这篇主要讲解的是Launcher程序的启动流程至于Launcher前面的系统进程启动流程大家可以看看我的Android应用跟系统之间通信什么时候用Socket什么时候用Binder结合源码详解有介绍。好了进入咱们今天的主题Launcher，注意本篇系统源码是基于Android4.4版本（也就是下面AMS、PMS的源码），先看看Launcher启动入口在哪里其实就在SystemServer中：

   public static void main(String[] args) {
        new SystemServer().run();
    }
    private void run() {
        ......   
        // Start services.
        try {
            ......   
            startOtherServices();
            ......   
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        } finally {
            traceEnd();
        }
        ......   
    }

没错就是它startOtherServices，来看一下源码run方法的startOtherServices：

 private void startOtherServices() {
     ......
     mActivityManagerService.systemReady(() -> {
            Slog.i(TAG, "Making services ready");
            traceBeginAndSlog("StartActivityManagerReadyPhase");
            mSystemServiceManager.startBootPhase(
                    SystemService.PHASE_ACTIVITY_MANAGER_READY);
            traceEnd();
            traceBeginAndSlog("StartObservingNativeCrashes");
            try {
                mActivityManagerService.startObservingNativeCrashes();
            } catch (Throwable e) {
                reportWtf("observing native crashes", e);
            }
            traceEnd();
            ......
        }
    }

我们再来看一看ActivityManagerService源码的systemReady方法吧：

public void systemReady(final Runnable goingCallback) {
     ......
     synchronized(this) {
            // Make sure we have no pre-ready processes sitting around.
            
            if (mFactoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL) {
                ResolveInfo ri = mContext.getPackageManager()
                        .resolveActivity(new Intent(Intent.ACTION_FACTORY_TEST),
                                STOCK_PM_FLAGS);
                CharSequence errorMsg = null;
                if (ri != null) {
                    ActivityInfo ai = ri.activityInfo;
                    ApplicationInfo app = ai.applicationInfo;
                    if ((app.flags&ApplicationInfo.FLAG_SYSTEM) != 0) {
                        mTopAction = Intent.ACTION_FACTORY_TEST;
                        mTopData = null;
                        mTopComponent = new ComponentName(app.packageName,
                                ai.name);
                    } else {
                        errorMsg = mContext.getResources().getText(
                                com.android.internal.R.string.factorytest_not_system);
                    }
                } else {
                    errorMsg = mContext.getResources().getText(
                            com.android.internal.R.string.factorytest_no_action);
                }
                if (errorMsg != null) {
                    mTopAction = null;
                    mTopData = null;
                    mTopComponent = null;
                    Message msg = Message.obtain();
                    msg.what = SHOW_FACTORY_ERROR_MSG;
                    msg.getData().putCharSequence("msg", errorMsg);
                    mHandler.sendMessage(msg);
                }
            }
        }

  synchronized (this) {
            if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {
                try {
                    List apps = AppGlobals.getPackageManager().
                        getPersistentApplications(STOCK_PM_FLAGS);
                    if (apps != null) {
                        int N = apps.size();
                        int i;
                        for (i=0; i<N; i++) {
                            ApplicationInfo info
                                = (ApplicationInfo)apps.get(i);
                            if (info != null &&
                                    !info.packageName.equals("android")) {
                                addAppLocked(info, false);
                            }
                        }
                    }
                } catch (RemoteException ex) {
                    // pm is in same process, this will never happen.
                }
            }

            // Start up initial activity.
            mBooting = true;
            
            try {
                if (AppGlobals.getPackageManager().hasSystemUidErrors()) {
                    Message msg = Message.obtain();
                    msg.what = SHOW_UID_ERROR_MSG;
                    mHandler.sendMessage(msg);
                }
            } catch (RemoteException e) {
            }

