RFR: 8316493: Make immutable maps @ValueBased

[Chen Liang]

On Thu, 7 Sep 2023 11:13:44 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR outlines a solution for making immutable maps `@ValueBased` by removing cacheing of certain values in `AbstractMap`.
> 
> By removing these caching fields in `AbstractMap`, we can make the immutable maps `@ValueBased` and at the same time, performance is likely improved because the JVM is probably able to optimize away object creation anyway via escape analysis. Also, all maps will occupy less space as we get rid of a number of objects and references stored for each map.
> 
> We need to benchmark this solution to better understand its implications.

src/java.base/share/classes/java/util/AbstractMap.java line 318:

> 316:      * and returned in response to all subsequent calls.  No synchronization
> 317:      * is performed, so there is a slight chance that multiple calls to this
> 318:      * method will not all return the same set.

The `@implSpec` above is now outdated since caching is removed.

src/java.base/share/classes/java/util/AbstractMap.java line 372:

> 370:      * returned in response to all subsequent calls.  No synchronization is
> 371:      * performed, so there is a slight chance that multiple calls to this
> 372:      * method will not all return the same collection.

Same outdated `@implSpec`

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PR Review Comment: https://git.openjdk.org/jdk/pull/15614#discussion_r1330809911
PR Review Comment: https://git.openjdk.org/jdk/pull/15614#discussion_r1330809969