Here is the short summary:
- For references, reads/writes are ALWAYS atomic (even in 64 bit implementations!)
- For
int
,char
,byte
,short
,boolean
,float
, reads/writes are ALWAYS atomic - For
double
andlong
, if they’revolatile
, reads/writes are ALWAYS atomic
Therefore there is only exception where reads/writes may not be atomic:
- For
double
andlong
, if they’re NOT declaredvolatile
, they’re NOT GUARANTEED to be atomic
So as far as atomicity of reading/writing shared data is concerned, you only need to make volatile
any double
or long
. Everything else is already guaranteed to be atomic, regardless of how many bits are used in actual implementation.
On the specification
Here’s the relevant section reproduced here for quick reference:
JLS 17.7 Non-atomic Treatment of
double
andlong
Some implementations may find it convenient to divide a single write action on a 64-bit
long
ordouble
value into two write actions on adjacent 32 bit values. For efficiency’s sake, this behavior is implementation specific; Java virtual machines are free to perform writes tolong
anddouble
values atomically or in two parts.For the purposes of the Java programming language memory model, a single write to a non-
volatile long
ordouble
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 ofvolatile long
anddouble
values are always atomic. Writes to and reads of references are always atomic, regardless of whether they are implemented as 32 or 64 bit values.VM implementors are encouraged to avoid splitting their 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.
See also
- JLS 17.6 Word Tearing – guarantees e.g. a
byte
can be updated without neighbor interference - JLS 8.3.1.4
volatile
Fields - Java Tutorials/Essential Classes/Concurrency/Atomic Variables
- Note that given
int i;
,i++
is NOT atomic!
- Note that given
Related questions
- Is there any point in using a volatile long?
- How to declare array elements volatile in Java?
- A
volatile long[]
isvolatile
reference to an array oflong
- The
long
elements themselves are notvolatile
- A