Raw JNI with async callbacks

JNI.cpp

static jclass callbacksClass;
static jobject callbacksInstance;

JNIEXPORT void JNICALL Java_com_example_NativeClass_nativeMethod(JNIEnv* env, jclass callingObject, jobject callbacks)
{
	// Cache the Java callbacks instance
	callbacksInstance = env->NewGlobalRef(callbacks);

	// Cache the Java callbacks class (in case of interface, this will be the concrete implementation class)
	jclass objClass = env->GetObjectClass(callbacks);
	// Check for null
	if (objClass)
	{
		callbacksClass = reinterpret_cast<jclass>(env->NewGlobalRef(objClass));
		env->DeleteLocalRef(objClass);
	}
}

// Example of callback lambdas
void callbacksLambdas()
{
		[env]() {
      // Invoke method called 'void onInitSucceeded()'
			jmethodID onSuccess = env->GetMethodID(callbacksClass, "onInitSucceeded", "()V");
			// Invoking the callback method on Java side
			env->CallVoidMethod(callbacksInstance, onSuccess);
		},
		// FailureCallback lambda
		[env](int errorCode, const std::string& message) {
		  // Invoke a method called 'void onInitFailed(int, String)'
			jmethodID onFailure = env->GetMethodID(callbacksClass, "onInitFailed", "(ILjava/lang/String;)V");
			// Prepare method paramters (note that it's not possible to release the jstring since we are returning it to Java side)
			jstring msg = env->NewStringUTF(message.c_str());
			jint err = errorCode;
			// Invoking the callback method on Java side
			env->CallVoidMethod(callbacksInstance, onFailure, err, msg);
		}
	);
}

void release()
{
  // Release the global references to prevent memory leaks
	env->DeleteGlobalRef(callbacksClass);
	env->DeleteGlobalRef(callbacksInstance);
}