Introduction

RabbitMQ is a message broker（消息中间件）:

• it accepts and forwards messages.
  • You can think about it as a post office: when you put the mail that you want posting in a post box, you can be sure that Mr. Postman will eventually deliver the mail to your recipient. In this analogy, RabbitMQ is a post box, a post office and a postman.
  • The major difference between RabbitMQ and the post office is that it doesn't deal with paper, instead it accepts, stores and forwards binary blobs of data ‒ messages.
  • RabbitMQ, and messaging in general, uses some jargon.
• According to "Hello world in RabbitMQ with Java"

Brief model (Product-Consume-Model)

Producing means nothing more than sending. A program that sends messages is a producer :

Produce

• A queue is the name for a post box which lives inside RabbitMQ. Although messages flow through RabbitMQ and your applications, they can only be stored inside a queue. A queue is only bound by the host's memory & disk limits, it's essentially a large message buffer. Many producers can send messages that go to one queue, and many consumers can try to receive data from one queue. This is how we represent a queue:

// queue_name
口口口口口口口口口口...... 
// I am sorry about that I am not good at painting :(

Consuming has a similar meaning to receiving. A consumer is a program that mostly waits to receive messages:

Consume

• Note that the producer, consumer, and broker do not have to reside on the same host; indeed in most applications they don't.

Start with "Hello,RabbitMQ"

• In this part of the tutorial we'll write two programs in Java; a producer that sends a single message, and a consumer that receives messages and prints them out. We'll gloss over some of the detail in the Java API, concentrating on this very simple thing just to get started. It's a "Hello World" of messaging.

1. Simple Send And Receive with Named Queue

• SimpleSender.java

package tech.shunzi.mq.demo;

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

import static tech.shunzi.mq.demo.MQConstants.QUEUE_NAME;

public class SimpleSender {

    public static void main(String[] argv) throws Exception
    {
        ConnectionFactory factory = new ConnectionFactory();

        // connect to a broker with its name or ip address
        factory.setHost("localhost");
        Connection connection = factory.newConnection();

        // create a channel, which is where most of the API for getting things done resides.
        Channel channel = connection.createChannel();

        // Declaring a queue is idempotent - it will only be created if it doesn't exist already.
        // String queue, boolean durable, boolean exclusive, boolean autoDelete, Map<String, Object> arguments
        channel.queueDeclare(QUEUE_NAME, false, false, false, null);

        String message = "Hello World!";
        // The message content is a byte array, so you can encode whatever you like there.
        // String exchange, String routingKey, BasicProperties props, byte[] body
        channel.basicPublish("", QUEUE_NAME, null, message.getBytes("UTF-8"));

        System.out.println(" [x] Sent '" + message + "'");

        channel.close();
        connection.close();
    }
}

• SimpleReceiver.java

package tech.shunzi.mq.demo;

import com.rabbitmq.client.*;

import java.io.IOException;
import java.io.UnsupportedEncodingException;

import static tech.shunzi.mq.demo.MQConstants.QUEUE_NAME;

public class SimpleReceiver {

    public static void main(String[] argv) throws Exception
    {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        Connection connection = factory.newConnection();
        Channel channel = connection.createChannel();

        channel.queueDeclare(QUEUE_NAME, false, false, false, null);
        System.out.println(" [*] Waiting for messages. To exit press CTRL+C");

        Consumer consumer = new DefaultConsumer(channel)
        {
            @Override
            public void handleDelivery(String consumerTag, Envelope envelope, AMQP.BasicProperties properties, byte[] body) throws IOException, UnsupportedEncodingException {
                String message = new String(body, "UTF-8");
                System.out.println(" [x] Received '" + message + "'");
            }
        };
        channel.basicConsume(QUEUE_NAME, true, consumer);
    }
}

• Key Config: ConnectionFactory, Host, Connection, Channel, Queue

2. Distribute time-consuming tasks among multiple workers

• NewTask.java

package tech.shunzi.mq.demo.multi.consumer;

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

public class NewTask {

    private static final String TASK_QUEUE_NAME = "task_queue";

    public static void main(String[] argv) throws Exception {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        Connection connection = factory.newConnection();
        Channel channel = connection.createChannel();

        channel.queueDeclare(TASK_QUEUE_NAME, true, false, false, null);

        // get the argv from the command line and format e.g.: xxx xxx xxx
        String message = getMessage(argv);

        // MessageProperties.PERSISTENT_TEXT_PLAIN can make sure message persistent
        // Marking messages as persistent doesn't fully guarantee that a message won't be lost.
        channel.basicPublish("", TASK_QUEUE_NAME, MessageProperties.PERSISTENT_TEXT_PLAIN, message.getBytes("UTF-8"));
        System.out.println(" [x] Sent '" + message + "'");

        channel.close();
        connection.close();
    }

    private static String getMessage(String[] strings) {
        if (strings.length < 1) {
            return "Hello World!";
        }
        return joinStrings(strings, " ");
    }

    private static String joinStrings(String[] strings, String delimiter) {
        int length = strings.length;
        if (length == 0)
            return "";
        StringBuilder words = new StringBuilder(strings[0]);
        for (int i = 1; i < length; i++) {
            words.append(delimiter).append(strings[i]);
        }
        return words.toString();
    }
}

• Worker.java

package tech.shunzi.mq.demo.multi.consumer;

import com.rabbitmq.client.*;

import java.io.IOException;

public class Worker {

    private static final String TASK_QUEUE_NAME = "task_queue";

    public static void main(String[] argv) throws Exception {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        final Connection connection = factory.newConnection();
        final Channel channel = connection.createChannel();

