基于之前对插件化的了解，大部分的插件对于资源都是合并式的，通过addAssetPath将插件的资源加入到assetManager中，这种方式会带来一个问题:resource id冲突。当然目前已经有方案去解决:aapt或者修改arsc文件。
VirtualApp中可以安装任意的第三方APK，如果采用合并式资源加载方案，那肯定也会有resource id冲突问题，但上面说的解决方案都需要第三方APK去配合修改，明显不能满足任意安装。

VirtualApp采用的是独立式资源加载方案，这样就不会有resource id冲突的问题。
在分析VirtualApp是怎么做到独立式资源加载前，需要先了解一下应用获取resources大概的流程。在自定义Application中，我们可以通过getResources去获取当前的资源，下面看看getResources调用流程：

最终会走到ContextWrapper中：

    public Resources getResources() {return mBase.getResources();}

发现是从mBase中拿到resources,那mBase从哪里来？也是在ContextWrapper类中：

 /*** Set the base context for this ContextWrapper.  All calls will then be* delegated to the base context.  Throws* IllegalStateException if a base context has already been set.* * @param base The new base context for this wrapper.*/protected void attachBaseContext(Context base) {if (mBase != null) {throw new IllegalStateException("Base context already set");}mBase = base;}

ContextWrapper的attachBaseContext方法是在哪里被调用的呢？base context几时被创建的？

具体调用流程分析参考：Android应用程序窗口（Activity）的运行上下文环境（Context）的创建过程分析
基于上面调用时序图，看看上述几个方法的实现
LoadedApk.makeApplication方法：

public Application makeApplication(boolean forceDefaultAppClass,Instrumentation instrumentation) {ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);app = mActivityThread.mInstrumentation.newApplication(cl, appClass, appContext);}

Instrumentation newApplication方法：

 public Application newApplication(ClassLoader cl, String className, Context context)throws InstantiationException, IllegalAccessException, ClassNotFoundException {return newApplication(cl.loadClass(className), context);}static public Application newApplication(Class<?> clazz, Context context)throws InstantiationException, IllegalAccessException, ClassNotFoundException {Application app = (Application)clazz.newInstance();app.attach(context);return app;}

Application attach方法：

/* package */ final void attach(Context context) {attachBaseContext(context);mLoadedApk = ContextImpl.getImpl(context).mPackageInfo;}

最终发现原来ContextWrapper 中的mBase是在LoadedApk.makeApplication创建的，

ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);

attachBaseContext方法中的base参数其实ContextImpl的实例。
现在转移到分析ContextImpl类
看看ContextImpl.createAppContext方法：

  static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo) {if (packageInfo == null) throw new IllegalArgumentException("packageInfo");return new ContextImpl(null, mainThread,packageInfo, null, null, 0, null, null, Display.INVALID_DISPLAY);}private ContextImpl(ContextImpl container, ActivityThread mainThread,LoadedApk packageInfo, IBinder activityToken, UserHandle user, int flags,Display display, Configuration overrideConfiguration, int createDisplayWithId) {mOuterContext = this;Resources resources = packageInfo.getResources(mainThread);mResources = resources;}public Resources getResources() {return mResources;}

发现resources来自packageInfo，packageInfo是LoadedApk的一个实例，
packageInfo是在ActivityThread的handleBindApplication方法中被创建的

 private void handleBindApplication(AppBindData data) {data.info = getPackageInfoNoCheck(data.appInfo, data.compatInfo);}public final LoadedApk getPackageInfoNoCheck(ApplicationInfo ai,CompatibilityInfo compatInfo) {return getPackageInfo(ai, compatInfo, null, false, true, false);}

来看看LoadedApk中的getResources方法是怎么实现的：

    public Resources getResources(ActivityThread mainThread) {if (mResources == null) {mResources = mainThread.getTopLevelResources(mResDir, mSplitResDirs, mOverlayDirs,mApplicationInfo.sharedLibraryFiles, Display.DEFAULT_DISPLAY, this);}return mResources;}public LoadedApk(ActivityThread activityThread, ApplicationInfo aInfo,CompatibilityInfo compatInfo, ClassLoader baseLoader,boolean securityViolation, boolean includeCode, boolean registerPackage) {setApplicationInfo(aInfo);}private void setApplicationInfo(ApplicationInfo aInfo) {final int myUid = Process.myUid();mAppDir = aInfo.sourceDir;mResDir = aInfo.uid == myUid ? aInfo.sourceDir : aInfo.publicSourceDir;mSplitAppDirs = aInfo.splitSourceDirs;mSplitResDirs = aInfo.uid == myUid ? aInfo.splitSourceDirs : aInfo.splitPublicSourceDirs;mOverlayDirs = aInfo.resourceDirs;mSharedLibraries = aInfo.sharedLibraryFiles;mDataDir = aInfo.dataDir;mLibDir = aInfo.nativeLibraryDir;}

最终resources是调用ActivityThread.getTopLevelResources方法创建的：

    /*** Creates the top level resources for the given package. Will return an existing* Resources if one has already been created.*/Resources getTopLevelResources(String resDir, String[] splitResDirs, String[] overlayDirs,String[] libDirs, int displayId, LoadedApk pkgInfo) {return mResourcesManager.getResources(null, resDir, splitResDirs, overlayDirs, libDirs,displayId, null, pkgInfo.getCompatibilityInfo(), pkgInfo.getClassLoader());}

ResourceManager根据APK的相关路径信息，读取到资源，最终生成Resources。

小结一下：Application调用getResources方法，最终获取到的是LoadedApk对象中根据APK相关路径创建的Resources对象。LoadedApk对象最早的创建是在ActivityThread.handleBindApplication时。
handleBindApplication是在绑定自定义Application时被调用。

分析了半天和VirtualApp的独立式资源加载方案有啥关系？？？
看段VirtualApp的代码

private AppBindData mBoundApplication;private final class AppBindData {String processName;ApplicationInfo appInfo;List<ProviderInfo> providers;Object info;}private void bindApplicationNoCheck(String packageName, String processName, ConditionVariable lock) {AppBindData data = new AppBindData();mBoundApplication = data;Context context = createPackageContext(data.appInfo.packageName);mBoundApplication.info = ContextImpl.mPackageInfo.get(context);mInitialApplication = LoadedApk.makeApplication.call(data.info, false, null);mirror.android.app.ActivityThread.mInitialApplication.set(mainThread, mInitialApplication);}

VirtualApp在运行第三方App时，会执行bindApplicationNoCheck去将第三方自定义Application绑定起来，

mInitialApplication = LoadedApk.makeApplication.call(data.info, false, null);

 mBoundApplication = data;Context context = createPackageContext(data.appInfo.packageName);mBoundApplication.info = ContextImpl.mPackageInfo.get(context);

   @Overridepublic Context createPackageContext(String packageName, int flags)throws NameNotFoundException {return createPackageContextAsUser(packageName, flags,mUser != null ? mUser : Process.myUserHandle());}@Overridepublic Context createPackageContextAsUser(String packageName, int flags, UserHandle user)throws NameNotFoundException {LoadedApk pi = mMainThread.getPackageInfo(packageName, mResources.getCompatibilityInfo(),flags | CONTEXT_REGISTER_PACKAGE, user.getIdentifier());if (pi != null) {ContextImpl c = new ContextImpl(this, mMainThread, pi, mActivityToken,user, flags, mDisplay, null, Display.INVALID_DISPLAY);if (c.mResources != null) {return c;}}}