Activity插件化原理第二种方案：Hook IActivityManager

温馨提示：阅读本文需要20-30分钟（一大波代码） 关注公众号：顾林海

今天，我们来解决一个问题：

Activity插件化原理第二种方案：Hook IActivityManager

人生一切难题，知识给你答案。

第一种方案Hook Instrumentation已经顺利完成，接下来我们看第二个方案，还是看这段代码：

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            //Activity启动
            options = transferSpringboardActivityOptions(options);
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
            if (ar != null) {
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }
            if (requestCode >= 0) {
                mStartedActivity = true;
            }

            cancelInputsAndStartExitTransition(options);
            windows.
        } else {
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }

复制代码

通过mInstrumentation的execStartActivity方法启动Activity，在execStartActivity方法中会获取AMS的代理，Android 7.0通过ActivityManagerNative的getDefault方法获取一个ActivityManagerProxy，这个ActivityManagerProxy内部封装了IBinder类型的ActivityManagerService的代理类，这样在应用程序进程中就可以通过这个ActivityManagerProxy与SystemServer进程的ActivityManagerProxy进行通信，而在Android 8.0去除了ActivityManagerProxy这个代理类，由IActivityManager代替，这里的IActivityManager.aidl通过AIDL工具自动生成IActivityManager.java。

Android 7.0-Activity启动

ActivityManager是一个和AMS相关联的类，它主要对运行中的Activity进行管理，ActivityManager中相关管理方法最终会通过ActivityManagerNative的getDefault方法来得到ActivityManagerProxy，再调用ActivityManagerProxy的相关管理方法，ActivityManagerProxy就是AMS的代理类，通过这个代理类就可以和AMS进行通信。

Android7.0的Activity启动过程会调用Instrumentation的execStartActivity方法，代码如下：

public ActivityResult execStartActivity(
        ...
        try {
            ...
            int result = ActivityManagerNative.getDefault()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target, requestCode, 0, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }
复制代码

在execStartActivity方法中会调用ActivityManagerNative的getDefault方法来获取ActivityManagerProxy，ActivityManagerProxy又是AMS的代理类，这样的话就可以通过ActivityManagerProxy向AMS发送startActivity的请求。

ActivityManagerNative的getDefault方法的代码如下：

private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
        protected IActivityManager create() {
            IBinder b = ServiceManager.getService("activity");
            ...
            IActivityManager am = asInterface(b);
            ...
            return am;
        }
    };

    static public IActivityManager getDefault() {
        return gDefault.get();
    }
复制代码

在getDefault方法中调用了gDefault的get方法，gDefault是一个单例类。通过ServiceManager的getService方法获取一个IBinder类型的AMS的引用，再将它通过asInterface方法转换成ActivityManagerProxy类型的对象。

asInterface方法：

static public IActivityManager asInterface(IBinder obj) {
        if (obj == null) {
            return null;
        }
        //检查本地进程是否有IActivityManager接口的实现
        IActivityManager in =
            (IActivityManager)obj.queryLocalInterface(descriptor);
        if (in != null) {
            return in;
        }
        //本地进程没有IActivityManager接口的实现，将IBinder类型的AMS引用封装成AMP
        return new ActivityManagerProxy(obj);
    }
复制代码

在asInterface方法中分两种情况，首先会检查本地进程是否有IActivityManager接口的实现，如果有就直接返回；如果没有，就将IBinder类型的AMS引用封装成ActivityManagerProxy。

public abstract class ActivityManagerNative extends Binder implements IActivityManager{
    ...
    class ActivityManagerProxy implements IActivityManager{
        public ActivityManagerProxy(IBinder remote){
            mRemote = remote;
        }
                ...
    }
    ...
}
复制代码

ActivityManagerProxy是ActivityManagerNative的内部类，在ActivityManagerProxy的构造方法中将AMS的引用赋值给变量mRemote，这样在ActivityManagerProxy中就可以使用AMS了。

继续回到Instrumentation的execStartActivity方法，代码如下：

public ActivityResult execStartActivity(
        ...
        try {
            ...
            int result = ActivityManagerNative.getDefault()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target, requestCode, 0, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }
复制代码

通过ActivityManagerNative的getDefault方法获取AMS的代理类ActivityManagerProxy，再调用ActivityManagerProxy的startActivity方法。

