如何使用 ExecutorService 进行轮询，直到结果到达

Question

我有一个场景，我必须轮询远程服务器检查任务是否已完成。完成后，我会进行不同的调用来检索结果。

我最初认为我应该使用SingleThreadScheduledExecutor 和scheduleWithFixedDelay 进行轮询：

ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
ScheduledFuture future = executor.scheduleWithFixedDelay(() -> poll(jobId), 0, 10, TimeUnit.SECONDS);

public void poll(String jobId) {
   boolean jobDone = remoteServer.isJobDone(jobId);
   if (jobDone) {
       retrieveJobResult(jobId);
   }
}

但由于我只能向scheduleWithFixedDelay 提供无法返回任何内容的Runnable，所以我不知道future 何时会完成，如果有的话。打电话给future.get() 是什么意思？我在等什么结果？

当我第一次检测到远程任务已经完成时，我想执行一个不同的远程调用并将其结果设置为future 的值。我想我可以为此使用 CompletableFuture，我会将其转发到我的 poll 方法，该方法又会将其转发到最终完成它的 retrieveTask 方法：

CompletableFuture<Object> result = new CompletableFuture<Object>();
ScheduledFuture future = executor.scheduleWithFixedDelay(() -> poll(jobId, result), 0, 10, TimeUnit.SECONDS);

public void poll(String jobId, CompletableFuture<Object> result) {
   boolean jobDone = remoteServer.isJobDone(jobId);
   if (jobDone) {
       retrieveJobResult(jobId, result);
   }
}

public void retrieveJobResult(String jobId, CompletableFuture<Object> result) {
    Object remoteResult = remoteServer.getJobResult(jobId);
    result.complete(remoteResult);
}

但这有很多问题。一方面，CompletableFuture 似乎并不适合这种用途。相反，我认为我应该做CompletableFuture.supplyAsync(() -> poll(jobId))，但是当我的CompletableFuture 被取消/完成时，我将如何正确关闭executor 并取消它返回的future？感觉轮询应该以完全不同的方式实现。

Answer 1

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;
}

Answer 2

在我看来，你比其他任何人都更担心一些风格问题。在java 8中，CompletableFuture有2个作用：一个是传统的future，它为任务执行和状态查询提供了一个异步源；另一个就是我们通常所说的promise。一个promise，如果你还不知道，可以被认为是future 的builder 和它的completion source。所以在这种情况下，直觉上需要一个承诺，这正是您在这里使用的情况。您担心的示例是向您介绍第一种用法的东西，而不是向您介绍的承诺方式。

接受这一点，你应该更容易开始处理你的实际问题。我认为 promise 应该有 2 个角色，一个是通知您的任务完成轮询，另一个是在完成时取消您的计划任务。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);
    }
}

Answer 3

我为此创建了一个通用实用程序，灵感来自this answer，使用Supplier<Optional<T>>，每个投票都可以返回Optional.empty()，直到值准备好。我还实现了timeout，以便在超过最大时间时抛出TimeoutException。

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<>();
    }
}