Question

我正在遵循RabbitMQ的本指南：https://www.rabbitmq.com/tutorials/tutorial-two-java.html。我想在一个队列中使用多个线程来模拟此功能。

如果在启动发件人之前先启动接收器，则它将按预期工作，如下所示：

[*] Rcvr1 Waiting for messages...
[*] Rcvr2 Waiting for messages...
[x] Rcvr1 Received 'Hello 0'
[x] Rcvr2 Received 'Hello 1'
[x] Rcvr1 Received 'Hello 2'
[x] Rcvr2 Received 'Hello 3'
[x] Rcvr1 Received 'Hello 4'
[x] Rcvr2 Received 'Hello 5'
[x] Rcvr1 Received 'Hello 6'
[x] Rcvr2 Received 'Hello 7'
[x] Rcvr1 Received 'Hello 8'
...

但是，首先启动我的接收方只会导致一个线程接收消息（最后一个线程将被启动）：

[*] Rcvr2 Waiting for messages...
[*] Rcvr1 Waiting for messages...
[x] Rcvr1 Received 'Hello 9'
[x] Rcvr1 Received 'Hello 10'
[x] Rcvr1 Received 'Hello 11'
[x] Rcvr1 Received 'Hello 12'
[x] Rcvr1 Received 'Hello 13'
[x] Rcvr1 Received 'Hello 14'
[x] Rcvr1 Received 'Hello 15'
...

有趣的是，如果我启动发送方，然后如上所述启动接收方，然后再次启动发送方（当接收方正在处理第一批数据时）。发送的第一条消息是串行处理的，而第二批消息是并行处理的，或者至少与其余线程一起处理。

 [*] Rcvr1 Waiting for messages...
 [*] Rcvr2 Waiting for messages...
 [x] Rcvr1 Received '[Batch 1] Hello 0'
 [x] Rcvr1 Received '[Batch 1] Hello 1'
 [x] Rcvr1 Received '[Batch 1] Hello 2'
 [x] Rcvr1 Received '[Batch 1] Hello 3'
 [x] Rcvr1 Received '[Batch 1] Hello 4'
 [x] Rcvr1 Received '[Batch 1] Hello 5'
 [x] Rcvr1 Received '[Batch 1] Hello 6'
 [x] Rcvr1 Received '[Batch 1] Hello 7'
 [x] Rcvr1 Received '[Batch 1] Hello 8'
 [x] Rcvr2 Received '[Batch 2] Hello 1'
 [x] Rcvr1 Received '[Batch 1] Hello 9'
 [x] Rcvr2 Received '[Batch 2] Hello 3'
 [x] Rcvr1 Received '[Batch 1] Hello 10'
 [x] Rcvr2 Received '[Batch 2] Hello 5'
 [x] Rcvr1 Received '[Batch 1] Hello 11'
 [x] Rcvr2 Received '[Batch 2] Hello 7'
 [x] Rcvr1 Received '[Batch 1] Hello 12'
 [x] Rcvr2 Received '[Batch 2] Hello 9'
 [x] Rcvr1 Received '[Batch 1] Hello 13'
 [x] Rcvr2 Received '[Batch 2] Hello 11'

RabbitMQ显然可以做到这一点，我不确定自己做错了什么。我的简单代码如下：

发件人

public class Send {

    private final static String QUEUE_NAME = "hello";

    public static void main(String[] argv) throws Exception {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        try (Connection connection = factory.newConnection();
             Channel channel = connection.createChannel()) {
            channel.queueDeclare(QUEUE_NAME, true, false, false, null);
            for(int x=0; x<100; x++) {
                String message = "Hello "+x;
                channel.basicPublish("", QUEUE_NAME, null, message.getBytes("UTF-8"));
                System.out.println(" [x] Sent '" + message + "'");
            }
        }
    }
}

接收器

package com.mawv.ingest.rabbitmq;

import com.rabbitmq.client.*;

import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;

public class Recv {

    private final static String QUEUE_NAME = "hello";

    public static void main(String[] argv) throws Exception {
        ThreadPoolExecutor rcvrPool = (ThreadPoolExecutor) Executors.newFixedThreadPool(10);
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");

        Runnable rcvr1 = () -> {
            try {
                Connection connection = factory.newConnection();
                Channel channel = connection.createChannel();
                channel.queueDeclare(QUEUE_NAME, true, false, false, null);

