How to refresh HashMap while clients read from it

Question

I have a static HashMap that is initialized on server startup. Clients initialize their data from this map when they login.
Now I need to refresh this map, but

Answer 1

Lets assume that "refresh" means that you want to replace all entries in the hashmap with a fresh set loaded from (say) a file.

If the set of keys in the new mapping is a superset of the keys in the original mapping, AND if you application doesn't care if clients can set part of the old mapping and part of the new mapping at the same time, then you could use a ConcurrentHashMap instead of a HashMap, and replace the entries with a sequence of put calls.

However, if keys are (or could be) different, or if the update needs to be atomic from the client's perspective then a ConcurrentHashMap is not going to work. Instead, you need to declare map as a volatile and implement your refresh() method as per your question.

As you point out, using synchronized (or a single-writer-multiple-reader lock) is liable to lead to a concurrency bottleneck.

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Note: using a volatile is likely to give better performance than using a ConcurrentHashMap even in the cases where the latter is a viable solution.