安卓系统启动流程解析

一直以来一直想有深入研究源码的想法，最近一段时间工作比较少，得以时间研究，有点心得就给大家分享，今天给大家带来的是安卓系统最开始的  

话不多说  首先放图

下面来详解这张图  

从系统的角度上来讲，Android系统的启动过程可以分为 bootloader 引导，装载和启动 linux内核 启动Android系统

BootLoader

bootloader 相当于电脑上的Bios 他的主要作用就是初始化基本的硬件设备，建立内存空间映射， 为装载linux内核准备好运行环境，当linux内核加载完毕之后，bootloder就会从内存中清除

对于FastBoot和Recover估计好多童鞋都不理解，fastboot是Android设计的一套通过usb来更新手机分区的映像协议，不过大部分厂商都搞掉了  google的nexus 上应该有的

Recovery模式是Android特有的升级系统，通过这个可以进行手机恢复出厂设置，或执行OTA,补丁和固件升级，实质是启动了一个文本模式的Linux。

bootloader启动后会向内存中装载boot.img镜像文件，这个镜像文件存放的是linux内核和一个根文件系统，linux内核进行初始化之后，装载完文件系统，就启动了init进程

Init进程

init进程是Linux创建的第一个进程，init进程会解析linux的脚本文件init.rc，根据这个文件的内容 init进程会装载Android的文件系统，创建系统目录，初始化属性系统，启动Android系统的重要的守护进程等，如上图，

下面简单的介绍一个下init进程fork出的几个重要的进程：

Zygote进程： 当init进程初始化结束的时候，会启动Zygote进程，看上图可以看出来 zygote进程扶着fork出应用进程，是所有进程的父进程，Zygote进程初始化的时候会创建Dalivik虚拟机，预装载系统的资源文件和java类，所有从Zygote进程fork出的用户进程将继承和共享这些资源，不用浪费时间重新加载（看到这里是不是感觉突然明白了什么？）干完这些之后，Zygote进程也将变为守护进程，负责响应启动APK应用的启动请求。

SystemServer进程：SystemServer进程是Zygote进程fork出的第一个进程，也是整个Android系统的核心进程，在SystemServer主要运行的是Binder服务，SystemServer首要启动本地服务 SensorService 接着启动 ActivityManagerService，WindowsManagerService，PackgeManagerService等在内的所有java服务。

MediaServer进程：也是由init进程启动，他包含了一些多媒体相关的本地Binder服务，包括CameraService，AudioFilingerService ，和AudioPolicyService。

当SystemServer加载完所有Java服务后最后会调用ActivityManagerService的SystemReady()方法，

        // 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);                }                try {                    if (networkScoreF != null) networkScoreF.systemReady();                } catch (Throwable e) {                    reportWtf("making Network Score Service ready", e);                }                try {                    if (networkManagementF != null) networkManagementF.systemReady();                } catch (Throwable e) {                    reportWtf("making Network Managment Service ready", e);                }                try {                    if (networkStatsF != null) networkStatsF.systemReady();                } catch (Throwable e) {                    reportWtf("making Network Stats Service ready", e);                }                try {                    if (networkPolicyF != null) networkPolicyF.systemReady();                } catch (Throwable e) {                    reportWtf("making Network Policy Service ready", e);                }                try {                    if (connectivityF != null) connectivityF.systemReady();                } catch (Throwable e) {                    reportWtf("making Connectivity Service ready", e);                }                try {                    if (audioServiceF != null) audioServiceF.systemReady();                } catch (Throwable e) {                    reportWtf("Notifying AudioService running", e);                }                Watchdog.getInstance().start();                // It is now okay to let the various system services start their                // third party code...                mSystemServiceManager.startBootPhase(                        SystemService.PHASE_THIRD_PARTY_APPS_CAN_START);                try {                    if (wallpaperF != null) wallpaperF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying WallpaperService running", e);                }                try {                    if (immF != null) immF.systemRunning(statusBarF);                } catch (Throwable e) {                    reportWtf("Notifying InputMethodService running", e);                }                try {                    if (locationF != null) locationF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying Location Service running", e);                }                try {                    if (countryDetectorF != null) countryDetectorF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying CountryDetectorService running", e);                }                try {                    if (networkTimeUpdaterF != null) networkTimeUpdaterF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying NetworkTimeService running", e);                }                try {                    if (commonTimeMgmtServiceF != null) {                        commonTimeMgmtServiceF.systemRunning();                    }                } catch (Throwable e) {                    reportWtf("Notifying CommonTimeManagementService running", e);                }                try {                    if (textServiceManagerServiceF != null)                        textServiceManagerServiceF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying TextServicesManagerService running", e);                }                try {                    if (atlasF != null) atlasF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying AssetAtlasService running", e);                }                try {                    // TODO(BT) Pass parameter to input manager                    if (inputManagerF != null) inputManagerF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying InputManagerService running", e);                }                try {                    if (telephonyRegistryF != null) telephonyRegistryF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying TelephonyRegistry running", e);                }                try {                    if (mediaRouterF != null) mediaRouterF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying MediaRouterService running", e);                }                try {                    if (mmsServiceF != null) mmsServiceF.systemRunning();                } catch (Throwable e) {                    reportWtf("Notifying MmsService running", e);                }            }        });

从上面的注释我们可以得出他是去打开lancher   具体的代码在这里

public void systemReady(final Runnable goingCallback) {        synchronized(this) {            if (mSystemReady) {                // If we're done calling all the receivers, run the next "boot phase" passed in                // by the SystemServer                if (goingCallback != null) {                    goingCallback.run();                }                return;            }            // Make sure we have the current profile info, since it is needed for            // security checks.            updateCurrentProfileIdsLocked();            .................................... 此处省略大量代码            // Start up initial activity.            mBooting = true;            <strong><u><span style="color:#000066;background-color: rgb(204, 204, 204);">startHomeActivityLocked(mCurrentUserId, "systemReady");</span></u></strong>            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);                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,                        INTERACT_ACROSS_USERS, AppOpsManager.OP_NONE,                        true, false, MY_PID, Process.SYSTEM_UID, UserHandle.USER_ALL);            } catch (Throwable t) {                Slog.wtf(TAG, "Failed sending first user broadcasts", t);            } finally {                Binder.restoreCallingIdentity(ident);            }            mStackSupervisor.resumeTopActivitiesLocked();            sendUserSwitchBroadcastsLocked(-1, mCurrentUserId);        }    }

这个里面最重要的就是标出的那一行，调用的方法是源码是在这里

  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;        }       <strong style="background-color: rgb(192, 192, 192);"> Intent intent = getHomeIntent();</strong>        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, reason);            }        }        return true;    }

看到这里大家明白了吧，这个getHomeIntent() 就是打开launcher应用的Inent

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;    }

其实在上一段代码中还要去PackageManager去取一下所有类型为ACTION_MAIN的intent 然后才开的Launcher。 

大致的流程就这些，中间有些部分会再以后的博文里接着分析，如果看到有什么错误的地方，都可以在下面评论中说出。希望我们共同进步！