将异步计算包装成同步（阻塞）计算

Question

类似的问题：

• Pattern for wrapping an Asynchronous JavaScript function to make it synchronous
• Wrapping an asynchronous method synchronously in C#

我有一个对象，我想向库客户端（尤其是脚本客户端）公开一个方法，如下所示：

interface MyNiceInterface
{
    public Baz doSomethingAndBlock(Foo fooArg, Bar barArg);
    public Future<Baz> doSomething(Foo fooArg, Bar barArg);
    // doSomethingAndBlock is the straightforward way;
    // doSomething has more control but deals with
    // a Future and that might be too much hassle for
    // scripting clients
}

但我可用的原始“东西”是一组事件驱动的类：

interface BazComputationSink
{
    public void onBazResult(Baz result);
}

class ImplementingThing
{
    public void doSomethingAsync(Foo fooArg, Bar barArg, BazComputationSink sink);
}

ImplementingThing 接受输入，做一些神秘的事情，比如在任务队列中排队，然后当结果出现时，sink.onBazResult() 被调用一个线程，该线程可能与 ImplementingThing.doSomethingAsync( ) 被调用。

有没有一种方法可以使用我拥有的事件驱动函数以及并发原语来实现 MyNiceInterface，以便脚本客户端可以愉快地等待阻塞线程？

编辑：我可以为此使用FutureTask 吗？

Answer 1

嗯，有一个简单的解决方案，比如：

public Baz doSomethingAndBlock(Foo fooArg, Bar barArg) {
  final AtomicReference<Baz> notifier = new AtomicReference();
  doSomethingAsync(fooArg, barArg, new BazComputationSink() {
    public void onBazResult(Baz result) {
      synchronized (notifier) {
        notifier.set(result);
        notifier.notify();
      }
    }
  });
  synchronized (notifier) {
    while (notifier.get() == null)
      notifier.wait();
  }
  return notifier.get();
}

当然，这假设您的 Baz 结果永远不会为空……

Answer 2

使用您自己的 Future 实现：

public class BazComputationFuture implements Future<Baz>, BazComputationSink {

    private volatile Baz result = null;
    private volatile boolean cancelled = false;
    private final CountDownLatch countDownLatch;

    public BazComputationFuture() {
        countDownLatch = new CountDownLatch(1);
    }

    @Override
    public boolean cancel(final boolean mayInterruptIfRunning) {
        if (isDone()) {
            return false;
        } else {
            countDownLatch.countDown();
            cancelled = true;
            return !isDone();
        }
    }

    @Override
    public Baz get() throws InterruptedException, ExecutionException {
        countDownLatch.await();
        return result;
    }

    @Override
    public Baz get(final long timeout, final TimeUnit unit)
            throws InterruptedException, ExecutionException, TimeoutException {
        countDownLatch.await(timeout, unit);
        return result;
    }

    @Override
    public boolean isCancelled() {
        return cancelled;
    }

    @Override
    public boolean isDone() {
        return countDownLatch.getCount() == 0;
    }

    public void onBazResult(final Baz result) {
        this.result = result;
        countDownLatch.countDown();
    }

}

public Future<Baz> doSomething(Foo fooArg, Bar barArg) {
    BazComputationFuture future = new BazComputationFuture();
    doSomethingAsync(fooArg, barArg, future);
    return future;
}

public Baz doSomethingAndBlock(Foo fooArg, Bar barArg) {
    return doSomething(fooArg, barArg).get();
}

该解决方案在内部创建一个 CountDownLatch，一旦接收到回调，它就会被清除。如果用户调用 get，CountDownLatch 用于阻塞调用线程，直到计算完成并调用 onBazResult 回调。 CountDownLatch 将确保如果在调用 get() 之前发生回调，则 get() 方法将立即返回结果。

Answer 3

googleguava library 有一个易于使用的 SettableFuture，它使这个问题变得非常简单（大约 10 行代码）。

public class ImplementingThing {

public Baz doSomethingAndBlock(Foo fooArg, Bar barArg) {
    try {
        return doSomething(fooArg, barArg).get();
    } catch (Exception e) {
        throw new RuntimeException("Oh dear");
    }
};

public Future<Baz> doSomething(Foo fooArg, Bar barArg) {
    final SettableFuture<Baz> future = new SettableFuture<Baz>();
    doSomethingAsync(fooArg, barArg, new BazComputationSink() {
        @Override
        public void onBazResult(Baz result) {
            future.set(result);
        }
    });
    return future;
};

// Everything below here is just mock stuff to make the example work,
// so you can copy it into your IDE and see it run.

public static class Baz {}
public static class Foo {}
public static class Bar {}

public static interface BazComputationSink {
    public void onBazResult(Baz result);
}

public void doSomethingAsync(Foo fooArg, Bar barArg, final BazComputationSink sink) {
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                Thread.sleep(4000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            Baz baz = new Baz();
            sink.onBazResult(baz);
        }
    }).start();
};

public static void main(String[] args) {
    System.err.println("Starting Main");
    System.err.println((new ImplementingThing()).doSomethingAndBlock(null, null));
    System.err.println("Ending Main");
}

