In my SpringBoot application I have a endpoint that needs to return data in an async way. The method is populating the data but when testing via postman it is returning 200 OK with blank data. I have tried the below 2 approaches but both are returning blank response. Requesting to please check and suggest.

Approach 1

@Async(value = "threadPoolTaskExecutor")
public List<Movie> getMoviesOfHeroASync() throws ExecutionException, InterruptedException {

    List<Movie> allMoviesOfHeroesList = new ArrayList<>();

    String heroes = "aaa;bbb;ccc";

    List<CompletableFuture<Map<String, List<Movie>>>> completableFutureList = Arrays.stream(heroes.split(";")).map(this::getMoviesOfHero).toList();

    CompletableFuture.allOf(completableFutureList.toArray(new CompletableFuture[0])).join();

    for(CompletableFuture<Map<String, List<Movie>>> completableFuture : completableFutureList){

        if(!completableFuture.get().isEmpty()) {

            completableFuture.get().values().stream().findAny().ifPresent(allMoviesOfHeroesList::addAll);
        }

    }

    return allMoviesOfHeroesList;

}

Approach 2

@Async(value = "threadPoolTaskExecutor")
public List<Movie> getMoviesOfHeroASync() throws ExecutionException, InterruptedException {

    List<Movie> allMoviesOfHeroesList = new ArrayList<>();

    String heroes = "aaa;bbb;ccc";

    List<CompletableFuture<Map<String, List<Movie>>>> completableFutureList = Arrays.stream(heroes.split(";")).map(this::getMoviesOfHero).toList();


    CompletableFuture.allOf(completableFutureList.toArray(new CompletableFuture[0]))
            .thenApplyAsync(value -> {
                for(CompletableFuture<Map<String, List<Movie>>> completableFuture : completableFutureList){

                    try {
                        if(!completableFuture.get().isEmpty()) {

                            completableFuture.get().values().stream().findAny().ifPresent(allMoviesOfHeroesList::addAll);
                        }
                    } catch (InterruptedException e) {
                        throw new RuntimeException(e);
                    } catch (ExecutionException e) {
                        throw new RuntimeException(e);
                    }

                }
                return null;
            });


    return allMoviesOfHeroesList;

}