Unit Test Json Serialization

If you're writing Spring Boot apps with Jackson, you'll eventually need to verify your DTOs serialize correctly. This gives you patterns for @JsonTest with JacksonTester, covering the essentials like field name mappings with @JsonProperty, custom serializers, nested objects, and polymorphic types. The examples are practical: testing that your API responses actually match what you expect, handling null fields properly, and validating date formats don't break between versions. It's focused on unit testing the JSON layer in isolation, not integration tests. Honestly, this is one of those things you set up once per project and then copy the pattern everywhere.