[concurrency-interest] Basic question on making a thread waiting for 2 mailboxes...

Hanson Char hanson.char at gmail.com
Tue Apr 24 13:08:01 EDT 2007

How about the use of ExecutorService.invokeAny with two Callables, each
trying to dequeue from a BlockingQueue (representing the mailboxes) ?

See DoubleQueueAccess.java below.  Note conceivably two new messages can
arrive at both queues/mailboxes simultaneously and both gets dequeued.  In
such case the result (in the form of AtomicReferenceArray) will contain both
messages.  Otherwise, only one of the two array elements contains a non-null

Hanson Char

import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicReferenceArray;

public class DoubleQueueAccess<T>
    private final ExecutorService es = Executors.newCachedThreadPool();
    private final BlockingQueue<T> q1, q2;

    public DoubleQueueAccess(BlockingQueue<T> q1, BlockingQueue<T> q2) {
        this.q1 = q1;
        this.q2 = q2;

    private static class Dequeue<T> implements Callable<T> {
        final BlockingQueue<T> q;
        final AtomicReferenceArray<T> result;
        final int pos;

        Dequeue(BlockingQueue<T> q, AtomicReferenceArray<T> result, int pos)
            this.q = q;
            this.result = result;
            this.pos = pos;

        public T call() throws InterruptedException {
            T element = q.take();
            result.set(pos, element);
            return element;

    public AtomicReferenceArray<T> dequeue() throws InterruptedException,
ExecutionException {
        final Collection<Dequeue<T>> callables = new
        final AtomicReferenceArray<T> result;
        callables.add(new Dequeue<T>(q1, result = new
AtomicReferenceArray<T>(2), 0));
        callables.add(new Dequeue<T>(q2, result, 1));
        return result;

On 4/23/07, Fred Janon <fjanon at gmail.com> wrote:
> Hi,
> I used to write real time apps using RSX11M+ (and VMS) that had mailboxes
> and events. There was a "wait for OR(events)". The task would wait for any
> event specified in the OR. Then I would associate an event with a mailbox,
> do that for another mailbox and when a message would arrive in one of the 2
> mailboxes, the task would awake. I am trying to do that in Java with
> threads.
> I have been reading "Java Threads",  "JCIP", "Java in a Nutshell" and I
> could think about a couple of classes I could use but I would appreciate
> some design advice... Or if there is an existing library that does that
> already does that, I would appreciate a reference, I did some research and
> did not find anything interesting.
> Basically I need 1 public mailbox and 1 private mailbox for a thread. The
> thread would be blocked until a message arrives in one of the mailboxes. The
> mailboxes must have multiple slots. I was thinking about using
> LinkedBlockingQueues as mailboxes since I don't want to block the sender (or
> avoid it) but I am a bit stuck on making the thread wait for a message in
> either queues.
> The next step would be to have a ThreadPool for the tasks, sharing a
> common public mailbox to process the messages and each have a private
> mailbox for system management messages. I guess switching to a single thread
> to a ThreadPool would be relatively easy?
> Thanks in advance.
> Fred
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