Make sure to use concurrent collection implementations for properties holding mutable state. Especially the ones making use of computeIfAbsent(…).
Usage of ConcurrentReferenceHashMap allows us to move on without additional changes as it allows to store null values, whereas ConcurrentHashMap would require us to store explicit NullValue placeholders for such cases, potentially causing trouble in downstream store specific projects.
JMH Benchmarks done with the Spring Data MongoDB mapping layer showed a potential performance loss of up to 5%. However a comparison between ConcurrentReferenceHashMap and ConcurrentHashMap did not show any significant difference in performance.
Original pull request: #259.
309cdc5e43