Do we need to make ConcurrentHashMap volatile?

Question

We have a shared ConcurrentHashMap which is read and written by 2 threads.

class Test {
    private ConcurrentHashMap<Object, Object> map = new ConcurrentHashMap<>();

    Object read() {
        return map.get(object);
    }

    void write(Object key, Object object) {
        map.put(key, object);
    }
}

Do we need to make the map volatile so that writes of one thread are seen by the reader threads as soon as possible?

Is it possible that a put to the map in one thread is not seen or seen very late by a different thread?

Same question for HashMap.

Answer 1

If you can make it final then do that. If you cannot make it final then yes you would need to make it volatile. volatile applies to the field assignment and if it's not final then there is a possibility (at least per the JMM) that a write of the CHM field by one thread may not be visible to another thread. To reiterate, this is the ConcurrentHashMap field assignment and not using the CHM.

That being said, you should really make it final.

Do we need to make the map volatile so that writes of one thread are seen by the reader threads as soon as possible?

If the writes you speak of are done using the mutation methods of the CHM itself (like put or remove) you making the field volatile doesn't have an effect. All memory visibility guarantees are done within the CHM.

Is it possible that a put to the map in one thread is not seen or seen very late by a different thread? Same question for HashMap.

Not for the ConcurrentHashMap. If you are using a plain HashMap concurrently, don't. See: http://mailinator.blogspot.com/2009/06/beautiful-race-condition.html