Android系统的启动过程

// frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
public static void main(String argv[]) {
    ZygoteServer zygoteServer = new ZygoteServer();
    ...
    final Runnable caller;
    try {
        ...
        RuntimeInit.enableDdms();
        boolean startSystemServer = false;
        String socketName = "zygote";
        String abiList = null;
        boolean enableLazyPreload = false;
        for (int i = 1; i < argv.length; i++) {
            if ("start-system-server".equals(argv[i])) {
                startSystemServer = true;
            } else if ("--enable-lazy-preload".equals(argv[i])) {
                enableLazyPreload = true;
            } else if (argv[i].startsWith(ABI_LIST_ARG)) {
                abiList = argv[i].substring(ABI_LIST_ARG.length());
            } else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
                socketName = argv[i].substring(SOCKET_NAME_ARG.length());
            } else {
                throw new RuntimeException("Unknown command line argument: " + argv[i]);
            }
        }
        ...
        zygoteServer.registerServerSocket(socketName);
        if (!enableLazyPreload) {
            preload(bootTimingsTraceLog);
        } else {
            Zygote.resetNicePriority();
        }
       ...
        if (startSystemServer) {
            Runnable r = forkSystemServer(abiList, socketName, zygoteServer);
            if (r != null) {
                r.run();
                return;
            }
        }
        caller = zygoteServer.runSelectLoop(abiList);
    } catch (Throwable ex) {
        Log.e(TAG, "System zygote died with exception", ex);
        throw ex;
    } finally {
        zygoteServer.closeServerSocket();
    }
    if (caller != null) {
        caller.run();
    }
}

// frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
private static Runnable forkSystemServer(String abiList, String socketName,
        ZygoteServer zygoteServer) {
    long capabilities = posixCapabilitiesAsBits(
        OsConstants.CAP_IPC_LOCK,
        OsConstants.CAP_KILL,
        OsConstants.CAP_NET_ADMIN,
        OsConstants.CAP_NET_BIND_SERVICE,
        OsConstants.CAP_NET_BROADCAST,
        OsConstants.CAP_NET_RAW,
        OsConstants.CAP_SYS_MODULE,
        OsConstants.CAP_SYS_NICE,
        OsConstants.CAP_SYS_PTRACE,
        OsConstants.CAP_SYS_TIME,
        OsConstants.CAP_SYS_TTY_CONFIG,
        OsConstants.CAP_WAKE_ALARM
    );
    /* Containers run without this capability, so avoid setting it in that case */
    if (!SystemProperties.getBoolean(PROPERTY_RUNNING_IN_CONTAINER, false)) {
        capabilities |= posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND);
    }
    /* Hardcoded command line to start the system server */
    String args[] = {
        "--setuid=1000",
        "--setgid=1000",
        "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1023,1032,3001,3002,3003,3006,3007,3009,3010",
        "--capabilities=" + capabilities + "," + capabilities,
        "--nice-name=system_server",
        "--runtime-args",
        "com.android.server.SystemServer",
    };
    ZygoteConnection.Arguments parsedArgs = null;

    int pid;

    try {
        parsedArgs = new ZygoteConnection.Arguments(args);
        ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
        ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);

        /* Request to fork the system server process */
        pid = Zygote.forkSystemServer(
                parsedArgs.uid, parsedArgs.gid,
                parsedArgs.gids,
                parsedArgs.debugFlags,
                null,
                parsedArgs.permittedCapabilities,
                parsedArgs.effectiveCapabilities);
    } catch (IllegalArgumentException ex) {
        throw new RuntimeException(ex);
    }

    /* For child process */
    if (pid == 0) {
        if (hasSecondZygote(abiList)) {
            waitForSecondaryZygote(socketName);
        }

        zygoteServer.closeServerSocket();
        return handleSystemServerProcess(parsedArgs);
    }

    return null;
}

// frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
Runnable r = forkSystemServer(abiList, socketName, zygoteServer);
if (r != null) {
    r.run();
    return;
}

// frameworks/base/core/java/com/android/internal/os/RuntimeInit$MethodAndArgsCaller.java
public void run() {
    try {
        mMethod.invoke(null, new Object[] { mArgs });
    } catch (IllegalAccessException ex) {
        throw new RuntimeException(ex);
    } catch (InvocationTargetException ex) {
        Throwable cause = ex.getCause();
        if (cause instanceof RuntimeException) {
            throw (RuntimeException) cause;
        } else if (cause instanceof Error) {
            throw (Error) cause;
        }
        throw new RuntimeException(ex);
    }
}

// frameworks/base/services/java/com/android/server/SystemServer.java
private void run() {
    try {
        ...
        Looper.prepareMainLooper();

