How to run multiple service calls in parallel using CompletableFuture?

Question

I am returning following response to the user

class FinalResponseDTO {
    List<Service1ResponseDTO> service1ResponseDTO;
    Long totalCount;
    List<Service2ResponseDTO> service2ResponseDTO;
}

As of now I am making three sequential calls to compute this FinalResponseDTO, each of this call can be run independent of others. I tried making three different CompletableFuture as:

CompletableFuture<List<Service1ResponseDTO> future1 = CompletableFuture.supplyAsync(() -> service1.callMethod1());
CompletableFuture<Long> future2 = CompletableFuture.supplyAsync(() -> service2.callMethod2());
CompletableFuture<Service2ResponseDTO> future3 = CompletableFuture.supplyAsync(() -> service3.callMethod3());

If I do CompletableFuture.allOf(future1, future2, future3).join(); or Should I call CompletableFuture.allOf(future1, future2, future3).get();? Even If I call either of these join or get then how should I construct FinalResponseDTO from it. I am new to the Java 8 concurrency features such as CompletableFuture and I am confused, as each return type of each of these future is different, how should I get combined responses of all such futures and then construct my final output?

Answer 1

From the Javadocs of CompletableFuture.allOf():

Returns a new CompletableFuture that is completed when all of the given CompletableFutures complete. If any of the given CompletableFutures complete exceptionally, then the returned CompletableFuture also does so, with a CompletionException holding this exception as its cause. Otherwise, the results, if any, of the given CompletableFutures are not reflected in the returned CompletableFuture, but may be obtained by inspecting them individually.

So when the combining CompletableFuture completes, you can inspect the values and construct the final response object using a simple constructor by applying a function that constructs the object:

CompletableFuture.allOf(future1, future2, future3).thenApply(v -> 
    new FinalResponseDTO(future1.join(), future2.join(), future3.join())
);