如何使用 CompletableFuture 并行运行多个服务调用？

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.getNow(null), future2.getNow(null), future3.getNow(null))
);

As implied, you should always check the Javadoc for the behavior of methods such as get() vs getNow() . I used the latter here in order to avoid exception handling, because the values would be guaranteed to be available after the allOf CompletableFuture is completed normally.

Answer 2

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

    CompletableFuture.allOf(future1, future2, future3).get();

    return new FinalResponseDTO(future1.join(), future2.join(), future3.join());

Note: supplyAsync runs on ForkJoinPool.commonPool() so it would be a good choice to provide your own executor eg Executors.newCachedThreadPool() :

CompletableFuture.supplyAsync(() -> action, executor);

Answer 3

Instead of using CompletableFuture.allOf , you could fall back to the somewhat hidden Applicative of the CompletableFuture :

static <T, R> CompletableFuture<R> alsoApply(CompletableFuture<T> future,     CompletableFuture<Function<T, R>> f) {
    return f.thenCompose(future::thenApply);
}

With this helper function you can execute futures in parallel threads:

CompletableFuture<String> future = alsoApply(
    CompletableFuture.supplyAsync(() -> "a"),
    CompletableFuture.supplyAsync(() -> "b")
.thenApply(b -> a -> a + b));

assertEquals("ab", future.get());

See this question and answer for where this is coming from and why and how it works.