Micronaut has been making waves in the Java ecosystem, and for good reason. It’s a modern, JVM-based framework that’s designed for building microservices and serverless applications. But what really sets it apart is its ability to create GraalVM native images, which can dramatically improve startup times and reduce memory usage.

Let’s dive into how we can implement GraalVM native images with Micronaut and see the magic unfold. Trust me, once you see the performance gains, you’ll wonder how you ever lived without it.

First things first, make sure you have GraalVM installed on your system. If you haven’t already, head over to the GraalVM website and follow their installation instructions. It’s pretty straightforward, but if you run into any issues, their community is super helpful.

Once you have GraalVM set up, let’s create a simple Micronaut application. We’ll use the Micronaut CLI to get started:

mn create-app com.example.demo
cd demo

This will create a basic Micronaut application for us to work with. Now, let’s add a simple controller to our app:

package com.example.demo;

import io.micronaut.http.annotation.*;

@Controller("/hello")
public class HelloController {

    @Get("/{name}")
    public String hello(String name) {
        return "Hello, " + name + "!";
    }
}

Nothing fancy, just a simple “Hello, World!” style endpoint. Now, let’s build our application as a native image. Add the following plugin to your build.gradle file:

plugins {
    id "com.github.johnrengelman.shadow" version "7.1.2"
    id "io.micronaut.application" version "3.7.0"
}

This plugin will help us create a native image. Now, let’s build our native image:

./gradlew nativeImage

Fair warning: this might take a while the first time you run it. Go grab a coffee, catch up on your favorite show, or do some stretches. Your computer’s about to do some heavy lifting.

Once it’s done, you’ll find your native image in the build/native-image directory. Run it like this:

./build/native-image/demo

Boom! Your application should start up almost instantly. We’re talking milliseconds here, not seconds. It’s like the difference between a cheetah and a sloth. Your app is now the cheetah.

But why stop there? Let’s make our application even more impressive. Micronaut has excellent support for reactive programming, which pairs beautifully with native images. Let’s update our controller to use reactive types:

package com.example.demo;

import io.micronaut.http.annotation.*;
import reactor.core.publisher.Mono;

@Controller("/hello")
public class HelloController {

    @Get("/{name}")
    public Mono<String> hello(String name) {
        return Mono.just("Hello, " + name + "!");
    }
}

Now our endpoint returns a Mono, which is a reactive type. This allows our application to handle more concurrent requests with fewer threads.

But wait, there’s more! Micronaut also has great support for serverless environments. Let’s modify our application to run as an AWS Lambda function. First, add the AWS Lambda dependency to your build.gradle:

dependencies {
    implementation("io.micronaut.aws:micronaut-function-aws-api-proxy")
}

Now, let’s create a Lambda handler:

package com.example.demo;

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
import io.micronaut.function.aws.MicronautRequestHandler;
import java.util.Map;

public class FunctionRequestHandler extends MicronautRequestHandler<Map<String, Object>, String> {
    @Override
    public String execute(Map<String, Object> input, Context context) {
        return "Hello, " + input.get("name") + "!";
    }
}

This handler will allow our application to run as a Lambda function. Now, when we build our native image, it will be compatible with AWS Lambda:

./gradlew buildNativeLambda

The resulting native image will be incredibly small and start up in mere milliseconds, making it perfect for serverless environments where cold start times are crucial.

Now, you might be thinking, “This is all great, but what about databases? Surely native images struggle with that?” Well, my friend, you’re in for a treat. Micronaut has excellent support for various databases, and it works beautifully with native images.

Let’s add a simple JPA entity and repository to our application. First, add the necessary dependencies to your build.gradle:

dependencies {
    implementation("io.micronaut.data:micronaut-data-hibernate-jpa")
    implementation("io.micronaut.sql:micronaut-jdbc-hikari")
    runtimeOnly("com.h2database:h2")
}

Now, let’s create a simple entity:

package com.example.demo;

import javax.persistence.*;

@Entity
public class Person {
    @Id
    @GeneratedValue
    private Long id;
    private String name;

    // Getters and setters omitted for brevity
}

And a repository to go with it:

package com.example.demo;

import io.micronaut.data.annotation.*;
import io.micronaut.data.repository.CrudRepository;

@Repository
public interface PersonRepository extends CrudRepository<Person, Long> {
    Person findByName(String name);
}

