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一：Hook點的選擇：

1 Hook的對象，稱之為Hook點；一般選擇容易找到的對象為Hook點。因此一般選擇靜態變量和單例。
2 在一個進程之內，靜態變量和單例變量是相對不容易發生變化的，因此非常容易定位，而普通的對象則要么無法標志，要么容易改變。根據這個原則找到所謂的Hook點。

二：Hook例子 - Hook Instrumentation.execStartActivity()

ContextImpl.startActivity()

@Override
public void startActivity(Intent intent, Bundle options) {
    warnIfCallingFromSystemProcess();
    if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
        throw new AndroidRuntimeException(
                "Calling startActivity() from outside of an Activity "
                + " context requires the FLAG_ACTIVITY_NEW_TASK flag."
                + " Is this really what you want?");
    }
    mMainThread.getInstrumentation().execStartActivity(
        getOuterContext(), mMainThread.getApplicationThread(), null,
        (Activity)null, intent, -1, options);
}

EvilInstrumentation： 繼承Instrumentation。

public class EvilInstrumentation extends Instrumentation {

    private static final String TAG = "EvilInstrumentation";

    // ActivityThread中原始的對象, 保存起來
    Instrumentation mBase;

    public EvilInstrumentation(Instrumentation base) {
        mBase = base;
    }

    public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {

        // Hook之前, XXX到此一游!
        Log.d(TAG, "\n執行了startActivity, 參數如下: \n" + "who = [" + who + "], " +
                "\ncontextThread = [" + contextThread + "], \ntoken = [" + token + "], " +
                "\ntarget = [" + target + "], \nintent = [" + intent +
                "], \nrequestCode = [" + requestCode + "], \noptions = [" + options + "]");

        // 開始調用原始的方法, 調不調用隨你,但是不調用的話, 所有的startActivity都失效了.
        // 由于這個方法是隱藏的,因此需要使用反射調用;首先找到這個方法
        try {
            Method execStartActivity = Instrumentation.class.getDeclaredMethod(
                    "execStartActivity",
                    Context.class, IBinder.class, IBinder.class, Activity.class, 
                    Intent.class, int.class, Bundle.class);
            execStartActivity.setAccessible(true);
            return (ActivityResult) execStartActivity.invoke(mBase, who, 
                    contextThread, token, target, intent, requestCode, options);
        } catch (Exception e) {
            // 某該死的rom修改了  需要手動適配
            throw new RuntimeException("do not support!!! pls adapt it");
        }
    }
}

設置 evilInstrumentation對象替代系統的Instrumentation

public static void attachContext() throws Exception{
    // 先獲取到當前的ActivityThread對象
    Class<?> activityThreadClass = Class.forName("android.app.ActivityThread");
    Method currentActivityThreadMethod = activityThreadClass.getDeclaredMethod("currentActivityThread");
    currentActivityThreadMethod.setAccessible(true);
    Object currentActivityThread = currentActivityThreadMethod.invoke(null);

    // 拿到原始的 mInstrumentation字段
    Field mInstrumentationField = activityThreadClass.getDeclaredField("mInstrumentation");
    mInstrumentationField.setAccessible(true);
    Instrumentation mInstrumentation = (Instrumentation) mInstrumentationField.get(currentActivityThread);

    // 創建代理對象
    Instrumentation evilInstrumentation = new EvilInstrumentation(mInstrumentation);

    // 偷梁換柱
    mInstrumentationField.set(currentActivityThread, evilInstrumentation);
}