Exchange JSON messages with Spring Boot AMQP and RabbitMQ

Setting up a Spring Boot application using AMQP with RabbitMQ is not really difficult, and there are many guides on the Internet showing you the basic setup. But creating an application making use of @RabbitListener annotations and producing and consuming messages in JSON format is trickier, so I would like to share with you a really simple but more serious approach that those hello-messaging apps.

This article covers:

• How to send/publish Java Objects as JSON messages using Spring Boot and RabbitMQ’s RabbitTemplate.
• How to read/consume JSON messages to Java Objects using Spring Boot and RabbitMQ’s @RabbitListener annotation.
• How to send and receive Java Objects through RabbitMQ using default Java serializer.

Introduction

As you probably know if you reached this article, AMQP is a specification, a protocol that defines messaging and patterns for communication between systems. RabbitMQ is a software that implements AMQP. You can find a lot of tutorials about these concepts on the Internet, apart from many useful books.

Goal

Within this article we’ll set up a Spring Boot application that uses RabbitMQ to send messages to a topic exchange formatted as JSON, and read them using two different queues:

• The first Queue will read the contents as a generic org.springframework.amqp.core.Message class.
• The second Queue will transform the JSON message to a specific class created by us.

I’ll also show you an alternative configuration to make this work without formatting the message to JSON, but nowadays this format is a good choice for intercommunication between applications and systems that may differ in programming languages, for instance.

I recommend you to check the implementation as you follow these instructions. You can do so by cloning the GitHub repository. Don’t forget to give it a star if you find it useful!

Setting up the project

pom.xml

Creating a simple Spring Boot application is pretty straightforward, in my case I use Spring Boot 1.5.10 and Maven. The only thing you need to do to make use of AMQP with RabbitMQ in Spring Boot is to include the corresponding library, Spring Rabbit:

The message

In this case, we’re going to send a really simple content, but enough to have an idea about how the message looks like in different formats. The class is called CustomMessage.

Pretty basic POJO. The only things to point out are the default constructor, which we need for JSON deserialization, and the toString() method, that will give us a human-readable representation of the objects.

Sending messages

As mentioned before, this example will use a topic exchange to send CustomMessage objects.

Some comments about this service:

• @Scheduled  annotation is used to schedule a method to be executed at a given time, or with a given frequency. In this case, this method will be executed every 3 seconds (until the end of the times or you stop the application, whatever happens earlier). You can check more info here.
• RabbitTemplate is used to convert and send a message using RabbitMQ. Is a helper class, as many other Template classes existing in Spring (such as JdbcTemplate , RestTemplate , etc.). Spring Boot creates a predefined one for you, but in this case, we need to tune it a little bit to make it use JSON converter when producing messages.
• The exchange and the routing key are AMQP concepts that I recommend you to read about that if you still don’t know what they do. As a summary, we are sending messages through a given channel (exchange), and the routing key allows us to filter for who are those generated. This sample application is not intended for showing advanced exchange / queues configuration so you will see that this is a basic setup.

Receiving messages

Of course, we need to build the other part of the communication too. For this post, I’ve created the receivers in the same application for better logistics (having a single repo), but you shouldn’t have problems to replicate this configuration with two different apps if you want. Actually, you could run this Spring Boot application twice and remove the producer in one instance, the consumers in another one, to make it more like a ‘real case’.

Here you can see our goal: the idea is to show at the same time how the same message, being sent by the CustomMessageSender class, can be received through two different queues. First queue (GENERIC) is receiving the message as a generic Message class, so there is no implicit conversion. Spoiler alert: when we log the message we should see something like a JSON string in the payload since it has not been converted.

On the other hand, the queue SPECIFIC is expecting a CustomMessage class (the method argument). That will trigger a logic inside Spring to find a converter from JSON to that specific class. This part should be easy to configure, but sometimes is the part in which we the developers struggle most. In this case is easy to guess that the text logged should be the CustomMessage’s implementation of toString().

Configuration

This is the most interesting part since it’s what is making everything work together. First, you will find some annotations in the class:

Apart from the one that makes Spring Boot works, @EnableRabbit is making sure that Rabbit Listeners and other Rabbit configurations are detected and processed. @EnableScheduling needs to be added to support the @Scheduled method for sending messages that we covered before in CustomMessageSender class.

Then you’ll find some constants and, next to them, the Exchange / Queues configuration:

These are the topic exchange, both queues and the bindings between them (both queues are bound to the same exchange with the same routing key). Just a sample configuration.

Down in the same class, we can find the configuration related to producing messages:

The only thing we’re doing here is overriding the default RabbitTemplate created by Spring Boot with another one that will use a message converter of type Jackson2JsonMessageConverter. Remember: this configuration only affects to messages being sent by RabbitTemplate.

And then we need to configure how our application should interpret the received messages. You need to implement a method from the interface  RabbitListenerConfigurer, responsible for handling the RabbitListener annotations. As you can see below, we need to configure the MessageHandlerMethodFactory to be not plainly the default one, but using a JSON message converter. Take into account that this converter is not the same as configured for producing messages since we’re using different classes.

Running the application

First, you need to make sure that the RabbitMQ server is running. In the repository, there is a docker-compose.yml file that you can use if you have Docker installed. Just run:

Also, if you want to use a configuration which is different from the default one, make sure you include those changes within the application.properties file (port number, authentication, etc.). You don’t need to do this if you use the default settings as they are set in this sample codebase.

Then you can run the application class from your IDE or by executing mvn spring-boot:run from the command line. When you do that, you should see these lines in the console:

Working as expected! Our listener for CustomMessage is showing us our implementation of toString() method. The last line though, the method listening for the Message class, is showing us the raw contents, which turns out to be –of course– JSON format.

Changing from JSON format to default

I invite you to comment the lines that set up the producer and consumer message converters (they are indicated in the source code). Then, remove the implements clause from the configuration class. What you will see then is the default configuration for Spring Boot, which is using a Java-specific format to encode messages.

As you can see in the headers of the message, now the format is  contentType=application/x-java-serialized-object. This is just another approach, which could turn out to be not the best one if you are using, for instance, different kind of applications (non-Java) for receiving messages.

Thanks and… do you want to know more?

That’s all, I hope you find this how-to guide useful. If you are interested in how to apply Event Driven patterns with RabbitMQ in a Microservice Architecture, and some other related topics like Service Discovery, Routing, Async vs. Sync communication, etc., you can get my book Learn Microservices with Spring Boot (also available on Apress), which will guide you step by step to get real experience.