java - 有没有简单的方法来改变未来> 走进未来?

Question

我有一些代码可以将请求提交给另一个线程，该线程可能会也可能不会将该请求提交给另一个线程。这会产生一个返回类型Future<Future<T>>。是否有一些不令人发指的方法可以立即将其转变为Future<T>等待整个未来链完成的过程？

我已经在使用 Guava 库来处理其他有趣的并发内容，并作为 Google Collections 的替代品，它运行良好，但我似乎找不到适合这种情况的东西。

Answer 1

另一种使用 guava 库的可能实现方式要简单得多。

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 增加Futures.dereference了这个功能。它需要一个ListenableFuture<ListenableFuture>，而不是一个普通的Future<Future>。（在平原上操作Future需要一个 makeListenable 调用，每个调用都需要一个用于任务生命周期的专用线程（方法的新名称更清楚地说明了这一点JdkFutureAdapters.listenInPoolThread）。）

Answer 3

我认为这是执行 Future 合约所能做的最好的事情。我采取了尽可能不聪明的策略，以确保它符合合同。不是特别是 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

您可以创建一个类，如：

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
        }
    }
    ...
}

您必须处理 get() 可以引发但其他方法不能引发的异常。

Answer 5

这是我第一次尝试它，但我确信它有很多问题。我很乐意将其替换为Futures.compress(f).

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);
        }
    }
}