Countdown latch does not get decremented within DeliverCallback

Question

I am trying to consume a message from a RabbitMQ channel using Java code. I invoke the myAction() method two times, and it works as expected only on the first one.

private Channel receiveChannel=//...;

String myAction() {
        var wrapper = new Object(){ String message = null; };
        final CountDownLatch countDownLatch = new CountDownLatch(1);
        DeliverCallback deliverCallback = (consumerTag, delivery) -> {
            wrapper.message = new String(delivery.getBody(), "UTF-8");
            countDownLatch.countDown();
            log.info("Received response '" + wrapper.message + "'");
        };

        receiveChannel.basicConsume(recvQueueKey, true, deliverCallback, consumerTag -> {});
        if(!countDownLatch.await(10000, TimeUnit.MILLISECONDS)) {
            fail();
        }
        return wrapper.message;
}

Basically, my goal is to consume and have myAction() return the message content. What happens in the second time I invoke myAction() is that log.info("Received response.. gets printed but the countDownLatch does not get decremented, causing fail() to be executed. Does anyone have any idea what I am missing? Is line with this, is it the proper way to consume and return the message content? Thank you for your help.

I'm no RabbitMQ expert, but are you sure that the second "Received.." message isn't being printed by the first deliverCallback? Try adding a unique ID to the callback when you create it. What is the lifetime of the consumer you are creating? — tgdavies, Jul 02 '20 at 11:27

score 0 · Answer 1 · answered Jul 02 '20 at 23:59

Here's an example which shows that the lifetime of callbacks is not limited to the scope in which you call basicConsume.

package org.example;

import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.DeliverCallback;

import java.io.IOException;
import java.util.concurrent.TimeoutException;

public class RabbitMQEg {

    private final static String QUEUE_NAME = "test";

    public static void main(String... args) throws IOException, TimeoutException, InterruptedException {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        try (Connection connection = factory.newConnection();
             Channel c = connection.createChannel()) {

            c.queueDeclare(QUEUE_NAME, false, false, false, null);

            setupConsumer(c, "Consumer 1");
            setupConsumer(c, "Consumer 2");

            Thread.sleep(1000);

            c.basicPublish("", QUEUE_NAME, null, "A".getBytes());
            Thread.sleep(1000);
            c.basicPublish("", QUEUE_NAME, null, "B".getBytes());
            Thread.sleep(1000);
            c.basicPublish("", QUEUE_NAME, null, "C".getBytes());

            Thread.sleep(1000);
        }
    }

    private static void setupConsumer(Channel c, String tag) throws IOException, TimeoutException {
            DeliverCallback deliverCallback = (consumerTag, delivery) -> {
                System.out.println("Received response '" + new String(delivery.getBody()) + "' " + tag + " on thread " + Thread.currentThread().getName());
            };

            c.basicConsume(QUEUE_NAME, true, deliverCallback, consumerTag -> {});
    }
}

The output I get is:

Received response 'A' Consumer 1 on thread pool-1-thread-5
Received response 'B' Consumer 2 on thread pool-1-thread-6
Received response 'C' Consumer 1 on thread pool-1-thread-7

Countdown latch does not get decremented within DeliverCallback

1 Answers1