Android(安卓)Apk加壳原理分析
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阅读本文前，建议读者首先阅读Android加壳原理，参考文章Android中的Apk的加固(加壳)原理解析和实现。如果没有看过这篇文章，本文理解起来比较困难。
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下面我们来分析脱壳代码为什么要这样写，核心脱壳代码在ProxyApplication类里面，首先执行成员方法attachBaseContext，然后执行成员方法onCreate。
那么attachBaseContext是什么时候被执行的呢，为什么先于onCreate执行呢？那就需要看Android的源码了，我们选用的是Android2.3源码。
我们首先看一张图，这张图表述了从桌面启动一个应用Activity的启动过程。
图 1
其中当执行到ApplicationThread.bindApplication时，会向ActivityThreadl类的Handler对象mH发送消息。
public final void bindApplication(String processName, ApplicationInfo appInfo, List providers, ComponentName instrumentationName, String profileFile, Bundle instrumentationArgs, IInstrumentationWatcher instrumentationWatcher, int debugMode, boolean isRestrictedBackupMode, Configuration config, Map services) { if (services != null) { // Setup the service cache in the ServiceManager ServiceManager.initServiceCache(services); } AppBindData data = new AppBindData(); data.processName = processName; data.appInfo = appInfo; data.providers = providers; data.instrumentationName = instrumentationName; data.profileFile = profileFile; data.instrumentationArgs = instrumentationArgs; data.instrumentationWatcher = instrumentationWatcher; data.debugMode = debugMode; data.restrictedBackupMode = isRestrictedBackupMode; data.config = config; queueOrSendMessage(H.BIND_APPLICATION, data); } queueOrSendMessage向ActivityThreadl类的Handler对象mH发送消息。
private final void queueOrSendMessage(int what, Object obj, int arg1, int arg2) { synchronized (this) { if (DEBUG_MESSAGES) Slog.v( TAG, "SCHEDULE " + what + " " + mH.codeToString(what) + ": " + arg1 + " / " + obj); Message msg = Message.obtain(); msg.what = what; msg.obj = obj; msg.arg1 = arg1; msg.arg2 = arg2; mH.sendMessage(msg); } }handler处理BIND_APPLICATION的流程如下。
private final class H extends Handler {...... } public void handleMessage(Message msg) { if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + msg.what); switch (msg.what) {...... case BIND_APPLICATION: AppBindData data = (AppBindData)msg.obj; handleBindApplication(data); break;...... } } 继续看handleBindApplication，其中data就是ApplicationThread.bindApplication生成的AppBindData对象。
private final void handleBindApplication(AppBindData data) {mBoundApplication = data;...... data.info = getPackageInfoNoCheck(data.appInfo);...... Application app = data.info.makeApplication(data.restrictedBackupMode, null); mInitialApplication = app;...... try { mInstrumentation.callApplicationOnCreate(app); } catch (Exception e) { if (!mInstrumentation.onException(app, e)) { throw new RuntimeException( "Unable to create application " + app.getClass().getName() + ": " + e.toString(), e); } } }首先把data赋值给了AppBindData对象mBoundApplication，然后通过getPackageInfoNoCheck得到的LoadedApk对象复制给data.info，之后调用data.info.makeApplication生成Application对象，我们下面来分析下data.info.makeApplication这个方法。
public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) { if (mApplication != null) { return mApplication; } Application app = null; String appClass = mApplicationInfo.className; if (forceDefaultAppClass || (appClass == null)) { appClass = "android.app.Application"; } try { java.lang.ClassLoader cl = getClassLoader(); ContextImpl appContext = new ContextImpl(); appContext.init(this, null, mActivityThread); app = mActivityThread.mInstrumentation.newApplication( cl, appClass, appContext); appContext.setOuterContext(app); } catch (Exception e) { if (!mActivityThread.mInstrumentation.onException(app, e)) { throw new RuntimeException( "Unable to instantiate application " + appClass + ": " + e.toString(), e); } } mActivityThread.mAllApplications.add(app); mApplication = app;...... return app; }首先通过mApplicationInfo.className获取application的名字，在本例中是ProxyApplication。然后通过getClassLoader获取了ClassLoader对象，那么我先来分析下getClassLoader的实现。
