25

我有一些CompletableFutures，我想并行运行它们，等待第一个正常返回。

我知道我可以使用CompletableFuture.anyOf等待第一个返回，但这会正常或异常返回。我想忽略异常。

List<CompletableFuture<?>> futures = names.stream().map(
  (String name) ->
    CompletableFuture.supplyAsync(
      () ->
        // this calling may throw exceptions.
        new Task(name).run()
    )
).collect(Collectors.toList());
//FIXME Can not ignore exceptionally returned takes.
Future any = CompletableFuture.anyOf(futures.toArray(new CompletableFuture<?>[]{}));
try {
    logger.info(any.get().toString());
} catch (Exception e) {
    e.printStackTrace();
}
4

5 回答 5

17
于 2015-12-08T19:03:12.780 回答
3

考虑到：

  1. Java 哲学的基础之一是防止或阻止不良的编程实践。

    （它在多大程度上成功地做到了这一点是另一个争论的主题；重点仍然是，这无疑是该语言的主要目标之一。）

  2. 忽略异常是一种非常糟糕的做法。

    异常应该总是被重新抛出到上面的层，或者被处理，或者至少被报告。具体来说，永远不应该默默地吞下异常。

  3. 应尽早报告错误。

    例如，查看运行时为了提供快速失败的迭代器所经历的痛苦，如果在迭代时修改了集合，则会抛出ConcurrentModificationException 。

  4. 忽略异常完成CompletableFuture意味着 a) 您没有尽早报告错误，并且 b) 您可能计划根本不报告错误。

  5. 不能简单地等待第一个非异常完成而不得不被异常完成所困扰并不会带来任何重大负担，因为您总是可以从列表中删除异常完成的项目，（同时不要忘记报告失败，对吗？）并重复等待。

因此，如果 Java故意遗漏了所寻求的功能，我不会感到惊讶，我愿意争辩说它的遗漏是正当的。

（对不起，Sotirios，没有规范的答案。）

于 2015-12-08T19:09:41.560 回答
2

嗯，这是框架应该支持的方法。首先，我认为CompletionStage.applyToEither做了类似的事情，但事实证明它没有。所以我想出了这个解决方案：

public static <U> CompletionStage<U> firstCompleted(Collection<CompletionStage<U>> stages) {
  final int count = stages.size();
  if (count <= 0) {
    throw new IllegalArgumentException("stages must not be empty");
  }
  final AtomicInteger settled = new AtomicInteger();
  final CompletableFuture<U> future = new CompletableFuture<U>();
  BiConsumer<U, Throwable> consumer = (val, exc) -> {
    if (exc == null) {
      future.complete(val);
    } else {
      if (settled.incrementAndGet() >= count) {
        // Complete with the last exception. You can aggregate all the exceptions if you wish.
        future.completeExceptionally(exc);
      }
    }
  };
  for (CompletionStage<U> item : stages) {
    item.whenComplete(consumer);
  }
  return future;
}

要查看它的实际效果，这里有一些用法：

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.BiConsumer;

public class Main {
  public static <U> CompletionStage<U> firstCompleted(Collection<CompletionStage<U>> stages) {
    final int count = stages.size();
    if (count <= 0) {
      throw new IllegalArgumentException("stages must not be empty");
    }
    final AtomicInteger settled = new AtomicInteger();
    final CompletableFuture<U> future = new CompletableFuture<U>();
    BiConsumer<U, Throwable> consumer = (val, exc) -> {
      if (exc == null) {
        future.complete(val);
      } else {
        if (settled.incrementAndGet() >= count) {
          // Complete with the last exception. You can aggregate all the exceptions if you wish.
          future.completeExceptionally(exc);
        }
      }
    };
    for (CompletionStage<U> item : stages) {
      item.whenComplete(consumer);
    }
    return future;
  }

  private static final ScheduledExecutorService worker = Executors.newSingleThreadScheduledExecutor();

  public static <U> CompletionStage<U> delayed(final U value, long delay) {
    CompletableFuture<U> future = new CompletableFuture<U>();
    worker.schedule(() -> {
      future.complete(value);
    }, delay, TimeUnit.MILLISECONDS);
    return future;
  }
  public static <U> CompletionStage<U> delayedExceptionally(final Throwable value, long delay) {
    CompletableFuture<U> future = new CompletableFuture<U>();
    worker.schedule(() -> {
      future.completeExceptionally(value);
    }, delay, TimeUnit.MILLISECONDS);
    return future;
  }

  public static void main(String[] args) throws InterruptedException, ExecutionException {
    System.out.println("Started...");

