Today, I am free to study Launcher's start-up time in the system start-up process. It is necessary to share the results of reading the code.

First of all, we know that the init process will be generated when the system starts. The init process will start daemons like usbd, adbd and so on. At the same time, it will start the Zygote process. Then the Zygote process will start the System Server process. This System Server is the process that starts most services. At the same time, registering these services in Service Manager, it should be thought that after the service starts, the system Server process will start. It will start SystemUI and Launcher, and then go to SystemServer.java to see:

 System.loadLibrary("android_servers");
        nativeInit();

        // Check whether we failed to shut down last time we tried.
        // This call may not return.
        performPendingShutdown();

        // Initialize the system context.
        createSystemContext();

        // Create the system service manager.
        mSystemServiceManager = new SystemServiceManager(mSystemContext);
        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

        // Start services.
        try {
            startBootstrapServices();
            startCoreServices();
            startOtherServices();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        }

        // For debug builds, log event loop stalls to dropbox for analysis.
        if (StrictMode.conditionallyEnableDebugLogging()) {
            Slog.i(TAG, "Enabled StrictMode for system server main thread.");
        }

As can be seen from the above code, the boot service will be started first, and then the core service will be started. In fact, these two kinds of services are to start platform services, and then to start other services. The other services are hardware services. Continue to look at the code startOther Services.

 final CommonTimeManagementService commonTimeMgmtServiceF = commonTimeMgmtService;
        final TextServicesManagerService textServiceManagerServiceF = tsms;
        final StatusBarManagerService statusBarF = statusBar;
        final AssetAtlasService atlasF = atlas;
        final InputManagerService inputManagerF = inputManager;
        final TelephonyRegistry telephonyRegistryF = telephonyRegistry;
        final MediaRouterService mediaRouterF = mediaRouter;
        final AudioService audioServiceF = audioService;
        final MmsServiceBroker mmsServiceF = mmsService;

        // We now tell the activity manager it is okay to run third party
        // code.  It will call back into us once it has gotten to the state
        // where third party code can really run (but before it has actually
        // started launching the initial applications), for us to complete our
        // initialization.
        mActivityManagerService.systemReady(new Runnable() {
            @Override
            public void run() {
                Slog.i(TAG, "Making services ready");
                mSystemServiceManager.startBootPhase(
                        SystemService.PHASE_ACTIVITY_MANAGER_READY);

                try {
                    mActivityManagerService.startObservingNativeCrashes();
                } catch (Throwable e) {
                    reportWtf("observing native crashes", e);
                }

                Slog.i(TAG, "WebViewFactory preparation");
                WebViewFactory.prepareWebViewInSystemServer();

                try {
                    startSystemUi(context);
                } catch (Throwable e) {
                    reportWtf("starting System UI", e);
                }
                try {
                    if (mountServiceF != null) mountServiceF.systemReady();
                } catch (Throwable e) {
                    reportWtf("making Mount Service ready", e);
                }

You can see the function startSystemUi in the above code, obviously to start the System UI, Launcher code should be about this place, continue to look at the method mActivityManagerService.systemReady

           int i;
                        for (i=0; i<N; i++) {
                            ApplicationInfo info
                                = (ApplicationInfo)apps.get(i);
                            if (info != null &&
                                    !info.packageName.equals("android")) {
                                addAppLocked(info, false, null /* ABI override */);
                            }
                        }
                    }
                } catch (RemoteException ex) {
                    // pm is in same process, this will never happen.
                }
            }

            // Start up initial activity.
            mBooting = true;
            startHomeActivityLocked(mCurrentUserId, "systemReady");

            try {
                if (AppGlobals.getPackageManager().hasSystemUidErrors()) {
                    Slog.e(TAG, "UIDs on the system are inconsistent, you need to wipe your"
                            + " data partition or your device will be unstable.");
                    mHandler.obtainMessage(SHOW_UID_ERROR_MSG).sendToTarget();
                }
            } catch (RemoteException e) {
            }

            if (!Build.isFingerprintConsistent()) {
                Slog.e(TAG, "Build fingerprint is not consistent, warning user");
                mHandler.obtainMessage(SHOW_FINGERPRINT_ERROR_MSG).sendToTarget();
            }

            long ident = Binder.clearCallingIdentity();
            try {
                Intent intent = new Intent(Intent.ACTION_USER_STARTED);

We see the startHomeActivityLocked method.

boolean startHomeActivityLocked(int userId, String reason) {
        if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
                && mTopAction == null) {
            // 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();
        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.instrumentationClass == null) {
                intent.setFlags(intent.getFlags() | Intent.FLAG_ACTIVITY_NEW_TASK);
                
            }
        }

        return true;
    }

Continue to look at the getHomeIntent method

                   UsageEvents.Event.MOVE_TO_BACKGROUND);
            }
            synchronized (stats) {
                stats.noteActivityPausedLocked(component.app.uid);
            }
        }
    }

    Intent getHomeIntent() {
        Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null);
        intent.setComponent(mTopComponent);
        if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
            intent.addCategory(Intent.CATEGORY_HOME);
        }
        return intent;
    }

    boolean startHomeActivityLocked(int userId, String reason) {
        if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
                && mTopAction == null) {
            // 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;

Here I think you can see that the getHome Intent method is to configure the Intent needed to start Launcher. mStackSupervisor. startHome Activity is to start Launcher application. StackSupervisor is essentially a class of activities started by the server, which has been analyzed before. If you are not familiar with the process of activity start, you will find that the process of activity start is not very familiar with. You can get to know it.