如何使用 ExecutorService 轮询，直到结果到达

我认为 CompletableFutures 是一个很好的方法：

ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();

private void run() {
    final Object jobResult = pollForCompletion("jobId1")
            .thenApply(jobId -> remoteServer.getJobResult(jobId))
            .get();

}

private CompletableFuture<String> pollForCompletion(final String jobId) {
    CompletableFuture<String> completionFuture = new CompletableFuture<>();
    final ScheduledFuture<Void> checkFuture = executor.scheduleAtFixedRate(() -> {
        if (remoteServer.isJobDone(jobId)) {
            completionFuture.complete(jobId);
        }
    }, 0, 10, TimeUnit.SECONDS);
    completionFuture.whenComplete((result, thrown) -> {
        checkFuture.cancel(true);
    });
    return completionFuture;
}

在我看来，你比其他任何风格问题都更担心一些风格问题。在 Java 8 中，有 2 个角色：一个是传统的 future，它为任务执行和状态查询提供异步源;另一个是我们通常所说的承诺。一个承诺，如果你还不知道，可以被认为是未来的建设者及其完成的来源。因此，在这种情况下，直观地需要一个承诺，这正是您在这里使用的确切情况。你所担心的例子是向你介绍第一个用法的东西，而不是承诺的方式。CompletableFuture

接受这一点，您应该更容易开始处理您的实际问题。我认为 promise 应该有 2 个角色，一个是通知你的任务完成轮询，另一个是在完成时取消你的预定任务。这里应该是最终的解决方案：

public CompletableFuture<Object> pollTask(int jobId) {
    CompletableFuture<Object> fut = new CompletableFuture<>();
    ScheduledFuture<?> sfuture = executor.scheduleWithFixedDelay(() -> _poll(jobId, fut), 0, 10, TimeUnit.SECONDS);
    fut.thenAccept(ignore -> sfuture.cancel(false));
    return fut;
}

private void _poll(int jobId, CompletableFuture<Object> fut) {
    // whatever polls
    if (isDone) {
        fut.complete(yourResult);
    }
}

我为此创建了一个通用实用程序，灵感来自这个答案，每个轮询都可以返回，直到值准备就绪。我还实现了 a，以便在超过最大时间时抛出 a。Supplier<Optional<T>>Optional.empty()timeoutTimeoutException

用法：

ScheduledExecutorService scheduledExecutor = Executors.newSingleThreadScheduledExecutor();
Supplier<Optional<String>> supplier = () -> remoteServer.isJobDone(jobId) ? Optional.of(jobId) : Optional.empty();
CompletableFuture<String> future = ScheduledCompletableFuture.builder(String.class)
   .supplier(supplier)
   .executorService(scheduledExecutor)
   .timeUnit(TimeUnit.SECONDS)
   .initialDelay(5)
   .period(5)
   .timeout(60 * 5)
   .build();

ScheduledCompletableFuture.java

public class ScheduledCompletableFuture {
    public static class ScheduledCompletableFutureBuilder<T> {
        private Supplier<Optional<T>> supplier;
        private ScheduledExecutorService executorService;
        private Long initialDelay;
        private Long period;
        private Long timeout;
        private TimeUnit timeUnit;

        public ScheduledCompletableFutureBuilder() {
        }

        public ScheduledCompletableFutureBuilder<T> supplier(Supplier<Optional<T>> supplier) {
            this.supplier = supplier;
            return this;
        }

        public ScheduledCompletableFutureBuilder<T> executorService(ScheduledExecutorService executorService) {
            this.executorService = executorService;
            return this;
        }

        public ScheduledCompletableFutureBuilder<T> initialDelay(long initialDelay) {
            this.initialDelay = initialDelay;
            return this;
        }

        public ScheduledCompletableFutureBuilder<T> period(long period) {
            this.period = period;
            return this;
        }

        public ScheduledCompletableFutureBuilder<T> timeout(long timeout) {
            this.timeout = timeout;
            return this;
        }

        public ScheduledCompletableFutureBuilder<T> timeUnit(TimeUnit timeUnit) {
            this.timeUnit = timeUnit;
            return this;
        }

        public CompletableFuture<T> build() {
            // take a copy of instance variables so that the Builder can be re-used
            Supplier<Optional<T>> supplier = this.supplier;
            ScheduledExecutorService executorService = this.executorService;
            Long initialDelay = this.initialDelay;
            Long period = this.period;
            Long timeout = this.timeout;
            TimeUnit timeUnit = this.timeUnit;

            CompletableFuture<T> completableFuture = new CompletableFuture<>();
            long endMillis = System.currentTimeMillis() + timeUnit.toMillis(timeout);
            Runnable command = () -> {
                Optional<T> optional = supplier.get();
                if (optional.isPresent()) {
                    completableFuture.complete(optional.get());
                } else if (System.currentTimeMillis() > endMillis) {
                    String msg = String.format("Supplier did not return a value within %s %s", timeout, timeUnit);
                    completableFuture.completeExceptionally(new TimeoutException(msg));
                }
            };
            ScheduledFuture<?> scheduledFuture = executorService.scheduleAtFixedRate(command, initialDelay, period, timeUnit);
            return completableFuture.whenComplete((result, exception) -> scheduledFuture.cancel(true));
        }
    }

    public static <T> ScheduledCompletableFutureBuilder<T> builder(Class<T> type) {
        return new ScheduledCompletableFutureBuilder<>();
    }
}