Question

You can ensure that changes one thread makes to a variable can be seen on other threads by making the variable volatile, or by having both threads synchronize on something. If the thing being changed is a java.util.ConcurrentHashMap, does it make sense to create a memory barrier by declaring the type of the variable holding this map as volatile, or are readers accessing the map (say via myMap.values()) going to get the latest possible view anyway? For context I have a heavy reading, light writing scenario where I am switching my lock free read solution to a ConcurrentHashMap.

Answer 1

ConcurrentHashMap guarantees that there is a happens-before relationship between writes and subsequent reads. So yes, when you read (get), you will see the most recent changes that have been “committed” (put has returned).

Note: this does not apply to iterators as explained in the javadoc.