java面试题：MQ、RabbitMQ面试题（面试必问，精华版）

在rabbitmq 3.5.7及以上的版本提供了一个插件（rabbitmq-delayed-message-exchange）来实现延迟队列功能。同时插件依赖Erlang/OPT 18.0及以上。
 

插件源码地址：
https://github.com/rabbitmq/rabbitmq-delayed-message-exchange

插件下载地址：
https://bintray.com/rabbitmq/community-plugins/rabbitmq_delayed_message_exchange

安装：

进入插件安装目录
{rabbitmq-server}/plugins/（可以查看一下当前已存在的插件）
下载插件
rabbitmq_delayed_message_exchange

wget https://bintray.com/rabbitmq/community-plugins/download_file?file_path=rabbitmq_delayed_message_exchange-0.0.1.ez

    1

（如果下载的文件名称不规则就手动重命名一下如：
rabbitmq_delayed_message_exchange-0.0.1.ez）
启用插件

rabbitmq-plugins enable rabbitmq_delayed_message_exchange

(关闭插件)
rabbitmq-plugins disable rabbitmq_delayed_message_exchange


插件使用

通过声明一个x-delayed-message类型的exchange来使用delayed-messaging特性
x-delayed-message是插件提供的类型，并不是rabbitmq本身的

// ... elided code ...
Map<String, Object> args = new HashMap<String, Object>();
args.put("x-delayed-type", "direct");
channel.exchangeDeclare("my-exchange", "x-delayed-message", true, false, args);
// ... more code ...


发送消息的时候通过在header添加"x-delay"参数来控制消息的延时时间

// ... elided code ...
byte[] messageBodyBytes = "delayed payload".getBytes("UTF-8");
Map<String, Object> headers = new HashMap<String, Object>();
headers.put("x-delay", 5000);
AMQP.BasicProperties.Builder props = new AMQP.BasicProperties.Builder().headers(headers);
channel.basicPublish("my-exchange", "", props.build(), messageBodyBytes);
// ... more code ...


使用示例：

消息发送端：

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

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

public class Send {
    // 队列名称
    private final static String EXCHANGE_NAME="delay_exchange";
    private final static String ROUTING_KEY="key_delay";

    @SuppressWarnings("deprecation")
    public static void main(String[] argv) throws Exception {
        /**
         * 创建连接连接到MabbitMQ
         */
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("192.168.12.190");
        factory.setUsername("admin");
        factory.setPassword("admin");
        Connection connection = factory.newConnection();
        Channel channel = connection.createChannel();

        SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");

        // 声明x-delayed-type类型的exchange
        Map<String, Object> args = new HashMap<String, Object>();
        args.put("x-delayed-type", "direct");
        channel.exchangeDeclare(EXCHANGE_NAME, "x-delayed-message", true,
                false, args);


        Map<String, Object> headers = new HashMap<String, Object>();
        //设置在2016/11/04,16:45:12向消费端推送本条消息
        Date now = new Date();
        Date timeToPublish = new Date("2016/11/04,16:45:12");

        String readyToPushContent = "publish at " + sf.format(now)
                + " \t deliver at " + sf.format(timeToPublish);

        headers.put("x-delay", timeToPublish.getTime() - now.getTime());

        AMQP.BasicProperties.Builder props = new AMQP.BasicProperties.Builder()
                .headers(headers);
        channel.basicPublish(EXCHANGE_NAME, ROUTING_KEY, props.build(),
                readyToPushContent.getBytes());

        // 关闭频道和连接
        channel.close();
        connection.close();
    }
}


消息接收端：

import java.text.SimpleDateFormat;
import java.util.Date;

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

public class Recv {

    // 队列名称
    private final static String QUEUE_NAME = "delay_queue";
    private final static String EXCHANGE_NAME="delay_exchange";

    public static void main(String[] argv) throws Exception,
            java.lang.InterruptedException {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("192.168.12.190");
        factory.setUsername("admin");
        factory.setPassword("admin");
        Connection connection = factory.newConnection();
        Channel channel = connection.createChannel();

        QueueingConsumer queueingConsumer = new QueueingConsumer(channel);

        channel.queueDeclare(QUEUE_NAME, true,false,false,null);
        channel.queueBind(QUEUE_NAME, EXCHANGE_NAME, "");
        channel.basicConsume(QUEUE_NAME, true, queueingConsumer);
        SimpleDateFormat sf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
        try {
            System.out.println("****************WAIT***************");
            while(true){
                QueueingConsumer.Delivery delivery = queueingConsumer
                        .nextDelivery(); //

                String message = (new String(delivery.getBody()));
                System.out.println("message:"+message);
                System.out.println("now:\t"+sf.format(new Date()));
            }

        } catch (Exception exception) {
            exception.printStackTrace();

        }

    }
}


启动接收端，启动发送端
运行结果：

****************WAIT***************
message:publish at 2016-11-04 16:44:16.887      deliver at 2016-11-04 16:45:12.000
now:    2016-11-04 16:45:12.023


结果显示在我们2016-11-04 16:45:12.023接收到了消息，距离我们设定的时间2016-11-04 16:45:12.023有23毫秒的延迟

Note：使用rabbitmq-delayed-message-exchange插件时发送到队列的消息数量在web管理界面可能不可见,不影响正常功能使用

Note ：使用过程中发现，当一台启用了rabbitmq-delayed-message-exchange插件的RAM节点在重启的时候会无法启动，查看日志发现了一个Timeout异常，开发者解释说这是节点在启动过程会同步集群相关数据造成启动超时，并建议不要使用Ram节点