Thread safe Hash Map?

I am writing an application which will return a HashMap to user. User will get reference to this MAP. On the backend, I will be running some threads which will update the Map.

What I have done so far?

I have made all the backend threads so share a common channel to update the MAP. So at backend I am sure that concurrent write operation will not be an issue.

Issues I am having

1. If user tries to update the MAP and simultaneously MAP is being updated at backend --> Concurrent write operation problem.
2. If use tries to read something from MAP and simultaneously MAP is being updated at backend --> concurrent READ and WRITE Operation problem.

Untill now I have not face any such issue, but i m afraid that i may face in future. Please give sugesstions.

I am using ConcurrentHashMap<String, String>.