In Micronaut applications, Singleton beans are typically initialized lazily by default. This deliberate choice aims to enhance the application startup speed. However, a drawback of this approach is that potential wiring errors may only surface when the bean is actually initialized. To mitigate this, we can employ a startup test.

Consider the following example:

@StartupTest
class MyAppStartupTest {
    @Inject
    private EmbeddedApplication application;

    @Test
    void applicationIsRunning() {
        assertThat(application.isRunning()).isTrue();
    }
}

Notice the StartupTest annotation? Its definition is as follows:

@MicronautTest(contextBuilder = TestEagerInitAppContextBuilder.class)
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface StartupTest {
}

Of particular note is the custom context builder referenced in the annotation, TestEagerInitAppContextBuilder

public class TestEagerInitAppContextBuilder extends DefaultApplicationContextBuilder {
    public TestEagerInitAppContextBuilder() {
        deduceEnvironment(false);
        eagerInitConfiguration(true);
        eagerInitSingletons(true);
    }
}

In this custom context builder, we explicitly disable automatic environment resolution while requesting eager initialization for both singletons and configuration beans. This strategic adjustment allows us to identify and rectify potential wiring errors early in the application lifecycle.