Messaging with RabbitMQ

This guide walks you through the process of setting up a RabbitMQ AMQP server that publishes and subscribes to messages and creating a Spring Boot application to interact with that RabbitMQ server.

What You Will Build

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

What You Need

About 15 minutes
A favorite text editor or IDE
JDK 11 or later
Gradle 4+ or Maven 3.2+
You can also import the code straight into your IDE:
- Spring Tool Suite (STS)
- IntelliJ IDEA
  - Set up the RabbitMQ server. See Set up the RabbitMQ Broker.

How to complete this guide

Like most Spring Getting Started guides, you can start from scratch and complete each step or you can bypass basic setup steps that are already familiar to you. Either way, you end up with working code.

To start from scratch, move on to Set up the RabbitMQ Broker.

To skip the basics, do the following:

Download and unzip the source repository for this guide, or clone it using Git: git clone https://github.com/spring-guides/gs-messaging-rabbitmq.git
cd into gs-messaging-rabbitmq/initial
Jump ahead to Starting with Spring Initializr.

When you finish, you can check your results against the code in gs-messaging-rabbitmq/complete.

Set up the RabbitMQ Broker

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

RabbitMQ is an AMQP server. The server is freely available at https://www.rabbitmq.com/download.html. You can download it manually or, if you use a Mac with Homebrew, by running the following command in a terminal window:

brew install rabbitmq

Unpack the server and launch it with default settings by running the following command in a terminal window:

rabbitmq-server

You should see output similar to the following:

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

You can also use Docker Compose to quickly launch a RabbitMQ server if you have Docker running locally. There is a docker-compose.yml in the root of the complete project in Github. It is very simple, as the following listing shows:

rabbitmq:
  image: rabbitmq:management
  ports:
    - "5672:5672"
    - "15672:15672"

With this file in the current directory, you can run docker-compose up to get RabbitMQ running in a container.

Starting with Spring Initializr

You can use this pre-initialized project and click Generate to download a ZIP file. This project is configured to fit the examples in this tutorial.

To manually initialize the project:

Navigate to https://start.spring.io. This service pulls in all the dependencies you need for an application and does most of the setup for you.
Choose either Gradle or Maven and the language you want to use. This guide assumes that you chose Java.
Click Dependencies and select Spring for RabbitMQ.
Click Generate.
Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.

If your IDE has the Spring Initializr integration, you can complete this process from your IDE.

You can also fork the project from Github and open it in your IDE or other editor.

Create a RabbitMQ Message Receiver

With any messaging-based application, you need to create a receiver that responds to published messages. The following listing (from src/main/java/com.example.messagingrabbitmq/Receiver.java) shows how to do so:

package com.example.messagingrabbitmq;

import java.util.concurrent.CountDownLatch;
import org.springframework.stereotype.Component;

@Component
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 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 lets it signal that the message has been 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. However, you need to:

Configure a message listener container.
Declare the queue, the exchange, and the binding between them.
Configure a component to send some messages to test the listener.

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

You will use RabbitTemplate to send messages, and you will register a Receiver with the message listener container to receive messages. The connection factory drives both, letting them connect to the RabbitMQ server. The following listing (from src/main/java/com.example.messagingrabbitmq/MessagingRabbitApplication.java) shows how to create the application class:

package com.example.messagingrabbitmq;

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.listener.SimpleMessageListenerContainer;
import org.springframework.amqp.rabbit.listener.adapter.MessageListenerAdapter;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class MessagingRabbitmqApplication {

  static final String topicExchangeName = "spring-boot-exchange";

  static final String queueName = "spring-boot";

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

  @Bean
  TopicExchange exchange() {
    return new TopicExchange(topicExchangeName);
  }

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

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

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

  public static void main(String[] args) throws InterruptedException {
    SpringApplication.run(MessagingRabbitmqApplication.class, args).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. For example, if spring-webmvc is on the classpath, this annotation 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 com/example package, letting it find the controllers.

The main() method uses Spring Boot’s SpringApplication.run() method to launch an application. Did you notice that there was not a single line of XML? There is no web.xml file, either. This web application is 100% pure Java and you did not 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 listens 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 that it invokes 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 do not send messages directly to queues. Instead, a message is sent to an exchange, which can go to a single queue or fan out to multiple queues, emulating the concept of JMS topics.

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.

In this case, we use a topic exchange, and the queue is bound with a routing key of foo.bar.#, which means that any messages sent with a routing key that begins with foo.bar. are routed to the queue.

Send a Test Message

In this sample, test messages are sent by a CommandLineRunner, which also waits for the latch in the receiver and closes the application context. The following listing (from src/main/java/com.example.messagingrabbitmq/Runner.java) shows how it works:

package com.example.messagingrabbitmq;

import java.util.concurrent.TimeUnit;

import org.springframework.amqp.rabbit.core.RabbitTemplate;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

@Component
public class Runner implements CommandLineRunner {

  private final RabbitTemplate rabbitTemplate;
  private final Receiver receiver;

  public Runner(Receiver receiver, RabbitTemplate rabbitTemplate) {
    this.receiver = receiver;
    this.rabbitTemplate = rabbitTemplate;
  }

  @Override
  public void run(String... args) throws Exception {
    System.out.println("Sending message...");
    rabbitTemplate.convertAndSend(MessagingRabbitmqApplication.topicExchangeName, "foo.bar.baz", "Hello from RabbitMQ!");
    receiver.getLatch().await(10000, TimeUnit.MILLISECONDS);
  }

}

Notice that the template routes the message to the exchange with a routing key of foo.bar.baz, which matches the binding.

In tests, you can mock out the runner so that the receiver can be tested in isolation.

Run the Application

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

Build an executable JAR

You can run the application from the command line with Gradle or Maven. You can also build a single executable JAR file that contains all the necessary dependencies, classes, and resources and run that. Building an executable jar makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.

If you use Gradle, you can run the application by using ./gradlew bootRun. Alternatively, you can build the JAR file by using ./gradlew build and then run the JAR file, as follows:

java -jar build/libs/gs-messaging-rabbitmq-0.1.0.jar

If you use Maven, you can run the application by using ./mvnw spring-boot:run. Alternatively, you can build the JAR file with ./mvnw clean package and then run the JAR file, as follows:

java -jar target/gs-messaging-rabbitmq-0.1.0.jar

The steps described here create a runnable JAR. You can also build a classic WAR file.

You should see the following output:

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

Summary

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