3.4 Injectable Container Types

In addition to being able to inject beans, Micronaut natively supports injecting the following types:

@Singleton
class AggregateEngine implements Engine {
  @Inject
  List<Engine> engines;
  @Override
  public void start() {
    engines.forEach(Engine::start);
  }
  …
}

Collection Ordering

When injecting a collection of beans, they are not ordered by default. Implement the Ordered interface to inject an ordered collection. If the requested bean type does not implement Ordered, Micronaut searches for the @Order annotation on beans.

The @Order annotation is especially useful for ordering beans created by factories where the bean type is a class in a third-party library. In this example, both LowRateLimit and HighRateLimit implement the RateLimit interface.

Factory with @Order

import io.micronaut.context.annotation.Factory;
import io.micronaut.core.annotation.Order;
import jakarta.inject.Singleton;
import java.time.Duration;
@Factory
public class RateLimitsFactory {
    @Singleton
    @Order(20)
    LowRateLimit rateLimit2() {
        return new LowRateLimit(Duration.ofMinutes(50), 100);
    }
    @Singleton
    @Order(10)
    HighRateLimit rateLimit1() {
        return new HighRateLimit(Duration.ofMinutes(50), 1000);
    }
}

Factory with @Order

import io.micronaut.context.annotation.Factory
import io.micronaut.core.annotation.Order
import jakarta.inject.Singleton
import java.time.Duration
@Factory
class RateLimitsFactory {
    @Singleton
    @Order(20)
    LowRateLimit rateLimit2() {
        new LowRateLimit(Duration.ofMinutes(50), 100);
    }
    @Singleton
    @Order(10)
    HighRateLimit rateLimit1() {
        new HighRateLimit(Duration.ofMinutes(50), 1000);
    }
}

Factory with @Order

import io.micronaut.context.annotation.Factory
import io.micronaut.core.annotation.Order
import java.time.Duration
import jakarta.inject.Singleton
@Factory
class RateLimitsFactory {
    @Singleton
    @Order(20)
    fun rateLimit2(): LowRateLimit {
        return LowRateLimit(Duration.ofMinutes(50), 100)
    }
    @Singleton
    @Order(10)
    fun rateLimit1(): HighRateLimit {
        return HighRateLimit(Duration.ofMinutes(50), 1000)
    }
}

When a collection of RateLimit beans are requested from the context, they are returned in ascending order based on the value in the annotation.

Injecting a Bean by Order

When injecting a single instance of a bean the @Order annotation can also be used to define which bean has the highest precedence and hence should be injected.