Question

我有一些代码向另一个线程提交请求，该线程可能会也可能不会将该请求提交给另一个线程。这会产生Future<Future<T>>的返回类型。是否有一些非令人发指的方法可以立即将其变为等待整个未来链完成的Future<T>？

我已经在使用Guava库来处理其他有趣的并发内容，并且作为Google Collections的替代品并且运行良好，但我似乎找不到适用于此案例的内容。

Answer 1

使用番石榴库的另一种可能的实现方法更简单。

import java.util.concurrent.*;
import com.google.common.util.concurrent.*;
import com.google.common.base.*;

public class FFutures {
  public <T> Future<T> flatten(Future<Future<T>> future) {
    return Futures.chain(Futures.makeListenable(future), new Function<Future<T>, ListenableFuture<T>>() {
      public ListenableFuture<T> apply(Future<T> f) {
        return Futures.makeListenable(f);
      }
    });
  }
}

Answer 2

Guava 13.0添加Futures.dereference来执行此操作。它需要ListenableFuture<ListenableFuture>，而不是普通的Future<Future>。（在普通Future上运行需要makeListenable调用，每个调用都需要一个专用线程来执行任务（通过方法的新名称JdkFutureAdapters.listenInPoolThread更清楚）。）

Answer 3

我认为这是实施未来合同所能做的最好的事情。我采取了尽可能不合格的方式，以确保它符合合同。特别是get with timeout的实现。

import java.util.concurrent.*;

public class Futures {
  public <T> Future<T> flatten(Future<Future<T>> future) {
    return new FlattenedFuture<T>(future);
  }

  private static class FlattenedFuture<T> implements Future<T> {
    private final Future<Future<T>> future;

    public FlattenedFuture(Future<Future<T>> future) {
      this.future = future;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
      if (!future.isDone()) {
        return future.cancel(mayInterruptIfRunning);
      } else {
        while (true) {
          try {
            return future.get().cancel(mayInterruptIfRunning);
          } catch (CancellationException ce) {
            return true;
          } catch (ExecutionException ee) {
            return false;
          } catch (InterruptedException ie) {
            // pass
          }
        }
      }
    }

    public T get() throws InterruptedException, 
                          CancellationException, 
                          ExecutionException 
    {
      return future.get().get();
    }

    public T get(long timeout, TimeUnit unit) throws InterruptedException, 
                                                     CancellationException, 
                                                     ExecutionException, 
                                                     TimeoutException 
    {
      if (future.isDone()) {
        return future.get().get(timeout, unit);
      } else {
        return future.get(timeout, unit).get(0, TimeUnit.SECONDS);
      }
    }

    public boolean isCancelled() {
      while (true) {
        try {
          return future.isCancelled() || future.get().isCancelled();
        } catch (CancellationException ce) {
          return true;
        } catch (ExecutionException ee) {
          return false;
        } catch (InterruptedException ie) {
          // pass
        }
      }
    }

    public boolean isDone() {
      return future.isDone() && innerIsDone();
    }

    private boolean innerIsDone() {
      while (true) {
        try {
          return future.get().isDone();
        } catch (CancellationException ce) {
          return true;
        } catch (ExecutionException ee) {
          return true;
        } catch (InterruptedException ie) {
          // pass
        }
      }
    }
  }
}

Answer 4

你可以创建一个类：

public class UnwrapFuture<T> implements Future<T> {
    Future<Future<T>> wrappedFuture;

    public UnwrapFuture(Future<Future<T>> wrappedFuture) {
        this.wrappedFuture = wrappedFuture;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        try {
            return wrappedFuture.get().cancel(mayInterruptIfRunning);
        } catch (InterruptedException e) {
            //todo: do something
        } catch (ExecutionException e) {
            //todo: do something
        }
    }
    ...
}

你必须处理get（）可以引发的异常，但其他方法不能。

Answer 5

这是我第一次尝试，但我确信它有很多错误。我很乐意用Futures.compress(f)之类的东西替换它。

public class CompressedFuture<T> implements Future<T> {
    private final Future<Future<T>> delegate;

    public CompressedFuture(Future<Future<T>> delegate) {
        this.delegate = delegate;
    }

    @Override
    public boolean cancel(boolean mayInterruptIfRunning) {
        if (delegate.isDone()) {
            return delegate.cancel(mayInterruptIfRunning);
        }
        try {
            return delegate.get().cancel(mayInterruptIfRunning);
        } catch (InterruptedException e) {
            throw new RuntimeException("Error fetching a finished future", e);
        } catch (ExecutionException e) {
            throw new RuntimeException("Error fetching a finished future", e);
        }
    }

    @Override
    public T get() throws InterruptedException, ExecutionException {
        return delegate.get().get();
    }

    @Override
    public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
        long endTime = System.currentTimeMillis() + unit.toMillis(timeout);
        Future<T> next = delegate.get(timeout, unit);
        return next.get(endTime - System.currentTimeMillis(), TimeUnit.MILLISECONDS);
    }

    @Override
    public boolean isCancelled() {
        if (!delegate.isDone()) {
            return delegate.isCancelled();
        }
        try {
            return delegate.get().isCancelled();
        } catch (InterruptedException e) {
            throw new RuntimeException("Error fetching a finished future", e);
        } catch (ExecutionException e) {
            throw new RuntimeException("Error fetching a finished future", e);
        }
    }

    @Override
    public boolean isDone() {
        if (!delegate.isDone()) {
            return false;
        }
        try {
            return delegate.get().isDone();
        } catch (InterruptedException e) {
            throw new RuntimeException("Error fetching a finished future", e);
        } catch (ExecutionException e) {
            throw new RuntimeException("Error fetching a finished future", e);
        }
    }
}

是否有一种简单的方法可以转变未来<future <t>＆gt;进入未来<t>？</t> </future <t>

5 个答案: