Sequential Async Join in Loop

Rule details

Rule ID	sequential-async-join-in-loop
Severity	WARNING — likely performance problem
Confidence	HIGH — structurally proven
Category	Loop amplifiers
Complexity	O(n·wait) → O(max-wait)

Description

Detects blocking calls (.join(), .get()) on futures inside loops. Awaiting each future sequentially negates the benefit of async execution — the total wall time becomes the sum of all individual waits instead of the maximum.

Typical

JavaJavaScript

for (CompletableFuture<PaymentResult> paymentFuture : paymentFutures) {
    PaymentResult result = paymentFuture.join(); // Blocks on each payment gateway response sequentially
    results.add(result);
}

for (const paymentPromise of paymentPromises) {
    const result = await paymentPromise; // Sequential await on payment gateway
    results.push(result);
}

After

JavaJavaScript

CompletableFuture.allOf(paymentFutures.toArray(new CompletableFuture[0])).join();
for (CompletableFuture<PaymentResult> paymentFuture : paymentFutures) {
    results.add(paymentFuture.join()); // All already completed
}

Use Promise.all to await all promises concurrently:

const results = await Promise.all(paymentPromises);

How Detection Works

1. Identifies loop constructs
2. Inside each loop, looks for blocking calls (.join(), .get())
3. Checks that the target variable name suggests a future or promise (future, promise, async, completable, deferred, task)

Suggestion

Collect all futures first, then call .join()/.get() outside the loop (e.g. CompletableFuture.allOf)