3.7.1.1 Eager Initialization of Singletons

Eager initialization of @Singleton beans maybe desirable in certain scenarios, such as on AWS Lambda where more CPU resources are assigned to Lambda construction than execution.

You can specify whether to eagerly initialize @Singleton-scoped beans using the ApplicationContextBuilder interface:

Enabling Eager Initialization of Singletons

public class Application {
    public static void main(String[] args) {
        Micronaut.build(args)
            .eagerInitSingletons(true) (1)
            .mainClass(Application.class)
            .start();
    }
}

When you use Micronaut in environments such as Serverless Functions, you will not have an Application class and instead you extend a Micronaut-provided class. In those cases, Micronaut provides methods which you can override to enhance the ApplicationContextBuilder

Override of newApplicationContextBuilder()

public class MyFunctionHandler extends MicronautRequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {
...
    @Nonnull
    @Override
    protected ApplicationContextBuilder newApplicationContextBuilder() {
        ApplicationContextBuilder builder = super.newApplicationContextBuilder();
        builder.eagerInitSingletons(true);
        return builder;
    }
    ...
}

@ConfigurationReader beans such as @EachProperty or @ConfigurationProperties are singleton beans. To eagerly init configuration but keep other @Singleton-scoped bean creation lazy, use eagerInitConfiguration:

Enabling Eager Initialization of Configuration

public class Application {
    public static void main(String[] args) {
        Micronaut.build(args)
            .eagerInitConfiguration(true) (1)
            .mainClass(Application.class)
            .start();
    }
}