        // Initialize native services.
        System.loadLibrary("android_servers");

        // 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);
        mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);
        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
        // Prepare the thread pool for init tasks that can be parallelized
        SystemServerInitThreadPool.get();
    } finally {
        traceEnd();  // InitBeforeStartServices
    }

    // Start services.
    try {
        traceBeginAndSlog("StartServices");
        startBootstrapServices();
        startCoreServices();
        startOtherServices();
        SystemServerInitThreadPool.shutdown();
    }
    ...
    // Loop forever.
    Looper.loop();
    throw new RuntimeException("Main thread loop unexpectedly exited");
}

// frameworks/base/services/java/com/android/server/SystemServer.java
private void startOtherServices() {
    ...
    mActivityManagerService.systemReady(() -> {
        mSystemServiceManager.startBootPhase(
                SystemService.PHASE_ACTIVITY_MANAGER_READY);
        try {
            mActivityManagerService.startObservingNativeCrashes();
        } catch (Throwable e) {
        }
        ...
        try {
            startSystemUi(context, windowManagerF);
        } catch (Throwable e) {
        }
        ...
    }, BOOT_TIMINGS_TRACE_LOG);
}

// frameworks/base/services/java/com/android/server/SystemServer.java
static final void startSystemUi(Context context, WindowManagerService windowManager) {
    Intent intent = new Intent();
    intent.setComponent(new ComponentName("com.android.systemui",
                "com.android.systemui.SystemUIService"));
    intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
    //Slog.d(TAG, "Starting service: " + intent);
    context.startServiceAsUser(intent, UserHandle.SYSTEM);
    windowManager.onSystemUiStarted();
}

// frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIService.java
public class SystemUIService extends Service {

    @Override
    public void onCreate() {
        super.onCreate();
        ((SystemUIApplication) getApplication()).startServicesIfNeeded();

        // For debugging RescueParty
        if (Build.IS_DEBUGGABLE && SystemProperties.getBoolean("debug.crash_sysui", false)) {
            throw new RuntimeException();
        }
    }
}

// frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIApplication.java
/**
 * The classes of the stuff to start.
 */
private final Class<?>[] SERVICES = new Class[] {
        Dependency.class,
        NotificationChannels.class,
        CommandQueue.CommandQueueStart.class,
        KeyguardViewMediator.class,
        Recents.class,
        VolumeUI.class,
        Divider.class,
        SystemBars.class,
        StorageNotification.class,
        PowerUI.class,
        RingtonePlayer.class,
        KeyboardUI.class,
        PipUI.class,
        ShortcutKeyDispatcher.class,
        VendorServices.class,
        GarbageMonitor.Service.class,
        LatencyTester.class,
        GlobalActionsComponent.class,
        RoundedCorners.class,
};

public void startServicesIfNeeded() {
    startServicesIfNeeded(SERVICES);
}

// frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIApplication.java
private void startServicesIfNeeded(Class<?>[] services) {
    ...
    final int N = services.length;
    for (int i = 0; i < N; i++) {
        Class<?> cl = services[i];
        long ti = System.currentTimeMillis();
        try {
            Object newService = SystemUIFactory.getInstance().createInstance(cl);
            mServices[i] = (SystemUI) ((newService == null) ? cl.newInstance() : newService);
        }

        mServices[i].mContext = this;
        mServices[i].mComponents = mComponents;
        mServices[i].start();

        // Warn if initialization of component takes too long
        ti = System.currentTimeMillis() - ti;
        if (ti > 1000) {
            Log.w(TAG, "Initialization of " + cl.getName() + " took " + ti + " ms");
        }
        if (mBootCompleted) {
            mServices[i].onBootCompleted();
        }
    }
    Dependency.get(PluginManager.class).addPluginListener(
            new PluginListener<OverlayPlugin>() {
                private ArraySet<OverlayPlugin> mOverlays;

                @Override
                public void onPluginConnected(OverlayPlugin plugin, Context pluginContext) {
                    StatusBar statusBar = getComponent(StatusBar.class);
                    if (statusBar != null) {
                        plugin.setup(statusBar.getStatusBarWindow(),
                                statusBar.getNavigationBarView());
                    }
                    // Lazy init.
                    if (mOverlays == null) mOverlays = new ArraySet<>();
                    if (plugin.holdStatusBarOpen()) {
                        mOverlays.add(plugin);
                        Dependency.get(StatusBarWindowManager.class).setStateListener(b ->
                                mOverlays.forEach(o -> o.setCollapseDesired(b)));
                        Dependency.get(StatusBarWindowManager.class).setForcePluginOpen(
                                mOverlays.size() != 0);

