Android Hook系统 Handler 消息实现

前言

主线程的Handler 主要依赖于 ActivityThread，Android是消息驱动，比如view的刷新，activity的创建等，如果能打印系统层Handler消息日志，就需要对于系统层的Handler 进行Hook

原理

ActivityThread中 mH对象主要负责整个主线程的事件传递，拿到mH传递的消息并打印出来，就可以，Handler的消息处理机制，会先处理Message的Callback 再处理handlerMessage

核心代码

ActivityThread.java

 final H mH = new H();

    class H extends Handler {
        public static final int BIND_APPLICATION        = 110;
        @UnsupportedAppUsage
        public static final int EXIT_APPLICATION        = 111;
        @UnsupportedAppUsage
        public static final int RECEIVER                = 113;

    ...
          public static final int EXECUTE_TRANSACTION = 159;

}

Handler.java

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

如果能给系统的Handler 设置上 callback ，那每次系统消息传递 就可以先回调我们的callback 

只要我们返回false ，系统正常运行，因为mH是静态的，通过代码反射invoke的就是系统的，

再反射获取 Handler的mCallback对象，设置我们自己的Callback实现类即可

源码

  private void hookSystemHandler() throws IllegalAccessException, NoSuchFieldException, ClassNotFoundException, NoSuchMethodException, InvocationTargetException {
        Class activityThreadClass = Class.forName("android.app.ActivityThread");

        //获取静态ActivityThread对象
        Object currentActivityThread = activityThreadClass.getDeclaredMethod("currentActivityThread").invoke(null);

        Field mHField = activityThreadClass.getDeclaredField("mH");
        mHField.setAccessible(true);
        //获取mH对象 系统的
        Object mH =  mHField.get(currentActivityThread);

        Field mCallbackField =Handler.class.getDeclaredField("mCallback");

        mCallbackField.setAccessible(true);
        //替换为我们的Callback
        mCallbackField.set(mH,new MyHookHandler());
    }



   class MyHookHandler implements Handler.Callback {
        public MyHookHandler() {
        }

        @Override
        public boolean handleMessage(@NonNull Message msg) {
            //我们的Handler 已经完成了替换
            //这里对所有系统消息进行拦截
            Log.d("tag-message",msg.toString());
            // 要保证系统运行正常 还得让原本handler 继续处理
//            originalHandler.handleMessage(msg);
            return false; //不拦截
        }
    }

最终效果

D  { when=-1ms what=159 obj=android.app.servertransaction.ClientTransaction@196c92db target=android.app.ActivityThread$H }

D  { when=-6ms what=159 obj=android.app.servertransaction.ClientTransaction@9bc76abb target=android.app.ActivityThread$H }

D  { when=-20ms what=159 obj=android.app.servertransaction.ClientTransaction@18771a77 target=android.app.ActivityThread$H }