Table of Contents

RabbitMQ and the Micronaut Framework - Event-Driven Applications

Use RabbitMQ to communicate between your Micronaut applications.

Authors: Iván López

Micronaut Version: 4.6.3

1. Getting Started

In this guide, we will create a Micronaut application written in Kotlin.

In this guide, we will create two microservices that will use RabbitMQ to communicate with each other in an asynchronous and decoupled way.

RabbitMQ is an open-source message-broker software that originally implemented the Advanced Message Queuing Protocol (AMQP) and has since been extended with a plug-in architecture to support Streaming Text Oriented Messaging Protocol (STOMP), Message Queuing Telemetry Transport (MQTT), and other protocols.

2. What you will need

To complete this guide, you will need the following:

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

4. Writing the application

Let’s describe the microservices you will build through the guide.

  • books - It returns a list of books. It uses a domain consisting of a book name and ISBN. It also publishes a message in RabbitMQ every time a book is accessed.

  • analytics - It connects to RabbitMQ to update the analytics for every book (a counter). It also exposes an endpoint to get the analytics.

Setup IntelliJ IDEA to develop Micronaut Applications.

4.1. Books microservice

Create the books microservice using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app --features=rabbitmq,reactor,graalvm example.micronaut.books --build=maven --lang=kotlin
If you don’t specify the --build argument, Gradle with the Kotlin DSL is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.
If you don’t specify the --test argument, JUnit is used for Java and Kotlin, and Spock is used for Groovy.

If you use Micronaut Launch, select Micronaut Application as application type and add the rabbitmq, reactor, and graalvm features.

The previous command creates a directory named books and a Micronaut application inside it with default package example.micronaut.

Create a BookController class to handle incoming HTTP requests into the books microservice:

books/src/main/kotlin/example/micronaut/BookController.kt
package example.micronaut

import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller("/books") (1)
class BookController(private val bookService: BookService) { (2)

    @Get (3)
    fun listAll(): List<Book> = bookService.listAll()

    @Get("/{isbn}") (4)
    fun findBook(isbn: String): Book? = bookService.findByIsbn(isbn)
}
1 The class is defined as a controller with the @Controller annotation mapped to the path /books.
2 Inject BookService using constructor injection.
3 The @Get annotation maps the listAll method to an HTTP GET request on /books.
4 The @Get annotation maps the findBook method to an HTTP GET request on /books/{isbn}.

The previous controller responds a List<Book>. Create the Book POJO:

books/src/main/kotlin/example/micronaut/Book.kt
package example.micronaut

import io.micronaut.serde.annotation.Serdeable

@Serdeable
data class Book(val isbn: String, val name: String)

To keep this guide simple there is no database persistence, and the list of books is kept in memory in BookService:

books/src/main/kotlin/example/micronaut/BookService.kt
package example.micronaut

import jakarta.annotation.PostConstruct
import jakarta.inject.Singleton

@Singleton
class BookService {

    private val bookStore: MutableList<Book> = mutableListOf()

    @PostConstruct
    fun init() {
        bookStore.add(Book("1491950358", "Building Microservices"))
        bookStore.add(Book("1680502395", "Release It!"))
        bookStore.add(Book("0321601912", "Continuous Delivery"))
    }

    fun listAll(): List<Book> = bookStore

    fun findByIsbn(isbn: String): Book? =
        bookStore.firstOrNull { it.isbn == isbn }
}

4.2. Analytics microservice

Create the analytics microservice using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app --features=rabbitmq,graalvm example.micronaut.analytics --build=maven --lang=kotlin
If you don’t specify the --build argument, Gradle with the Kotlin DSL is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.
If you don’t specify the --test argument, JUnit is used for Java and Kotlin, and Spock is used for Groovy.

If you use Micronaut Launch, select Micronaut Application as application type and add the kafka and graalvm features.

To keep this guide simple there is no database persistence, and the books analytics is kept in memory in AnalyticsService:

analytics/src/main/kotlin/example/micronaut/AnalyticsService.kt
package example.micronaut

import java.util.concurrent.ConcurrentHashMap
import jakarta.inject.Singleton

@Singleton
class AnalyticsService {

    private val bookAnalytics: MutableMap<Book, Long> = ConcurrentHashMap() (1)

    fun updateBookAnalytics(book: Book) { (2)
        bookAnalytics.compute(book) { k, v ->
            if (v == null) return@compute 1L else return@compute v + 1
        }
    }

    fun listAnalytics(): List<BookAnalytics> = (3)
            bookAnalytics.entries.map { (key, value) -> BookAnalytics(key.isbn, value) }
}
1 Keep the books analytics in memory.
2 Initialize and update the analytics for the book passed as parameter.
3 Return all the analytics.

