Do we need to synchronize access to an array when the array variable is volatile?

Question

I have a class containing a volatile reference to an array:

private volatile Object[] objects = new Object[100];

Now, I can guarantee that, only one thread (call it writer) can write to the array. For example,

objects[10] = new Object();

All other threads will only read values written by the writer thread.

Question: Do I need to synchronize such reads and writes in order to ensure memory consistency?

I presume, yes I should. Because it would not be useful from performance standpoint if JVM provides some kind of memory consistency guarantees when writing to an array. But I'm not sure about that. Didn't find anything helpful in documentation.

Answer 1

You may use AtomicReferenceArray:

final AtomicReferenceArray<Object> objects = new AtomicReferenceArray<>(100);

// writer
objects.set(10, new Object());

// reader
Object obj = objects.get(10);

This will ensure atomic updates and happens-before consistency of read/write operations, the same as if each item of array was volatile.