Get Thread JNIEnv

This is an example shows how:

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved)
{
    cached_jvm = jvm; /* cache the JavaVM pointer */
    return JNI_VERSION_1_2;
}

static void Some_Function()
{
    (*cached_jvm)->AttachCurrentThread(cached_jvm, &env, NULL);
    (*env)->MonitorEnter(env, callbackInst);
    
    jclass cls = (*env)->GetObjectClass(env, callbackInst);
    jmethodID id = (*env)->GetMethodID(env, cls, "callback", "(I)V");
    (*env)->CallVoidMethod(env, callbackInst, id, (int)l);

    (*env)->MonitorExit(env, callbackInst);
}