It would have to be very quick as it would need the thread to be started before your object has finished being constructed. So it seems unlikely. What's not clear to me from a quick reading of the code is whether the store is actually correct. Although it is only being filled by a single thread that isn't the one that is going to read it on shutdown and I'm not clear on whether there is anything forcing a barrier before the onExit thread has starts reading the hash maps on the nodes.

Actually, is there anything stopping the sampler from taking another sample while onExit is running?