消息有序指可以按照消息的发送顺序来消费。RocketMQ可以保证消息是有序的。

在MQ模型中，序列需要通过3个阶段来保证：

1）发送消息时保持顺序。

2）消息的存储顺序与发送顺序相同。

3）当消息被消费时，它们的保存顺序与它们存储的顺序相同。

发送时保持顺序是指对于有顺序要求的消息，用户应该在同一个线程中同步发送。 存储和发送的顺序是一样的，它要求在同一个线程中发送的消息A和B必须在空间A之前存储。 消费和存储的一致性要求消息A和B到达Consumer后必须按照A和B的顺序进行处理。

1. 生产者

创建一个OrderProducer类：

import org.apache.rocketmq.client.exception.MQBrokerException;
import org.apache.rocketmq.client.exception.MQClientException;
import org.apache.rocketmq.client.producer.DefaultMQProducer;
import org.apache.rocketmq.client.producer.MessageQueueSelector;
import org.apache.rocketmq.client.producer.SendResult;
import org.apache.rocketmq.common.message.Message;
import org.apache.rocketmq.common.message.MessageQueue;
import org.apache.rocketmq.remoting.common.RemotingHelper;
import org.apache.rocketmq.remoting.exception.RemotingException;

import java.io.UnsupportedEncodingException;
import java.util.List;

public class OrderProducer {

    public static void main(String[] args) throws MQClientException, UnsupportedEncodingException, RemotingException, InterruptedException, MQBrokerException {
        //1. 创建消息生产者producer，并制定生产者组名
        DefaultMQProducer producer = new DefaultMQProducer("demo_producer_group");
        //2. 指定Nameserver地址
        producer.setNamesrvAddr("192.168.195.128:9876");
        //3. 启动producer
        producer.start();
        //4. 创建消息
        for (int i = 0; i < 5; i++) {
            Message msg = new Message("TopicOrderDemo",
                    "Tag1",
                    "Key" + i,
                    ("HelloWorld" + i).getBytes(RemotingHelper.DEFAULT_CHARSET));
            //5. 发送消息
            SendResult result = producer.send(msg, new MessageQueueSelector() {
                /**
                 *
                 * @param mqs：发送的消息信息
                 * @param msg：消息对象
                 * @param arg：指定对应的队列下标
                 * @return
                 */
                @Override
                public MessageQueue select(List<MessageQueue> mqs, Message msg, Object arg) {
                    //获取队列的下标
                    Integer index = (Integer) arg;
                    //获取对应下标的队列
                    return mqs.get(index);
                }
            }, 1);
            System.out.println(result);
        }
        //6. 关闭producer
        producer.shutdown();
    }
}

输出：

SendResult [sendStatus=SEND_OK, msgId=C0A8017A385818B4AAC250DAEAF70000, offsetMsgId=C0A8C38000002A9F000000000000086F, messageQueue=MessageQueue [topic=TopicOrderDemo, brokerName=rocketmq-nameserver1, queueId=1], queueOffset=0]

SendResult [sendStatus=SEND_OK, msgId=C0A8017A385818B4AAC250DAEB130001, offsetMsgId=C0A8C38000002A9F0000000000000937, messageQueue=MessageQueue [topic=TopicOrderDemo, brokerName=rocketmq-nameserver1, queueId=1], queueOffset=1]

SendResult [sendStatus=SEND_OK, msgId=C0A8017A385818B4AAC250DAEB1C0002, offsetMsgId=C0A8C38000002A9F00000000000009FF, messageQueue=MessageQueue [topic=TopicOrderDemo, brokerName=rocketmq-nameserver1, queueId=1], queueOffset=2]

SendResult [sendStatus=SEND_OK, msgId=C0A8017A385818B4AAC250DAEB1E0003, offsetMsgId=C0A8C38000002A9F0000000000000AC7, messageQueue=MessageQueue [topic=TopicOrderDemo, brokerName=rocketmq-nameserver1, queueId=1], queueOffset=3]

SendResult [sendStatus=SEND_OK, msgId=C0A8017A385818B4AAC250DAEB210004, offsetMsgId=C0A8C38000002A9F0000000000000B8F, messageQueue=MessageQueue [topic=TopicOrderDemo, brokerName=rocketmq-nameserver1, queueId=1], queueOffset=4]

2. 消费者

创建一个OrderConsumer类：

import org.apache.rocketmq.client.consumer.DefaultMQPushConsumer;
import org.apache.rocketmq.client.consumer.listener.*;
import org.apache.rocketmq.client.exception.MQClientException;
import org.apache.rocketmq.common.message.MessageExt;
import org.apache.rocketmq.remoting.common.RemotingHelper;

import java.io.UnsupportedEncodingException;
import java.util.List;

public class OrderConsumer {
    public static void main(String[] args) throws MQClientException {
        //1. 创建消费者Consumer，制定消费者组名
        DefaultMQPushConsumer consumer = new DefaultMQPushConsumer("demo_producer_group");
        //2. 指定Nameserver地址
        consumer.setNamesrvAddr("192.168.195.128:9876");
        //3. 订阅主题Topic和Tag
        consumer.subscribe("TopicOrderDemo", //要消费的主题
                "*"); //过滤规则
        //4. 创建消息监听
        consumer.setMessageListener(new MessageListenerOrderly() {
            @Override
            public ConsumeOrderlyStatus consumeMessage(List<MessageExt> msgs, ConsumeOrderlyContext context) {
                //读取消息
                for (MessageExt msg : msgs) {
                    //获取主题
                    String topic = msg.getTopic();
                    //获取标签
                    String tags = msg.getTags();
                    //获取信息
                    byte[] body = msg.getBody();
                    try {
                        String result = new String(body, RemotingHelper.DEFAULT_CHARSET);
                        System.out.println("OrderConsumer消费信息——Topic: " + topic + ", Tags: " + tags + ", Result: " + result);
                    } catch (UnsupportedEncodingException e) {
                        e.printStackTrace();
                        return ConsumeOrderlyStatus.SUSPEND_CURRENT_QUEUE_A_MOMENT;
                    }
                }
                return ConsumeOrderlyStatus.SUCCESS;
            }
        });
        //5.启动消费者consumer
        consumer.start();
    }
}

输出：

OrderConsumer消费信息——Topic: TopicOrderDemo, Tags: Tag1, Result: HelloWorld0

OrderConsumer消费信息——Topic: TopicOrderDemo, Tags: Tag1, Result: HelloWorld1

OrderConsumer消费信息——Topic: TopicOrderDemo, Tags: Tag1, Result: HelloWorld2

OrderConsumer消费信息——Topic: TopicOrderDemo, Tags: Tag1, Result: HelloWorld3

OrderConsumer消费信息——Topic: TopicOrderDemo, Tags: Tag1, Result: HelloWorld4