Hi Stuart
Looks like in you new webrev. The wait will continue to go loop until
systemStub is not null. If it’s what you meant to do that?
Thank you
Tristan
On Jan 30, 2014, at 10:57 AM, Stuart Marks <stuart.ma...@oracle.com> wrote:
> Hi all, wow, lots of comments on this. Let me see if I can tackle them in one
> message.
>
> Quick aside before I get to the issues: my priorities for this code are
> correctness and maintainability, possibly at the expense of performance. In
> other words I'm willing to make the code more complex so that it's correct,
> but I'm less willing to make it more complex in order to make it go faster.
>
> (Tristan, David) Making 'initialized' be volatile. As things stand, as David
> has pointed out (thanks!) it's not necessary for it to be volatile. There are
> other circumstances (see below) where it would be necessary to make it
> volatile, though.
>
> (Alan, Paul) We could convert to double-checked locking and avoid a
> synchronization in the common case, paying only a volatile read. Something
> like,
>
> volatile boolean initialized = false;
> ...
> private void awaitInitialized() {
> if (!initialized) {
> synchronized (this) {
> try {
> while (!initialized) {
> wait();
> } catch (InterruptedException ie) {
> Thread.currentThread().interrupt();
> }
> }
> }
> }
>
> I *think* that's right. (Is it?) I don't know whether this performs any
> better, or if it does, whether it's worth the additional complexity. I had to
> squint at this for a while to convince myself it's correct.
>
> I am fairly sure this is not a performance-critical area of code. (Famous
> last words, I know.) The other threads that can be active here are handling
> remote calls, so they're already doing network I/O, unmarshalling, and
> dispatching to the RMI thread pool. Compared to this, the incremental cost of
> a synchronization block seems inconsequential. I don't have much intuition
> about how much we'd save by substituting a volatile read for a full
> synchronization in the common case, but given that this is within the context
> of a fairly expensive operation, it doesn't seem like it's worth it to me.
>
> (Alan, Paul) Calling awaitInitialized isn't strictly necessary anywhere
> except for the equals(NAME) case of lookup(). Yes, that's right. I added it
> everywhere because of a possibly misguided sense of completeness and
> consistency. One could essentially redefine awaitInitialized() to protect
> just the systemStub field, not the "entire" object, whose only state is that
> field anyway. Also, see below regarding renaming this method.
>
> (Alan) Use systemStub == null as the condition instead of !initialized. I
> considered at first this but it got confusing really fast. Take a look:
>
> private final ActivationSystem systemStub;
>
> SystemRegistryImpl(..., systemStub) {
> ...
> this.systemStub = systemStub;
> notifyAll();
> ...
> }
>
> private synchronized void awaitInitialized() {
> ...
> while (systemStub == null) {
> wait();
> }
> ...
> }
>
> We rely on systemStub to be initialized at object creation time (before
> construction!) to its default value of null. I think this is right. The
> constructor then initializes the blank final to non-null and notifies.
>
> Then, awaitInitialized seems straightforward until you see that the condition
> is waiting for the value of a final variable to change! JLS sec 17.5 [1]
> allows all sorts of optimizations for final fields, but they all are
> qualified with what is essentially a safe publication requirement on the
> reference:
>
> An object is considered to be completely initialized when its constructor
> finishes. A thread that can only see a reference to an object after that
> object has been completely initialized is guaranteed to see the correctly
> initialized values for that object's final fields.
>
> [1] http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.5
>
> Unfortunately this code *unsafely* publishes a reference to 'this' which is
> the root of this whole problem. Under these circumstances I'd prefer to be
> really conservative about the code here. I can't quite convince myself that a
> condition loop waiting for a final field to change value is safe. That's why
> I added a separate field.
>
> I can see removing the boolean and using systemStub == null as the condition
> making things simpler, since there are fewer variables to reason about, but
> only if systemStub is made non-final. Making it non-final prevents any
> unwanted optimizations resulting from it being final. Having it be final
> doesn't add much anyway. I'll also move all accesses to systemStub within
> synchronized blocks so there is no question about visibility. (As a result,
> awaitInitialized now gets turned into getSystemStub.)
>
> (Peter) Should the interrupt break out of the loop even though the condition
> isn't satisfied? This is a good point. Usually I think of interrupt as
> wanting to avoid waiting indefinitely for the condition to become true (which
> is the point of supporting interruption) but in this case the condition will
> always occur in a timely fashion. So I'll accept the suggestion to save the
> interrupt state and let the condition loop terminate.
>
> Updated webrev here:
>
> http://cr.openjdk.java.net/~smarks/reviews/8023541/webrev.1/
>
> Thanks,
>
> s'marks