public ClassLoader getClassLoader() { synchronized (this) { if (mClassLoader != null) { return mClassLoader; }...... }返回到makeApplication，继续看，首先生成了ContextImpl对象，最终调用了mActivityThread.mInstrumentation.newApplication来生成Application对象，并把生成的Context对象放到这个Application对象中。这部分可参考博客Android中Context详解 ---- 你所不知道的Context。再附一张Context类图，帮大家理解。
那么我们讲了这么多，到底是什么时候执行的ProxyApplication类的方法attachBaseContext的呢？答案就在mActivityThread.mInstrumentation.newApplication，我们继续分析此方法。
public Application newApplication(ClassLoader cl, String className, Context context) throws InstantiationException, IllegalAccessException, ClassNotFoundException { return newApplication(cl.loadClass(className), context); }其中context对象就是我们刚刚生成的，继续分析newApplication方法。
static public Application newApplication(Class<?> clazz, Context context) throws InstantiationException, IllegalAccessException, ClassNotFoundException { Application app = (Application)clazz.newInstance(); app.attach(context); return app; }这个函数首先生成了一个Application对象app，然后调用了他的方法attach，我们来分析这个方法。
final void attach(Context context) { attachBaseContext(context); }答案在此揭晓，此时调用了ProxyApplication类的方法attachBaseContext，注意此时还没有调用ProxyApplication类的方法onCreate。
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知道了执行到ProxyApplication类的方法attachBaseContext之前的流程，我们接下来重点分析下这个方法。
protected void attachBaseContext(Context base) {super.attachBaseContext(base);try {......// 配置动态加载环境Object currentActivityThread = RefInvoke.invokeStaticMethod("android.app.ActivityThread", "currentActivityThread",new Class[] {}, new Object[] {});//获取主线程对象 http://blog.csdn.net/myarrow/article/details/14223493String packageName = this.getPackageName();//当前apk的包名//下面两句不是太理解HashMap mPackages = (HashMap) RefInvoke.getFieldOjbect("android.app.ActivityThread", currentActivityThread,"mPackages");WeakReference wr = (WeakReference) mPackages.get(packageName);//创建被加壳apk的DexClassLoader对象 加载apk内的类和本地代码（c/c++代码）DexClassLoader dLoader = new DexClassLoader(apkFileName, odexPath,libPath, (ClassLoader) RefInvoke.getFieldOjbect("android.app.LoadedApk", wr.get(), "mClassLoader"));//base.getClassLoader(); 是不是就等同于 (ClassLoader) RefInvoke.getFieldOjbect()? 有空验证下//?//把当前进程的DexClassLoader 设置成了被加壳apk的DexClassLoader ----有点c++中进程环境的意思~~RefInvoke.setFieldOjbect("android.app.LoadedApk", "mClassLoader",wr.get(), dLoader);Log.i("demo","classloader:"+dLoader);......} catch (Exception e) {Log.i("demo", "error:"+Log.getStackTraceString(e));e.printStackTrace();}}省略部分的代码还请大家参考Android中的Apk的加固(加壳)原理解析和实现。
首先通过反射调用了ActivityThread类的currentActivityThread方法，该方法是静态的，返回当前的ActivityThread，代码如下：
public static final ActivityThread currentActivityThread() { return sThreadLocal.get(); }然后再获取ActivityThread的成员变量mPackages，mPackages也位于frameworks\base\core\java\android\app\ActivityThread.java中：
final HashMap> mPackages = new HashMap>(); 然后根据要加载的apk，也就是实际要执行的apk，生成DexClassLoader对象，其中parentClassLoader就是刚刚获取的LoadedApk对象中的mClassLoader变量。
大家可能会有个疑问，这里获取的LoadedApk对象和data.info对象是一样的么？答案是一样的，代码的关键在handleBindApplication中getPackageInfoNoCheck，代码如下：
private final LoadedApk getPackageInfo(ApplicationInfo aInfo, ClassLoader baseLoader, boolean securityViolation, boolean includeCode) { synchronized (mPackages) { WeakReference ref; if (includeCode) { ref = mPackages.get(aInfo.packageName); } else { ref = mResourcePackages.get(aInfo.packageName); } LoadedApk packageInfo = ref != null ? ref.get() : null; if (packageInfo == null || (packageInfo.mResources != null && !packageInfo.mResources.getAssets().isUpToDate())) { if (localLOGV) Slog.v(TAG, (includeCode ? "Loading code package " : "Loading resource-only package ") + aInfo.packageName + " (in " + (mBoundApplication != null ? mBoundApplication.processName : null) + ")"); packageInfo = new LoadedApk(this, aInfo, this, baseLoader, securityViolation, includeCode && (aInfo.flags&ApplicationInfo.FLAG_HAS_CODE) != 0); if (includeCode) { mPackages.put(aInfo.packageName, new WeakReference(packageInfo)); } else { mResourcePackages.put(aInfo.packageName, new WeakReference(packageInfo)); } } return packageInfo; } } 这里生成了一个LoadedApk对象，并以当前包名为键，LoadedApk对象为值存入了mPackages这个HashMap中，并且返回LoadedApk对象，并赋值给data.info。
