unit-test-json-serialization_skill

Overview

This skill provides practical patterns for unit testing JSON serialization and deserialization using Spring's @JsonTest and Jackson. It focuses on fast, focused tests for POJOs, custom serializers/deserializers, date/time formatting, polymorphic types, nested structures, and null handling without loading a full Spring context. Examples and pitfalls are included to make tests reliable and maintainable.

How this skill works

Annotate tests with @JsonTest to auto-configure JacksonTester for type-safe JSON assertions. Use JacksonTester to write objects to JSON and parse JSON back to objects, combine JsonPath extractions for targeted assertions, and verify behaviors like @JsonProperty mappings, @JsonIgnore exclusions, custom JsonSerializer output, and temporal formatting. Tests run quickly because only JSON-related beans are loaded.

When to use it

• Validating DTO mapping for REST requests and responses
• Verifying custom JsonSerializer/JsonDeserializer implementations
• Ensuring date/time fields serialize to and parse from expected formats
• Testing nested objects, collections, and polymorphic types
• Checking null handling and presence/absence of JSON properties

Best practices

• Annotate with @JsonTest and inject JacksonTester<T> for focused tests
• Always test both serialization and deserialization for critical fields
• Use JsonPath assertions to check specific paths instead of string matching
• Cover nulls, empty collections, and missing fields explicitly
• Test custom serializers and @JsonProperty mappings with dedicated examples

Example use cases

• Unit test a UserDto to ensure field names and types map correctly to JSON
• Verify a custom money serializer emits a '$99.99' string for BigDecimal
• Deserialize an array of UserDto and assert order and values
• Confirm LocalDateTime fields serialize as ISO-8601 and parse back
• Validate polymorphic PaymentMethod deserialization using type metadata