RFR: 8256167: Convert JDK use of `Reference::get` to `Reference::refersTo` [v2]

[Kim Barrett]

On Fri, 4 Dec 2020 22:38:24 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> This patch replaces some uses of `Reference::get` to `Reference::refersTo` to avoid keeping a referent strongly reachable that could cause unnecessary delay in collecting such object.   I only made change in some but not all classes in core libraries when working with Kim on `Reference::refersTo`.    The remaining uses are left for the component owners to convert at appropriate time.
>
> Mandy Chung has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 15 commits:
> 
>  - revise matchsKey to check equality if the reference is not cleared
>  - fix typo
>  - update copyright end year and fixup grammatical nit
>  - Merge branch 'master' of https://github.com/openjdk/jdk into refersTo
>  - 8256167: Convert JDK use of  to
>  - Merge branch 'master' of https://github.com/openjdk/jdk into refersTo
>  - Merge branch 'master' of https://github.com/openjdk/jdk into refersTo
>  - minor cleanup
>  - Merge branch 'refersto' of https://github.com/kimbarrett/openjdk-jdk into refersTo
>  - improve refersTo0 descriptions
>  - ... and 5 more: https://git.openjdk.java.net/jdk/compare/d8ac76fa...72947eb3

Marked as reviewed by kbarrett (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/1609