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;@UnsupportedAppUsagepublic static final int EXIT_APPLICATION = 111;@UnsupportedAppUsagepublic 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CallbackmCallbackField.set(mH,new MyHookHandler());}class MyHookHandler implements Handler.Callback {public MyHookHandler() {}@Overridepublic 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 }
)
