.i(TAG, "Making services ready");    mSystemServiceManager.startBootPhase(            SystemService.PHASE_ACTIVITY_MANAGER_READY);    //...    }    //...}

来看ActivityManagerService.systemReady()

public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {     //...mStackSupervisor.resumeFocusedStackTopActivityLocked();//...}

再来看ActivityStackSupervisor.resumeFocusedStackTopActivityLocked

boolean resumeFocusedStackTopActivityLocked(            ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {     //...    final ActivityRecord r = mFocusedStack.topRunningActivityLocked();    if (r == null || r.state != RESUMED) {             mFocusedStack.resumeTopActivityUncheckedLocked(null, null);    } else if (r.state == RESUMED) {             mFocusedStack.executeAppTransition(targetOptions);    }    return false;}

再来看ActivityStack.resumeTopActivityUncheckedLocked

boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {             //...        boolean result = false;        try {                 mStackSupervisor.inResumeTopActivity = true;            result = resumeTopActivityInnerLocked(prev, options);        } finally {                 mStackSupervisor.inResumeTopActivity = false;        }//...        return result;    }

再来看resumeTopActivityInnerLocked

private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {          //...     if (!hasRunningActivity) {              return resumeTopActivityInNextFocusableStack(prev, options, "noMoreActivities");     }     //...}

接着来看resumeTopActivityInNextFocusableStack

private boolean resumeTopActivityInNextFocusableStack(ActivityRecord prev,            ActivityOptions options, String reason) {     //...// Only resume home if on home display return isOnHomeDisplay() &&          mStackSupervisor.resumeHomeStackTask(prev, reason);}

这里的mStackSupervisor.resumeHomeStackTask就是真正启动Launcher App的入口了

boolean resumeHomeStackTask(ActivityRecord prev, String reason) {         //...    return mService.startHomeActivityLocked(mCurrentUser, myReason);}

这里会通过ActivityManagerService.startHomeActivityLocked

来看ActivityManagerService

boolean startHomeActivityLocked(int userId, String reason) {             //...        Intent intent = getHomeIntent();        ActivityInfo aInfo = resolveActivityInfo(intent, STOCK_PM_FLAGS, userId);        if (aInfo != null) {             //创建Launcher启动所需的Intent            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) {                     intent.setFlags(intent.getFlags() | Intent.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;                //启动Launcher                mActivityStarter.startHomeActivityLocked(intent, aInfo, myReason);            }        } else {                 Slog.wtf(TAG, "No home screen found for " + intent, new Throwable());        }        return true;    }

可以看到，这里通过Intent，调用mActivityStarter.startHomeActivityLocked

来看ActivityStarter

void startHomeActivityLocked(Intent intent, ActivityInfo aInfo, String reason) {     //将Launcher放入HomeStack中，HomeStack是在ActivityManagerService中定义的用于存储Launcher的变量        mSupervisor.moveHomeStackTaskToTop(reason);        mLastHomeActivityStartResult = startActivityLocked(null /*caller*/, intent,                null /*ephemeralIntent*/, null /*resolvedType*/, aInfo, null /*rInfo*/,                null /*voiceSession*/, null /*voiceInteractor*/, null /*resultTo*/,                null /*resultWho*/, 0 /*requestCode*/, 0 /*callingPid*/, 0 /*callingUid*/,                null /*callingPackage*/, 0 /*realCallingPid*/, 0 /*realCallingUid*/,                0 /*startFlags*/, null /*options*/, false /*ignoreTargetSecurity*/,                false /*componentSpecified*/, mLastHomeActivityStartRecord /*outActivity*/,                null /*inTask*/, "startHomeActivity: " + reason);        if (mSupervisor.inResumeTopActivity) {                 // If we are in resume section already, home activity will be initialized, but not            // resumed (to avoid recursive resume) and will stay that way until something pokes it            // again. We need to schedule another resume.            mSupervisor.scheduleResumeTopActivities();        }    }

在这里就会去Manifest中寻找Launcher应用，并启动Launcher应用，接下来的启动过程就和Activity的启动过程类似了。

其他

参考
Zygote进程浅析
SystemServer进程浅析
Init进程源码分析
开机SystemServer到ActivityManagerService启动过程分析

其他

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