Question

I’ve been trying to get familiar with JNI and its functionality. I am trying to figure out how could I possibly keep reference to the Java environment. I am first calling native method that stores creates “NewGlobalRef” from the jobject and it works fine for a while. But then all the sudden I receive fatal error and log. I am pretty sure garbage collector removes something, even if I am keeping reference to the object all the time in both sides.

What I am doing atm is this

I call native method

myCoolObject.nativeMethod(otherCoolObject);

I am keeping reference to otherCoolObject all the time on java side. What the native method does is

JNIEXPORT void JNICALL Java_HelloWorld_nativeMethod(JNIEnv *env, jobject obj , jobject otherCoolObject) {
    cout << "Called native method!" << endl;
    coolObj = env->NewGlobalRef(otherCoolObject);
    clazz = (*env).GetObjectClass(coolObj;
    callbackEnv = env;
    methodId = (*env).GetMethodID(clazz, "callback", "(II)V");
    jint x = 1;
    jint y = 2;
    (*callbackEnv).CallVoidMethod(coolObj, methodId, x, y);
}

and I can call the method after this for a while but all the sudden it gives me fatal error I told you about

Answer 1

You can’t reuse your JNIenv, cause it’s specific to the function that created it (it’s thread, scope and all).
for getting a valid JNIenv for calling an java method from your callback you have to
use JNI_GetCreatedJavaVMs() and AttachCurrentThread()

more information here:
https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html