data.info = getPackageInfoNoCheck(data.appInfo);回到attachBaseContext中，最后把这个新生成DexClassLoader对象赋值给LoadedApk对象的mClassLoader变量，也就是更新了这个mClassLoader变量。
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执行完mActivityThread.mInstrumentation.newApplication，返回到makeApplication，继续执行下面两句代码：
mActivityThread.mAllApplications.add(app); mApplication = app;
mInitialApplication = app;0x04
执行完data.info.makeApplication，我们返回到handleBindApplication(代码请参考上面)，继续执行mInstrumentation.callApplicationOnCreate(app)，此时ProxyApplication类的onCreate方法开始执行。
@Overridepublic void onCreate() {{//loadResources(apkFileName);Log.i("demo", "onCreate");// 如果源应用配置有Appliction对象，则替换为源应用Applicaiton，以便不影响源程序逻辑。String appClassName = null;try {ApplicationInfo ai = this.getPackageManager().getApplicationInfo(this.getPackageName(),PackageManager.GET_META_DATA);Bundle bundle = ai.metaData;if (bundle != null && bundle.containsKey("APPLICATION_CLASS_NAME")) {appClassName = bundle.getString("APPLICATION_CLASS_NAME");//className 是配置在xml文件中的。} else {Log.i("demo", "have no application class name");return;}} catch (NameNotFoundException e) {Log.i("demo", "error:"+Log.getStackTraceString(e));e.printStackTrace();}//有值的话调用该ApplicaitonObject currentActivityThread = RefInvoke.invokeStaticMethod("android.app.ActivityThread", "currentActivityThread",new Class[] {}, new Object[] {});Object mBoundApplication = RefInvoke.getFieldOjbect("android.app.ActivityThread", currentActivityThread,"mBoundApplication");Object loadedApkInfo = RefInvoke.getFieldOjbect("android.app.ActivityThread$AppBindData",mBoundApplication, "info");//把当前进程的mApplication 设置成了nullRefInvoke.setFieldOjbect("android.app.LoadedApk", "mApplication",loadedApkInfo, null);Object oldApplication = RefInvoke.getFieldOjbect("android.app.ActivityThread", currentActivityThread,"mInitialApplication");//http://www.codeceo.com/article/android-context.htmlArrayList mAllApplications = (ArrayList) RefInvoke.getFieldOjbect("android.app.ActivityThread",currentActivityThread, "mAllApplications");mAllApplications.remove(oldApplication);//删除oldApplicationApplicationInfo appinfo_In_LoadedApk = (ApplicationInfo) RefInvoke.getFieldOjbect("android.app.LoadedApk", loadedApkInfo,"mApplicationInfo");ApplicationInfo appinfo_In_AppBindData = (ApplicationInfo) RefInvoke.getFieldOjbect("android.app.ActivityThread$AppBindData",mBoundApplication, "appInfo");appinfo_In_LoadedApk.className = appClassName;appinfo_In_AppBindData.className = appClassName;Application app = (Application) RefInvoke.invokeMethod("android.app.LoadedApk", "makeApplication", loadedApkInfo,new Class[] { boolean.class, Instrumentation.class },new Object[] { false, null });//执行 makeApplication（false,null）RefInvoke.setFieldOjbect("android.app.ActivityThread","mInitialApplication", currentActivityThread, app);......app.onCreate();}}首先appClassName为MyApplication，然后依然是通过反射调用了ActivityThread类的currentActivityThread方法，该方法是静态的，返回当前的ActivityThread对象。
再通过反射获取当前ActivityThread对象的mBoundApplication变量，这个mBoundApplication对象还记得么？是在handleBindApplication被赋值的。
private final void handleBindApplication(AppBindData data) {mBoundApplication = data;......}data.info = getPackageInfoNoCheck(data.appInfo);...... Application app = data.info.makeApplication(data.restrictedBackupMode, null);继续执行到如下函数：
