What is the difference between compareAndExchange vs compareAndExchangeAcquire

Question

Here is a snippet from Java library:

public final boolean compareAndExchangeAcquire(boolean expectedValue, boolean newValue) {
    return (int)VALUE.compareAndExchangeAcquire(this,
                                                (expectedValue ? 1 : 0),
                                                (newValue ? 1 : 0)) != 0;
}

It is from AtomicBoolean class. How can a cast to int return a boolean ?

My main question: What is the difference between compareAndExchange vs compareAndExchangeAcquire ?

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In layman terms: statements written prior to xxxAcquire and after xxxRelease is free to reorder while applying xxx .

Answer 1

The last part of the code you posted is != 0 . With clarifying variable:

int a = (int)VALUE.compareAndExchangeAcquire(this,
                                                (expectedValue ? 1 : 0),
                                                (newValue ? 1 : 0));
return a != 0;

Of course the != operator returns a boolean.

As for the second part of the question:

Also, what is the difference between compareAndExchange vs compareAndExchangeAcquire?

Firstly some required reading: https://stackoverflow.com/a/16181675/3424746

From the above answer you should understand that compilers/processors can reorder loads/stores, and the restrictions that acquires and releases place on those. Compare and exchange is most likely implemented with a CAS instruction, which can be viewed as a load+store. compareAndExchangeAcquire and compareAndExchangeRelease add the release/acquire semantics to the CAS/load+stores in question. In other words you can use these to prevent certain reorderings, or allow certain reorderings.