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Re: Making one or more threads wait for another to produce a value or fail

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On Tue, 31 May 2011 16:46:55 +0100, Tom Anderson wrote:

> On Tue, 31 May 2011, Peter Duniho wrote:
> 
>> (Alternatively, you could provide a custom implementation of Future<V>
>> that doesn't wrap a Runnable like FutureTask<V>, letting your thread
>> continue even after delivering the Future<V>…but such an implementation
>> would be more complicated than just using the wait()/notifyAll()
>> pattern,
> 
> Not *that* much more complicated, i don't think. Both varieties of get
> map on to calls to their corresponding variety of wait in a loop guarded
> by a call to isDone, and isDone maps on to a check on whether the
> verdict is delivered. cancel/isCancelled would require code over and
> above the simplest wait/notify implentation, but not a lot.
> 
> The thing that bugs me is that if this is so simple, and as generally
> useful as i think it is, why isn't it in the JDK already?

I don't know. But it is available in at least one other JVM language's 
standard library: Clojure has functions called "promise" and "deliver" 
for exactly this sort of scenario. Under the hood it combines a 
CountDownLatch with a class instance variable to hold the result and 
implements Clojure's IDeref interface. The Java equivalent would just be 
some class Promise<T> with internal value and CountDownLatch and deliver 
and get methods. Deliver would decrement the CountDownLatch and set the 
value cell; get would see if the CountDownLatch was zero, block if it 
wasn't, and return the value cell's contents if it was. It would also 
throw InterruptedException and maybe have a version of get accepting a 
timeout.

Something like:

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class Promise<T> {
    T value;
    CountDownLatch cdl = new CountDownLatch(1);

    public void deliver (T value) {
        this.value = value;
        cdl.countDown();
    }

    public T get () throws InterruptedException {
        cdl.await();
        return value;
    }

    public T get (long timeout, TimeUnit unit)
      throws InterruptedException {
        if (cdl.await(timeout, unit)) {
            return value;
        } else {
            return null;
        }
    }

    public T get (long timeout, TimeUnit unit, T to)
      throws InterruptedException {
        if (cdl.await(timeout, unit)) {
            return value;
        } else {
            return to;
        }
    }

    public T getEx (long timeout, TimeUnit unit)
      throws InterruptedException {
        if (cdl.await(timeout, unit)) {
            return value;
        } else {
            throw new InterruptedException();
        }
    }
}

Untested and no Javadoc but it *should* work. In particular, the docu for 
CountDownLatch says that value setting should "happen-before" value 
getting. The first timeout-accepting get method will return null on 
timeout. The second accepts a third parameter for the object to return on 
timeout. The getEx method throws an InterruptedException if it times out.

I dedicate the above code into the public domain, so that it may 
expeditiously find its way into JDK 8 perhaps sometime around 2040 
without legal obstacles. :)

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Thread

Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-05-31 15:00 +0100
  Re: Making one or more threads wait for another to produce a value or fail Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-05-31 07:14 -0700
    Re: Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-05-31 16:46 +0100
      Re: Making one or more threads wait for another to produce a value or fail Deeyana <d.awlberg@hotmail.invalid> - 2011-05-31 19:23 +0000
        Re: Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-06-01 22:12 +0100
      Re: Making one or more threads wait for another to produce a value or fail Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-05-31 20:45 -0700
        Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-05-31 21:37 -0700
          Re: Making one or more threads wait for another to produce a value or fail Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-06-01 07:18 -0700
            Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-06-01 09:01 -0700
        Re: Making one or more threads wait for another to produce a value or fail Lawrence D'Oliveiro <ldo@geek-central.gen.new_zealand> - 2011-06-01 16:40 +1200
          Re: Making one or more threads wait for another to produce a value or fail "John B. Matthews" <nospam@nospam.invalid> - 2011-06-01 11:22 -0400
            Re: Making one or more threads wait for another to produce a value or fail Lawrence D'Oliveiro <ldo@geek-central.gen.new_zealand> - 2011-06-02 10:15 +1200
              Re: Making one or more threads wait for another to produce a value or fail "John B. Matthews" <nospam@nospam.invalid> - 2011-06-01 21:38 -0400
        Re: Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-06-01 22:07 +0100
          Re: Making one or more threads wait for another to produce a value or fail Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-06-01 22:46 -0700
  Re: Making one or more threads wait for another to produce a value or fail markspace <-@.> - 2011-05-31 08:26 -0700
    Re: Making one or more threads wait for another to produce a value or fail Tom Anderson <twic@urchin.earth.li> - 2011-05-31 19:13 +0100
  Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-05-31 09:06 -0700
    Re: Making one or more threads wait for another to produce a value or fail Paul Cager <paul.cager@googlemail.com> - 2011-05-31 10:08 -0700
      Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-05-31 10:39 -0700
        Re: Making one or more threads wait for another to produce a value or fail Paul Cager <paul.cager@googlemail.com> - 2011-05-31 16:24 -0700
  Re: Making one or more threads wait for another to produce a value or fail Lawrence D'Oliveiro <ldo@geek-central.gen.new_zealand> - 2011-06-01 12:33 +1200
    Re: Making one or more threads wait for another to produce a value or fail "John B. Matthews" <nospam@nospam.invalid> - 2011-06-01 00:38 -0400
  Re: Making one or more threads wait for another to produce a value or fail markspace <-@.> - 2011-06-02 14:10 -0700
    Re: Making one or more threads wait for another to produce a value or fail markspace <-@.> - 2011-06-02 14:25 -0700
    Re: Making one or more threads wait for another to produce a value or fail Patricia Shanahan <pats@acm.org> - 2011-06-02 14:43 -0700

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