Question

I want to pass a pre-built Map to multiple threads that are going to only read the values from the Map simultaneously. Do I need to worry about concurrent access and use the ConcurrentHashMap instead of HashMap in this case?

Answer 1

You still need to make sure that there is a “happens-before” relationship between the thread that creates and initializes the Map, and the threads that subsequently read it. If there is no such relationship, it is theoretically possible for the reading threads to see stale data.

If you create and populate the Map in the thread that starts the other threads … before it does this … you should be fine.

Another simple way to ensure safety is to do something like this:

public class Foo {
    private final Map map;

    public Foo(....) {
        map = new HashMap();
        // populate
    }

    public Map getMap() {
        // If the 'map' is final, and nothing changes it apart from 
        // the constructor, this method doesn't need to be synchronized.
    }

Both of these will result in a “happens-before” relationship between the creation / initialization, and each thread’s first use of the Map.

You could also declare map in the above to be volatile, but is likely to cause more cache flushes than strictly need to occur; i.e. you’ll take a small performance hit.