Answer 4

一个很简单的例子，只是为了理解CountDownLatch，没有任何 额外的代码。

java.util.concurrent.CountDownLatch 是一种并发构造，它允许一个或多个线程等待给定的一组操作完成。

CountDownLatch 使用给定的计数进行初始化。通过调用countDown() 方法会减少此计数。等待此计数达到零的线程可以调用await() 方法之一。调用await() 会阻塞线程，直到计数为零。

下面是一个简单的例子。 Decrementer 在CountDownLatch 上调用countDown() 3 次后，等待的 Waiter 从await() 调用中释放。

您也可以提一些TimeOut 等待。

CountDownLatch latch = new CountDownLatch(3);

Waiter      waiter      = new Waiter(latch);
Decrementer decrementer = new Decrementer(latch);

new Thread(waiter)     .start();
new Thread(decrementer).start();

Thread.sleep(4000);
public class Waiter implements Runnable{

    CountDownLatch latch = null;

    public Waiter(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Waiter Released");
    }
}

//-------------

public class Decrementer implements Runnable {

    CountDownLatch latch = null;

    public Decrementer(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {

        try {
            Thread.sleep(1000);
            this.latch.countDown();

            Thread.sleep(1000);
            this.latch.countDown();

            Thread.sleep(1000);
            this.latch.countDown();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Reference

如果您不想使用CountDownLatch，或者您的要求与 Facebook 相同，但功能不同。表示如果正在调用一种方法，则不要调用另一种方法。

在这种情况下你可以声明一个

private volatile Boolean isInprocessOfLikeOrUnLike = false;

然后您可以在方法调用的开头检查是否为false，然后调用方法，否则返回.. 取决于您的实现。

Answer 5

根据 Paul Wagland 的回答，这是一个更通用的解决方案：

public abstract class AsyncRunnable<T> {
    protected abstract void run(AtomicReference<T> notifier);

    protected final void finish(AtomicReference<T> notifier, T result) {
        synchronized (notifier) {
            notifier.set(result);
            notifier.notify();
        }
    }

    public static <T> T wait(AsyncRunnable<T> runnable) {
        final AtomicReference<T> notifier = new AtomicReference<>();

        // run the asynchronous code
        runnable.run(notifier);

        // wait for the asynchronous code to finish
        synchronized (notifier) {
            while (notifier.get() == null) {
                try {
                    notifier.wait();
                } catch (InterruptedException ignore) {}
            }
        }

        // return the result of the asynchronous code
        return notifier.get();
    }
}

这是一个如何使用它的例子::

    String result = AsyncRunnable.wait(new AsyncRunnable<String>() {
        @Override
        public void run(final AtomicReference<String> notifier) {
            // here goes your async code, e.g.:
            new Thread(new Runnable() {
                @Override
                public void run() {
                    finish(notifier, "This was a asynchronous call!");
                }
            }).start();
        }
    });

可以在此处找到更详细的代码版本：http://pastebin.com/hKHJUBqE

编辑： 与该问题相关的示例是：

public Baz doSomethingAndBlock(final Foo fooArg, final Bar barArg) {
    return AsyncRunnable.wait(new AsyncRunnable<Baz>() {
        @Override
        protected void run(final AtomicReference<Baz> notifier) {
            doSomethingAsync(fooArg, barArg, new BazComputationSink() {
                public void onBazResult(Baz result) {
                    synchronized (notifier) {
                        notifier.set(result);
                        notifier.notify();
                    }
                }
            });
        }
    });
}

Answer 6

这对于 RxJava 2.x 来说非常简单：

try {
    Baz baz = Single.create((SingleEmitter<Baz> emitter) ->
            doSomethingAsync(fooArg, barArg, result -> emitter.onSuccess(result)))
            .toFuture().get();
} catch (InterruptedException e) {
    e.printStackTrace();
} catch (ExecutionException e) {
    e.printStackTrace();
}

或者没有 Lambda 表示法：

Baz baz = Single.create(new SingleOnSubscribe<Baz>() {
                @Override
                public void subscribe(SingleEmitter<Baz> emitter) {
                    doSomethingAsync(fooArg, barArg, new BazComputationSink() {
                        @Override
                        public void onBazResult(Baz result) {
                            emitter.onSuccess(result);
                        }
                    });
                }
            }).toFuture().get();

---

更简单：

Baz baz = Single.create((SingleEmitter<Baz> emitter) ->
                doSomethingAsync(fooArg, barArg, result -> emitter.onSuccess(result)))
                .blockingGet();

---

Kotlin 版本：

val baz = Single.create<Baz> { emitter -> 
    doSomethingAsync(fooArg, barArg) { result -> emitter.onSuccess(result) } 
}.blockingGet()

Answer 7

最简单的方法（对我有用）是

1. 创建阻塞队列
2. 调用异步方法 - 使用向阻塞队列提供结果的处理程序。

3. 轮询队列（这就是 你阻止）的结果。

   public Baz doSomethingAndBlock(Foo fooArg, Bar barArg) throws InterruptedException {
       final BlockingQueue<Baz> blocker = new LinkedBlockingQueue();
       doSomethingAsync(fooArg, barArg, blocker::offer);
       // Now block until response or timeout
       return blocker.poll(30, TimeUnit.SECONDS);
   }