Unit Test Mapper Converter

This handles the unglamorous but critical work of testing MapStruct mappers and custom converters. It walks you through validating field mappings, null handling, bidirectional transformations, and nested object conversions with concrete patterns for each scenario. The troubleshooting is practical: what to do when generated classes are missing, how to catch asymmetries in entity-to-DTO roundtrips, and why your custom expressions might be failing silently. Includes a complete executable test example with imports so you're not hunting down AssertJ syntax. Honest take: mapper tests feel tedious until a silent mapping bug corrupts data in production, then you wish you'd written them from the start.