Provide a configuration option to enable lazy initialisation

[wilkinsona]

The bean factory that is used by default in a Spring Boot application performs eager initialisation of all singleton beans. There has been some interest in performing initialisation lazily so we want to provide a configuration option that will allow users to opt in to this behaviour. When enabled, beans will be initialised on first use. In a web application, this will mean that a large number of beans are initialised on first request rather than during the application's startup.

The main benefit of lazy initialisation is that initial startup time is reduced. However, this comes at a cost:

1. Handling of HTTP requests may take longer while any deferred initialisation occurs
2. Failures that would normally occur at startup will now not occur until later

[bishoybasily]

hi @wilkinsona ,
i'm just asking if something like this can do the job ?

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Import(ProcessorConfiguration.class)
public @interface EnableLazyInitByDefault { }

public class ProcessorConfiguration {

    @Bean
    public FactoryProcessor factoryProcessor() {
        return new FactoryProcessor();
    }

}

public class FactoryProcessor implements BeanFactoryPostProcessor {

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {

        for (String s : beanFactory.getBeanDefinitionNames()) {
            BeanDefinition beanDefinition = beanFactory.getBeanDefinition(s);
            if (beanDefinition instanceof AnnotatedBeanDefinition) {
                AnnotatedBeanDefinition annotatedBeanDefinition = (AnnotatedBeanDefinition) beanDefinition;
                MethodMetadata methodMetadata = annotatedBeanDefinition.getFactoryMethodMetadata();
                if (methodMetadata != null)
                    beanDefinition.setLazyInit(!methodMetadata.getDeclaringClassName().startsWith("org.springframework"));
            }
        }

    }

}

i know it's already assigned, i'm just validating my understanding!
thank you.

[wilkinsona]

Yeah, that's a large part of it. We're currently planning on enabling lazy init of all beans, not just those in a particular package space. There's also a bit of work to do to make things lazy in a WebFlux app. If you'd like to see what the code currently looks like, it is available in this branch.

[wilkinsona]

This needs some reference documentation then we're happy for it to be merged.

[wilkinsona]

Reopening as DevTools restart does not work with lazy initialization enabled.

[tkvangorder]

Hi Andy,

I am glad to see this in the official codebase.

Back when I had originally purposed the idea in the context of integration tests,
we had run into some issues with the Spring Integration DSL:

#9685

Once we migrate to Boot 2.2, will we still have to exclude the flows or was there some other workaround?

Thanks!