ActivityManagerProxy的startActivity方法：

public int startActivity(IApplicationThread caller, String callingPackage, Intent intent,
            String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle options) throws RemoteException {
        ...
        //向AMS发送START_ACTIVITY_TRANSACTION类型的进程间通信请求
        mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);
        reply.readException();
        int result = reply.readInt();
        reply.recycle();
        data.recycle();
        return result;
    }
复制代码

在ActivityManagerProxy的startActivity方法中，通过mRemote，也就是AMS的引用，向服务端的AMS发送一个START_ACTIVITY_TRANSACTION类型的进程间通信请求，服务端AMS就会从Binder线程池中读取客户端发来的数据，最终会调用ActivityManagerNative的onTransact方法。

@Override
public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
throws RemoteException {
    switch (code) {
        case START_ACTIVITY_TRANSACTION:{
            ...
            int result = startActivity(app, callingPackage, intent, resolvedType,
            ...
            return true;
        }
        ...
    }

    return super.onTransact(code, data, reply, flags);
}
复制代码

同时ActivityManagerService继承自ActivityManagerNative，因此onTransact方法的switch语句的START_ACTIVITY_TRANSACTION分支会调用AMS的startActivity。

public final class ActivityManagerService extends ActivityManagerNative implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {

    @Override    public final int startActivity(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
           int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
            return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode, startFlags, profilerInfo, bOptions,
                UserHandle.getCallingUserId());
      }
}
复制代码

Android 8.0-Activity启动

Android8.0和7.0的Activity启动过程都会调用Instrumentation的execStartActivity方法，代码如下：

public ActivityResult execStartActivity(
        Context who, IBinder contextThread, IBinder token, String target,
        Intent intent, int requestCode, Bundle options) {
        ...
        try {
            ...
            int result = ActivityManager.getService()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target, requestCode, 0, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }
复制代码

在execStartActivity方法中通过ActivityManager的getService获取IActivityManager对象，并通过IActivityManager对象的startActivity方法通知AMS启动Activity。

public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }

    private static final Singleton<IActivityManager> IActivityManagerSingleton =
            new Singleton<IActivityManager>() {
                @Override
                protected IActivityManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };
复制代码

IActivityManagerSingleto是一个单例类，在它的create方法中获取iBinder类型的AMS引用，接着通过AIDL，将AMS应用转换成IActivityManager类型的对象。相比于Android 7.0来说，这里去掉了ActivityManagerProxy这个代理类，由IActivityManager代替，这里的IActivityManager.aidl通过AIDL工具自动生成IActivityManager.java。

通过Android 7.0和Android 8.0的Activity启动流程可以得出Activity插件化的另一种方案：Hook IActivityManager，通过动态代理实现。

首先创建占坑Activity：

public class StubActivity extends AppCompatActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_stub);
    }
}
复制代码

创建插件Activity：

public class TargetActivity extends AppCompatActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_target);
    }
}
复制代码

并在AndroidManifest.xml中注册占坑Activity：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="com.glh.haiproject01">

    <application
        android:name=".MyApplication"
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        tools:ignore="AllowBackup,GoogleAppIndexingWarning">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".StubActivity" />
    </application>

</manifest>
复制代码

在AndroidManifest.xml中没有注册插件Activity，这时如果启动插件Activity会报错。

接着开始Hook IActivityManager，创建代理类IActivityManagerProxy：

public class IActivityManagerProxy implements InvocationHandler {

    private Object mActivityManager;

    public IActivityManagerProxy(Object activityManager){
        this.mActivityManager=activityManager;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if("startActivity".equals(method.getName())){
            //拦截startActivity
            Intent intent=null;
            int index=0;
            for(int i=0,length=args.length;i<length;i++){
                if(args[i] instanceof Intent){
                    index=i;
                    break;
                }
            }
            //获取插件Activity的Intent
            intent= (Intent) args[index];
            //创建占坑Activity的Intent
            Intent subIntent=new Intent();
            subIntent.setClassName("com.glh.haiproject01","com.glh.haiproject01.StubActivity");
            //保存插件Activity的Intent
            subIntent.putExtra(HookHelper.REQUEST_TARGET_INTENT_NAME,intent);
            //替换为占坑Activity
            args[index]=subIntent;
        }
        return method.invoke(mActivityManager,args);
    }
}

