Unit Testing JSON Serialization with @JsonTest

Test JSON serialization and deserialization of POJOs using Spring's @JsonTest. Verify Jackson configuration, custom serializers, and JSON mapping accuracy.

When to Use This Skill

Use this skill when:

Testing JSON serialization of DTOs
Testing JSON deserialization to objects
Testing custom Jackson serializers/deserializers
Verifying JSON field names and formats
Testing null handling in JSON
Want fast JSON mapping tests without full Spring context

Setup: JSON Testing

Maven

<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>
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
</dependency>

Gradle

dependencies {
  implementation("org.springframework.boot:spring-boot-starter-json")
  implementation("com.fasterxml.jackson.core:jackson-databind")
  testImplementation("org.springframework.boot:spring-boot-starter-test")
}

Basic Pattern: @JsonTest

Test JSON Serialization

import org.springframework.boot.test.autoconfigure.json.JsonTest;
import org.springframework.boot.test.json.JacksonTester;
import org.junit.jupiter.api.Test;
import static org.assertj.core.api.Assertions.*;

@JsonTest
class UserDtoJsonTest {

  @Autowired
  private JacksonTester<UserDto> json;

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

    org.assertj.core.data.Offset 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)
      .isNotNull()
      .hasFieldOrPropertyWithValue("id", 1L)
      .hasFieldOrPropertyWithValue("name", "Alice")
      .hasFieldOrPropertyWithValue("email", "alice@example.com")
      .hasFieldOrPropertyWithValue("age", 25);
  }

  @Test
  void shouldHandleNullFields() throws Exception {
    String json_content = "{\"id\":1,\"name\":null,\"email\":\"alice@example.com\",\"age\":null}";

    UserDto user = json.parse(json_content).getObject();

    assertThat(user.getName()).isNull();
    assertThat(user.getAge()).isNull();
  }
}

Testing Custom JSON Properties

@JsonProperty and @JsonIgnore

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

  @JsonProperty("total_amount")
  private BigDecimal amount;

  @JsonIgnore
  private String internalNote;

  private LocalDateTime createdAt;
}

@JsonTest
class OrderJsonTest {

  @Autowired
  private JacksonTester<Order> json;

  @Test
  void shouldMapJsonPropertyNames() throws Exception {
    String json_content = "{\"order_id\":123,\"total_amount\":99.99,\"createdAt\":\"2024-01-15T10:30:00\"}";

    Order order = json.parse(json_content).getObject();

    assertThat(order.getId()).isEqualTo(123L);
    assertThat(order.getAmount()).isEqualByComparingTo(new BigDecimal("99.99"));
  }

  @Test
  void shouldIgnoreJsonIgnoreAnnotatedFields() throws Exception {
    Order order = new Order(123L, new BigDecimal("99.99"));
    order.setInternalNote("Secret note");

    JsonContent<Order> result = json.write(order);

    assertThat(result.json).doesNotContain("internalNote");
  }
}

Testing List Deserialization

JSON Arrays

@JsonTest
class UserListJsonTest {

  @Autowired
  private JacksonTester<List<UserDto>> json;

  @Test
  void shouldDeserializeUserList() throws Exception {
    String jsonArray = "[{\"id\":1,\"name\":\"Alice\"},{\"id\":2,\"name\":\"Bob\"}]";

    List<UserDto> users = json.parseObject(jsonArray);

    assertThat(users)
      .hasSize(2)
      .extracting(UserDto::getName)
      .containsExactly("Alice", "Bob");
  }

  @Test
  void shouldSerializeUserListToJson() throws Exception {
    List<UserDto> users = List.of(
      new UserDto(1L, "Alice"),
      new UserDto(2L, "Bob")
    );

    JsonContent<List<UserDto>> result = json.write(users);

    result.json.contains("Alice").contains("Bob");
  }
}

Testing Nested Objects

Complex JSON Structures

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

public class Category {
  private Long id;
  private String name;
}

public class Review {
  private String reviewer;
  private int rating;
  private String comment;
}

@JsonTest
class ProductJsonTest {

  @Autowired
  private JacksonTester<Product> json;