Now, let’s update our controller to use this repository:

package com.example.demo;

import io.micronaut.http.annotation.*;
import reactor.core.publisher.Mono;

@Controller("/hello")
public class HelloController {

    private final PersonRepository repository;

    public HelloController(PersonRepository repository) {
        this.repository = repository;
    }

    @Get("/{name}")
    public Mono<String> hello(String name) {
        return Mono.fromCallable(() -> {
            Person person = repository.findByName(name);
            if (person == null) {
                person = new Person();
                person.setName(name);
                repository.save(person);
                return "Hello, " + name + "! Nice to meet you.";
            } else {
                return "Welcome back, " + name + "!";
            }
        });
    }
}

This controller now uses the repository to check if a person exists, and if not, creates a new one. And the best part? All of this works seamlessly with native images.

Now, you might be wondering about testing. Don’t worry, Micronaut has got you covered there too. Let’s add a simple test for our controller:

package com.example.demo;

import io.micronaut.http.client.HttpClient;
import io.micronaut.http.client.annotation.Client;
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import org.junit.jupiter.api.Test;

import jakarta.inject.Inject;

import static org.junit.jupiter.api.Assertions.assertEquals;

@MicronautTest
public class HelloControllerTest {

    @Inject
    @Client("/")
    HttpClient client;

    @Test
    public void testHelloEndpoint() {
        String response = client.toBlocking().retrieve("/hello/John");
        assertEquals("Hello, John! Nice to meet you.", response);

        response = client.toBlocking().retrieve("/hello/John");
        assertEquals("Welcome back, John!", response);
    }
}

This test will ensure our controller is working as expected, even with the database interaction. And yes, you guessed it - this test will work with native images too!

Now, let’s talk about configuration. Micronaut uses a application.yml file for configuration, which works great with native images. Here’s an example configuration:

micronaut:
  application:
    name: demo
datasources:
  default:
    url: jdbc:h2:mem:devDb;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
    driverClassName: org.h2.Driver
    username: sa
    password: ''
jpa:
  default:
    properties:
      hibernate:
        hbm2ddl:
          auto: update

This configuration sets up our H2 database and configures Hibernate to automatically update our schema.

One of the coolest things about Micronaut is its ahead-of-time (AOT) compilation. This means that a lot of the reflection and runtime analysis that traditional frameworks do is done at compile time with Micronaut. This is a big part of what makes Micronaut so fast and compatible with GraalVM native images.

But what if you need to use a library that relies heavily on reflection? No problem! Micronaut provides a way to hint to GraalVM about classes that need reflection. You can do this by creating a file named reflect-config.json in your src/main/resources/META-INF/native-image directory:

[
  {
    "name" : "com.example.ReflectionHeavyClass",
    "allDeclaredConstructors" : true,
    "allPublicConstructors" : true,
    "allDeclaredMethods" : true,
    "allPublicMethods" : true,
    "allDeclaredFields" : true,
    "allPublicFields" : true
  }
]

This tells GraalVM to include full reflection support for the specified class in the native image.

Now, let’s talk about deployment. One of the great things about native images is how easy they make deployment. You don’t need to worry about JVM versions or classpath issues - everything your application needs is contained in a single binary.

For example, you could create a minimal Docker image like this:

FROM oracle/graalvm-ce:latest as graalvm
COPY . /home/app/demo
WORKDIR /home/app/demo
RUN native-image --no-server -cp build/libs/*.jar

FROM frolvlad/alpine-glibc
EXPOSE 8080
COPY --from=graalvm /home/app/demo/demo .
ENTRYPOINT ["./demo"]

This creates a tiny Docker image containing just your native image and the minimal runtime it needs.

In conclusion, Micronaut’s support for GraalVM native images is a game-changer. It allows you to create incredibly fast, resource-efficient applications that start up in milliseconds. Whether you’re building microservices, serverless functions, or traditional web applications, Micronaut and GraalVM native images can give you a significant performance boost.

Remember, though, that native images aren’t always the right choice. They have longer build times and can be more difficult to debug. But for many applications, especially those where startup time and memory usage are critical, they can be a fantastic option.

So go ahead, give it a try. Build your next project with Micronaut and GraalVM native images. I promise you’ll be impressed by the results. And who knows? You might just become the office hero when your application starts up faster than anyone can say “Java.”