I’m writing a simple Android application in which some of the actions are implemented in JNI. The task is to perform some actions in a separate thread in the native part and then display the result in the corresponding field of the user interface.

public class MainActivity extends AppCompatActivity {
    private static Handler staticHandler;
...
    private ActivityMainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        staticHandler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                binding.multilineText.setText(msg.what);
            }
        };

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());
...       
        Button buttonStartThreads = findViewById(R.id.button2);
        buttonStartThreads.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                start_runners();
            }
        });
    }

    @Override
    protected void onDestroy() {
        staticHandler = null;
        super.onDestroy();
    }
    //Java method I'm calling from JNI
    public static void sendMessage(int v){
        Log.e("My Name->", "sendMessage called from JNI(" + v+ ")");
        //staticHandler.sendEmptyMessage(0); // just the what() is filled with the id
    }

...
    public native void start_runners();

}

The native part has a method sendMessageFromJni, which calls SendMessage from the Java part.

void sendMessageFromJni(const char* buffer, int bufferlen) {
    JNIEnv * env;

    int getEnvStat = javaVM->GetEnv((void **)&env, JNI_VERSION_1_4);
    if (getEnvStat == JNI_EDETACHED) {
        if (javaVM->AttachCurrentThread( &env, NULL) != 0) {
            LOGE("Failed to attach");
            return;
        }
    } 

    int value = 7;
    jclass cls = env->FindClass("com/example/myapplication/MainActivity"); //<- return NULL

    if(cls == NULL)
        LOGE("cls == NULL");

    jmethodID methodId = env->GetStaticMethodID(cls, "sendMessage", "(I)V");
    env->CallStaticVoidMethod(cls, methodId, value);
}

So the call chain looks like this:
sendMessageFromJni (C++) -> SendMessage (Java) -> handler.handleMessage (Java)

The problem is that if I uncomment the call in

    public static void sendMessage(int v){
        Log.e("My Name->", "sendMessage called from JNI(" + v+ ")");
        //staticHandler.sendEmptyMessage(0); // just the what() is filled with the id
    }

env->FindClass() starts retuning NULL in the native part.

I can’t figure out what the problem is