Non-Null check on nested collection mapping

I have a set of classes like the following

ClassRoot, ClassA, ClassB

ClassRootDto, ClassADto, ClassBDto

where

ClassRoot has a list of ClassA, and ClassA has a list of ClassB

ClassRootDto has a list of ClassADto, and ClassADto has a list of ClassBDto

I tried to map a ClassRoot instance to a ClassRootDto instance

The list of ClassADto instances is explicit set to the field of ClassRootDto instance without any problem.

However, when set the list of ClassBDto to the field of a ClassADto instance, the generated code does a null check on the target field.

Why are there different null-check behaviors for different level of mappings?

My current solution:
Initialize an empty ClassBDto list in ClassADto field

My expectation
I expect there should be no null-check on target field when doing collection mapping

首先复现 issue 中描述的 ClassRoot、ClassA 和 ClassB 嵌套映射，然后检查为 ClassBDto collection 赋值的生成 mapper。将这段生成的代码与 ClassADto collection 的赋值进行比较，并确定 target null 检查是否应有所不同。当预期行为得到明确并由回归测试覆盖时，即表示完成。