This guide walks you through the process of setting up a RabbitMQ AMQP server that publishes and subscribes to messages.

What you’ll build

You’ll build an application that publishes a message using Spring AMQP’s RabbitTemplate and subscribes to the message on a POJO using MessageListenerAdapter.

What you’ll need

+ - RabbitMQ server (installation instructions below)

Set up RabbitMQ broker

Before you can build your messaging application, you need to set up the server that will handle receiving and sending messages.

RabbitMQ is an AMQP server. The server is freely available at http://www.rabbitmq.com/download.html. You can download it manually, or if you are using a Mac with homebrew:

brew install rabbitmq

Unpack the server and launch it with default settings.

rabbitmq-server

You should see something like this:

            RabbitMQ 3.1.3. Copyright (C) 2007-2013 VMware, Inc.
##  ##      Licensed under the MPL.  See http://www.rabbitmq.com/
##  ##
##########  Logs: [email protected]
######  ##        [email protected]
##########
            Starting broker... completed with 6 plugins.

Create a RabbitMQ message receiver

With any messaging-based application, you need to create a receiver that will respond to published messages.

src/main/java/hello/Receiver.java

package hello;

import java.util.concurrent.CountDownLatch;

public class Receiver {

	private CountDownLatch latch = new CountDownLatch(1);

	public void receiveMessage(String message) {
		System.out.println("Received <" + message + ">");
		latch.countDown();
	}

	public CountDownLatch getLatch() {
		return latch;
	}

}

The Receiver is a simple POJO that defines a method for receiving messages. When you register it to receive messages, you can name it anything you want.

For convenience, this POJO also has a CountDownLatch. This allows it to signal that the message is received. This is something you are not likely to implement in a production application.

Register the listener and send a message

Spring AMQP’s RabbitTemplate provides everything you need to send and receive messages with RabbitMQ. Specifically, you need to configure:

  • A message listener container

  • Declare the queue, the exchange, and the binding between them

Spring Boot automatically creates a connection factory and a RabbitTemplate, reducing the amount of code you have to write.

You’ll use RabbitTemplate to send messages, and you will register a Receiver with the message listener container to receive messages. The connection factory drives both, allowing them to connect to the RabbitMQ server.

src/main/java/hello/Application.java

package hello;

import java.util.concurrent.TimeUnit;

import org.springframework.amqp.core.Binding;
import org.springframework.amqp.core.BindingBuilder;
import org.springframework.amqp.core.Queue;
import org.springframework.amqp.core.TopicExchange;
import org.springframework.amqp.rabbit.connection.ConnectionFactory;
import org.springframework.amqp.rabbit.core.RabbitTemplate;
import org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer;
import org.springframework.amqp.rabbit.listener.adapter.MessageListenerAdapter;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class Application implements CommandLineRunner {

	final static String queueName = "spring-boot";

	@Autowired
	AnnotationConfigApplicationContext context;

	@Autowired
	RabbitTemplate rabbitTemplate;

	@Bean
	Queue queue() {
		return new Queue(queueName, false);
	}

	@Bean
	TopicExchange exchange() {
		return new TopicExchange("spring-boot-exchange");
	}

	@Bean
	Binding binding(Queue queue, TopicExchange exchange) {
		return BindingBuilder.bind(queue).to(exchange).with(queueName);
	}

	@Bean
	SimpleMessageListenerContainer container(ConnectionFactory connectionFactory, MessageListenerAdapter listenerAdapter) {
		SimpleMessageListenerContainer container = new SimpleMessageListenerContainer();
		container.setConnectionFactory(connectionFactory);
		container.setQueueNames(queueName);
		container.setMessageListener(listenerAdapter);
		return container;
	}

    @Bean
    Receiver receiver() {
        return new Receiver();
    }

	@Bean
	MessageListenerAdapter listenerAdapter(Receiver receiver) {
		return new MessageListenerAdapter(receiver, "receiveMessage");
	}

    public static void main(String[] args) throws InterruptedException {
        SpringApplication.run(Application.class, args);
    }

    @Override
    public void run(String... args) throws Exception {
        System.out.println("Waiting five seconds...");
        Thread.sleep(5000);
        System.out.println("Sending message...");
        rabbitTemplate.convertAndSend(queueName, "Hello from RabbitMQ!");
        receiver().getLatch().await(10000, TimeUnit.MILLISECONDS);
        context.close();
    }
}

@SpringBootApplication is a convenience annotation that adds all of the following:

  • @Configuration tags the class as a source of bean definitions for the application context.

  • @EnableAutoConfiguration tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.

  • Normally you would add @EnableWebMvc for a Spring MVC app, but Spring Boot adds it automatically when it sees spring-webmvc on the classpath. This flags the application as a web application and activates key behaviors such as setting up a DispatcherServlet.

  • @ComponentScan tells Spring to look for other components, configurations, and services in the hello package.

The main() method uses Spring Boot’s SpringApplication.run() method to launch an application. Did you notice that there wasn’t a single line of XML? No web.xml file either. This web application is 100% pure Java and you didn’t have to deal with configuring any plumbing or infrastructure.

The bean defined in the listenerAdapter() method is registered as a message listener in the container defined in container(). It will listen for messages on the "spring-boot" queue. Because the Receiver class is a POJO, it needs to be wrapped in the MessageListenerAdapter, where you specify it to invoke receiveMessage.

JMS queues and AMQP queues have different semantics. For example, JMS sends queued messages to only one consumer. While AMQP queues do the same thing, AMQP producers don’t send messages directly to queues. Instead, a message is sent to an exchange, which can go to a single queue, or fanout to multiple queues, emulating the concept of JMS topics. For more, see Understanding AMQP.

The message listener container and receiver beans are all you need to listen for messages. To send a message, you also need a Rabbit template.

The queue() method creates an AMQP queue. The exchange() method creates a topic exchange. The binding() method binds these two together, defining the behavior that occurs when RabbitTemplate publishes to an exchange.

Spring AMQP requires that the Queue, the TopicExchange, and the Binding be declared as top level Spring beans in order to be set up properly.

The main() method starts that process by creating a Spring application context. This starts the message listener container, which will start listening for messages. There is a run() method which is then automatically executed: it then retrieves the RabbitTemplate from the application context, waits five seconds, and sends a "Hello from RabbitMQ!" message on the "spring-boot" queue. Finally, it closes the Spring application context and the application ends.

You should see the following output:

Sending message...
Received <Hello from RabbitMQ!>

Summary

Congratulations! You’ve just developed a simple publish-and-subscribe application with Spring and RabbitMQ. There’s more you can do with Spring and RabbitMQ than what is covered here, but this should provide a good start.