Create the Book POJO used by AnalyticsService:

analytics/src/main/kotlin/example/micronaut/Book.kt
package example.micronaut

import io.micronaut.serde.annotation.Serdeable

@Serdeable
data class Book(val isbn: String, val name: String)

The previous service responds a List<BookAnalytics>. Create the BookAnalytics POJO:

analytics/src/main/kotlin/example/micronaut/BookAnalytics.kt
package example.micronaut

import io.micronaut.serde.annotation.Serdeable

@Serdeable
data class BookAnalytics(val bookIsbn: String, val count: Long)

Write a test:

analytics/src/test/kotlin/example/micronaut/AnalyticsServiceTest.kt
package example.micronaut

import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test
import jakarta.inject.Inject

@MicronautTest (1)
class AnalyticsServiceTest {

    @Inject (2)
    lateinit var analyticsService: AnalyticsService

    @Test
    fun testUpdateBookAnalyticsAndGetAnalytics() {
        val b1 = Book("1491950358", "Building Microservices")
        val b2 = Book("1680502395", "Release It!")

        analyticsService.updateBookAnalytics(b1)
        analyticsService.updateBookAnalytics(b1)
        analyticsService.updateBookAnalytics(b1)
        analyticsService.updateBookAnalytics(b2)

        val analytics = analyticsService.listAnalytics()

        assertEquals(2, analytics.size)
        assertEquals(3, findBookAnalytics(b1, analytics).count)
        assertEquals(1, findBookAnalytics(b2, analytics).count)
    }

    private fun findBookAnalytics(b: Book, analytics: List<BookAnalytics>): BookAnalytics {
        val ba : BookAnalytics? = analytics.filter { (bookIsbn) -> bookIsbn == b.isbn }.firstOrNull()
        return ba ?: throw RuntimeException("Book not found")
    }
}
1 micronaut-test-junit5 is added automatically to build.gradle (or pom.xml) when creating an application with the CLI. For more information, see the documentation.
2 Just inject the collaborator and @MicronautTest will take care of everything.

Create a Controller to expose the analytics:

analytics/src/main/kotlin/example/micronaut/AnalyticsController.kt
package example.micronaut

import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller("/analytics")
class AnalyticsController(private val analyticsService: AnalyticsService) {

    @Get
    fun listAnalytics(): List<BookAnalytics> = (1)
        analyticsService.listAnalytics()
}
1 Just expose the analytics.

The application doesn’t expose the method updateBookAnalytics created in AnalyticsService. This method will be invoked when reading messages from RabbitMQ.

To run the tests:

analytics
./mvnw test

Modify the Application class to use dev as a default environment:

The Micronaut framework supports the concept of one or many default environments. A default environment is one that is only applied if no other environments are explicitly specified or deduced.

analytics/src/main/kotlin/example/micronaut/Application.kt
package example.micronaut

import io.micronaut.context.env.Environment.DEVELOPMENT
import io.micronaut.runtime.Micronaut.build

fun main(args: Array<String>) {
    build()
            .args(*args)
            .packages("example.micronaut")
            .defaultEnvironments(DEVELOPMENT)
            .start()
}

Create src/main/resources/application-dev.yml. The Micronaut framework applies this configuration file only for the dev environment.

analytics/src/main/resources/application-dev.yml
micronaut:
  server:
    port: 8081 (1)
1 Start the analytics microservice on port 8081.

5. Test Resources

When the application is started locally — either under test or by running the application — resolution of the rabbitmq.uri property is detected and the Test Resources service will start a local RabbitMQ docker container, and inject the properties required to use this as the message broker.

When running under production, you should replace this property with the location of your production message broker via an environment variable.

RABBITMQ_URI=amqp://production-server:5672

For more information, see the RabbitMQ section of the Test Resources documentation.

6. Running the application

Run the books microservice:

books
./mvnw mn:run
16:35:55.614 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 576ms. Server Running: http://localhost:8080

Run analytics microservice:

analytics
./mvnw mn:run
16:35:55.614 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 623ms. Server Running: http://localhost:8081

You can run curl commands to test the application:

curl http://localhost:8080/books
[{"isbn":"1491950358","name":"Building Microservices"},{"isbn":"1680502395","name":"Release It!"},{"isbn":"0321601912","name":"Continuous Delivery"}]
curl http://localhost:8080/books/1491950358
{"isbn":"1491950358","name":"Building Microservices"}
curl http://localhost:8081/analytics
[]

Please note that getting the analytics returns an empty list because the applications are not communicating to each other (yet).

