Hi Stuart,
I wonder whether you should replace the assert in the
constructor by an explicit null check:
- assert systemStub != null
+ if (systemStub == null) throw new NullPointerException();
The reason I see is that before your change, an object constructed
with a null systemStub would have sooner or later failed in NPE.
Now with your change, an object constructed with a null system
stub will block - waiting forever for system stub to become not
null.
The question of course is whether throwing NPE in the constructor
would cause any compatibility issues. Passing the JCK might help
to figure it out.
Best regards,
-- daniel
On 1/30/14 3:57 AM, Stuart Marks 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
