Question

I’m trying to implement Future.get(long, TimeUnit) in terms of TimeUnit.timedWait(Object, long).

It’s not clear how to use TimeUnit.timedWait(Object, long) in a way that handles spurious wakeups without losing the nanosecond component of TimeUnit. Normally you’d do something like this:

public V get(long timeout, TimeUnit unit)
{
  long expirationTime = new Date().getTime() + unit.toMillis(timeout);
  while (!condition)
  {
    long timeLeft = expirationTime - new Date().getTime();
    if (timeLeft <= 0)
      throw new TimeoutException();
    unit.timedWait(object, timeLeft);
  }
}

but you lose the nanosecond component. If everyone is simply dropping the nanosecond component then what is the point of TimeUnit even supporting nanoseconds and offering TimeUnit.timedWait()?

Answer 1

CountDownLatch seems to be the easiest way to implement this:

public class MutableFuture<T> implements Future<T>
{
  private final CountDownLatch done = new CountDownLatch(1);
  private T value;
  private Throwable throwable;

  public synchronized boolean isDone()
  {
    return done.getCount() == 0;
  }

  public synchronized T get(long timeout, TimeUnit unit)
    throws InterruptedException, ExecutionException, TimeoutException
  {
    if (!done.await(timeout, unit))
      throw new TimeoutException();
    if (throwable != null)
      throw new ExecutionException(throwable);
    return value;
  }

  // remaining methods left as exercise to the reader :)
}

CountdownLatch is not vulnerable to spurious wakeups (because it can check the latch state internally before returning).