6.1. Books microservice

Via Test Resources the Micronaut application will connect to a RabbitMQ instance running inside Docker so it is not necessary to add anything to application.yml. In case you want to change the configuration, add the following:

books/src/main/resources/application-prod.yml
rabbitmq:
  uri: amqp://rabbitmq-server:5672

6.1.1. Create RabbitMQ exchange, queue and binding

Before being able to send and receive messages using RabbitMQ it is necessary to define the exchange, queue and binding. One option is create them directly in the RabbitMQ Admin UI available on port 15672.

Use guest for both username and password.

Another option is to create them programmatically. Create the class ChannelPoolListener:

books/src/main/kotlin/example/micronaut/ChannelPoolListener.kt
package example.micronaut

import com.rabbitmq.client.BuiltinExchangeType
import com.rabbitmq.client.Channel
import io.micronaut.rabbitmq.connect.ChannelInitializer
import java.io.IOException
import jakarta.inject.Singleton

@Singleton
class ChannelPoolListener : ChannelInitializer() {

    @Throws(IOException::class)
    override fun initialize(channel: Channel, name: String) {
        channel.exchangeDeclare("micronaut", BuiltinExchangeType.DIRECT, true) (1)
        channel.queueDeclare("analytics", true, false, false, null) (2)
        channel.queueBind("analytics", "micronaut", "analytics") (3)
    }
}
1 Define an exchange named micronaut. From the producer point of view everything is sent to the exchange with the appropriate routing key
2 Define a queue named analytics. The consumer will listen for messages in that queue.
3 Define a binding between the exchange and the queue using the routing key analytics.

6.1.2. Create RabbitMQ client (producer)

Let’s create an interface to send messages to RabbitMQ. The Micronaut framework will implement the interface at compilation time:

books/src/main/kotlin/example/micronaut/AnalyticsClient.kt
package example.micronaut

import io.micronaut.rabbitmq.annotation.Binding
import io.micronaut.rabbitmq.annotation.RabbitClient

@RabbitClient("micronaut") (1)
interface AnalyticsClient {

    @Binding("analytics") (2)
    fun updateAnalytics(book: Book) (3)
}
1 Set the exchange used to send the messages.
2 Set the routing key.
3 Send the Book POJO. The Micronaut framework will automatically convert it to JSON before sending it.

6.1.3. Send Analytics information automatically

Sending a message to RabbitMQ is as simple as injecting AnalyticsClient and calling updateAnalytics method. The goal is to do it automatically every time a book is returned, i.e., every time there is a call to http://localhost:8080/books/{isbn}. To achieve this we will create an Http Server Filter. Create the AnalyticsFilter class:

books/src/main/kotlin/example/micronaut/AnalyticsFilter.kt
package example.micronaut

import io.micronaut.http.HttpRequest
import io.micronaut.http.MutableHttpResponse
import io.micronaut.http.annotation.Filter
import io.micronaut.http.filter.HttpServerFilter
import io.micronaut.http.filter.ServerFilterChain
import reactor.core.publisher.Flux
import reactor.core.publisher.Mono
import org.reactivestreams.Publisher

@Filter("/books/?*") (1)
class AnalyticsFilter(private val analyticsClient: AnalyticsClient) (3)
    : HttpServerFilter { (2)

    override fun doFilter(request: HttpRequest<*>, (4)
                          chain: ServerFilterChain): Publisher<MutableHttpResponse<*>> =
            Flux
                    .from(chain.proceed(request)) (5)
                    .flatMap { response: MutableHttpResponse<*> ->
                        Mono.fromCallable {
                            val book = response.getBody(Book::class.java) (6)
                            book.ifPresent { book: Book -> analyticsClient.updateAnalytics(book) } (7)

                            response
                        }
                    }
}
1 Annotate the class with @Filter and define the ANT Matcher pattern to intercept all the calls to the desire URI.
2 The class needs to implement HttpServerFilter.
3 Constructor injection for RabbitMQ AnalyticsClient.
4 Override doFilter method.
5 Execute the request. This will call the controller action.
6 Get the response from the controller and return the body as a Book.
7 If the book is found, use RabbitMQ client to send a message.

6.2. Analytics microservice

6.2.1. Create RabbitMQ exchange, queue and binding

As we already did in Books Microservice, let’s create the class ChannelPoolListener to define the exchange, queue and binding:

analytics/src/main/kotlin/example/micronaut/ChannelPoolListener.kt
package example.micronaut

import com.rabbitmq.client.BuiltinExchangeType
import com.rabbitmq.client.Channel
import io.micronaut.rabbitmq.connect.ChannelInitializer
import java.io.IOException
import jakarta.inject.Singleton

@Singleton
class ChannelPoolListener : ChannelInitializer() {

    @Throws(IOException::class)
    override fun initialize(channel: Channel, name: String) {
        channel.exchangeDeclare("micronaut", BuiltinExchangeType.DIRECT, true)
        channel.queueDeclare("analytics", true, false, false, null)
        channel.queueBind("analytics", "micronaut", "analytics")
    }
}
Instead of copy-paste the class in every project it would be better to create a new Gradle (or Maven) module and share it among all the microservices.

