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android系统Context初始化过程

1、system context的初始化过程 1.1 SystemServer.createSystemContext()

private void createSystemContext() { ActivityThread activityThread = ActivityThread.systemMain(); mSystemContext = activityThread.getSystemContext(); mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar); }

ActivityThread管理着一个应用进程的主线程，用来调度和执行运行在该进程中的Activities，Broadcasrs以及其他的相关操作。在android中，每个应用都运行在一个独立的进程中，在这个进程中至少含有一个主线程，这个主线程由ActivityThread来管理,ActivityThread本身运行在主线程中。
1.2 ActivityThread.systemMain()
public static ActivityThread systemMain() { // The system process on low-memory devices do not get to use hardware // accelerated drawing, since this can add too much overhead to the // process. if (!ActivityManager.isHighEndGfx()) {// 对于低内存的设备，直接关闭硬件加速功能 HardwareRenderer.disable(true); } else { HardwareRenderer.enableForegroundTrimming(); } // 创建ActivityThread 实例 ActivityThread thread = new ActivityThread(); thread.attach(true); return thread; }

这个方法主要作用：硬件加速的开关，ActivityThread的初始化。
1.3 ActivityThread.attach()
private void attach(boolean system) { sCurrentActivityThread = this; mSystemThread = system; if (!system) {// 非系统应用 ViewRootImpl.addFirstDrawHandler(new Runnable() { @Override public void run() { ensureJitEnabled(); } }); // DDMS中显示的进程名字 android.ddm.DdmHandleAppName.setAppName("
", UserHandle.myUserId()); RuntimeInit.setApplicationObject(mAppThread.asBinder()); // 得到ActivityManagerService对象 final IActivityManager mgr = ActivityManagerNative.getDefault(); try {// mAppThread是一个ApplicationThread mgr.attachApplication(mAppThread); } catch (RemoteException ex) { // Ignore } // Watch for getting close to heap limit. // DC回收机制:当应用消耗的内存大于总内存的75%的时候，就要开始进行资源的回收了 BinderInternal.addGcWatcher(new Runnable() { @Override public void run() { if (!mSomeActivitiesChanged) { return; } Runtime runtime = Runtime.getRuntime(); long dalvikMax = runtime.maxMemory(); long dalvikUsed = runtime.totalMemory() - runtime.freeMemory(); if (dalvikUsed > ((3*dalvikMax)/4)) { if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024) + " total=" + (runtime.totalMemory()/1024) + " used=" + (dalvikUsed/1024)); mSomeActivitiesChanged = false; try { mgr.releaseSomeActivities(mAppThread); } catch (RemoteException e) { } } } }); } else {//系统应用 // Don't set application object here -- if the system crashes, // we can't display an alert, we just want to die die die. // 显示在DDMS中的进程名 android.ddm.DdmHandleAppName.setAppName("system_process", UserHandle.myUserId()); try { // 初始化Instrumentation mInstrumentation = new Instrumentation(); // 生成一个Context,是一个ContextImpl ContextImpl context = ContextImpl.createAppContext( this, getSystemContext().mPackageInfo); // Application初始化 mInitialApplication = context.mPackageInfo.makeApplication(true, null); // 执行Application.onCreate() mInitialApplication.onCreate(); } catch (Exception e) { throw new RuntimeException( "Unable to instantiate Application():" + e.toString(), e); } }// add dropbox logging to libcore DropBox.setReporter(new DropBoxReporter()); // 应用对内存的管理 ViewRootImpl.addConfigCallback(new ComponentCallbacks2() { @Override public void onConfigurationChanged(Configuration newConfig) { synchronized (mResourcesManager) { // We need to apply this change to the resources // immediately, because upon returning the view // hierarchy will be informed about it. if (mResourcesManager.applyConfigurationToResourcesLocked(newConfig, null)) { // This actually changed the resources!Tell // everyone about it. if (mPendingConfiguration == null || mPendingConfiguration.isOtherSeqNewer(newConfig)) { mPendingConfiguration = newConfig; sendMessage(H.CONFIGURATION_CHANGED, newConfig); } } } } @Override public void onLowMemory() { } @Override public void onTrimMemory(int level) { } }); }

因为systemMain传过来的参数为true，因而这里的attach主要作用是初始化Instrumentation，创建Context(ContextImpl),然后通过Instrumentation创建Application并执行Application.onCreate().
这里先来理清下面几个类的作用：Instrumentation，ContextImpl ，Application，**LoadedApk **
Instrumentation：对于一个应用进程，该类会优先被创建出来，然后通过他来创建其他组件；另外，它还是系统与组件交互的桥梁，因而通过他可以监听组件和系统之间的各种交互了。
**Context ** ： 是一个接口，通过他能够访问整个应用的全局信息如资源了类了等等，它还可以启动activity、broadcast以及接收intents。
Application：保存了整个应用的状态。
**LoadedApk **：保存了整个APK的相关信息。
1.4 ActivityThread.getSystemContext()
public ContextImpl getSystemContext() { synchronized (this) { if (mSystemContext == null) { mSystemContext = ContextImpl.createSystemContext(this); } return mSystemContext; } }

