Java volatile reference vs. AtomicReference

Question

Is there any difference between a volatile Object reference and AtomicReference in case I would just use get() and set()-

Answer 1

There are several differences and tradeoffs:

1. Using an AtomicReference get/set has the same JMM semantics as a volatile field(as the javadoc states), but the AtomicReference is a wrapper around a reference, so any access to the field involves a further pointer chase.

2. The memory footprint is multiplied (assuming a compressed OOPs environment, which is true for most VMs):

   • volatile ref = 4b
   • AtomicReference = 4b + 16b (12b object header + 4b ref field)

3. AtomicReference offers a richer API than a volatile reference. You can regain the API for the volatile reference by using an AtomicFieldUpdater, or with Java 9 a VarHandle. You can also reach straight for sun.misc.Unsafe if you like running with scissors. AtomicReference itself is implemented using Unsafe.

So, when is it good to choose one over the other:

• Only need get/set? Stick with a volatile field, simplest solution and lowest overhead.
• Need the extra functionality? If this is a performance(speed/memory overhead) sensitive part of your code make a choice between AtomicReference/AtomicFieldUpdater/Unsafe where you tend to pay in readability and risk for your performance gain. If this not a sensitive area just go for AtomicReference. Library writers typically use a mix of these methods depending on targeted JDKs, expected API restrictions, memory constraints and so on.