Java Reference
In-Depth Information
If the read of
dataReady
by thread-2 yields
true
, then the write of
dataReady
by thread-1 happened-before that read. Since the write to
data
by thread-1 happens-before the write to
dataReady
by thread-1, it follows
that it also happens-before the read of
dataReady
by thread-2, hence the
read of
data
by thread-2. In short, if thread-2 sees
dataReady
is
true
then
it is guaranteed to see the new
Data
object created by thread-1. This is
even though
data
is not
itself a volatile variable.
[2]
The same would be true if
dataReady
were not volatile but instead had synchronized
get
and
set
methods.
Finally, note that the actual execution order of instructions and memory
accesses can be in any order as long as the actions of the thread appear
to that thread as if program order were followed, and provided all values
read are allowed for by the memory model. This allows the program-
mer to fully understand the semantics of the programs they write, and
it allows compiler writers and virtual machine implementors to perform
complex optimizations that a simpler memory model would not permit.
This discussion gives you an idea of how the memory model interacts
with multithreading. The full details of the memory model are beyond
the scope of this topic. Fortunately, if you pursue a straightforward lock-
ing strategy using the tools in this topic, the subtleties of the memory
model will work for you and your code will be fine.
