Tip #40 (Spring) Waiting for a Combo of CompletableFutures

In the last tip, we learned how to wait for a single CompletableFuture using the join() method.

Tip #39 (Spring) Ensure Changes Only on Successful Kafka Message Delivery

But, imagine your new scenario: You’re in a loop, sending multiple messages, and now you need to wait for all CompletableFutures to make sure everything was delivered.

Without any changes, you’d end up waiting for each message before sending the next one, right?

For this situation, we can use CompletableFuture.allOf to create a single CompletableFuture that completes only when all the given CompletableFutures are done. If the definition sounds strange, don’t worry—the code is simple:

your class {
   List<CompletableFuture> futures = new ArrayList<>();
   loop to send a lot of messages (
       var completableFuture = kafkaProducer.send(event, topic);
       completableFuture
             .thenAcceptAsync(result -> myJob.setStatus(NEW_STATUS))
             .exceptionallyAsync(this::onFailure);
       futures.add(completableFuture);
   )
   CompletableFuture.allOf(futures.toArray(new CompletableFuture[0])).join();
-- continue your code here
}


private Void onFailure(@NotNull Throwable t) {
  log.warn("Unable to send message to Kafka. cause {}", t.getMessage());
  return null;
}

Now we’ve removed join() from the loop. Instead, we create a new CompletableFuture using allOf(), and then call join() on this combined future.

With this setup, you may want to save or update all myJob instances in the repository once the status updates are complet

Do you use CompletableFuture? Let me know!

Happy coding!