Clarify configuration lite support and offer an opt-in option

[snicoll]

The Spring Boot team has been investigating startup time improvements by not relying on the proxy capabilities of configuration classes. Turns out that not using that stuff (basically switching all auto-configurations and related imports to "lite mode") makes quite a difference spring-projects/spring-boot#9068 (comment)

I am not a big fan that we were looking for @Bean methods in any component, treating them in lite mode if they don't have the @Configuration stereotype. I was wondering if we would be open to revisit this in the 5.2 line.

Being able to opt-out from the proxy mode using the @Configuration stereotype would be ideal. We are not keen to remove @Configuration and use the lite mode as it stands as:

• That's not representative of what a good citizen of the framework should do (using the right stereotype)
• We have an annotation processor that identifies configuration classes to extract conditions so that we can back off (without loading the ASM metadata or the class) when a static condition does not match (i.e. class not on the classpath)

Having a way to opt-out would be also a nice opportunity to clarify the behaviour and stop scanning components for bean factory methods.

[wilkinsona]

I'd like to echo Stephane's dislike of using @Bean on any component. I find the behaviour surprising and it's caused confusion in the past. I too would prefer that you have to opt in to @Bean method support via a stereotype annotation that explicitly indicates that a class is a configuration class. Once you've opted in to being a configuration class and @Bean methods defining beans, we'd then need a way to opt out of proxying. Given that the use of proxies is more than an implementation detail and is already something that a user needs to think about (@Configuration classes cannot be final, for example), I wonder if a proxy attribute on @Configuration that defaults to true would suffice here.

In terms of the benefits of not proxying @Configuration classes, the table in the linked Spring Boot issue shows some of the performance benefits in terms of startup time. There are also other benefits with both metaspace and code cache usage being reduced due to fewer classes being loaded and subsequently processed by the JIT.

@philwebb raised another benefit of being able to identify a configuration class where @Bean methods should be supported but where proxying should not be used. It would allow an application to be started in a mode where the configuration classes were proxied anyway, but purely for the purpose of catching and warning about accidental interdependencies between @Bean methods. I modified Framework to use this technique to identify @Configuration classes in Spring Boot that currently rely on proxying and it proved very useful.

[sbrannen]

Historically related issues:

• spring-test allows you to reference configuration classes that aren't annotated with @Configuration [SPR-9051] #13690
• Improve documentation for @Bean 'lite' mode and annotated class support in the TestContext Framework [SPR-9401] #14037
• Document @Bean 'lite' mode and annotated classes in the reference manual [SPR-9425] #14061

[sbrannen]

While discussing this with @wilkinsona, I proposed we name the boolean flag in @Configuration something like proxyMethodInvocations = false analogous to proxyTargetClass in other places.

He came back with "proxyBeanMethodInvocations or perhaps proxyBeanMethods to make it clear that it’s only @Bean methods that are affected."

I tend to like @Configuration(proxyBeanMethods = false) to turn off the default behavior.

[jhoeller]

I have an implementation ready to be committed now, using a proxyBeanMethods flag as suggested above. (Even if this is technically not "proxying" in the AOP sense but rather method interception through overriding in the same instance, I suppose it can still be called proxying in a general sense.)

Whatever we may feel about it, I can't see how we'd limit @Bean methods to @Configuration classes at this point... without breaking plenty of people who intentionally or unintentionally don't use the @Configuration stereotype for some of their @Bean-declaring classes. This has been out there for 10 years and I am not inclined to break this, even if it is now possible to effectively get the same behavior using a new flag on @Configuration... Arguably the very fact that this is now a well-defined mode of regular configuration classes makes it easier to understand that the same behavior is applicable to individual @Bean methods outside of configuration classes as well, with less surprise that there is no method interception through CGLIB subclasses there, and that this is a proper variant of operation.

As for the annotation detection overhead, let's tackle this along with our other issues for it. Once we have a method-level indication in the components index, we'll get efficient skipping not only for @Bean checks but also for @EventListener, @Scheduled, @Transactional, the caching annotations etc. And if we introduce corresponding programmatic hints at the bean definition level, those should be equally applicable to the detection of all those method-level annotations so that we never reflectively iterate the methods of the affected component classes - not for @Bean detection and not for AOP proxying either.

[wilkinsona]

Really pleased to see this land. Thanks!

[sbrannen]

@jhoeller, shall we reopen this issue to update the Reference Manual, or are you planning on doing that in a new issue?

[wilkinsona]

I've noticed an interesting side-effect of this change that I hadn't anticipated.

When I switch from @Configuraton to @Configuration(proxyBeanMethods = false) the bean definitions for the class's @Bean methods change from having a Class beanClass to having a String beanClass. This became apparent due to FactoryBeans now being instantiated more aggressively as this type determination shortcut no longer happens. In the scenario where the change became apparent, this change in behaviour results in a BeanCurrentlyInCreationException. I can work around the problem by enabling bean method proxying on the @Configuration class that defines the factory bean but it feels like that shouldn't be necessary.

[jhoeller]

@wilkinsona A strange effect indeed, I'll have a look at it this morning... I'm into that area anyway with the remainder of #22420.