Object oldApplication = RefInvoke.getFieldOjbect("android.app.ActivityThread", currentActivityThread,"mInitialApplication");//http://www.codeceo.com/article/android-context.htmlArrayList mAllApplications = (ArrayList) RefInvoke.getFieldOjbect("android.app.ActivityThread",currentActivityThread, "mAllApplications");mAllApplications.remove(oldApplication);//删除oldApplication
回到onCreate中继续看，我们先过掉几行代码，直接看makeApplication生成新的application对象。在解释这个对象的生成过程中，我们会讲解在生成此对象前的一些操作的意义。
既然要重新生成，那么我们首先看一下makeApplication的实现：
public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) { if (mApplication != null) { return mApplication; } Application app = null; String appClass = mApplicationInfo.className; if (forceDefaultAppClass || (appClass == null)) { appClass = "android.app.Application"; } try { java.lang.ClassLoader cl = getClassLoader(); ContextImpl appContext = new ContextImpl(); appContext.init(this, null, mActivityThread); app = mActivityThread.mInstrumentation.newApplication( cl, appClass, appContext); appContext.setOuterContext(app); } catch (Exception e) { if (!mActivityThread.mInstrumentation.onException(app, e)) { throw new RuntimeException( "Unable to instantiate application " + appClass + ": " + e.toString(), e); } } mActivityThread.mAllApplications.add(app); mApplication = app;...... return app; } 然后需要获取ApplicationInfo对象mApplicationInfo的成员变量className，因为现在我要启动是被加壳的apk中MyApplication，所以我们要把名字设置为MyApplication。这就是下面几行代码的作用。
ApplicationInfo appinfo_In_LoadedApk = (ApplicationInfo) RefInvoke.getFieldOjbect("android.app.LoadedApk", loadedApkInfo,"mApplicationInfo");ApplicationInfo appinfo_In_AppBindData = (ApplicationInfo) RefInvoke.getFieldOjbect("android.app.ActivityThread$AppBindData",mBoundApplication, "appInfo");appinfo_In_LoadedApk.className = appClassName;appinfo_In_AppBindData.className = appClassName;最后返回到onCreate方法中，把新生成的application对象赋值给ActivityThread对象的mInitialApplication变量。
对了，还有一点忘记说了，在makeApplication时，getClassLoader获得的ClassLoader对象，已经被替换为DexClassLoader对象，这个对象加载的是被加壳的apk。
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那么MyApplication类什么时候执行onCreate呢?答案在ProxyApplication类的onCreate方法最后，会调用app.onCreate()。
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那么什么时候开启MainActivtiy呢？怎么样开启的呢？
我们再一次看图1，从桌面启动一个应用Activity的启动过程，怎么开启的MainActivity呢？
private final void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) { ...... Activity a = performLaunchActivity(r, customIntent);......}private final Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")"); ActivityInfo aInfo = r.activityInfo; if (r.packageInfo == null) { r.packageInfo = getPackageInfo(aInfo.applicationInfo, Context.CONTEXT_INCLUDE_CODE); } ComponentName component = r.intent.getComponent(); if (component == null) { component = r.intent.resolveActivity( mInitialApplication.getPackageManager()); r.intent.setComponent(component); } if (r.activityInfo.targetActivity != null) { component = new ComponentName(r.activityInfo.packageName, r.activityInfo.targetActivity); } Activity activity = null; try { java.lang.ClassLoader cl = r.packageInfo.getClassLoader(); activity = mInstrumentation.newActivity( cl, component.getClassName(), r.intent); r.intent.setExtrasClassLoader(cl); if (r.state != null) { r.state.setClassLoader(cl); } } catch (Exception e) { if (!mInstrumentation.onException(activity, e)) { throw new RuntimeException( "Unable to instantiate activity " + component + ": " + e.toString(), e); } } try { ...... mInstrumentation.callActivityOnCreate(activity, r.state);} catch {} }代码位于frameworks\base\core\java\android\app\ActivityThread.java。
此时获取的Classloader对象已经被替换为DexClassLoader对象，这个对象加载的是被加壳的apk。
但是此时获取的activity信息为什么是MainActivity呢？答案在AndroidManifest.xml里面。
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