Is it possible to create a new jobject of a java listener in JNI? - java-native-interface

Question

In Android Studio MainActivity, I write something like
int itemA_num = 0;
int itemB_num = 0;
ABCListener mabclistener = new ABCListenter() {
#Override
public void onEventActivated(CustomResult result) {
//do sth secret e.g.
itemA_num ++;
}
}
ABCobject mabcobject = (ABCobject) findviewById(R.id.abcobject1);
mabcobject.setListener(mabcListener);
I don't want people to decompile my APK and modify the code by amending the value or adding something like this:
ABCListener mabclistener = new ABCListenter() {
#Override
public void onEventActivated(CustomResult result) {
//do sth secret e.g.
itemA_num += 10000; //possibly some general name read by those guys and modified as int1 +=10000;
itemB_num += 500; //possibly some general name read by those guys and added this line int2 +=500;
}
}
So I want to use JNI with Cmake. Inside a .cpp file, I want to create the Class Object, findviewById, setListener and create the ABCListener.
I know using the format
jclass cls = (jclass) env->FindClass("abc/def/GHI");
jmethodID mid = (jmethod) env->GetMethodID(cls, "methodname", "(xxxx)yyy");
jobject obj = (jobject) env->CallObjectMethod(cls, mid, params);
However, if I want to write code about ABCListener and make a jobject of it, I don't know how and where to tell the machine I am going to write some code relating to #Override public void onEventActivated(CustomResult result) { ... }. I also want to add some lines of code inside the response in JNI.
I have found a website "similar" to this question but it is from 2011 and about Runnable. https://community.oracle.com/tech/developers/discussion/2298550/overriding-interface-methods-via-jni
I don't know if it still works in 2021.

Answer 1

First, define a new class on the Java side:
class NativeABCListener implements ABCListener {
#Override public native void onEventActivated(CustomResult result);
}
Next, create an instance of NativeABCListener, either in Java or in native code, and attach it to your mabcobject. You know how to do this so I will not repeat it.
On the native side, you simply define a C++ method with the appropriate name:
JNIEXPORT void JNICALL Java_your_package_NativeABCListener_onEventActivated(JNIEnv *env, jobject thiz, jobject result) {
...
}
If you need multiple ABCListeners that do different things, you can choose to create multiple NativeABCListener classes (each with their own corresponding native function), or you can modify NativeABCListener to store a C++ function pointer in a Java long field. In the ..._onEventActivated function you then extract the field from thiz and call it like a regular functino pointer.