code sample for https://stackoverflow.com/questions/50164786/jni-call-c-method-from-java-using-void-pointers: C++ side

example.c

#include <jni.h>

JNIEXPORT void JNICALL
Java_com_example_thunks_SomeClass_callback(JNIEnv *env, jobject obj, jint arg1, jlong arg2)
{
    jclass SomeClass = (*env)->GetObjectClass(env, obj);
    jfieldID nativeCallback_ID = (*env)->GetFieldID(env, SomeClass, "m_nativeCallback", "J"); // it would be wise to cache this ID
    void (*callbackPtr)(int, long) = (void (*)(int, long)) (*env)->GetLongField(env, object, nativeCallback_ID);
    callbackPtr(arg1, arg2);
}

// the callbacks - may even be result of dlsym()
void callback1(int arg1, long arg2)
{
    …
}

void callback2(int arg1, long arg2)
{
    …
}

// this is not a Java method!!

void registerSomeJavaCallback(JNIEnv *env, jobject obj, jlong fpointer)
{
    // set the chosen callback for an existing object of SomeClass
    // jclass SomeClass = (*env)->FindClass(env, "com/example/thunks/SomeClass"); // we could find the class by name, too
    jclass SomeClass = (*env)->GetObjectClass(env, object);
    jfieldID nativeCallback_ID = (*env)->GetFieldID(env, SomeClass, "m_nativeCallback", "J");
    (*env)->SetLongField(env, object, nativeCallback_ID, fpointer);
}

// example of calling this function:
// note that we must use `long long`, not `int` here:
registerSomeJavaCallback(theCurrentThreadEnv, someObj, (int64_t)&callback1);