Spring Boot BackgroundPreinitializer

[Sebastien Deleuze]

Hi,

I would like to bring to the attention of the working group
https://github.com/spring-projects/spring-boot/blob/main/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/BackgroundPreinitializer.java
that we are using to speedup Spring Boot startup time on machines with at
least 2 cores on the JVM.

It creates and drops instances of popular but costly to initialize
instances (Jackson, Hibernate Validator, etc.) and invoke some JDK methods (
StandardCharsets.UTF_8.name(), ZoneId.systemDefault()) to pre-load related
classes and pre-initialize related static fields on a dedicated thread. I
think with the AOT cache, the class-preloading effect is not visible
anymore, maybe the pre-initialization of static fields still has an impact.

Ideally, I would hope that this kind of optimization will not be needed
anymore with Leyden AOT cache in order to avoid creating instances that we
drop, avoid to throw related exceptions when the libraries are not in the
classpath, etc.

I would like to ask feedback on those 3 points:
 - It may be interesting to compare what BackgroundPreinitializer does at
runtime and the effect of AOT cache on related classes as we know those
ones are impacting badly the startup time of typical enterprise
applications and see if AOT cache could be refined with optimizations
similar to what BackgroundPreinitializer does at runtime.
- If we can't optimize more automatically, I am wondering if the idea of
giving the possibility to frameworks to specify a list of costly classes
safe to initialize in the AOT cache makes sense or not (Spring Boot could
configure the one present in BackgroundPreinitializer for example).
 - It could be potentially useful to publish an API to know if AOT caching
is enabled or not to allow us to disable this kind of mechanism in that
case, something simple and fast like a method returning a boolean.

 Any thoughts?

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