            long ident = Binder.clearCallingIdentity();
            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, mCurrentUserId);
                broadcastIntentLocked(null, null, intent,
                        null, null, 0, null, null, null, AppOpsManager.OP_NONE,
                        false, false, MY_PID, Process.SYSTEM_UID, mCurrentUserId);
                intent = new Intent(Intent.ACTION_USER_STARTING);
                intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY);
                intent.putExtra(Intent.EXTRA_USER_HANDLE, mCurrentUserId);
                broadcastIntentLocked(null, null, intent,
                        null, new IIntentReceiver.Stub() {
                            @Override
                            public void performReceive(Intent intent, int resultCode, String data,
                                    Bundle extras, boolean ordered, boolean sticky, int sendingUser)
                                    throws RemoteException {
                            }
                        }, 0, null, null,
                        android.Manifest.permission.INTERACT_ACROSS_USERS, AppOpsManager.OP_NONE,
                        true, false, MY_PID, Process.SYSTEM_UID, UserHandle.USER_ALL);
            } finally {
                Binder.restoreCallingIdentity(ident);
            }
            mStackSupervisor.resumeTopActivitiesLocked();
            sendUserSwitchBroadcastsLocked(-1, mCurrentUserId);
        }

}

找到mStackSupervisor.resumeTopActivitiesLocked()：

    boolean resumeTopActivitiesLocked() {
        return resumeTopActivitiesLocked(null, null, null);
    }

    boolean resumeTopActivitiesLocked(ActivityStack targetStack, ActivityRecord target,
            Bundle targetOptions) {
        if (targetStack == null) {
            targetStack = getFocusedStack();
        }
        boolean result = false;
        for (int stackNdx = mStacks.size() - 1; stackNdx >= 0; --stackNdx) {
            final ActivityStack stack = mStacks.get(stackNdx);
            if (isFrontStack(stack)) {
                if (stack == targetStack) {
                    result = stack.resumeTopActivityLocked(target, targetOptions);
                } else {
                    stack.resumeTopActivityLocked(null);
                }
            }
        }
        return result;
    }

接下来我们再看看ActivityStack的resumeTopActivityLocked方法：

 final boolean resumeTopActivityLocked(ActivityRecord prev) {
        return resumeTopActivityLocked(prev, null);
    }

 final boolean resumeTopActivityLocked(ActivityRecord prev, Bundle options) {
    ......
      if (next == null) {
            // There are no more activities!  Let's just start up the
            // Launcher...
            ActivityOptions.abort(options);
            if (DEBUG_STATES) Slog.d(TAG, "resumeTopActivityLocked: No more activities go home");
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return mStackSupervisor.resumeHomeActivity(prev);
        }
    }

看到返回的是ActivityStackSupervisor的resumeHomeActivity方法：

    boolean resumeHomeActivity(ActivityRecord prev) {
        moveHomeToTop();
        if (prev != null) {
            prev.task.mOnTopOfHome = false;
        }
        ActivityRecord r = mHomeStack.topRunningActivityLocked(null);
        if (r != null && r.isHomeActivity()) {
            mService.setFocusedActivityLocked(r);
            return resumeTopActivitiesLocked(mHomeStack, prev, null);
        }
        return mService.startHomeActivityLocked(mCurrentUser);
    }

这里返回的是ActivityManagerService的startHomeActivityLocked方法：

  boolean startHomeActivityLocked(int userId) {
        if (mHeadless) {
            // Added because none of the other calls to ensureBootCompleted seem to fire
            // when running headless.
            ensureBootCompleted();
            return false;
        }

        if (mFactoryTest == SystemServer.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);
                mStackSupervisor.startHomeActivity(intent, aInfo);
            }
        }

        return true;
    }

终于来到了要点了getHomeIntent：

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

这里启动的程序就是Launcher，如果对Launcher程序感兴趣我可以贴上Launcher的AndroidManifest.xml源码：

<?xml version="1.0" encoding="utf-8"?>
<!--

-->
<manifest
    xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.android.launcher3">

    <permission
        android:name="com.android.launcher3.permission.PRELOAD_WORKSPACE"
        android:permissionGroup="android.permission-group.SYSTEM_TOOLS"
        android:protectionLevel="system|signature" />
    <permission
        android:name="com.android.launcher.permission.INSTALL_SHORTCUT"
        android:permissionGroup="android.permission-group.SYSTEM_TOOLS"
        android:protectionLevel="dangerous"
        android:label="@string/permlab_install_shortcut"
        android:description="@string/permdesc_install_shortcut" />
    <permission
        android:name="com.android.launcher.permission.UNINSTALL_SHORTCUT"
        android:permissionGroup="android.permission-group.SYSTEM_TOOLS"
        android:protectionLevel="dangerous"
        android:label="@string/permlab_uninstall_shortcut"
        android:description="@string/permdesc_uninstall_shortcut"/>
    <permission
        android:name="com.android.launcher3.permission.READ_SETTINGS"
        android:permissionGroup="android.permission-group.SYSTEM_TOOLS"
        android:protectionLevel="normal"
        android:label="@string/permlab_read_settings"
        android:description="@string/permdesc_read_settings"/>
    <permission
        android:name="com.android.launcher3.permission.WRITE_SETTINGS"
        android:permissionGroup="android.permission-group.SYSTEM_TOOLS"
        android:protectionLevel="normal"
        android:label="@string/permlab_write_settings"
        android:description="@string/permdesc_write_settings"/>

