java concurrency: multi-producer one-consumer

Question

I have a situation where different threads populate a queue (producers) and one consumer retrieve element from this queue. My problem is that when one of these elements are retrieved from the queue some is missed (missing signal?). The producers code is:

class Producer implements Runnable {

    private Consumer consumer;

    Producer(Consumer consumer) { this.consumer = consumer; }

    @Override
public void run() {
    consumer.send("message");
  }
}

and they are created and run with:

ExecutorService executor = Executors.newSingleThreadExecutor();
for (int i = 0; i < 20; i++) {
  executor.execute(new Producer(consumer));
}

Consumer code is:

class Consumer implements Runnable {

private Queue<String> queue = new ConcurrentLinkedQueue<String>();

void send(String message) {
    synchronized (queue) {
        queue.add(message);
        System.out.println("SIZE: " + queue.size());
        queue.notify();
    }
}

@Override
public void run() {
    int counter = 0;
    synchronized (queue) {
    while(true) {
        try {
            System.out.println("SLEEP");
                queue.wait(10);
        } catch (InterruptedException e) {
                Thread.interrupted();
        }
        System.out.println(counter);
        if (!queue.isEmpty()) {             
            queue.poll();
            counter++;
        }
    }
    }
}

}

When the code is run I get sometimes 20 elements added and 20 retrieved, but in other cases the elements retrieved are less than 20. Any idea how to fix that?

Answer 1

I'd suggest you use a BlockingQueue instead of a Queue. A LinkedBlockingDeque might be a good candidate for you.

Your code would look like this:

void send(String message) {
    synchronized (queue) {
        queue.put(message);
        System.out.println("SIZE: " + queue.size());
    }
}

and then you'd need to just

queue.take()

on your consumer thread

The idea is that .take() will block until an item is available in the queue and then return exactly one (which is where I think your implementation suffers: missing notification while polling). .put() is responsible for doing all the notifications for you. No wait/notifies needed.