About Unsafe.getObjectVolatile usage

Question

As known, the arrays, even when volatile, do not provide volatile semantics when reading or writing elements, concurrent accessing the k-th element of the array requires an explicit volatile read. This volatile read is performed through the Unsafe.getObjectVolatile().

I want to know , only declaring one object, not array without volatile, using Unsafe.getObjectVolatile() to read it can get the same effect as declaring it with volatile?

Answer 1

Just look at the implementation of java.util.concurrent.atomic.AtomicReferenceArray

It internally uses a private final Object[] array field and does fenced accesses to it through unsafe, e.g. here is the element setter:

public final void set(int i, E newValue) {
    unsafe.putObjectVolatile(array, checkedByteOffset(i), newValue);
}

Do note that the internal and unsupported Unsafe API will probably go away/become inaccessible with java 9, so if you want to use it prepare for some forward-porting effort once varhandles arrive. And/or consider implementing a fallback codepath.