unit-test-json-serialization

This skill provides patterns for unit testing JSON serialization and deserialization in Spring Boot applications using Spring's @JsonTest slice and Jackson. It solves the problem of verifying that DTOs map correctly to and from JSON, that custom serializers behave as intended, and that field mappings, date formats, and polymorphic types are handled reliably, without spinning up a full application context.

The skill explains the JacksonTester-based workflow step by step: annotate the test class with @JsonTest, autowire a JacksonTester for the target type, assert serialization output with extractingJsonPath* helpers, parse JSON back into objects to assert state, validate round-trips, and cover edge cases like null values, missing fields, empty collections, and invalid JSON. It includes concrete, copy-ready examples for Maven and Gradle setup, basic serialize/deserialize tests, @JsonProperty and @JsonIgnore field mapping, nested objects and lists, LocalDateTime formatting, custom JsonSerializer implementations, and polymorphic deserialization with @JsonTypeInfo and @JsonSubTypes. It also recommends adding validation checkpoints to ensure each assertion fails meaningfully on wrong data.

It targets Java and Spring Boot developers writing or hardening JSON-related unit tests. Typical use cases include testing REST DTO contracts, verifying custom Jackson serializers and deserializers, confirming that ignored fields never leak into output, and validating type discrimination for polymorphic payloads.