I’ve been building REST APIs with Micronaut for about a year now, and I keep getting asked why I didn’t just use Spring Boot. Here’s the honest answer.

What Micronaut does differently

Micronaut does dependency injection and AOP at compile time, not runtime. That means:

  • Faster startup — typically under a second, compared to 5-10s for equivalent Spring apps
  • Lower memory footprint — critical when you’re running many microservices in Kubernetes
  • No reflection overhead — better performance in steady state

For container-heavy architectures where you’re scaling pods up and down constantly, those startup times matter a lot.

A minimal example

Here’s what a basic controller looks like:

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@Controller("/api/users")
public class UserController {

    private final UserService userService;

    public UserController(UserService userService) {
        this.userService = userService;
    }

    @Get("/{id}")
    public HttpResponse<User> getUser(@PathVariable Long id) {
        return userService.findById(id)
            .map(HttpResponse::ok)
            .orElse(HttpResponse.notFound());
    }

    @Post
    public HttpResponse<User> createUser(@Body @Valid UserRequest request) {
        User created = userService.create(request);
        return HttpResponse.created(created);
    }
}

If you’ve used Spring before, this will look familiar. The annotations are similar, the mental model is similar. The difference is all under the hood.

Where it shines

  • Serverless — cold starts are dramatically shorter
  • Microservices on Kubernetes — faster pod readiness means quicker rollouts
  • GraalVM native images — Micronaut was designed with this in mind from day one

Where Spring still wins

  • Ecosystem. Spring has a library for everything. Micronaut is catching up but it’s not there yet.
  • Team familiarity. If your team already knows Spring inside and out, retraining has a cost.
  • Stack Overflow answers. Still much more for Spring problems.

My take

Pick Micronaut when startup time, memory, or native compilation matters. Pick Spring when ecosystem depth or team familiarity matters. There’s no universally right answer — and that’s fine.