On Mon, 7 Dec 2020 22:14:28 GMT, David Holmes <dhol...@openjdk.org> wrote:
>> Ok, I understand now. `ReferenceType.instances()` only counts objects >> "reachable for the purposes of garbage collection". This change in behavior >> does concern me a little bit. I think the expectation is that the instances >> created by `ClassType.newInstance()` will not show up in this count unless >> `disableCollection()` is called, even when under a "suspend all". Clearly >> that's the expectation of this test, so the question is whether or not it is >> a reasonable expectation. >> >> Note that `ClassType.newInstance()` says nothing about the state of the >> returned object w.r.t. GC. It makes no mention of the need to call >> `disableCollection()` before resuming the VM, so I guess this gives us some >> wiggle room. However, the argument against the object being strongly >> reachable is comes from user asking the question "who has the strong >> reference that makes it strongly reachable?". It's not obvious to the user >> why there is a strong reference, and why it seemingly goes a way once >> `VM.resumeAll()` is called. >> >> I still think overall this is the right approach (baring a better approach >> being presented), but we may need to include some spec clarifications, and >> be prepared for some push back if this breaks anything. > > I don't follow your reasoning here Chris. All ObjectReferences can be GC'd at > any time unless GC has been disallowed. So a reference create via newInstance > is no different to any other reference. If it is currently reachable then > instances() should return it. Are you treating "reachable for the purposes of > garbage collection" as-if it said "strongly reachable"? It doesn't so I think > you are reading too much into this. I think there is a lot of flexibility in > this API in terms of what it may return regarding weak references. I read "reachable for the purposes of garbage collection" as not including objects reachable only via weak reference. So if the only reference to an object is a weak reference, which is normally what you have after calling `ClassType.newInstance()`, then the object is not considered reachable. At the very least, his is how `ReferenceType.instances()` is implemented, and is based on JVMTI [FollowReferences](https://docs.oracle.com/en/java/javase/14/docs/specs/jvmti.html#FollowReferences)(). So given that, the expectation would be that an object returned `ClassType.newInstance()` would not be counted by `ReferenceType.instances()` unless something is done to add a strong reference to the object, such as calling `ObjectReference.disableCollection()`. Now with Per's changes a strong reference is also created with doing a VM.suspend(). The test doesn't expect this behavior, and it's understandable why. ------------- PR: https://git.openjdk.java.net/jdk/pull/1595
