Java – Hashmap concurrency issue


I have a Hashmap that, for speed reasons, I would like to not require locking on. Will updating it and accessing it at the same time cause any issues, assuming I don't mind stale data?

My accesses are gets, not iterating through it, and deletes are part of the updates.

Best Answer

Yes, it will cause major problems. One example is what could happen when adding a value to the hash map: this can cause a rehash of the table, and if that occurs while another thread is iterating over a collision list (a hash table "bucket"), that thread could erroneously fail to find a key that exists in the map. HashMap is explicitly unsafe for concurrent use.

Use ConcurrentHashMap instead.