        // make the queue durable.   true
        channel.queueDeclare(TASK_QUEUE_NAME, true, false, false, null);
        System.out.println(" [*] Waiting for messages. To exit press CTRL+C");

        // producer can only send a message to a consumer.   balanced load
        channel.basicQos(1);

        final Consumer consumer = new DefaultConsumer(channel) {
            @Override
            public void handleDelivery(String consumerTag, Envelope envelope, AMQP.BasicProperties properties, byte[] body) throws IOException {
                String message = new String(body, "UTF-8");

                System.out.println(" [x] Received '" + message + "'");
                try {
                    // simulate a task which cost some time.
                    doWork(message);
                } finally {
                    System.out.println(" [x] Done");
                    // false means need ack to verify.
                    channel.basicAck(envelope.getDeliveryTag(), false);
                }
            }
        };
        channel.basicConsume(TASK_QUEUE_NAME, false, consumer);
    }

    private static void doWork(String task) {
        for (char ch : task.toCharArray()) {
            if (ch == '.') {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException _ignored) {
                    Thread.currentThread().interrupt();
                }
            }
        }
    }
}

• Some key functions details: interface Channel.java

    /**
     * Declare a queue
     * @see com.rabbitmq.client.AMQP.Queue.Declare
     * @see com.rabbitmq.client.AMQP.Queue.DeclareOk
     * @param queue the name of the queue
     * @param durable true if we are declaring a durable queue (the queue will survive a server restart)
     * @param exclusive true if we are declaring an exclusive queue (restricted to this connection)
     * @param autoDelete true if we are declaring an autodelete queue (server will delete it when no longer in use)
     * @param arguments other properties (construction arguments) for the queue
     * @return a declaration-confirm method to indicate the queue was successfully declared
     * @throws java.io.IOException if an error is encountered
     */
    Queue.DeclareOk queueDeclare(String queue, boolean durable, boolean exclusive, boolean autoDelete,
                                 Map<String, Object> arguments) throws IOException;
                                 
    /**
     * Publish a message.
     *
     * Publishing to a non-existent exchange will result in a channel-level
     * protocol exception, which closes the channel.
     *
     * Invocations of <code>Channel#basicPublish</code> will eventually block if a
     * <a href="http://www.rabbitmq.com/alarms.html">resource-driven alarm</a> is in effect.
     *
     * @see com.rabbitmq.client.AMQP.Basic.Publish
     * @see <a href="http://www.rabbitmq.com/alarms.html">Resource-driven alarms</a>
     * @param exchange the exchange to publish the message to
     * @param routingKey the routing key
     * @param props other properties for the message - routing headers etc
     * @param body the message body
     * @throws java.io.IOException if an error is encountered
     */
    void basicPublish(String exchange, String routingKey, BasicProperties props, byte[] body) throws IOException; 
    
    /**
     * Request a specific prefetchCount "quality of service" settings
     * for this channel.
     *
     * @see #basicQos(int, int, boolean)
     * @param prefetchCount maximum number of messages that the server
     * will deliver, 0 if unlimited
     * @throws java.io.IOException if an error is encountered
     */
    void basicQos(int prefetchCount) throws IOException;
    
    /**
     * Acknowledge one or several received
     * messages. Supply the deliveryTag from the {@link com.rabbitmq.client.AMQP.Basic.GetOk}
     * or {@link com.rabbitmq.client.AMQP.Basic.Deliver} method
     * containing the received message being acknowledged.
     * @see com.rabbitmq.client.AMQP.Basic.Ack
     * @param deliveryTag the tag from the received {@link com.rabbitmq.client.AMQP.Basic.GetOk} or {@link com.rabbitmq.client.AMQP.Basic.Deliver}
     * @param multiple true to acknowledge all messages up to and
     * including the supplied delivery tag; false to acknowledge just
     * the supplied delivery tag.
     * @throws java.io.IOException if an error is encountered
     */
    void basicAck(long deliveryTag, boolean multiple) throws IOException;  
    
    /**
     * Start a non-nolocal, non-exclusive consumer, with
     * a server-generated consumerTag.
     * @param queue the name of the queue
     * @param autoAck true if the server should consider messages
     * acknowledged once delivered; false if the server should expect
     * explicit acknowledgements
     * @param callback an interface to the consumer object
     * @return the consumerTag generated by the server
     * @throws java.io.IOException if an error is encountered
     * @see com.rabbitmq.client.AMQP.Basic.Consume
     * @see com.rabbitmq.client.AMQP.Basic.ConsumeOk
     * @see #basicConsume(String, boolean, String, boolean, boolean, Map, Consumer)
     */
    String basicConsume(String queue, boolean autoAck, Consumer callback) throws IOException;

• autoAck:
  • true - The msg will be acknowledged once delivered
  • false - The server expects explicit acknowledgements. (It means that manual ack publish is necessary. basicAck())
• durable: (Producer & Consumer Both Config this)
  • true - To make sure that RabbitMQ will never lose our queue. And it only makes sense when first create/setup.
• BasicProperties props :
  • MessageProperties.PERSISTENT_TEXT_PLAIN - Make sure the message persistent. Attention: The persistence guarantees aren't strong, but it's more than enough for our simple task queue. One situation is that message is stored in cache and hasn't be stored on disk.
• prefetchCount: maximum number of messages that the server will deliver
  • 1 - This tells RabbitMQ not to give more than one message to a worker at a time. Or, in other words, don't dispatch a new message to a worker until it has processed and acknowledged the previous one. Instead, it will dispatch it to the next worker that is not still busy.
  • ...