When should you use StructuredTaskScope instead of CompletableFuture in Java 21?

Both APIs manage concurrent asynchronous work, but they have different designs, guarantees, and ideal use cases. Java 21 introduces StructuredTaskScope as the preferred model when running on virtual threads.

// CompletableFuture — unstructured, manual cancellation
CompletableFuture  userF  = CompletableFuture.supplyAsync(() -> fetchUser(id));
CompletableFuture orderF = CompletableFuture.supplyAsync(() -> fetchOrder(id));
// If fetchUser throws, fetchOrder may continue running — thread leak possible
String result = userF.thenCombine(orderF,
    (u, o) -> u.name() + " / " + o.total()).join();

// StructuredTaskScope — structured, automatic cleanup
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    var userTask  = scope.fork(() -> fetchUser(id));
    var orderTask = scope.fork(() -> fetchOrder(id));
    scope.join().throwIfFailed();  // if either fails, closes both
    String result2 = userTask.get().name() + " / " + orderTask.get().total();
} // scope.close() cancels any still-running tasks

// Choose CompletableFuture when:
// - Integrating with existing CF-based APIs
// - You need complex transformation pipelines on the result
// - You cannot use preview features

// Choose StructuredTaskScope when:
// - You want automatic cancellation and no thread leaks
// - Code runs on virtual threads (Java 21)
// - You want structured, readable concurrent code

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