CompletableFuture in loop: How to collect all responses and handle errors

Question

I am trying to call a rest api for PUT request in a loop. Each call is a CompletableFuture. Each api call returns an object of type RoomTypes.RoomType

• I want to collect the responses (both successful and error responses) in different lists. How do I achieve that? I am sure I cannot use allOf because it would not get all the results if any one call fails to update.

• How do I log errors/exception for each call?

public void sendRequestsAsync(Map<Integer, List> map1) {
    List<CompletableFuture<Void>> completableFutures = new ArrayList<>(); //List to hold all the completable futures
    List<RoomTypes.RoomType> responses = new ArrayList<>(); //List for responses
    ExecutorService yourOwnExecutor = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());

    for (Map.Entry<Integer, List> entry :map1.entrySet()) { 
        CompletableFuture requestCompletableFuture = CompletableFuture
                .supplyAsync(
                        () -> 
            //API call which returns object of type RoomTypes.RoomType
            updateService.updateRoom(51,33,759,entry.getKey(),
                           new RoomTypes.RoomType(entry.getKey(),map2.get(entry.getKey()),
                                    entry.getValue())),
                    yourOwnExecutor
            )//Supply the task you wanna run, in your case http request
            .thenApply(responses::add);

    completableFutures.add(requestCompletableFuture);
}

Answer 1

You can simply use allOf() to get a future that is completed when all your initial futures are completed (exceptionally or not), and then split them between succeeded and failed using Collectors.partitioningBy():

List<CompletableFuture<RoomTypes.RoomType>> completableFutures = new ArrayList<>(); //List to hold all the completable futures
ExecutorService yourOwnExecutor = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());

for (Map.Entry<Integer, List> entry : map1.entrySet()) {
    CompletableFuture<RoomTypes.RoomType> requestCompletableFuture = CompletableFuture
            .supplyAsync(
                    () ->
                //API call which returns object of type RoomTypes.RoomType
                updateService.updateRoom(51, 33, 759, entry.getKey(),
                        new RoomTypes.RoomType(entry.getKey(), map2.get(entry.getKey()),
                                entry.getValue())),
                    yourOwnExecutor
            );

    completableFutures.add(requestCompletableFuture);
}

CompletableFuture.allOf(completableFutures.toArray(new CompletableFuture[0]))
        // avoid throwing an exception in the join() call
        .exceptionally(ex -> null)
        .join();
Map<Boolean, List<CompletableFuture<RoomTypes.RoomType>>> result =
        completableFutures.stream()
                .collect(Collectors.partitioningBy(CompletableFuture::isCompletedExceptionally)));

The resulting map will contain one entry with true for the failed futures, and another entry with false key for the succeeded ones. You can then inspect the 2 entries to act accordingly.

Note that there are 2 slight changes compared to your original code:

• requestCompletableFuture is now a CompletableFuture<RoomTypes.RoomType>
• thenApply(responses::add) and the responses list were removed

Concerning logging/exception handling, just add the relevant requestCompletableFuture.handle() to log them individually, but keep the requestCompletableFuture and not the one resulting from handle().