A very welcome change overall. However, I have concerns about the semantic change to the PhantomReference specification. I propose that PhantomReference semantics remain unchanged, and that PhantomReference:RefersTo should return true only for null.
See more in comment at https://bugs.openjdk.java.net/browse/JDK-8188055?focusedCommentId=14329319&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14329319 > On Apr 7, 2020, at 5:25 PM, Kim Barrett <kim.barr...@oracle.com> wrote: > > [Note review on both core-libs and hotspot-gc-dev lists; try not to lose > either when replying.] > > Please review a new function: java.lang.ref.Reference.refersTo. > > This function is needed to test the referent of a Reference object > without artificially extending the lifetime of the referent object, as > may happen when calling Reference.get. Some garbage collectors > require extending the lifetime of a weak referent when accessed, in > order to maintain collector invariants. Lifetime extension may occur > with any collector when the Reference is a SoftReference, as calling > get indicates recent access. This new function also allows testing > the referent of a PhantomReference, which can't be accessed by calling > get. > > The new function uses a native method whose implementation is in the > VM so it can use the Access API. It is the intent that this function > will be intrinsified by optimizing compilers like C2 or graal, but > that hasn't been implemented yet. Bear that in mind before rushing > off to change existing uses of Reference.get. > > CR: > https://bugs.openjdk.java.net/browse/JDK-8188055 > https://bugs.openjdk.java.net/browse/JDK-8241029 (CSR) > > Webrev: > https://cr.openjdk.java.net/~kbarrett/8188055/open.04/ > > Testing: > mach5 tier1 > > Locally (linux-x64) verified the new test passes with various garbage > collectors.
