Replace deprecated Unsafe API

[Horcrux7]

I get the follow on the console with Java 23:

WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::getLong has been called by com.kenai.jffi.UnsafeMemoryIO$UnsafeMemoryIO64 (file:.../jffi.jar)
WARNING: Please consider reporting this to the maintainers of class com.kenai.jffi.UnsafeMemoryIO$UnsafeMemoryIO64
WARNING: sun.misc.Unsafe::getLong will be removed in a future release

If I look into the source of Unsafe you should use the class VarHandle which exists since Java 9.

[headius]

Thanks for the issue! Yes, we know we should move to Java 9 VarHandle, but since this library still supports 8 we need to be able to work both ways. Perhaps you can help with a VarHandle patch?

[Horcrux7]

@headius Thanks for the very fast feedback. If you want support the out dated Java 8 in the future then a possible solution for Java 8 support and the use of VarHandle in newer Java versions can be a multi-release jar file.

Perhaps you can help with a VarHandle patch?

No, I have self never use Unsafe or VarHandle. I know that it exists. This is all. This is not a good base to write a patch.

[headius]

Warnings show up for JRuby users so we'll need a fix sooner than later: jruby/jruby#8696

[headius]

I started looking into the code and it's not actually something we can use VarHandle for. The code in question is actually for doing IO to and from native memory given a pointer value. We will need to replace it with Project Panama's foreign memory API conditionally.

We do, however, do have a property to disable Unsafe use at least for this case: jffi.unsafe.disabled. If you set that to false it will switch to a slower (by how much I don't know) JNI-based version of MemoryIO and it won't produce the warnings.

[headius]

A second option is this JVM flag you can use to disable the warning: --sun-misc-unsafe-memory-access=allow.

We are debating whether we fix this or just push forward with the FFM-based jnr-ffi in https://github.com/JornVernee/jnr-ffi/tree/panama-22-unsquashed.