1.5 ContextImpl.createSystemContext()
static ContextImpl createSystemContext(ActivityThread mainThread) { // LoadedApk初始化 LoadedApk packageInfo = new LoadedApk(mainThread); // ContextImpl初始化 ContextImpl context = new ContextImpl(null, mainThread, packageInfo, null, null, false, null, null, Display.INVALID_DISPLAY); // 资源信息相关 context.mResources.updateConfiguration(context.mResourcesManager.getConfiguration(), context.mResourcesManager.getDisplayMetricsLocked()); return context; }

1.6 系统APK(framework-res.apk)对应的LoadedApk初始化
/** * Create information about the system package. *system package : framework-res.apk,packagename为android * Must call {@link #installSystemApplicationInfo} later. */ LoadedApk(ActivityThread activityThread) { mActivityThread = activityThread; mApplicationInfo = new ApplicationInfo(); // packageName为"android",这个APK为framework-res.apk mApplicationInfo.packageName = "android"; mPackageName = "android"; mAppDir = null; mResDir = null; mSplitAppDirs = null; mSplitResDirs = null; mOverlayDirs = null; mSharedLibraries = null; mDataDir = null; mDataDirFile = null; mLibDir = null; mBaseClassLoader = null; mSecurityViolation = false; mIncludeCode = true; mRegisterPackage = false; mClassLoader = ClassLoader.getSystemClassLoader(); mResources = Resources.getSystem(); }

1.7 ContextImpl初始化
/** *ActivityThread mainThread = mainThread *LoadedApk packageInfo = packageInfo *boolean restricted = false *int createDisplayWithId = Display.INVALID_DISPLAY */ private ContextImpl(ContextImpl container, ActivityThread mainThread, LoadedApk packageInfo, IBinder activityToken, UserHandle user, boolean restricted, Display display, Configuration overrideConfiguration, int createDisplayWithId) { mOuterContext = this; mMainThread = mainThread; // ActivityThread赋值 mActivityToken = activityToken; mRestricted = restricted; if (user == null) { user = Process.myUserHandle(); } mUser = user; mPackageInfo = packageInfo; // LoadedApk赋值 mResourcesManager = ResourcesManager.getInstance(); final int displayId = (createDisplayWithId != Display.INVALID_DISPLAY) ? createDisplayWithId : (display != null) ? display.getDisplayId() : Display.DEFAULT_DISPLAY; CompatibilityInfo compatInfo = null; if (container != null) { compatInfo = container.getDisplayAdjustments(displayId).getCompatibilityInfo(); } if (compatInfo == null) { compatInfo = (displayId == Display.DEFAULT_DISPLAY) ? packageInfo.getCompatibilityInfo() : CompatibilityInfo.DEFAULT_COMPATIBILITY_INFO; } mDisplayAdjustments.setCompatibilityInfo(compatInfo); mDisplayAdjustments.setConfiguration(overrideConfiguration); mDisplay = (createDisplayWithId == Display.INVALID_DISPLAY) ? display : ResourcesManager.getInstance().getAdjustedDisplay(displayId, mDisplayAdjustments); // resources初始化 Resources resources = packageInfo.getResources(mainThread); if (resources != null) { if (displayId != Display.DEFAULT_DISPLAY || overrideConfiguration != null || (compatInfo != null && compatInfo.applicationScale != resources.getCompatibilityInfo().applicationScale)) { resources = mResourcesManager.getTopLevelResources(packageInfo.getResDir(), packageInfo.getSplitResDirs(), packageInfo.getOverlayDirs(), packageInfo.getApplicationInfo().sharedLibraryFiles, displayId, overrideConfiguration, compatInfo); } } mResources = resources; // 赋值if (container != null) { mBasePackageName = container.mBasePackageName; mOpPackageName = container.mOpPackageName; } else { mBasePackageName = packageInfo.mPackageName; ApplicationInfo ainfo = packageInfo.getApplicationInfo(); if (ainfo.uid == Process.SYSTEM_UID && ainfo.uid != Process.myUid()) { // Special case: system components allow themselves to be loaded in to other // processes.For purposes of app ops, we must then consider the context as // belonging to the package of this process, not the system itself, otherwise // the package+uid verifications in app ops will fail. mOpPackageName = ActivityThread.currentPackageName(); } else { mOpPackageName = mBasePackageName; } }mContentResolver = new ApplicationContentResolver(this, mainThread, user); }

主要是相关变量初始化，涉及的相关类关系如下：
android系统Context初始化过程
文章图片
QQ截图20160804185935.png 【android系统Context初始化过程】SystemServer.createSystemContext()分析到这里就基本结束了，从这里可以看到，SystemServer.createSystemContext()主要是创建了ActivityThread，获取了Context，这个Context是系统进程（framework-res.apk）运行的环境。另外，从代码中也可以看到:
1)ActivityThread中用了一个集合来保存Application，说明一个进程里面可以包含多个Application;
具体实现：在APK的AndroidManifext.xml中配置标签的android:sharedUserId和配置标签的android:process相同即可，这样配置之后，这些APK就能够运行在同一个进程，实现数据共享；
2)Application是在LoadedApk中通过Intrumentation创建出来的(LoadedApk.makeApplication()-->Instrumentation.newApplication()-->Application.attach())，Application通过attach()方法持有LoadedApk对象，Application和LoadedApk是一一对应的关系。

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