Is setting a HashMap thread safe?

Question

I have a HashMap in my program which is accessed by multiple threads, and is occasionally set by a single thread.

For example:

Map

        

Answer 1

First off, Java's HashMap class is not thread safe, so there are no guarantees when reads and writes are happening concurrently.

However, since reads and writes to references in Java are atomic, then the pattern you described could be thread-safe as long as the refresh code is not mutating the old map. For example, the following would be fine:

// this refresh code would be thread-safe
Map copy = new HashMap(myMap);
copy.put(x, y); // change the map
myMap = copy;

// you might also consider
myMap = Collections.unmodifiableMap(copy);
// to make sure that the non-thread-safe map will never be mutated

One thing to consider with this pattern is that you may want the myMap field to be declared as volatile so that all threads will get the most recent version of myMap whenever they read from that variable.

Finally, as other posters have mentioned ConcurrentHashMap may be a better approach depending on the complexity of the refresh code. One disadvantage of ConcurrentHashMap is that it doesn't offer any way to batch the operations, so you'd have to make sure that the state at every point during the refresh process was valid for the rest of your application to consume.