Question

The java memory model mandates that writing a int is atomic: That is, if you write a value to it (consisting of 4 bytes) in one thread and read it in another, you will get all bytes or none, but never 2 new bytes and 2 old bytes or such.

This is not guaranteed for long. Here, writing 0x1122334455667788 to a variable holding 0 before could result in another thread reading 0x112233440000000 or 0x0000000055667788.

Now the specification does not mandate object references to be either int or long-sized. For type safety reasons I suspect they are guaranteed to be written atomically, but on a 64bit VM these references could be very well 64bit values (merely memory addresses).

Now here are my questions:

• Are there any memory model specs covering this (that I haven’t found)?
• Are long-writes suspect to be atomic on 64bit VMs?
• Are VMs forced to map references to 32bit?

Regards,
Steffen

Answer 1

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.