Java CompletableFuture.allOf() 没有找到任何数组元素

Question

一个非常简单的代码片段如下：

String[] list = {"a", "b", "c"};
List<CompletableFuture<String>> completableFutureList = new ArrayList<>();
for (String s : list) {
    completableFutureList.add(CompletableFuture.supplyAsync(() -> s)
            .thenApply(String::toUpperCase));
}
CompletableFuture<String>[] a = completableFutureList
        .toArray(new CompletableFuture[completableFutureList.size()]);
System.out.println(a.length);
CompletableFuture.allOf(a).whenComplete((r, e) -> {
    if (null != r) {
        System.out.println(r);
    } else {
        throw new RuntimeException(e);
    }
});

我希望程序应该打印 "A B C"。但实际上什么也没有打印。为什么以及如何解决这个问题？

Answer 1

引用 CompletableFuture.allOf() 方法的 Javadoc（强调我的）：

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. If no CompletableFutures are provided, returns a CompletableFuture completed with the value {@code null}.

所以我认为您需要在 whenComplete() 回调中手动查询它们（例如使用 a[0].get()）。像这样：

CompletableFuture.allOf(a).whenComplete((r, e) -> {
    for (CompletableFuture<String> future : a) {
        try {
           System.out.println(future.get());
        }
        catch (InterruptedException | ExecutionException e1) {
           e1.printStackTrace();
        }
    }
});

Java CompletableFuture.allOf() 没有找到任何数组元素

Java CompletableFuture.allOf() doesn't find any array elements

java

future