Path: csiph.com!x330-a1.tempe.blueboxinc.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!border3.nntp.dca.giganews.com!Xl.tags.giganews.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local2.nntp.dca.giganews.com!nntp.earthlink.com!news.earthlink.com.POSTED!not-for-mail
NNTP-Posting-Date: Tue, 31 May 2011 11:06:06 -0500
Date: Tue, 31 May 2011 09:06:00 -0700
From: Patricia Shanahan <pats@acm.org>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.17) Gecko/20110414 Thunderbird/3.1.10
MIME-Version: 1.0
Newsgroups: comp.lang.java.programmer
Subject: Re: Making one or more threads wait for another to produce a value or fail
References: <alpine.DEB.2.00.1105311443530.28134@urchin.earth.li>
In-Reply-To: <alpine.DEB.2.00.1105311443530.28134@urchin.earth.li>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Message-ID: <WZidnVKVjbRyjHjQnZ2dnUVZ_tSdnZ2d@earthlink.com>
Lines: 59
X-Usenet-Provider: http://www.giganews.com
NNTP-Posting-Host: 75.8.126.96
X-Trace: sv3-ZarE/DrBW5uuGB0unLun73qyMKLwcJYLons/yoUAhL23/PstGtfvL2OI5c39VD0wTVtgXf5kABsa2G7!Acfsk58hnqdpSnBpvUjw31E3hlw/XxPumNaiasi2e1mr1gvoOfqxIWajLkUAVxvtTgsuTkcLSThO!tmQ0Jkdd5m0AXYml94KOx27hfHXeqshUVezkaiVYFls=
X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers
X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly
X-Postfilter: 1.3.40
X-Original-Bytes: 3247
Xref: x330-a1.tempe.blueboxinc.net comp.lang.java.programmer:4801

On 5/31/2011 7:00 AM, Tom Anderson wrote:
> The scenario:
>
> Penelope is a widow, or at least her husband isn't around any more
> (she's not sure which; long story). There are 108 suitors who would like
> to marry her. She hasn't decided which one she'll marry. So, the 108
> suitors are sitting about waiting for her to decide. It's possible that
> instead of deciding to marry one of them, she'll deliver some other,
> exceptional, verdict (eg "turns out my husband is still alive, and will
> now murder you all").
>
> Penelope is a thread, as are her suitors. Penelope has plenty to do
> after she delivers her verdict, so the verdict is not a return value -
> it's a value she'll pass to a method. In code, this looks something like:
>
> class Penelope implements Runnable {
> public void run() {
> try {
> Verdict v = ... ;
> DELIVER(v);
> }
> catch (Exception e) {
> DELIVER_EXCEPTION(e);
> }
> }
> }
>
> class Suitor implements Runnable() {
> public void run() {
> try {
> Verdict v = AWAIT();
> }
> catch (Exception e) {
> // alas
> }
> }
> }
>
> There has got to be something in java.util.concurrent that she can use
> to deliver her verdict. What?
>
> In terms of synchronisation, CountdownLatch with a count of 1 does it -
> the suitors await, and Penelope counts down. But it has no way to pass a
> value.

In any case, I would wrap the verdict delivery issues up in a class so
that neither Penelope nor the suitors need to know about the
synchronization implementation.

Within that class, I would probably use ordinary fields to represent the
verdict, exceptional or otherwise.

One way to do the synchronization would be a semaphore that is initially
zero, but with a large number of permits added when Penelope calls a
setVerdict method. The getVerdict method that the suitors call would
wait to get a permit, record the verdict, and put the permit back so
there is no possibility of running out of permits.

Patricia