These tutorials target Micronaut Framework 3. Read, Guides for Micronaut Framework 4.

Access a database with Micronaut Data R2DBC

Learn how to access a database with Micronaut R2DBC repositories.

Authors: Graeme Rocher

Micronaut Version: 3.9.2

1. Getting Started

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

The application exposes some REST endpoints and stores data in a MySQL database using Micronaut Data R2DBC.

What is R2DBC?

The Reactive Relational Database Connectivity (R2DBC) project brings reactive programming APIs to relational databases.

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

Create an application using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app example.micronaut.micronautguide \
    --features=data-r2dbc,flyway,mysql,test-resources,graalvm,jdbc-hikari,serialization-jackson \
If you don’t specify the --build argument, Gradle is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.

The previous command creates a Micronaut application with the default package example.micronaut in a directory named micronautguide.

If you use Micronaut Launch, select Micronaut Application as application type and add data-r2dbc, flyway, mysql, test-resources, graalvm, jdbc-hikari, and serialization-jackson features.

If you have an existing Micronaut application and want to add the functionality described here, you can view the dependency and configuration changes from the specified features and apply those changes to your application.

4.1. Data Source configuration

Define the R2DBC and the JDBC datasource in src/main/resources/application.yml (the latter is needed for Flyway migrations).

      dialect: MYSQL
    dialect: MYSQL
Only the dialect is defined. The remainder of the values (including the database URL etc.) will automatically be populated by the Test Resources integration, which uses Testcontainers.

When deploying to production, the datasource connection properties and r2dbc connection properties can be specified externally (using environment variables for example).

4.2. Database Migration with Flyway

We need a way to create the database schema. For that, we use Micronaut integration with Flyway.

Flyway automates schema changes, significantly simplifying schema management tasks, such as migrating, rolling back, and reproducing in multiple environments.

Add the following snippet to include the necessary dependencies:


We will enable Flyway in application.yml and configure it to perform migrations on one of the defined data sources.

      enabled: true (1)
1 Enable Flyway for the default datasource.
Configuring multiple data sources is as simple as enabling Flyway for each one. You can also specify directories that will be used for migrating each data source. Review the Micronaut Flyway documentation for additional details.

Flyway migration will be automatically triggered before your Micronaut application starts. Flyway will read migration commands in the resources/db/migration/ directory, execute them if necessary, and verify that the configured data source is consistent with them.

Create the following migration files with the database schema creation:



During application startup, Flyway will execute the SQL file and create the schema needed for the application.

4.3. Domain

Create the domain entity:

package example.micronaut.domain

import io.micronaut.serde.annotation.Serdeable

data class Genre(
    var id: Long? = null,
    var name: String
You could use a subset of supported JPA annotations instead by including the following compileOnly scoped dependency: jakarta.persistence:jakarta.persistence-api.

4.4. Repository Access

Next, create a repository interface to define the operations to access the database. Micronaut Data will implement the interface at compilation time:

package example.micronaut

import example.micronaut.domain.Genre
import reactor.core.publisher.Mono
import javax.transaction.Transactional
import javax.validation.constraints.NotBlank

@R2dbcRepository(dialect = Dialect.MYSQL) (1)
abstract class GenreRepository : ReactorPageableRepository<Genre, Long> { (2)
    abstract fun save(@NotBlank name: String): Mono<Genre>

    open fun saveWithException(@NotBlank name: String): Mono<Genre> {
        return save(name)
            .then(Mono.error(DataAccessException("test exception")))

    abstract fun update(@Id id: Long, @NotBlank name: String): Mono<Long>
1 @R2dbcRepository with a specific dialect.
2 Genre, the entity to treat as the root entity for the purposes of querying, is established either from the method signature or from the generic type parameter specified to the GenericRepository interface.

The repository extends from ReactorPageableRepository. It inherits the hierarchy ReactorPageableRepositoryReactorCrudRepositoryReactiveStreamsCrudRepositoryGenericRepository.

Repository Description


A repository that supports pagination. It provides findAll(Pageable) and findAll(Sort).


A repository interface for performing CRUD (Create, Read, Update, Delete). It provides methods such as findAll(), save(Genre), deleteById(Long), and findById(Long).


A root interface that features no methods but defines the entity type and ID type as generic arguments.

4.5. Controller

Micronaut validation is built on the standard framework – JSR 380, also known as Bean Validation 2.0.

Hibernate Validator is a reference implementation of the validation API. Micronaut has built-in support for validation of beans that are annotated with javax.validation annotations.

The necessary dependencies are included by default when creating a new application, so you don’t need to add anything else.

Create a class to encapsulate the Update operations:

package example.micronaut

import io.micronaut.serde.annotation.Serdeable
import javax.validation.constraints.NotBlank

@Serdeable (1)
data class GenreUpdateCommand(
    val id: Long,
    @field:NotBlank val name: String
1 Declare the @Serdeable annotation at the type level in your source code to allow the type to be serialized or deserialized.

