Agent Skills: Unit Testing JSON Serialization with `@JsonTest`

Unit Testing JSON Serialization with @JsonTest

Overview

Provides patterns for unit testing JSON serialization and deserialization using Spring's @JsonTest and Jackson. Covers POJO mapping, custom serializers, field name mappings, nested objects, date/time formatting, and polymorphic types.

When to Use

• Testing JSON serialization/deserialization of DTOs
• Verifying custom Jackson serializers/deserializers
• Validating @JsonProperty, @JsonIgnore, and field name mappings
• Testing date/time format handling (LocalDateTime, Date)
• Testing null handling and missing fields
• Testing polymorphic type deserialization

Instructions

1. Annotate test class with @JsonTest → Enables JacksonTester auto-configuration
2. Autowire JacksonTester for target type → Provides type-safe JSON assertions
3. Test serialization → Call json.write(object) and assert JSON paths with extractingJsonPath*
4. Test deserialization → Call json.parse(json) or json.parseObject(json) and assert object state
5. Validate round-trip → Serialize, then deserialize, verify same data (if object is properly comparable)
6. Test edge cases → Null values, missing fields, empty collections, invalid JSON
7. Add validation checkpoints: After each assertion, verify the test fails meaningfully with wrong data

Examples

Maven Setup

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-json</artifactId>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>

Gradle Setup

dependencies {
  implementation("org.springframework.boot:spring-boot-starter-json")
  testImplementation("org.springframework.boot:spring-boot-starter-test")
}

Basic Serialization and Deserialization

@JsonTest
class UserDtoJsonTest {

  @Autowired
  private JacksonTester<UserDto> json;

  @Test
  void shouldSerializeUserToJson() throws Exception {
    UserDto user = new UserDto(1L, "Alice", "alice@example.com", 25);
    JsonContent<UserDto> result = json.write(user);

    result
      .extractingJsonPathNumberValue("$.id").isEqualTo(1)
      .extractingJsonPathStringValue("$.name").isEqualTo("Alice")
      .extractingJsonPathStringValue("$.email").isEqualTo("alice@example.com")
      .extractingJsonPathNumberValue("$.age").isEqualTo(25);
  }

  @Test
  void shouldDeserializeJsonToUser() throws Exception {
    String json_content = "{\"id\":1,\"name\":\"Alice\",\"email\":\"alice@example.com\",\"age\":25}";
    UserDto user = json.parse(json_content).getObject();

    assertThat(user.getId()).isEqualTo(1L);
    assertThat(user.getName()).isEqualTo("Alice");
    assertThat(user.getEmail()).isEqualTo("alice@example.com");
    assertThat(user.getAge()).isEqualTo(25);
  }

  @Test
  void shouldHandleNullFields() throws Exception {
    String json_content = "{\"id\":1,\"name\":null,\"email\":\"alice@example.com\"}";
    UserDto user = json.parse(json_content).getObject();
    assertThat(user.getName()).isNull();
  }
}

Custom JSON Properties

public class Order {
  @JsonProperty("order_id")
  private Long id;

  @JsonProperty("total_amount")
  private BigDecimal amount;

  @JsonIgnore
  private String internalNote;
}

@JsonTest
class OrderJsonTest {

  @Autowired
  private JacksonTester<Order> json;

  @Test
  void shouldMapJsonPropertyNames() throws Exception {
    String json_content = "{\"order_id\":123,\"total_amount\":99.99}";
    Order order = json.parse(json_content).getObject();
    assertThat(order.getId()).isEqualTo(123L);
    assertThat(order.getAmount()).isEqualByComparingTo(new BigDecimal("99.99"));
  }

  @Test
  void shouldIgnoreJsonIgnoreFields() throws Exception {
    Order order = new Order(123L, new BigDecimal("99.99"));
    order.setInternalNote("Secret");
    assertThat(json.write(order).json).doesNotContain("internalNote");
  }
}

Nested Objects

public class Product {
  private Long id;
  private String name;
  private Category category;
  private List<Review> reviews;
}

@JsonTest
class ProductJsonTest {

  @Autowired
  private JacksonTester<Product> json;

  @Test
  void shouldSerializeNestedObjects() throws Exception {
    Product product = new Product(1L, "Laptop", new Category(1L, "Electronics"));
    JsonContent<Product> result = json.write(product);

    result
      .extractingJsonPathNumberValue("$.category.id").isEqualTo(1)
      .extractingJsonPathStringValue("$.category.name").isEqualTo("Electronics");
  }

