Learn MICRONAUT with Real Code Examples

import io.micronaut.http.annotation.*;
import javax.inject.Singleton;

@Singleton
class CounterService {
    private int count = 0;
    public int getCount() { return count; }
    public int increment() { return ++count; }
    public int decrement() { return --count; }
    public int reset() { count = 0; return count; }
}

@Controller("/counter")
class CounterController {
    private final CounterService service;

    CounterController(CounterService service) {
        this.service = service;
    }

    @Get
    public int getCount() { return service.getCount(); }

    @Post("/increment")
    public int increment() { return service.increment(); }

    @Post("/decrement")
    public int decrement() { return service.decrement(); }

    @Post("/reset")
    public int reset() { return service.reset(); }
}

Demonstrates a simple Micronaut REST API with a counter using Controller and in-memory state.

import io.micronaut.http.annotation.*;
import io.micronaut.runtime.Micronaut;

@Controller("/hello")
class HelloController {
    @Get
    public String hello() { return "Hello World"; }
}

public class Application {
    public static void main(String[] args) {
        Micronaut.run(Application.class);
    }
}

A minimal Micronaut API returning 'Hello World'.

import io.micronaut.http.annotation.*;
import java.util.Map;

@Controller("/echo")
class EchoController {
    @Post
    public Map<String, Object> echo(@Body Map<String, Object> payload) {
        return payload;
    }
}

Echoes back JSON sent in POST request.

import io.micronaut.http.annotation.*;

@Controller("/greet")
class GreetController {
    @Get
    public String greet(@QueryValue(defaultValue="Guest") String name) {
        return "Hello " + name;
    }
}

Greets user using query parameters.

import io.micronaut.http.annotation.*;

@Controller("/users")
class UserController {
    @Get("/{id}")
    public String getUser(@PathVariable int id) {
        return "User " + id;
    }
}

Returns info for a user based on path variable.

import io.micronaut.http.annotation.*;
import java.util.concurrent.*;

@Controller("/async")
class AsyncController {
    @Get
    public CompletableFuture<String> asyncHello() {
        return CompletableFuture.supplyAsync(() -> {
        try { Thread.sleep(1000); } catch (InterruptedException e) {}
        return "Async Hello";
        });
    }
}

Asynchronous endpoint returning a delayed response.

import io.micronaut.http.annotation.*;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.server.exceptions.ExceptionHandler;
import javax.inject.Singleton;
import io.micronaut.http.HttpRequest;
import io.micronaut.http.HttpStatus;
import io.micronaut.web.router.exceptions.NoRouteFoundException;

@Singleton
class NotFoundHandler implements ExceptionHandler<NoRouteFoundException, HttpResponse> {
    @Override
    public HttpResponse handle(HttpRequest request, NoRouteFoundException exception) {
        return HttpResponse.status(HttpStatus.NOT_FOUND).body("Custom 404 Not Found");
    }
}

Custom error handling for 404 requests.

import io.micronaut.http.annotation.*;
import javax.inject.Singleton;
import java.util.Map;

@Singleton
class CounterService {
    private int count = 0;
    public int getCount() { return count; }
    public int increment() { return ++count; }
    public int decrement() { return --count; }
    public int reset() { count = 0; return count; }
}

@Controller
class CombinedController {
    private final CounterService service;
    CombinedController(CounterService service) { this.service = service; }

    @Get("/counter") public int getCount() { return service.getCount(); }
    @Post("/counter/increment") public int increment() { return service.increment(); }
    @Post("/counter/decrement") public int decrement() { return service.decrement(); }
    @Post("/counter/reset") public int reset() { return service.reset(); }

    @Post("/echo") public Map<String,Object> echo(@Body Map<String,Object> payload) { return payload; }
    @Get("/greet") public String greet(@QueryValue(defaultValue="Guest") String name) { return "Hello " + name; }
}

Multiple endpoints: counter, echo, and greeting combined.

import io.micronaut.http.annotation.*;

@Controller("/health")
class HealthController {
    @Get
    public String health() { return "OK"; }
}

Simple health check endpoint returning status.

import io.micronaut.http.annotation.*;
import java.util.Map;

@Controller("/json")
class JsonController {
    @Post
    public String postJson(@Body Map<String,Object> payload) {
        return "Received: " + payload.toString();
    }
}

Receives JSON and responds with confirmation message.