复制代码

IActivityManagerProxy代理类的invoke非常简单，就是将插件Activity的Intent替换为占坑Activity的Intent，并保存插件Activity的Intent，方便后续还原。

private void hookIActivityManager(){
        Object defaultSingleton;
        if(Build.VERSION.SDK_INT==26){
            //Android 8.0
            defaultSingleton=getIActivityManagerSingleton();
        }else{
            defaultSingleton=getDefault();
        }
        try {
            Class<?> singletonClazz=Class.forName("android.util.Singleton");
            Field instanceField=singletonClazz.getDeclaredField("mInstance");
            instanceField.setAccessible(true);
            //获取defaultSingleton中IActivityManager类型的mInstance成员变量
            Object iActivityManager=instanceField.get(defaultSingleton);

            Class<?> iActivityManagerClazz=Class.forName("android.app.IActivityManager");
            Object proxy=Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),
                    new Class<?>[]{iActivityManagerClazz},new IActivityManagerProxy(iActivityManager));
            //替换为代理类IActivityManagerProxy
            instanceField.set(defaultSingleton,proxy);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }

    private Object getIActivityManagerSingleton(){
        try {
            Class<?> activityManagerClazz=Class.forName("android.app.ActivityManager");
            Field field=activityManagerClazz.getDeclaredField("IActivityManagerSingleton");
            field.setAccessible(true);
            return field.get(null);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        return null;
    }

    private Object getDefault(){
        try {
            Class<?> activityManagerClazz=Class.forName("android.app.ActivityManagerNative");
            Field field=activityManagerClazz.getDeclaredField("gDefault");
            field.setAccessible(true);
            return field.get(null);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        return null;
    }
复制代码

以上代码主要就是将Singleton的IActivityManager类型的成员变量mInstance替换成通过动态代理对象。当我们向AMS请求启动Activity时，会执行代理类IActivityManagerProxy的invoke方法进行狸猫换太子。

当AMS通过验证后需要还原插件Activity的Intent，在Android 8.0和Android 7.0(其他版本源码有可能不同)中，AMS通过Binder跨进程调用scheduleLaunchActivity，scheduleLaunchActivity方法在应用程序进程的Binder线程池中，通过mH发送LAUNCH_ACTIVITY消息（100）切换到主线程中创建并启动Activity。

mH的类型是ActivityThread中的内部类H，H继承自Handler并实现了handleMessage方法，在Handler源码中，有这么一段代码：

public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
复制代码

当mCallback不为null时会调用handleMessage方法，mCallback类型是Callback接口，因此我们可以Hook Callback，用自定的Callback替换Handler的mCallback。

代理Callback：

public class CallBackProxy implements Handler.Callback {

    private Handler mHandler;

    public CallBackProxy(Handler handler){
        this.mHandler=handler;
    }

    @Override
    public boolean handleMessage(Message msg) {
        if(msg.what==100){
            Object o=msg.obj;
            try {
                Field field=o.getClass().getDeclaredField("intent");
                field.setAccessible(true);
                //获取占坑Activity的Intent
                Intent intent= (Intent) field.get(o);
                //获取之前保存的插件Activity的Intent
                Intent targetIntent=intent.getParcelableExtra(HookHelper.REQUEST_TARGET_INTENT_NAME);
                //将占坑的Activity替换为插件Activity
                intent.setComponent(targetIntent.getComponent());
            } catch (NoSuchFieldException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }

        }
        mHandler.handleMessage(msg);
        return true;
    }
}
复制代码

将Handler的mCallback替换为代理类CallBackProxy：

private  void hookHandler(){
        try {
            Class<?> activityThreadClazz=Class.forName("android.app.ActivityThread");
            Field currentActivityThreadField=activityThreadClazz.getDeclaredField("sCurrentActivityThread");
            currentActivityThreadField.setAccessible(true);
            Object currentActivityThread=currentActivityThreadField.get(null);

            Field handlerField=activityThreadClazz.getDeclaredField("mH");
            handlerField.setAccessible(true);
            Handler mH= (Handler) handlerField.get(currentActivityThread);

            Field callbackField=Handler.class.getDeclaredField("mCallback");
            callbackField.setAccessible(true);
            //Handler的mCallback替换为CallBackProxy
            callbackField.set(mH,new CallBackProxy(mH));
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }

复制代码

至此，Hook IActivityManager方案已经完成，在主界面中启动插件Activity：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        findViewById(R.id.btn_startActivity).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent=new Intent(MainActivity.this,TargetActivity.class);
                startActivity(intent);
            }
        });
    }
}
复制代码

运行效果：