                System.out.println(" [*] Rcvr1 Waiting for messages...");
                DeliverCallback deliverCallback = (consumerTag, delivery) -> {
                    Envelope envelope = delivery.getEnvelope();
                    String message = new String(delivery.getBody(), "UTF-8");
                    System.out.println(" [x] Rcvr1 Received '" + message + "'");
                    long deliveryTag = envelope.getDeliveryTag();
                    channel.basicAck(deliveryTag, true);
                    try {
                        Thread.sleep(1000);
                    } catch (Exception ex) { }

                };
                channel.basicConsume(QUEUE_NAME, false, deliverCallback, consumerTag -> {  });

            } catch(Exception ex){
                ex.printStackTrace();
            }
        };
        Runnable rcvr2 = () -> {
            try {
                Connection connection = factory.newConnection();
                Channel channel = connection.createChannel();
                channel.queueDeclare(QUEUE_NAME, true, false, false, null);

                System.out.println(" [*] Rcvr2 Waiting for messages...");
                DeliverCallback deliverCallback = (consumerTag, delivery) -> {
                    Envelope envelope = delivery.getEnvelope();
                    String message = new String(delivery.getBody(), "UTF-8");
                    System.out.println(" [x] Rcvr2 Received '" + message + "'");
                    long deliveryTag = envelope.getDeliveryTag();
                    channel.basicAck(deliveryTag, true);
                    try {
                        Thread.sleep(1000);
                    } catch (Exception ex) {
                    }
                };
                channel.basicConsume(QUEUE_NAME, false, deliverCallback, consumerTag -> {
                });
            } catch(Exception ex){
                ex.printStackTrace();
            }
        };
        rcvrPool.execute(rcvr1);
        rcvrPool.execute(rcvr2);

    }
}

我也捆绑了这个示例，正如他们描述的那样，并且看到了相同的结果。 https://self-learning-java-tutorial.blogspot.com/2015/09/rabbitmq-one-producer-and-multiple.html

我假设我的设置不正确。

Answer 1

根据RabbitMQ API：

“虽然一个通道可以被多个线程使用，但是重要的是要确保只有一个线程一次执行一个命令。命令的并行执行可能会导致抛出UnexpectedFrameError”

首先，我认为您应该为不同的线程使用不同的渠道。

最后，我认为第一个线程被终止是因为它处于空闲状态，因此只有第二个线程处于活动状态并且可以完成整个工作。在这种情况下，一个线程就足够了。

看看Java 8的ThreadPoolExecutor api：

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadPoolExecutor.html

例如，您可以找到：

“默认情况下，甚至核心线程也仅在新任务到达时才最初创建和启动，但是可以使用prestartCoreThread（）或prestartAllCoreThreads（）方法动态地覆盖它。如果使用非空队列”

和

“如果当前池中有多个corePoolSize线程，则多余的线程将在空闲时间超过keepAliveTime时终止（请参阅getKeepAliveTime（TimeUnit））。”

您应该使用prestartAllCoreThreads（）或prestartCoreThreads（）来使核心线程处于空闲状态，或者使用getKeepAliveTime（TimeUnit）使核心线程即使处于空闲状态也可以保持活动状态。

Answer 2

好像我缺少关键通道配置。这解决了我的问题：

channel.basicQos（1）;

这是RabbitMQ不得不说的。

公平派遣

您可能已经注意到调度仍然无法完全正常进行   如我们所愿。例如在有两个工人的情况下   消息很重，甚至消息很轻，一位工人就会   不断忙碌，另一人几乎不做任何工作。好，   RabbitMQ对此一无所知，仍会派遣   消息均匀。

之所以会这样，是因为RabbitMQ只是在   消息进入队列。它不看数量   消费者未确认的消息。它只是盲目派遣   发送给第n个消费者的每第n条消息。

为了克服这一点，我们可以将basicQos方法与   prefetchCount = 1设置。这告诉RabbitMQ不要给超过   一次向工人发送一条消息。换句话说，就是不要派遣   向工作人员发送新消息，直到它处理并确认   前一个。相反，它将分派给下一个工作人员   还不忙。

一个RabbitMQ队列上有多个使用者

2 个答案: