CompletableFuture同时执行多个任务合并返回

//同时执行多个任务，并等待执行完毕后，将多个任务的返回值合并返回
CompletableFuture<List<Integer>> future1 = CompletableFuture.supplyAsync(() -> Lists.newArrayList(1));
CompletableFuture<List<Integer>> future2 = CompletableFuture.supplyAsync(() -> Lists.newArrayList(2,1));
CompletableFuture<List<Integer>> future3 = CompletableFuture.supplyAsync(() -> Lists.newArrayList(3));
ArrayList<CompletableFuture<List<Integer>>> completableFutures = Lists.newArrayList(future1, future2, future3);
CompletableFuture<List<Integer>> listCompletableFuture1 = CompletableFuture
                .allOf(completableFutures.toArray(new CompletableFuture[completableFutures.size()]))
                .thenApply(v -> completableFutures.stream()
                        .map(CompletableFuture::join)
                        .collect(Collectors.toList())
                )
                .thenApply(lists -> lists.stream().flatMap(Collection::stream).collect(Collectors.toList()));
List<Integer> join = listCompletableFuture1.join();
System.out.println(join);