is Java HashSet thread-safe for read only?

Question

If I have an instance of an HashSet after I ran it through Collections.unmodifiableSet(), is it thread-safe?

I\'m asking this since Set documentation states that it\'s n

Answer 1

I don't believe it is thread safe just because you run Collections.unmodifiableSet(). Even though the HashSet if fully initialized and you marked it as unmodifiable, doesn't mean that those changes will be visible to other threads. Even worse, in the absence of synchronization, a compilier is allowed to re-order instructions, which could mean that not only does a reading thread see missing data but it can also see the hashset in a wierd state. Therefore you will need some synchronization. I believe one way around this is to create the hashset as final and to fully initialize it in the constructor. Here is a good article on the JMM http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html. Read the section on How do final fields work under the new JMM?

The ability to see the correctly constructed value for the field is nice, but if the field itself is a reference, then you also want your code to see the up to date values for the object (or array) to which it points. If your field is a final field, this is also guaranteed. So, you can have a final pointer to an array and not have to worry about other threads seeing the correct values for the array reference, but incorrect values for the contents of the array. Again, by "correct" here, we mean "up to date as of the end of the object's constructor", not "the latest value available".