Getting started with Micronaut

Micronaut is a new JVM-based framework that is designed for building twelve-factor microservice applications.

The project was initiated by OCI and the team that built the Grails framework, officially announced by Graeme Rocher at Greach Conference 2018, and publicly open-sourced under Apache License 2 on May 23rd, 2018. The first milestone release towards version 1.0.0 was released today, on May 30th, 2018. As the official guide states, "Micronaut is developed by the creators of the Grails framework and takes inspiration from lessons learnt over the years building real-world applications from monoliths to microservices using Spring, Spring Boot and Grails."

Micronaut's benefits include a fast startup time and low memory footprint that not only allow the development of serverless applications.

The low overhead is achieved by using a compile-time Dependency Injection (DI) and Aspect-Oriented Programming (AOP) API that does not use reflection. Reflection-based Inversion of Control (IoC) frameworks load and cache reflection data for every field, method and constructor. With Micronaut, your application startup time and memory consumption are not bound to the size of your codebase.

Micronaut comes with a HTTP server and HTTP client. Micronaut's HTTP server makes it as easy as possible to expose APIs that can be consumed by HTTP clients. Non-blocking reactivity is supported with the use of Netty. HTTP clients can be build declaratively with the use of annotations, which are implemented at compile-time, reducing memory consumption. The Grails' ORM framework GORM is officially supported which provides a rich set of APIs for accessing SQL databases like PostgreSQL with Hibernate as well as Neo4J, Redis, MongoDB, GraphQL and Cassandra databases.

Service discovery (Consul, Eureka), client side load balancing, circuit breaker, configuration sharing (Consul, AWS/GCP planned), distributed tracing tools (such as Zipkin), support for serverless computing with AWS Lambda are featured.

Over all, Micronaut offers a pretty similar programming model to Spring Boot and Grails.

Let's start

We can start with the official example projects on GitHub to get a glimpse on how to develop with Micronaut. They run with Java 8 and the provided Gradle wrapper (or Gradle 4.7). Note that the hello-world-kotlin sample does not currently use the same port as the others (i.e. port 8080), but a random port.

For folks who are keen on building from source, snapshots of Micronaut are available on Sonatype. Otherwise you can download the pre-built 1.0.0.M1 binary from GitHub.

There is a Micronaut Command Line Interface (CLI) that we can use to setup projects on our own. The CLI zip of version 1.0.0-SNAPSHOT can also be found at Sonatype. To create a new project we can install the Micronaut Command Line Interface (CLI) with SDKman:

$ sdk install micronaut

Afterwards, we verify that Micronaut CLI was installed:

$ mn --version
| Micronaut Version: 1.0.0.M1
| JVM Version: 1.8.0_172

If you got any errors with Java 9 or 10 so far, try to switch to the Java version that worked for me, i.e. Java 8 (1.8.0_172) (for example via sdk install java 8.0.172-zulu).

$ java -version
openjdk version "1.8.0_172"
OpenJDK Runtime Environment (Zulu (build 1.8.0_172-b01)
OpenJDK 64-Bit Server VM (Zulu (build 25.172-b01, mixed mode)

To try out the Groovy samples or develop with Groovy, setup Groovy (with SDKman: sdk install groovy) and check if it's working:

$ groovy -version
Groovy Version: 2.5.0 JVM: 1.8.0_172 Vendor: Azul Systems, Inc. OS: Mac OS X

Afterwards we can create a hello-world server application following the official documentation or try some commands out on our own:

$ mn create-app micronaut

Note that we can use the interactive shell to create beans, clients, controllers and jobs from there on:

$ mn
| Starting interactive mode...
| Enter a command name to run. Use TAB for completion:

create-bean          create-client        create-controller    create-job           help
mn> create-controller -lang kotlin hello
| Rendered template Controller.kt to destination src/main/kotlin/micronaut/HelloController.kt
| Rendered template to destination src/test/java/micronaut/
mn> create-job -lang groovy hello
| Rendered template Job.groovy to destination src/main/groovy/micronaut/HelloJob.groovy
mn> create-bean hello -lang groovy
| Rendered template Bean.groovy to destination src/main/groovy/micronaut/Hello.groovy

The CLI helps us with commands and their parameters:

mn> help create-job
| Command: create-job
| Description:
Creates a job with scheduled method

| Usage:
mn create-job [JOB NAME]

| Arguments:
* Job Name - The name of the job class to create (REQUIRED)

| Flags:
* force - Whether to overwrite existing files
* lang - The language used for the job (options: groovy, kotlin, java)

Ok, so far we played around with the CLI. We will now build our own hello-world application:

Building our own Hello World application

Let's start by creating our project using the Micronaut CLI:

$ mn create-app helloworld

Main class

The executable Java Main class is already available:


package helloworld;

import io.micronaut.runtime.Micronaut;

public class Application {

    public static void main(String[] args) {;

We can run the application through this class if we use our preferred Java IDE (i.e. IDEA 😉) or with ./gradlew run using the Gradle wrapper for development.

