On Wed, 16 Jul 2025 02:00:48 GMT, Sergey Bylokhov <s...@openjdk.org> wrote:
>> I think that would be OK, meaning probably harmless. But I don't know if >> this case actually requires it. >> Where I've seen a problem like this a Java object is used at an earlier >> point in the method and not again and so becomes eligible to be freed, but >> some native resource it held is needed later but is already freed along with >> its no longer references holder. >> >> Here, if this object (the JRSUIControl) becomes unreachable during >> construction, and so the referent and disposer are also unreachable outside >> this object, they are still going to be live until we reach code in the >> constructor which uses them and since the referent and the disposer are >> passed into addRecord they'll be live there until stored. >> >> But these cases are subtle, maybe I'm missing something. Can you expand on >> the scenario ? >> Or maybe there's some other patterns where we use Disposer that it is needed >> ? > > I think I understand why reachabilityFence is used > [here](https://github.com/openjdk/jdk/blob/a5c9bc70324693e9d0b25bb2c51b91dfc750c453/src/java.base/share/classes/jdk/internal/ref/PhantomCleanable.java#L84). > That code first registers the object in the cleanup queue and then continues > executing the rest of the constructor. As far as I understand, the object > could be collected between the registration and the end of the constructor so > the cleaner will be called on a partially constructed object. > > If that assumption is correct then in the change above the code: > nativeMap = new HashMap<Key, DoubleValue>(); > changes = new HashMap<Key, DoubleValue>(); > might never be executed and the native disposer will be called before, not an > issue in this particular case but good to know. > Or I missing something. I think that's right. At first I thought you were referring to the constructor of the thing that wants to use the cleaner, but you mean the constructor of the Cleanable itself may be still under construction when the referent is collected and installing the reachabilityFence at the end of the constructor prevents that. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/26331#discussion_r2211601222
