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", "[email protected]", 25);
    JsonContent<UserDto> result = json.write(user);

    result
      .extractingJsonPathNumberValue("$.id").isEqualTo(1)
      .extractingJsonPathStringValue("$.name").isEqualTo("Alice")
      .extractingJsonPathStringValue("$.email").isEqualTo("[email protected]")
      .extractingJsonPathNumberValue("$.age").isEqualTo(25);
  }

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

    assertThat(user.getId()).isEqualTo(1L);
    assertThat(user.getName()).isEqualTo("Alice");
    assertThat(user.getEmail()).isEqualTo("[email protected]");
    assertThat(user.getAge()).isEqualTo(25);
  }

  @Test
  void shouldHandleNullFields() throws Exception {
    String json_content = "{\"id\":1,\"name\":null,\"email\":\"[email protected]\"}";
    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