Scoped values (preview)

Key points

• ThreadLocal is mutable and must be cleaned up, or a pooled thread leaks it to the next task.
• It also copies into every child thread, which is unaffordable when threads are free.
• A scoped value is immutable and bound for the duration of one call: ScopedValue.where(V, x).run(...).
• It unbinds automatically at the end of the block - there is no remove() to forget.
• Child threads inside the scope share the binding by reference rather than copying it.

Example

// Preview API in Java 21 - requires --enable-preview.
public class Main {
    static final ScopedValue<String> CURRENT_USER = ScopedValue.newInstance();

    static void handleRequest() {
        // No parameter threading, and nothing to clean up
        System.out.println("serving " + CURRENT_USER.get());
    }

    public static void main(String[] args) {
        ScopedValue.where(CURRENT_USER, "ada").run(() -> {
            handleRequest();
            ScopedValue.where(CURRENT_USER, "bob").run(Main::handleRequest);  // nested rebind
            handleRequest();                                                  // back to ada
        });
        // Outside the scope CURRENT_USER is unbound - calling get() throws.
    }
}

Scoped values swap ThreadLocal's mutable per-thread slot for an immutable per-call binding.