AtomicInteger and volatile

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?

Solution 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.

Solution 2

I'd say no, it's not thread-safe, if you define thread-safe as having the same result under single threaded mode and multithreaded mode. In single threaded mode, the count will never go greater than 10, but in multithreaded mode it can.

The issue is that get and incrementAndGet is atomic but an if is not. Keep in mind that a non-atomic operation can be paused at any time. For example:

1. count = 9 currently.
2. Thread A runs if(count.get() <10) and gets true and stopped there.
3. Thread B runs if(count.get() <10) and gets true too so it runs count.incrementAndGet() and finishes. Now count = 10.
4. Thread A resumes and runs count.incrementAndGet(), now count = 11 which will never happen in single threaded mode.

If you want to make it thread-safe without using synchronized which is slower, try this implementation instead:

class A{

final AtomicInteger count;

void someMethod(){
// do something
  if(count.getAndIncrement() <10){
      // safe now
  } else count.getAndDecrement(); // rollback so this thread did nothing to count
}

Solution 3

To maintain the original semantics, and support multiple threads, you could do something like:

public class A {

    private AtomicInteger count = new AtomicInteger(0);

    public void someMethod() {

        int i = count.get();
        while (i < 10 && !count.compareAndSet(i, i + 1)) {
            i = count.get();
        }

    }

}

This avoids any thread ever seeing count reach 10.

Solution 4

Answer is there in this code

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/concurrent/atomic/AtomicInteger.java

This is source code of AtomicInteger. The value is Volatile. So,AtomicInteger uses Volatile inside.

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Comments

[Achow]

Ok, I have amended the question a little bit, can u please explain if that would hold good?

[Jon Skeet]

@anirbanchowdhury: Why would you use intValue() rather than get?

[Achow]

Rite, my mistake, have updated the original question to get()

[Thomas]

Thanks a ton.Exactly this is what I was looking for

[n0rm1e]

The real field holding the value in an AtomicInteger is, unsurprisingly, volatile!