java.lang.InstantiationError after upgrading to Spring Boot 2.4.4 (spring data commons 2.4.6)

In a Spring Boot application we use Spring Data Mongo to map some documents. Some of them have embedded documents with a hierarchy, e.g.:

@Document("projects")
@TypeAlias("project")
data class Project(
       ...
       val budget: ProjectBudget)

abstract class ProjectBudget {

    abstract fun getCurrentBudget(): MonetaryAmount?

}

@TypeAlias("projectProjectBudget")
class ClosedProjectBudget(... ) : ProjectBudget() {}

Up to Spring Boot 2.4.3 when retrieving a Project instance the corresponding ProjectBudget subclass what built correctly. However after the upgrade to Boot 2.4.4 we get this exception:

Caused by: java.lang.InstantiationError: xxx.ProjectBudget
	at xxx.ProjectBudget_Instantiator_srp3na.newInstance(Unknown Source)
	at org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator$EntityInstantiatorAdapter.createInstance(ClassGeneratingEntityInstantiator.java:238)
	at org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator.createInstance(ClassGeneratingEntityInstantiator.java:87)
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:344)

There seems to be a regression when instantiating subclasses.

[mp911de]

This looks actually as if the subclass isn’t registered with MappingContext. To properly resolve the target class using TypeAlias, it must be already present in the context.

Can you isolate the issue and provide a minimal reproducer?

[mp911de]

I entirely missed the fact of the custom Mongo configuration MongoDbClientConfiguration hence I relied on Spring Boot to perform the entity scan.

I moved the ticket into Spring Data MongoDB. The difference comes from #3592 where we removed @Persistent as marker annotation from the entity scan in Spring Data MongoDB. Previously, this arrangement worked because @TypeAlias is meta-annotated with @Persistent and so entities annotated with @TypeAlias are picked up as entities during the scan.

ServiceProjectBudget and ProjectProjectBudget aren’t subtypes of Project hence the @Document annotation doesn’t apply these types. So annotating both types with @Document is the way to go and it worked previously rather by accident and not by intent.