Summary

Make java.lang.ScopedValue a permanent API.

Problem

The motivation section of JEP 506 explains the issues with ThreadLocal and the motivation for introducing a new API.

Solution

Introduce a simple API that allows a value to be safely and efficiently shared to methods without using method parameters. The unfolding execution of the methods define a dynamic scope. This scope is not just that of a single thread: it extends to allow scoped values in a parent thread to be automatically inherited by child threads created with java.util.concurrently.StructuredTaskScope.

The classes in the API are as follows:

java.lang.ScopedValue
java.lang.ScopedValue.Carrier
java.lang.ScopedValue.CallableOp

The 3 classes are in JDK 24 as preview APIs. The only API change that is proposed for JDK 25 is to change the ScopedScope::orElse(V) method to disallow null. Additional, the @since tag on the 3 classes is updated to be 25 (as required by JEP 12).

Specification

Remove the @PreviewFeature annotations. The only other change in this PR is to remove the permission to pass null to orElse().

The diff is attached.