Question

I know volatile allows for visibility, AtomicInteger allows for atomicity.
So if I use a volatile AtomicInteger, does it mean I don’t have to use any more synchronization mechanisms?

Eg.

class A {

    private volatile AtomicInteger count;

    void someMethod(){
        // do something
        if(count.get() < 10) {
            count.incrementAndGet();
        }
}

Is this threadsafe?

Answer 1

I believe that Atomic* actually gives both atomicity and volatility. So when you call (say) AtomicInteger.get(), you’re guaranteed to get the latest value. This is documented in the java.util.concurrent.atomic package documentation:

The memory effects for accesses and updates of atomics generally follow the rules for volatiles, as stated in section 17.4 of The Java™ Language Specification.

• get has the memory effects of reading a volatile variable.
• set has the memory effects of writing (assigning) a volatile variable.
• lazySet has the memory effects of writing (assigning) a volatile variable except that it permits reorderings with subsequent (but not previous) memory actions that do not themselves impose reordering constraints with ordinary non-volatile writes. Among other usage contexts, > – lazySet may apply when nulling out, for the sake of garbage collection, a reference that is never accessed again.
• weakCompareAndSet atomically reads and conditionally writes a variable but does not create any happens-before orderings, so provides no guarantees with respect to previous or subsequent reads and writes of any variables other than the target of the weakCompareAndSet.
• compareAndSet and all other read-and-update operations such as getAndIncrement have the memory effects of both reading and writing volatile variables.

Now if you have

volatile AtomicInteger count;

the volatile part means that each thread will use the latest AtomicInteger reference, and the fact that it’s an AtomicInteger means that you’ll also see the latest value for that object.

It’s not common (IME) to need this – because normally you wouldn’t reassign count to refer to a different object. Instead, you’d have:

private final AtomicInteger count = new AtomicInteger();

At that point, the fact that it’s a final variable means that all threads will be dealing with the same object – and the fact that it’s an Atomic* object means they’ll see the latest value within that object.