Java Reference
In-Depth Information
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 imple-
mented 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 effi-
ciency'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.