The registration of more than one ConfigurationClassPostProcessor
results in the double-enhancement of @Configuration classes, i.e. a
two-deep CGLIB subclass hierarchy is created.
As a side-effect of changes introduced in 3.1 M2 fixing SPR-8080, this
behavior now results in an infinite loop at CGLIB callback processing
time, leading to a StackOverflowException which is then suppressed by
the container, and ultimately results in the user being presented with
an unintuitive "Bean 'x' is not already in creation" exception.
This fix introduces a marker interface 'EnhancedConfiguration' to be
implemented by all generated @Configuration subclasses. The
configuration class enhancer can then behave in an idempotent fashion
by checking to see whether a candidate @Configuration class is already
assignable to this type i.e. already enhanced and ignore it if so.
Naturally, users should avoid registering more than one
ConfigurationClassPostProcessor, but this is not always possible. As
with the case in point, SPR-8824 originates from problems with
spring-data-neo4j, which explicitly registers its own
ConfigurationClassPostProcessor. The user has little control over this
arrangement, so it is important that the framework is defensive as
described above.
Issue: SPR-8824
224cf11fcb