    /*
    // Looks like applyToEither doesn't work as expected
    CompletableFuture<Integer> a = CompletableFuture.completedFuture(99);
    CompletableFuture<Integer> b = Main.<Integer>completedExceptionally(new Exception("Exc")).toCompletableFuture();
    System.out.println(b.applyToEither(a, x -> x).get()); // throws Exc
    */

    try {
      List<CompletionStage<Integer>> futures = new ArrayList<>();
      futures.add(Main.<Integer>delayedExceptionally(new Exception("Exception #1"), 100));
      futures.add(Main.<Integer>delayedExceptionally(new Exception("Exception #2"), 200));
      futures.add(delayed(1, 1000));
      futures.add(Main.<Integer>delayedExceptionally(new Exception("Exception #4"), 400));
      futures.add(delayed(2, 500));
      futures.add(Main.<Integer>delayedExceptionally(new Exception("Exception #5"), 600));
      Integer value = firstCompleted(futures).toCompletableFuture().get();
      System.out.println("Completed normally: " + value);
    } catch (Exception ex) {
      System.out.println("Completed exceptionally");
      ex.printStackTrace();
    }

    try {
      List<CompletionStage<Integer>> futures = new ArrayList<>();
      futures.add(Main.<Integer>delayedExceptionally(new Exception("Exception B#1"), 400));
      futures.add(Main.<Integer>delayedExceptionally(new Exception("Exception B#2"), 200));
      Integer value = firstCompleted(futures).toCompletableFuture().get();
      System.out.println("Completed normally: " + value);
    } catch (Exception ex) {
      System.out.println("Completed exceptionally");
      ex.printStackTrace();
    }

    System.out.println("End...");
  }

}
于 2015-12-08T19:01:03.303 回答
0

对上面的代码进行了一些更改，允许测试第一个结果是否符合预期。

public class MyTask implements Callable<String> {

    @Override
    public String call() throws Exception {
        int randomNum = ThreadLocalRandom.current().nextInt(5, 20 + 1);
        for (int i = 0; i < randomNum; i++) {
            TimeUnit.SECONDS.sleep(1);
        }
        return "MyTest" + randomNum;
    }
}


public class CompletableFutureUtils {

    private static <T> T resolve(FutureTask<T> futureTask) {
        try {
            futureTask.run();
            return futureTask.get();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    private static <V> boolean predicate(Predicate<V> predicate, V v) {
        try {
            return predicate.test(v);
        } catch (Exception e) {
            return false;
        }
    }

    public static <T> void cancel(List<FutureTask<T>> futureTasks) {
        if (futureTasks != null && futureTasks.isEmpty() == false) {
            futureTasks.stream().filter(f -> f.isDone() == false).forEach(f -> f.cancel(true));
        }
    }

    public static <V> CompletableFuture<V> supplyAsync(List<FutureTask<V>> futureTasks, Predicate<V> predicate) {
        return supplyAsync(futureTasks, predicate, null);
    }

    public static <V> CompletableFuture<V> supplyAsync(List<FutureTask<V>> futureTasks, Predicate<V> predicate,
            Executor executor) {
        final int count = futureTasks.size();
        final AtomicInteger settled = new AtomicInteger();
        final CompletableFuture<V> result = new CompletableFuture<V>();
        final BiConsumer<V, Throwable> action = (value, ex) -> {
            settled.incrementAndGet();
            if (result.isDone() == false) {
                if (ex == null) {
                    if (predicate(predicate, value)) {
                        result.complete(value);
                        cancel(futureTasks);
                    } else if (settled.get() >= count) {
                        result.complete(null);
                    }
                } else if (settled.get() >= count) {
                    result.completeExceptionally(ex);
                }
            }
        };
        for (FutureTask<V> futureTask : futureTasks) {
            if (executor != null) {
                CompletableFuture.supplyAsync(() -> resolve(futureTask), executor).whenCompleteAsync(action, executor);
            } else {
                CompletableFuture.supplyAsync(() -> resolve(futureTask)).whenCompleteAsync(action);
            }
        }
        return result;
    }
}

public class DemoApplication {
    public static void main(String[] args) {
        List<FutureTask<String>> tasks = new ArrayList<FutureTask<String>>();
        for (int i = 0; i < 2; i++) {
            FutureTask<String> task = new FutureTask<String>(new MyTask());
            tasks.add(task);
        }
        Predicate<String> test = (s) -> true;
        CompletableFuture<String> result = CompletableFutureUtils.supplyAsync(tasks, test);
        try {
            String s = result.get(20, TimeUnit.SECONDS);
            System.out.println("result=" + s);
        } catch (Exception e) {
            e.printStackTrace();
            CompletableFutureUtils.cancel(tasks);
        }
    }
}

调用非常重要，CompletableFutureUtils.cancel(tasks);因此当发生超时时，它将取消后台任务。

于 2021-08-05T15:02:29.240 回答
0

我发现 Vertx - CompositeFuture.any 方法在这种情况下非常有用。它专为完全相同的情况而设计。当然你必须用户vertx定义Future。 Vertx CompositeFuture API 文档

于 2021-08-12T10:18:01.407 回答