[mperor]

What you'll learn

• How to bind a value for a thread with ScopedValue.where(KEY, value).run(...) / .call(...), and that it is only visible inside that call
• Why isBound() and get() reflect the binding's lifetime, throwing NoSuchElementException once it's gone
• How nesting where(...).run(...) rebinds a ScopedValue only for the nested call, restoring the outer value afterward
• Why ScopedValue has no set()/remove(), unlike the mutable, leak-prone ThreadLocal

Why it matters

ScopedValue (finalized in JDK 25 via JEP 506) gives you immutable, structured context propagation without the risk of forgetting a ThreadLocal.remove() in a pooled thread, and it composes safely with virtual threads and structured concurrency.

Read more

JEP 506: Scoped Values

Quiz

What does this print?

ScopedValue.where(USER, "John Doe").run(() -> {
    System.out.println(USER.get());
    ScopedValue.where(USER, "Jane Doe").run(() -> System.out.println(USER.get()));
    System.out.println(USER.get());
});

Answer

John Doe
Jane Doe
John Doe

The nested run() only rebinds USER for its own body; once it returns, the outer binding is restored unchanged.

Note on testing: JDK 25 (required by this project's Gradle toolchain) could not be provisioned in this sandbox — the toolchain auto-download host (api.foojay.io) and direct JDK download hosts are blocked by the environment's network policy. The ScopedValue API used here is unchanged from its JDK 21 preview form (just finalized, no --enable-preview needed), so every test method's logic was independently verified by compiling and running an equivalent standalone program with javac --release 21 --enable-preview / java --enable-preview, confirming the exact assertions in this PR. Opening as a draft; CI will run the real build on JDK 25.

Closes #87

[github-actions]

Test Results

266 tests  +4   263 ✅ +4   5s ⏱️ -1s
 78 suites ±0     3 💤 ±0 
 78 files   ±0     0 ❌ ±0 

Results for commit 836dda6. ± Comparison against base commit b209ba5.

This pull request skips 1 and un-skips 1 tests.

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testSerializationSafe()

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testThreadSafe()