RFR: 8331670: Deprecate the Memory-Access Methods in sun.misc.Unsafe for Removal

[Maurizio Cimadamore]

On Fri, 10 May 2024 10:06:55 GMT, Alan Bateman <alanb at openjdk.org> wrote:

> This is the implementation changes for JEP 471.
> 
> The methods in sun.misc.Unsafe for on-heap and off-heap access are deprecated for removal. This means a removal warning at compile time. No methods have been removed. A deprecated message is added to each of the methods but unlikely to be seen as the JDK does not generate or publish the API docs for this class.
> 
> A new command line option --sun-misc-unsafe-memory-access=$value is introduced to allow or deny access to these methods. The default proposed for JDK 23 is "allow" so no change in behavior compared to JDK 22 or previous releases.
> 
> A new test is added to test the command line option settings. The existing micros for FFM that use Unsafe are updated to suppress the removal warning at compile time. A new micro is introduced with a small sample of methods to ensure the changes doesn't cause any perf regressions.
> 
> For now, the changes include the update to the man page for the "java" command. It might be that this has to be separated out so that it goes with other updates in the release.

src/jdk.unsupported/share/classes/sun/misc/Unsafe.java line 1764:

> 1762:     }
> 1763:     // set to true by first usage of memory-access method
> 1764:     private static @Stable boolean memoryAccessWarned;

Very nice code. The fact that you only need to report first use (and not first use per module, like in FFM) simplify things a bit (and, most importantly, avoids the need to make all unsafe methods caller sensitive!).

src/jdk.unsupported/share/classes/sun/misc/Unsafe.java line 1869:

> 1867:                     default -> {
> 1868:                         // emit warning
> 1869:                         log("--sun-misc-unsafe-memory-access ignored, value '" + value +

The FFM code throws if an unknown value is passed. Here we log. Should we try to be more consistent?

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PR Review Comment: https://git.openjdk.org/jdk/pull/19174#discussion_r1601362348
PR Review Comment: https://git.openjdk.org/jdk/pull/19174#discussion_r1601364811