Create GenreController, a controller that exposes a resource with the common CRUD operations:

package example.micronaut

import example.micronaut.domain.Genre
import io.micronaut.http.HttpHeaders
import io.micronaut.http.HttpResponse
import io.micronaut.http.HttpStatus
import io.micronaut.http.MutableHttpHeaders
import io.micronaut.http.MutableHttpResponse
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Delete
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Post
import io.micronaut.http.annotation.Put
import io.micronaut.http.annotation.Status
import reactor.core.publisher.Mono
import javax.validation.Valid
import javax.validation.constraints.NotBlank

@Controller("/genres") (1)
open class GenreController(private val genreRepository: GenreRepository) {     (2)

    @Get("/{id}") (3)
    fun show(id: Long): Mono<Genre> {
        return genreRepository
            .findById(id) (4)

    @Put (5)
    open fun update(@Body @Valid command: GenreUpdateCommand): Mono<HttpResponse<*>> { (6)
        return genreRepository.update(,
                    .header(HttpHeaders.LOCATION, location(
            ) (7)

    @Get("/list") (8)
    open fun list(@Valid pageable: Pageable): Mono<List<Genre>> { (9)
        return genreRepository.findAll(pageable)
            .map { it.content }

    @Post (10)
    open fun save(@NotBlank @Body("name") name: String): Mono<HttpResponse<Genre>> {

    @Post("/ex") (11)
    open fun saveExceptions(@NotBlank @Body name: String): Mono<MutableHttpResponse<Genre>> {
        return genreRepository.saveWithException(name)

    @Delete("/{id}") (12)
    fun delete(id: Long): Mono<Void> {
        return genreRepository.deleteById(id)

    private fun createGenre(genre: Genre) : MutableHttpResponse<Genre> {
        return HttpResponse
            .headers { headers: MutableHttpHeaders ->

    private fun location(id: Long?): URI {
        return URI.create("/genres/$id")

    private fun location(genre: Genre): URI {
        return location(
1 The class is defined as a controller with the @Controller annotation mapped to the path /genres.
2 Use constructor injection to inject a bean of type GenreRepository.
3 Maps a GET request to /genres/{id}, which attempts to show a genre. This illustrates the use of a URL path variable.
4 Returning an empty Reactor Mono when the genre doesn’t exist makes the Micronaut framework respond with 404 (not found).
5 Maps a PUT request to /genres, which attempts to update a genre.
6 Adds @Valid to any method parameter that requires validation. Use a POJO supplied as a JSON payload in the request to populate command.
7 The Mono.thenReturn(..) method is used to return a NO_CONTENT response only if the update was successful. Headers are easily added by the HttpResponse API.
8 Maps a GET request to /genres/list, which returns a list of genres.
9 You can bind Pageable as a controller method argument. Check the examples in the following test section and read the Pageable configuration options. For example, you can configure the default page size with the configuration property
10 Maps a POST request to /genres, which attempts to save a genre. This example uses the method to obtain the result of the call to save(String name) and map it to a HTTP CREATED response if the save operation was successful.
11 Maps a POST request to /ex, which generates an exception. This example demonstrates how to handle exceptions with Mono.onErrorReturn(..), which simply returns a NO_CONTENT response if an error occurs. More complex use cases (like logging the exception) can be handled with Mono.onErrorResume(..).
12 Maps a DELETE request to /genres/{id}, which attempts to remove a genre. This illustrates the use of a URL path variable. The Mono.then() method is called to discard the result and return an empty Mono only if the call to deleteById is successful.

4.6. Writing Tests

Create a test to verify the CRUD operations:

package example.micronaut

import example.micronaut.domain.Genre
import io.micronaut.core.type.Argument
import io.micronaut.http.HttpHeaders
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpStatus
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.http.client.exceptions.HttpClientResponseException
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import jakarta.inject.Inject
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Assertions.assertNotNull
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.assertThrows

@MicronautTest(transactional = false) (1)
class GenreControllerTest(@Client("/") val client: HttpClient) { (2)

    fun testFindNonExistingGenreReturn404() {
        val thrown = assertThrows<HttpClientResponseException>{

        assertEquals(HttpStatus.NOT_FOUND, thrown.status)

    fun testGenreCrudOperations() {

        var request = HttpRequest.POST("/genres", mapOf("name" to "DevOps")) (3)
        var response = client.toBlocking().exchange(request,

        assertEquals(HttpStatus.CREATED, response.status)
        assertEquals("/genres/1", response.header(HttpHeaders.LOCATION))

        request = HttpRequest.POST("/genres", mapOf("name" to "Microservices")) (3)
        response = client.toBlocking().exchange(request,

        assertEquals(HttpStatus.CREATED, response.status)
        assertEquals("/genres/2", response.header(HttpHeaders.LOCATION))

        var genre = client.toBlocking().retrieve("/genres/2", (4)


        var cmdRequest = HttpRequest.PUT("/genres", GenreUpdateCommand(2, "Micro-services"))
        response = client.toBlocking().exchange(cmdRequest) (5)