$ ./gradlew run

> Task :run
13:52:11.961 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 1121ms. Server Running: http://localhost:47743

Note that the server starts at a random port by default without further configuration. We can change that by adding the server port configuration to our existing src/main/resources/application.yml:

        name: helloworld
        port: 8888

Just stop the server by pressing CTRL+C, because we haven't implemented any endpoint to talk to yet.

HTTP endpoint

Next, we create a controller that is responsible for the /hello and subordinate paths:


package helloworld;

import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.reactivex.Single;

public class HelloController {

    public Single<String> hello(String name) {
        return Single.just("Hello " + name + "!");

The controller accepts GET requests for /hello/{name} where {name} is the dynamic route parameter that is injected to our "hello" method which is annotated with the @Get annoation. A GET request to /hello/Jonas/ should return "Hello Jonas!".

To test the endpoint, we can implement a HTTP client (helloworld.HelloClient) class in the src/test folder of our project now.

A reactive Java HTTP Client


package helloworld;

import io.micronaut.http.annotation.Get;
import io.micronaut.http.client.Client;
import io.reactivex.Single;

import javax.validation.constraints.NotBlank;

public interface HelloControllerClient {

    Single<String> hello(@NotBlank String name);

The client is generated at compile time using this declarative approach.

The HTTP Client Test

We can test our hello world endpoint with this Java JUnit test:


package helloworld;

import io.micronaut.context.ApplicationContext;
import io.micronaut.runtime.server.EmbeddedServer;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import static org.junit.Assert.assertEquals;

public class HelloControllerTest {

    private EmbeddedServer server;
    private HelloControllerClient client;

    public void setup() {
        this.server =;
        this.client = server.getApplicationContext().getBean(HelloControllerClient.class);

    public void shouldReturnHello() {
        String response = client.hello("Jonas").blockingGet();
        assertEquals(response, "Hello Jonas!");

    public void cleanup() {

The executable application

To run the application via an executable or deploy the application, we create a runnable JAR file by running ./gradlew assemble.

The JAR file is created at build/libs and executed from the project's root folder with this command:

java -jar build/libs/helloworld-0.1-all.jar

The server will start and be available on localhost on port 8888 (http://localhost:8888/hello/Jonas), as we configured above.

The file size is 11.1 MB. The startup time is about 1 second.

Notes on testing

Using the generic HTTP client

We do not have to implement our own declarative HTTP client for testing. We can use the default provided with Micronaut like this:


package helloworld;

import io.micronaut.context.ApplicationContext;
import io.micronaut.http.HttpRequest;
import io.micronaut.http.client.HttpClient;
import io.micronaut.runtime.server.EmbeddedServer;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;


import static org.junit.Assert.assertEquals;

public class HelloControllerAlternativeTest {

    private EmbeddedServer server;
    private HttpClient client;

    public void setup() {
        this.server =;
        this.client = HttpClient.create(this.server.getURL());

    public void shouldReturnHello() {
        String response = client.toBlocking()
        assertEquals(response, "Hello Jonas!");

    public void cleanup() throws IOException {

Using the Spock framework for testing

To alternatively use the Spock framework, which is also my favorite testing framework, we apply the Gradle Groovy plugin and add the testCompile dependency to Spock in our Gradle build:

apply plugin: "groovy"

dependencies {
    testCompile "org.spockframework:spock-core:1.1-groovy-2.4"

Afterwards, we can implement our Specification:


package helloworld

import io.micronaut.context.ApplicationContext
import io.micronaut.runtime.server.EmbeddedServer
import spock.lang.AutoCleanup
import spock.lang.Shared
import spock.lang.Specification
import spock.lang.Unroll

class HelloControllerSpec extends Specification {

    EmbeddedServer embeddedServer =

    HelloControllerClient client = embeddedServer

    void "test hello #name response"() {
        client.hello(name).blockingGet() == 'Hello Jonas!'
        name = 'Jonas'

Spock creates a HTML test report by default which we can view by opening the build/reports/tests/test/index.html with a browser after we ran the tests with ./gradlew test.

What's next

This is just the start. The full code example of our hello-world application is available at GitHub.

I recommend the talk "Launching the Micro Future: Groovy, Grails and Project Particle" by Graeme Rocher which is available on YouTube. Graeme demonstrates a lot of the public examples and provides more details.

Álvaro Sánchez talked about "Reactive microservices with Micronaut" at Greach 2018.

There is an official guide and API reference as well as some official implementation guides – as of writing primarily to different authentication mechanisms.

I am sure we will be provided with more information and see further progress after GR8Conf 2018, which is currently taking place. For example, there is a workshop held by Álvaro Sánchez today.

Watch #micronautfw on Twitter for more details. You can also follow me there.