Use computeIfAbsent to simplify some code, in addition, the content of some Javadoc has been adjusted.

Comment From: mdeinum

@sbrannen wasn't this computeIfAbsent a cause for some race conditions/wierd behavior in other parts of the framework?

Comment From: sbrannen

@sbrannen wasn't this computeIfAbsent a cause for some race conditions/wierd behavior in other parts of the framework?

Yes, computeIfAbsent() can lead to issues if the mappingFunction calls back into the same map. See https://github.com/spring-projects/spring-framework/issues/25801#issuecomment-698346498 for details.