  @Test
  void shouldDeserializeNestedObjects() throws Exception {
    String json_content = "{\"id\":1,\"name\":\"Laptop\",\"category\":{\"id\":1,\"name\":\"Electronics\"}}";
    Product product = json.parse(json_content).getObject();
    assertThat(product.getCategory().getName()).isEqualTo("Electronics");
  }

  @Test
  void shouldHandleListOfNestedObjects() throws Exception {
    String json_content = "{\"id\":1,\"reviews\":[{\"rating\":5},{\"rating\":4}]}";
    Product product = json.parse(json_content).getObject();
    assertThat(product.getReviews()).hasSize(2);
  }
}

Date/Time Formatting

@JsonTest
class DateTimeJsonTest {

  @Autowired
  private JacksonTester<Event> json;

  @Test
  void shouldFormatDateTimeCorrectly() throws Exception {
    LocalDateTime dt = LocalDateTime.of(2024, 1, 15, 10, 30, 0);
    json.write(new Event("Conference", dt))
      .extractingJsonPathStringValue("$.scheduledAt").isEqualTo("2024-01-15T10:30:00");
  }
}

Custom Serializers

public class CustomMoneySerializer extends JsonSerializer<BigDecimal> {
  @Override
  public void serialize(BigDecimal value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
    gen.writeString(value == null ? null : String.format("$%.2f", value));
  }
}

@JsonTest
class CustomSerializerTest {

  @Autowired
  private JacksonTester<Price> json;

  @Test
  void shouldUseCustomSerializer() throws Exception {
    json.write(new Price(new BigDecimal("99.99")))
      .extractingJsonPathStringValue("$.amount").isEqualTo("$99.99");
  }
}

Polymorphic Deserialization

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
@JsonSubTypes({
  @JsonSubTypes.Type(value = CreditCard.class, name = "credit_card"),
  @JsonSubTypes.Type(value = PayPal.class, name = "paypal")
})
public abstract class PaymentMethod { }

@JsonTest
class PolymorphicJsonTest {

  @Autowired
  private JacksonTester<PaymentMethod> json;

  @Test
  void shouldDeserializeCreditCard() throws Exception {
    String json_content = "{\"type\":\"credit_card\",\"id\":\"card123\"}";
    assertThat(json.parse(json_content).getObject()).isInstanceOf(CreditCard.class);
  }

  @Test
  void shouldDeserializePayPal() throws Exception {
    String json_content = "{\"type\":\"paypal\",\"id\":\"pp123\"}";
    assertThat(json.parse(json_content).getObject()).isInstanceOf(PayPal.class);
  }
}

Best Practices

• Test serialization AND deserialization for complete coverage
• Verify JSON paths individually rather than comparing full JSON strings
• Test null handling explicitly — null fields may be included or excluded depending on @JsonInclude
• Use extractingJsonPath* methods for precise field assertions
• Test round-trip: serialize an object, deserialize the JSON, verify the result matches
• Validate edge cases: empty strings, empty collections, deeply nested structures
• Group related assertions in a single test for clarity

Constraints and Warnings

• @JsonTest loads limited context: Only JSON-related beans; use @SpringBootTest for full Spring context
• Jackson version: Ensure annotation versions match the Jackson version in use
• Date formats: ISO-8601 is default; use @JsonFormat for custom patterns
• Null handling: Use @JsonInclude(Include.NON_NULL) to exclude nulls from serialization
• Circular references: Use @JsonManagedReference/@JsonBackReference to prevent infinite loops
• Immutable objects: Use @JsonCreator + @JsonProperty for constructor-based deserialization
• Polymorphic types: @JsonTypeInfo must correctly identify the subtype for deserialization to work

Debugging Workflow

When a JSON test fails, follow this workflow:

| Failure Symptom | Common Cause | How to Verify | |----------------|--------------|---------------| | JsonPath assertion fails | Field name mismatch | Check @JsonProperty spelling matches JSON key | | Null expected but got value | @JsonInclude(NON_NULL) configured | Verify annotation on field/class | | Deserialization returns wrong type | Missing @JsonTypeInfo | Add type info property to JSON or configure subtype mapping | | Date format mismatch | Format string incorrect | Confirm @JsonFormat(pattern=...) matches expected string | | Missing field in output | @JsonIgnore or transient modifier | Check field for @JsonIgnore or transient keyword | | Nested object is null | Inner JSON missing or malformed | Log parsed JSON; verify inner structure matches POJO | | JsonParseException | Malformed JSON string | Validate JSON syntax; check for unescaped characters |

Validation checkpoint after fixing: Re-run the test — if it passes, write a complementary test for the opposite case (e.g., if you fixed null handling, add a test for non-null values to prevent regression).

References

• Spring @JsonTest Documentation
• Jackson ObjectMapper
• Jackson Annotations