RFR: 8052260: Reference.isEnqueued() spec does not match the long-standing behavior returning true iff it's in the ref queue

[Kim Barrett]

On Tue, 8 Dec 2020 02:36:01 GMT, Mandy Chung <mchung at openjdk.org> wrote:

> `Reference::isEnqueued` method was never implemented as it was initially specified since 1.2. The specification says that it tests if this reference object has been enqueued, but the long-standing behavior is to test if this reference object is in its associated queue, only reflecting the state at the time when this method is executed. The implementation doesn't do what the specification promised, which might cause serious bugs if unnoticed. For example, an application that relies on this method to release critical resources could cause serious performance issues, in particular when this method is misused on Reference objects without an associated queue.  `Reference::refersTo(null)` is the better recommended way to test if a Reference object has been cleared.
> 
> This proposes to deprecate `Reference::isEnqueued`.  Also the spec is updated to match the long-standing behavior.
> 
> CSR: https://bugs.openjdk.java.net/browse/JDK-8189386

This change looks good.  There are a couple of tests that are using isEnqueued.
vmTestbase/gc/gctests/ReferencesGC/ReferencesGC.java
jdk/java/lang/ref/ReferenceEnqueue.java
https://bugs.openjdk.java.net/browse/JDK-8257876
(I'm working on this.)

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Marked as reviewed by kbarrett (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1684