6.2.2. Create RabbitMQ consumer

Create a new class to act as a consumer of the messages sent to RabbitMQ by the Books Microservice. The Micronaut framework will implement the consumer at compile time. Create AnalyticsListener:

analytics/src/main/kotlin/example/micronaut/AnalyticsListener.kt
package example.micronaut

import io.micronaut.context.annotation.Requires
import io.micronaut.context.env.Environment
import io.micronaut.rabbitmq.annotation.Queue
import io.micronaut.rabbitmq.annotation.RabbitListener

@Requires(notEnv = [Environment.TEST]) (1)
@RabbitListener (2)
class AnalyticsListener(private val analyticsService: AnalyticsService) { (3)

    @Queue("analytics") (4)
    fun updateAnalytics(book: Book) =
        analyticsService.updateBookAnalytics(book) (5)
}
1 Do not load this bean for the test environment. This enable us to run the tests without having a RabbitMQ instance running.
2 Annotate the class with @RabbitListener to indicate that this bean will consume messages from RabbitMQ.
3 Constructor injection for AnalyticsService.
4 Annotate the method with @Queue. This listener will listen to messages in analytics queue.
5 Call the previously created method to update the analytics for the book.

6.3. Running the application

Run books microservice:

books
./mvnw mn:run
16:35:55.614 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 576ms. Server Running: http://localhost:8080

Execute a curl request to get one book:

curl http://localhost:8080/books/1491950358
{"isbn":"1491950358","name":"Building Microservices"}

Open RabbitMQ Admin UI on http://localhost:15672 and use guest for both username and password. Select queues and analytics queue. You can see that there is a message in the queue.

rabbitmq message

Expand the "Get messages" option and get one message. You can see all the information: exchange, routing key, and the `payload serialized to JSON:

rabbitmq message detail

Run analytics microservice:

analytics
./mvnw mn:run
16:35:55.614 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 623ms. Server Running: http://localhost:8081

The application will consume and process the message automatically after the startup. Go to RabbitMQ Admin UI and check that the message has been consumed:

rabbitmq message consumed

Now, run a curl to get the analytics:

curl http://localhost:8081/analytics
[{"bookIsbn":"1491950358","count":1}]

7. Generate a Micronaut Application Native Executable with GraalVM

We will use GraalVM, an advanced JDK with ahead-of-time Native Image compilation, to generate a native executable of this Micronaut application.

Compiling Micronaut applications ahead of time with GraalVM significantly improves startup time and reduces the memory footprint of JVM-based applications.

Only Java and Kotlin projects support using GraalVM’s native-image tool. Groovy relies heavily on reflection, which is only partially supported by GraalVM.

7.1. GraalVM Installation

The easiest way to install GraalVM on Linux or Mac is to use SDKMan.io.

Java 21
sdk install java 21.0.5-graal

For installation on Windows, or for a manual installation on Linux or Mac, see the GraalVM Getting Started documentation.

The previous command installs Oracle GraalVM, which is free to use in production and free to redistribute, at no cost, under the GraalVM Free Terms and Conditions.

Alternatively, you can use the GraalVM Community Edition:

Java 21
sdk install java 21.0.2-graalce

7.2. Native Executable Generation

To generate a native executable using Maven, run:

./mvnw package -Dpackaging=native-image

The native executable is created in the target directory and can be run with target/micronautguide.

It is possible to customize the name of the native executable or pass additional build arguments using the Maven plugin for GraalVM Native Image building. Declare the plugin as following:

pom.xml
<plugin>
    <groupId>org.graalvm.buildtools</groupId>
    <artifactId>native-maven-plugin</artifactId>
    <version>0.10.3</version>
    <configuration>
        <!-- <1> -->
        <imageName>mn-graalvm-application</imageName> (1)
        <buildArgs>
              <!-- <2> -->
          <buildArg>-Ob</buildArg>
        </buildArgs>
    </configuration>
</plugin>
1 The native executable name will now be mn-graalvm-application.
2 It is possible to pass extra build arguments to native-image. For example, -Ob enables the quick build mode.

Start the native executables for the two microservices and run the same curl request as before to check that everything works with GraalVM.

8. Next Steps

Read more about RabbitMQ support in the Micronaut framework.

9. License

All guides are released with an Apache license 2.0 license for the code and a Creative Commons Attribution 4.0 license for the writing and media (images…​).