Android 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/14223493
String 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。
@Override
public 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Applicaiton
Object 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$AppBindData",
mBoundApplication, "info");
//把当前进程的mApplication 设置成了null
RefInvoke.setFieldOjbect("android.app.LoadedApk", "mApplication",
loadedApkInfo, null);
Object oldApplication = RefInvoke.getFieldOjbect(
"android.app.ActivityThread", currentActivityThread,
"mInitialApplication");
//http://www.codeceo.com/article/android-context.html
ArrayList mAllApplications = (ArrayList) RefInvoke
.getFieldOjbect("android.app.ActivityThread",
currentActivityThread, "mAllApplications");
mAllApplications.remove(oldApplication);//删除oldApplication
ApplicationInfo appinfo_In_LoadedApk = (ApplicationInfo) RefInvoke
.getFieldOjbect("android.app.LoadedApk", loadedApkInfo,
"mApplicationInfo");
ApplicationInfo appinfo_In_AppBindData = (ApplicationInfo) RefInvoke
.getFieldOjbect("android.app.ActivityThread$AppBindData",
mBoundApplication, "appInfo");
appinfo_In_LoadedApk.className = appClassName;
appinfo_In_AppBindData.className = appClassName;
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.html
ArrayList 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。