Effective Java

---

CHAPTER 11
CONCURRENCY
314
Note that both the write method (
requestStop
) and the read method (
stop-
Requested
) are synchronized. It is 
not
sufficient to synchronize only the write
method! 
Synchronization is not guaranteed to work unless both read and
write operations are synchronized.
Occasionally a program that synchronizes
only writes (or reads) may 
appear
to work on some machines, but in this case,
appearances are deceiving.
The actions of the synchronized methods in 
StopThread
would be atomic
even without synchronization. In other words, the synchronization on these
methods is used 
solely
for its communication effects, not for mutual exclusion.
While the cost of synchronizing on each iteration of the loop is small, there is a
correct alternative that is less verbose and whose performance is likely to be
better. The locking in the second version of 
StopThread
can be omitted if
stopRequested
is declared volatile. While the 
volatile
modifier performs no
mutual exclusion, it guarantees that any thread that reads the field will see the
most recently written value: