ControllerTest - GSON VS ObjectMapper

The problematic code

@AutoConfigureMockMvc  
@WebMvcTest(UserController.class)  
public class UserControllerTest {  
  
  @Autowired  
  MockMvc mockMvc;  
  
  @MockitoBean  
  UserService userService;  
  
  @MockitoBean  
  UserStatusService userStatusService;  
  
  private final Gson gson = new Gson();  
 
  @DisplayName("유저 Id로 UserStatus 업데이트 ")
  @Test
  void updateUserStatusByUserId_returnsUserStatusDto() throws Exception {
    // ================== given ==================
    UUID userId = UUID.randomUUID();
    Instant newLastActiveAt = Instant.now().minusSeconds(10);
    UserStatusUpdateRequest request = new UserStatusUpdateRequest(
        newLastActiveAt
    );
 
    UserStatusDto expectedUserStatusDto = new UserStatusDto(
        UUID.randomUUID(),
        userId,
        newLastActiveAt
    );
 
    when(userStatusService.updateByUserId(userId, request))
        .thenReturn(expectedUserStatusDto);
 
    // ================== when & then ==================
    mockMvc.perform(patch("/api/users/" + userId + "/userStatus")
            .contentType(MediaType.APPLICATION_JSON)
            .content(gson.toJson(request)))
        .andExpect(status().isOk())
        .andExpect(jsonPath("$.userId").value(userId.toString()))
        .andExpect(jsonPath("$.lastActiveAt").value(newLastActiveAt.toString()));
  }
 

public record UserStatusUpdateRequest(  
    @NotNull(message = "마지막 활동 시간은 필수입니다.")  
    @PastOrPresent(message = "마지막 활동 시간은 현재 또는 과거여야 합니다.")  
    Instant newLastActiveAt  
) {  
  
}

Error Message:

com.google.gson.JsonIOException: Failed making field 'java.time.Instant#seconds' accessible; either increase its visibility or write a custom TypeAdapter for its declaring type.

The problem:

• Gson and Jackson (ObjectMapper) have fundamentally different default strategies for how they “read” your Java objects to turn them into JSON
• Gson
  • Since Java 9 the Java platform has become much stricter about security and encapsulation. Core Java classes like java.time.Instant are “locked” and don’t allow reflection to access their private fields (like seconds)
  • So if you’re using GSON you would have to make a custom TypeAdapter
• Jackson
  • By default, Jackson’s main strategy is to use public accessor methods (getters/setters) and specialized modules
  • It has a powerful module system. Spring Boot automatically includes a critical module called JavaTimeModule (jackson-datatype-jsr310).
    • How It Works: This module is an expert on handling Java 8+ Date and Time API classes. It knows the correct, public way to serialize an Instant

Feature	Gson (Default)	Jackson / ObjectMapper
Primary Method	Reflection (accessing private fields)	Accessors (using public getters) & Modules
Java 9+ Compatibility	Can fail on core JDK classes due to encapsulation.	Works perfectly due to modules that use the public API.
Spring Boot Default	No	✅ Yes

After (Solution) - use ObjectMapper

@AutoConfigureMockMvc  
@WebMvcTest(UserController.class)  
public class UserControllerTest {  
  
  @Autowired  
  MockMvc mockMvc;  
  
  @MockitoBean  
  UserService userService;  
  
  @MockitoBean  
  UserStatusService userStatusService;  
  
  @Autowired  
  private ObjectMapper objectMapper;
 
 
  @DisplayName("유저 Id로 UserStatus 업데이트 ")
  @Test
  void updateUserStatusByUserId_returnsUserStatusDto() throws Exception {
    // ================== given ==================
    UUID userId = UUID.randomUUID();
    Instant newLastActiveAt = Instant.now().minusSeconds(10);
    UserStatusUpdateRequest request = new UserStatusUpdateRequest(
        newLastActiveAt
    );
 
    UserStatusDto expectedUserStatusDto = new UserStatusDto(
        UUID.randomUUID(),
        userId,
        newLastActiveAt
    );
 
    when(userStatusService.updateByUserId(userId, request))
        .thenReturn(expectedUserStatusDto);
 
    // ================== when & then ==================
    mockMvc.perform(patch("/api/users/" + userId + "/userStatus")
            .contentType(MediaType.APPLICATION_JSON)
            .content(objectMapper.writeValueAsString(request)))
        .andExpect(status().isOk())
        .andExpect(jsonPath("$.userId").value(userId.toString()))
        .andExpect(jsonPath("$.lastActiveAt").value(newLastActiveAt.toString()));
  }
}