but java's hashmap still resizes when breaching the loadfactor. so any similar implementation, even for singlethreaded cases can't achieve (soft)realtime. I am not aware of any linear hashing or similar approach for in memory hash tables to mitigate such latency spikes.

of course it does; it's possible to pre-allocate the entire table though (c-tor with size, rounded to the next power of 2, compensated by the load factor). Massive waste of memory but no resizes.