    <permission
        android:name="com.android.launcher3.permission.RECEIVE_LAUNCH_BROADCASTS"
        android:protectionLevel="signature"
        />

    <uses-permission android:name="android.permission.CALL_PHONE" />
    <uses-permission android:name="android.permission.SET_WALLPAPER" />
    <uses-permission android:name="android.permission.SET_WALLPAPER_HINTS" />
    <uses-permission android:name="android.permission.VIBRATE" />
    <uses-permission android:name="android.permission.BIND_APPWIDGET" />
    <uses-permission android:name="android.permission.GET_ACCOUNTS" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="com.android.launcher.permission.READ_SETTINGS" />
    <uses-permission android:name="com.android.launcher.permission.WRITE_SETTINGS" />
    <uses-permission android:name="com.android.launcher3.permission.READ_SETTINGS" />
    <uses-permission android:name="com.android.launcher3.permission.WRITE_SETTINGS" />
    <uses-permission android:name="com.android.launcher3.permission.RECEIVE_LAUNCH_BROADCASTS" />

    <application
        android:name="com.android.launcher3.LauncherApplication"
        android:label="@string/application_name"
        android:icon="@mipmap/ic_launcher_home"
        android:hardwareAccelerated="true"
        android:largeHeap="@bool/config_largeHeap"
        android:supportsRtl="true">
        <activity
            android:name="com.android.launcher3.Launcher"
            android:launchMode="singleTask"
            android:clearTaskOnLaunch="true"
            android:stateNotNeeded="true"
            android:theme="@style/Theme"
            android:windowSoftInputMode="adjustPan"
            android:screenOrientation="nosensor">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.HOME" />
                <category android:name="android.intent.category.DEFAULT" />
                <category android:name="android.intent.category.MONKEY"/>
            </intent-filter>
        </activity>

        <activity
            android:name="com.android.launcher3.ToggleWeightWatcher"
            android:label="@string/toggle_weight_watcher"
            android:enabled="@bool/debug_memory_enabled"
            android:icon="@mipmap/ic_launcher_home">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.DEFAULT" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <activity
            android:name="com.android.launcher3.WallpaperPickerActivity"
            android:theme="@style/Theme.WallpaperCropper"
            android:label="@string/pick_wallpaper"
            android:icon="@mipmap/ic_launcher_wallpaper"
            android:finishOnCloseSystemDialogs="true"
            android:process=":wallpaper_chooser">
            <intent-filter>
                <action android:name="android.intent.action.SET_WALLPAPER" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>

        <activity
            android:name="com.android.launcher3.WallpaperCropActivity"
            android:theme="@style/Theme.WallpaperCropper"
            android:label="@string/crop_wallpaper"
            android:icon="@mipmap/ic_launcher_wallpaper"
            android:finishOnCloseSystemDialogs="true"
            android:process=":wallpaper_chooser">
            <intent-filter>
                <action android:name="android.service.wallpaper.CROP_AND_SET_WALLPAPER" />
                <category android:name="android.intent.category.DEFAULT" />
                <data android:mimeType="image/*" />
            </intent-filter>
        </activity>

        <!-- Debugging tools -->
        <activity
            android:name="com.android.launcher3.MemoryDumpActivity"
            android:theme="@android:style/Theme.NoDisplay"
            android:label="@string/debug_memory_activity"
            android:enabled="@bool/debug_memory_enabled"
            android:excludeFromRecents="true"
            android:icon="@mipmap/ic_launcher_home"
            >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.DEFAULT" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <service android:name="com.android.launcher3.MemoryTracker"
            android:enabled="@bool/debug_memory_enabled"
            >
        </service>