        assertEquals(HttpStatus.NO_CONTENT, response.status())

        genre = client.toBlocking().retrieve("/genres/2",


        request = HttpRequest.GET("/genres/list")
        var genres = client.toBlocking().retrieve(request, Argument.listOf(

        assertEquals(2, genres.size)

        request = HttpRequest.POST("/genres/ex", mapOf("name" to "Microservices"))
        response = client.toBlocking().exchange(request)

        assertEquals(HttpStatus.NO_CONTENT, response.status)

        request = HttpRequest.GET("/genres/list")
        genres = client.toBlocking().retrieve(request, Argument.listOf(

        assertEquals(2, genres.size)

        request = HttpRequest.GET("/genres/list?size=1")
        genres= client.toBlocking().retrieve(request, Argument.listOf(

        assertEquals(1, genres.size)
        assertEquals("DevOps", genres[0].name)

        request = HttpRequest.GET("/genres/list?size=1&sort=name,desc")
        genres= client.toBlocking().retrieve(request, Argument.listOf(

        assertEquals(1, genres.size)
        assertEquals("Micro-services", genres[0].name)

        request = HttpRequest.GET("/genres/list?size=1&page=2")
        genres= client.toBlocking().retrieve(request, Argument.listOf(

        assertEquals(0, genres.size)

        for (i in 1..2) {
            request = HttpRequest.DELETE("/genres/$i")
            response = client.toBlocking().exchange(request)

            assertEquals(HttpStatus.NO_CONTENT, response.status)

        request = HttpRequest.GET("/genres/list")
        genres = client.toBlocking().retrieve(request, Argument.listOf(

        assertEquals(0, genres.size)


1 Annotate the class with @MicronautTest so the Micronaut framework will initialize the application context and the embedded server. More info.
2 Inject the HttpClient bean and point it to the embedded server.
3 Creating HTTP Requests is easy thanks to the Micronaut framework fluid API.
4 If you care just about the object in the response, use retrieve.
5 Sometimes, receiving just the object is not enough, and you need information about the response. In this case, instead of retrieve, use the exchange method.

5. Testing the Application

To run the tests:

./mvnw test

6. Running the Application

To run the application, use the ./mvnw mn:run command, which starts the application on port 8080.

7. Testing Running API

Save one genre, and your genre table will now contain an entry.

curl -X "POST" "http://localhost:8080/genres" \
     -H 'Content-Type: application/json; charset=utf-8' \
     -d $'{ "name": "music" }'

8. Test Resources

When the application is started locally — either under test or by running the application — resolution of the datasource URL is detected and the Test Resources service will start a local MySQL docker container, and inject the properties required to use this as the datasource.

For more information, see the JDBC section or R2DBC section of the Test Resources documentation.

9. Connecting to a MySQL database

Previously, we connected to a MySQL database, which Micronaut Test Resources started for us.

However, it is easy to connect to an already existing database. Let’s start a database and connect to it.

Execute the following command to run a MySQL container:

docker run -it --rm \
    -p 3306:3306 \
    -e MYSQL_DATABASE=db \
    -e MYSQL_USER=sherlock \
    -e MYSQL_PASSWORD=elementary \
If you are using macOS on Apple Silicon – e.g. M1, M1 Pro, etc. – Docker might fail to pull an image for mysql:8. In that case substitute mysql:oracle.

Database Migrations tools, such Flyway need a configured JDBC datasource. Export several environment variables:

export DATASOURCES_DEFAULT_URL=jdbc:mysql://localhost:3306/db

Micronaut Framework populates the properties datasources.default.url, datasources.default.username and datasources.default.password with those environment variables' values. Learn more about JDBC Connection Pools.

For R2DBC, export serveral environment variables:

export R2DBC_DATASOURCES_DEFAULT_URL=jdbc:mysql://localhost:3306/db

Micronaut Framework populates the properties r2dbc.datasources.default.url, r2dbc.datasources.default.username and r2dbc.datasources.default.password with those environment variables' values.

You can run the application and test the API as it was described in the previous sections. However, when you run the application, Micronaut Test Resources does not start a MySQL container because you have provided values for r2dbc.datasources.default. and datasources.default. properties.

10. Generate a Micronaut Application Native Executable with GraalVM

We will use GraalVM, the polyglot embeddable virtual machine, to generate a native executable of our Micronaut application.

Compiling native executables ahead of time with GraalVM 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.

10.1. Native executable generation

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

Java 11
sdk install java 22.3.r11-grl
If you still use Java 8, use the JDK11 version of GraalVM.
Java 17
sdk install java 22.3.r17-grl

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

After installing GraalVM, install the native-image component, which is not installed by default:

gu install native-image

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.

You can execute the genres endpoints exposed by the native image, for example:

curl localhost:8080/genres/list

11. Next steps

12. Help with the Micronaut Framework

The Micronaut Foundation sponsored the creation of this Guide. A variety of consulting and support services are available.