Micronaut

Micronaut processes annotations at compile time rather than runtime reflection, eliminating Spring’s slow startup and high memory usage. It supports Java, Kotlin, and Groovy and provides first-class support for GraalVM native images, serverless functions, and service discovery. Its compile-time approach means startup in milliseconds — ideal for Lambda functions and containerised microservices.

Quick start

sdk install micronaut
mn create-app com.example.my-app --build=gradle --lang=kotlin
cd my-app
./gradlew run

// src/main/kotlin/com/example/PostController.kt
import io.micronaut.http.annotation.*
import io.micronaut.http.HttpStatus
import jakarta.inject.Singleton

data class Post(val id: Int, val title: String, val body: String)

@Singleton
class PostService {
    private val posts = mutableListOf<Post>()

    fun findAll(): List<Post> = posts

    fun save(post: Post): Post {
        val saved = post.copy(id = posts.size + 1)
        posts.add(saved)
        return saved
    }
}

@Controller("/posts")
class PostController(private val service: PostService) {

    @Get
    fun list() = service.findAll()

    @Post
    @Status(HttpStatus.CREATED)
    fun create(@Body post: Post) = service.save(post)
}

When to use

Micronaut is ideal for Java/Kotlin microservices that need fast startup, low memory footprint, and cloud-native features. It’s particularly strong for serverless (AWS Lambda) and Kubernetes deployments where container startup time matters. Quarkus is its closest competitor with similar compile-time philosophy. For teams with deep Spring Boot expertise, the migration cost to Micronaut may not be worthwhile unless startup time is a bottleneck.