In Java, is it safe to change a reference to a HashMap read concurrently

Question

I hope this isn\'t too silly a question...

I have code similar to the following in my project:

public class ConfigStore {

    public static class Co         


        

Answer 1

Your code is fine. You need volatile, otherwise your code would be 100% thread-safe (updating a reference is atomic), however the change might not be visible to all the threads. It means some threads will still see the old value of store.

That being said volatile is obligatory in your example. You might consider AtomicReference, but it won't give you anything more in your case.

You cannot trade correctness for performance so your second question is not really valid. It will have some performance impact, but probably only during update, which happens very rarely as you said. Basically JVM will ensure the change is visible to all the threads by "flushing" it, but after that it will be accessible as any other local variable (up until next update).

BTW I like Config class being immutable, please also consider immutable Map implementation just in case.