具有不同类型的Java ExecutorService invokeAll [重复]

Question

我使用的是 Java 1.7。

我正在尝试同时执行两种方法。但是，每种方法都有不同的返回类型。

我有以下：

private ExecutorService executorService = Executors.newFixedThreadPool(2);

private void executeConcurrentTasks(final boolean autoBookingEnabled, final TripDTO tripDTO, final ApprovalRequestDetails approvalRequestDetails, final ApprovalRequest approvalRequest, final QuoteResponse quoteResponse, final TripRequirementsResponse tripRequirementsResponse, final List<List<Evaluator>> evaluatorsList, final List<EvaluationApprovalTree> evaluationTree, final Long memberId, final Map<String,Object> properties, final HttpSession session) {

    List<Callable<?>> tasks = Arrays.asList(
        new Callable<BookServicesResponse>() {
            public BookServicesResponse call() throws Exception {
                BookServicesResponse autoBookResponse = bookServices(autoBookingEnabled, tripDTO, approvalRequestDetails, approvalRequest, quoteResponse, memberId, properties, session);
                return autoBookResponse;
            }
        },
        new Callable<ApprovalResponse>() {
            public ApprovalResponse call() throws Exception {
                ApprovalResponse approvalResponse = submitApproval(approvalRequest, tripDTO, tripRequirementsResponse, evaluatorsList, evaluationTree, memberId, properties, session);
                return approvalResponse;
            }
        });

    List<Future<?>> futures = executorService.invokeAll(tasks); // compile error
    
    for (Future<?> future : futures) {
        try {
            Object obj = future.get();
            if (obj instanceof BookServicesResponse) {
                // ...
            } else if (obj instanceof ApprovalResponse) {
                // ...
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }

}

编译错误：

问题

是否有一种通用的方式来处理这两个方法的调用，使得返回的期货列表每个都有不同的对象？

Answer 1

因为invokeAll 需要List<? extends Callable<T>> 而不是List<? extends Callable<? extends T>> 类型的参数，所以它不起作用。您可以认为这是 API 的一个缺点。

为了解决这个问题，您可以使用List<Callable<Object>> 或使用BookServicdsResponse 和ArrovalResponse 的公共超类（在此示例中我将使用Response）：

List<Callable<Response>> tasks = Arrays.asList(
        new Callable<Response>() {
            public Response call() throws Exception {
                BookServicesResponse autoBookResponse = bookServices(autoBookingEnabled, tripDTO, approvalRequestDetails, approvalRequest, quoteResponse, memberId, properties, session);
                return autoBookResponse;
            }
        },
        new Callable<Response>() {
            public Response call() throws Exception {
                ApprovalResponse approvalResponse = submitApproval(approvalRequest, tripDTO, tripRequirementsResponse, evaluatorsList, evaluationTree, memberId, properties, session);
                return approvalResponse;
            }
        });

    List<Future<Response>> futures = executorService.invokeAll(tasks); // compile error

您也可以像这样使用“不安全”演员表：

List<Future<?>> futures = executorService.invokeAll((List<Callable<Object>>) tasks);

由于您只会从Callables 中检索元素，因此这不会导致不需要的ClassCastExceptions。

Answer 2

我不知道这是否是最好的解决方案，它实际上并没有回答如何使用泛型来获得不同可调用对象列表的问题。但我最终使用了两个单独的 Callables 和 Futures。

    Callable<BookServicesResponse> taskBook = new Callable<BookServicesResponse>() {
            public BookServicesResponse call() throws Exception {
                BookServicesResponse autoBookResponse = bookServices(autoBookingEnabled, tripDTO, approvalRequestDetails, approvalRequest, quoteResponse, memberId, properties, session);
                return autoBookResponse;
            }
        };

    Callable<ApprovalResponse> taskApprove = new Callable<ApprovalResponse>() {
            public ApprovalResponse call() throws Exception {
                ApprovalResponse approvalResponse = submitApproval(approvalRequest, tripDTO, tripRequirementsResponse, evaluatorsList, evaluationTree, memberId, properties, session);
                return approvalResponse;
            }
        };

    Future<BookServicesResponse> futureBook = executorService.submit(taskBook);
    Future<ApprovalResponse> futureApprove = executorService.submit(taskApprove);