倒计时锁存器不会在 DeliverCallback 中递减答案

【问题标题】：Countdown latch does not get decremented within DeliverCallback倒计时锁存器不会在 DeliverCallback 中递减
【发布时间】：2020-07-02 11:00:38
【问题描述】：

我正在尝试使用 Java 代码使用来自 RabbitMQ 通道的消息。我调用了 myAction() 方法两次，它只在第一次时按预期工作。

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;
}

基本上，我的目标是消费并让myAction() 返回消息内容。我第二次调用myAction() 时发生的情况是log.info("Received response.. 被打印但countDownLatch 没有递减，导致fail() 被执行。有谁知道我错过了什么？符合这一点，是不是消费和返回消息内容的正确方式？感谢您的帮助。

【问题讨论】：

我不是 RabbitMQ 专家，但你确定第二个“Received..”消息没有被第一个 DeliverCallback 打印出来吗？尝试在创建回调时为其添加唯一 ID。您正在创建的消费者的生命周期是多久？
感谢您的回复。是的，我确定

标签： java rabbitmq countdownlatch

【解决方案1】：

这是一个例子，它表明回调的生命周期并不局限于你调用basicConsume的范围。

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 -> {});
    }
}

我得到的输出是：

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

【讨论】：