Volatile arrays in Java

[Roman Romm]

You are writing on this page that :
"declaring an array volatile does not give volatile access to its fields"
and also
it is unsafe to call arr[x] = y on an array (even if declared volatile) in one thread and then expect arr[x] to return y from another thread

if I have such an class :

public class Test {

int[] numbers = new int[Integer.MAX_VALUE];

void increment(int ind){
numbers[ind]++;
}

int get(int ind){
return numbers[ind];
}
}

and some reader threads , "get" and only one writer "increment".
I though not require further synchronization in this class in a case of MRSW.
But you claim that the elements inside the array could be cached. Is this right ?

Thanks

[Neil Coffey]

With a volatile array, the "volatile" characteristic applie sto the reference, not individual elements in the array.

So if you want accesses to elements to be thread-safe, you generally need to synchronize on accesses in some other way.

The easiest way is to use one of the atomic array classes.

[Magnus Eriksson]

Studying the description of volatile in the Java language spec you absolutely get the impression that "volatile" only apply to a single primitive value (int, boolean etc) and a pointer is of course a special case of this. From this one can assume that volatile on an array ONLY protects reading / updating the array pointer and nothing else!