I think the real problem is this DTOMapper. Sounds like a really sad architecture/ framework. It should have never been created at first place, then no one would have idea of testing it.
I know - if you follow "industry standard" of having anemic domain model with javabeans - you quickly get that abominations - your choice.
Don't get me started with the new industry standard: annotate-all-the-things.
See youtube.com/watch?v=-6zT60l5hDc for a good presentation about it. Maybe you are familiar ;).
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