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

If it's ok that clients have stale data, then all you need to do is create a new map and point your static reference at it. If a new client comes along while you're doing this then they get the stale data and no harm is done, if they turn up after the switch (reassignment) to the new values has occurred then they will get the new values. Job done.

If it's not ok then you will also, probably, have to inform other clients that existed before the update about the change. In which case you want to use the observer pattern for the updates. In this pattern it's fine if the client connects during the update, because they will be updated as soon as possible after the update is complete.

BTW: in all cases, you really shouldn't be using 'static' for anything. It'll only lead to problems down the line. Rather, create a non-static singleton that holds the map and inject that into your clients/services/whatever.