                    }
                }
            }, OverlayPlugin.class, true /* Allow multiple plugins */);

    mServicesStarted = true;
}

private final Class<?>[] SERVICES_PER_USER = new Class[] {
        Dependency.class,
        NotificationChannels.class,
        Recents.class
};

public class SystemBars extends SystemUI {
    private static final String TAG = "SystemBars";
    private static final boolean DEBUG = false;
    private static final int WAIT_FOR_BARS_TO_DIE = 500;

    // in-process fallback implementation, per the product config
    private SystemUI mStatusBar;

    @Override
    public void start() {
        createStatusBarFromConfig();
    }

    private void createStatusBarFromConfig() {
        final String clsName = mContext.getString(R.string.config_statusBarComponent);
        Class<?> cls = null;
        try {
            cls = mContext.getClassLoader().loadClass(clsName);
        }
        try {
            mStatusBar = (SystemUI) cls.newInstance();
        }
        mStatusBar.mContext = mContext;
        mStatusBar.mComponents = mComponents;
        mStatusBar.start();
    }
}

// frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/StatusBar.java
...

// frameworks/base/core/service/java/android/server/am/ActivityManagerService.java
public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {
    traceLog.traceBegin("PhaseActivityManagerReady");
    synchronized(this) {
        if (mSystemReady) {
            if (goingCallback != null) {
                goingCallback.run();
            }
            return;
        }
        ...
        mSystemReady = true;
    }
    ...
    ArrayList<ProcessRecord> procsToKill = null;
    synchronized(mPidsSelfLocked) {
        for (int i=mPidsSelfLocked.size()-1; i>=0; i--) {
            ProcessRecord proc = mPidsSelfLocked.valueAt(i);
            if (!isAllowedWhileBooting(proc.info)){
                if (procsToKill == null) {
                    procsToKill = new ArrayList<ProcessRecord>();
                }
                procsToKill.add(proc);
            }
        }
    }
    synchronized(this) {
        if (procsToKill != null) {
            for (int i=procsToKill.size()-1; i>=0; i--) {
                ProcessRecord proc = procsToKill.get(i);
                removeProcessLocked(proc, true, false, "system update done");
            }
        }
        mProcessesReady = true;
    }
    ...
    retrieveSettings();
    final int currentUserId;
    synchronized (this) {
        currentUserId = mUserController.getCurrentUserIdLocked();
        readGrantedUriPermissionsLocked();
    }

    if (goingCallback != null) goingCallback.run(); // 来自SystemServer
    ...
    mSystemServiceManager.startUser(currentUserId);
    synchronized (this) {
        startPersistentApps(PackageManager.MATCH_DIRECT_BOOT_AWARE);
        mBooting = true;
        if (UserManager.isSplitSystemUser() &&
                Settings.Secure.getInt(mContext.getContentResolver(),
                     Settings.Secure.USER_SETUP_COMPLETE, 0) != 0) {
            ComponentName cName = new ComponentName(mContext, SystemUserHomeActivity.class);
            try {
                AppGlobals.getPackageManager().setComponentEnabledSetting(cName,
                        PackageManager.COMPONENT_ENABLED_STATE_ENABLED, 0,
                        UserHandle.USER_SYSTEM);
            }
        }
        startHomeActivityLocked(currentUserId, "systemReady");
        ...
        try {
            Intent intent = new Intent(Intent.ACTION_USER_STARTED);
            intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY | Intent.FLAG_RECEIVER_FOREGROUND);
            intent.putExtra(Intent.EXTRA_USER_HANDLE, currentUserId);
            broadcastIntentLocked(...);
            intent = new Intent(Intent.ACTION_USER_STARTING);
            intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY);
            intent.putExtra(Intent.EXTRA_USER_HANDLE, currentUserId);
            broadcastIntentLocked(...);
        } finally {
            Binder.restoreCallingIdentity(ident);
        }
        mStackSupervisor.resumeFocusedStackTopActivityLocked();
        mUserController.sendUserSwitchBroadcastsLocked(-1, currentUserId);
    }
}

// frameworks/base/core/service/java/android/server/am/ActivityManagerService.java
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;
}
    
boolean startHomeActivityLocked(int userId, String reason) {
    if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
            && mTopAction == null) {
        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 beinginstrumented.
        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 actuallylaunched.
            final String myReason = reason + ":" + userId + ":" + resolvedUserId;
            mActivityStarter.startHomeActivityLocked(intent, aInfo, myReason);
        }
    } else {
        Slog.wtf(TAG, "No home screen found for " + intent, new Throwable());
    }

    return true;
}