Using Poor Man's DI to inject helper class dependencies
I don't see anything wrong with your implementation - if it works for you, and does not interfere with the system - go for it!

I will even go further and claim that the guard conditions in your internal constructor are redundant because the constructor will only be used by the single public constructor, or by a test which:


Either counts on the helper - in which case a NullPointerException will occur and fail the test, or
Does not need the helper, in which case there is no need to mock it...


Final note: I guess that the public constructor actually uses resourceService and serializer in some code you deleted from it, otherwise, the guard conditions there are also redundant...