Why is reference assignment atomic in Java?

Question

As far as I know reference assignment is atomic in a 64 bit JVM. Now, I assume the jvm doesn\'t use atomic pointers internally to model this, since otherwise there would be no n

Answer 1

First of all, reference assignment is atomic because the specification says so. Besides that, there is no obstacle for JVM implementors to fulfill this constraint, as 64 Bit references are usually only used on 64 Bit architectures, where atomic 64 Bit assignment comes for free.

Your main confusion stems from the assumption that the additional “Atomic References” feature means exactly that, due to its name. But the AtomicReference class offers far more, as it encapsulates a volatile reference, which has stronger memory visibility guarantees in a multi-threaded execution.

Having an atomic reference update does not necessarily imply that a thread reading the reference will also see consistent values regarding the fields of the object reachable through that reference. All it guarantees is that you will read either the null reference or a valid reference to an existing object that actually was stored by some thread. If you want more guarantees, you need constructs like synchronization, volatile references, or an AtomicReference.

AtomicReference also offers atomic update operations like compareAndSet or getAndSet. These are not possible with ordinary reference variables.