        <!-- Intent received used to prepopulate the default workspace. -->
        <receiver
            android:name="com.android.launcher3.PreloadReceiver"
            android:permission="com.android.launcher3.permission.PRELOAD_WORKSPACE">
            <intent-filter>
                <action android:name="com.android.launcher3.action.PRELOAD_WORKSPACE" />
            </intent-filter>
        </receiver>

        <!-- Intent received used to install shortcuts from other applications -->
        <receiver
            android:name="com.android.launcher3.InstallShortcutReceiver"
            android:permission="com.android.launcher3.permission.INSTALL_SHORTCUT">
            <intent-filter>
                <action android:name="com.android.launcher3.action.INSTALL_SHORTCUT" />
            </intent-filter>
        </receiver>

        <!-- Intent received used to uninstall shortcuts from other applications -->
        <receiver
            android:name="com.android.launcher3.UninstallShortcutReceiver"
            android:permission="com.android.launcher3.permission.UNINSTALL_SHORTCUT">
            <intent-filter>
                <action android:name="com.android.launcher3.action.UNINSTALL_SHORTCUT" />
            </intent-filter>
        </receiver>

        <!-- New user initialization; set up initial wallpaper -->
        <receiver
            android:name="com.android.launcher3.UserInitializeReceiver"
            android:exported="false">
            <intent-filter>
                <action android:name="android.intent.action.USER_INITIALIZE" />
            </intent-filter>
        </receiver>

        <receiver android:name="com.android.launcher3.PackageChangedReceiver" >
            <intent-filter>
                <action android:name="android.intent.action.PACKAGE_CHANGED"/>
                <action android:name="android.intent.action.PACKAGE_REPLACeD"/>
                <action android:name="android.intent.action.PACKAGE_REMOVED"/>
                <data android:scheme="package"></data>
            </intent-filter>
        </receiver>

        <!-- The settings provider contains Home's data, like the workspace favorites -->
        <provider
            android:name="com.android.launcher3.LauncherProvider"
            android:authorities="com.android.launcher3.settings"
            android:exported="true"
            android:writePermission="com.android.launcher3.permission.WRITE_SETTINGS"
            android:readPermission="com.android.launcher3.permission.READ_SETTINGS" />

        <meta-data android:name="android.nfc.disable_beam_default"
                       android:value="true" />
    </application>
</manifest>

再看看frameworks\base\services\java\com\android\server\pm\PackageManagerService.java：

 if (pkg.packageName.equals("android")) {
            synchronized (mPackages) {
                if (mAndroidApplication != null) {
                    Slog.w(TAG, "*************************************************");
                    Slog.w(TAG, "Core android package being redefined.  Skipping.");
                    Slog.w(TAG, " file=" + mScanningPath);
                    Slog.w(TAG, "*************************************************");
                    mLastScanError = PackageManager.INSTALL_FAILED_DUPLICATE_PACKAGE;
                    return null;
                }

                // Set up information for our fall-back user intent resolution activity.
                mPlatformPackage = pkg;
                pkg.mVersionCode = mSdkVersion;
                mAndroidApplication = pkg.applicationInfo;

                if (!mResolverReplaced) {
                    mResolveActivity.applicationInfo = mAndroidApplication;
                    mResolveActivity.name = ResolverActivity.class.getName();
                    mResolveActivity.packageName = mAndroidApplication.packageName;
                    mResolveActivity.processName = "system:ui";
                    mResolveActivity.launchMode = ActivityInfo.LAUNCH_MULTIPLE;
                    mResolveActivity.flags = ActivityInfo.FLAG_EXCLUDE_FROM_RECENTS;
                    mResolveActivity.theme = com.android.internal.R.style.Theme_Holo_Dialog_Alert;
                    mResolveActivity.exported = true;
                    mResolveActivity.enabled = true;
                    mResolveInfo.activityInfo = mResolveActivity;
                    mResolveInfo.priority = 0;
                    mResolveInfo.preferredOrder = 0;
                    mResolveInfo.match = 0;
                    mResolveComponentName = new ComponentName(
                            mAndroidApplication.packageName, mResolveActivity.name);
                }
            }
        }

这里是存储Launcher信息的，大家可以看到Launcher在system_server进程中开了个子进程"system:ui"进程，感兴趣的可以下载Android系统源码看完整源码详解，这里就先讲到这。