Implementation of Scoped Values (Preview)

This is the CSR for JEP 446: Scoped Values (Preview).

Summary

Introduce scoped values, which enable the sharing of immutable data within and across threads. They are preferred to thread-local variables, especially when using large numbers of virtual threads.

Problem

The motivation in the JEP explains the disadvantages of alternative approaches to Scoped Values at some length.

Solution

Add a simple API that allows the one-way transmission of data from caller to callees within the same 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 StructuredTaskScope.

The preview APIs proposed are:

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

In conjunction with JEP 453, remove module jdk.incubator.concurrent.

Specification

The specdiffs are attached.