  @Test
  void shouldSerializeNestedObjects() throws Exception {
    Category category = new Category(1L, "Electronics");
    Product product = new Product(1L, "Laptop", category);

    JsonContent<Product> result = json.write(product);

    result
      .extractingJsonPathNumberValue("$.id").isEqualTo(1)
      .extractingJsonPathStringValue("$.name").isEqualTo("Laptop")
      .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())
      .isNotNull()
      .hasFieldOrPropertyWithValue("name", "Electronics");
  }

  @Test
  void shouldHandleListOfNestedObjects() throws Exception {
    String json_content = "{\"id\":1,\"name\":\"Laptop\",\"reviews\":[{\"reviewer\":\"John\",\"rating\":5},{\"reviewer\":\"Jane\",\"rating\":4}]}";

    Product product = json.parse(json_content).getObject();

    assertThat(product.getReviews())
      .hasSize(2)
      .extracting(Review::getRating)
      .containsExactly(5, 4);
  }
}

Testing Date/Time Formatting

LocalDateTime and Other Temporal Types

@JsonTest
class DateTimeJsonTest {

  @Autowired
  private JacksonTester<Event> json;

  @Test
  void shouldFormatDateTimeCorrectly() throws Exception {
    LocalDateTime dateTime = LocalDateTime.of(2024, 1, 15, 10, 30, 0);
    Event event = new Event("Conference", dateTime);

    JsonContent<Event> result = json.write(event);

    result.extractingJsonPathStringValue("$.scheduledAt").isEqualTo("2024-01-15T10:30:00");
  }

  @Test
  void shouldDeserializeDateTimeFromJson() throws Exception {
    String json_content = "{\"name\":\"Conference\",\"scheduledAt\":\"2024-01-15T10:30:00\"}";

    Event event = json.parse(json_content).getObject();

    assertThat(event.getScheduledAt())
      .isEqualTo(LocalDateTime.of(2024, 1, 15, 10, 30, 0));
  }
}

Testing Custom Serializers

Custom JsonSerializer Implementation

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

public class Price {
  @JsonSerialize(using = CustomMoneySerializer.class)
  private BigDecimal amount;
}

@JsonTest
class CustomSerializerTest {

  @Autowired
  private JacksonTester<Price> json;

  @Test
  void shouldUseCustomSerializer() throws Exception {
    Price price = new Price(new BigDecimal("99.99"));

    JsonContent<Price> result = json.write(price);

    result.extractingJsonPathStringValue("$.amount").isEqualTo("$99.99");
  }
}

Testing Polymorphic Deserialization

Type Information in JSON

@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 {
  private String id;
}

@JsonTest
class PolymorphicJsonTest {

  @Autowired
  private JacksonTester<PaymentMethod> json;

  @Test
  void shouldDeserializeCreditCard() throws Exception {
    String json_content = "{\"type\":\"credit_card\",\"id\":\"card123\",\"cardNumber\":\"****1234\"}";

    PaymentMethod method = json.parse(json_content).getObject();

    assertThat(method).isInstanceOf(CreditCard.class);
  }

  @Test
  void shouldDeserializePayPal() throws Exception {
    String json_content = "{\"type\":\"paypal\",\"id\":\"pp123\",\"email\":\"user@paypal.com\"}";

    PaymentMethod method = json.parse(json_content).getObject();

    assertThat(method).isInstanceOf(PayPal.class);
  }
}

Best Practices

Use @JsonTest for focused JSON testing
Test both serialization and deserialization
Test null handling and missing fields
Test nested and complex structures
Verify field name mapping with @JsonProperty
Test date/time formatting thoroughly
Test edge cases (empty strings, empty collections)

Common Pitfalls

Not testing null values
Not testing nested objects
Forgetting to test field name mappings
Not verifying JSON property presence/absence
Not testing deserialization of invalid JSON

Troubleshooting

JacksonTester not available: Ensure class is annotated with @JsonTest.

Field name doesn't match: Check @JsonProperty annotation and Jackson configuration.

DateTime parsing fails: Verify date format matches Jackson's expected format.

unit-test-json-serialization