Is writing a reference atomic on 64bit VMs

Reading/writing references always atomic

See JLS section 17.7: Non-atomic Treatment of double and long

For the purposes of the Java programming language memory model, a single write to a non-volatile long or double value is treated as two separate writes: one to each 32-bit half. This can result in a situation where a thread sees the first 32 bits of a 64-bit value from one write, and the second 32 bits from another write.

Writes and reads of volatile long and double values are always atomic.

Writes to and reads of references are always atomic, regardless of whether they are implemented as 32-bit or 64-bit values.

Some implementations may find it convenient to divide a single write action on a 64-bit long or double value into two write actions on adjacent 32-bit values. For efficiency's sake, this behavior is implementation-specific; an implementation of the Java Virtual Machine is free to perform writes to long and double values atomically or in two parts.

Implementations of the Java Virtual Machine are encouraged to avoid splitting 64-bit values where possible. Programmers are encouraged to declare shared 64-bit values as volatile or synchronize their programs correctly to avoid possible complications.

(Emphasis added)

AtomicReference

If you want to coordinate between old and new values, or want specific memory effects, use the class AtomicReference.

For example, AtomicReference::getAndSet returns the old value while setting the new value atomically, eliminating any chance of another thread having intervened between the two steps. Uses volatile memory semantics.