ScopedValueUsageExample.java

import java.util.concurrent.*;

/**
 * Using JDK manually built from main branch.
 *
 * The only change from JDK 20 is the package to import
 * (we no longer need to use add-module jdk.incubator.concurrent).
 *
 * **Not released in the build yet**
 * Run: `java --source 21 --enable-preview ScopedValueUsageExample.java`
 */
public class ScopedValueUsageExample {
	final static ScopedValue<String> MAIN_SCOPE = ScopedValue.newInstance();

	public static void main(String[] args) {
		System.out.println("Starting scoped value");

		// we can share a value from here
		ScopedValue.where(MAIN_SCOPE, "message from main")
			.run(() -> {
				var worker = new Worker();
				worker.execute();
			});
		System.out.println("main ending");
	}
}

class Worker {
	final static ScopedValue<String> WORKER_SCOPE = ScopedValue.newInstance();

	public void execute() {
		// accessing the value from the scope
		System.out.println("shared value from main: " + ScopedValueUsageExample.MAIN_SCOPE.get());

		// === Nested Scope ===
		// we can create a nested scope
		ScopedValue.where(WORKER_SCOPE, "message from worker")
			.run(() -> {
				// the outmost scope will still works
				var messageFromMain = ScopedValueUsageExample.MAIN_SCOPE.get();
				var messageFromWorker = WORKER_SCOPE.get();
				System.out.println("shared value to inner scope from main: " + messageFromMain);
				System.out.println("shared value to inner scope from worker: " + messageFromWorker);
			});

		// we cannot access it from outside its scope (NoSuchElementException)
		// var invalidAccess = WORKER_SCOPE.get();

		// === Rebinded Scope Value ===
		// we can create a new scope that rebinds a new value to the created scope (when using the same variable)
		ScopedValue.where(ScopedValueUsageExample.MAIN_SCOPE, "message from worker over main")
			.run(() -> {
				// the outmost scope will still works
				var rebindedMessage = ScopedValueUsageExample.MAIN_SCOPE.get();
				System.out.println("shared value from shadowed scope: " + rebindedMessage);
			});

		// but the original scope from main will still have its initial value (immutable)
		System.out.println("shared value from main after all scopes: " + ScopedValueUsageExample.MAIN_SCOPE.get());
	}
}