Well, at the absolute minimum the variable should be volatile, so any
changes are visible among all threads.
Since increment is the only mutating method, this must be synchronized.
This is a cheap form of multi read, single write threading, although one
must be careful, as this only works on primitives and immutable object
references, since only the reference itself is being updated.
If it was a reference to a mutable object (or long), then all methods
would need to be synchronized.
Cheers,
Peter.
Patricia Shanahan wrote:
I've found something I think is a problem in
com.sun.jini.test.impl.outrigger.matching.StressTest, but it does not
seem to be the problem, or at least not the only problem, causing the
test hang I'm investigating. It's difficult to test, so I'd like a
review of my reasoning. This is a question for those who are familiar
with the Java memory model.
Incidentally, if we went to 1.5 as the oldest supported release, this
could be replaced by an AtomicInteger.
In the following inner class, I think the methods reset and getCount
should be synchronized. As the comments note, the operations they
perform are atomic. My concern is the lack of a happens-before
relationship between those two methods and the increment method. As
far as I can tell, there is nothing forcing the change in the counter
due to an increment to become visible to a getCount call in another
thread.
private class Counter {
/**
* Internal counter variable.
*/
private int _count = 0;
/**
* Constructor. Declared to enforce protection level.
*/
Counter() {
// Do nothing.
}
/**
* Resets internal counter to zero.
*/
void reset() {
// Integer assignment is atomic.
_count = 0;
}
/**
* Increments internal counter by one.
*/
synchronized void increment() {
++_count;
}
/**
* Returns current value of this <code>Counter</code> object.
*/
int getCount() {
// Returning an integer is atomic.
return _count;
}
}
