Getting started with Micronaut

Spring Boot is our weapon of choice when developing microservices. It comes with lots of useful features out of the box and it allows us to achieve positive results swiftly. However, when it first came out, microservices weren’t a thing yet, and that entails certain drawbacks. That’s why we used our last brown bag session to address these drawbacks, and fiddled with Micronaut. But let’s start from the beginning.

Spring Boot

There are always advantages and disadvantages derived from using one framework or the other, or just not using any at all, and Spring Boot is no exception to this rule. For us, three of its pain points are:

  • Slow test execution
  • Slow application startup
  • Memory consumption

Slow test execution

Our microservices contain a number of tests, and most of them include at least a couple hundred integration tests. Despite the fact that each test is executed in the order of milliseconds, running all our tests takes a couple of minutes, and executing a test class individually takes an average of 30 seconds.

The reason behind this is that before executing our integration tests, Spring Boot needs to carry out some additional work such as loading Spring’s ApplicationContext and WebApplicationContext.

Now think about a developer who is working on a new feature and who is running tests over and over again. Can you picture their desperation while waiting for the test execution to finish?

Slow application start-up

Similarly, starting a microservice is relatively slow, and one of the reasons is that there are lots of things happening at runtime. For example, the bytecode of each and every Spring bean is read, and that of course adds up to the start-up time.

In a microservices environment, where individual applications need to be deployed quite often, especially when following some kind of continuous deployment approach, this may become inconvenient.

Memory consumption

Although Spring Boot memory performance can be improved for example by playing with different JVM parameters, in general Spring Boot applications have a relatively high memory footprint.

In our environment, a microservice requires a couple of hundred megabytes of memory, and just like time is money, so is memory. In other words, the more microservices you have, the more memory you need, and that doesn’t come for free.

We used our last brown bag session to address these pain points, and fiddle with Micronaut.


Micronaut is an Open Source JVM framework comparable to Spring Boot, specifically designed to solve microservice problems. It’s being developed by the inventors of Grails. So, if you come from a Spring Boot, Grails, or similar background, the learning curve should be pretty smooth.

This is how Micronaut tackles the pain points we have with Spring Boot:

  • Test execution. A lot of the heavy work that Spring Boot does at runtime, is done at compile time on Micronaut. As a consequence, test suite setup is minimal and way faster.
  • Application start-up. For the same reason as above, starting an application only takes a couple of seconds, and that doesn’t change dramatically when increasing the number of classes.
  • Memory consumption. While with Spring Boot we talk about a couple of hundred megabytes per microservice, Micronaut is happy with tens of megabytes.

Unfortunately, there aren’t many benchmarks out there yet, but I can recommend the following Micronaut presentation, where some interesting numbers are explained:

After all this theory, let’s now jump right into it and write our first REST client with Micronaut.

Example: GitHub REST client

In this example I’ll write a simple REST client with Kotlin that makes use of the GitHub GraphQL API, and exposes a REST endpoint.


Now, we’re ready to create a skeleton project by running:

mn create-app demo -l kotlin

GitHub client

Let’s start by writing a client that is able to interact with GitHub’s API:

interface GithubClient {

    fun queryGitHubApi(
            query: String,
            @Header(value = "Authorization") auth: String,
            @Header(value = "User-Agent") userAgent: String
    ): String

The class above defines an @Client annotated interface that points to a specific API ( and contains an @Post annotated method which takes three parameters: the GraphQL query we want to execute, an Authorization header that should be fed with our GitHub API token, and a mandatory User-Agent header.

Configuration file

Communication with GitHub’s API must be authenticated, which means that we need to use a token. In order to use this token, we can benefit from configuration files. For this, we first need to add a new section to our application.yml:

    token: d3397be89674134d5045d0ae57018b54092dac92

Afterwards, we create a configuration class that enables us to read the token:

const val GITHUB_PROPERTIES_PREFIX = "github"

class GitHubConfiguration {
    lateinit var token: String

This way, GitHubConfiguration gets the token injected from application.yml.

REST controller

The last step of this HelloWorld example is to create a REST controller that calls our GitHub client.

For this example, I’ll create a controller that contains a method which returns a list of open source licenses known to GitHub:

class GithubController(
        val githubClient: GithubClient,
        val gitHubConfiguration: GitHubConfiguration
) {

    fun retrieveOpenSourceLicenses(): String {
        val query: String = """
              licenses {

        return githubClient.queryGitHubApi(
                "bearer ${gitHubConfiguration.token}",


Here we’re exposing a /github/licenses endpoint that will call the GitHub client using the token from the configuration file.


Now we have everything we need to try out our new REST client. First, we need to start the application, which on my laptop takes 1843 milliseconds:

[main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 1843ms. Server Running: http://localhost:8080

As you can see, the server is running on port 8080, so we can now call the previously created endpoint and see if we achieve the desired result. I’ll use HTTPie for that:

http GET :8080/github/licenses

We can also shorten the response by collecting only license names by means of jq:

http GET :8080/github/licenses | jq .data.licenses[].name

And this is the result:

"GNU General Public License v3.0"
"The Unlicense"
"GNU Lesser General Public License v2.1"
"GNU Affero General Public License v3.0"
"MIT License"
"GNU General Public License v2.0"
"BSD 2-Clause \"Simplified\" License"
"Apache License 2.0"
"GNU Lesser General Public License v3.0"
"Eclipse Public License 2.0"
"Mozilla Public License 2.0"
"BSD 3-Clause \"New\" or \"Revised\" License"


Carrying out too much work at runtime has certain downsides that need to be taken into account. The concepts software developers work with nowadays are different from the ones that were considered when some of the most popular frameworks kicked off.

However, I would like to stress the fact that Micronaut is just starting. It was announced a couple of months ago and it may not feel production ready yet. Other frameworks, such as Spring Boot, have an impressibe set of features and community support that can’t be overlooked. Nevertheless, Micronaut is an interesting piece of technology that is definitely worth keeping track of.


Here’s a collection of resources that may be interesting if you want to play around with Micronaut:

About the author

Xavier Alvarez is a Java and Kotlin Backend Developer, and an Ubuntu fanboy.